Direct Measurements of Polyelectrolyte Brush Responses using Atomic Force and Optical Microscopy

Direct Measurements of Polyelectrolyte Brush Responses using Atomic Force and Optical Microscopy Dissertation Von der Universität Bayreuth zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung Fakultät für Biologie, Chemie und Geowissenschaften Lehrstuhl Physikalische Chemie II von Johann Erath Diplom-Physiker geboren in Wasserlos, Alzenau Erstgutachter: Prof. Dr. Andreas Fery Zweitgutachter: Prof. Dr. Georg Papastavrou Dissertation eingereicht: 02.07.2013 Wissenschaftliches Kolloquium: 11.11.2013 I Die vorliegende Arbeit wurde in der Zeit von Juli 2009 bis Juli 2013 am Lehrstuhl Physikalische Chemie II unter der Betreuung von Prof. Dr. Andreas Fery an der Universität Bayreuth angefertigt. Vollständiger Abdruck der von der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.). Dissertation eingereicht: 02.07.2013 Zulassung durch die Prüfungskommission: 10.07.2013 Wissenschaftliches Kolloquium: 11.11.2013 Amtierender Dekan: Prof. Dr. Rhett Kempe Prüfungsausschuss: Prof. Dr. Andreas Fery (Erstgutachter) Prof. Dr. Georg Papastavrou (Zweitgutachter) Prof. Dr. Josef Breu (Vorsitz) Prof. Dr. Andreas Greiner Contents List of Publications 1 1 Overview 3 1.1 Outline.................................... 5 1.2 Content of the Individual Chapters . . . . . . . . . . . . . . . . . . . . 5 1.3 Individual Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1.4 References.................................. 24 2 Theory and Status of the Field 25 2.1 Introduction................................. 27 2.2 From Polymers to Polyelectrolyte Brushes . . . . . . . . . . . . . . . . 28 2.2.1 NeutralPolymers.......................... 29 2.2.2 Polyelectrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2.3 Self Assembly of Polyelectrolytes . . . . . . . . . . . . . . . . . 38 2.2.4 Functionalization of Surfaces with Polyelectrolytes . . . . . . . . 41 2.2.5 PolymerBrushes .......................... 42 2.3 Surface and Interfacial Forces . . . . . . . . . . . . . . . . . . . . . . . 54 2.3.1 The Derjaguin Approximation . . . . . . . . . . . . . . . . . . . 55 2.3.2 Van der Waals Interactions . . . . . . . . . . . . . . . . . . . . . 56 2.3.3 Interactions of Charged Systems . . . . . . . . . . . . . . . . . . 57 2.3.4 Capillary Interactions . . . . . . . . . . . . . . . . . . . . . . . . 58 2.3.5 Steric Interactions . . . . . . . . . . . . . . . . . . . . . . . . . 60 2.3.6 Contact Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . 63 2.4 Experimental Methods: Atomic Force and Optical Microscopy . . . . . 69 III CONTENTS IV 2.4.1 Atomic Force Microscopy (AFM) . . . . . . . . . . . . . . . . . 69 2.4.2 Optical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 76 2.5 References.................................. 81 3 Soft Colloidal Probe AFM 95 3.1 Introduction................................. 97 3.2 Experimental ................................ 99 3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 3.4 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 107 3.5 References.................................. 109 3.A Supporting Information . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 4 Mechanoresponsive Polyelectrolyte Brushes 119 4.1 Introduction................................. 121 4.2 Experimental ................................ 121 4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 4.4 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 126 4.5 References.................................. 128 4.A Supporting Information . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 5 Phototunable Surface Interactions 145 5.1 Introduction................................. 147 5.2 Experimental ................................ 149 5.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 5.4 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 156 5.5 References.................................. 159 5.A Supporting Information . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 6 Interactions of Spherical Polyelectrolyte Brushes 167 6.1 Introduction................................. 169 6.2 Experimental ................................ 171 6.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 6.4 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 182 6.5 References.................................. 186 CONTENTS V 7 Swelling Behavior of Block Copolymer Micelles 193 7.1 Introduction................................. 195 7.2 Experimental ................................ 197 7.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 7.4 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 211 7.5 References.................................. 213 7.A Supporting Information . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 8 Further Perspectives 223 8.1 Direct Measurements of Contact Stresses of Soft Materials . . . . . . . 225 8.2 Contact and Adhesion of Biomimetic Patterned Adhesives . . . . . . . 227 8.3 Tuning the Response of Mechanoresponsive Brushes . . . . . . . . . . . 239 8.3.1 Understanding of the Mechanoresponse . . . . . . . . . . . . . . 240 8.3.2 Change of the Detection Scheme . . . . . . . . . . . . . . . . . . 244 8.4 References.................................. 248 9 Summary 251 10 Zusammenfassung 257 A Theory of Polymer Brushes 265 A.1 References.................................. 270 B A Little Coding with Igor 271 Danke 273 List of Figures 1.1 The soft colloidal probe technique . . . . . . . . . . . . . . . . . . . . . 8 1.2 Mechanoresponsive surfaces . . . . . . . . . . . . . . . . . . . . . . . . 11 1.3 Phototunable surface interactions . . . . . . . . . . . . . . . . . . . . . 14 1.4 Interactions of Spherical Polyelectrolyte Brushes . . . . . . . . . . . . . 16 1.5 Swelling of Block Copolymer Micelles . . . . . . . . . . . . . . . . . . . 18 2.1 Responsivesystems............................. 27 2.2 Polymersystems .............................. 29 2.3 PotentialofaPE.............................. 35 2.4 Adsorption of polyelectrolytes . . . . . . . . . . . . . . . . . . . . . . . 39 2.5 Self assembly of polyelectrolytes . . . . . . . . . . . . . . . . . . . . . . 40 2.6 Functionalization of surfaces with polyelectrolytes . . . . . . . . . . . . 43 2.7 Different types of polymer brushes . . . . . . . . . . . . . . . . . . . . . 44 2.8 Preparation of polymer brushes . . . . . . . . . . . . . . . . . . . . . . 46 2.9 Grafted polymer chains . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 2.10 Parameter of a cationic polymer brush . . . . . . . . . . . . . . . . . . 48 2.11 Brush properties as a function of salt concentration . . . . . . . . . . . 52 2.12 Phase diagram of polymer brushes . . . . . . . . . . . . . . . . . . . . . 53 2.13 The Derjaguin approximation . . . . . . . . . . . . . . . . . . . . . . . 55 2.14 Interactions of charged Surfaces . . . . . . . . . . . . . . . . . . . . . . 59 2.15 Interactions of polymer brushes . . . . . . . . . . . . . . . . . . . . . . 62 2.16Contactparameters............................. 64 2.17 Contact mechanics of elastic bodies . . . . . . . . . . . . . . . . . . . . 67 2.18 Availability of contact mechanic models . . . . . . . . . . . . . . . . . . 68 VII ABBREVIATIONS AND SYMBOLS XIV IIonic strength aKuhn length MMolecular weight NNumber of monomeres NA Numerical aperture Permittivity LpPersistence lenth νPoisson ratio UPotential PPressure RRadius, reduced radius KReduced modulus nRefractive index fres Resonance frequency kSpring constant iStrain in i direction σiStress in i direction σSSurface charge density µTTabor parameter TTemperature wWork of adhesion EYoung’s modulus List of Publications 1. Characterization of Adhesion Phenomena and Contact of Surfaces by Soft Colloidal Probe AFM. Erath, J., Schmidt, S., and Fery, A., Soft Matter, 2010. 6(7): p. 1432-1437. 2. Direct Correlation between Local Pressure and Fluorescence Output in Mechanoresponsive Polyelectrolyte Brushes. Bunsow, J., Erath, J., Biesheuvel, P. M., Fery, A., Huck, W. T. S., Angewandte Chemie-International Edition, 2011. 50(41): p. 9629-9632. 3. Sensitive as Human Skin: Polymer Surfaces with High Precision Pressure Detection. Fery, A. and Erath, J., International Journal of Materials Research, 2011. 102(12): p. 1524-1525. 4. Tuning of the Elastic Modulus of Polyelectrolyte Multilayer Films built up from Polyanions Mixture. Trenkenschuh, K., Erath, J., Kuznetsov, V., Gensel, J., Boulmedais, F., Schaaf, P., Papastavrou, G., Fery, A., Macromolecules, 2011. 44(22): p. 8954-8961. 5. Adsorption of Spherical Polyelectrolyte Brushes: from Interactions to Surface Patterning. Hanske, C., Erath, J., Kuehr, C. , Trebbin, M., Schneider, C., Wittemann, A., Fery, A., Zeitschrift Für Physikalische Chemie - International Journal of Research in Physical Chemistry and Chemical Physics, 2012. 226(7-8): p. 569-584. 6. Reversible Swelling Transitions in Stimuli-Responsive Layer-by-Layer Films containing Block Copolymer Micelles. Gensel, J., Dewald, I., Erath, J.,Betthausen, E., Mueller, A. H. E., Fery, A., Chemical Science, 2013. 4(1): p. 325-334. 1 LIST OF PUBLICATIONS 2 7. Clay-Based Nanocomposite Coating for Flexible Optoelectronics Applying Commercial Polymers. Kunz, D.A., Schmid, J., Feicht, P., Erath, J., Fery, A., Breu J., ACS Nano, 2013. 7(5): p. 4275-80. 8. In-plane Modulus of Singular 2:1-Clay Lamellae Applying a Simple Wrinkling Technique. Kunz, D., Erath, J., Kluge, D., Thurn, H.; Putz, B.; Fery, A., Breu, J., ACS Applied Materials and Interfaces, 2013. 5: p. 5851-5855 9. Phototunable Surface Interactions. Erath, J., Cui, J., Schmid, J., Kappl, M., del Campo, A., Fery, A., Langmuir, 2013. 29: p. 12138-12144 Achievements •Best lecture award: Soft colloidal probe AFM: A new method for the investigation of adhesion and contact of soft surfaces, ACS Spring Meeting 2010 (Biofunctional Architectures Symposium), San Francisco, USA, March 21-15, 2010 •Best poster award: Correlation of Local Pressure and Optical Response of Mechanoresponsive Polyelectrolyte Brushes, International Conference on Scanning Probe Microscopy on Soft Polymeric Materials: SPM on SPM 2012, Kerkrade, The Netherlands, September 23-26, 2012 •Highlighted Publication: Direct Correlation between Local Pressure and Fluorescence Output in Mechanoresponsive Polyelectrolyte Brushes, published in Angewandte Chemie international Edition, 2011 was highlighted in: Nature Materials, 10, 724, 2011 1 Overview 3 CHAPTER 1. OVERVIEW 5 1.1 Outline This thesis addresses direct measurements of the response from polyelectrolyte (PE) layers, composed of polyelectrolyte brushes. In particular, systems that are studied are polyelectrolyte brushes on hard substrates and layers that are built up of colloidal building blocks, i.e. spherical polyelectrolyte brushes and PE micelles composed of double-end-tethered annealed polyelectrolyte brushes. For the investigation of these so-called ”smart” coatings, coatings that can switch their properties in response to external stimuli (or vice versa), atomic force (AFM) and optical microscopy was used. New techniques by means of combination of AFM and optical microscopy were developed. Also, established physico-chemical techniques were used to explore and characterize properties of the polymer brush systems. Smart coatings are an interdisciplinary research field and everyone has something to bring to the table1: A chemist is for example interested in developing new synthetic methodologies or in studying polymerization in the confined dimensions of a thin film; an engineer is interested in understanding transport phenomena and barrier properties of new coatings and to design new devices; a biologist is interested in biomimetic systems that enable the replication of in-vivo conditions and cellular interactions; a nanotechnology oriented scientist thinks about the unique nanoscale dimension by which structure-property relationships can be derived; and a physicist is interested in interfacial phenomena, in particular the understanding of conformational changes and the resulting response. We, me and my cooperation partners, addressed all of these points. By combining the capabilities of the involved groups in polymer synthesis, chemical characterization, atomic forceand optical microscopy, and micro-mechanical modeling, we developed new polymer brush systems that show unique properties, characterized these systems, emerged an understanding of the observed response and related this to possible applications. Examples are the rational design of sensors, actuators, and reversible adhesives. 1.2 Content of the Individual Chapters After an introduction Chapter 2 gives a review of the status of research on PE layers and of their theoretical treatment. In particular the most important aspects of PEs, functionalization of surfaces with PEs, and properties of polymer brushes are discussed. Furthermore the experimental techniques, i.e. AFM and optical microscopy 1The following passage is adapted from Ref. [1]. CHAPTER 1. OVERVIEW 6 and fundamentals of interaction forces and contact mechanics that are relevant to the experiments presented in this thesis are introduced. The thesis consist of five individual chapters that present issued publications in Chapters five to nine and work in progress that is presented as drafts in further perspectives (Chapter 8). The first paper (Chapter 3) presents a novel method that can be used to study adhesion and contact phenomena of surfaces based on a soft colloidal probe (SCP), attached to an AFM cantilever using the Johnson, Kendall, and Roberts (JKR) approach. In the second part (Chapter 4), a novel method is established to map contact stresses with unprecedented precision using mechanosensitive polymer brush layers. This system is calibrated using the SCP probe technique, introduced in Ch. 3. Further, polymer brushes can be used for the design of responsive layer systems and to tune surface properties, such as wettability, adhesion, and friction. Chapter 5 presents an approach for gradual tuning of surface interactions based on photo-responsive polymer brushes. Also, responsive layers can be built from colloidal building blocks. Interaction properties of spherical polymer brushes (SPBs) with multilayers as a function of ionic strength are studied in Chapter 6. The measurement results can be used to explain and to regulate the absorption behavior of SPBs and to design functional layer systems. Further, highly sensitive coatings are designed, based on block copolymer micelles. These coatings are investigated with respect to their swelling behavior which depends on pH and ionic strength (Chapter 7). Also the resulting changes in their porosity and mechanical properties are studied. In Chapter 8 further perspectives for mechanoresponsive systems are adressed. After discussions about further possibilities for direct measurements of contact stresses of soft materials (Ch.8.1), mechanoresponsive polymer brush systems are used in Ch. 8.2 to study contact and adhesion of biomimetic adhesives. Ch. 8.3 shows possibilities to enhance the sensitivity and resolution of the mechanoresponsive polymer brushes by rational design of the brush layers. Soft Colloidal Probe AFM Chapter 3: ”Soft Colloidal Probe AFM” [2] introduces a novel technique to characterize adhesion and contact on the micron scale. Such phenomena are important for all kinds of soft matter interactions. Current issues of research are interface phenomena in biological systems, as cell migration or cellular CHAPTER 1. OVERVIEW 7 differentiation. Understanding the adhesive properties of cells to the substrate will help to control such behavior. Another important research field is miniaturization of components. The performance of nano and micronscale components is determined by their interfacial properties. Such, for materials selection, device design, and performance accurate determination of the interfacial properties is necessary. Also of growing interest are interfacial properties of complex synthetic systems, e.g. polymer brushes, multilayers and patterned surfaces, since such smart coatings can tailor surface properties like wettability, adhesion, permeability or optical features. Requirements for the characterization of adhesion and contact phenomena are that information on the micron-sized contact zone and on dynamics of contact formation is accessible. Here, we introduce a novel approach for the investigation of such phenomena of soft matter surfaces that combines advantages of a macro scale method, the so-called ”JKR apparatus” and a micro-scale method, namely colloidal probe (CP) atomic force microscopy (AFM). In this soft colloidal probe (SCP) AFM technique an elastomeric colloidal probe, made of polydimethylsiloxane (PDMS) is attached to an AFM cantilever, rendering the contact area between probe and sample much larger as compared to standard CPs (e.g. composed of silicon or glass). This allows to determine the contact behavior of the probe, i.e. the contact area, via interferometry as a function of applied load (Figure 1.1A). The load can be controlled with subnanonewton precision using the AFM feedback loop. We could show that the contact situation can be described using a contact mechanics model developed by Johnson, Kendall, and Roberts (JKR). In order to establish the technique, we developed a protocol for the SCP preparation and solved the problem of optical lever sensitivity determination for cantilevers functionalized with soft probes. SCPs made of PDMS with a diameter in the order of 10 µmand a Young’s modulus in the order of 1 MPa were prepared via suspension polymerization of the precursor polymer in tenside solution. SCPs where attached to the cantilever in order to ensure an adequate (large enough) contact area between the particle and the cantilever. In order to measure adhesion energies, we pressed the SCP against the substrate of choice and recorded the contact area by micro interferometry (i.e. reflection interference contrast microscopy: RICM) in situ. Fitting the data with the JKR theory yields the adhesion energy: the contact area (a) can be described as a function of applied load (P), elastic properties (K) and work of adhesion (w), a=f(P, K, w), and all parameters except the work of adhesion are known (Figure 1.1B). We tested this method at ambient conditions as well as in aqueous media on well-known surface chemistries CHAPTER 1. OVERVIEW 8 A B Figure 1.1: The soft colloidal probe technique: 1.1A Experimental setup, 1.1B Analysis of the thermodynamic work of adhesion CHAPTER 1. OVERVIEW 9 and can clearly separate the contributions of capillary forces in air, hydration forces, and hydrophobic interactions in water. Full ensemble of data (for every load wcan be determined) and the large contact area make the SCP approach an outstanding method for adhesion measurements with an enhanced sensitivity. Additionally it is possible to study soft matter contact situations at controlled conditions on the micron scale. This fact can be exploited to investigate stress sensitive systems, because the local stress can be determined from JKR theory for an adjusted applied load. In the contact zone the sample under investigation is exposed to various pressures and its response can be analyzed. Mechanoresponsive Polyelectrolyte Brushes Chapter 4: ”Mechanoresponsive Polyelectrolyte Brushes” [3] introduces a promising technique for local detection of stress distributions with outstanding resolution. Therefore stress is translated by a mechanoresponsive polyelectrolyte brush into an optical output. Accurate knowledge of stress distribution in the contact area is crucial for understanding soft matter contact situations. The key challenge in the experimental studies of stress distributions in soft matter contacts is the demand of combining high stress sensitivity (on the order of kPa) with high lateral resolution (below micrometer). Classical solutions, such as stress sensors (often called pressure sensors) using the deflection of mechanical elements like membranes as a means for quantifying stresses are reaching fundamental limits in terms of the lateral dimensions. Even most sophisticated microelectromechanical system approaches (MEMS) have so far only reached the pressure sensitivity for lateral dimensions of >> 10 µm. Mechanoresponsive materials even in their early stages of developments, overcome these fundamental limitations. In these systems, a mechanical stimulus directly affects the electrical, chemical or optical property of a material sensor. For these material based approaches, the limiting factor in terms of lateral resolution is how locally the material responds to external pressure and how accurately these changes can be read out. Polymer brushes are particularly interesting in this respect, since they consist of individual, surface grafted, but not laterally crosslinked polymers. The weak lateral coupling, indeed, is a necessary condition for high lateral resolution. At the same time, polymer brushes are themselves soft matter systems and thus match the typical range of elastic properties and deformability, allowing for suitable sensitivity. The key challenge however is to modify the polymer brushes such that their compression state can CHAPTER 1. OVERVIEW 16 A B C Figure 1.4: Interactions of Spherical Polyelectrolyte Brushes: 1.4A Design of the SPBs, 1.4B Interactions of SPB with PE functionalized substrates, 1.4C Adsorption behavior of SPBs depending on the salt concentration. CHAPTER 1. OVERVIEW 17 action to external stimuli are a very attractive research field regarding its potential applications (as shown above). Examples are drug delivery, microfluidic systems, cell tissue engineering, as well as sensing, or actuation. Our goal was to create highly responsive and stable coatings. Therefore we used micells that are composed of a hydrophobic polybutadiene core, an annealed anionic poly(methacrylic acid) polymer brush shell and a quenched cationic corona of quaternized poly(2-(dimethylamino)ethyl methacrylate) as building block. These micelles were assembled from solution (pH 4 buffer, where the shell is uncharged) with a quenched polyanion (anionic poly(sodium 4-styrenesulfonate: PSS) into multilayers as scatched in Figure 1.5A using the layer-by-layer (LBL) approach. This system combines the advantages of the highly responsive annealed polymer brushes, the functionality and internal hierarchy of colloidal building blocks and the simple preparation procedure of LBL films. These multilayer are studied with respect to morphology, porosity swelling degree and the corresponding mechanical properties dependent on the composition of the film and the surrounding medium. Using ellipsometry, AFM imaging and force spectroscopy we followed the pH triggered reversible swelling and contraction of the multilayer films and the resulting mechanical properties. Also, the dependence on the number of deposition steps was investigated. We could show that morphology and porosity strongly depend on the number of deposition steps. The porosity can be tuned between 0% and 50% for 20 or 1 deposition steps, respectively. The porosity has a big influence on the water uptake and the corresponding swelling behavior. We could vary water uptake by around two orders of magnitude and the swelling degree by more than three orders of magnitude. The swelling decreases with increasing film thickness. Pore opening and closing and the resulting degree of swelling can be regulated by the solution pH (between pH 4 and pH 12). We observed a 6-fold increase in film thickness. This could be associated to an increase in Young’s modulus from a few kPa to hundreds of kPa (Figure 1.5B). Further Perspectives Chapter 8: ”Further Perspectives” addresses new aspects of the mechanoresponsive systems based on cationic polymer brushes (Ch. 4). Understanding the nature and the distribution of stresses at the contacts of deformable solids is fundamental to the fields of soft mechanics and adhesion. The results of mechanoresponsive systems based on CHAPTER 1. OVERVIEW 18 A B Figure 1.5: Swelling of Block Copolymer Micelles: 1.5A Micelles are composed of a hydrophobic polybutadiene core, an annealed anionic poly(methacrylic acid) polymer brush shell and a quenched cationic corona of quaternized poly(2-(dimethylamino)ethyl methacrylate). These micelles are assembled in layer-by-layer films using PSS. 1.5B Response of block copolymer micelles to pH. CHAPTER 1. OVERVIEW 19 cationic polymer brushes are very promising, and several fundamental aspects can be addressed to fully unfold the potential of this detection scheme (Ch.8.1). Also, such surfaces are of particular interest for the understanding of bioinspired reversible adhesives as will be discussed in Ch.8.2. Further, possible enhancement of the sensitivity and resolution of the mechanoresponse by rational design of brush layers is discussed in Ch.8.3. CHAPTER 1. OVERVIEW 21 1.3 Individual Contributions This work is the outcome of close collaborations and knowledge transfer between different groups and individual contributions of diverse persons besides the author. Soft Colloidal Probe AFM •I developed the method, performed and analyzed all experiments, and wrote the manuscript. •S. Schmidt helped to develop the method and corrected the manuscript. •A. Fery analyzed the results, helped with discussions, and correcting the manuscript. Mechanoresponsive Polyelectrolyte Brushes •I developed the method for stress detection, performed and analyzed all experiments for stress detection, wrote parts of the manuscript and corrected the manuscript. •J. Bünsow developed the synthesis protocol for the mechanoresponsive polymer brushes, characterized the polymer brushes, helped to develop the method and with the experiments for stress detection, wrote parts of the manuscript, and corrected the manuscript. •P. M. Biesheuvel helped to analyze and discuss the results. •A. Fery analyzed the results, helped with discussions, and corrected the manuscript. •W.T.S. Huck analyzed the results and finalized the manuscript. Photo-Tunable Surface Interactions •I performed and analyzed all experiments for the characterization of surface properties of the polymer brush, and wrote the manuscript. •J. Cui synthesized and characterized the brushes substrates, and corrected the manuscript. •J. Schmid helped with force spectroscopy measurements and the development of the condensation microscopy technique. CHAPTER 1. OVERVIEW 22 •M. Kappl helped with the AFM measurements, participated in discussions, and corrected the manuscript. •A. del Campo analyzed the results, helped with discussions, and corrected the manuscript. •A. Fery analyzed the results, helped with discussions, and corrected the manuscript. Interactions of Spherical Polyelectrolyte Brushes •I performed the AFM interaction measurements, analyzed these experiments, was involved in scientific discussions, wrote parts of the manuscript, and corrected the manuscript. •C. Hanske performed adsorption experiments, the micro contact printing, analyzed these experiments, was involved in scientific discussions, and wrote the manuscript. •C. Kühr and C. Schneider synthesized and characterized the SPBs and the microparticles. •M. Trebbin produced a special designed stamp for micro contact printing using soft lithography. •A. Wittemann developed the synthesis protocol for the SPBs, was involved in scientific discussions, wrote parts of the manuscript, and helped correcting the manuscript. •A. Fery analyzed the results, helped with discussions, and corrected the manuscript. Swelling of Block Copolymer Micelles •I performed colloidal probe AFM measurements, was involved in scientific discussion, wrote parts of the manuscript, and corrected the manuscript. •J. Gensel and I. Dewald performed most of the experiments, and analyzed these experiments. J. Gensel wrote the manuscript. •E. Betthausen conducted the synthesis and characterization of the polymer used, was involved in scientific discussions, and corrected the manuscript. CHAPTER 1. OVERVIEW 23 •A. H. E. Müller helped with discussions, and corrected the manuscript. •A. Fery analyzed the results, helped with discussions, and corrected the manuscript. Further perspectives Direct measurements of contact stresses of soft materials for rational design of reversible adhesives •I wrote the manuscript. •M. Chaudurhi helped with discussions. •A. del Campo helped with discussions. •A. Fery helped with discussions. Contact and Adhesion of Biomimetic Patterned Adhesives •I developed the method to study stress distributions of biomimetic contacts, performed and analyzed the experiments and wrote the manuscript. •D. Drotlef produced masters for the biomimetic substrates and performed the adhesion and the SEM measurements. •I. Dewald synthesized and characterized the mechanoresponsive brush substrates. •J. Bünsow helped to develop the method to study stress distributions of biomimetic contacts, developed the synthesis protocol for the mechanoresponsive brush substrates, and corrected the manuscript. •M. Chaudurhi helped with discussions. •A. del Campo analyzed the results, helped with discussions, and corrected the manuscript. •A. Fery analyzed the results, helped with discussions, and corrected the manuscript. CHAPTER 1. OVERVIEW 24 Tuning the response of mechanoresponsive brushes •I performed and analyzed all experiments, developed the theoretical models, and wrote the manuscript. •J. Neubauer and I. Dewald synthesized the brush substrates. J. Neubauer helped with force spectroscopy experiments and the analysis of the data. •S. Block helped to analyze the experiments, and developed the theoretical model. •J. Bünsow helped with discussions. •S. Carregal and W. Parak developed the synthesis protocol for attachment of SNARF molecules. •A. del Campo helped with discussions. •A. Fery helped with discussions. 1.4 References [1] Knoll. Functional Polymer Films. Vol. 1. Wiley-VCH. Weinheim, Germany, 2011. [2] J. Erath, S. Schmidt, and A. Fery. “Characterization of adhesion phenomena and contact of surfaces by soft colloidal probe AFM”. In: Soft Matter 6.7 (2010), pp. 1432–1437. [3] J. Bunsow et al. “Direct Correlation between Local Pressure and Fluorescence Output in Mechanoresponsive Polyelectrolyte Brushes”. In: Angewandte ChemieInternational Edition 50.41 (2011), pp. 9629–9632. [4] J. Erath et al. “Phototunable surface interactions”. In: Langmuir 29.39 (2013), pp. 12138–44. [5] C. Hanske et al. “Adsorption of Spherical Polyelectrolyte Brushes: from Interactions to Surface Patterning”. In: Zeitschrift Fur Physikalische ChemieInternational Journal of Research in Physical Chemistry and Chemical Physics 226.7-8 (2012), pp. 569–584. [6] J. Gensel et al. “Reversible swelling transitions in stimuli-responsive layer-bylayer films containing block copolymer micelles”. In: Chemical Science 4.1 (2013), pp. 325–334. 2 Theory and Status of the Field 25 CHAPTER 2. THEORY AND STATUS OF THE FIELD 32 chain are parameters to characterize PEs [16]. The bulk concentration Cjof species j leads to an ionic strength I(in mol/l) of I=1 2X j z2 jCj(~r),(2.9) and the local charge density ρ(~r)is related to the local ion concentration cjby ρ(~r) = eX j zjcj(~r),(2.10) where eis the elementary charge, cjlocal ion concentration, and zjthe valency of species j. For PEs, the excluded volume (monomer-monomer repulsion) is much larger compared to neutral polymers due to the electrostatic Coulomb potential that determines the conformation and interactions of the polymer. The Coulomb potential u(~r)is determined by the Poisson equation for electrostatics ∇2u(~r) = −ρ(~r) ,(2.11) where 3is the dielectric permeability. Dissolving the PE in aqueous (salt) solution (electrolyte solution) leads to immobilized counter-charges, i.e. counterions that maintain electric neutrality and are attracted by the charged units of the PE. Attraction leads to screening of the Coulomb interactions. The distribution of the mobile counterions is governed by the electric field around the PE and the balance of electrostatic energy and entropic contributions (S∝kBTln(r)) [22]. The Coloumb potential of the PE is determined by the Poisson equation Eq. 2.11 and depends on its geometry u(~r)∝         1/r for a point like system rfor a planar system ln(r)for a line like system. (2.12) For a point like system, the entropic contribution of the energy is larger compared to the electrostatic energy and counterions are unbound. For a planar system most counterions are bound to the surface and form the so called Gouy-Chapman layer (Eq. 3·0≡ CHAPTER 2. THEORY AND STATUS OF THE FIELD 33 2.28). In case of a line like charge the balance depends on the charge density (both contributions ∝ln r). If the PE system (i.e. a charged surface) is in thermodynamic equilibrium the resulting charge density of the ions at position ~r follows a Boltzmann-like behavior ρj(~r) = Cjexp(−uij(~r) kBT).(2.13) The many-body interactions uij can be averaged and approximated by a mean field potential for low molecular weight and weakly charged PEs. Following the theory of Debye Hückel (DH) the mean field potential can be expressed as umean =zjehφ(~r)i(2.14) where inter-particle correlations are neglected and hφ(~r)iis a time-averaged potential (each counterion interacts with a diffuse cloud of the other counterions). Additionally, electro neutrality can be assumed X j zjeCj = 0.(2.15) Insertion into Eq. 2.11 yields the Poisson Boltzmann equation (PB equation), connecting the electrostatic potential to the charge density of the ions ∇2umean =X j zje Cjexp(−umean kBT).(2.16) For weak potentials umean << kBT/zjr, this equation can be expanded using a Taylor series and then linearized. This results in the Debye Hückel equation [23]: ∇2umean =1 λ2 D umean,(2.17) with the so called ”Debye length” λD λD=sKbT Pj(z2 jCj(~r))e2=rKbT 2Ie2.(2.18) CHAPTER 2. THEORY AND STATUS OF THE FIELD 34 The Debye Hückel equation (2.17) can be solved (PE on a surface) using umean(~r) = u0exp(−r λD ).(2.19) This (2.19) shows that the Debye length can be interpreted as a screening length of the Coloumb potential (Figure 2.3). Assuming an n:n salt solution (AnBn→An++Bn−) the Debye length is λD=0.304 nm n√I. For r << λDthe electrostatic interactions are purely Coulomb and for r >> λD, the electrostatic interactions are screened completely and the behavior of the PE converges to a neutral polymer. If the electrolyte solution screens the electrostatic repulsion, the PE starts to coil. Two boundary conditions have to be fulfilled. The first condition demands that the total charge (surface charge plus the charge of the mobile ions) must be zero [24]. The surface charge density σand the distribution of the ions ρare related in the Graham equation, what can be deducted from electro neutrality conditions of the system σ=−Z∞ 0 ρdr =p8cKBTsinh eumean 2KBT≈umean λD .(2.20) Furthermore the potential has to vanish for large distances. For nonlinear PB theory one can show that u(~r) = 2KBT ze ln "1 + ξexp(−r λD 1−ξexp(−r λD#(2.21) ≈4KBT ze ξexp(−r λD )for r >> λD,(2.22) with ξ= tanh(zeuo)/(4KBT)[24]. In summary the potential of a PE decays with the debye length. The prefactor depends on the geometry of the object and the boundary conditions (see Ch. 2.3 for more details on the interactions of two charged surfaces). However, these approximations fail in case of strong charged PEs where counterions condense to reduce the electrostatic potential. That meanes the counterions become trapped by the PE in order to balance the electrostatic energy by a decrease in entropy. That effect is called ”counterion condensation” [25, 26]. Depending on the corresponding parameters, Coulomb interactions or the loss of entropy dominate and determine the counterion-distribution [16]. If the electrostatic energy (Eq. 2.12) is small compared to entropic contributions, counterions cannot be stabilized and no counterion condensation occurs. That is the case if the number of charges per unit length (Γ) CHAPTER 2. THEORY AND STATUS OF THE FIELD 35 A B Figure 2.3: 2.3A Potential of a PE: Umean(r)for (U0= 66 mV,T= 293.15 K and ionic strength I= 0.1−0.001 M). Inset displays a log plot for these potentials. 2.3B Debye length λDfor a 1:1 salt as a function of the ionic strength I. CHAPTER 2. THEORY AND STATUS OF THE FIELD 36 is smaller than one charge per Bjerrum length Γ<1/lB. The Bjerrum length lBis defined as the length at which two electron charges have an interaction energy in the magnitude of the thermal energy lB=e2 4πKbT.(2.23) In pure water at standard lab pressure and temperature lB≈0.7 nm. If the electrostatic energy is large compared to entropic contributions (if the number of charges per unit length is higher than one charge per Bjerrum length Γ>1/lB), counterion condensation to the PE occurs to reduce the charge density until a maximum of one charge per Bjerrum length ρmax =e/lbis reached. Uncompensated counterions can be described using the DH approach. The effect of counterion condensation can be described using the model proposed by Oosawar 4[27]. Due to electrostatic effects, PEs are quite stiff. This stiffnes effect can be described using an electrostatic persistence length, ”Odijk length” Lp. One can show that the persistence length Lpof the PE, describing its stiffness, is proportional to the Debye length λ2 Dfor flexible poyelectrolytes [28, 29, 30]. If the PE is diluted in an electrolyte solution of low salt concentration, the polymer is strongly stretched (L=Lmax/(Γ/lB), with Lthe length of the stretched PE, Lmax the maximal length aligning the monomer units of the polymer, and Γthe number of charges per unit length (vice versa for high concentrated electrolyte solutions). The excluded volume for PEs results in an increased excluded volume compared to neutral polymers due to the electrostatic interactions (v2∝LpλD). In case of annealed PEs, both the ionic strength and the pH of the solution strongly affect the properties of the polymer. The pH determines the degree of dissociation and thus the actual charge density. This dependence makes such systems interesting for many applications [31]. The chemical equilibrium of annealed PEs is described by a modified version of the Henderson-Hasselbalch equation [16] which relates the pH of the solution and the fraction of charged groups. For molecular acids this results in: pKa(app)=pH +log1−α α(2.24) 4The PE is locally stiffen and can be approximated as a cylinder, that traps all countions inside a cylindric cell [27] CHAPTER 2. THEORY AND STATUS OF THE FIELD 37 where αdescribes the actual degree of dissociation and the pKais defined by the law of mass. Overbeek showed that the experimentally observed apparent pKa(app)(Eq. 2.24) of an annealed PEs, depends on the intrinsic value pK0 a pKa(app)=pK0 a+1 ln(10)RT dG dα (2.25) with the term dG/dα describing the work necessary to carry charges against the electrostatic attraction from a PE to infinite distance [32]. In other words, the second term represents the shift in the dissociation constant due to changes in the electrostatic free energy of a PE upon variation of the number of charged groups [1]. In contrast to neutral polymers (Ch. 2.2.1), the structural properties of adsorbed PEs are mostly dominated by electrostatic interactions. A charged surface can be neutralized by a oppositely charged PE, which is entropically favorable and therefore promotes PE adsorption [20, 33, 34, 35, 36]. Several theoretical approaches like self-consistent field theory (SCF), Monte-Carlo simulations, or scaling approaches have been applied to describe the adsorption behavior. Possible conformations depending on the adsorption energy are so called trains (all PEs are in contact with the substrate), loops (parts of the PE are not in contact with the substrate), and tails (non-adsorbed ends of the PE) as sketched in Figure 2.4A [37, 38]. One adsorbed layer of PEs has a thickness in the order of the chain diameter (≈1 nm). The adsorption is accomplished by a confinement of the PE, which involves an increase in free energy. For compensation of this increase an additional attractive interaction must stabilize the adsorption. The driving forces of adsorption are the gain of entropy by complexation with oppositely charged surfaces and release of counterions as well as the release of solvent molecules, on the fulfillment of electro neutrality. However, often more PEs are adsorbed than necessary for electroneutrality, which is called ”charge overcompensation”. This can lead to a charge reversal of the surface. Since electrostatic interactions are dominant, parameters like surface charge, ionic strength, pH and the architecture of the PE govern the adsorption. It depends on the balance between electrostatic and non-electrostatic interactions whether an increase in salt concentration leads to an increase or decrease in adsorption [39]. Two regimes were proposed to describe this effect. In the so called ”screening reduced” adsorption regime (high surface charge, low charge density of the PE, weak non-electrostatic contribution), Coulomb interactions between segments and the surface dominate. If the attraction between polyelectrolyte and surface is purely electrostatic only this regime CHAPTER 2. THEORY AND STATUS OF THE FIELD 38 is valid. Several adsorbed layers of PE are possible, due to the long range nature of the Coulomb interactions. The adsorbed amount decreases with an increase in ionic strength due to screening effects. The PE can be released from the surface when a critical salt concentration is reached. In the regime of ”screening enhanced” adsorption (generally quenched PEs) non-electrostatic interactions (short range interactions) between the segments and the surface are dominant. The adsorption increases with ionic strength because salt screens the repulsion between the equally charged groups on the polymer. In the intermediate case, when both forces are of roughly equal importance, changing the salt concentration will hardly affect the adsorption [39]. The different adsorption regimes are shown in Figure 2.4B The adsorbed layer thickness can be calculated by minimizing the free energy [40, 41]. Assuming that the Debye length λDis larger than the adsorbed layer thickness d(valid for not too high ionic strength, see 2.18), two regimes can be obtained for the layer thickness: one for relatively large salt concentrations (or rather stiff polymers) and small layer thickness and one where the layer thickness is larger than the persistence length but smaller than the screening length d∝       ln(lBσSΓL2 P lBσSΓL1/3 P3/5 for d<λD< Lp LP lBσSΓ1/3for Lp< d < λD (2.26) with σSthe charge density of the surface, Γthe charge density of the chain, and Lp the effective persistence length [41]. 2.2.3 Self Assembly of Polyelectrolytes If PEs are dissolved in aqueous solution with suitable counterparts that are oppositely charged, they form aggregates due to electrostatic interactions (other intermolecular forces are possible as well) [42]. These aggregates can build up interpolyelectrolyte complexes (IPECs), which form a new class of macromolecules [43, 44]. The complexation is driven by the release of counterions which leads to an increase in entropy. Several theories and studies exist to describe the features and physical properties of the resulting IPECs [45]. IPECs can consist of a PE with a second oppositely charged PE, with low molecular counterions, ionic surfactants, colloidal particles, and others (Figure 2.5A). These IPECS are interesting for numerous applications. Examples are the build up of polyelectrolyte multilayers, where many oppositely charged PEs are adsorbed in layers in altering order [46]. CHAPTER 2. THEORY AND STATUS OF THE FIELD 39 A B Figure 2.4: Adsorption of PEs: 2.4A Adsorbed polyelectrolyte chain form trains, loops and tails. 2.4B Different Adsorption regimes of PEs depending on charge density and salt concentration: (I) low charge density, low salt concentration: formation of loops and tails, high adsorbed amount; (II) PE has the same charge as the substrate, low salt concentration: PE releases from the surface; (III) strong charge density, low salt concentration: PE lies flat and stable on the surface, weaker adsorption as in (I); (IV) low charge density, high salt concentration: screening of electrostatic interactions, adsorption amount can increase or decrease; (V) PE has the same charge as the substrate, high salt concentration: electrostatic repulsion is screened, adsorption is possible; (VI) high charge density, high salt concentration: similar to (I) due to screening of electrostatic interactions; (VII) above a critical salt concentration all PEs release from the surface. CHAPTER 2. THEORY AND STATUS OF THE FIELD 40 If the polymer consists of two or more blocks, that are distinguishable by the chemical and physical properties and linked by covalent bonds, the system phase separates and can self-assemble to complex structures like spheres, cylinders, or lamellae. These different architectures result in a big number of functionalities, as reported in Ref. [47]. The geometry and the physical properties of these complex systems are tunable by parameters like block length, number of blocks, solvent quality, ionic strength, and -in case of annealed PEspH. The complexity of the structures and the number of different morphologies increase drastically with the number of blocks, as for example described for ABC triblock terpolymers in Ref. [48]. When a block copolymer is dissolved at a concentration exceeding the critical micellar concentration (cmc) and if one of the blocks is soluble whereas the other is not they form micellar structures with a solvophobic core and corona pointing into the solution (Figure 2.5B)[49]. A B Figure 2.5: Self assembly of polyelectrolytes: 2.5A Formation of an interpolyelectrolyte complexes (reproduced from [44] c RSC). 2.5B Formation of a block copolymers micelle. CHAPTER 2. THEORY AND STATUS OF THE FIELD 41 2.2.4 Functionalization of Surfaces with Polyelectrolytes There are many possibilities to functionalize surfaces with PEs (Figure 2.6). It is beyond the scope of this work to explain all possible techniques in detail. Here an overview over several techniques is given, following the scope made in Ref. [1]. Two ways of surface functionalizations can be distinguished: physical or chemical functionalization. In case of physical functionalization the molecules are coupled to the substrate via physical interactions, while chemical functionalization involves covalent attachement of molecules [50]. Often the Langmuir Blodgett technique was extensively used for functionalization of surfaces with polymers [51] (Figure 2.6A). After equilibration of polymer monolayers at an air-liquid interface the PEs were transferred to a substrate of choice by dipping the substrate into the liquid. Using several dipping steps, multilayers can be realized. However, this technique has some limitations with respect to type, topography, and size of the substrate and requires special equipment. Another approach to produce smart surfaces is based on self-assembly of monolayers (SAM) due to chemical adsorption (Figure 2.6B) [52, 53, 54, 55]. Multilayers can be obtained by targeted molecular design of the monolayers. However, these films are limited to certain classes of covalent or coordinative chemistry. An elegant approach for the formation of functional surfaces, is the layer-by-layer deposition technique (LBL) (Figure 2.6C). Here oppositely charged PEs (or other molecules) are deposited as films via dipping, spin, or spray coating in altering order onto the sample substrate. This process is driven by electrostatic interactions and the gain of entropy by release of counterions and solvent molecules and complex formation (see Ch. 2.2.3) [56]. The technique was established by Decher [57]5and has nowadays a lot of applications, particularly in the development of responsive coatings [59, 60]. This is due to advantages like easy handling, low cost equipment, and no restrictions with respect to topography, geometry, chemistry, and size of the substrates. In addition, the thickness of the film and the resulting charge of the sample can be adjusted by the number of deposition steps (1 up to around 1000), the PE used (e.g. molecular weight [61]), and the properties of the solution (concentration, ionic strength, pH [62, 63] and dipping time) [64, 62]. For example LBLs are used for biomedical applications and cell substrates [65, 66], for coatings (walls) of microcapsules that can be used for e.g. drug delivery [67, 68], controling surface wettability [69], for the design of optical sensors [70], for the preperation of light emmiting diodes [71], or fuel cell membranes [72]. The 5already first reported by Iler [58] CHAPTER 2. THEORY AND STATUS OF THE FIELD 48 the polymer brush properties are functions of the distance from the surface. For more details, the reader is referred to [82]. The discussion so far was for neutral polymers. If the monomer units are charged and the grafting density is high, a PE polymer brush is formed. For system neutrality, counterions that lead to an osmotic pressure are present in the brush (see Ch. 2.2.2). The polymer chains are stretched by segment-segment interactions and electrostatic (Coulomb) interactions in equilibrium with the elastic free energy gained by the entropic restoring force of the chain (see Ch. 2.2.1). These PE brushes can be either quenched (charges are fixed inside the brush), or annealed (charges are mobile in the brush). The charge can be accounted for the degree of ionization α. The main important parameters for PE brushes 1/σ and αare combined in the so called ”Gouy-Chapman” length Λ = σ lBNα (2.28) that defines the characteristic thickness of the counterion cloud (Figure 2.10). If the Figure 2.10: Parameters of a cationic polymer brush in salt solution chains are densly packed (σsmall) and strongly charged (large α) the Gouy-Chapman length is small and can be smaller than the brush height H(Λ< H). In that case the counterions are trapped inside the brush and compensate the immobilized charges of the polyions. If the grafting is relative spare and the degree of ionization is low, the Guoy-Chapman length becomes larger than the brush height H(Λ> H). In that case, the counterions spread into the solution beyond the edge of the brush. CHAPTER 2. THEORY AND STATUS OF THE FIELD 49 The analytic expression for the charge and counterion distributions and the structure of the PE brush can be calculated using self-consistent field theory (SCF) [106, 107, 108, 109]. If the solution is salted, an additional screening of the Coulomb interactions between chain segments is provided by coions and counterions of the salt. This screening can be described by the Debye screening length λD(see Eq. 2.18). For an understanding of the physical behavior and the internal structure of polymer brushes different models and approaches can be used. The first time, the influence of confinement on endgrafted polymer chains was studied by Alexander [110] and de Gennes [111] (AdG model). Using scaling arguments and describing the brush segments as so called ”blobs”7they could show that the density of the brush reaches a homogenous plateau for σ < z < H and drops down quickly for z > H, where zis the distance from the surface. This behavior can be described by a step profile (see Figure 2.9C) ρ∝N Hσ (2.29) for 0< z < H and zero elswhere. This corresponds to a brush height of H∝Nσ−1/3.(2.30) The brush height scales linearly with the length of the attached polymer. To describe the physics of polymer brushes, models based on these ideas and new approaches like SCF theories or numerical methods were applied. Also, the curvature of the substrate, polydispersity, changes in the environment, and other aspects were taken into account. To describe all of this is much beyond of the scope of this work and the reader is referred for example to Ref. [77]. In the following, just some aspects will be discussed following the conclusions of [107, 108, 109] (for details see Appendix A). Different models are valid for different limits of behavior. In case of local compensation of immobilized charges by mobile ions, the ”Local Electron neutrality Approximation” (LEA) can be used [107]. This approximation is applicable if Coulomb interactions in the polymer brush are screened by trapped counterions or by added salt on a scale smaller than the brush thickness. On the contrary, if the system is unable to retain counterions inside the brush, the system can be approximated by a capacitor model [112]. In general (most of the experimental systems), just a partial charge neutralization 7one blob contains a polymer segment that behaves like an ideal polymer CHAPTER 2. THEORY AND STATUS OF THE FIELD 50 takes place and a fraction of mobile ions spreads beyond the edge of the brush. In that case, Self Consistent Field (SCF) is the theory of choice [106, 107, 108, 109]. The LEA assumes that the charge and the force is locally balanced inside the brush (see Appendix A for more details). In case of a neutral polymer brush it is sufficient to balance between the steric excluded volume interactions Fconc and the restoring force associated with the loss in entropy of the polymer chain upon stretching Fconf (also often called as elastic energy). Solving this balance results in a scaling law for the brush height, depending on the solvent quality. In case of a good solvent, the brush height Hscales as follows [107]: H∝Nσ−1/3,(2.31) which is the same as the result of AdG (eq. 2.30). In case of PE brushes, the short range interactions are weak compared to electrostatic forces. Here electrostatic interactions Fion are balanced by Fconf . Additional balance of the charges leads to two different scaling regimes for PE brushes in a salt solution where the brush height scales linearly with the polymer contour length [107]: H∝Naα1/2,for cS< cCI (2.32) H∝N(a2α2σ−1C−1 S)1/3,for cS> cCI,(2.33) where CSis the salt concentration and CCI the concentration of the counterions. PE brushes that can be described by eq. 2.32 are called ”osmotic brush” (OsB). This is the case if the salt concentration of the solution is low and the concentration of counterions inside the brush is equal to that of the immobilized charge. An important feature of the OsB is that the average thickness is independent of the grafting density (Eq. 2.32). Above a certain salt concentration, salt ions dominate over the immobilized charges inside the brush (Eq. 2.33) and a so called ”salted brush” (SB) is formed. Here, the brush height decreases continuously with increaseing salt concentration and grafting density due to screening effects. Only free counterions contribute to the osmotic pressure. The transition from the OSB regime to the SB regime occurs when the bulk salt concentration equals the concentration of the free mobile counterions. Equations 2.32 and 2.33 are quite general and apply to both quenched and annealed PE brushes [107]. For quenched brushes this is the final result. For the annealed case, the situation is more complex. The dissociation degree and the apparent pKa of the PE brush depend on its local electric field (environment), in particular on the solution CHAPTER 2. THEORY AND STATUS OF THE FIELD 51 pH, the ionic strength, and on the grafting density. In case of weak polyacid brushes, the pKa shifts to higher values compared to the pKa of the polymer in bulk solution (or vice versa for basic brushes). Adding salt to the solution shifts the apparent pKa to lower values (close to pKa in bulk solution). The degree of dissociation within the brush is close to zero at low salt concentrations. It increases in the OsB regime and reaches the bulk level (αBof an individual polyacid molecule immersed in the solution [113]) in the salted brush regime. Also, the brush height is affected by the ionic strength. Using scaling models or SCF one can show that the brush height (of an annealed polymer brush) passes a maximum that is located at the OsB/SB transition [107, 113, 114, 115, 116] (Figure 2.11). In addition, for annealed PE brushes the brush height is a function of the solution pH. The height increases for basic PEs with increasing pH and decreases under acidic conditions (or vice versa for acidic PEs). In addition to the OsB and the SB regime, four other brush regimes can be distinguished which are seperated by the grafting density and the degree of ionization. Figure 2.12 summarizes these behaviors in phase diagrams for the salt-free and the salted case [107]. •Low grafting density (small 1/σ); electrostatic interactions weak (small α) compared to the volume interactions (Fion << Fconc) lead to grafted individual neutral coils (NC: mushroom regime H∝aN3/5ν1/5 2) or isolated charged chains in salted water (SC), respectively. •Low grafting density (small 1/σ), high electrostatic interactions (big α) lead to isolated grafted polyions stretched due to intramolecular Coulomb repulsion (IS: H∝aNα2/3). •High grafting density, weak electrostatic interactions (small α) compared to volume interactions (Fion << Fconc) lead to a quasineutral regime, meaning that the brush behaves like a neutral brush where the brush height is balanced by the equlibrium of entropic stretching and steric interactions (NB: H∝Nσ−1/3). •Intermediate regime (charged Pincus brush (PB): H∝a2N3ασ−1): mobile ions are distributed in the space above the grafting surface (Λ> H). This regime appears just in salt free solutions. The above described LEA donot provide information on the intrinsic structure of free and confined brushes as a function of the brush parameters (grafting density, molecular CHAPTER 2. THEORY AND STATUS OF THE FIELD 52 Figure 2.11: Brush properties as a function of salt concentration: Brushheight H for quenched (solid line) and annealed (dotted line) brushes and degree of dissociation within the brush αfor quenched (solid line) and annealed (dotted line) brushes (adapted from ([113] c American Chemical Society) weight, degree of ionization, solution properties). To fulfill these requirements a theory is needed wich works without a priori assumptions and that give analytic expressions for the density profile of the monomers, the equilibrium distribution for the mobile ions inside and outside the brush, and the average thickness. An ansatz to solve this problem can be the self-consistent field theory (SCF [108, 109, 116]). Here, the intrinsic structure is obtained by minimization of the total free energy ftot. The total free energy has three repulsive contributions: 1) The conformation free energy which describes the steric repulsion between chain segments (fconc), 2) the free energy associated to the entropy Sion confining counterions to a layer of thickness H:−KbTSion, and 3) the direct electrostatic contribution fion if the PE brush is not locally electro neutral throughout the system. The attractive contribution to the total free energy ftot is the entropic free energy loss fconf [108]. ftot =fconf +fconc +fion −KbTSion.(2.34) Minimization (of this functional) and full expressions of the individual terms lead to functions of the polymer density profile inside the brush (parabolic form as scatched in Figure 2.9C, of the brush height, and of the charge density inside and outside the brush and thus the specific electrostatic potentials. For details see Ref. [108, 109, 116]. Also, the surface pressure, the conformation and the behavior of PE brushes in confinement can be obtained. CHAPTER 2. THEORY AND STATUS OF THE FIELD 53 A B C Figure 2.12: Phase diagram of polymer brushes (reproduced from [107] c American Chemical Society): Type of polymer brush depending on the grafting density σ−1and degree of dissociation αfor the salt free case 2.12A and the salted case 2.12B. 2.12C Height as a function of the grafting density σ−1along the black dotted line of 2.12B. (Quenched polymer brush: solid lines, annealed polymer brush: dotted line) CHAPTER 2. THEORY AND STATUS OF THE FIELD 54 Also, PE brushes can be modeled using numerical methods, for example molecular dynamics (MD) simulations [117]. Results from SCF and MD simulations show the same behavior for polymer brush parameters. 2.3 Surface and Interfacial Forces Interactions and adhesion play a major roles in natural science since they control surface properties and are crucial for many applications especially for objects or effects on the colloidal scale (order of microns). Examples are coatings of surfaces with polymers for surface protection [118], composites [119], paintings and adhesives (or nonsticking surfaces). Examples in nature are cell adhesion [120, 121], receptor-ligand interactions [122], and the effect of contact shape of animal pads on their sticking behavior [123]. Details on theoretical aspects are explained elsewhere [33, 124, 125, 24, 126, 127]. Here, just an overview is given with emphasis explaining the observed systems and the used techniques that are presented in the individual chapters. The presentation is oriented on [33, 124, 125, 24, 126, 127] and the cited literature. Surface and interfacial forces are e.g. ionic, metallic, or covalent bonds, van der Waals forces, electrostatic or magnetic interactions as well as solvent forces like hydrophobic and hydrophilic interactions, hydrogen bonding and capillary forces. The type of interaction force depends on the type of the interacting material, its environment and the distance (long range 1−100 nm or short range <1 nm). When two bodies are brought into contact, the interaction forces lead to adhesion and the bodies can deform. The adhesion is characterized by the stress and the work needed to separate the interfaces, plus aspects like mechanical interlocking and interpenetration [126]. The total adhesion force (force needed to separate two bodies) is the superposition of all repulsive and attractive forces. The adhesion energy (also called Duprés work of adhesion) is the sum of the surface energies γof the contacting surfaces iand j(in medium m) lowered by the interfacial energy γij [24, 126] wadh =γim +γjm −γij.(2.35) If wadh is positive the bodies attract each other. If wadh is negative they repel each other. Measured results are in general lower than this theoretical value due to surface heterogeneities, roughness and wetting effects. CHAPTER 2. THEORY AND STATUS OF THE FIELD 55 2.3.1 The Derjaguin Approximation Interaction forces F(D)of two bodies of any shape can be related to the interaction energy per unit area w(D)between two planar plates as a function of the separation distance Dand the material properties using the Derjaguin approximation8[128, 129]. For two interacting spheres with radii Riand Rjit can be shown that F(D)≈Z∞ D 2πRP(D∗)dD∗= 2πRw(D),(2.36) where R=RiRj/(Ri+Rj)is the reduced radius and P(D)is the normal force per unit area acting between two planar surfaces (Figure 2.13). This approximation is valid for any type of interaction if the curvature of the probes is large compared to the separation distance and the conformation of the interacting system is independent of the distance from the surface (e.g. not valid for SPBs). Figure 2.13: The Derjaguin approximation: Two interacting spheres with radii R1 and R2<< R1at separation distance D(adapted from [128] c Elsevier). 8This is helpful as in many cases it is much easier to model the interactions for the planar case and measure forces between spheres or cylinders. CHAPTER 2. THEORY AND STATUS OF THE FIELD 56 2.3.2 Van der Waals Interactions Van der Waals (vdW) forces are of universal importance since they exist between any combination of molecules and surfaces independent of the charge, the material, or the surrounding medium [24, 126]. They are the sum of diverse dipole-dipole interactions, i.e. Keesom, Debye and London interactions. Keesom interactions describe the interplay between constant dipoles of the molecules dependent on the orientation and on the absolute temperature. Debye interactions take into account dipole induced interactions with a constant charge dependent on the orientation. London dispersion interactions act between all molecules and have quantum mechanical origin. They can be described by fluctuation induced dipoles. All potentials describing these interactions have a 1/r6 dependence, where ris the distance of the interacting molecules: UvdW(r) = −CD+CK+CL r6=−CVdW r6(2.37) and CD,CK,CLare proportionality constants considering the Debye (D), Keesom (K) and London (L) contribution. They account the charge, the polarity, and the optical properties of the molecules or atoms, and the surrounding medium. In case of macroscopic bodies, the force depends on the local distance of the bodies (determined by the geometry). The interactions can be approximated using the Hammaker approach (or more complex approaches like the Lifschitz theory or spectral methods). For example, for the interaction of two spherical objects with radii Riand Rjat distance Dthe VdW potential and the resulting vdW force between two spheres can be approximated with UvdW(D) = −AR 6D, ⇒F(D) = AR 6D2(2.38) where A=π2CVdWρiρjis the Hammakar constant (with ρthe molecular density of the material) and R=RiRj/(Ri+Rj)is the reduced radius of the system. In more extended theories, Ais a complex function of the temperature, the dielectric properties, and the absorption frequencies of the material. Typical values of the Hamaker constant of condensed phases in vacuum are about 10−19 J. CHAPTER 2. THEORY AND STATUS OF THE FIELD 57 2.3.3 Interactions of Charged Systems If the bodies are charged, long range Coulomb interactions play a dominant role. The potential is determined by the Poisson equation (Eq. 2.11). For two point charges, the interaction potential is given by a 1/r dependency which can either be attractive or repulsive depending on the signs of the charges (Eq. 2.12). For macroscopic bodies the potential results, depending on the geometry, in diverse proportionalities of the distance r(eq. 2.12). In solvent, most surfaces are charged, since it is energetically favorable to charge the surface with respecrt to the thermal energy KBT[33]. This occurs by dissociation of ionizable groups or by the adsorption of charged species (Ch.2.2.2). For electroneutrality, oppositely charged counterions are immobilized. This charging results in an electric double layer consisting of the so called inner ”Stern” or ”Helmholtz” layer where counterions are bound close to the surface and the diffuse Gouy-Chapman layer consisting of a diffuse counterion atmosphere [126] (Figure 2.14). In electrolyte solutions the potential in the diffuse layer decays exponentially (far away from the surface). This behavior is described quantitavely by the Debye screening length and is determined by the PB equation (U∝e−Dλ−1 D, see Eq. 2.16). The PB equation relates the ion distribution to the surface potential, by using the electro-neutrality condition. If two charged surfaces are brought near to each other, the ion distribution overlap. Therefore an increase of the osmotic pressure due to the increase of mixing entropy of the ion clouds occures and the electrostatic double layer force arise (Figure 2.14A). One can show that it is enough to consider the counterion distribution at the midplane of both surfaces and the surface charge density σS(contact value theorem 9). In addition to electro-neutrality, the surface charge density σSof the interacting objects influences the potential. That leads to additional boundary conditions. Three types can be identified: 1) Constant charge (cc: the surface charge density is constant. Solving the PB equation results in a distance dependence between σS,0[24]), 2) the constant potential U0(cp): the surface potential is independent of the distance, and 3) the constant regulation approximation (cr: the surface charge depends on the charge density and on the distance). Taking into account these conditions, the electrostatic double layer force between two 9general and also valid for other interactions CHAPTER 2. THEORY AND STATUS OF THE FIELD 64 limit of 0.5 [24], whereas glass has a Poisson ratio of around 0.1 for example. Contact Mechanic Models The first model to describe the contact between two nonadhering, isotropic, linear elastic spheres was given by H. Hertz in 1881[143]. H. Hertz calculated the deformation δand contact area aas a function of applied load F, geometrical terms (R=RiRj/(Ri+ Rj)), and material parameters as accounted for the reduced modulus K K=4 31−ν2 i Ei +1−ν2 j Ej−1 .(2.54) The resulting contact parameters as well as the stress distribution σ(r)in the contact zone (as a function of distance from the axial center r) are summarized in Figure 2.17 and Tab. 2.2 [127, 128]. Applying some simple conversions, the force to achieve a certain deformation δcan be obtained by FHertz(δ) = KR1/2δ3/2.(2.55) Taking adhesion in the contact area into account, Johnson, Kendall and Roberts (JKR) Figure 2.16: Contact parameters of a sphere pressed against a flat substrate developed a more realistic model for soft contacts [144]. By balancing surface energies and the elastic potential, they calculated the contact area and the deformation as a function of applied load and surface energy per unit area w, as well as the resulting stress distribution (Figure 2.17 and Tab. 2.2). The JKR theory is valid for soft samples CHAPTER 2. THEORY AND STATUS OF THE FIELD 65 with a large reduced radius and large adhesion forces. In case of zero external load, finite contact radius, deformation, and stress are observed due to adhesion [127] a0JKR =6πwR2 K1/3 ,(2.56) at a deformation of δJKR 0=a2 0 R−2 3r6πwa0 K.(2.57) Due to the strength of adhesion, the interacting bodies still adhere while pulling (negative loads) until a critical force Fadh is reached (see Tab. 2.2). It should be noted that the expression for the adhesion force is independent of the elastic properties of the material. Furthermore, the aspect of experimentally observed adhesion hysteresis can be explained using the JKR theory. The contact area is larger when unloading than in the loading case until rupture at a critical contact radius of ac= 0.63a0and a (negative) deformation of δc=−(πw2R/12K)1/3. At the same time Derjaguin, Muller, und Toporov (DMT) developed an alternative model where adhesion is present around the contact zone [145]. The DMT model assumes that the surface profile is the same as for Hertzian contacts. Adhesion is included by an additional load caused by the surface forces around the contact area. The DMT theory can be applied to contacts of stiff samples with small radii and small adhesion. In case of zero external load, finite contact radius, deformation, and stress occur aDMT 0=2πwR2 K1/3 ,(2.58) at a deformation of δDMT 0=a2 0 R.(2.59) Due to adhesion, the interacting bodies adhere while pulling (negative loads) until a critical force is reached. The deformation parameters are summarized in Figure 2.17 and Tab. 2.2. If the interaction energy is negligible or for very high loads (F > 103πwR), the results of JKR and DMT models reduce to the equations given by the Hertz model. As mentioned above, both the JKR and the DMT model are valid for different limits of material parameters. For quantification of the validity of the particular model, Tabor introduced ”the Tabor parameter” which is defined by the ratio between neck height CHAPTER 2. THEORY AND STATUS OF THE FIELD 66 Table 2.2: Expressions for contact parameters of different contact mechanic models. Adapted from Ref. [128]. Hertz JKR DMT a3FR K R KF+ 3πRw +p6πRwF + (3πRw)2F R K+2πwR2 K δa2 R=F2 K2R1/3a2 R−q8πaw 3K a2 R=(F+2πRw)2 K2R1/3 σ3Ka 2πR p1−r2/a23Ka 2πR p1−r2/a2−q3Kw 2πa 1 √1−r2/a2 3Ka 2πR p1−r2/a2 Fadh 0−3 2πwR −2πwR at critical deformation due to adhesion and the range of surface forces z0[146] µT=16Rw2 9K2z3 01/3 .(2.60) For µT<< 1, the DMT model is valid and for µT>> 1JKR theory applies. Maugis introduced a more general theory, which describes the transition range between the JKR and the DMT model and applies to all materials from large hard spheres with high surface energy to small soft bodies with low surface energies. The Maugis theory describes interactions by a Dugdale model and results in expressions for the contact parameters as a function of the so-called ”Maugis parameter” (µM≈1.16µT). For analytic expressions of the contact parameters, the reader is referred to Ref. [127, 147]. Figure 2.18 shows an overview for the availability of the presented contact mechanic models. A general expression for the contact parameters of two axisymmetric elastic bodies (i.e. sphere, parabola, or cone) was given by Sneddon’s solution [127]. The reader should notice that the contact radius must be known for these expressions. Also, one can show for any punch that the load displacement can be written in the form F(δ) = αδn,(2.61) with αincluding material parameters and ndependent on the geometry (n= 1 for flat cylinders, n= 2 for cones, n= 1.5for paraboloids i.e. spheres) [128]. The expressions of the stress distributions presented above (see Tab. 2.2) have limitations in their phyical meanings, since the stresses in the descriptions are infinite at the edge of contact. This unphysical situation can be resolved using additional parameters. One can show that the peak stress can only be in the order of the elastic modulus for CHAPTER 2. THEORY AND STATUS OF THE FIELD 67 A B C Figure 2.17: Contact mechanics of elastic bodies: 2.17A Contact radius as a function of applied load, 2.17B Deformation as a function of applied load for a soft sphere (R= 10 µm,E= 1 MPa,w= 10 mJ/m2) pressed against a hard substrate (dotted lines show the hysteresis character of the JKR theory). 2.17C Stress in the contact zone of a soft sphere for an applied load of F= 1 µN(dotted lines show the contact radius). CHAPTER 2. THEORY AND STATUS OF THE FIELD 68 Figure 2.18: Availability of contact mechanic models (reproduced from [148] c Elsevier) soft materials or the predicted VdW stress of the interface. Above, only total linear elastic deformations were treated. For indentations where plastic deformation or viscoelastic phenomena occur the situation is even more complex due to nonlinear behavior. Approaches to model these contact problems were developed for example by Oliver and Pharr [149]. Also, heterogeneities and roughness are neglected in the continuum elastic theories described above and should be taken into account for a full realistic description [150, 151]. CHAPTER 2. THEORY AND STATUS OF THE FIELD 69 Table 2.3: Experimental methods to study responsive layers, FTIR: Fourier transform infrared spectroscopy, QMB: Quartz crystal micro balance, SFA: Surface force apparatus, AFM: Atomic force microscope, JKR: JKR apparatus properties methods chemical structure FTIR, QMB, spectroscopy thickness and density scattering techniques, ellipsometry, reflectometry spectroscopy, microscopy, SFA, AFM surface interactions and mechanics SFA, AFM, JKR, microscopy 2.4 Experimental Methods: Atomic Force and Optical Microscopy For characterization and understanding of responsive surfaces experimental techniques are necessary. Novel scanning probe microscopy approaches and optical methods can give insights into the behavior of surfaces and interfaces [125]. Exemplary experimental approaches to study the chemical structure, the thickness and density of responsive PE surface, as well as surface interactions and mechanical properties are summarized in Table 2.3 [1, 17]. These methods are accomplished by new theoretical approaches and modeling techniques. This section focus on the used techniques in this thesis, in particular atomic force and optical microscopy. The discussion is oriented on [124, 125, 24, 126] and the cited literature. 2.4.1 Atomic Force Microscopy (AFM) Several characterization methods are known to study the response of smart coatings (Table 2.3). In this thesis, the focus is set on changes of interactions and mechanical properties of such systems on the colloidal scale. One of the most suitable techniques for surface characterization in this regime is atomic force microscopy (AFM) [152]. Complementary methods can be e.g. the surface force apparatus [153, 154], the JRK apparatus [155], or optical or magnetic tweezers [156]. The AFM was invented by Binnig et al. in the 1980s [157] on the basis of the scanning tunneling microscope. Nowadays, the AFM has many fields of applications [158]. Besides the original intention of imaging surface topographies, the AFM allows for detecting surface and interfacial forces, molecular interactions and characterization of the mechanical and the electrical properties of the surface of the studied material, just to name a few. CHAPTER 2. THEORY AND STATUS OF THE FIELD 70 Figure 2.19: Working principle of an AFM The working principle of an AFM is based on interactions between the sample and a probe as a function of their distance. The probe is commonly a cantilever which deflects towards or away from the surface depending on attractive or repulsive forces. The deflection can be detected at high precision using several methods. The most common detection method is the optical lever approach where a laser beam is focused on the backside of the cantilever and is reflected to a position-sensitive photodiode (Figure 2.19). The position-sensitivity is achieved by subdivision of the detector (typically quarterized), which results in different intensities on the individual areas and finaly in a current signal. The current can be transformed into the required information, in particular height changes (these can be further converted to interaction forces, hardness, and roughness.) The acting force can be calculated from the deflection of the cantilever by Hooks law F(δc) = kcδc,(2.62) where kcis the spring constant and δcthe deflection of the cantilever. The relative position of cantilever and sample is controlled by piezoelectric elements with a precession of 0.1 nm in x-, y-, and zdirection. Typical cantilevers are made from CHAPTER 2. THEORY AND STATUS OF THE FIELD 71 silicon or silicon nitrid and have a spring constant in the order of (0.001 −100) N/m. For enhancement of the reflectivity of the cantilever, the backside of the cantilever can be coated with a metal. The advantages of an AFM are its high spatial resolution on the nm scale (determined by the convolution of the tip and the sample), high sensitivity towards forces in the range from pN up to some µN, and a resolution for deformations smaller than 1 nm (determined by the spring constant of the cantilever). These pros and the ability to study nearly any kind of solid (or liquid) interface in various environments in a big temperature range have made the AFM one of the most popular tools in surface science. Also, other surface properties can bes studied, as for example electrical properties by controling an electric potential applied to the cantilever. For instance lateral electric properties are important in the semiconductor and hard disc industry [159]. Drawbacks of the AFM are that the technique is relatively slow and limited to surface (rather than bulk) properties. Additionally, the operating distances are limited by the used piezos (typically in the range of 10 µmfor z-, and 100 µmin xand ydirection). Imaging AFM For imaging, (typically) sharp tip cantilevers are scanned in xand ydirection over the sample substrate. Three main operation modes can be distinguished: 1) the contact mode, 2) the noncontact mode, and 3) the tapping mode. In contact mode, interaction forces are detected while the cantilever tip remains in contact with the sample during scanning. The resulting deflection (constant height) or the applied force to keep the deflection constant (constat force), are used as the signal. Using a feedback loop the signal can be directly converted into a topographic image. The noncontact and the tapping mode are dynamic modes. Here the cantilever is oscillated near its resonance frequency and shifts of amplitude and frequency are detected. In case of the noncontact mode, shifts of the resonance frequency are detected while moving the cantilever above the surface (no contact). The tapping mode combines benefits of both, the contact and non-contact mode, by oscillating the cantilever near its resonance frequency, while allowing for small impacts of the cantilever tip into the sample. The resulting image of these modes is mathematically a convolution of the sample and the probe. As a result the spatial resolution is limited by the geometry of cantilever tip11. An overview of the different imaging modes is given in Ref. [158, 160, 161]. 11Additional factors are external and internal vibrations and the damping quality of the cantilever. CHAPTER 2. THEORY AND STATUS OF THE FIELD 72 Force Spectroscopy Besides imaging, the AFM can be used for direct force measurements [128, 152]. For this, the cantilever is driven in zdirection towards or away from the surface. The cantilever deflection δcis recorded as a function of the piezo hub Z. This outcome can be transformed into a force versus distance (FD) curve (often named force-separation or force-indentation curve, depending on the experiment). For that purpose the voltage of the photodiode must be related to the force acting on the cantilever. If no artifacts occurs (e.g. due to very large cantilever deflections), the deflection of the cantilever is linear proportional to the voltage of the photodiode V δc=SV, (2.63) where Sis the so-called ”inverted optical lever sensitivity” (InvOLS). If the spring constant of the cantilever kcis known precisely12 (from calibration measurements, see below), the deflection can be transformed into the acting force using Hooks law (eq. 2.62) F(Z) = kcSV. (2.64) The separation of a cantilever from the surface Dis related to the movement of the z-piezo Zas D=Z−(δC+δS),(2.65) where δcis the cantilever deflection and δSthe deformation of the sample surface. FD curves reflect the contributions of surface interactions and Hooks law of the cantilever. A schematic example of a FD curve is shown in Figure 2.20. When the cantilever is far away from the surface, no interactions occure and cantilever deflection is zero. This part of the FD curve is called ”baseline”. If the cantilever comes close to the surface, the cantilever will start to bend due to surface forces. In case of attractive forces, the cantilever will bend towards the surface and jump into contact (jtc) when the gradient of the attractive forces overcomes the spring constant of the cantilever. When the surface forces are repulsive (e.g. due to electrostatic diffuse layer froces, Ch. 2.3.3) the cantilever starts to bend away from the surface which results in an increase of the detected force. If the cantilever movement is sustained, the cantilever will deflect as described by Eq. 2.62 in combination with the deformation of the sample (Tab. 2.2). In case of a hard substrate (stiffness of the cantilever is much smaller than the stiffness of the substrate), 12Manufacture values are typically not realible, see below for several calibration methods CHAPTER 2. THEORY AND STATUS OF THE FIELD 73 this results in the constant compliant region, where probe and sample surface move in parallel. At a certain point, the piezo movement is turned back and the cantilever retracts. Trace and retrace in contact are named as contact part, contain information on the mechanics of the sample. They can be described by contact mechanical models (Ch. 2.3.6). In case of attractive forces (adhesion between the sample and the probe), the cantilever will stay in contact until the restoring force of the cantilever overcomes the adhesion force Fadh and the cantilever jumps out of contact (joc) (For expressions of Fadh see Tab. 2.2). The included area between trace and retrace of the FD curve contains information on the work of adhesion (adhesion hysteresis). A B Figure 2.20: Force distance curve: 2.20A Cantilever movement, 2.20B Detected raw data, top (schematic of hard tip interacting with a hard substrate) converted to a force distance curve, bottom. The presented FD curve (Figure 2.20) is just a simplified example. For complete interpretation of the experimentally detected FD curves, including all surface and molecular interactions, mechanical properties (as introduced in Ch. 2.3) and hydrodynamic effects of the sample, the reader is referred to Ref. [128, 152]. In these reviews also many CHAPTER 2. THEORY AND STATUS OF THE FIELD 80 with I1and I2the intensities of the reflected light of the substrate and the sample, h(x)the local height of the specimen, k= 2πn/λ the wave number of the light and δ accounting for possible phase shifts. Reconstruction of the specimen is possible by the use of an arccos-trafo of Eq. 2.74 that can be adopted stepwise to the interference pattern. For that purpose, a rough knowledge of the sample geometry is needed, in order to consider the right curvature of the object. For absolute distance measurements, a certain reference point must be known. Without this knowledge, the absolute distance perpendicular to the surface can onloy be measured with two-color RICM [179]. Figure 2.23: Principle of reflection interference contrast microscopy (RICM): I0incident light, I1reflected light from the substrate, I2reflected light from the sample and interference pattern (of a glass sphere) on the detector. CHAPTER 2. THEORY AND STATUS OF THE FIELD 81 2.5 References [1] Knoll. Functional Polymer Films. Vol. 1. Wiley-VCH. Weinheim, Germany, 2011. [2] T. P. Russell. “Surface-responsive materials”. In: Science 297.5583 (2002), pp. 964– 967. [3] A. Sidorenko et al. “Switching of polymer brushes”. In: Langmuir 15.24 (1999), pp. 8349–8355. [4] K. Glinel et al. “Responsive polyelectrolyte multilayers”. In: Colloids and Surfaces a-Physicochemical and Engineering Aspects 303.1-2 (2007), pp. 3–13. [5] M. A. C. Stuart et al. “Emerging applications of stimuli-responsive polymer materials”. In: Nature Materials 9.2 (2010), pp. 101–113. [6] C. D. H. Alarcon, S. Pennadam, and C. Alexander. “Stimuli responsive polymers for biomedical applications”. In: Chemical Society Reviews 34.3 (2005), pp. 276– 285. [7] A. L. Black, J. M. Lenhardt, and S. L. Craig. “From molecular mechanochemistry to stress-responsive materials”. In: Journal of Materials Chemistry 21.6 (2011), pp. 1655–1663. [8] E. Cabane et al. “Stimuli-Responsive Polymers and Their Applications in Nanomedicine”. In: Biointerphases 7.1-4 (2012), pp. 1–27. [9] J. Bunsow, T. S. Kelby, and W. T. S. Huck. “Polymer Brushes: Routes toward Mechanosensitive Surfaces”. In: Accounts of Chemical Research 43.3 (2010), pp. 466–474. [10] A. Kumar, H. A. Biebuyck, and G. M. Whitesides. “Patterning Self-Assembled Monolayers - Applications in Materials Science”. In: Langmuir 10.5 (1994), pp. 1498–1511. [11] Y. Liu et al. “Controlled switchable surface”. In: Chemistry-a European Journal 11.9 (2005), pp. 2622–2631. [12] G. Pasparakis and M. Vamvakaki. “Multiresponsive polymers: nano-sized assemblies, stimuli-sensitive gels and smart surfaces”. In: Polymer Chemistry 2.6 (2011), pp. 1234–1248. CHAPTER 2. THEORY AND STATUS OF THE FIELD 82 [13] I. Tokarev, M. Motornov, and S. Minko. “Molecular-engineered stimuli-responsive thin polymer film: a platform for the development of integrated multifunctional intelligent materials”. In: Journal of Materials Chemistry 19.38 (2009), pp. 6932– 6948. [14] F. Zhou and W. T. S. Huck. “Surface grafted polymer brushes as ideal building blocks for ”smart” surfaces”. In: Physical Chemistry Chemical Physics 8.33 (2006), pp. 3815–3823. [15] Omar Azzaroni. “Polymer brushes here, there, and everywhere: Recent advances in their practical applications and emerging opportunities in multiple research fields”. In: Journal of Polymer Science Part a-Polymer Chemistry 50.16 (2012), pp. 3225–3258. [16] H. Dautzenberg. Polyelectrolyts. Hanser. Munich, Germany, 1994. [17] H.S. Nalwa. Handbook of Polyelectrolytes and Their Applications. American Scientific Publishers. USA, 2002. [18] M. Rubinstein and R. H. Collby. Polymer Physics. Vol. 1. Oxford University Press. NY, USA, 2003. [19] G. Strobel. The Physics of Polymers: Concepts for Understanding Their Structures and Behavior. Springer. Heidelberg, Germany, 2007. [20] A. V. Dobrynin and M. Rubinstein. “Theory of polyelectrolytes in solutions and at surfaces”. In: Progress in Polymer Science 30.11 (2005), pp. 1049–1118. [21] C. Wood et al. “From conditioning shampoo to nanomechanics and haptics of human hair”. In: Journal of cosmetic science 62.2 (2011), pp. 259–64. [22] M. Gueron and G. Weisbuch. “Poly-Electrolyte Theory .1. Counterion Accumulation, Site-Binding, and Their Insensitivity to Poly-Electrolyte Shape in Solutions Containing Finite Salt Concentrations”. In: Biopolymers 19.2 (1980), pp. 353–382. [23] P. Debye and E. Hueckel. In: Physikalische Zeitschrift (1923). [24] H. J. Butt and M. Kappl. Surface and Interfacial Forces. Weinheim, Germany: Wiley-VCH, 2010. [25] G. S. Manning. “Limiting Laws and Counterion Condensation in Polyelectrolyte Solutions .I. Colligative Properties”. In: Journal of Chemical Physics 51.3 (1969), pp. 924–. CHAPTER 2. THEORY AND STATUS OF THE FIELD 83 [26] G. S. Manning and B. H. Zimm. “Cluster Theory of Polyelectrolyte Solutions .I. Activity Coefficients of Mobile Ions”. In: Journal of Chemical Physics 43.12 (1965), pp. 4250–. [27] A. Deshkovski, S. Obukhov, and M. Rubinstein. “Counterion phase transitions in dilute polyelectrolyte solutions”. In: Physical Review Letters 86.11 (2001), pp. 2341–2344. [28] B. Y. Ha and D. Thirumalai. “Electrostatic Persistence Length of a Polyelectrolyte Chain”. In: Macromolecules 28.2 (1995), pp. 577–581. [29] T. Odijk. “Polyelectrolytes near the Rod Limit”. In: Journal of Polymer Science Part B-Polymer Physics 15.3 (1977), pp. 477–483. [30] T. Odijk and A. C. Houwaart. “Theory of Excluded-Volume Effect of a Polyelectrolyte in a 1-1 Electrolyte Solution”. In: Journal of Polymer Science Part B-Polymer Physics 16.4 (1978), pp. 627–639. [31] E. Raphael and J. F. Joanny. “Annealed and Quenched Polyelectrolytes”. In: Europhysics Letters 13.7 (1990), pp. 623–628. [32] J. T. G. Overbeek. “The Dissociation and Titration Constants of Polybasic Acids”. In: Bulletin Des Societes Chimiques Belges 57.5-6 (1948), pp. 252–261. [33] S. Block. On surface forces and morphology of linear polyelectrolytes physisorbed onto oppositely charged surfaces. Greifswald, Germany, 2010. [34] G. J. Fleer. Polymers at interfaes. Springer. Berlin, Germany, 1993. [35] G. J. Fleer, J. Scheutjens, and M. A. C. Stuart. “Theoretical Progress in Polymer Adsorption, Steric Stabilization and Flocculation”. In: Colloids and Surfaces 31 (1988), pp. 1–29. [36] M. A. C. Stuart. “Poly-Electrolyte Adsorption”. In: Journal De Physique 49.6 (1988), pp. 1001–1008. [37] J. M. H. M. Scheutjens and G. J. Fleer. “Statistical-Theory of the Adsorption of Interacting Chain Molecules .1. Partition-Function, Segment Density Distribution, and Adsorption-Isotherms”. In: Journal of Physical Chemistry 83.12 (1979), pp. 1619–1635. [38] J.M.H.M. Scheutjens and G. J. Fleer. “Statistical-Theory of the Adsorption of Interacting Chain Molecules .2. Train, Loop, and Tail Size Distribution”. In: Journal of Physical Chemistry 84.2 (1980), pp. 178–190. CHAPTER 2. THEORY AND STATUS OF THE FIELD 84 [39] H. G. M. Vandesteeg et al. “Polyelectrolyte Adsorption - a Subtle Balance of Forces”. In: Langmuir 8.10 (1992), pp. 2538–2546. [40] R. R. Netz and D. Andelman. “Neutral and charged polymers at interfaces”. In: Physics Reports-Review Section of Physics Letters 380.1-2 (2003), pp. 1–95. [41] R. R. Netz and J. F. Joanny. “Adsorption of semiflexible polyelectrolytes on charged planar surfaces: Charge compensation, charge reversal, and multilayer formation”. In: Macromolecules 32.26 (1999), pp. 9013–9025. [42] J. Kotz, S. Kosmella, and T. Beitz. “Self-assembled polyelectrolyte systems”. In: Progress in Polymer Science 26.8 (2001), pp. 1199–1232. [43] V. A. Kabanov and A. B. Zezin. “Soluble Interpolymeric Complexes as a New Class of Synthetic Poly-Electrolytes”. In: Pure and Applied Chemistry 56.3 (1984), pp. 343–354. [44] Dmitry V. Pergushov, Axel H. E. Mueller, and Felix H. Schacher. “Micellar interpolyelectrolyte complexes”. In: Chemical Society Reviews 41.21 (2012), pp. 6888– 6901. [45] V. A. Kabanov. “Polyelectrolyte complexes in solution and in the condensed phase”. In: Uspekhi Khimii 74.1 (2005), pp. 5–23. [46] S. A. Sukhishvili, E. Kharlampieva, and V. Izumrudov. “Where polyelectrolyte multilayers and polyelectrolyte complexes meet”. In: Macromolecules 39.26 (2006), pp. 8873–8881. [47] F. H. Schacher, P. A. Rupar, and I. Manners. “Functional Block Copolymers: Nanostructured Materials with Emerging Applications”. In: Angewandte ChemieInternational Edition 51.32 (2012), pp. 7898–7921. [48] C. A. Fustin, V. Abetz, and J. F. Gohy. “Triblock terpolymer micelles: A personal outlook”. In: European Physical Journal E 16.3 (2005), pp. 291–302. [49] G. Riess. “Micellization of block copolymers”. In: Progress in Polymer Science 28.7 (2003), pp. 1107–1170. [50] J. Ruhe et al. “Polyelectrolyte brushes”. In: Polyelectrolytes with Defined Molecular Architecture I. Vol. 165. Advances in Polymer Science. Berlin, Germany: Springer-Verlag, 2004. [51] A. Ulman. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self Assembly. Academic Press. San Diego, USA, 1991. CHAPTER 2. THEORY AND STATUS OF THE FIELD 85 [52] J. C. Love et al. “Self-assembled monolayers of thiolates on metals as a form of nanotechnology”. In: Chemical Reviews 105.4 (2005), pp. 1103–1169. [53] F. Schreiber. “Structure and growth of self-assembling monolayers”. In: Progress in Surface Science 65.5-8 (2000), pp. 151–256. [54] A. Ulman. “Formation and structure of self-assembled monolayers”. In: Chemical Reviews 96.4 (1996), pp. 1533–1554. [55] G. M. Whitesides and B. Grzybowski. “Self-assembly at all scales”. In: Science 295.5564 (2002), pp. 2418–2421. [56] G. Decher. “Multilayer Thin Films”. In: (2012). [57] G. Decher. “Fuzzy nanoassemblies: Toward layered polymeric multicomposites”. In: Science 277.5330 (1997), pp. 1232–1237. [58] R. K. Iler. “Multilayers of Colloidal Particles”. In: Journal of Colloid and Interface Science 21.6 (1966), pp. 569–. [59] K. Ariga, J. P. Hill, and Q. Ji. “Layer-by-layer assembly as a versatile bottomup nanofabrication technique for exploratory research and realistic application”. In: Physical Chemistry Chemical Physics 9.19 (2007), pp. 2319–2340. [60] P. T. Hammond. “Form and function in multilayer assembly: New applications at the nanoscale”. In: Advanced Materials 16.15 (2004), pp. 1271–1293. [61] Z. J. Sui, D. Salloum, and J. B. Schlenoff. “Effect of molecular weight on the construction of polyelectrolyte multilayers: Stripping versus sticking”. In: Langmuir 19.6 (2003), pp. 2491–2495. [62] S. T. Dubas and J. B. Schlenoff. “Swelling and smoothing of polyelectrolyte multilayers by salt”. In: Langmuir 17.25 (2001), pp. 7725–7727. [63] S. S. Shiratori and M. F. Rubner. “pH-dependent thickness behavior of sequentially adsorbed layers of weak polyelectrolytes”. In: Macromolecules 33.11 (2000), pp. 4213–4219. [64] S. T. Dubas and J. B. Schlenoff. “Factors controlling the growth of polyelectrolyte multilayers”. In: Macromolecules 32.24 (1999), pp. 8153–8160. [65] T. Boudou et al. “Multiple Functionalities of Polyelectrolyte Multilayer Films: New Biomedical Applications”. In: Advanced Materials 22.4 (2010), pp. 441–467. [66] L. Richert et al. “Layer by layer buildup of polysaccharide films: Physical chemistry and cellular adhesion aspects”. In: Langmuir 20.2 (2004), pp. 448–458. CHAPTER 2. THEORY AND STATUS OF THE FIELD 86 [67] L. L. del Mercato et al. “LbL multilayer capsules: recent progress and future outlook for their use in life sciences”. In: Nanoscale 2.4 (2010), pp. 458–467. [68] G. Sukhorukov, A. Fery, and H. Mohwald. “Intelligent microand nanocapsules”. In: Progress in Polymer Science 30.8-9 (2005), pp. 885–897. [69] L. Zhai et al. “Stable superhydrophobic coatings from polyelectrolyte multilayers”. In: Nano Letters 4.7 (2004), pp. 1349–1353. [70] S. Srivastava and N. A. Kotov. “Composite Layer-by-Layer (LBL) Assembly with Inorganic Nanoparticles and Nanowires”. In: Accounts of Chemical Research 41.12 (2008), pp. 1831–1841. [71] A. C. Fou et al. “Fabrication and properties of light-emitting diodes based on self-assembled multilayers of poly(phenylene vinylene)”. In: Journal of Applied Physics 79.10 (1996), pp. 7501–7509. [72] J. L. Lutkenhaus and P. T. Hammond. “Electrochemically enabled polyelectrolyte multilayer devices: from fuel cells to sensors”. In: Soft Matter 3.7 (2007), pp. 804–816. [73] J. Cho et al. “Nanoporous block copolymer micelle/micelle multilayer films with dual optical properties”. In: Journal of the American Chemical Society 128.30 (2006), pp. 9935–9942. [74] N. Ma et al. “Polymer micelles as building blocks for layer-by-layer assembly: An approach for incorporation and controlled release of water-insoluble dyes”. In: Chemistry of Materials 17.20 (2005), pp. 5065–5069. [75] B. D. Gates et al. “New approaches to nanofabrication: Molding, printing, and other techniques”. In: Chemical Reviews 105.4 (2005), pp. 1171–1196. [76] Y. N. Xia and G. M. Whitesides. “Soft lithography”. In: Angewandte ChemieInternational Edition 37.5 (1998), pp. 551–575. [77] R Advincula et al. Polymer brushes. Wiley-VCH. Weinheim, Germany, 2004. [78] S. T. Milner. “Polymer Brushes”. In: Science 251.4996 (1991), pp. 905–914. [79] W. J. Brittain and S. Minko. “A structural definition of polymer brushes”. In: Journal of Polymer Science Part a-Polymer Chemistry 45.16 (2007), pp. 3505– 3512. [80] B. Zhao and W. J. Brittain. “Polymer brushes: surface-immobilized macromolecules”. In: Progress in Polymer Science 25.5 (2000), pp. 677–710. CHAPTER 2. THEORY AND STATUS OF THE FIELD 87 [81] X. Dai et al. “Direct Visualization of Reversible Switching of Micropatterned Polyelectrolyte Brushes on Gold Surfaces Using Laser Scanning Confocal Microscopy”. In: Langmuir 24.22 (2008), pp. 13182–13185. [82] M. Ballauff. “Spherical polyelectrolyte brushes”. In: Progress in Polymer Science 32.10 (2007), pp. 1135–1151. [83] M. Ballauff and O. Borisov. “Polyelectrolyte brushes”. In: Current Opinion in Colloid and Interface Science 11.6 (2006), pp. 316–323. [84] R. Barbey et al. “Polymer Brushes via Surface-Initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties, and Applications”. In: Chemical Reviews 109.11 (2009), pp. 5437–5527. [85] L. Ionov et al. “Gradient mixed brushes: "Grafting to" approach”. In: Macromolecules 37.19 (2004), pp. 7421–7423. [86] O. Azzaroni et al. “Switching the properties of polyelectrolyte brushes via "Hydrophobic collapse"”. In: Macromolecules 38.24 (2005), pp. 10192–10199. [87] G. Reiter, P. Auroy, and L. Auvray. “Instabilities of thin polymer films on layers of chemically identical grafted molecules”. In: Macromolecules 29.6 (1996), pp. 2150–2157. [88] F. Xia et al. “Dual-responsive surfaces that switch superhydrophilicity and superhydrophobicity”. In: Advanced Materials 18.4 (2006), pp. 432–+. [89] J. Klein et al. “Reduction of Frictional Forces between Solid-Surfaces Bearing Polymer Brushes”. In: Nature 370.6491 (1994), pp. 634–636. [90] U. Raviv et al. “Lubrication by charged polymers”. In: Nature 425.6954 (2003), pp. 163–165. [91] P. Pincus. “Colloid Stabilization with Grafted Polyelectrolytes”. In: Macromolecules 24.10 (1991), pp. 2912–2919. [92] L. Sun, G. L. Baker, and M. L. Bruening. “Polymer brush membranes for pervaporation of organic solvents from water”. In: Macromolecules 38.6 (2005), pp. 2307–2314. [93] S. Moya et al. “Locking and unlocking of polyelectrolyte brushes: toward the fabrication of chemically controlled nanoactuators”. In: Angewandte ChemieInternational Edition 44.29 (2005), pp. 4578–4581. [94] S. Santer and J. Ruhe. “Motion of nano-objects on polymer brushes”. In: Polymer 45.25 (2004), pp. 8279–8297. CHAPTER 2. THEORY AND STATUS OF THE FIELD 88 [95] Y. Mei et al. “High catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes”. In: Langmuir 21.26 (2005), pp. 12229–12234. [96] R. Iwata et al. “Control of nanobiointerfaces generated from well-defined biomimetic polymer brushes for protein and cell manipulations”. In: Biomacromolecules 5.6 (2004), pp. 2308–2314. [97] A. Wittemann and M. Ballauff. “Interaction of proteins with linear polyelectrolytes and spherical polyelectrolyte brushes in aqueous solution”. In: Physical Chemistry Chemical Physics 8.45 (2006), pp. 5269–5275. [98] W. Senaratne, L. Andruzzi, and C. K. Ober. “Self-assembled monolayers and polymer brushes in biotechnology: Current applications and future perspectives”. In: Biomacromolecules 6.5 (2005), pp. 2427–2448. [99] S. Edmondson, V. L. Osborne, and W. T. S. Huck. “Polymer brushes via surfaceinitiated polymerizations”. In: Chemical Society Reviews 33.1 (2004), pp. 14–22. [100] X. Y. Huang and M. J. Wirth. “Surface-initiated radical polymerization on porous silica”. In: Analytical Chemistry 69.22 (1997), pp. 4577–4580. [101] V. Coessens, T. Pintauer, and K. Matyjaszewski. “Functional polymers by atom transfer radical polymerization”. In: Progress in Polymer Science 26.3 (2001), pp. 337–377. [102] K. Matyjaszewski and J. H. Xia. “Atom transfer radical polymerization”. In: Chemical Reviews 101.9 (2001), pp. 2921–2990. [103] N. Cheng et al. “The effect of [Cu-I]/[Cu-II] ratio on the kinetics and conformation of polyelectrolyte brushes by atom transfer radical polymerization”. In: Macromolecular Rapid Communications 27.19 (2006), pp. 1632–1636. [104] X. Guo and M. Ballauff. “Spatial dimensions of colloidal polyelectrolyte brushes as determined by dynamic light scattering”. In: Langmuir 16.23 (2000), pp. 8719– 8726. [105] P.G. de Gennes. Scaling Concepts in Polymer Physics. Vol. 1. Cornell University Press. NY, USA, 1979. [106] R. Israels et al. “Charged Polymeric Brushes - Structure and Scaling Relations”. In: Macromolecules 27.12 (1994), pp. 3249–3261. [107] E. B. Zhulina, T. M. Birshtein, and O. V. Borisov. “Theory of Ionizable Polymer Brushes”. In: Macromolecules 28.5 (1995), pp. 1491–1499. CHAPTER 2. THEORY AND STATUS OF THE FIELD 89 [108] E. B. Zhulina and O. V. Borisov. “Structure and interaction of weakly charged polyelectrolyte brushes: Self-consistent field theory”. In: Journal of Chemical Physics 107.15 (1997), pp. 5952–5967. [109] E. B. Zhulina, J. K. Wolterink, and O. V. Borisov. “Screening effects in a polyelectrolyte brush: Self-consistent-field theory”. In: Macromolecules 33.13 (2000), pp. 4945–4953. [110] S. Alexander. “Adsorption of Chain Molecules with a Polar Head a-Scaling Description”. In: Journal De Physique 38.8 (1977), pp. 983–987. [111] P. G. de Gennes. “Conformations of Polymers Attached to an Interface”. In: Macromolecules 13.5 (1980), pp. 1069–1075. [112] O. V. Borisov and E. B. Zhulina. “Structure of weakly charged polyelectrolyte brushes: Monomer density profiles”. In: Journal De Physique Ii 7.3 (1997), pp. 449–458. [113] R. Israels, F. A. M. Leermakers, and G. J. Fleer. “On the Theory of Grafted Weak Polyacids”. In: Macromolecules 27.11 (1994), pp. 3087–3093. [114] M. Biesalski, D. Johannsmann, and J. Ruhe. “Synthesis and swelling behavior of a weak polyacid brush”. In: Journal of Chemical Physics 117.10 (2002), pp. 4988–4994. [115] P. M. Biesheuvel. “Ionizable polyelectrolyte brushes: brush height and electrosteric interaction”. In: Journal of Colloid and Interface Science 275.1 (2004), pp. 97–106. [116] E. B. Zhulina and O. V. Borisov. “Poisson-Boltzmann Theory of pH-Sensitive (Annealing) Polyelectrolyte Brush”. In: Langmuir : the ACS journal of surfaces and colloids 27.17 (2012), pp. 10615–33. [117] A. Kumar. Molecular dynamics simulations of polyelectrolyte brushes. Golm, Germany, 2006. [118] W. Funke. “Problems and progress in organic coatings science and technology”. In: Progress in Organic Coatings 31.1-2 (1997), pp. 5–9. [119] L. T. Drzal and M. Madhukar. “Fiber Matrix Adhesion and Its Relationship to Composite Mechanical-Properties”. In: Journal of Materials Science 28.3 (1993), pp. 569–610. CHAPTER 3. SOFT COLLOIDAL PROBE AFM 97 Abstract We present a method based on colloidal probe atomic force microscopy (AFM) to measure adhesion energies and to study other contact phenomena of surfaces. The method employs an elastomeric colloidal probe, rendering the contact area between probe and sample much larger as compared to standard atomic force microscopy techniques. The technique allows us to determine the contact area via microinterferometry and measure the applied forces at the same time. The adhesion properties can then be accessed by using the Johnson-Kendall-Roberts (JKR) approach, i.e. measuring (a) the contact area as a function of applied load, and (b) the elastic parameters and the thermodynamic work of adhesion. We test this method in ambient conditions as well as in aqueous media on well-known surface chemistries, and can clearly characterize the contributions of capillary in air, hydration forces and hydrophobic interactions in water. This novel method provides a means to study the contact behavior of soft colloids and enhanced sensitivity for adhesion measurements. 3.1 Introduction Adhesion and contact phenomena are important in many branches of nature and technology. Two centuries of research have proven their significance in fields such as nano(bio)technology [1, 2, 3] and biophysics [4, 5]. For example, adhesion forces determine cell differentiation [6] and allow lizards to climb sheer walls [7, 8]. Adhesion technology is vital in coatings, composite materials [9, 10] or adhesives [11]. There are still many open questions on adhesion and contact phenomena of soft matter systems. Current research focuses on the behavior of (bio)polymers in water [12, 13, 14], biosystems that make use of various adhesion strategies on multiple length scales [4] or dynamic phenomena like rearrangement of polymers inside the contact area [15, 16] or switchable polymer brushes that change their adhesion properties by changing their environmental conditions [17]. To tackle such challenges several methods suitable for different length scales were developed [18, 19, 20]. With regard to the size of contact areas that are accessible, existing methods can be grouped into macroand microscalecontact methods. Therefore, developments in adhesion measurement methods allowed the study of macroand microscale-contact phenomena: macro-scale contacts are characterized using Peel and torsion tests [20, 21, 22] and wetting methods like contact-angle measurements [23, 24]. For ultra-thin films, the surface force apparatus (SFA) [25, 26, 27] provides an unparalleled sensitivity and vertical resolution on the CHAPTER 3. SOFT COLLOIDAL PROBE AFM 98 nanometre-scale for macro-scale contacts. The so-called JKR apparatus takes advantage of the fact that the contact area for large elastomeric lenses can be modeled using the Johnson-Kendall-Roberts theory [28]. Here, adhesion energies are derived using the dependence of contact area on the applied load. This method has been successfully applied through the last years, for example as described in Ref. [20] and [29, 30, 31, 32]. The advantage of macro-scale methods is the well-defined contact area. However, these methods are limited to smooth and chemically homogenous samples over a few hundred µm2. On the microand nanoscale the resolution of an atomic force microscope (AFM) [33] is required in order to study surface interactions. [18] Here, surface interactions are measured by force-distance measurements, [34] where a sharp AFM tip (typical radius of curvature in the range of 5−20 nm) is brought into contact with the sample. In this mode usually the pull-off force of the AFM probe from the sample surface is used as a measure of the surface-probe interactions. While this technique allows access to extremely small contact areas, the contact area is rather illdefined and cannot be independently determined. The reason is that the shape of AFM tips cannot be accurately controlled in the manufacturing process or even during measurement and therefore, AFM tips do not satisfy the demand for a well-defined contact geometry. A major step towards solving this problem was the introduction of the colloidal probe-AFM technique [35] by Ducker et al. [36] and Butt [37] in the 1990s. Colloidal particles of several micrometres in size are attached to AFM cantilevers. Typically silica spheres or plastic beads with a diameter in the range of 1−50 mm and with surface roughnesses of only a few nanometres are used [18, 35]. While the probe geometry is well defined as compared to standard AFM tips, the contact radii with hard substrates are still in the order of 100 nm and thus hard to determine in situ. Because of that major drawback, one still has to rely on assumptions rather than direct measurement of the contact area which is problematic for determining the work of adhesion. In the last years the colloidal probe technique was extended to measure interaction forces or to describe the contact behavior between other kinds of probes, for example segments of hairs [38] or soft probes like droplets [39, 40, 41, 42] or bubbles [43]. In this paper we describe a novel technique combining the advantages of the JKR method with the colloidal probe AFM approach. We modify the colloidal probe technique to obtain a larger contact area between probe and sample by using soft colloidal particles, made of cross-linked polydimethylsiloxane (PDMS). The elastic properties, the size of the probe and thus the size of the contact area can be controlled by varying the preparation methodology of the soft probes. Under suitable conditions the contact CHAPTER 3. SOFT COLLOIDAL PROBE AFM 99 areas are sufficiently large to allow in situ measurement using microinterferometry [44, 45]. Consequently, the adhesion properties of the system can be accessed by using the JKR approach. 3.2 Experimental Materials and methods The instrumental setup relies on a combination of interference microscopy and AFM as sketched in Figure 3.1 and the use of novel probe particles. In the following, we summarize the probe preparation and mounting procedure, cantilever calibration, as well as technical details of the setup. Fabrication and characterization of soft colloidal probes (SCPs) The soft probes were prepared via cross-linking droplets of the precursor polymer in solution. PDMS (Sylgard 184 kit) was purchased from Dow Corning, USA. First the prepolymer was mixed with the curing agent using a 10 : 3 ratio. Then, a Milli-Q water dispersion containing 5 wt% PDMS precursor and 0.1 wt% sodium dodecyl sulfate (CAS number 151-21-3, Sigma-Aldrich, Germany) was prepared. After curing for three days at room temperature, the cross-linked PDMS droplets were extracted by freezedrying. The resulting particles were characterized with an inverted optical microscope (Axiovert 200, Zeiss, Germany) to determine particle sizes. Using AFM force-distance measurements (Nanowizard I, JPK Instruments AG, Germany) the Young’s modulus was determined as well. The mean diameter of the particles was in a range of 10 to 30 µmand the corresponding Young’s modulus in the order of 1 MPa. Figure 3.1: Schematic of the experimental setup: Combination of AFM and optics. CHAPTER 3. SOFT COLLOIDAL PROBE AFM 100 Mounting the probes and calibration The colloidal particles were attached with epoxy resin (UHU schnellfest, Germany) on pre-calibrated AFM cantilevers (7−28 Nm1, tipples NSC 12, Micromash, Estonia) using a micromanipulator (MP-285, Shutter Instrument, USA). The force constant was detected with the thermal noise method, introduced by Hutter and Bechhoefer [46]. A typical PDMS particle attached to a cantilever is shown in Figure 3.2. Deliberately, we established a large contact area between cantilever and particle, such that during measurement, the particle deformation took place mainly at the substrate-probe interface (see Figure 3.3A). After attachment, the SCP was washed in Milli-Q water and dried by a stream of nitrogen. Afterwards, the SCP was treated for 40 s in 1 mbar air plasma at 0.1 kW intensity (PDC-32 G plasma cleaner, Harrick, USA) in order to remove surface impurities and to provide a well-defined silica-like layer on the SCP surface. Under those conditions the thickness of the oxide layer is on the nanoscale [47]. Thus, the elastic properties of the probe are not significantly affected by this process. A B Figure 3.2: Attached PDMS particle on cantilever: 3.2A light microscopy image and 3.2B REM image. Force spectroscopy A commercial AFM (Nanowizard I, JPK Instruments AG, Germany) was used for all AFM measurements. SCPs were mounted into the AFM setup and the optical lever sensitivity was detected (see Results and discussion section: Boundary conditions, calibration and modeling of the SCP). The maximum applied force was between 1600 nN and 2500 nN. For the JKR approach, the load was changed stepwise in intervals of CHAPTER 3. SOFT COLLOIDAL PROBE AFM 101 A B Figure 3.3: 3.3A Schematic of the contact behavior of the systems, probe-substrate and probe-cantilever; 3.3B sketch illustrating the contact parameters: the dotted line represents the undeformed probe, the solid line represents the deformed probe. around 200 nN. The presented measurements show the mean value of four different detection positions of the probe on the substrate. The force-distance curves were recorded using the following parameters: the speed of the piezo actuator was set to 8µms−1. The force-distance curves were measured on different spots using the force-mapping mode of the AFM. Measurements were undertaken on at least 64 spots on a 100 µm2 grid on the substrate. Reflection interference contrast microscopy We used reflection interference contrast microscopy [48, 49] (RICM) to evaluate the contact area of the SCPs with a hard substrate (functionalized glass surface) in situ. During RICM, the sample was illuminated with monochromatic light in reflection geometry. Light was reflected by the substrate and the SCP interface. Due to phase shift and the path length difference between the reflected beams an interference pattern similar to Newton rings can be observed. This interference pattern provides information about the local distance between the SCP and the substrate, as well as the contact area. To enhance the interference pattern we used the antiflex technique introduced by Ploem [50]. For illumination we used a Hg-vapor lamp with two different monochromators (481 nm and 546 nm). A Zeiss Antiflex 63 X NO 1.25 oil-immersion objective, additional polarizers to avoid internal reflections and a Zeiss AxiocamHRm camera were used to image the RICM patterns. Details of the calibration of the RICM setup and the evaluation process of the contact area are summarized in Ch. 3.A and further CHAPTER 3. SOFT COLLOIDAL PROBE AFM 102 details of the setup are reported in Ref.[44, 45, 51] and [52]. Test surface preparation To test the method, we studied samples with known surface chemistry. Here, symmetrically functionalized surfaces (SCP-substrate), i.e. hydrophilic-hydrophilic and hydrophobic-hydrophobic systems, were studied. The surfaces were modified via silanisation agents by gas phase deposition. Hydrophilic surfaces were prepared using [hydroxy(polyethyleneoxy)propyl]triethoxysilane (8-12EO), 50% ethanol (ABCR, Germany); hydrophobic surfaces were prepared using (heptadecafluoro-1,1,2,2-tetrahydrodecyl) dimethylchlorosilane (ABCR, Germany). 3.3 Results and Discussion Boundary conditions, calibration and modeling of the SCP The deformation of the SCP can only be observed at the side of the probe facing the substrate. Therefore, the main deformation of the soft probe should be located in the contact regime of probe and surface. This can be ensured by rendering the contact area of particle and cantilever much larger than the contact area of probe and surface (see Figure 3.3A). Hence we used SCPs where the contact between colloidal particle and cantilever was adequate (large enough) after the attachment. In order to perform AFM force-distance measurements the spring constant of the cantilever as well as its inverted optical lever sensitivity (InvOLS) are required. The InvOLS is the proportionality constant between photodiode signal and the cantilever deflection. Measuring both parameters allows calculation of the InvOLS. In the case of a SCP-modified cantilever this leads to some special challenges: Usually, the cantilever deflection is determined by pressing the cantilever on a hard, non-deformable substrate. In this case the piezo displacement is identical to the cantilever deflection and the InvOLS can be directly read from force-distance curves recorded on hard substrates. This is not possible with an SCP attached to the cantilever. Therefore, we developed two alternative approaches to determine InvOLS: for measurements in air, we used a noncontact approach to determine InvOLS [53]. We use cantilevers for which the spring constant has been determined ex situ and carry out a spring constant determination using the thermal noise method in situ. As this method requires the InvOLS as an input parameter, it can be used to derive it by adjusting the InvOLS until it matches the known spring constant (see Ch. 3.A). In liquid the thermal noise method cannot be used due to CHAPTER 3. SOFT COLLOIDAL PROBE AFM 103 high damping [18]. In this case, we detected the optical lever sensitivity by analysis of the slope of a force-distance curve in contact region, while pressing the apex of the cantilever against a sharp edge. In order to derive adhesion energies the system has to be modelled using the proper contact-mechanic theory [54]. The first analytical description of contact between two isotropic, homogeneous, linear elastic bodies, but without adhesion contribution, was given by Hertz. [55] Regarding the adhesion contribution there are several theories which apply for certain limits of contact behavior. The respective limiting cases are described by the elastic and adhesive properties of the system [54]. Contact parameters like contact radius, deformation, pressure distribution, etc. are functions of load, elastic parameters of the system and the adhesion interaction. The two most common theories were developed by Johnson, Kendall and Roberts [28] (JKR theory) and Derjaguin, Muller and Toporov [56] (DMT theory). Tabor [57] showed that it is possible to separate (to quantify) the different limits of contact behavior with a parameter µT (respectively Maugis [58] λ∝µT) by comparing the elastic deformation caused by the surface interactions with the range of surface forces: µT=16Rw2 9K2z3 01/3 (3.1) where Ris the effective radius of curvature, Kis the effective elastic modulus of the system, wis the thermodynamic work of adhesion and z0is the equilibrium separation of the surfaces (interatomic equilibrium distance for solid-solid interactions in the Lennard-Jones potential, typically on the range of 0.3to 0.5 nm). Rand Kare given by: 1 R=1 R1 +1 R2 and 1 K=3 41−ν2 1 E1 +1−ν2 2 E2(3.2) here R1and R2are the radii of curvature of the interacting surfaces, E1and E2are their Young’s moduli, and ν1and ν2their Poisson ratios (Figure 3.3B. In the case of soft solids, large radius of curvature and large adhesion energy (µT>5), the JKR theory is valid. For stiff solids, small radius of curvature and weak energy of adhesion (µT<5) the DMT theory is valid [54]. To analyze our data we have to decide first of all which theory is suitable to describe our system. We are interested in flat, hard surfaces interacting with the soft probes. For an elastomeric sphere (R1,E1,ν1= 0.5) touching such a surface (R2→ ∞,E2>> E1), the effective radius of curvature Rreduces to R1and the effective modulus of the system Kequals 16/9E1(see Eq. 3.2). With a probe radius in the range of R1= 10 µm, a CHAPTER 3. SOFT COLLOIDAL PROBE AFM 104 Young’s modulus on the order of E1= 1 MPa and a typical thermodynamic work of adhesion in the range of w= 10 mJm−2the Tabor parameter (Eq. 3.1) lies in the range of µT>50. Therefore, we are in the limit of the JKR theory. Following the JKR theory the contact radius between two spheres pressed together by a load Pis given by: a3=R KP+ 3πRw +p6πRwP + (3πRw)2(3.3) Adhesion measurements JKR describe the contact area as a function of load P, elastic properties K, and the adhesion energy w:a=f(P, K, w), see Eq. 3.3. In order to obtain the adhesion energy we press the SCP against the substrate of choice, which can be done with subnanonewton precision using the AFM feedback loop. Simultaneously, we record the adhesion area by microinterferometric imaging. The data are collected at discrete load-force intervals of ∆P > 100 nN. Figure 3.4 shows a typical experiment of a hydrophilic functionalized SCP against a hydrophilic hard substrate in water. The JKR fit, as shown in Figure 3.4B, was computed using Eq. 3.3, yielding the adhesion energy and the Young’s modulus as fit parameters. To crosscheck the Young’s modulus we detected force-distance curves of SCPs on a hard substrate. The force-distance curves were transformed into force-deformation curves of the soft probe by subtracting the effect of the cantilever deflection. Then we fitted the contact region of the force deformation curves with the Hertz model [55]. Both moduli are in the same range (variation <25%). This is in agreement with findings reported in literature, e.g. Ref. [59] To test the method further, we investigated surfaces with known surface chemistry. Adhesion interaction of symmetrically functionalized surfaces, hydrophilic probe-hydrophilic substrate and hydrophobic probe-hydrophobic substrate, was investigated and compared to literature results. Hydrophilic interactions For the hydrophilic probe and the hydrophilic substrate in water we expect relatively small adhesion forces due to hydrophilic interactions [60]. The JKR approach leads to values of wwater = 1.8±0.3 mJm−2(Figure 3.5A) using a particle with a radius of R= 10.9µmcontact with the hydrophilic surface. The Young’s modulus of the particle detected by the JKR fit was E= 0.7±0.1 MPa. The value of 1.8 mJm−2is at the detection limit of adhesion energies for the SCP method at this stage, but in a reasonable range considering that the hydrophilic surface might be slightly contaminated. Values CHAPTER 3. SOFT COLLOIDAL PROBE AFM 105 A B C Figure 3.4: Analysis of the thermodynamic work of adhesion wby extraction of the contact radius afor varying loads P. 3.4A Force-distance curve of a hydrophilic soft colloidal probe against a hydrophilic surface (solid line represents approach cycle, dotted line represents the retraction cycle, and dashed lines show readings of the contact area as shown in 3.4B. 3.4B RICM images of the SCP allowed determination of the contact radius. 3.4C The plot of aagainst Pfollows the JKR predictions. The thermodynamic work w is extracted from the JKR fits (cross-makers, X, denote experimental data, black line denotes the JKR fit). CHAPTER 3. SOFT COLLOIDAL PROBE AFM 112 [34] B. Cappella and G. Dietler. “Force-distance curves by atomic force microscopy”. In: Surface Science Reports 34.1-3 (1999), pp. 1–+. [35] M. Kappl and H. J. Butt. “The colloidal probe technique and its application to adhesion force measurements”. In: Particle and Particle Systems Characterization 19.3 (2002), pp. 129–143. [36] W. A. Ducker, T. J. Senden, and R. M. Pashley. “Direct Measurement of Colloidal Forces Using an Atomic Force Microscope”. In: Nature 353.6341 (1991), pp. 239–241. [37] H. J. Butt. “Measuring Electrostatic, Vanderwaals, and Hydration Forces in Electrolyte-Solutions with an Atomic Force Microscope”. In: Biophysical Journal 60.6 (1991), pp. 1438–1444. [38] C. Wood et al. “AFM Based Single Hair Interaction Measurements”. In: SOFW 11 (2009). [39] R. R. Dagastine et al. “Forces between two oil drops in aqueous solution measured by AFM”. In: Journal of Colloid and Interface Science 273.1 (2004), pp. 339–342. [40] R. R. Dagastine et al. “Dynamic forces between two deformable oil droplets in water”. In: Science 313.5784 (2006), pp. 210–213. [41] R. Manica et al. “Dynamics of interactions involving deformable drops: Hydrodynamic dimpling under attractive and repulsive electrical double layer interactions”. In: Langmuir 23.2 (2007), pp. 626–637. [42] Grant B. Webber et al. “Measurements of dynamic forces between drops with the AFM: novel considerations in comparisons between experiment and theory”. In: Soft Matter 4.6 (2008), pp. 1270–1278. [43] I. U. Vakarelski et al. “Bubble colloidal AFM probes formed from ultrasonically generated bubbles”. In: Langmuir 24.3 (2008), pp. 603–605. [44] F. Dubreuil, N. Elsner, and A. Fery. “Elastic properties of polyelectrolyte capsules studied by atomic-force microscopy and RICM”. In: European Physical Journal E 12.2 (2003), pp. 215–221. [45] A. Fery and R. Weinkamer. “Mechanical properties of microand nanocapsules: Single-capsule measurements”. In: Polymer 48.25 (2007), pp. 7221–7235. [46] J. L. Hutter and J. Bechhoefer. “Calibration of Atomic-Force Microscope Tips”. In: Review of Scientific Instruments 64.7 (1993), pp. 1868–1873. CHAPTER 3. SOFT COLLOIDAL PROBE AFM 113 [47] N. Bowden et al. “The controlled formation of ordered, sinusoidal structures by plasma oxidation of an elastomeric polymer”. In: Applied Physics Letters 75.17 (1999), pp. 2557–2559. [48] A. S. G. Curtis. “Mechanism of Adhesion of Cells to Glass - Study by Interference Reflection Microscopy”. In: Journal of Cell Biology 20.2 (1964), pp. 199–215. [49] M. Kuhner and E. Sackmann. “Ultrathin hydrated dextran films grafted on glass: Preparation and characterization of structural, viscous, and elastic properties by quantitative microinterferometry”. In: Langmuir 12.20 (1996), pp. 4866– 4876. [50] J. Ploem. “Reflection contrast microscopy as a tool for investigation of the attachment of living cells to a glass surface”. In: Mononuclear Phagocytes in Immunity, Infection, and Pathology, Blackwell scientific publications. (1975). [51] S. Schmidt, M. Nolte, and A. Fery. “Single-colloidal-particle microcontact printing”. In: Physical Chemistry Chemical Physics 9.36 (2007), pp. 4967–4969. [52] J. K. Ferri et al. “Separating membrane and surface tension contributions in Pickering droplet deformation”. In: Soft Matter 4.11 (2008), pp. 2259–2266. [53] M. J. Higgins et al. “Noninvasive determination of optical lever sensitivity in atomic force microscopy”. In: Review of Scientific Instruments 77.1 (2006). [54] K. L. Johnson and J. A. Greenwood. “An adhesion map for the contact of elastic spheres”. In: Journal of Colloid and Interface Science 192.2 (1997), pp. 326–333. [55] H. Hertz. “Ueber die Beruehrung fester elastischer Koerper”. In: Journal fuer die reine und angewandte Mathematik 92 (1881). [56] B. V. Derjaguin, V. M. Muller, and Y. P. Toporov. “Effect of Contact Deformations on Adhesion of Particles”. In: Journal of Colloid and Interface Science 53.2 (1975), pp. 314–326. [57] D. Tabor. “Surface Forces and Surface Interactions”. In: Journal of Colloid and Interface Science 58.1 (1977), pp. 2–13. [58] D. Maugis. “Adhesion of Spheres - the JKR-DMT Transition Using a Dugdale Model”. In: Journal of Colloid and Interface Science 150.1 (1992), pp. 243–269. [59] S. A. Chizhik et al. “Micromechanical properties of elastic polymeric materials as probed by scanning force microscopy”. In: Langmuir 14.10 (1998), pp. 2606– 2609. CHAPTER 3. SOFT COLLOIDAL PROBE AFM 114 [60] Israelachvili J.N. Intermolecular and Surface Forces. 2nd ed. Amsterdam, The Netherlands: Academic Press, 2007. [61] P. Warszynski et al. “Interpretation of adhesion force between self-assembled monolayers measured by chemical force microscopy”. In: Colloids and Surfaces a-Physicochemical and Engineering Aspects 214.1-3 (2003), pp. 61–75. [62] G. Papastavrou, S. Akari, and H. Mohwald. “Interactions between hydrophilic and hydrophobic surfaces on microscopic scale and the influence of air bubbles as observed by scanning force microscopy in aqueous and alcoholic mediums”. In: Europhysics Letters 52.5 (2000), pp. 551–556. [63] S. Ohnishi et al. “Characterization of fluorocarbon monolayer surfaces for direct force measurements”. In: Langmuir 16.6 (2000), pp. 2722–2730. [64] S. Ohnishi, V. V. Yaminsky, and H. K. Christenson. “Measurements of the force between fluorocarbon monolayer surfaces in air and water”. In: Langmuir 16.22 (2000), pp. 8360–8367. [65] E. R. Beach et al. “Pull-off force measurements between rough surfaces by atomic force microscopy”. In: Journal of Colloid and Interface Science 247.1 (2002), pp. 84–99. [66] S. Biggs and G. Spinks. “Atomic force microscopy investigation of the adhesion between a single polymer sphere and a flat surface”. In: Journal of Adhesion Science and Technology 12.5 (1998), pp. 461–478. [67] Y. I. Rabinovich et al. “Adhesion between nanoscale rough surfaces - II. Measurement and comparison with theory”. In: Journal of Colloid and Interface Science 232.1 (2000), pp. 17–24. [68] D. M. Schaefer et al. “Surface-Roughness and Its Influence on Particle Adhesion Using Atomic-Force Techniques”. In: Journal of Adhesion Science and Technology 9.8 (1995), pp. 1049–1062. [69] P. Silberzan et al. “Study of the Self-Adhesion Hysteresis of a Siloxane Elastomer Using the Jkr Method”. In: Langmuir 10.7 (1994), pp. 2466–2470. [70] A. K. Dillow and M. Tirrell. “Targeted cellular adhesion at biomaterial interfaces”. In: Current Opinion in Solid State and Materials Science 3.3 (1998), pp. 252–259. [71] E. L. Florin, V. T. Moy, and H. E. Gaub. “Adhesion Forces between Individual Ligand-Receptor Pairs”. In: Science 264.5157 (1994), pp. 415–417. CHAPTER 3. SOFT COLLOIDAL PROBE AFM 115 [72] E. Sackmann. “Supported membranes: Scientific and practical applications”. In: Science 271.5245 (1996), pp. 43–48. [73] F. Carrillo et al. “Nanoindentation of polydimethylsiloxane elastomers: Effect of crosslinking, work of adhesion, and fluid environment on elastic modulus”. In: Journal of Materials Research 20.10 (2005), pp. 2820–2830. 3.A Supporting Information RICM In order to reconstruct the height profile of the object from the RICM images, three different approaches are possible: The simple theory (considers light which enters the sample in direction normal to the plane of the substrate), the finite aperture theory (correction for finite aperture effects) and the non-local theory (correction for finite aperture effects and corrections due to curved interfaces) [49]. To get the intensity distribution over the distance rfrom raw data we take the average of intensity profiles over an angle ϕ, shown in Figure 3.6A. For analysis of the intensity distribution we used the simple theory with correction factors for finite aperture and geometry effects [52]. To obtain the correction factors, we imaged glass beads (Polysciences Inc., Warrington, PA) on a glass surface in RICM mode. We recorded 10 glass beads with a diameter in the range of 30 −50 µmand extracted the intensity profile, see Figure 3.6B. Using the profiles, we reconstructed the shape of the beads and compared it to the calculated shapes (Rby measuring their size with light microscope), and determined the correction factors corresponding to our experimental setup presented in Figure 3.6C. Optical Lever Sensitivity In order to accurately determine the force from cantilever deflection the optical lever sensitivity is required. However, since we attached soft probes on the cantilevers, we cannot use the standard procedure in order to measure the optical lever sensitivity, i.e. force-distance measurements on a hard substrate to obtain the cantilever deflection from the z-piezo displacement. Here we use the spring constant of the cantilever, which was measured before, in combination with the Thermal Noise Method to determine the optical lever sensitivity [53]. CHAPTER 3. SOFT COLLOIDAL PROBE AFM 116 A B C Figure 3.6: Evaluation of the RICM immages: 3.6A RICM image of a glass bead, 3.6B extracted intensity profile, 3.6C reconstruction of bead profile CHAPTER 3. SOFT COLLOIDAL PROBE AFM 117 The determination process is shown in the following flow chart: 1. Detection of spring constant of cantilever (without soft probe) ⇒kc 2. Detection of sensitivity of cantilever with soft probe onto a hard substrate ⇒S0 3. Thermal Noise Method with soft probe on cantilever ⇒k0 c 4. If ⇒k0 c6=kcchange sensitivity and go back to 3. If ⇒k0 c=kcsensitivity of the current setup. In liquid the Thermal Noise Method cannot1be used due to high damping [18]. In this case, we detect the optical lever sensitivity by using the standard procedure, i.e. forcedistance measurement on a hard substrate, but avoiding deformation of the colloidal probe. This can be done by approaching the very tip of the cantilever apex onto a sharp edge, not touching the soft colloidal probe. Any steep lithographic surface can be used as a sharp edge, here we use a cantilever chip (CSC38, Îij-mash, Estonia) glued onto a glass surface. Force spectroscopy mode In addition to the JKR approach shown in the main text, the soft colloidal probe setup also allows to obtain the adhesion from AFM force-distance curves. Form force-distance curves as shown in Figure 3.7 we determine the work of adhesion Wadh (area of the force curve under the baseline [34]). Simultaneously we measure the contact radius a of the adhesion area at maximum load via RICM. The adhesion energy per unit area wcan then be calculated according to dividing Wadh by πa2. For statistics, we detect the force-distance curves in mapping mode, defining a 100 µm2grid on the substrate with at least 64 data points. Afterwards, the force-displacement measurements are done on each point of the grid. For the hydrophilic system in water we found wwater = (0.9±0.1) mJ/m2, and in air wair = (40 ±10) mJ/m2. For the hydrophobic probes in water we obtained wwater = (45 ±10) mJ/m2and wair = (11 ±5) mJ/m2in air. These values agree well with values reported in the literature. The capillary force is a line effect, so in air win units J/m2represents a reference value. 1can just approximately used (included after publication) CHAPTER 3. SOFT COLLOIDAL PROBE AFM 118 Figure 3.7: Analysis of the thermodynamic work of adhesion: Collect force distance curve, evaluating work of adhesion Wadh (gray area) and measuring simultaneously the particle-surface contact area at maximum load by micro interferometry (RICM). 4 Direct Correlation between Local Pressure and Fluorescence Output in Mechanoresponsive Polyelectrolyte Brushes Reproduced by permission of John Wiley and Sons, Angewandte Chemie-International Edition, 2011. 50(41): p. 9629-9632. Copyright c (2011) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Bunsow, J., Erath, J., Biesheuvel, P. M., Fery, A., Huck, W. T. S., Direct Correlation between Local Pressure and Fluorescence Output in Mechanoresponsive Polyelectrolyte Brushes. Angewandte Chemie-International Edition, 2011. 50(41): p. 9629-9632. 119 CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 121 4.1 Introduction In recent years, there has been huge progress in the development of stimuli-responsive polymeric materials [1], and especially mechanoresponsive polymers, which convert mechanical stimuli into optical, electrical or chemical signals are a particularly attractive class of materials [2, 3, 4, 5]. An ultimate goal of such materials would be to emulate the unique responsiveness of human skin, which can detect gentle touches of around 1 kPa at a spatial resolution of about 40 µm[6]. Here, we introduce a new concept for optical force mapping based on mechanoresponsive polyelectrolyte brushes, which in addition to their response to force also respond to changes in the chemical environment [7]. Dense, strong polyelectrolyte brushes are hard to compress due to the increase of the osmotic pressure of the counter-ions and the excluded volume interactions between the individual chains [8, 9]. Thus, they are not mechanically responsive per se, and to generate an optical signal a ”mechanophore” [10] needs to be introduced. Previously, we used a pH-sensitive dye to act as a mechanosensitive building block, where the dissociation constant of the dye was a function of brush compression [7]. However, the dye was only infiltrated into the brush and not a covalent part of it. Furthermore, only qualitative information on the correlation between pressure and optical response was obtained. The key advance in the present work is the quantitative characterization of the mechanoresponsive properties of polyelectrolyte brushes functionalized by covalent attachment of fluorescent dye molecules. We have determined a response function I(p), which correlates local fluorescence intensity (I) to local pressure (p) and we have found an excellent pressure sensitivity in the order of 10 kPa and a lateral resolution better than 1µm. 4.2 Experimental Full experimental details on polymer brush formation and characterization and determination of fluorescent monomer loading in brushes as well as a detailed description of soft colloidal probe experiments and JKR analysis is available in the Supporting Information section 4.A. CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 128 Acknowledgements W.T.S.H. gratefully acknowledges funding from the Friedrich Wilhelm Bessel Award of the Humboldt Foundation. J.B. thanks the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for funding. J.E. and A.F. gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Forschergruppe 608: TP: Fe 600/10-1). We thank Petra Zippelius for helpful assistance in carrying out some of the experiments. 4.5 References [1] M. A. Cohen Stuart et al. “Emerging applications of stimuli-responsive polymer materials”. In: Nature Materials 9.2 (2010), pp. 101–113. [2] J. Buensow, T. S. Kelby, and W. T. S. Huck. “Polymer Brushes: Routes toward Mechanosensitive Surfaces”. In: Accounts of Chemical Research 43.3 (2010), pp. 466–474. [3] M. M. Caruso et al. “Mechanically-Induced Chemical Changes in Polymeric Materials”. In: Chemical Reviews 109.11 (2009), pp. 5755–5798. [4] J. M. Lenhardt et al. “Trapping a Diradical Transition State by Mechanochemical Polymer Extension”. In: Science 329.5995 (2010), pp. 1057–1060. [5] K. M. Wiggins et al. “Mechanical Reconfiguration of Stereoisomers”. In: Journal of the American Chemical Society 132.10 (2010), pp. 3256–+. [6] V. Maheshwari and R. Saraf. “Tactile devices to sense touch on a par with a human finger”. In: Angewandte Chemie-International Edition 47.41 (2008), pp. 7808–7826. [7] O. Azzaroni et al. “Mechanically induced generation of counterions inside surfacegrafted charged macromolecular films: Towards enhanced mechanotransduction in artificial systems”. In: Angewandte Chemie-International Edition 45.44 (2006), pp. 7440–7443. [8] U. Raviv et al. “Lubrication by charged polymers”. In: Nature 425.6954 (2003), pp. 163–165. [9] E. B. Zhulina, O. V. Borisov, and T. M. Birshtein. “Static forces in confined polyelectrolyte layers”. In: Macromolecules 33.9 (2000), pp. 3488–3491. CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 129 [10] M. J. Kryger et al. “Masked Cyanoacrylates Unveiled by Mechanical Force”. In: Journal of the American Chemical Society 132.13 (2010), pp. 4558–+. [11] J. Erath, S. Schmidt, and A. Fery. “Characterization of adhesion phenomena and contact of surfaces by soft colloidal probe AFM”. In: Soft Matter 6.7 (2010), pp. 1432–1437. [12] I. U. Vakarelski et al. “Deformation and adhesion of elastomer microparticles evaluated by AFM”. In: Langmuir 17.16 (2001), pp. 4739–4745. [13] P. Silberzan et al. “Study of the Self-Adhesion Hysteresis of a Siloxane Elastomer Using the Jkr Method”. In: Langmuir 10.7 (1994), pp. 2466–2470. [14] R. F. Chen and J. R. Knutson. “Mechanism of Fluorescence Concentration Quenching of Carboxyfluorescein in Liposomes - Energy-Transfer to Nonfluorescent Dimers”. In: Analytical Biochemistry 172.1 (1988), pp. 61–77. [15] F. Caruso et al. “Fluorescence studies of the binding of anionic derivatives of pyrene and fluorescein to cationic polyelectrolytes in aqueous solution”. In: Macromolecules 31.21 (1998), pp. 7365–7377. [16] P. Attard and J. L. Parker. “Deformation and Adhesion of Elastic Bodies in Contact”. In: Physical Review A 46.12 (1992), pp. 7959–7971. [17] K. L. Johnson, K. Kendall, and A. D. Roberts. “Surface Energy and Contact of Elastic Solids”. In: Proceedings of the Royal Society of London Series a-Mathematical and Physical Sciences 324.1558 (1971), pp. 301–. [18] M. Biesalski, D. Johannsmann, and J. Ruhe. “Electrolyte-induced collapse of a polyelectrolyte brush”. In: Journal of Chemical Physics 120.18 (2004), pp. 8807– 8814. [19] T. S. Kelby and W. T. S. Huck. “Controlled Bending of Microscale Au-Polyelectrolyte Brush Bilayers”. In: Macromolecules 43.12 (2010), pp. 5382–5386. [20] O. Azzaroni et al. “Switching the properties of polyelectrolyte brushes via "Hydrophobic collapse"”. In: Macromolecules 38.24 (2005), pp. 10192–10199. [21] M. Husseman et al. “Controlled synthesis of polymer brushes by "Living" free radical polymerization techniques”. In: Macromolecules 32.5 (1999), pp. 1424– 1431. [22] A. M. Jonas et al. “Thermo-responsive polymer brushes with tunable collapse temperatures in the physiological range”. In: Macromolecules 40.13 (2007), pp. 4403– 4405. CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 130 4.A Supporting Information Experimental Materials METAC (80 wt.%in water), AEMA, HEMA, CuBr, CuCl, CuCl2, CuBr2, 2,2’-bipyridyl (bipy), triethylamine, 5(6)-carboxyfluorescein N-hydroxysuccinimide ester (CF-NHS) and sodium dodecyl sulfate were purchased from Sigma Aldrich and used as received unless otherwise stated. NaCl and NaClO4were from Gruessing (Germany). Prior to polymerisation, METAC and HEMA were activated by passing over a neutral alumina column. Triethylamine was distilled from KOH and stored over molecular sieve (4 Å, Sigma Aldrich). CuBr and CuCl were stored under vacuum. High purity water of a resistance of 18.2 MΩcm was obtained from a Millipore Synergy system. The ATRP initiator 2-bromo-2-methylpropionic acid 3-trichlorosilanlypropyl ester was synthesised according to Ref. [21, 22]. Round cover slips (ø= 24 mm) were purchased from VWR and cleaned by 10 min sonication in ethanol. 5(6)-carboxyfluorescein Nhydroxysuccinimide ester was stored at −20 ◦C. The PDMS precursor kit Sylgard 184 was obtained from Dow Corning (USA). Initiator Immobilisation on Glass Glass cover slips were cleaned in air plasma (Emitech, model K1050X) for 10 min at a power of 100 W.50.4µLtriethylamine were mixed with 30 mL dry toluene and 10 µL of the ATRP initiator were added under shaking. The mixture was transferred to the petri dish containing the glass slides and initiator immobilisation proceeded under gentle shaking (50 rpm) over night. The modified glass slides were rinsed with toluene, ethanol and water, dried with nitrogen and baked in an oven at 150◦Cfor 3 to 4 hours. Synthesis of Polymer Brushes METAC (7 g, 30 mmol), AEMA (0.5 g, 3 mmol), bipy (214 mg, 1.4 mmol) and CuCl2 (3.6 mg, 0.03 mmol) were dissolved in a mixture of water (2.4 mL) and isopropanol (9 mL). The solution was degassed with N2for 30 min. CuBr (80 mg, 0.56 mmol) and initiatormodified glass cover slips were placed in separate Schlenk tubes and degassed by evacuating and flushing with N2in at least three subsequent cycles. CuBr was dissolved in the monomer solution by sonication. The reactant mixture was added to the cover slips and kept at room temperature under N2 for 5 hours. Samples were CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 131 rinsed with ethanol, water, and acetone; and sonicated in water for 10 mins to remove non-covalently attached polymer. Samples were dried with N2and kept under N2until further use. Attachment of Carboxyfluorescein CF-NHS was dissolved in dimethylsulfoxide at a concentration of 1 mg/mL. Brushes on glass slides were immersed in 2 mL 0.1 M aqueous NaHCO3and 200 µlof the dye stock solution were added under shaking. Dye immobilisation was allowed to proceed in the dark over night under shaking (100 rpm). Samples were rinsed with water and sonicated for 10 min in 1 M NaCl and water, respectively, to remove non-covalently attached dye. Synthesis of neutral P(HEMA-co-AEMA) brushes Hydroxyethylmethacrylate (HEMA) was purchased from Sigma Aldrich and activated by passing over neutral alumina. P(HEMA-co-AEMA) brushes were prepared from a solution of HEMA (6 mL, 50 mmol), AEMA (0.9 g, 5 mmol), bipy (163 mg, 2 mmol) and CuBr2 (24 mg, 0.1 mmol) in water (6.5 mL). After degassing with nitrogen for 30 min, the monomer solution was added to degassed CuCl (37 mg, 0.35 mmol) in a Schlenk tube. The reactant mixture was added to the cover slips and kept at room temperature under N2 for 5 hours. Samples were rinsed with ethanol, water, and acetone; and sonicated in water for 10 mins to remove noncovalently attached polymer. Samples were dried with N2and kept under N2until further use. Compression Experiments Compression experiments were carried out with a combination of an AFM (MFP 3D I,Asylum Research, USA) and a CLSM (LSM710, Zeiss, Germany). Using the SCP technique, we compressed the polyelectrolyte brushes in the direction normal to the surface and recorded the optical response via CLSM. Measurements were performed in a 0.1 ml liquid droplet. Samples were stored in water to guarantee full hydration of the brushes. After measurements in 0.1 M NaClO4solution, samples were regenerated by extensive rinsing in 1 M NaCl and water. AFM PDMS beads with diameters of 20 to 40 µmwere prepared according to Ref. [11], transferred to a polystyrene dish by dip-coating, and glued with an epoxy resin (UHU CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 132 schnellfest, Germany) to pre-calibrated cantilevers (728 N/m, NSC 12, tipless, noAl, Micromash, Estonia) using a micromanipulator (MP-285, Shutter Instrument, USA) and an inverted optical microscope (Axiovert 200, Zeiss, Germany). Force constants of the cantilevers were determined by the thermal noise method. Using AFM forcedistance measurements and Hertz analysis (neglecting adhesion and deformation of the brush), the YoungâĂŹs Modulus of the bead was determined to be in the order of 1 MPa. The SCP was washed with water, dried in a stream of nitrogen and mounted into the AFM holder. Prior to every measurement, the optical lever sensitivity (INVOLS) was determined via a noncontact approach with an error for the applied load of about 15 %. The SCP was pressed against the substrate with a defined force using the AFM feedback loop. Data were collected at discrete load-force intervals of 500 nN with a maximum force of 8µN. All data are averaged values of measurements on at least three different lateral positions on the substrate. CLSM The CLSM was equipped with an EC ”Plan-Neofluar” 20x/0.50 M27 objective. Images were acquired with a resolution of (0.104X0.104) µm2at a pinhole size of 1 airy unit (= 4.4µm). Fluorescence was excited at a wavelength of 488 nm and emission was detected from 492 to 625 nm. The focus plane was set to the plane of maximum fluorescence intensity. For P(METAC-co-AEMA) brushes, the master gain (a measure of the power applied to the photodetector) was set to 650 at a laser power of 5%. Neutral brushes showed a higher fluorescence intensity and strong bleaching which made it necessary to reduce the laser power to 0.5%. Images were processed with an ImageJ macro. The radial intensity distribution from the center of the compression zone to the undisturbed background was extracted by averaging the grey value of the images over an angle phi (see below). Characterization of cationic and neutral brushes Growth kinetics of cationic homo-and copolymer brushes Growth kinetics of cationic homo-and copolymer brushes: Figure 4.5 Film thickness and fluorescence intensity of cationic and neutral brushes Correlation of film thickness as a function of salt concentration and fluorescence intensity of cationic and neutral brushes see Figure 4.6. CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 133 Figure 4.5: Growth kinetics of PMETAC (black ) and P(METAC-co-AEMA) (red ) with an AEMA concentration of 10 mol −%in the feed. The dry film thickness d increases linearly with polymerisation time. Growth kinetics of homoand copolymer brushes are very similar. CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 134 A B Figure 4.6: 4.6A Film thickness measured by AFM in liquid of P(METAC-co-AEMA) (black ) and P(HEMA-co-AEMA) (red ◦) brushes in water, NaCl solutions and 0.1 M NaClO4solution after the covalent attachment of carboxyfluorescein. Values on the left side refer to water and NaCl solutions of various concentrations; the value on the right side was measured in 0.1 mol/Laqueous NaClO4. The thickness of cationic brushes depends strongly on the salt concentration whereas neutral brushes show little response to the addition of salt. 4.6B Fluorescence intensity of uncompressed brushes immersed in water and salt solutions. Cationic brushes show a strong decrease of fluorescence intensity with increasing salt concentration; the fluorescence intensity of neutral brushes is much less affected by the addition of salt. CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 135 Apparent dye concentration The apparent dye concentration in the brushes was measured by UV-vis spectroscopy using the following procedure. Calibration was realized by dissolving 0.002−0.08 mmol/L carboxyfluorescein in water. Spectra were acquired in a wavelength regime from 400 −600 nm. The peak height at 480 nm was plotted versus the bulk concentration and the slope of a linear fit to the data was determined to be 8.2. With Beer’s law 4.3 Eλ=−lg I1 I0 =λcd, (4.3) where Eλis the extinction, I1and I0are the intensities of transmitted and irradiated light, respectively, λis the molar absorbtivity, cis the dye concentration, and dis the path length (here: thickness of cuvette, 0.1 dm) it follows that λ= 82 dm2/mmpl. A droplet of water was added to the surface of glass cover slips covered with brushes. Samples were sandwiched between two PDMS slides and mounted in a home-built holder with a hole (Ø≈1 cm) through which the light beam could pass. The maximum intensity was detected at 480 nm. The dye concentration was calculated from capp =Eλ λdbrush ,(4.4) where capp is the apparent dye concentration in the brush and dbrush is the thickness of the brush in water. pKa of carboxyfluorescein attached to charged and neutral brushes The pKa of carboxyfluorescein covalently attached to brushes was measured with the CLSM by detecting the fluorescence intensity of brushes in contact with diluted HCl and NaOH solutions whose total salt concentration was adjusted to 10 mmol/Lby addition of NaCl. Titration curves were fitted with a sigmoidal fit using Microcal Origin 6.0. The inflection point of the fit is the pKa of the dye bound to the brush. (see Figure 4.7) Film thickness and fluorescence intensity of cationic and neutral brushes Correlation of film thickness as a function of salt concentration and fluorescence intensity of cationic and neutral brushes see Figure 4.6. CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 136 A B Figure 4.7: Fluorescence intensity, normalized to maximum value, of two titration experiments on 4.7A cationic P(METAC-co-AEMA) (squares) and 4.7B neutral P(HEMA-co-AEMA) (circles)) brushes as a function of pH. Straight lines represent sigmoidal fits to the curves. The inflection point of the fit curve corresponds to the pKa of the dye in the brush. The average pKa was 1 and 3.8 in charged and neutral brushes, respectively. Bulk Experiments of CF-solutions Variation of NaCl concentration The fluorescence of 0.03 mM CF-solution is not affected by the salt concentration. Dependence of fluorescence on pH The pKa of carboxyfluorescein in bulk solution was determined by fluorescence spectroscopy of a solution of 0.03 mmol/Ldye in diluted HCl and NaOH solutions with an ionic strength 0.01 M NaCl (see Figure 4.8). Self quenching of CF Selfquenching of CF, see Figure 4.9. Quenching of the dye by METAC Quenching of CF by the monomer of the polymer brush (METAC), see Figure 4.10. CHAPTER 4. MECHANORESPONSIVE POLYELECTROLYTE BRUSHES 137 Figure 4.8: Change of pH, CF concentration: 0.03 mM, salt concentration: 0.01 M NaCl; Decrease of fluorescence by decrease of pH, pKa of bulk solution: 5.2. (Lit.: pKa of bulk solution: a: 6.5) Details on force experiments Force-distance curves Exemplary force distance curve of a soft colloidal probe interacting with the brush, see Figure 4.11. Adhesion in water and NaCl solutions Interaction engergy between probe and brush, depending on the solution conditions, see Figure 4.12. Hysteresis Adhesion hysteresis between probe and brush, see Figure 4.13 Correlation of intensity and pressure profile Details on JKR theory [17] The system PDMS bead pressed against a substrate functionalized with a P(METACco-AEMA) brush can be described by contact-mechanic theory. H. Hertz presented the first analytic description of two isotropic, homogeneous linear elastic bodies in contact, but without adhesion. If one takes adhesion into account there are several models which apply for certain conditions of contact behavior. The two most common theories were developed by Johnson, Kendall and Roberts (JKR theory) and Derjaguin, Muller and CHAPTER 8. FURTHER PERSPECTIVES 240 The sensitivity of the response and the force range can possibly be extended by combining other mechanophores with alternative brush compositions and architectures [23]. For instance annealed polyelectrolyte brushes allow externall triggering of the mechanochemical response by pH or ionic strength, or vice versa. That means that vertical compression of the brush could lead to a shift of the dissociation equilibrium. This variation could be measured using pH-sensitive indicators for example. An alternative could be to use polyelectrolyte gel films rather than polymer brushes. Polymer gels systems could allow alternative routes for surface functionalization and the usage of other mechanophores. They are flexible in handling and steric hindrance for the mechanophore could be overcome. Also, measurements in air could be possible. For the understanding of the processes in strongly compressed PMETAC brushes, we combined AFM force spectroscopy with polymer brush theory. First experiments are underway to study the effect of brush parameters on the compressibility. These experiments are evaluated on basis of AdG theory (developed by Alexander and de Gennes [29, 30]). Also, the relation between response and compressibility, i.e. the state of quenching as a function of applied force is investigated. Further other types of response will be addressed. A more specific response than quenching is possible by using dyes, which show a shift in the emission spectrum (rather than a shift in intensity) in response to changes in the chemical environment. For instance seminaphtharhodafluor (SNARF), which responds to changes of the dissociation equilibrium. For this purpose, we have established a synthesis protocol for the attachment of SNARF dye to PMETAC polymer brushes. In order to quantify the response of SNARF dye, the detection scheme was changed from fluorescence intensity mapping to mapping of spectroscopic properties. 8.3.1 Understanding of the Mechanoresponse For the design of mechanoresponsive systems based on polymer brushes we have to understand how the brush is affected by the environment and the applied force, and how the response is related to the compression state of the brush. Brush Compresssion We performed (first) force spectroscopic experiments to gain information about the compressibility of cationic PMETAC polymer brushes with a covalently attached 5(6)carboxyfluorescein (CF) [1] dependent on the ionic strength. We used the colloidal probe CHAPTER 8. FURTHER PERSPECTIVES 241 (CP) technique, where a CP was attached to an AFM cantilever using micromanipulation. In a further step the CP was functionalized with a 3-Aminopropyl di isopropyl ethoxysilane (97%) to avoid electrostatic attraction between the CP and the cationic brush. After silanization force distance data were recorded in a droplet of aqueous NaCl solution. As expected, we could observe that the repulsive forces increase (strength and range) as the ionic strength Iis decreased. This can be attributed to multiple origins. On the one hand the electrostatic interactions are screened. In addition the brush swells and steric interactions are more dominant. In a first approximation, we analyzed the measurements in terms of the AdG theory for an asymmetric situation (brush/ no brush; electro static interactions are neglected). F(D) 2πR =2kBTH 35σ3/2"7D H−5/4 +D H7/4 −12#for D < H, (8.1) As it can be seen in Figure 8.7 the AdG model fits well to the recorded data (For the Figure 8.7: Force measurements at PMETAC brushes: Force profiles for NaCl salt concentrations of 0.1 M and 0.01 M Measured data are firtted with the AdG model. fitting procedure we included a displacement parameter δbecause we could not reach the constant compliance regime: D=D0−δ). The resulting brush parameters are physically in the right order. The brush height was 150 ±30 nm, and the separation distance of grafting points σ1/2= 15 ±3 nm for a salt concentration of 0.1 M. For a concentration of 0.01 M the brush height was 300 ±50 nm, and a separation distance of grafting points σ1/2= 19 ±4 nm. The obtained values for the brush height are slightly to large compared to values detected using ellipsometry [1]. However this can CHAPTER 8. FURTHER PERSPECTIVES 242 attributed to the electrostatic repulsion of the sample and the probe. To analyze the effect of swelling, we normalized the force profiles as suggested by S. Block [31]. The AdG model predicts that the steric interactions scale linear with the brush height. The brush height again scales with the salt concentration as H∝I−1/3. Obviously, the force profiles should show the same behavior if they are plotted over DI1/3, where Dis the separation distance of the probe and the brush. Normalization of the resulting force profiles by I1/3should lead to a collapse of the curves for different salt concentrations onto a master curve. Following this procedure the effect of swelling and the resulting steric interactions could be confirmed as pointed out in Figure 8.8 (The same procedure can be applied for mean field (MF) theories (and others). For the MF approach the force profiles must be normalized by I2/3. However, rescaling is just sucessful if the force profiles are shifted). Figure 8.8: Normalization of the force profiles using the AdG model: Force profiles collapse onto a master curve if they are normalized as predicted by the AdG model. Inset shows rescaling using a mean field model. Rescaling in that case is just sucessfull if the force profiles are shifted Interpretation of the Response Function In our previous study on mechanoresponsive polymer brushes we analyzed the mechanoresponse in the contact zone of a soft colloidal probe that is pressed against the PMETAC polymer brush layer. In our case, we could observe fluorescence quenching as response to pressure. From these measurements we could determine a response function I(p), which correlates local fluorescence intensity (I) to local pressure (p). The pressure is calculated with use of the JKR theory. To relate the mechanoresponse, i.e. state CHAPTER 8. FURTHER PERSPECTIVES 243 of quenching to the applied force and the compressibility of the polymer brush, we can assume that the applied pressure pin the contact zone of the colloidal probe is proportional to the osmotic pressure inside the polymer brush p(D) = posm =kBT σ3/2"D L−9/4 −D L3/4#.(8.2) Intermolecular deactivation processes, such as quenching, can be described using the Stern-Volmer equation [32]. I0 IQ = 1 + KcQ,(8.3) where I0is the intensity of fluorescence without quenching, IQis the intensity with the quencher of the concentration cQand Kis the quencher rate. We could show in our previous study [1] that the fluorescence intensity of 5(6)carboxyfluorescein (CF) is mainly quenched by METAC molecules (KcMETAC >> 1). Thus the intensity of fluorescein with METAC can be expressed by IMetac =I0 1 + KcMetac ≈I0 KcMetac =I0σ KNMetac D⇒I∝D. (8.4) This assumption allows to interpret the detected ”inverse” response function p(I)directly as force-distance profiles. For a confirmation of this hypothesis we fitted the recorded response functions with Eq.8.2 where Dis replaced by I. The shown curves Figure 8.9: Interpretation of the Response Function: Response functions for H2O, 0.1 M NaCl and 1 M NaCl solution: datapoints (reproduced from Ref. [1]) fitted with AdG profiles (solid lines). CHAPTER 8. FURTHER PERSPECTIVES 244 (Figure 8.9) demonstrate that the measured data points are located on the falling edge of the AdG profiles. However, according to the fit the grafting density decreases with decreasing salt concentration. Further experimnts are necessary to varify these presumptions. Conclusion and Outlook In summary, we investigated the steric interactions of mechanoresponsive polymer brush systems. We could show that the AdG model can describe the steric interactions in the brush layers. Further experiments are necessary to confirm the observed behavior. In future, we will perform force spectroscopic measurements on patterned substrate (areas with and areas without brush) to achieve an internal reference of the brush height. Also, this will help to separate long and short range interactions. The effect of brush parameters on force profiles should be studied as well. For this purpose the brush height can be varied by a change of the polymerization time. The grafting density can be varied using different silanes as initiators or blocking the initiator (grafting sites) using suitable blocking molecules (Also other variations of the synthesis protocol are possible, e.g. type ligand, solvent). This will help to understand the influence of the brush parameters on compressibility and the mechanoresponse. Further force experiments and rescaling evaluation can help to prove experimentally, the validity and limitation of the existing theories on polymer brushes. 8.3.2 Change of the Detection Scheme A more specific detection scheme than fluorescence quenching will help to understand how mechanical energy converts into changes of the chemical environment inside the brush. Here we introduce an alternative to fluorescence quenching using a pH sensitive fluorophore, i.e. seminaphtharhodafluor (SNARF). The physical properties of this type of dye are triggered by the dissociation equilibrium inside the brush. Variation of this equilibrium results in changes of the emission spectra. Dye Attachment The target was to bind SNARF molecules to the primary amine of the polymer PMETAC brushes, like shown in Figure 8.10. For this purpose, we used NHS-SNARF so that the succinimidyl ester reacts with the primary amine forming a peptide bond. After synthesis of the copolymer PMETAC brush (on glass slides) as established in Ref. [1], a functionalized glass slide was sonicated for 10 min in 1 M aqueous NaCl solution, CHAPTER 8. FURTHER PERSPECTIVES 245 washed with water and dried in a N2stream. The NHS-SNARF (Invitrogen # S22801) was treated with 10 µldimethylsulfoxid (DMSO) and incubated at room temperature for ca. 5 min. The dye solution was mixed with water to get a final dye concentration of 0.281 mM, and was filled on the bottom of a centrifuge cap. A glass slide (with the copolymer PMETAC polymer brush) was placed in the cap, with the polymer film towards the dye solution. After reaction time over night at room temperature the glass slide was rinsed with water and sonicated 2 times, each ca. 5 min, in 1 M aqueous NaCl solution. Finally, the slide was washed with water and dried in a N2-stream. Figure 8.10: Attachement of SNARF to polymer brushes Characterization For the characterization of the dye attachment and of the response of the polymer brushes with covalent attached SNARF dye we performed fluorescence microscopy and fluorescence spectroscopy. The functionalized glass slides were placed on a fluorescence microscope and images were take in a range of pH 4 to pH 9 (pH solutions with different buffers listed in Tab. 8.1). A color change is clearly visible (Figure 8.11). This can be related to a shift in the emission spectra of the substrates. To quantify this alteration we analyzed the ratio of the intensity from the red channel Ir(red fluorescent emission) of the fluorescent microscope and Iythe intensity of the yellow channel, following Ref. [33, 34] (Figure 8.12). From these calibration curves we could determine pKa of the covalently attached SNARF [33, 34]. The mean value of three slides could be determined to pKa = 9 ±1. Furthermore, we performed fluorescence spectroscopy. For this purpose the functionalized glass slides were analyzed using a confocal microscope in spectroscopy mode. Here, for each wavelength interval (3 nm) an intensity image was detected to create CHAPTER 8. FURTHER PERSPECTIVES 246 Table 8.1: Buffers that were used to study the response of PMETAC-SNARF brushes on pH. pH composition 4 citric acid / NaCl / NaOH 5 citric acid / NaOH 6 citric acid / NaOH 7 KH2PO4/ Na2HPO4 8 Na2B4O7/ HCl 9 Na2B4O7/ KCl 10 Na2B4O7/ NaOH 11 Na2B4O7/ NaOH / KCl 12 Na2HPO4/ NaOH A B C D E F G H I Figure 8.11: Fluorescence Microscope Images of SNARF functionalized brushes: pH4 (8.11A) - pH12 (8.11I) CHAPTER 8. FURTHER PERSPECTIVES 247 Figure 8.12: Calibration curve of the SNARF functionalized brushes spectral profiles of the dye. For two pH values a clear change of the emission spectra could be identified (Figure 8.13). Figure 8.13: Emission spectra of SNARF functionalized brushes at pH7 and pH10 Conclusion and Outlook In summary, we have established a protocol to attach the pH-sensitive dye seminaphtharhodafluor covalently to PMETAC polymer brushes. The obtained substrates were characterized with respect to functionalization and the resulting fluorescence properties as a function of pH. Further studies will verify these results. Follow up projects are planned to study the chemical environment inside polyelectrolyte brushes, with and CHAPTER 8. FURTHER PERSPECTIVES 248 without compression, by analysis of fluorescence properties of the brush depending on the conformation of the polymer brush. A quantitative evaluation on the effect of these chemical modifications on response, in particular pressure sensitivity could be carried out in these experiments. In that context, annealed PE polymer brushes and polyelectrolyte gels could be tested as well. Beside the question about compressibility and potential responses such an approach can help to ”look” inside a brush and, thereby, allows proving the existing theories on polymer brushes. 8.4 References [1] J. Buensow et al. “Direct Correlation between Local Pressure and Fluorescence Output in Mechanoresponsive Polyelectrolyte Brushes”. In: Angewandte Chemie-International Edition 50.41 (2011), pp. 9629–9632. [2] M. K. Chaudhury and G. M. Whitesides. “How to Make Water Run Uphill”. In: Science 256.5063 (1992), pp. 1539–1541. [3] M. K. Chaudhury and P. S. Goohpattader. “Noise-activated dissociation of soft elastic contacts”. In: European Physical Journal E 35.12 (2012). [4] A. Ghatak et al. “Peeling from a biomimetically patterned thin elastic film”. In: Proceedings of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences 460.2049 (2004), pp. 2725–2735. [5] B. D. Grashoff C.and Hoffman et al. “Measuring mechanical tension across vinculin reveals regulation of focal adhesion dynamics”. In: Nature 466.7303 (2010), 263–U143. [6] E. Arzt, S. Gorb, and R. Spolenak. “From micro to nano contacts in biological attachment devices”. In: Proceedings of the National Academy of Sciences of the United States of America 100.19 (2003), pp. 10603–10606. [7] A. del Campo et al. “Patterned surfaces with pillars with controlled 3D tip geometry mimicking bioattachment devices”. In: Advanced Materials 19.15 (2007), pp. 1973–+. [8] A. del Campo, C. Greiner, and E. Arzt. “Contact shape controls adhesion of bioinspired fibrillar surfaces”. In: Langmuir 23.20 (2007), pp. 10235–10243. CHAPTER 8. FURTHER PERSPECTIVES 249 [9] L. F. Boesel et al. “Gecko-Inspired Surfaces: A Path to Strong and Reversible Dry Adhesives”. In: Advanced Materials 22.19 (2010), pp. 2125–2137. [10] D.-M. Drotlef et al. “Insights into the Adhesive Mechanisms of Tree Frogs using Artifi cial Mimics”. In: Advanced Functional Materials (2012). [11] C. Greiner, A. del Campo, and E. Arzt. “Adhesion of bioinspired micropatterned surfaces: Effects of pillar radius, aspect ratio, and preload”. In: Langmuir 23.7 (2007), pp. 3495–3502. [12] J. Cui, V. San Miguel, and A. del Campo. “Light-Triggered Multifunctionality at Surfaces Mediated by Photolabile Protecting Groups”. In: Macromolecular Rapid Communications 34.4 (2012), pp. 310–329. [13] C. Y. Hui et al. “Design of biomimetic fibrillar interfaces: 2. Mechanics of enhanced adhesion”. In: Journal of the Royal Society Interface 1.1 (2004), pp. 35– 48. [14] Y. Menguec et al. “Gecko-Inspired Controllable Adhesive Structures Applied to Micromanipulation”. In: Advanced Functional Materials 22.6 (2012), pp. 1246– 1254. [15] M. P. Murphy, B. Aksak, and M. Sitti. “Gecko-inspired Directional and Controllable Adhesion”. In: Small 5.2 (2009), pp. 170–175. [16] D. Paretkar et al. “Bioinspired pressure actuated adhesive system”. In: Materials Science andja Engineering C-Materials for Biological Applications 31.6 (2011), pp. 1152–1159. [17] J. Yu et al. “Gecko-Inspired Dry Adhesive for Robotic Applications”. In: Advanced Functional Materials 21.16 (2011), pp. 3010–3018. [18] G. Carbone and E. Pierro. “Sticky Bio-inspired Micropillars: Finding the Best Shape”. In: Small 8.9 (2012), pp. 1449–1454. [19] G. Carbone, E. Pierro, and S. N. Gorb. “Origin of the superior adhesive performance of mushroom-shaped microstructured surfaces”. In: Soft Matter 7.12 (2011), pp. 5545–5552. [20] M. Schargott, V. L. Popov, and S. Gorb. “Spring model of biological attachment pads”. In: Journal of Theoretical Biology 243.1 (2006), pp. 48–53. [21] R. Spolenak, S. Gorb, and E. Arzt. “Adhesion design maps for bio-inspired attachment systems”. In: Acta Biomaterialia 1.1 (2005), pp. 5–13. CHAPTER 9. SUMMARY 256 surfaces has been investigated. The observations are correlated to force-spectroscopic measurements. For this purpose micron sized model particles were prepared which could be used as probes in a colloidal probe AFM setup. These probes were used to measure the interactions between these ”micro SPBs” and positively or negatively charged surfaces. We showed that the adhesive properties of the SPBs can be controlled by the ionic strength of the surrounding solution and the substrate charge. At low ionic strength, the negatively charged SPBs preferably adsorb onto the positively charged substrates. Above a critical ionic strength, this selectivity is lost. Due to secondary interactions the particles adsorb in these condition on both systems equally. The observations have been used to build up hierarchical structures from SPBs, which are adsorbed from a suspension. The hierarchy can be controlled by the micro-contact printing technique. Such systems can be exemplarily used for optical applications, if a metal core is employed. Furthermore, highly active surfaces have been prepared on basis of polyelectrolytecopolymer micelles. For this, a triblock terpolymer is used (BMAADq: polybutadiene (B), poly(methacrylic acid) (MAA), and quaternized poly(2-(dimethylamino)ethyl methacrylate) (Dq)) which forms micelles in solution composed of a hydrophobic core, a middle block of an annealed polymer brush, and a positively charged corona. These micelles have been disposed in multilayers using the layer-by-layer method with a negative polymer. The advantage of the micelles used in this approach is that the central block is shielded by the other two blocks from complexation with the multilayer. Because of the central block, the micelles and thus the multilayers are strongly dependent on the pH of the environment. At a low pH, the middle block contracts, whereas it is highly swollen at high pH. In this thesis we investigate the swelling behavior of the multilayer-system on the number of deposition steps and as a function of the pH value. Further, we correlate the swelling behavior with porosity and mechanical properties of the multilayers. Due to their high sensitivity, these responsive layers are suitable for the design of actuators. In summary, this work bridges from fundamental polymer chemistry (chemistry of polymer brushes) over physics (contact mechanics, adhesion, and polymer physics) to potential applications (sensors and actuators). Scientific input has been provided for the research on polymer brushes, mechanoresponsive systems, force sensing, and active materials, as well as contact mechanics, colloidal arrangement, and (bio) adhesion. 10 Zusammenfassung 257 CHAPTER 10. ZUSAMMENFASSUNG 259 In dieser Arbeit werden sogenannte ”geladene Polymer Bürsten” untersucht. Dabei liegt der Fokus auf der Untersuchung und dem Verständnis der schaltbaren Eigenschaften dieser Polymer Oberflächen. Dazu werden sowohl neue Methoden aus Kombination von Kraft Spektroskopie und optischer Mikroskopie eingeführt, sowie bereits etablierte physikalisch-chemische Techniken verwendet. Polymer Bürsten bilden sich aus, wenn viele Polymerketten in einem Mindestabstand an einer Substrat Oberfläche angebunden werden, so dass deren Wechselwirkungspotentiale überlappen. Dadurch richten sich die Polymere aus und bilden einen responsiven Film. Das Anbinden der Polymerketten erfolgt in dieser Arbeit mit der sogenannten ”Grafting from” Methode, bei der eine Polymerisation direkt von der Oberfläche ausgeführt wird. Die Synthese der Polymer Bürsten ist sehr flexibel im Hinblick auf ihre molekulare Architektur, da jede Art von Polymerisation anwendbar ist, solange der benötigte Initiator an die Oberfläche gebunden werden kann. Des Weiteren hängt die Konformation und damit die physikalischen Eigenschaften der Polymer Bürsten, stark von deren Umgebung ab, insbesondere bei geladenen Polymer Bürsten. Außerdem ist es möglich funktionelle Gruppen an die Polymere anzubinden, so dass diese durch externe Stimuli geschaltet werden können. Damit erweisen sich geladene Polymer Bürsten als exzellente Bausteine für schaltbare Oberflächen. Schalten lassen sich Oberflächeneigenschaften, wie zum Beispiel Adhäsion, Ladung, Reibung, Mechanik, optische Eigenschaften, Porosität oder Biokompatibilität. Dieses Schalten kann durch Veränderung der Umgebung wie Ionenstärke, pH-Wert oder Temperatur und durch externe Stimuli wie elektrische oder magnetische Felder, Druck oder Licht ausgelöst werden. Damit eignen sich solche Systeme ideal, um Sensoren auf Umgebungseigenschaften oder externe Stimuli bzw. Aktuatoren aufzubauen. Diese lassen sich zum Beispiel in der Stammzellenforschung einsetzen um Zellen zu stimulieren und deren Wachstum und Eigenschaften zu kontrollieren. Für solche Anwendungen ist ein fundamentales Verständnis der Polymer Bürsten, deren Schalteigenschaften und den damit verbundenen Oberflächeneigenschaften notwendig. Im ersten Teil dieser Arbeit werden neue Messmethoden für die Untersuchung von schaltbaren Systemen vorgestellt. Desweiteren werden geladene Polymer Bürsten für den Aufbau von Sensoren und für die gezielte Einstellung von Oberflächeneigenschaften verwendet. CHAPTER 10. ZUSAMMENFASSUNG 260 Es ist die kolloidale Kraft-Spektroskopie mit weichen Partikeln (engl. Soft colloidal probe: SCP) entwickelt worden. Diese Methode kombiniert die Vorteile eines JKRMessgeräts (große Kontaktfläche) und der AFM Kraftspektroskopie (hohe Kraftauflösung). Durch ein am vorderen Ende einer AFM-Blattfeder (Cantilever) angebrachtes weiches Partikel aus Polydimethylsiloxan (PDMS) wird ein definierter mikroskopischer Kontakt erzeugt. Dazu wird das Partikel mit kontrollierter Kraft gegen eine Oberfläche gefahren, wodurch sich eine Kontaktfläche ausbildet. Diese kann mittels Mikrointerferometrie beobachtet werden. Mit Hilfe von Kontakt-Mechanik Modellen lassen sich so aus den gemessenen Größen verschiedene Parameter wie etwa die Adhäsionsenergie pro Einheitsfläche und Spannungsverteilung in der Kontaktzone bestimmen. Diese Methode ist an verschiedenen Modellsystemen mit bekannter Oberflächenchemie zur Bestimmung der Adhäsionsenergie pro Einheitsfläche getestet worden. Dabei kann klar zwischen kapillaren, hydrophilen und hydrophoben Wechselwirkungen unterschieden werden. Des Weiteren ermöglicht die SCP Methode durch ihre hohe Sensitivität die Untersuchung eines breiten Spektrums an Systemen und die Aufklärung wissenschaftlicher Fragen. Kenntnisse über die Spannungsverteilung innerhalb eines Kontakts sind fundamental für das Verständnis von Kontaktphänomenen, insbesondere auf der kolloidalen Skala. Auf der Basis von mechanoresponsiven Bürstensystemen ist ein Sensoren entwickelt worden, der seine optischen Eigenschaften als Funktion der angelegten Kraft ändert. Dies ermöglicht Spannungsverteilungen mit hoher lateraler Auflösung (<1µm) und hoher Druckempfindlichkeit (<10 kPa) zu detektieren. Dieser Sensor besteht aus kationischen, fluoreszenzmarkierten Polyelektrolyt Bürsten (Poly [2 - (Methacryloyloxy) ethyl] trimethyl-ammoniumchlorid (PMETAC) Bürsten, markiert mit einem kovalent angebundenen Fluoreszenzfarbstoff, 5 (6) Carboxyfluorescein (CF)). Die Polymer Bürsten ändern ihre Fluoreszenzeigenschaften abhängig von ihrem Kompressionszustand. Dieses System ist mit der SCP Methode untersucht und kalibriert worden. Dazu wir das weiche Partikel mit der Polymer Bürste in Kontakt gebracht. Unter Kompression bildet sich eine Kontaktfläche aus. Die optischen Signale aus der Kontaktfläche, Fluoreszenzänderungen, können mit einem Konfokalen Mikroskop mit hoher Auflösung detektiert werden. Dabei kann in der Kontaktfläche eine lokal vom Ort abhängige Fluoreszenzintensitäts-Verteilung beobachtet werden. Um den Verlauf dieser Verteilung zu verstehen, ist die Kontaktzone mit einem Kontaktmechanik Modell, entwickelt von Johnson, Kendall und Roberts, beschrieben worden. Dabei zeigt sich, dass unter Kompression die Fluoreszenzintensität kleiner wird. An- CHAPTER 10. ZUSAMMENFASSUNG 261 dererseits nimmt die Fluoreszenzintensität zu, wenn man an den Bürsten zieht. Mit diesen Überlegungen ist es möglich eine Antwortfunktion zu bestimmen, die der lokalen Fluoreszenzintensität einen lokalen Druck zuordnet. Physikalisch/Chemisch kann die Fluoreszenzänderungen einem ”Quenching Mechanismus” zugeordnet werden. Außerdem ist das Signal reversibel und stabil. All diese Eigenschaften machen ein solches mechanoresponsives System zu einer einzigartigen Basis für den Aufbau von Druck Sensoren. Darüber hinaus werden drei Perspektiven der mechanoresponsiven Polymer Bürsten diskutiert: 1) Analytische Beschreibung des Zusammenhangs von Kompression und Änderung des Fluoreszenzsignals; 2) Alternative Detektionsmechanismen wie Änderung des Emissionsspektrums als Funktion der Bürstenkompression durch Modifikation des Mechanophors und 3) Anwendungsmöglichkeiten auf die Untersuchung von biomimetischen Systemen, wie zur Aufklärung der bemerkenswerten Hafteigenschaften von Geckofüßen. Licht kann nicht nur als Antwortfunktion verwendet werden, sondern auch als Trigger. Es ist ein System von Polymer Bürsten etabliert worden, das aufgrund der Einstrahlung von Licht seine chemische Struktur ändert. Diese Polymer Bürsten (PNVOCMA) bestehen aus einem Methacrylat Rückgrat mit ionisierbarem -COOH Seitengruppen, die durch Bestrahlung des photosensitiven 6-nitroveratryloxy Carbonyl (NVOC) frei werden und eine Poly (Methacrylsäure) (PMAA) Bürste ausbilden. Dadurch ändert sich die Oberfläche von neutral zu negativ geladen und wird hydrophil. Mit diesen photoresponsiven Systemen ist es möglich Oberflächeneigenschaften nicht nur zwischen ihren Extremwerten hin und her zu schalten, sondern diese durch Anpassung der Belichtungszeit und Intensität definiert einzustellen und damit für die gewünschte Anwendung zu optimieren. In dieser Arbeit wird gezeigt, wie sich die Belichtungszeit auf den Umwandlungszustand der Polymer Bürste auswirkt und wiederum was für einen Einfluss das auf Benetzbarkeit, Adhäsion und Reibung hat. Mit Zunahme der Belichtungszeit wird die Wasseraufnahme der Bürste größer. Außerdem verändern sich mit der Photokonversion auch die Oberflächenkräfte. Mit Zunahme der Bestrahlungsdauer werden Reichweite und Stärke von repulsiven Wechselwirkungen zwischen der Polymer Bürste und einer Cantileverspitze aus Siliziumdioxid größer und die Hafteigenschaften nehmen ab. Andererseits wird die Reibung zwischen der Sonde und dem Substrat mit Zunahme der Belichtungszeit größer, was durch Substrateffekte erklärt werden kann. Diese photoresponsiven Systeme können zum Beispiel dazu verwendet werden, um Wasserkondensa- CHAPTER 10. ZUSAMMENFASSUNG 262 tion und die Bewegung von Flüssigkeiten zu manipulieren. Dies kann in ”Wasserernte-” oder Mikrofluidik-Systemen eingesetzt werden. Eine weitere Anwendung ist das gezielte Ankoppeln oder Abtrennen von chemischen Verbindungen oder biologischen Objekten. Dies kann beispielsweise in ”Lab-on-Chip”-Geräten verwendet werden. Alternativ zu den gezeigten Ansätzen von Polymer Bürsten auf flachen Substraten lassen sich responsive Systeme auch aus kolloidalen Bausteinen aufbauen. Der zweite Teil dieser Arbeit befasst sich mit dem Aufbau von Schaltbaren Systemen aus solchen Bausteinen. Dies bringt einige Vorteile mit sich: Die Handhabung von kolloidalen Bausteinen ist in der Regel einfach, bzw. lässt sich die Anordnung dieser durch gezielte Selbstorganisation gut kontrollieren. Außerdem können die kolloidalen Bausteine diverse Funktionalitäten besitzen und durch den kolloidalen Charakter ganz neue Eigenschaften generieren, beispielsweise die Lokalisierung von Licht oder kollektive optische Prozesse. Voraussetzung für solche Anwendungen ist das Verständnis der Wechselwirkungseigenschaften mit dem Substrat und des Verhaltens solcher Bausteine abhängig von ihrer Umgebung. In dieser Arbeit ist das Adsorptionsverhalten von geladenen sphärische Polymer Bürsten (SPBs) bestehend aus einem Polystyrene (PS) Kern und angebundenen Polystyrene Sulfonate (PSS) Ketten) auf geladenen Oberflächen untersucht und dieses mit kraftspektroskopischen Messungen korreliert worden. Dazu sind Modell-Partikel hergestellt worden, die als kolloidale Sonden an einem AFM-Cantilever verwendet werden können. Mit diesen Sonden und Messung der Wechselwirkungen zwischen diesen ”Mikro-SPBs” und positiv bzw. negativ geladenen Oberflächen wird gezeigt, dass die Hafteigenschaften der SPBs durch Ionenstärke und Ladung des Substrats gesteuert werden können. Bei geringer Ionenstärke adsorbieren die negativ geladenen SPBs vorzugsweise auf den positiv geladenen Substraten. Ab einer kritischen Ionenstärke geht diese Selektivität aber verloren und die Partikel adsorbieren aufgrund sekundärer Wechselwirkungen auf beiden Systemen gleich gut. Dieses Verständnis wird weiter dazu genutzt, um hierarchische Strukturen aus SPBs aufzubauen, die aus einer Suspension adsorbiert werden. Die Strukturierung kann hierbei durch die Mikrokontaktdruck-Technik gesteuert werden. Verwendet man metallische Kerne (anstatt der bei unserem Modellsystem verwendeten PS Kerne) lassen sich für optische Anwendungen interessante Strukturen aufbauen. CHAPTER 10. ZUSAMMENFASSUNG 263 Weiterhin sind aktive Oberflächen durch Polyelektrolyt CopolymerMizellen hergestellt worden. Dazu werden Triblockterpolymere verwendet (BMAADq: Polybutadienblock (B), einem pH-sensitiven Polymethacrylsäure-Mittelblock (MAA) und einem permanent geladenen Block aus quarternisiertem Poly(2-dimethylamino)ethylmethacrylat) (Dq)), welche in Lösung Mizellen mit einem hydrophobischen Kern, einen Mittel-Block aus einer schwachen Polymer Bürste und einer positiv geladenen Korona ausbilden. Diese Mizellen werden mit der ”Layer-by-Layer-Methode” mit einem negativ geladenem Partner-Polymer in Multilagen angeordnet. Durch den Mittelblock sind die Mizellen, und damit auch die Multilagen stark von dem pH Wert der Umgebung abhängig. Bei einem niedrigen pH Wert zieht sich der Mittelblock zusammen, wogegen er bei einem hohen pH Wert stark gequollen ist. Der Vorteil der hier verwendeten Mizellen ist, dass der Mittelblock durch die anderen beiden Blöcke von der Komplexierung in der Multilage abgeschirmt ist. In dieser Arbeit haben wir das Quellverhalten dieser Systeme in den Multilagen untersucht und dieses abhängig von Anzahl der Multilagen und pH-Wert quantifiziert. In einem weiteren Schritt haben wir dieses Verhalten mit Porosität und mechanischen Eigenschaften der Multilagen korreliert. Durch ihre hohe Sensitivität eignen sich diese responsiven Lagen sehr gut als Aktuator-Materialien. Zusammenfassend wurde in dieser Arbeit eine Brücke von Polymer Chemie (Chemie der Polymerbürsten) über die Physik (Kontaktmechanik, Adhäsion und Polymerphysik) zur potentiellen Anwendung (Sensoren und Aktuatoren) geschlagen. Dabei wurden wissenschaftliche Beiträge zur Forschung an Polymer Bürsten, mechanoresponsiven und aktiven Materialien, Krafterkennung und Kontaktmechanik, kolloidaler Anordnung und (Bio-) Haftung geleistet. A Theory of Polymer Brushes 265 Danke! Wie sagt man nach so vielen Jahren am besten allen Danke, die zum Gelingen eines Projekts, wie der vorliegenden Arbeit beigetragen haben? Um nicht den ein oder anderen zu vergessen möchte ich mich erst einmal grundsätzlich bei ALLEN bedanken mit denen ich zusammen arbeiten durfte. So nun der Reihe nach: Andreas Fery, Danke für die super Betreuung, die vielen interessanten Diskussionen, die Freiheit mich entfalten zu können und auch dafür, das ein oder andere Projekt mit anstoßen zu dürfen. Es hat mir ehrlich immer viel Spaß gemacht mit Dir zu arbeiten. Meistens hat es sich daher gar nicht wie Arbeit angefühlt. Außerdem Danke, dass ich so viele Konferenzen besuchen und dadurch die Welt besser kennenlernen durfte. Sybille Zimmermann, du weißt, ohne Dich wäre der Lehrstuhl nicht so wie er ist. Du hältst deine ”Familie” echt super zusammen. Danke für alles was du für mich erledigt hast. Johanna Bünsow, dir möchte ich Danke sagen für Deine Einführung in die Welt der Polymer-Bürsten, Deine Geduld, wenn ich mal etwas nicht gleich verstanden habe und natürlich das Korrekturlesen dieser Arbeit. Ich freue mich einfach, dass wir auf dem Weg der Wissenschaft Freunde geworden sind, juhu! An dieser Stelle möchte ich auch Wilhelm Huck danken, der viel zu meinem PolymerBürsten Verständnis beigetragen hat und für die gute Kooperation. Arancha del Campo, Danke dass ich bei Euch am MPI für Polymer-Forschung in Mainz immer willkommen war, die nette Atmosphäre und die vielen interessanten Diskussionen. Außerdem natürlich für die gute Zusammenarbeit an unseren Projekten. Hier auch Danke an Dirk, Michael und Jiaxi für die tolle Zusammenarbeit. 273 DANKE 274 Danke an Stephan Block, für tolle Disussionen und den regen E-Mail Kontakt. Ein besonderer Dank gilt all meinen Kollegen aus der Physikalische Chemie II. Ich hab hier bei euch immer eine super Arbeitsatmosphäre und Hilfsbereitschaft vorgefunden und fand es immer toll, mein Wissen so gut wie möglich mit Euch zu teilen. Ich denke, die Kombination Chemie/ Physik war nicht schlecht, und jeder hatte etwas davon. Ich hab jedenfalls viel von Euch gelernt! Besonders möchte ich mich bei Daniel bedanken, für die vielen Momente, die wir zusammen hatten, Melani für die vielen interessanten Diskussionen und Hilfestellungen bei verschiedensten Fragen, Ben dafür, dass Du so bist, wie Du bist und für das Korrekturlesen von Abschnitten dieser Arbeit, Chris dafür, dass Du immer positiv warst und vor Ideen sprühst, Christoph für die tolle Zusammenarbeit und die Unternehmungen die wir zusammen hatten, Bernhard für die Einführung in die Kletterei, Martin für Deine Gesellschaft, Inna für interessante Diskussionen und für das Korrekturlesen von Abschnitten dieser Arbeit, Max, Moriz und Jens für die Zusammenarbeit und die tollen Sprüche. Georg Papastavrou danke ich für die vielen interessanten Diskussionen. Außerdem möchte ich Alex und Stephan dafür danken, dass Ihr mich in die PC II eingeführt habt, Julia und Katja für die lustigen und interessanten Gespräche und Öznur für die Freundschaft. Danke Euch allen für Eure Motivation, die interessanten Diskussionen und den Spaß, den wir zusammen hatten. Arbeiten geht bei mir aber nicht ohne Freunde und Unternehmungen außerhalb der Uni. Bei allen meinen Freunden aus der Physik, Sport und den Anderen (Ihr wisst schon, wen ich so meine...) möchte ich mich für all die Jahre voller Spaß, Sport und natürlich gute Zusammenarbeit bedanken und dafür, dass ich durch Euch den Blick für die wesentlichen Dinge behalten habe. So, jetzt kommen wir zu den wichtigsten Menschen in meinem Leben. Erst einmal ein riesengroßer Dank, den man gar nicht in Worte fassen kann, an meine neu dazugewonnene Familie (Wagner, Schulte), dafür, dass ich mich bei Euch geborgen fühle. Der größte Dank zum Schluss gilt Euch, meiner Familie daheim und Dir, Susanne, meiner kleinen Fee. Danke Susanne für die unglaublichen Jahre und Momente, die wir zusammen hatten, und dass Du mich so nimmst, wie ich bin! Danke, Mama, Papa, T, und Regina für Eure moralische Unterstützung in allen Lebenslagen, durch die ich der werden konnte, der ich jetzt bin. Ich bin ganz zufrieden damit! Erklärung Die vorliegende Arbeit wurde von mir selbstständig verfasst und ich habe dabei keine anderen als die angegebenen Hilfsmittel und Quellen benutzt. Ferner habe ich nicht versucht, anderweitig mit oder ohne Erfolg eine Dissertation einzureichen oder mich der Doktorprüfung zu unterziehen. Johann Erath 275