Hybrids Based on Layered Silicates

Hybrids based on layered Silicates Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) im Promotionsprogramm Polymer Science an der Bayreuther Graduiertenschule für Mathematik und Naturwissenschaften (BayNAT) der Universität Bayreuth Vorgelegt von Stephan Weiß geboren in Coburg Bayreuth, 2013 Die vorliegende Arbeit wurde in der Zeit von Januar 2009 bis Februar 2013 in Bayreuth am Lehrstuhl Makromolekulare Chemie II unter Betreuung von Herrn Prof. Dr. Axel H. E. Müller angefertigt. Vollständiger Abdruck der von der Graduiertenschule BayNAT der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.). Promotionsgesuch eingereicht am: 02.05.2013 Zulassung durch das Leitungsgremium: 07.05.2013 Wissenschaftliches Kolloquium: 11.06.2013 Amtierender Direktor: Prof. Dr. F.X. Schmid Prüfungsausschuß: Prof. Dr. Axel. H. E. Müller (Erstgutachter) Prof. Dr. Adreas Fery (Zweitgutachter) Prof. Dr. Rainer Schobert (Vorsitz) Prof. Dr. Josef Breu Für meine wundervolle Familie. »Nichts schockiert mich. Ich bin Wissenschaftler.« Indiana Jones »What surprises me most of all in mankind, is that man will lose their health in order to get money and then they lose that money in order to recover their health. At the same time while worrying about the future, they forget to live the present, this way they end up living in neither the present nor the future. They live as if they are never going to die and they die as if they have never lived.« Buddha Table of Contents Table of Contents SUMMARY .............................................................................................................................. 1 GLOSSARY .............................................................................................................................. 5 1 CHAPTER 1: INTRODUCTION ......................................................................... 7 1.1 Layered silicates ........................................................................................................................ 7 1.1.1 Natural montmorillonite ....................................................................................................... 9 1.1.2 Synthetic hectorite (Na-fluorohectorite) ............................................................................. 10 1.1.3 Natural kaolinite .................................................................................................................. 11 1.2 Organic/inorganic hybrid nanoparticles .................................................................................. 12 1.2.1 Clay based hybrid nanoparticles .......................................................................................... 13 1.2.2 Patchy nanoparticles ........................................................................................................... 14 1.2.3 Janus nanoparticles ............................................................................................................. 14 1.3 (Clay reinforced) nanocomposites .......................................................................................... 16 1.3.1 Nanocomposites based on homopolymers ......................................................................... 17 1.3.2 Nanocomposites based on polymer blends ........................................................................ 17 1.4 Motivation and objective of this thesis ................................................................................... 18 2 CHAPTER 2: EXPERIMENTAL PART AND METHODS ............................................ 20 2.1 Materials ................................................................................................................................ 20 2.2 Instrumentation ...................................................................................................................... 21 2.2.1 Nuclear magnetic resonance spectroscopy (NMR) ............................................................. 21 2.2.2 Size exclusion chromatographie (SEC) ................................................................................. 22 2.2.3 Transmission electron microscopy (TEM) ........................................................................... 22 2.2.4 Scanning electron microscope (SEM) .................................................................................. 22 2.2.5 Thermogravimetric analysis (TGA) ...................................................................................... 22 2.2.6 Dynamic light scattering ...................................................................................................... 22 2.2.7 Stability measurements (LUMiFuge®) ................................................................................. 23 2.2.8 Charge titration stability analysis (Stabisizer®) ................................................................... 23 2.2.9 Dynamic-mechanical analysis (DMA) and tensile tests ....................................................... 23 2.2.10 Powder X-ray diffraction (PXRD) ..................................................................................... 24 2.2.11 Fourier-transform infrared spectroscopy (FT-IR) ............................................................ 24 2.2.12 Specific surface area measurements .............................................................................. 24 Table of Contents 2.3 Tailoring of stacks height and stiffness of fluorohectorite ....................................................... 24 2.4 Synthesis of the copolymers for surface modification ............................................................ 25 2.4.1 Synthesis of the catechol-modified PMMA copolymer (PCM) ............................................ 25 2.4.2 Synthesis of poly(2-(2-bromoisobutyryloxy)ethyl methacrylate)-stat-(2dimethyl(amino)ethyl methacrylate) (MI) via Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization ....................................................................................................................... 26 2.4.3 Synthesis of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) based diblocks via RAFT ............................................................................................................................................. 27 2.5 Clay surface modification ........................................................................................................ 28 2.5.1 Surface modification of K-fluorohectorite and surface-initiated Atom Transfer Radical Polymerization (si-ATRP) of methyl methacrylate (MMA) ................................................................ 28 2.5.2 Surface modification of montmorillonite (MMT) ................................................................ 29 2.5.3 Surface modification of kaolinite ........................................................................................ 30 2.6 Preparation of clay/polymer nanocomposites ........................................................................ 31 2.6.1 Embedding of K-fluorohectorite/PMMA hybrid particles (hybrid-hect) into a PMMA Matrix for tensile testing ............................................................................................................................... 31 2.6.2 Preparation of hybrid-clay/polystyrene (PS)/PMMA nanocomposite samples for TEM analysis ............................................................................................................................................. 31 2.6.3 Preparation of hybrid-MMT/PS/PMMA nanocomposite samples for DMA ........................ 32 3 CHAPTER 3: HYBRID MICA-LIKE PARTICLES BASED ON HIGH ASPECT RATIO FLUOROHECTORITE ............................................................................................................. 33 3.1 Preparation of tailored mica-like K-fluorohectorite/PMMA hybrid particles .......................... 33 3.1.1 Tailoring of a high aspect ratio mica-like nanofiller ............................................................ 33 3.1.2 Selective surface modification of the mica-like nanofiller .................................................. 34 3.1.3 Surface-initiated ATRP of MMA ........................................................................................... 38 3.2 Mechanical properties of the clay/PMMA nanocomposites .................................................... 44 3.3 Conclusion .............................................................................................................................. 46 4 CHAPTER 4: PATCHY HYBRID PARTICLES BASED ON POLYMER GRAFTED MONTMORILLONITE (MMT) ............................................................................................. 47 4.1 Preparation of patchy hybrid particles based on MMT ........................................................... 47 4.1.1 Synthesis of DMAEMA based diblock copolymers via sequential RAFT polymerization ..... 48 4.1.2 Solution behaviour of the diblock copolymers .................................................................... 52 4.1.3 Modification of the basal planes ......................................................................................... 54 Table of Contents 4.2 Mechanical properties of the clay/PMMA/PS nanocomposites .............................................. 57 4.2.1 Preparation of clay/PS/PMMA blends ................................................................................. 57 4.2.2 DMA of the blends ............................................................................................................... 60 4.3 Conclusion .............................................................................................................................. 61 5 CHAPTER 5: HYBRID JANUS PARTICLES BASED ON POLYMER MODIFIED KAOLINITE ............................................................................................................................. 62 5.1 Preparation of hybrid janus particles based on kaolinite ........................................................ 62 5.1.1 Synthesis of the copolymers PCM and D16-b-S115 ................................................................ 64 5.1.2 Modification of the kaolinite basal planes .......................................................................... 66 5.2 TEM Analysis of the morphology of the hybrid-kaolinite / PMMA/ PS nanocomposites ......... 72 5.3 Conclusion .............................................................................................................................. 75 6 CHAPTER 6: REFERENCES ......................................................................................... 76 ACKNOWLEDGEMENTS .................................................................................................. 82 Chapter 1 7 1 Chapter 1: Introduction 1.1 Layered silicates In mankind's early history, the utilization of new materials lead to major technological progress, from the stoneover the bronzeto the copper-age, the new materials enabled better tools, better hygiene and better protection and also often decided the fate of civilizations. Modern society in contrast needs highly specialized materials, often tailored to a single task, to face the challenges of its rapidly advancing technological sectors like automotive, aerospace, hygiene, energy and engineering. Living in the age of polymers, there is already a large supply of basic and advanced materials to choose from, but the introduction of nanotechnology, and with it nanoparticles, opened up a vast range of possibilities for better and novel materials, where even the cheapest basic polymers, which make up most of the daily-use items around the world, can be mixed with a small amount of nanosized objects to enhance and alter their properties significantly1. Clay minerals have been used since thousands of years by cultures worldwide as ceramics, but due to their manifold other properties, e.g. high water adsorption, capacity for cation exchange, non-Newtonian fluid behavior they found their way into modern applications, like filtration, purification, encapsulation of wastes, drilling and gardening. A recently advanced topic and one of growing interest is their use in polymeric materials as cheap and versatile nanofillers to enhance their toughness, flame retardency and gas barrier properties. All of these properties are a direct result from their unique layered sheet-like structure after which they are named and classified. Gary W. Beall and Clois E. Powell ask as an introductory question to their book2: „Can one imagine the utility of a dispersed-phase reinforcement for polymers that has a thickness of 1 nm, a plate-like morphology with minimal dimensions of 150 to 200 nm, robust with a modulus of 180 GPa, non-toxic [...], a surface area in excess of 750 m²/g, a charge suitable for altering its hydrophilic balance at will, and a refractive index similar to polymer so that the nanoparticle will appear transparent in the Introduction 8 polymer composite? How difficult would it be to prepare such a particle?” As the authors are discussing naturally occurring layered silicates, this rhetorical question amplifies the potential which lies in these clay minerals found all over the world in sedimentary rocks like bentonites or kaoline. Layers are held together by van-der-Waals forces, hydrogen bonds and electrostatic interactions forming large stacks (tactoids) in the dry state. Each layer is roughly 1 nm in height and is made up from two different basic building blocks3, namely [TO4]- tetrahedra and [M(O,OH)6]-octahedra. Most commonly tetrahedral cations are Si4+, Al3+ and Fe3+, while octahedra feature Al3+, Fe3+, Mg2+, Fe2+ or Li+. Each corner of one polyhedron is occupied by O2-, OHor Fanions. Tetrahedra are connected via three shared corners and form a two-dimensional hexagonal lattice structure (Figure 1.1). The fourth apical corner acts as connection to the octahedral layer. Octahedra are connected to each other by shared edges and the upper and lower triangular sides of all tetrahedra lie in plane respectively. Figure 1.1 Schematic representation of a 2:1 layered Silicate. T= Tetrahedron, O=octahedron. Reprinted with permission from reference [4]. There are two important types of layered silicates, in both cases each lamella consists of one octahedral layer connected either on one side to a tetrahedral layer (named 1:1 layered silicate, e.g. kaolinite, chapter 1.1.3) or sandwich-like on both sides (called 2:1 layered silicate, e.g. montmorillonite or hectorite, chapter 1.1.1 and Chapter 1 9 1.1.2). This has a major influence on the way how each silicate compensates its layer charge resulting from isomorphous substitution. Any cation in the tetrahedral or octahedral layer which is replaced by a cation of lower valency will contribute to a permanent net layer charge, ζ, which is compensated by counter-ions close to the layer. The amount of (exchangeable) counter-ions is denoted as cation exchange capacity (CEC). In the case of 2:1 silicates the cations reside on the external basal planes and in the interlayer spaces. The interlayer distance varies with the cation species and its degree of hydration between 9.1 Å and 18.0 Å. A 1:1 silicate does not have any cations in its interlayer space and compensates its charge only at the external tetrahedral layer. In the case where the open spaces of all octahedrons are filled with cations the layer is called trioctahedral or brucitic (e.g. hectorite, see chapter 1.1.2), while an occupation of only 2/3 is named dioctahedral or gibbsitic (e.g. montmorillonite and kaolinite, chapter 1.1.1 and 1.1.3) (Figure B7c). As a result cations of higher valency (Al3+ vs. Mg2+) are incorporated into dioctahedral structures to compensate for the layer charge. With natural silicates varying degrees of isomorphous substitution occur in each layer, depending on the conditions under which they were formed. 1.1.1 Natural montmorillonite Montmorillonite (MMT) is a natural 2:1 layered silicate from the smectite group with the dioctahedral structure (Na, Ca)0.3(Al, Mg)2(Si4O10)(OH)2·nH2O. It is found all over the world5. It is an alteration product of volcanic tuff and ash, forming bentonite beds, and of wall rocks bordering hydrothermal mineral deposits. It forms under alkaline conditions of poor drainage, with Mg, Ca, Na, and K remaining in the soil. As it is a natural product it contains impurities, most commonly feldspar, quartz, mica, carbonate and hydroxycarbonate, which have to be removed prior to commercial application. Furthermore, all of its properties depend on the conditions it was formed under, varying with its origin. E.g. the CEC reaches from 90 up to 150 meq/100g as the negative charge is distributed inhomogenously inside each layer and between layers, resulting in inhomogeneous surface coverage with counterions, Introduction 10 sometimes resulting in clusters forming around spots with high density of isomorphous substitution6, 7. Counterions usually are hydrated sodium or calcium cations, and increasing hydration fosters desaggregation of tactoids and partial exfoliation. Delamination into singular layers of 1 nm height is only observed after ion exchange with Li and removal of amorphous binders. Combined with a lateral dimension of up to over 300 nm, they can reach aspect ratios, α, of up to 300 in theory. Though, these single sheets lose their intrinsic stiffness and start to curl and break under shear (e.g. during mixing), their practical aspect ratio after processing is usually not higher than 1008. Nevertheless, due to its easy mining and processing MMT rapidly became the commercially most attractive clay as an additive for polymeric matrices in the last decades and there is a range of companies, supplying MMT with different grades of purity, dimensions and CEC. In its pristine form it is only miscible with hydrophilic polymers, such as poly(ethylene oxide) and poly(vinyl alcohol)9, 10. To render MMT miscible with hydrophobic polymers, alkali counterions classically are exchanged with cationic-organic surfactants, such as alkylammonium salts11, 12. 1.1.2 Synthetic hectorite (Na-fluorohectorite) Hectorite is a 2:1 layered silicate commonly of the structure Na0.3(Mg,Li)3Si4O10(OH, F)2. Natural hectorite belongs to the smectite group as well and is related to montmorillonite, but has a trioctahedral structure. Its natural variant suffers from the same impurities and inhomogenities described in 1.1.1. To omit those disadvantages, classical solid-state reactions and melt synthesis have been used to produce artificial hectorite. High temperatures lead to statistical distribution of isomorphous substitution, generating a homogeneous layer charge. Until recently this procedure has been very expensive and industrially inapplicable. Development of a new synthetic route in powerful high frequency furnaces by Hussein Kalo at the department of Inorganic Chemistry I of University of Bayreuth under supervision of Prof. Breu allows for production quantities of kilograms with a price of 18 €/kg. But price is not the only benefit; by synthesis it is possible to create much larger platelets, leading to huge practical aspect ratios α of up to 20000 in case Chapter 1 11 of Li-fluorohectorite or annealed Na-fluorohectorite (hectorite where part of the octahedral O-atoms have been replaced by F-atoms is called fluorohectorite). With homogeneity in surface charge and far less impurities than natural clay13 it is possible to control alternation between a highly hydrated ‘shear-labile’ state and a nonhydrated ’shear-stiff’, mica-like state by simple cation exchange. This transition between hydration states cannot be observed for natural MMT due to heterogeneity of charge density and lower layer charge. Cation exchange toward Mgfluorohectorite gives a highly hydrated and therefore ‘shear-labile’ state, enabling exfoliation by application of shear forces in a stirred media mill14. A subsequent cation exchange with K+ ions yielded a collapsed non-swollen, ‘shear-stiff´, mica-like material. Powder x-ray diffraction (PXRD) measurements showed that collapsed stacks will not exchange interlayer cations and thus reactivity is restricted to external basal planes. This prevents the curvature observed in delaminated single sheets and should substantially increase potential reinforcement effects15. These unique properties have led to a renewed academic interest to develop hybrid materials based on synthetic hectorite for industrial applications. 1.1.3 Natural kaolinite Kaolinite is a 1:1 silicate, with the formula unit of Al2Si2O5(OH)4. Tactoid height ranges from 70 nm to 100 nm and lateral extension varies between 500 nm and 15 µm, depending strongly on its origin. Kaolinite has several features not found in 2:1 silicates. Single lamellae in tactoids are not held together by van-der-Waals forces but by strong hydrogen bonds between µ-hydroxide-groups of the octahedral layer and the silicon network of the tetrahedra (Figure 1.2)16, resulting in much smaller interlamellar distances of 7.2 Å. This makes intercalation difficult and restricts it to only a small range of neutral molecules with high dipolar moments like Dimethyl sulfoxide (DMSO)17 and N-Methylformamide (NMF)18. Introduction 12 Figure 1.2 Schematic represantation of 1:1 layered silicate (kaolinite). OS= Octahedral surface, TS= tetrahedral surface. Reprinted with permission from reference [14]. The most interesting feature in the particle architecture is the preservation of its polar lamellar structure throughout the tactoid, which means each particle has two chemically distinct surfaces, which can also be selectively targeted for modification to create Janus structures (see chapter 1.2.3). A combination of both features (no intercalation and chemically distinct external surfaces) leads to the interesting fact that the negative charge generated by isomorphous substitution of Si4+ against Al3+ in the tetrahedral layer can only be compensated by counterions at the tetrahedral surface (abbreviated TS), which means the outermost tetrahedral layer of a tactoid19, 20. Natural counter ions are sodium and calcium, which are easily replaced by other ions respective to their comparatively low CEC, which lies at ~2.6 meq/100g. Recent studies show that the octahedral layer can be selectively addressed by molecules bearing a catechol moiety21. Those groups most likely will undergo a condensation reaction and bind covalently to the µ-hydroxide groups of the octahedral surface (called OS), similar to what was observed with alcohols and structurally related aluminum oxide surfaces22, 23. 1.2 Organic/inorganic hybrid nanoparticles Nanomaterials have, by definition, at least one dimension in the nanometer scale (<100 nm) and show novel properties strongly influenced by the large surface to Chapter 1 13 volume ratio. The synthesis, characterization, and applications of nanoparticles are among the most important sections of the wide range of nanotechnology. In recent years, nanoparticles have gained tremendous attention as the transition from microparticles to nanoparticles was seen to lead to immense changes in the physical and chemical properties of a material. Due to the vast increase in surface area to volume ratio gained from this step down in length scale, surface atoms and their effects now play a dominant role over bulk atoms. Especially when introduced into composites, the huge specific interphase area alters the properties of the matrix considerably. Research started in the 1980s with nanoparticles made from one material24-26, but it was quickly discovered, that adding a shell around the core particle gives rise to new materials only possible by combination of both properties27-29. The name “core/shell” particles was adopted for materials consisting of a inorganic/organic core of different shapes and an inorganic or organic shell. Applications are manifold30 and advances in surface modification techniques allow for ever new combinations of core properties and shell properties. A recent example for the multifunctionality of inorganic core/ polymeric shell hybrids are superparamagnetic and fluorescent CdSe(ZnS) nanoparticles coated with protective silica and bearing a polymeric thermo-responsive poly(N-isopropylacrylamide) shell31. 1.2.1 Clay based hybrid nanoparticles Clay particles are well suited inorganic cores for the creation of hybrid particles. Even the symmetrical 2:1 structure of smectites already provides two chemically different reactive sites for attaching a shell: basal surfaces and edges. The previously described inherent negative layer charge of layered silicates enables facile modification of the basal surface and interlayer spaces with organic molecules bearing a positively charged group by simple cation exchange. Modification of edge located silanol groups with silicon halides, acid halides or silazanes leads to stronger covalent bonds. Both sites can be accessed to go beyond simple alkyl ammonium surfactant modification by attaching polymers with tailored properties via controlled polymerization of suitable monomers and chain length. Classically smectite-based Introduction 14 hybrid nanoparticles are implemented into polymeric materials to enhance toughness, flame retardancy and gas barrier properties. 1.2.2 Patchy nanoparticles For mixtures of polymer species of different polarity it is energetically favourable to segregate and form domains of their single polymer species, respectively. If bound to an inflexible core, complete phase segregation becomes inherently difficult and formation of compartmentalized (patchy) shells can be observed. There has been a recent breakthrough in creation of multicompartment micelles by hierarchical self-assembly of ABC triblock terpolymers32 and patchy wormlike crystalline core micelles made from ABC triblocks with crystallisable core33. These groups report on the potential of patchy particles for hierarchical step-growth polymerization of multicompartment micelles into “micron-scaled segmented supracolloid polymers”32 and their use as super surfactants close to pure Janus colloids in surface activity, while usually being less complicated to produce. On the core/shell particle side there are few examples utilizing the promising potential of a patchy shell. Furthermore, most of the produced patchy particles are spherical in nature34. As shown by Schmelz et al. patchy particles made from triblock terpolymers with crystallized middle block act as giant surfactants in mixtures of immiscible fluids, reducing the surface tension with an effect comparable to that of Janus cylinders35. In this thesis we use disc-like montmorillonite 2:1 layered silicates as core to investigate the influence of a patchy shell on its interfacial behavior in an immiscible polymer blend. 1.2.3 Janus nanoparticles Particles which embed exactly two distinct sides or surfaces of different chemical property and/or polarity into one structure are called Janus particles, named after the Roman god Janus with two faces and whose name is used symbolically for entities showing character or behavior of two incompatible sides. This noncentrosymmetric appearance leads to a unique set of characteristics regarding Chapter 1 15 material properties and self-assembly behavior36. A whole spectrum of different Janus particle architectures is known. Janus particles can be categorized according to their dimensions. There are three-dimensional spherical particles, two versions of two-dimensional disc-like structures and two different one-dimensional cylinders. While their overall geometry can be simple and symmetric, the lack of chemical centrosymmetry proved to be the biggest challenge in their preparation. The pioneers in the field of Janus particles were Casagrande and Veyssié. They embedded half of a mesoscopic glass bead into a substrate and then silylated the other half. As the amount of particles producible by those syntheses was very limited, all their methods had the major drawback that they were not applicable on a larger scales37, 38. Recently the application of photopolymerization and photolithographic polymerization to microfluidic devices enabled an even higher degree of control and structural variety. The microfluidic device sends a two-phase stream into a channel. There it is cut into droplets by an aqueous crossflow, containing surfactants to stabilize the resultant particles. Then a photopolymerization locks the shape of the biphasic particles. Unfortunately this method is not able to create particles with submicron dimensions yet39. Another interesting approach was developed by Müller and coworkers36 , using the selfassembly behavior of triblock terpolymers. Triblock polymers with phase-separating outer blocks will undergo self-assembly upon film casting and form nanometerscaled bulk structures, which can be locked by crosslinking the inner part (in this case polybutadiene). Upon dissolution of the polymer, the crosslinked part will preserve its bulk shape and thus will yield non-centrosymmetric particles. By defined engineering of the terpolymer composition the bulk structure and thus the resulting particle shape and size can be controlled. All these approaches have in common, that they start with symmetrical systems and break those apart into non-symmetrical particles or complicatedly synthesize non symmetrical building blocks to start with instead of applying intrinsically polar particles like kaolinite. As described in chapter 1.1.3 it is possible to address each side individually in solution, facilitating the creation of disc-like Janus particles based on a layered silicate, even in large quantities. Introduction 16 Several fields of application result from the unique structural properties of Janus particles. They have evoked great academic interest, as they represent a class of particles with extraordinary self-assembly behavior. Fundamental understanding of self-assembly processes is attributed with the possibility to create new functionalities not present in the individual building blocks by assembling them into hierarchical superstructures. Their enormous surface activity puts them into the focus of industrial applications as super-surfactants and structuring agents in polymer blends. Furthermore the anisotropic character of single Janus particles is used for optical and analytical probes in confined space 40, 41, medical sensors for cell targeting42 and switchable electro-optical devices39. . 1.3 (Clay reinforced) nanocomposites The commercial breakthrough of clay reinforced nanocomposites happened in the early 1990´s when Toyota researchers published their work on nylon-6-clay thermoplastic nanocomposite technology43, 44. The key aspect was a fundamental improvement of properties at minimal loading. At only 4.2 wt% clay the modulus doubled, strength increased by 50 % and the heat distortion temperature (HDT) increased by 80 °C compared to neat polymer. Toyota still holds a broad range of patents in this technological field. Nevertheless academic and industrial interest is still strong and research and development of clay reinforced nanocomposites is growing. For most applications it is necessary to organophilize the clay surface to increase the compatibility with the matrix and enable a good dispersability. A well known commercial brand of organophilized MMT is the Cloisite product family by Rockwood Additives, which has been optimized for application in aliphatic polymer matrices. On their product webpage it is claimed that their clay based products can act as a new flame retardant approach, increase modulus and tensile strength, improve barrier properties, increase dimensional stability, are thermoplastic recyclable, improve clarity, increase HDT, reinforce and lower density45, while at a much lower loading (3-5 wt%) compared to conventional fillers (20-60%). However, as most of Chapter 2 23 2.2.7 Stability measurements (LUMiFuge®) The stability measurements were performed in a LUMiFuge® 114 (LUM) with a variable rotation frequency of 300, 600, 900 rpm (rounds per minute) and different time intervals of 200 s, 300 s, and 900 s, respectively. Kaolinite suspensions (0.25 wt%) in THF and water were placed in tubes in horizontal positions on the disc of the LUMiFuge®. During the horizontal rotation of this disc the transparencies of the suspensions were measured in the area between the menisci and the sediment. The mean transparency of the whole area was determined. The transparency was measured in time intervals of 10 s while increasing rotation speed stepwise. High turbidity, even after applying centrifugal forces indicates a stable suspension. 2.2.8 Charge titration stability analysis (Stabisizer®) Determination of point of zero charge of the clay platelets was done using a Stabisizer® (Particle Metrix GmbH). Therefore the microionic clouds of localized particles are displaced by flow induced via a piston. The generated potential is measured and used for monitoring titration with monoand polycationic species. After complete replacement of displaceable sodium ions by immobile cations the point of zero charge is reached. Four separate solutions containing the polycationic MI solution (1 g/l) in deionized water (DI) were prepared with different pH values using acetic acid (100 %) in order to protonize the amine functions. The point of zero charge for the clay basal surface was measured using a charge titration stability analyzer. 2.2.9 Dynamic-mechanical analysis (DMA) and tensile tests Dynamic-mechanical analysis (DMA) experiments are carried out in the tension mode at a constant force of 5 N and a temperature range from 30 to 140 °C using a Mettler Toledo DMA/STDA 861e. The heating rate is 5 °C/min and the test specimens is approximately 25 mm in length, 6 mm in width and 1 mm in thickness. Tensile modulus, tensile strength and elongation at break were measured using a Universal Tensile Tester according to ISO 527 applying a strain rate of 1 mm/min. For Experimental Part and Methods 24 each material at least 8 samples were tested. The elongation at break was determined by a macro-displacement-transducer. 2.2.10 Powder X-ray diffraction (PXRD) The powder X-ray diffraction (PXRD) patterns were recorded in reflection mode using nickel filtered Cu-Kα radiation λ =1.54187 Å on a Bragg-Brentano-geometry diffractometer (PANalytical Xpert-Pro) equipped with an X′Celerator Scientific RTMS detector. 2.2.11 Fourier-transform infrared spectroscopy (FT-IR) The particles powders of untreated and treated clay were characterized with a Nicolet FTIR 460 (Thermo Nicolet Corp.). The transmittance absorption spectra were scanned 64 times at 4 cm-1 spectral resolution at room temperature. 2.2.12 Specific surface area measurements The specific surface area of a freeze-dried K-hect sample was calculated from the N2 adsorption/desorption isotherms using the Brunauer-Emmett-Teller (BET) equation. Measurements were carried out on a Quantachrome Nova 2000e analyzer. 2.3 Tailoring of stacks height and stiffness of fluorohectorite The aqueous dispersion of synthetic Na-hect was transferred to a highly hydrated ‘shear-labile’ state by exchanging the interlayer Na+ with Mg2+ cations (Mg-hect). The aqueous dispersion of Mg-hect was processed in a stirred media mill (LabStar LS1) for 60 minutes in order to exfoliate the tactoid stacks by applying shear forces. The degree of exfoliation was controlled by the number of milling passages14. Subsequently, the clay was transferred into a collapsed and non-swollen ‘shear-stiff’ mica-like material (K-hect) with no intracrystalline reactivity by exchanging Mg2+ with K+ cations56. All exchanging procedures were followed by washing several times with water to remove chlorine ions. Chapter 2 25 2.4 Synthesis of the copolymers for surface modification 2.4.1 Synthesis of the catechol-modified PMMA copolymer (PCM) 3,4-Dibenzoxybenzoic acid 1 and 3-hydroxypropylbenzoate 2 were prepared following literature procedures (scheme 5.1).57 3-Methacryloyloxypropyl-3,4-dibenzoxybenzoate 3 Compound 2 (2.11 g, 5.38 mmol) was dissolved in dry DCM (20 ml) and cooled in an ice bath. Et3N (1.12 ml, 8.07 mmol) and methacryloyl chloride (626 µl, 6.47 mmol) were added and the reaction mixture was stirred at room temperature for 3 h. After washing with water the aqueous phase was extracted with DCM and the combined organic phases were dried over Na2SO4, filtered and concentrated in vacuum. The residue was purified by column chromatography (silica gel 60, ethyl acetate/nhexane 1:2, v/v). Yield: 1.64 g (3.57 mmol, 67%); colorless oil; Rf = 0.63 (ethyl acetate/n-hexane 1:2); νmax (ATR)/cm-1: 3032, 2963, 1711, 1636, 1599, 1510, 1454, 1427, 1380, 1321, 1266, 1204, 1163, 1130, 1104, 1038, 1006, 944, 815, 761, 734, 695; 1H NMR (300 MHz, CDCl3):  1.92 (3 H, s), 2.0-2.2 (2 H, m), 4.28 (2 H, t, 3J 6.3 Hz), 4.36 (2 H, t, 3J 6.3 Hz), 5.18 (2 H, s), 5.21 (2 H, s), 5.5-5.6 (1 H, m), 6.0-6.1 (1 H, m), 6.91 (1 H, d, 3J 9.0 Hz), 7.3-7.5 (10 H, m), 7.6-7.7 (2 H, m); 13C NMR (75.5 MHz, CDCl3):  18.3, 28.2, 61.3, 61.4, 70.8, 71.2, 113.2, 115.6, 123.0, 124.0, 125.6, 127.1, 127.4, 127.9, 128.0, 128.5, 128.6, 128.9, 136.2, 136.5, 136.8, 148.3, 153.0, 166.1, 167.3; m/z (%) 461 (13) [M+], 460 (47) [M+], 369 (6), 317 (8), 225 (17), 181 (27), 127 (12), 91 (100). Copolymer PCBM 4 Methyl methacrylate (650 mg, 6.52 mmol), compound 3 (100 mg, 0.22 mmol) and dodecanethiol (26 mg, 0.13 mmol) were dissolved in dry THF (3 ml) under argon atmosphere and AIBN (10 mg) was added to the reaction mixture, which was stirred under reflux for 5 h. The solution was poured into cyclohexane (100 ml) and the appearing colorless precipitate was collected and precipitated once more from an acetone/cyclohexane mixture. Yield: 710 mg; colorless solid; νmax (ATR)/cm-1: 2996, Experimental Part and Methods 26 2952, 1722, 1601, 1484, 1448, 1432, 1385, 1363, 1268, 1241, 1189, 1144, 989, 965, 911, 842, 761, 748, 698; 1H NMR (300 MHz, acetone-d6):  0.8-1.0 (33 H, m), 1.8-2.0 (24 H, m), 3.61 (30 H, s), 4.1-4.2 (2 H, m), 4.3-4.4 (2 H, m), 5.2-5.3 (4 H, m), 7.1-7.7 (13 H, m). Copolymer PCM 5 Compound 4 (580 mg) was dissolved in dioxane/methanol (40 ml, 1:1), flushed with argon and 10% Pd/C (80 mg) was added. The argon atmosphere was replaced by hydrogen gas and the reaction mixture was stirred at room temperature for 5 h. The suspension was filtered over celite and the filtrate was concentrated in vacuum. The oily residue was triturated with n-hexane and dried in vacuum. Yield: 500 mg; offwhite solid; νmax (ATR)/cm-1: 3392, 2996, 2950, 1725, 1605, 1480, 1444, 1386, 1270, 1239, 1191, 1146, 1121, 988, 965, 889, 873, 842, 765, 750; 1H NMR (300 MHz, DMSO-d6):  0.5-0.9 (33 H, m), 1.6-2.0 (24 H, m), 3.55 (30 H, s), 4.0-4.1 (2 H, m), 4.24.3 (2 H, m), 6.81 (1 H, d, 3J 7.9 Hz), 7.2-7.4 (2 H, m), 9.32 (1 H, s), 9.81 (1 H, s); 13C NMR (75.5 MHz, DMSO-d6):  16.1, 18.4, 27.5, 43.9, 51.6, 53.7, 60.8, 115.2, 116.3, 120.5, 121.8, 145.0, 150.4, 165.5, 176.2, 176.9, 177.3. 2.4.2 Synthesis of poly(2-(2-bromoisobutyryloxy)ethyl methacrylate)-stat- (2-dimethyl(amino)ethyl methacrylate) (MI) via Reversible AdditionFragmentation Chain Transfer (RAFT) polymerization To a 100 ml round bottom flask, equipped with rubber septum, 1.7 g (6.1 mmol) of BIEM, 6.7 g (42.8 mmol) of DMAEMA, 270 mg (1.2 mmol) of 2-cyano-2-propyl benzodithioate (CPBT), 100 mg (0.6 mmol) of AIBN, 40 ml of DMSO as solvent and 2 ml of anisole as internal standard were added. After three freeze-pump-thaw cycles the reaction was placed into an oil bath at 70 °C for 4 h to reach a conversion of 54% as determined by 1H NMR spectroscopy. The resulting polymer solution was cooled down, exposed to air and dialysed against dioxane until no monomer related peaks at 5.8-6.4 ppm were detected by NMR spectroscopy. Mn = 9000 g/mol and Mw = 16000 g/mol was determined via SEC with DMAc as eluent and a DMAEMA calibration. The final polymer is from here on referred to as maco-initiator (MI). 1H Chapter 2 27 NMR (300 MHz, CDCl3, δ in ppm): 4.4 – 4.1 (R-C(-CH3)-COO-CH2-CH2-OOC-C(CH3)2-Br), 4.0 (R-C(-CH3)-COO-CH2-CH2-N-(CH3)2), 2.6 (R-C(-CH3)-COO-CH2-CH2-N-(CH3)2), 2.2 (RC(-CH3)-COO-CH2-CH2-N-(CH3)2), 1.9 (R-C(-CH3)-COO-CH2-CH2-OOC-C(CH3)2-Br), 1.8 (RC(-CH3)-COO-CH2-CH2-N-(CH3)2). 2.4.3 Synthesis of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) based diblocks via RAFT All polymerizations were carried out at 80°C in septum sealed flasks. Detailed amounts of reactants are listed in Table 1. In each case DMAEMA precursors were prepared by placing 1,4-dioxane, DMAEMA, AIBN, the chain transfer agent (CTA), 2cyano-2-propyl benzodithioate, and 1,3,5-trioxane in the reaction flask. Nitrogen flow was established for 20 min at room temperature and then the polymerization was initiated by heating the flask in an oil-bath. The precursor solution was transferred to a degassed and heated solution of 1,4-dioxane, second monomer and 1,3,5-trioxane after 4h at a typical conversion of DMAEMA of above 90 %. The reaction was terminated by cooling in an ice bath and exposure to atmospheric oxygen. The polymers were purified by precipitation into a non-solvent (isopropanol for PS containing diblocks and cyclohexane for PMMA containing diblocks) and freeze-dried from 1,4-dioxane. Final polymers are abbreviated D17-b-M300 in the case of poly(2-(dimethylamino)ethyl methacrylate)17-block-poly(methyl methacrylate)300 and D17-b-S360 in the case of poly(2-(dimethylamino)ethyl methacrylate)17-blockpolystyrene300. Table 1 Applied amounts of chemicals in the preparation of the PDMAEMA-macro-CTA and the diblock-copolymers in 1,4-dioxane . Formula a DMAEMA / mg, mmol CTA / mg, mmol AIBN / mg, mmol Monomer / g, mmol Solvent/ ml D17-b-M300 380; 2.5 54; 0.25 15; 0.09 12.0; 120 30 D16-b-S360 1083; 6.9 149; 0.67 38; 0.24 35.2; 338 30 D16-b-S115 1402; 9.1 100; 0.45 26; 0.18 18.8; 181 30 a repeating units of DMAEMA calculated by subtracting the molecular weight of CPBDT (221.00 g/mol) and dividing by the molecular weight of DMAEMA (157.21 g/mol), repeating units of the second block Experimental Part and Methods 28 were calculated by subtracting the molecular weight of DMAEMA block, acronyms: D: DMAEMA, , S: styrene, M: MMA Figure 3.1 a) Synthesis of the statistical copolymer (PDB) and characterization via b) 1H-NMR spectrum and c) SEC trace with DMAc as an eluent. 2.5 Clay surface modification 2.5.1 Surface modification of K-fluorohectorite and surface-initiated Atom Transfer Radical Polymerization (si-ATRP) of methyl methacrylate (MMA) The external surface of K-hect (10 g) was modified with the MI (320 mg) in DI water at pH=6.8. Subsequently, the flocculated hydrophobic nanoplatelets (O-hect) were centrifuged and redispersed in THF. The grafting of MMA was initiated from O-hect via a copper mediated ATRP in the presence of EBiB as a free sacrificial initiator. All experiments were performed under inert atmosphere in a conventional run procedure58, 59; A dispersion of the O-hect (10 g; calculated 390 µmol of initiating 4 3 2 1 0 chemical shift [ppm] 20 22 24 26 28 30 32 34 elution volume [ml] b) c) a) Chapter 2 29 sites) in 400 ml THF, MMA (164.5 g; 1.462 mol) and EBiB (19 mg; 97.5 µmol) were added to a flask and sealed with a rubber septum. The reaction mixture was degassed three times by freeze-pump-thaw cycles and filled with argon. In a separate flask a stock solution of PMDETA, Cu(I)Cl and Cu(II)Cl2 (338 mg; 975 µmol, 115.8 mg; 1.17 mmol and 39.2 mg; 292.5 µmol) in 20 ml anisole was degassed for 30 min under argon. Finally, 10ml of the stock solution was introduced to the reaction flask by a syringe. The reaction flask was immersed in an oil bath at 80 °C. Samples were withdrawn at various times to monitor the reaction kinetics and it was stopped after 300 min by cooling and exposing to air. The final hybrids of K-hect with a polymeric shell of PMMA chains with an average DP of 380 (hybrid-hect (DP 380)) were centrifuged and washed several times with THF. As a reference a nanofiller with a commercial surfactant, dodecylamine, was used after protonation using one equivalent of HCl (0.1 mol). Standard procedures were used to exchange the K+ cations with the organic cation (C12 ammonium chloride)60. After ion exchange, the modified nanoplatelets (C12-hect) were centrifuged and washed several times with DI water, ethanol, and THF. 2.5.2 Surface modification of montmorillonite (MMT) The diblock copolymer solutions of D17-b-M300 and D17-b-S360 for surface modification of MMT were prepared in two different ways: 1. Solutions with pre-formed micelles of D17-b-M300 and D17-b-S360 were prepared by dissolving the polymer in THF and adding water (pH=6.5) dropwise until turbidity occurred. 2. Molecularly dispersed solutions were prepared by adding the freeze dried polymer into THF and stirring until no solids were visible anymore and the transparent solution had a slightly pink colour. The polymer solutions (30 mg, 1 mg/mL) were added to a dispersion of MMT in water (100 mg, 5 mg/mL, pH=6.5) using a cannula and stirred over night. Final MMT/PS/PMMA hybrid particles (hybrid-MMT) were purified by removing nonanchored polymer via centrifugation at 4000 rpm, decantation of the supernatant and redispersion in water (1x) and THF (3x) using ultrasonication. Experimental Part and Methods 30 2.5.3 Surface modification of kaolinite It is possible to modify each side, the tetrahedral surface and the octahedral surface (TS and OS), of the kaolinite specifically and individually without influencing the other side as shown in a previous publication21. The order of modification chosen, starting with D16-b-S115 has a purely practical purpose, as the DMAEMA block of the D16-b-S115 is charged at pH 6 and thus kaolinite can be modified in aqueous suspension where it is dispersed best. After cation exchange the unilaterally modified kaolinite can be dispersed in THF more easily than unmodified kaolinite, as seen in Fig. 5.4 (stability measurements). PCM is soluble in THF, but not in water. Nevertheless pristine kaolinite can be modified by PCM as first step as well, but for that kaolinite has to be dispersed in THF by vigorous stirring first. Modification of TS 100 mg D16-b-S115 were dissolved in 30 ml THF. 400 mg of the kaolinite was suspended in 30 ml of water (pH~ 5.5, degree of protonation of the DMAEMA block ~80 %61). After 20 min of stirring a complete flocculation of the kaolinite was achieved and the suspension was washed ten times with THF to remove the excess of D16-b-S115. The hybrid was dispersed and stored in THF to prevent drying. Modification of OS 100 mg of PCM was dissolved in 20 ml dry THF under argon atmosphere in a Schlenk flask. 400 mg kaolinite (pristine or already unilaterally modified hybrid) was dispersed in the PCM THF solution by vigorous stirring over night at 60 °C. After the reaction the kaolinite was washed ten times with THF to remove the excess of PCM and then dispersed in THF to give hybrid-kaolinite. Chapter 2 31 2.6 Preparation of clay/polymer nanocomposites 2.6.1 Embedding of K-fluorohectorite/PMMA hybrid particles (hybrid-hect) into a PMMA Matrix for tensile testing Two different K-fluorohectorite/PMMA hybrid nanoparticles were evaluated as nanofillers in a PMMA matrix: hybrid-hect (DP 380) and C12-hect were mixed with a solution of PMMA in THF (5 wt. -% clay loading), respectively. To ensure good distribution of the clay in the polymer matrix both dispersions were placed in an overhead shaker overnight. Both dispersions were film casted and dried in a vacuum oven first at 90 °C for 14 h followed by drying at 140 °C for another 18 h. Neat PMMA was treated in a similar way before melt compounding. The dried nanocomposite materials were melt-compounded in a discontinuous counter-rotating twin-screw microcompounder (DSM Xplore, 15 ml microcompounder) at a temperature of 190 °C, a mixing speed of 210 rpm and a mixing time of 3 min. The material was added stepwise to the running microcompounder and during each cycle a batch of 7.5 g was processed. After extrusion, the melt was injection-moulded with a microinjector (DSM Xplore 12 ml injection moulding machine; melt temperature: 190 °C; mould temperature: 40 °C; injection pressure: 8 bar) into dumbbell specimens (75 mm × 5 mm × 2 mm) for tensile testing. 2.6.2 Preparation of hybrid-clay/polystyrene (PS)/PMMA nanocomposite samples for TEM analysis PS and PMMA in the ratio of 1:2 were dissolved in THF. The polymer content of the solutions was 10 wt%. 50 mg of hybrid-clay (hybrid-MMT or hybrid-kaolinite) was suspended in 10 ml of the PS/PMMA solutions by 15 minutes of strong shearing (Heidolph Silent Crusher, 16.000 rpm) at 30 °C, resulting in 5 wt% hybrid-clay in the final, dry blend. A film was cast by letting the solvent evaporate slowly from the mixture in a glass vial. The resulting dry film was cut with an ultramicrotome and examined via TEM. Experimental Part and Methods 32 2.6.3 Preparation of hybrid-MMT/PS/PMMA nanocomposite samples for DMA The PS/PMMA matrix containing the hybrids is cast into glass petri dishes and dried in a vacuum oven. The polymer is crushed and melt pressed into DMA mould samples using hot plates from P/O/Weber co. (Germany). The samples are melted at 200 °C without pressure for about 6 minutes, removing last traces of THF, then heat pressed for 5 minutes using 70-75 kN and finally cold pressed for 3 minutes using 3040 kN. Chapter 3 39 clay surface. The ATRP technique allows for preparation of polymers with narrow molecular weight distributions and precise control over the architecture. Table 3.1 gives the number and weight average molecular weights Mn and Mw, respectively, of the free PMMA grown in solution determined by SEC as a function of reaction time. The differences between theoretical and experimental values can be attributed to an initiation efficiency of less than 100%. Table 3.1 Molecular weights of the free PMMA chains as a function of polymerization time in THF [MMA] : [PMDETA] : [EBiB/MI] : [Cu(I)Cl] : [Cu(II)Cl2] (3000 : 2 : 0.2/0.8 : 1.2 : 0.3) Reaction time (min) Conv.a (%) DP a Mn a (kg/mol) DP b Mn b (kg/mol) Mw b (kg/mol) PDI b 10 1 36 3.5 84 8.4 10.0 1.2 20 2 72 7.2 100 10.0 13.0 1.3 30 4 120 12.0 150 15.0 17.0 1.1 60 5 143 14.3 190 19.0 23.0 1.2 120 7 203 20.3 310 31.0 36.0 1.2 300 9 263 26.3 380 38.0 50.0 1.3 a determined by 1H NMR spectroscopy. b determined by SEC with THF as eluent and PMMA standard calibration. The reaction shows a controlled character with a very good polydispersity index. Fig. 3.6 (left) shows first-order kinetic of ln[M0]/[M] as a function of time, whereas, polymerization rate slowed down after 120 min. This indicates that the number of the initiating species remained approximately constant up to 120 min reaction time and then started to decrease. Furthermore, by plotting the molecular weight versus conversion as in Fig. 3.6 (right), a linear increase ascertained the controlled behaviour of the reaction. Based on a surface area of 68 m²/g of K-hect we can calculate a grafting density of 0.08 chains per nm² using the following formula: Hybrid mica-like particles 40 Where 𝜌 is the grafting density (chains per nm2), MP is the molar amount of PMMA chains with a DP of 380, NA is the Avogadro constant and SA is the surface area in m2/g. Figure 3.6 (▲) First-order kinetic plot for the polymerization of PMMA. (■) Evolution of the molecular weight with conversion of PMMA. For a qualitative analysis of the grafted polymer hybrid, FT-IR spectra of neat PMMA and hybrid nanofiller with grafted PMMA chains are compared in Fig. 3.7. Characteristic CH vibrations (2800-3000 cm-1), C=O vibration (1727 cm-1) and C-O vibration (1263 cm-1) confirm the presence of PMMA on the surface even after extensive washing with THF. Chapter 3 41 Figure 3.7 FT-IR spectra of neat PMMA (black), surface modified O-hect (red) and hybrid nanofiller (blue). The amount of surface grafted PMMA was examined by TGA (Fig. 3.8). As expected, nanocomposites with longer polymer chains showed a higher weight loss of organic material. O-hect was found to have 3.1 % loss of volatile materials after heating up to 400 °C (the weight loss prior to 100 °C is neglected due to the traces of solvent). The grafted hybrids showed a weight loss of 15.2 % and 32.4 % for grafted PMMA chains with a DP of 150 and 380, respectively. Hybrid mica-like particles 42 100 200 300 400 500 600 700 0 20 40 60 80 100 hybrid (DP 150) neat PMMA hybrid (DP 380) weight (%) temperature (°C) O-hect Figure 3.8 TGA measurements showing weight loss versus temperature for O-hect (red), hybrid with PMMA DP 150 (blue), hybrid with PMMA DP 380 (green) and neat PMMA (black). Powder X-ray diffraction (PXRD) was used to track any changes in the interlamellar spacing during preparation of the hybrid nanofiller. The PXRD patterns of exfoliated K-hect and the hybrid nanofiller are shown in Fig. 3.9. Chapter 3 43 5 6 7 8 9 10 intensity (a.u.) 2 (°) d (001): 1 nm Figure 3.9 PXRD patterns of the (001) peak of K-hect (─), C12-hect (∙∙∙) and of hybrid nanofiller (DP 380) (---). The recorded PXRD patterns showed no shifting of the characteristic (001) sharp reflection at d001= 9.9 Å which is typical for non-hydrated mica-like clay. This observation assures that all treatments including surface grafting do not have any influence on the interlamellar structure. The morphological changes of the clay’s external surface were analyzed using scanning electron microscopy (SEM). In Fig. 3.10 a) a high aspect ratio of K-hect can be seen after exfoliation, having a large lateral extension and smooth surface. In comparison to O-hect (Fig. 3.10 b), the surface grafted hybrid nanofiller has a rougher and coarse surface (Fig. 3.10 c). The pattern is typical for collapsed PMMA chains in a dried state forming mushroom-like structures, as expected for the achieved grafting density. Hybrid mica-like particles 44 Figure 3.10 SEM images of a) exfoliated K-hect, b) surface morphology prior of O-hect c) Surface of hybrid nanofiller covered with collapsed polymer chains above entanglement length. *(Arrows indicate clay fragments from milling treatment, actual polymers are the smaller structures on the surface). 3.2 Mechanical properties of the clay/PMMA nanocomposites The mechanical properties of neat PMMA and PMMA/clay nanocomposites determined by tensile evaluation tests at a 5 wt% clay loading are presented in Fig.6. With incorporation of hybrid clay particles the tensile modulus of PMMA/clay nanocomposites showed a significant enhancement compared to neat PMMA. The improvement for C12-hect is already 45 % and hybrid-hect (DP 380) almost doubled the tensile modulus with an improvement of 84 %. The addition of the nanoplatelets has no significant influence on the tensile strength. The elongation at break is reduced by 35% in case of C12-hect and on the contrary shows an increase of 18% for the hybrid-hect (DP 380). Chapter 3 45 Figure 3.9 Young’s modulus (orange, striped), tensile strength (grey) and elongation at break (blue) of neat PMMA and two different clay/PMMA nanocomposites. The incorporation of high aspect ratio nanoplatelets into a PMMA matrix already leads to a significant reinforcement effect compared to neat PMMA as demonstrated by Fischer et al. in 201269. The optimization of the preparation method through solution blending eliminates any agglomerates formed in a normal melt blending process, which usually are the cause for a significant reduction in the tensile strength70. The absence of such agglomerates reduces any local stress concentration in the matrix and thus the tensile strength of the composite is not affected by the addition of the nanofiller. Significant shifts in the elongation at break behaviour were observed depending on the type of nanofiller. Usually, the addition of rigid nanofillers to a brittle matrix leads to an increase in modulus at the expense of strength, strain and toughness as embrittlement takes place71. This behaviour was observed for the incorporation of C12-hect, which increased the modulus by 45 %, but reduced the elongation at break by 35 %. The clay acts as a barrier and thus restricts the sliding of polymer chains among each other. On the contrary a significant increase in modulus and elongation at break was observed for Hybrid mica-like particles 46 the novel hybrid nanofiller, which emphasizes the importance of the addition of polymeric chains matching the polarity of the matrix onto the hybrid’s surface. This leads to a better adhesion of the nanofiller to the matrix, which generates an effective stress transfer from the matrix to the nanofiller while still participating in the sliding of polymer chains among each other and consequently leads to a significant increase in both, the modulus and elongation at break without sacrificing tensile strength. 3.3 Conclusion Surface-initiated ATRP was successfully employed to graft PMMA chains from the external basal planes of a shear-stiff, mica-like K-hect to create novel hybrid nanofiller. The employed synthetic fluorohectorite was characterized by a high aspect ratio and homogeneity of layer charge, while the multiple anchoring groups of the synthesized macroinitiator enabled a strong adhesion to the clay’s surface. The kinetic study of si-ATRP of PMMA confirmed a controlled polymerization in a linear variation. Furthermore, grafted polymer chains allow for stable dispersions of the hybrids in various organic solvents. The obtained hybrid nanofiller shows a strong reinforcing effect after being compounded into a PMMA matrix due to the synergistic effect of inherent shear stiffness and huge lateral extension of the clay itself and the optimized interface between the hybrid nanofiller and the matrix through addition of a tailored polymeric shell. Chapter 4 47 4 Chapter 4: Patchy hybrid particles based on polymer grafted montmorillonite (MMT) 4.1 Preparation of patchy hybrid particles based on MMT To compatibilize an immiscible binary polymer blend, a hybrid clay particle with homogeneous shell as used in the last chapter is not perfect. Based on the assumption, that a particle with a binary polymer shell should be drawn towards the interface if placed into a mixture of two immiscible components (polymers in a blend or liquids of different polarity) by the Pickering effect, we created disc-like particles with compartmentalized surface, bearing patches of polymer, each matching one of the components of an immiscible blend (Figure 4.1). We hypothesize, that the fraction of polymer chains with unfavourable interactions would collapse and stay close to the solid surface of the hybrid disc, whereas polymer chains with favourable interaction would protrude into the matrix. Fig. 4.1 General approach to patchy hybrid nanodiscs via grafting of diblock copolymers onto clay surface and selective collapse of chains at an interface. Patchy Hybrid Particles 48 4.1.1 Synthesis of DMAEMA based diblock copolymers via sequential RAFT polymerization Fig. 4.2 RAFT-polymerization of DMAEMA using 2-cyano-2-propyl benzodithioate (CPBDT) as chain transfer agent As shown in chapter 3 and in literature, positively charged poly(2- (dimethylamino)ethyl methacrylate) (PDMAEMA) can firmly attach to negatively charged surfaces, like those of layered silicates or colloidal silica64, 72. In this chapter, protonated PDMAEMA is incorporated into a diblock copolymer and, as it can be charged positively depending on the pH or permanently by quaternization, is used as a flexible cationic anchoring group to equip clay particles with a polymeric shell. The polymer of DMAEMA is easily accessible via RAFT polymerization and can undergo copolymerization with a range of different monomers greatly contributing to the flexibility of modifying clay surfaces with polymers of different polarities, responsiveness and sensitivities. It served as an anchor to the MMT external planes via cation exchange. The block-length was kept short and stayed in the range of 14 to 20 repeating units for all diblock-copolymers, as with increasing length of chain it becomes more likely to crosslink several platelets by the same anchoring block. Controlled radical polymerization by the RAFT-process guarantees low polydispersities of the obtained polymers which is a precondition to generate well defined microphase-seperated solutions and thus control over patch size. Also RAFT polymerization is applicable to various monomers and the synthesis of blockcopolymers is facile. Chapter 4 55 Fig. 4.8 SEM images of hybrids based on hectorite (left) and MMT (right) modified with a 1:1 (molar) mixture of D17-b-M300 and D16-b-S360 in THF By this approach it was possible to modify several types of clay (for kaolinite see chapter 5): The small particle (tactoid) size and agglomeration into band-like structures or self-supporting clay films upon drying makes it difficult to visualize the successful grafting and its patchiness in general on the surface of MMT. Since no individual platelets are visible, but rather agglomerates of several µm in diameter, it is impossible to distinguish polymer from platelets. As a proof of principle larger and stiffer synthetic clay was used. The same shear-stiff K-fluorohectorite from chapter 3 was modified via “grafting onto”. Qualitative analysis of SEM images showed less grafting density and more inhomogeneous surface coverage (Fig. 4.8) as compared to experiments with polymeric shells attached via a “grafting from” technique (Fig. 3.10). While SEM analysis of the synthetic clays delivered evidence for successful grafting of polymer onto the clay surface, it was not possible to prove grafting of both polymer species in this way. 1H-NMR analysis was used to determine whether preferential adsorption of either of the polymers occurred. Ratios between peaks of the 5 aromatic protons in the PS block (δ=6.4-7.2 ppm) and the 3 protons of the CH3Oester group of the PMMA block (δ=3.6 ppm) where compared, between the solution used to modify the clay and the supernatant after centrifugation for the first Patchy Hybrid Particles 56 time. Both ratios of PS to PMMA were roughly the same before and after modification (deviation of less than 10%), which leads to the assumption that adsorption is primarily controlled by the PDMAEMA block, which is of comparable length in all cases, and there is no significant preferential adsorption depending on the species of the second block with the given species. -100 0 100 200 300 400 500 600 700 800 80 85 90 95 100 Weight [%] Temperature [°c] Fig. 4.9 TGA results showing the temperature dependent weights of different hybrid particles: unmodified MMT (solid gray), D16-b-S360 modified MMT (solid black), D17-bM300 modified MMT (striped gray), patchy hybrid MMT modified with both aforementioned diblock copolymers (solid red) To quantify the amount of surface-bound polymer a thermogravimetric analysis (TGA) was performed (Fig. 4.9): unmodified MMT showed a total mass loss of 2 %, while all hybrids showed a mass loss of around 12-15 % in the most relevant region (200-500 °C), which gives a calculated grafting density of 0.5 units of DMAEMA per nm² for completely exfoliated MMT using the following formula: Where; ρ is the grafting density (initiator per nm2), MD is the average molar amount of DMAEMA units per g of hybrid, NA is the Avogadro constant and SA is the surface Chapter 4 57 area of 68 m2/g. The calculated value is around half of the cation exchange capacity (CEC) value provided by the company for the employed MMT (PGV) in a theoretical delaminated state and. The discrepancy can be explained by incomplete exfoliation. 4.2 Mechanical properties of the clay/PMMA/PS nanocomposites 4.2.1 Preparation of clay/PS/PMMA blends Dually modified PS/PMMA-patchy hybrid particles, with the polymer patches on each side should be able to selectively collapse or extend to match the surface tension of the polymer phase they reside in, were tested as compatibilizers in films of PS/PMMA blends cast from THF, a good solvent for both polymers and a good dispersant for the hybrids. Located at an interface, each side should collapse the incompatible polymer chains and extend the compatible ones into the matrix, forming a Janus-like structure. For comparison and to be able to estimate the effect of the patchy character additional blends with homogeneously modified clay (either PS or PMMA as shell) and blends with unmodified MMT were prepared by solvent casting under the same conditions. A PS/PMMA ratio of 1:2 (wt/wt) was chosen. The Flory-Huggins parameter for a blend of this molecular weight is χSM = 0.041 at 20 °C74 indicating its incompatibility. The samples for transmission electron microscopy (TEM) were prepared by casting the polymer solution with dispersed clay into a glass vial followed by slow drying and microtome cutting. The difference between PS and PMMA is clearly visible in the TEM images (Fig. 4.9) even without selective staining. Dark grey areas result from stronger electron contrast of PS and light grey areas from PMMA, which is more easily damaged by the electron beam. The clay particles appear even darker, almost black, and their profile shapes are clearly visible due to their strong contrast, the completely white regions are holes in the film introduced during ultra microtome cutting. Patchy Hybrid Particles 58 We are aware of the fact that solvent evaporation will trap the system in a metastable state and such prepared films can only show a qualitative aspect of compatibilization achieved by our hybrid particles. For pure PS/PMMA blend films of comparable molecular weight without compatibilizer, it is known, that large (several µm in diameter) spherical domains of the minority phase inside a matrix formed by the majority phase result from phase segregation51. From literature on classical organoclays (e.g. Cloisite 20A), it is known, that modification with simple alkyl chains will lead to dispersion only in the PMMA phase of a PS/PMMA blend and formation of clusters in PS homopolymer blends.75 Analysis of blends mixed with PS-grafted MMT (modified with D16-b-S360) showed that the hybrids without exception stay in the PS phase or assemble at the interface, though no strong tendency for interfacial interaction is observed (Fig. 4.10 a). Similar results are obtained for hybrids based on PMMA-grafted MMT (modified with D17-b-M300). Both hybrids stay in the phase of the polymer their surface is modified with. Though being the minority phase, we can find prominently huge domains of PMMA, filled with randomly oriented hybrid platelets (Fig. 4.10 b). The observed polygonal shape of polymer domains can be attributed to increased viscosity of the filler-rich phase rather than interfacial activity, as there are only a few platelets directly assembled at the interface. Fig. 4.10 TEM images of 2:1 (wt/wt) PS/PMMA blend films a) showing D16-b-S360 modified hybrids in the PS phase only, b) D17-b-M300 modified hybrids in PMMA phase. The fraction of the added hybrid is 5 wt% and the scale bar represents 1 µm. b ) a ) Chapter 4 59 Blends compatibilized with patchy hybrid particles (shell based on modification with a 1:1 (molar) mixture of D16-b-S360 and D17-b-M300) show a completely different structure. Hybrids are randomly oriented and distributed over the whole blend in both phases and the interface (Fig. 4.11 a). The domain size is reduced compared to blends compatibilized with unmodified and single-polymer-species modified clay. Domain shapes are completely irregular (Fig. 4.11 b), following the shape of platelets where they reside in the interface. As not all of the interfacial area is covered, the hybrids act as a physical barrier and increase viscosity, preventing the formation of large spherical domains (Fig. 4.11 b). Regarding the structure of the polymeric shell of the patchy hybrids, we expect three different cases: a hybrid residing in the PS phase will have its PMMA chains collapsed near the clay surface, screened by extended PS chains, interacting with the matrix. Hybrids in the PMMA phase will show opposite behaviour, where PMMA chains are extended and PS chains are collapsed. In an interface, a hybrid would show both aforementioned behaviours at once, according to the polymer phase the respective side is facing. Fig. 4.11 TEM images of 2:1 (wt/wt) PS/PMMA blend films a) showing patchy hybrids with PMMA and PS shell in the PS phase, the PMMA phase and the interface, b) close up. The fraction of the added patchy hybrids is 5 wt% and the scale bar represents 1 µm. As the blend morphology is created under influence of THF as a solvent and is fixated only after slow evaporation of the solvent, the shell of each hybrid has enough time a ) b ) Patchy Hybrid Particles 60 and is flexible enough to extend and collapse its polymer chains to match the interfacial tension / surface energy of the polymer phase it resides in. Compared to a pure Janus or Pickering particle it can find its energetic minimum not only at an interface, but inside one of the phases as well. 4.2.2 DMA of the blends From the variety of DMA data, the storage modulus, E', is plotted in dependence of the heating temperature, T. (Fig. 4.12). 20 40 60 80 100 120 140 160 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 Storage Modulus E' [GPa] Temperature [°C] Fig. 4.12 DMA results showing the temperature dependend storage moduli of 2:1 PS:PMMA blends: uncompatibilized pure blend (solid black), blend compatibilized with 5 wt% alkylammonium-modified PGV (dotted gray) and compatibilized with patchy hybrids of PGV (solid red). The storage modulus of a pure, uncompatibilized 2:1 PS/PMMA blend is 3.2 ±0.1 GPa at 35 °C, compatibilization with a C12-alkylammonium modified MMT resulted in a storage modulus increase of ~7 % to 3.4 ±0.1 GPa, while compatibilization by a patchy hybrid resulted in an increase of 17 % to 3.9 ± 0.1 GPa (Fig. 4.12). Thus, a significantly higher degree of reinforcement is observed by compatibilizing the blend with patchy hybrids than with simple organoclay. Chapter 4 61 4.3 Conclusion The experimental data presented confirm the successful grafting of different patches of two diblock copolymer species onto the surface of clays from the smectite group, creating novel patchy hybrid particles. We used a versatile and simple approach to synthesize functional diblock copolymers which consist of a short anchoring block to attach to the clay surface and a longer block adding the desired polarity or functionality. A combination of two or more of different diblock copolymers grafted onto clay can lead to a dynamic shell which is able to adapt to environments of different polarity and even show interfacial activity in immiscible polymer blends. The dispersion in both phases and the interface of an immiscible polymer blend indicates the relevance of the theoretical concept of selective polymer chain collapse and extension in the polymeric shell of the particles and leads to a reinforcing effect, shown in an increase of up to 17 % in Young’s-modulus. While the presented method did not lead to Janus-type behaviour, characterized by preferable presence in the interface, the method should allow further interesting combinations of different diblock copolymers to functionalize the patchy shell of the hybrid particles, opening up new fields unreachable with homogeneously modified particles. Hybrid Janus Particles 62 5 Chapter 5: Hybrid Janus particles based on polymer modified kaolinite This chapter is the result of the cooperation with Dunja Hirseman and major parts of this chapter were published in Polymer 2013, 54, 1388-1396 under the title: “Hybrid Janus particles based on polymer-modified kaolinite” by Stephan Weiss, Dunja Hirsemann, Bernhard Biersack, Mazen Ziadeh, Axel H.E. Müller, Josef Breu, Text and respective figures are adapted and reprinted with permission. Copyright 2013 Elsevier 5.1 Preparation of hybrid janus particles based on kaolinite The investigations of randomly compartmentalized, patchy hybrid particles based on MMT showed interfacial activity too low to attach to the interphase between PS and PMMA in an immiscible blend of both. One of the reasons we found was the flexibility of the particle shell, with immiscible patches collapsing and miscible patches extending into the matrix, thus keeping the hybrid particle flexible enough to adapt to its surroundings, independent of where it was located. The next logical step was to create disc-like Janus hybrid particles, with a shell comprised of exactly two chemically distinct, oppositely located compartments, increasing intrinsic polarity of the hybrid and targeting the interphase of the blend as the energetically most favourable location. 2:1 Smectites, like hectorite and montmorillonite are not well suited to create Janus particles, as it is very tedious to selectively address each of their external basal planes individually. However, due to its polar crystal structure, the two opposing external basal planes of kaolinite, TS and OS are truncated by distinct functional groups and may selectively be modified by simple cation exchange and covalent grafting via catechol groups, respectively, making it the perfect base for a disc-like hybrid Janus particle. As an example we chose poly((2dimethylamino)ethyl methacrylate)-block-polystyrene (D16-b-S115) and poly(3-(2,3dihydroxy-benzoyloxy)propyl methacrylate)-stat-(methyl methacrylate)) (PCM). Chapter 5 63 HO OH O O OO *3 O N O20 O O * 90 stat block * 115 H (H2C)3 Fig. 5.1 Structure of poly(3-(2,3-dihydroxybenzoyloxy)propyl methacrylate)-stat-(methyl methacrylate) (PCM) (top) and poly(2-(dimethylamino)ethyl methacrylate)-blockpolystyrene (D16-b-S115) cations (bottom). PCM is a statistical copolymer, while D16-b-S115 is a block copolymer (Fig. 5.1). Consequently, both modifiers interact with the kaolinite basal planes in a different manner (Fig. 5.2). PCM will likely be close to the OS, forming short loops or lying flat. In contrast, the polystyrene block might arrange brush-like on the TS of the kaolinite. The resulting Janus particles are tailored for compatibilizing PS-PMMA or industrially more relevant PPE–SAN (poly(2,6-dimethyl-1,4-phenylene ether) (PPE), poly(styreneco-acrylonitrile) (SAN)) blends50, 51. Hybrid Janus Particles 64 Fig. 5.2 Schematic picture of a) pristine kaolinite, b) modified with D16-b-S115 on the tetrahedral surface (TS), c) further modified with PCM on the opposite octahedral surface (OS) and d) embedding of the final hybrid particle at the interface in a PSPMMA blend. 5.1.1 Synthesis of the copolymers PCM and D16-b-S115 Synthesis of the catechol-modified poly(methyl methacrylate) copolymer (PCM) Initially, a suitable catechol modified methacrylate monomer was prepared for copolymerization with methyl methacrylate (MMA). 3,4-Dibenzoxy-(3hydroxypropyl)benzoate 2 was obtained from 3,4-dibenzoxybenzoic acid 1.57 Reaction of 2 with methacryloyl chloride gave the mixed diester 3 (Scheme 5.1). OBn OBn CO2H OBn OBn O O OH OBn OBn O O O O 12 3 (i) (ii) Scheme 5.1 Synthesis of the catechol monomer. Reagents and conditions: (i) SOCl2, CH2(CH2OH)2, Et3N, THF / DCM, r.t., 5 h, 51%; (ii) CH2C(CH3)COCl, Et3N, DCM, r.t., 3 h, 67%. Monomer 3 was copolymerized with a 30-fold excess of MMA by free radical polymerization using AIBN as initiator and dodecanethiol as transfer agent to gain control and reduce molecular weight, giving copolymer 4. 1H NMR spectroscopy and Chapter 5 71 the statistical copolymer PCM stays comparatively close to the surface. Moreover, the hydrated inorganic cations residing at unmodified TS contribute to an efficient electrostatic stabilization of PCM-kaolinite in water. Alternatively, the stability of PCM-kaolinite might be explained by the formation of sandwich structures as depicted in Fig. 6a (blue framed inset). For such sandwich structures only the hydrophilic TS are exposed to the aqueous media. Such polymer-bridged sandwich structures would not be expected for D16-b-S115-kaolinite because the long PSbrushes will hamper dimer-formation sterically. In THF (Fig. 5.6b) both, the PCM-kaolinite (Fig. 5.6b, blue) as well as the D16-b-S115kaolinite (Fig. 5.6b, red), showed good stability which in turn is comparable to that of the dually modified D16-b-S115/PCM-kaolinite (Fig. 5.6b, black). This suggests that even the short PCM loops at the OS were able to assure a good stability in THF and expectedly the longer chains perform as well. Moreover, it would be expected that sandwich structures of D16-b-S115and PCM-kaolinite are formed (Fig. 5.6b, red and blue squares). Fig. 5.6 Integrated transparency of 0.25 wt% suspensions in a) water and b) THF of pristine kaolinite (pink), PCM-kaolinite (blue), D16-b-S115-kaolinite (red) and D16-b-S115/PCMkaolinite (black) under time dependent centrifugal forces of 300 rpm, 600 rpm, and 900 rpm. In summary, the stabilities in waterand THF-suspensions observed for the different kaolinite samples are in line with a specific modification of TS and OS by D16-b-S115 and PCM, respectively, and strongly support the Janus character of D16-b-S115/PCMkaolinite. Hybrid Janus Particles 72 5.2 TEM Analysis of the morphology of the hybrid-kaolinite / PMMA/ PS nanocomposites Dually modified D16-b-S115/PCM-kaolinite, where the surface tensions of the opposing basal surfaces are fine-tuned to match PS and PMMA, respectively, was tested as compatibilizer in films of incompatible PS-PMMA blends cast from THF, similar to the process used in chapter 4.3.4. For comparison and to be able to estimate the effect of the Janus character in excess of the pure Pickering effect additional blends with unilaterally modified and blends with unmodified kaolinite were prepared by solvent casting under the same conditions. A PS/PMMA ratio of 1:2 (wt/wt) was chosen. The samples for transmission electron microscopy (TEM) were prepared by casting the polymer solution with dispersed clay into a glass vial followed by slow drying and microtome cutting. All images are unstained. Dark grey areas result from stronger electron contrast of PS and light grey areas from PMMA. The kaolinite particles appear even darker, almost black, and their shapes are clearly visible due to their strong contrast, the completely white regions are holes in the film, introduced during ultra microtome cutting. Similar to what was the case with experiments conducted in chapter 4.3.4 we are aware of the fact that solvent evaporation will trap the system in a metastable state and such prepared films can only show the qualitative aspect of compatibilization achieved by our hybrid particles. To determine industrially relevant quantitative effects, like mechanical properties of compatibilized blends, it is necessary to conduct extrusion experiments and mechanical tests. For pure PS/PMMA blend films of comparable molecular weight that contain no compatibilizers, it is known, that large (several µm in diameter) spherical domains of the minority phase inside a matrix formed by the majority phase result from phase segregation.51 With unmodified kaolinite we observe macrophase separation (Fig. 5.7a). No dispersion is achieved, only large aggregates of clay particles can be found, separating from the matrix, trapped inside the polymer phase where they happen to be upon drying (Fig. 5.7a). This behaviour is expected due to the clay’s hydrophilic Chapter 5 73 nature (charged on one side and polar hydroxy groups on the other side) and the fact that it does not form stable dispersions in THF (and thus is hard to disperse in the Polymer mixture to start with). In another experiment we modified the TS of kaolinite with dodecylamine, which is comparable in structure to the alkyl ammonium salts used to prepare commercial organoclay like the widely used Cloisite 20A. Here we can observe clustering in the PMMA phase (Fig. 5.7b). Like in the Lumifuge experiments we expect the kaolinite to form sandwich structures with the alkyl chains of the organophilized TS aggregated via hydrophobic interactions in the inside and the polar OS at the outside or the other way round. In none of the cases we could find a surface which has high compatibility with any of the polymer phases and thus is not dispersed homogeneously. This observation is in good agreement with literature about other organoclay (e.g. Cloisite 20A), which disperses only in the PMMA phase of a PS/PMMA blend and forms strong clusters in PS homopolymer blends.75 Obviously modification of one side is not sufficient to align the particles at the interface under these conditions. Hybrid Janus Particles 74 Fig. 5.7 TEM images of 3:7 (wt/wt) PS/PMMA blend films. a) with pristine kaolinite, b) with unilaterally organophilized kaolinite, c) with D16-b-S115/PCM-kaolinite, and d) close up at an inter-face. The fraction of the clay is 5wt% and the scale bar represents 500 nm. In contrast, in the film prepared with the Janus-type D16-b-S115/PCM-kaolinite (Fig. 5.7c, d) the kaolinite particles are assembled exactly at the interface between both polymer phases. A nearly full coverage of the interface by compatibilizer is realized. Due to the Janus character of the modified kaolinite the interfacial tension of the platelets in the blend interface should be very low. Therefore, the assembly of the particles at the interface is energetically highly favored. As a consequence, the PS domains are no longer spherical but appear polygonal following the shape of the clay platelets (Fig. 5.7c, d). Chapter 5 75 5.3 Conclusion A synopsis of all experimental data presented confirms that the external basal planes of kaolinite platelets can be selectively addressed by polystyrene and PMMA, similar to what has been studied in detail for the molecular modification with Ru(bpy)32+ and a phosphorous-labelled catechol (3-Diphenylphosphinyloxypropyl-3,4dihydroxybenzoate) in literature21. Janus-type D16-b-S115/PCM-kaolinite platelets obtained by dual modification showed interfacial activity in a solvent-cast PS/PMMA blend film. Obviously, blend preparation via melt extrusion would be advantageous. Work in that direction is on the way but the results obtained by solvent-casting already give a strong indication on the efficiency of the hybrid Janus particles as blend compatibilizers. While the presented work represents a proof of principle, the approach is, of course, highly modular and should allow for facile and affordable fine-tuning of appropriate compatibilizers for a broad range of blend systems. The intrinsically polar structure of kaolinite serves as versatile core of these Janus platelets. Adjustment of the surface tensions of both basal planes can easily and selectively be tailored for each specific blend composition. Moreover, particle size distribution and morphology (aspect ratio) may be varied over a wide range by the choice of the kaolinite source. Furthermore, that concept is not restricted to kaolinite but can be transferred to any other inorganic material which possesses a polar crystal structure and where opposing crystal faces are truncated by chemically different functional groups, paving the way to selective modification. 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