Coumarin based light responsive healable waterborne polyurethanes

Coumarin based light responsive healable waterborne polyurethanes Robert H. Aguirresarobe, Loli Martin*, Nora Aramburu, Lourdes Irusta and Maria Jose Fernandez-Berridi POLYMAT, Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country, UPV/EHU, Spain. * Macrobehavior-Mesostructure-Nanotechnology and NMR SGiker service of UPV/EHU. Polytechnic School, UPV/EHU. Spain. Email address: [email protected] Abstract Waterborne polyurethanes containing coumarin as chain extenders were successfully synthesized. Taking advantage of the photoinduced [2+2] cycloaddition reaction of coumarin moieties, this work evaluated the healing abilities of these materials. The photochemical process was analyzed by UV spectroscopy and the cycloaddition reaction kinetics was studied. In addition, the healing abilities of the synthesized waterborne polyurethanes were determined by means of scratch refilling studies and strain-stress tests. The irradiation conditions as well as the polymeric chain mobility on the fracture and the capacity to generate polymer networks by coumarin linkages were demonstrated to be the main factors affecting the healing process. The behavior of these systems was also determined under the direct irradiation of sunlight. Keywords Coumarin, waterborne polyurethanes, sunlight, light responsive, healing. 2 Highlights Synthesis of healable waterborne polyurethanes with coumarin moieties incorporated in the polymeric backbone. Evaluation of coumarin reversible photodimerization reaction by UV spectroscopy and determination of the cycloaddition reaction kinetics. Dependence of the radiation conditions on the scratch filling abilities of the polymeric coatings. Influence of the polyurethane physical properties on the healing efficiency of the polymeric systems. Study of coumarin cycloaddition reaction under sunlight conditions 3 1. Introduction Polyurethanes are polymers that can be easily modified by the introduction of different monomer structures. As a consequence, their physical properties can be tailored to those required in the final product. Hence, they have been widely used in the development of intrinsic healable materials [1–3]. The intrinsic healing strategy is based on the incorporation of healing active elements in polymer backbones so that the chemical nature of these elements determines the autonomic or non-autonomic nature of the healing process. However, the introduction of active monomers into the polyurethane structure is not easy. On the one hand, these elements should be capable of reacting with isocyanate groups and consequently the selected active monomers should have hydrogen-containing nucleophiles in their structure. On the other hand, there should be a sufficient amount of active elements on the surface of the damage without compromising the physical properties of the polyurethane. Therefore, it is necessary to reach a balance between the healing abilities and the desired physical properties of the final product. It is well known that coumarin derivatives show the reversible photoinduced [2+2] cycloaddition reaction [4,5], which enables the joining or separation of the coumarin molecules in a controlled way, selecting the appropriate irradiation conditions. Thus, according to Scheme 1, when irradiating coumarin molecules with UV light below 300 nm, the [2+2] cycloaddition reaction takes place and a cyclobutane is formed between two molecules. In contrast, the irradiation of these dimers with UV light at wavelengths higher than 300 nm provokes the rupture of the cyclobutane ring, leading to the restoration of the original coumarin structures. Scheme 1. Coumarin photoinduced reactions 4 As reported in literature, coumarin monomers have been incorporated into polyurethane chains both as chain ends and/or chain extenders, giving the possibility of reversibly increasing the molecular weight of the polymer chains or crosslinking samples [6–8]. Taking advantage of this process, in this work the incorporation of coumarin monomers as chain extenders in waterborne polyurethanes is described. Lightresponsive properties of coumarin monomers have demonstrated to provide repairing abilities to polymer structures [9–12]. However, to the best of our knowledge, this is the first time that polyurethanes with optically healable abilities have been synthesized using a methodology which leads to stable waterbased dispersions. This strategy offers the possibility of using environmentally friendly materials with healing properties in a wide range of applications such as coatings for automobiles, textiles, furniture and wood. 2. Experimental 2.1. Materials Isophorone diisocyanate (IPDI), 2-bis(hydroxymethyl) propionic acid (DMPA), 1,4-butanediol (BD), polypropylene glycol (PPG) (Mn 1000 g mol-1), triethylamine (TEA), dibutyltin diacetate (DBTDA), acetone, phloroglucinol, ethyl acetonate, 1,4 dioxane, concentrated sulfuric acid, potassium carbonate, dimethylformamide and 2-bromoethanol were purchased from Sigma-Aldrich Chemical Corporation. All materials were used as received. 2.2. Synthesis of 5, 7-bis(2-hydroxyethoxy)-4-methylcoumarin (DHEMC) The synthesis of DHEMC was carried out in two separate reactions, following the procedure described in literature [13,12]. In the first one, 5,7-dihydroxy-4methylcoumarin (DHMC) was obtained from the reaction between phloroglucinol and ethyl acetoacetate in acid conditions. In the second step, the final product was obtained by the modification of the phenolic groups of DHMC with bromoethanol. 2.3. Synthesis of waterborne polyurethane The synthetic way to obtain PPGDHEMC polymer is shown in Scheme 2. 5 Polyol, Internal emulsifier (DMPA) and TEA were poured together with DBTDA (800 ppm) and acetone (36 g) into a flask reactor. When the reaction temperature reached 56 ºC, IPDI was added and the reaction was maintained for 3 h. In a further step, the polymer chains were extended using the appropriate BD amount, leaving some free isocyanate groups. This reaction was carried out for one additional hour. Afterwards, DHEMC was added to the reaction mixture in order to obtain stoichiometric ratio of NCO/OH groups. The reaction was stopped when the infrared absorbance of the NCO groups (around 2200 cm-1) was negligible. Table 1 summarizes the formulations used in the different reactions. Scheme 2. Synthetic pathway to obtain waterborne PPGDHEMC polyurethanes Table 1 Amount of reagents used in the synthesis of different PPGDHEMC Sample Polyol DMPA IPD TEA BD Coumarin monomer mmol mmol mol(%) (wt%) PPG1DHEMC 22.5 11 56.5 15 21.5 1.5 1.32 1.08 PPG5DHEMC 22.5 11 56.5 15 14.5 8.5 7.52 5.91 PPG10DHEMC 22.5 11 56.5 15 6 17 15.04 11.38 PPG15DHEMC 22.5 11 56.5 15 0 23 20.35 14.98 In order to obtain the waterborne polyurethane dispersion, the solid content of the reaction mixture was adjusted to 60% using acetone. The emulsification 6 process was carried out at room temperature and the mechanical stirring was raised to 400 rpm to help the dispersion process. 25 g of the reaction mixture was incorporated into the reactor and water (60 g) was added drop-wise at 1 mL/min. After water addition, the stirring was kept at the same rate for an additional 30 min. Finally acetone was removed using distillation equipment at 30 ºC. The solid content of the resulting dispersion was 20%. 2.4. Evaluation of healing properties The evaluation of the healing properties was determined taking into account two different applications of these materials, polymer coatings and films. For polymer coatings, crack refilling experiments were performed using optical microscopy for different irradiation conditions. The quantification of the healing process of polymer films was established by stress-strain tests irradiating diecut samples at 365 nm for 24 hours, 12 hours in each side of the film, in order to maximize the crosslinking degree and to provide them with structural integrity. Three different independent healing events were performed cutting the specimens perpendicularly to the applied stress and subsequent fractures were cut at 45º degrees with respect to the first. After each fracture the specimens were repaired under the appropriate healing conditions. 2.5. Instrumentation 1H liquid Nuclear Magnetic Resonance (NMR) spectra were obtained in a Fourier Transform Bruker 300 MHz spectrometer (model Avance 300 DPX). Dynamic Light Scattering (DLS) measurements were carried out using 90Plus (Brookhaven) Particle Size Analyzer in order to obtain the diameter of the particles, Dp, as an average of eight measurements. The reversible photoreaction was monitored by UV-vis spectrometry in a Helios Omega UV-Vis spectrometer (Thermo Scientific). The measurements were made from films obtained by casting a 0.1wt% acetone solution onto the outside wall of a quartz cell. The samples were irradiated both at 365 nm (0.8 mWcm-2) and 254 nm (0.16 mWcm-2), using a Vilber Lourmat VL-6LC lamp and the UV-Vis spectra were recorded at different times. Tensile tests were carried out on a universal Instron 5569 tensile test machine. A load cell of 100 N was used, the initial distance between clamps was fixed at 30 mm and the experiments were 7 performed using a crosshead-speed of 20 mm/min. The filling abilities of different healable systems were evaluated in a Nikon Eclipse E6000 optical microscope equipped with a temperature controller unit Mettler Toledo FP90. AFM images were obtained in a Dimension ICON Nanoscope V (Bruker) device, using TESP-V2 tips (f=320 kHz, k=40 N/m). The images were obtained in 1 μm x 1 μm frames. 3. Results and Discussion 3.1. Characterization 1H NMR was used to characterize the final product (Figure 1). As can be seen, all signals have been correctly assigned to the corresponding protons. The region between 3.5 and 4.5 ppm is particularly significant as it confirms the incorporation of the coumarin group in the polyurethane chain. Table 2 summarizes the particle size values for all formulations, where it can be seen that the particle size increases with coumarin content. In addition, all formulations were stable for at least one week. Figure 1. 1H NMR of PPG10DHME system 8 Table 2 Particle size of PPGDHEMC Coumarin % wt 1.5 5 10 15 Dp (nm) 50 49 77 115 3.2. Reversible photoinduced cycloaddition reaction UV visible spectrometry was used to follow the [2+2] cycloaddition reaction of coumarin moieties and the dimerization degree (p) was calculated from the absorbance at 320 nm as a function of time, using the methodology reported elsewhere [9,12,14], and described by equation 1, where At corresponds to the absorbance at 320 nm at time t and A0 the original absorbance at 320 nm before irradiation at 365 nm. Dimerization degree = 1-At/A0 ( 1 ) Figure 2 shows the dimerization degree as a function of time for different coumarin content PPGDHEMC samples for the first irradiation cycle. Figure 2. First cycle kinetic plots for PPGDHEMC samples containing different amounts of coumarin. As can be seen the reaction progresses at the same velocity independently of the DHEMC content during the first 45 minutes. However, at longer times a 9 slight dependence of the reaction evolution on the coumarin concentration is observed. These results clearly differ from those obtained previously for PPGHEMC systems where the coumarin moieties were inserted at the polymeric chain ends [15]. The difference of the phase-separated morphology of both systems can be responsible for the observed behavior. Figure 3 shows the AFM phase images of PPGHEMC and PPGDHEMC samples containing different amounts of coumarin, before being exposed to 365 nm UV light. Figure 3. AFM Phase images of polyurethanes containing different amounts of coumarin as chain ends (PPGHEMC) and chain extenders (PPGDHEMC) As can be seen, PPGDHEMC systems present a less homogeneous morphology compared to PPGHEMC samples, with brighter and darker regions attributable to polyurethane hard and soft domains [16,17]. Taking into account that coumarin moieties tend to aggregate in the hard domains, the photochemical process occurs preferably in these domains, although the presence of coumarin outside these domains should not be dismissed. In order to better understand the influence of the coumarin concentration, a quantitative analysis of the reaction kinetcs was performed, following the same procedure carried out for coumarin end-capped polyurethanes (PPGHEMC) [15]. In contrast to PPGHEMC systems, which fitted to a pseudo-second order kinetics, coumarin chain extended polyurethanes did not appropriately fit any simple kinetics laws. However, taking into consideration the phase separated 16 The evolution of the cycloaddition reaction was followed by UV spectroscopy and Figure 8 shows the reaction kinetics plots. The cycloaddition reaction in March took place in 3 hours whereas nearly 80 per cent of conversion was reached in 30 minutes when the sample was exposed in July. This fact is directly related to the UV-A light intensity measured in both experiments: 0.59 mW/cm2 (March) and 3.1 mW/cm2 (July). However the maximum conversion was similar in both cases. 4. Conclusions Coumarin moieties were successfully introduced as a chain extender into waterborne polyurethane backbones (WPUs) at three different concentrations. The reversible photodimerization reaction of these moieties was analyzed by UV spectroscopy and the kinetics of the cycloaddition reaction was determined. Due to the phase-separated morphology of the systems, the reaction kinetics was independent of coumarin content at least during the first stages. The incorporation of the coumarin monomer provides healing ability to WPUs. The healing efficiency shown by these systems depends on their physical properties. 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