Letter pubs.acs.org/macroletters
Multiple Hydrogen-Bond Array Reinforced Cellular Polymer Films from Colloidal Crystalline Assemblies of Soft Latex Particles Yunhua Chen,† Samuel T. Jones,† Ian Hancox,† Richard Beanland,‡ Edward J. Tunnah,† and Stefan A. F. Bon*,† Departments of †Chemistry and ‡Physics, University of Warwick, Coventry CV4 7AL, United Kingdom S Supporting Information *
ABSTRACT: Waterborne polymer films made from soft polymer latex dispersions generally suffer from deterioration of chemical resistance and physical barrier properties under high humidity conditions and upon solvent exposure. Here we demonstrate the fabrication of robust polyhedral cellular polymer films from poly(methyl methacrylate-co-butyl acrylate) latexes, which were made by emulsion polymerization using a 2-ureido-4-pyrimidinone (UPy) functional methacrylate comonomer. Multiple hydrogen bond (MHB) arrays provided by UPy groups arrest the film formation process thereby creating a cellular reinforcement. The cellular polymer films exhibit impressive physical and mechanical properties. Upon solvent exposure, the films show colloidal crystalline-type Bragg diffraction features and do not suffer excessive and deteriorative uptake of water and, more remarkably, can absorb high amounts of organic solvents, thereby turning into an organogel with preservation of shape, up to a 14-fold volumetric swelling ratio of the polymer films in case of chloroform. film. Indeed, such “cellular” films have been made from soft polymer latexes functionalized with a substantial amount of carboxylic acid groups7,8 or hydrogen bonding ureido groups.9 These films, however, suffer from considerable water uptake in humid environments as a result of the hydrophilic nature of the carboxylic acid or ureido network. An alternative approach toward cellular polymer films was reported by Creton et al.10 and coined a “soft−soft nanocomposite” in which a percolating cross-linked phase was obtained through covalent bond formation between ketone carboxyl shell functionalized latex particles and a water-soluble hydrazide. Weaver and co-workers recently reported on the reversible fabrication of supracolloidal structures made from emulsion droplets11 and porous scaffolds constructed from polymer particles12 accomplished through hydrogen-bond directed assembly of carboxylic acid and ethylene oxide surface functionalized colloidal building blocks. Here we demonstrate the fabrication of robust polyhedral cellular polymer films that do not suffer excessive and deteriorative uptake of water and resist destruction and show impressive swelling characteristics upon exposure to a variety of organic solvents. The key to these enhanced properties is the use of a strongly self-complementary multiple hydrogen bond array (MHB) conveniently introduced in the form of a functionalized methacrylate-based comonomer (illustrative monomer unit, see Figure 1) through emulsion polymerization.
n the area of polymer films for the coatings and adhesives industries there is a longstanding environmental drive to switch from solvent-based to waterborne systems, thereby greatly reducing the VOC (volatile organic content), HAPs (hazardous air pollutant) emissions and potentially the overall carbon footprint.1,2 Whereas advances in performance characteristics of waterborne coatings have been made in recent years, key drawbacks such as deterioration of chemical resistance and physical barrier properties under high humidity conditions and upon solvent exposure persist. The traditional objective in waterborne polymer films formed from soft polymer latex dispersions (soft meaning the polymer is above its glass transition temperature) is to ultimately achieve a coherent homogeneous film.3,4 Failure to achieve this desired morphology leaves interstitial weak points within the film, lowering the overall barrier properties and making the film vulnerable to chemical breakdown. The film formation process typically goes through three characteristic stages: (1) concentration and ordering/packing of the individual latex particles, (2) direct contact between and subsequent deformation of the particles into polyhedral cells, and (3) disappearance of the boundaries between individual particles through interdiffusion of polymer chains.5,6 The (surface) characteristics of the individual latex particles play a key role in the process. One can envisage that soft polymer particles with functional groups that are able to undergo a strong attractive interaction act like a molecular type of Velcro, thereby having the ability to arrest film formation in stage two. This would lead to a closed-cell polyhedral morphology of the
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© 2012 American Chemical Society
Received: March 16, 2012 Accepted: April 16, 2012 Published: April 19, 2012 603
dx.doi.org/10.1021/mz300126u | ACS Macro Lett. 2012, 1, 603−608
ACS Macro Letters
Letter
generic procedure reported by Meijer and co-workers. Noteworthy is that isolated solid PEGMA-UPy monomer (2) after evaporation of chloroform was hard to redissolve. We observed the same issue with UPy functionalized α,ω-telechelic “soft” polymer chains, made from both hydroxyl-functional Kraton and poly(dimethylsiloxane) macromolecules. UPy monomer (2) was therefore kept as a solution in chloroform and used as such in the emulsion polymerizations. Low-solids content (ca. 5.2 wt %) “soft” polymer latexes were prepared by soap-free emulsion polymerization using a 50:50 molar ratio (44:56 wt % ratio) of methyl methacrylate (MMA) and butyl acrylate (BA) both in absence and in presence of approximately 1.5 mol % (based on total amount of monomer) of UPy monomer (2, used as a 0.12 g mL−1 solution in chloroform). The 44:56 wt % mixture of MMA and BA was chosen to warrant a low glass transition temperature with a predictive value (Fox equation using Tg(PMMA) = 105 °C and Tg(PBA) = −54 °C) of −4 °C. The polymer latexes were thoroughly dialyzed to remove impurities and in case of the UPy functionalized ones also to remove the chloroform. Monodisperse particle size distributions were obtained for both the reference bare “soft” latex particles and the UPy functionalized latex particles (dispersities of 0.020 and 0.007, respectively) with z-average diameters of 378 and 330 nm correspondingly, as measured by dynamic light scattering (DLS) in water (Figures S4 and S5, Supporting Information). The purified latexes were stable without any observed aggregation up to background electrolyte concentrations of NaCl of about 0.1 M. An approximate value for the number of UPy groups per latex particle can be calculated (assume polymer density is 1.1 g cm−3, average molar mass of (2) = 819.3 g mol−1) and yields approximately 1.5 million groups for the number of UPy groups per polymer latex particle, which agrees with an overall concentration of 0.13 M of UPy groups in each particle. To obtain an estimate for the overall amount of UPy groups incorporated into the latex particles we carried out UV−vis absorption measurements. UV−vis spectra were recorded on the films casted from the two respective polymer latexes, one of which contained the UPy functionality. As calibration standards we used solutions of UPy monomer (2) in chloroform (Figures S6 and S7, Supporting Information). We chose a wavelength of 250 nm to determine the overall concentration of UPy groups, as in the region 240−255 nm the absorption behavior is nearly independent of the tautomeric conformation and assembly behavior of the UPy group31 (note that for values
