Nanofiller Modification of Phase Stability in PS/PMMA Blends
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Nanofiller Modification of Phase Stability in PS/PMMA Blends G. Ji, F. Clement and E.P. Giannelis Department of Materials Science and Engineering Cornell University, Ithaca, NY 14853
ABSTRACT The effect of nanoparticles on the phase stability of an immiscible PS/PMMA blend was studied in thin film samples. Using AFM and selective dissolution we find that nanoparticles affect the domain evolution of the polymer blend. Annealing of the pure PS/PMMA blend at 190 °C leads to PS drops that protrude from the PMMA surface. Films containing the nanoparticles show similar surface features but these features become less regular and tend to get shallower and larger as the amount of nanoparticles increases. The characteristic length of the PS drops in the pure blend film is ~ 7 microns and they cover 22 % of the surface. The corresponding numbers for the film containing 10% nanoparticles are 15 microns and 65%, respectively.
INTRODUCTION Several applications require blending of polymers with dissimilar properties for optimum performance. Because of the often chemically different nature of the polymers and the low entropy of mixing, blends tend to be immiscible. Lowering the interfacial tension between the different components does improve mixing but the stability of the dispersion also depends on kinetics. Thus, the long-term stability of these dispersions is based on the balance between thermodynamics and kinetics [1]. Copolymers or other surfactants have frequently been used as “compatibilizers” to lower the interfacial tension between two polymers. To circumvent the slow diffusion of these molecules, compatibilizers have been sometimes produced reactively at the interface. For example reactive blending via transesterification involves the formation of block A-B copolymer chains from A and B homopolymers right at the interface [1,2]. In this paper we report on the use of nanoparticles (nanoclays) as the means to improve compatibilization and wetting between two immiscible polymers, polystyrene, PS and polymethylmethacrylate, PMMA. The presence of particles has already been shown to perturb the phase separation of an immiscible blend [3]. Additionally, Walheim et al., Affrossman et al. and Tanaka et al. have studied the evolution of the domain structure of PS/PMMA thin films using atomic force microscopy, AFM [4-6]. Here we show using the same technique how nanoparticles can affect domain evolution between the two polymers. Nanoclay refers to 2:1 layered silicates. Their crystal structure consists of layers made up by sandwiching an octahedral layer of alumina or magnesia to two tetrahedral layers of silica. Stacking of the layers leads to regular intervals that can accommodate (intercalate) both small molecules and macromolecules. Pristine nanoclays usually contain Li or Na cations between the layers. An ion exchange reaction with cationic KK10.1.1
surfactants renders their surfaces organophilic, which improves the miscibility with and allows intercalation of different polymers [7]. EXPERIMENTAL DETAILS Materials. Polysty
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