Asymmetric miscibility in random copolymer/homopolymer blends: Monomeric size and polarity

A compositionally symmetric random copolymer of styrene and methyl methacry late (S-r-MMA) was previously found to exhibit preferential miscibility wit h poly(methyl methacrylate) (PMMA) as compared with polystyrene (PS). This asymmetric miscibility of a random copolymer with its related homopolymers is not predicted by the Copolymer Effect theory, and its underlying causes are unclear in the absence of strong intermolecular interactions. In this p aper, correlations were explored between asymmetric miscibility in random c opolymer/homopolymer blends and the physical characteristics of the monomer ic units. Four copolymer systems were studied, including S-r-MMA, which sys tematically varied the size and polarity of the monomeric units. A new tech nique, cross-sectional microscopy of thick bilayers, was developed to deter mine coexistence compositions in these blends based on linearized mass bala nces. In addition to S-r-MIMA, a second compositionally symmetric random co polymer, poly(styrene-mn-ethyl methacrylate), was found to exhibit preferen tial miscibility with poly(ethyl methacrylate) over polystyrene. Two other copolymer systems, poly(styrene-ran-butyl methacrylate) and poly(styrene-ra n-4 vinylpyridine), exhibit comparable miscibility with their related homop olymers. Comparing the monomer characteristics to both our miscibility and infrared spectroscopy results suggests that differences in the size and pol arity of the monomeric units may induce asymmetric miscibility in random co polymers. Finally? deuteration within the S-r-MMA system was sufficient to eliminate the asymmetric miscibility behavior.