Kinetic effects on phase heterogeneity in bisphenol-A polycarbonate/poly(methyl methacrylate) blends

Optical microscopy and d.s.c. were performed on PC/PMMA blends cast with THF at 50°C to demonstrate that the domain sizes were dependent on the film thickness, which should help provide yet stronger evidence that the blends were not of a single-phase nature with thermodynamic molecular mixing, but that the phase behaviour was governed by the kinetics of solvent evaporation rate. While thin blends were totally amorphous and possessed ultra micro heterogeneity, thicker-film blends possessed much greater scales of heterogeneity and developed solvent-induced PC crystallinity, which could suppress the T_g of the PC phase. This study further proved that the equilibrium phase behaviour of PC/PMMA was thermodynamically immiscible, and that the common thermodynamic factors such as solvent types and temperature could only affect the scales of heterogeneity in such systems. Cases influenced by solvent type and temperature were discussed to illustrate the dependence of heterogeneity on such factors. Also, given the same thermodynamic factors, kinetic factors can influence the scales of phase heterogeneity.