COMPATIBILIZERS FOR MELT BLENDING - PREMADE BLOCK-COPOLYMERS

Poly(methyl methacrylate) (PMMA) was melt mixed 30:70 into polystyrene (PS) with and without symmetric P(S-b-MMA) diblock copolymers. The mo lecular weight of the components was varied. After 5 min of shear mixi ng, the PMMA was dispersed into roughly spherical, submicron particles . Particle size was measured by light scattering and transmission elec tron microscopy. As little as 1% copolymer led to a significant reduct ion in PMMA particle size, although larger amounts were needed to make the particles stable to annealing (180 degrees C for 15 min). The pri nciple role of block copolymers in controlling morphology appears to b e in preventing coalescence. Preventing dynamic coalescence leads to s ize reduction, while preventing static coalescence results in stabilit y or compatibilization. We estimate that less than 5% of the interface needs to be covered to prevent dynamic coalescence while similar to 2 0% is necessary to impart static stability. Mobility, critical micelle concentration, and molecular weight of the block copolymer also appea r to be important. Lowering the molecular weight of the PMMA phase fro m 43 000 to 11 000 resulted in dramatically lower particle size (700 v s 60 nm). These variables are discussed in terms of a qualitative bala nce between rate of diffusion and rate of area generation during blend ing.