Phase behavior of a binary mixture of a block copolymer with lower disorder-to-order transition and a homopolymer

Abstract

The phase diagrams for a binary mixture of symmetric polystyrene-block-poly(n-butyl methacrylate) copolymer (PS-b-PnBMA) exhibiting lower disorder-to-order transition (LDOT) and PS homopolymer and a binary mixture of PS-b-PnBMA and PnBMA homopolymer were investigated by using polarizing optical microscopy, turbidity measurement, small-angle X-ray scattering, rheology, and transmission electron microscopy. Emphasis was placed on the effect of the molecular weight of homopolymers on the phase behavior of the mixtures. We found that when the molecular weight of a homopolymer was smaller than the half of molecular weight of the corresponding block, LDOT of the mixture increased (thus miscibility was enhanced). However, the changes of microdomains for the mixtures containing PS homopolymer are different from those for the mixtures containing PnBMA homopolymer, especially when the molecular weight of homopolymer becomes large. For instance, for the mixture of PS-b-PnBMA and PnBMA with a molecular weight of 65000, lamellar microdomains structure was destroyed even at the addition of 20 wt % of PnBMA homopolymer. On the other hand, for the mixture of PS-b-PnBMA and PS with a molecular weight 68000, the lamellar microdomain structure was maintained up to 40% of PS homopolymer. The phase diagrams of the mixtures were compared with predictions developed based on the compressible random phase approximation.