Phase Separation in Styrene-α-Methyl Styrene Block Copolymers

Abstract

In previous work from this laboratory, it has been shown that diblock and triblock copolymers of styrene, S, and α-methy1 styrene, MS, are homogeneous up to considerably higher molecular weights than are mixtures of the corresponding homopolymers (1,2). Homogeneity was inferred from the presence of a single Tg, intermediate between those of polystyrene, PS, and of poly (α-methyl styrene), PMS, in the samples. The glass transition temperatures were measured using DTA, DSC, and dilatometry. The same techniques showed that, if low molecular weight PS or PMS was mixed with one of the block copolymers, phase separation was always enhanced (3). The molecular weight and composition at which microphase separation occurred in the block copolymers could be predicted quite well using a theoretical treatment developed by one of us (4). The data indicated that the interaction parameter between PS and PMS was between 0.0030 and 0.0036 for samples with M_w/M_n < 1.3. Block copolymer samples with broader molecular weight distributions showed enhanced microphase separation, i.e., they exhibited microphase separation even when their M_w was so low that homogenity was predicted theoretically (4). Furthermore, the addition of homopolymer to the block copolymers always enhanced microphase separation, even though it was predicted (5) that the presence of very low molecular weight homopolymer should serve to compatibilize the system.