Compositional Dependence of the Microphase Separation Transition in Styrene−Butadiene−Styrene Triblock Copolymer/Polystyrene Blends

The change in the microphase separation transition (MST) temperature of a styrene−butadiene−styrene (SBS) triblock copolymer induced by the addition of polystyrene (PS) was investigated by small-angle X-ray scattering. It was found that the transition temperature was determined from the molecular weight ( M H ) of the added PS in relation to that of the corresponding blocks ( M A ) in the copolymer. The MST temperature decreased with added PS if M H / M A < 1/4, while it increased with added PS when M H / M A > 1/4. Analysis of the theoretical expression based on the random phase approximation showed exactly the same tendency of change in the transition temperatures as that observed experimentally. The interaction parameter, χ SB, obtained by nonlinear fitting of the scattering profiles of SBS/PS blends in the disordered state, was found to be a function of temperature and composition. Composition fluctuations were found to exist in SBS/PS blends, increasing with increasing addition of PS but diminishing with increasing molecular weight of the added PS.