COMPOSITIONAL DEPENDENCE OF THE MICROPHASE SEPARATION TRANSITION IN STYRENE-BUTADIENE-STYRENE TRIBLOCK COPOLYMER POLYSTYRENE BLENDS/

The change in the microphase separation transition (MST) temperature o f a styrene-butadiene-styrene (SBS) triblock copolymer induced by the addition of polystyrene (PS) was investigated by small-angle X-ray sca ttering. It was found that the transition temperature was determined f rom 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 a dded PS when M(H)/M(A) > 1/4 Analysis of the theoretical expression ba sed on the random phase approximation showed exactly the same tendency of change in the transition temperatures as that observed experimenta lly. The interaction parameter, chi(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 in creasing addition of PS but diminishing with increasing molecular weig ht of the added PS.