Dynamic Mechanical Properties of Polystyrene Containing Microspherical Inclusions of Polybutadiene: Influence of Domain Boundaries and Rubber Molecular Weight

A model set of well-characterized anionically polymerized diblock copolymer-homopolymer blends was prepared from styrene and butadiene. These materials contain microspherical polybutadiene domains whose phase boundary thickness, size, size distribution, and spatial packing are all known from previous electron microscopy and small-angle neutron scattering studies of the same set of samples. Dynamic mechanical properties were determined in a tensile mode at 3.5 Hz between -140 and +110 °C. Increasing the volume fraction of interfacial material in these composites produces no change in the storage modulus and a small increase in the level of viscoelastic loss in the region between the pure-component glass transition temperatures. Inclusion of polybutadiene in a glassy polystyrene matrix results in a significant lowering of the rubber glass transition temperature, T[formula omitted], which can be explained on the basis of negative pressure resulting from differential contraction. Composites containing lower molecular weight polybutadienes exhibit smaller T[formula omitted] depressions, suggesting the possibility of cavitation in the rubber in these materials.