Morphology and thermomechanical properties of blends of styrene–butadiene–styrene triblock copolymer with polystyrene and syndiotactic polybutadiene homopolymers

The reinforcing effect of a dispersed phase of polystyrene or syndiotactic 1,2-polybutadiene on the mechanical properties of a styrene–butadiene–styrene triblock copolymer (SBS) has been studied. The blends of any of the homopolymers with the styrene–butadiene–styrene triblock copolymer present phase separation. The morphology of the blends has been characterized by transmission electron microscopy. The blends with polystyrene homopolymer show a dispersion of small homopolymer domains in the SBS matrix with poor adhesion between the polystyrene homopolymer domains and the polybutadiene blocks of SBS. This conclusion can be reached through the low, or nearly null, reinforcing effect in the low deformation regime studied by dynamic-mechanical experiments. The blends of SBS with syndiotactic polybutadiene on the contrary, show a characteristic increase of the modulus in the temperature region in which the SBS is in the rubber state and the syndiotactic polybutadiene is in the glassy state. Some cavitation phenomena in this region, due to the separation of both phases under deformation, cannot be ruled out. Dynamic-mechanical relaxation spectrum of the blends has been analyzed with the help of a simple block model.