Modification of styrene-ethylene/butylene-styrene copolymer microstructure by polystyrene homopolymer and evolution of a cocontinuous blend morphology

Styreneethylene/butylenestyrene (SEBS) copolymer was blended with the homopolymer polystyrene (hPS) in a counter-rotating twin-screw extruder to prepare hPS/SEBS blends. The morphology of the hPS/SEBS blends was studied by environmental scanning electron microscopy (ESEM) and by transmission electron microscopy (TEM). TEM imaging of pure SEBS revealed a microphase-separated morphology, showing ordered regions or grains where the discrete styrene blocks formed hexagonally packed cylindrical structures within the ethylene/butylene matrix. Blending of small quantities of the homopolymer hPS showed an interesting result. The TEM images of the SEBS-rich blends revealed that the added hPS was absorbed within the bulk of the SEBS phase resulting in a new SEBS-rich phase, and the absorbed hPS was found to join the styrene cylinders of pure SEBS thereby modifying its original microstructure. As the hPS concentration in the blends was increased further, the ESEM and TEM images showed that the excess homopolymer hPS phase separated, forming a cocontinuous morphology with the hPS-modified SEBS-rich phases. Linear viscoelastic tests on the SEBS-rich blends also revealed the existence of a synergistic effect, showing increased elastic behavior compared with that of pure SEBS. Small-angle X-ray scattering results showed an additional periodicity in the SEBS-rich phase because of the modification of the SEBS microstructure by hPS. POLYM. ENG. SCI., 52:2559-2572, 2012. (C) 2012 Society of Plastics Engineers