Compatibilization of polypropylene–polystyrene blends: Part 2, crystallization behavior and mechanical properties

Reactor produced blends of polypropylene (PP) and polystyrene (PS) are obtained by graft copolymerization of styrene onto polypropylene chains. This technique generates simultaneously a graft copolymer (PP-g-PS) and polystyrene homopolymer. The resulting blends, however, have a low impact resistance and have to be modified with the addition of rubbery toughening agents, such as an ethylene propylene copolymer (EPR) or a styrene-b-ethylene-alt-butylene-b-styrene (SEBS) triblock copolymer, in a downstream compounding operation. Part 1 of this work, which has been accepted for publication in the British Journal “Plastics, Rubber, and Composites”, was concerned with the compatibilization efficiency and the effect of mixing intensity on the morphological and rheological properties. Part 2 of the study deals with interactions of the components of the blends. These interactions were assessed by monitoring the crystallization behavior and mechanical properties. Mixing experiments were conducted on a proprietary twin screw mixing element evaluator (TSMEE) and also on a commercial TSE-30 extruder. Differential scanning calorimetry (DSC) was used to determine the transition temperatures and the crystallinity of the blends specimens after annealing. The results show that both the free polystyrene and the graft copolymer, PP-g-PS, act as nucleators for the polypropylene phase, thus increasing both the degree of crystallinity and the crystallization temperature of the blends. While a good correlation was found between modulus and crystallinity, the factors affecting fracture behavior were less clearly discernible. © 2000 John Wiley & Sons, Inc. Adv Polym Techn 19: 180–193, 2000