Compatibilization of polypropylene - Polystyrene blends: Part 2, crystallization behavior and mechanical properties

Reactor produced blends of polypropylene (PP) and polystyrene (PS)are obtai ned by graft copolymerization of styrene onto polypropylene chains. This te chnique generates simultaneously a graft copolymer (PP-g-PS) and polystyren e homopolymer. The resulting blends, however, have a low impact resistance and have to be modified with the addition of rubbery toughening agents, suc h as an ethylene propylene copolymer (EPR) or a styrene-b-ethylene-alt-buty lene-b-styrene (SEBS) triblock copolymer, in a downstream compounding opera tion. Part 1 of this work, which has been accepted for publication in the B ritish Journal "Plastics, Rubber, and Composites", was concerned with the c ompatibilization efficiency and the effect of mixing intensity on the morph ological and rheological properties. Part 2 of the study deals with interac tions of the components of the blends. These interactions were assessed by monitoring the crystallization behavior and mechanical properties. Mixing e xperiments were conducted on a proprietary twin screw mixing element evalua tor (TSMEE) and also on a commercial TSE-30 extruder. Differential scanning calorimetry (DSC) was used to determine the transition temperatures and th e crystallinity of the blends specimens after annealing. The results show t hat both the free polystyrene and the graft copolymer, PP-g-PS, act as nucl eators for the polypropylene phase, thus increasing both the degree of crys tallinity and the crystallization temperature of the blends. While a good c orrelation was found between modulus and crystallinity, the factors affecti ng fracture behavior were less clearly discernible. (C) 2000 John Wiley & S ons, Inc.