Block Copolymer Micelle Toughened Isotactic Polypropylene

Abstract

A series of compositionally symmetric poly(ethylene-alt-propylene)-b-poly(ethylene-ran-ethyl ethylene) (PEP-PEEE) diblock copolymers were synthesized by anionic polymerization of isoprene and butadiene followed by heterogeneous catalytic hydrogenation. These materials were melt blended with isotactic polypropylene (iPP) at concentrations between 1.25 and 20 wt %, and the resulting morphologies were investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Uniformly dispersed micelles with an average diameter of approximately 100 nm were obtained in blends with PEP-PEEE molecular weights of 100 and 240 kg/mol. Tensile tests showed a 25-fold increase in the strain at break at a concentration of just 2.5 wt % with no discernible loss in elastic modulus or tensile strength. Blends containing 5 and 10 wt % micelles displayed 5 and 12 times the Izod impact strength of the pure iPP, respectively. Conversely, blends prepared with PEP homopolymer formed larger particles (â‰0.5 μm in diameter) accompanied by marginal improvement in the tensile and impact properties at the same concentrations. The toughening mechanisms were investigated using electron microscopy, which revealed cavitation-induced shear yielding and multiple crazing in the iPP/PEP-PEEE blends, and limited plastic deformation in the iPP/PEP blend. A published theory that accounts for this type of toughening has been employed to model the rubber modified solids when subjected to different stress states leading to predictive criteria for cavitation and shear yielding. These results establish the relationship between rubber particle size and toughness in rubber-modified iPP.