(766c) Toughening Isotactic Polypropylene with Block Copolymer Micelles

A series of compositionally symmetric poly((ethylene-alt-propylene)-b-(ethylene-ran-ethyl ethylene)) (PEP-b-PEEE) block copolymers (BCPs) with molecular weights ranging from 50 to 240 kg/mol were synthesized by anionic polymerization of isoprene and butadiene followed by heterogeneous catalytic hydrogenation. These materials were melt blended with isotactic polypropylene (iPP) at loadings from 1.25-20 wt.% and the resulting morphologies were investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Discrete BCP micelles with an average diameter of approximately 100 nm were uniformly dispersed in iPP matrix of 5 wt.% blends. Tensile testing was used to study the blend mechanical properties. The nano-sized BCP micelles led to a 25-fold increase in the strain at break (ε_b ≈ 20% for pure iPP), compared to ε_b ≈ 100% in the blend containing 5 wt.% PEP homopolymer (a model analogue of ethylene propylene (EPR) rubber). A BCP loading of just 2.5 wt.% produced an optimal combination of high toughness with little or no discernable loss in elastic modulus or tensile strength. The blend mechanical properties also have been evaluated with notched Izod impact testing. Blends containing 5 wt.% and 10 wt.% BCP displayed impact strength that was 5 and 12 times that of the pure iPP (23.6 J/m), again significantly outperforming the PEP based counterpart. The toughening mechanisms have been investigated using electron microscopy, which revealed cavitation induced shear yielding and multiple crazing. A published theory that accounts for this type of toughening has been employed to model yielding the rubber modified solids when subjected to different stress states leading to predictive criteria for cavitation and shear yielding.