The role of high molecular weight chains in flow-induced crystallization precursorstructures

Abstract

Flow-induced crystallization in a bimodal polyethylene blend was investigatedby means of in situ shear-WAXD (wide-angle x-ray diffraction) and shear-SAXS(small-angle x-ray scattering) techniques. The blend contained a low molecular weight(Mw = 50 000 g mol−1 andpolydispersity = 2)polyethylene copolymer matrix (MB-50k) with 2 mol% of hexene, and a nearly monodisperse high molecularweight (Mw = 161 000 g mol−1 and polydispersity = 1.1) hydrogenated polybutadiene component (MD-161k), which has the microstructure of anethylene–butene copolymer with 4 mol% butene. At the experimental temperatures of 112 and115 °C, MB-50k exhibited faster crystallization kinetics and higher crystallinity due to higherchain mobility and higher ethylene content than those of the MB-50k/MD-161k blend.However, both WAXD and SAXS results indicated that the high molecular weightcomponent (MD-161k) is responsible for the formation of more highly oriented crystals,which we relate to a shear-induced precursor scaffold. Values of the lamellar long period inall experimental runs were found to slightly decrease in the beginning of crystallization andthen reached a plateau value. Vonk’s method for single lamella scattering was employed toestimate the lamellar thickness in the MB-50k/MD-161k blend at high temperature(115 °C), where the lamellar thickness was also found to decrease in the beginning and remainedabout constant afterward. Twisted lamellar structures were observed in all formedkebabs.