Effects of Flow History on the Crystallization of Polyethylenes with Different Molecular Weight Distributions
Yuji NAKASHIMA, Satoshi HASEGAWA, Akihiko TAKADA and Yoshiaki TAKAHASHI
Abstract:Effects of shear flow history on the degree of crystallization of high density polyethylenes with broad molecular weight distributions are examined by conventional DSC measurements in connection with slight differences in molecular weight distributions. It was observed that the shear rate where the degree of crystallization starts to increase is related to the shear rate where the shear thinning is observed. By comparing the data for samples showing almost the same viscosity behavior but having slightly different molecular weight distributions, it was observed that the degree of crystallization for the sample containing small amounts of longer chains become slightly higher, which is consistent with the idea that the small portions of long chains become crystalline precursors. It was also observed that a long time, longer than the mechanical relaxation time, is needed to erase the effects of flow. Key Words:Shear flow, High density polyethylene, Crystallization, Molecular weight distribution, Shear thinning