Cold-Drawn Material As Model Material for the Environmental Stress Cracking (ESC) Phenomenon in Polyethylene. A Raman Spectroscopy Study of Molecular Stress Induced by Macroscopic Strain in Drawn Polyethylenes and Their Relation to Environmental Stress Cracking

Abstract

Raman spectroscopy was used to study strain-induced molecular stress in cold-drawn polyethylenes, which were being used as a model system for fibrils present in the crazes formed during environmental stress crack resistance (ESCR) tests. The molecular stress was measured at 240 K in order to minimize relaxation phenomena. Molecular stress was related to macroscopic strain and, by correcting for differences in E-moduli, to true stress. In this paper, the measured molecular stress is related to ESCR values and sample characteristics. It was observed that good ESCR materials showed a lower molecular stress than worse ESCR materials at the same macroscopic strain level. It was also observed that the molecular weight has a major effect on the observed molecular stress per macroscopic strain (molecular stress per macroscopic strain decreases with increasing Mw), whereas the effect of chain branching is smaller (molecular stress per macroscopic strain decreases with chain branching).