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

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 M w), whereas the effect of chain branching is smaller (molecular stress per macroscopic strain decreases with chain branching).