Craze morphology and molecular orientation in the slow crack growth failure of polyethylene

An optical microscopy study and a micro-Raman spectroscopy study were carried out on polyethylene samples subjected to an environmental stress crack resistance (ESCR) test. The aim was to elucidate the molecular deformation mechanisms associated with the failure process. It has been shown that in the early stages of the ESCR test, in a regime of low local stress, failure in the craze occurs via a brittle process with limited ductility and with molecular orientation being detected. As the experiment progresses, however, extensive fibrillation takes place. The molecular orientation in these fibrils was found to be comparable to that measured in cold-drawn samples. Moreover, the fibril molecular orientation decreased from the crack to craze tip and was found to be higher in the midrib part of the fibril (fibril failure point). As a consequence, fibril creep is the most likely mechanism of failure in the craze. Microscopy and Raman measurements showed that the extent of the brittle process is molecular weight-dependent, that is, the brittle process seems to operate longer at higher molecular weights. These observations are in agreement with a previous work which showed that the molecular stress per macroscopic strain/stress decreases with increasing molecular weight, therefore holding the high molecular weight craze in a regime of low local stress for longer testing times. Fibrils spanning the craze are envisaged as the anchor points that hold the structure during the process of failure. (C) 2000 John Wiley & Sons, Inc.