Morphological characterisation of the crystalline structure of cold-drawn HDPE used as a model material for the environmental stress cracking (ESC) phenomenon

Abstract

This study comprises a detailed morphological study of cold-drawn polyethylene materials by Raman spectroscopy and other techniques. Cold-drawn polyethylene is recently being used as a model material for studying and characterising the environmental stress cracking resistance (ESCR) behaviour of polyethylene grades. The cold-drawn structure was shown to be highly oriented and a large decrease in the Raman orthorhombic crystallinity was observed. Also other Raman and i.r. vibrational split modes pointed to the orthorhombic crystallinity decrease. No corresponding crystallinity changes were seen using the i.r. active -CH 2- bending doublet or DSC. WAXS and Raman did not give evidence for large scale phase transformation from orthorhombic to monoclinic or triclinic. The results suggest an ill-defined orthorhombic crystalline structure with dislocations and disrupted crystals formed by cold-drawing, probably as a result of molecules being pulled through the crystals. In situ Raman straining experiments were carried out on the cold-drawn material at 240 K (in order to suppress molecular relaxation). Further orthorhombic crystalline disruption was evident with strain. No disruption was seen in fibrils created during environmental stress crack resistance (ESCR) tests carried out at 348 K. Temperature was confirmed as an important factor in determining the crystalline phase recovery of the orthorhombic crystallinity and disappearance of the monoclinic phase occurred when the cold-drawn structure was annealed.