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

This study comprises a detailed morphological study of cold-drawn polyethyl ene materials by Raman spectroscopy and other techniques. Cold-drawn polyet hylene is recently being used as a model material for studying and characte rising the environmental stress cracking resistance (ESCR) behaviour of pol yethylene 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 orthorhomb ic crystallinity decrease. No corresponding crystallinity changes were seen using the i.r. active -CH2- bending doublet or DSC. WAXS and Raman did not give evidence for large scale phase transformation from orthorhombic to mo noclinic or triclinic. The results suggest an ill-defined orthorhombic crys talline structure with dislocations and disrupted crystals formed by cold-d rawing, probably as a result of molecules being pulled through the crystals . In situ Raman straining experiments were carried out on the cold-drawn ma terial at 240 K (in order to suppress molecular relaxation). Further orthor hombic crystalline disruption was evident with strain. No disruption was se en in fibrils created during environmental stress crack resistance (ESCR) t ests carried out at 348 K. Temperature was confirmed as an important factor in determining the crystalline phase recovery of the orthorhombic crystall inity and disappearance of the monoclinic phase occurred when the cold-draw n structure was annealed. (C) 1999 Published by Elsevier Science Ltd. All r ights reserved.