Microstructure and Crystallinity of Polyolefins Oriented via Solid-state Stretching at an Elevated Temperature

Abstract

Different polyolefines (low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and isotactic polypropylene (iPP)) were oriented via solid-state stretching at an elevated temperature. In order to investigate orientation-induced changes in microstructure and crystallinity, optical microscopy (OM), scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC) were employed. To quantify the degree of molecular orientation WAXS data were used to calculate Hermans orientation function (f(c)); the results reinforce the morphological picture obtained from OM and SEM confirming Peterlins molecular model of drawing. Furthermore, orientation-induced changes in the crystalline phase, especially in its volume and perfection, were observed by DSC and WAXS. Comparison between these data showed that the biggest change in the degree of crystallinity was achieved during the transformation of the initial lamellar into fibrillar structure. After completion of this transformation further orientation, which occurs through plastic deformation of fibre structure, introduces only minor changes in crystallinity. The overall orientation-induced behaviour hasnt been considerably influenced by the structural differences amongst polyolefins.