Uniaxially Oriented High Density Polyethylene

Abstract

The molecular relaxation behavior of an ice-quenched high density polyethylene (HDPE) subjected to solid-state stretching at elevated temperature (100 degrees C) to various draw ratios (up to lambda=13.7) was examined by means of dielectric spectroscopy. All relaxation zones (alpha, beta and gamma, in order of decreasing temperature) between 25 K and melting temperature were studied in the frequency range from 1 kHz to 1 MHz. The changes observed in different dielectric relaxations were related to the orientation-induced modifications of the structural and morphological parameters. In order to investigate orientation-induced structural changes, optical microscopy (OM), scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS), and differential scanning calorimetry (DSC) were employed. Hermans orientation function (f(c)) was used to quantify the degree of crystal orientation. Complete disappearance of the already weak beta relaxation with orientation is attributed to the increase in crystallinity, but the contribution due to a more restricted chain segment mobility in the interlamellar regions of oriented specimens should also be taken into account. Presented results also reveal two different orientation-induced dynamics in the evolution of the dielectric alpha and gamma relaxations connected with the main transformation stages in the drawing of crystalline polymers. The transformation of the initial isotropic into a fully oriented fibrillar structure introduces large changes in the dielectric relaxation spectra of HDPE, especially in the alpha relaxation zone; by contrast, with further increase in the draw ratio much smaller changes are observed due to the deformation of the fiber structure by longitudinal sliding motions of microfibrils and/or by fibrils slipping past each other.