Structural Changes and Dielectric Relaxation Behavior of 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 °C) to various draw ratios (up to Λ=13.7) was examined by means of dielectric spectroscopy. All relaxation zones (α, β and γ, 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. Herman's orientation function (f_c) was used to quantify the degree of crystal orientation. Complete disappearance of the already weak β 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 α and γ 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 α 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.