Dynamic Mechanical Properties of Oriented Polymers.

Abstract

The effects of processing conditions and gamma-ray irradiation on the morphology, melting behavior, and dynamic mechanical properties of oriented linear polyethylene and other semicrystalline polymers were studied. Three different processing methods, including solid-state extrusion (by Capillary Rheometer), shear-crystallization (by Couette Rheometer), and cold drawing (by Instron Tensile Machine) were used to prepare oriented samples. The dynamic mechanical studies of solid state extruded polymers revealed that the a relaxation (which includes two mechanisms: grain boundary movement at low temperatures and intracrystal shear movement at high temperatures) is sensitive to the extrusion temperature while the effect of extrusion rate is negligible. In the case of shear-crystallized linear polyethylene, both shear rate and shear temperature play an important role in determining thermal stability, stiffness, and crystal relaxation in sheared samples. We found that SIC (strain-induced crystallization) crystals can exist in samples held at shear temperature (135(DEGREES)C) for 30 minutes. In the gamma-ray irradiation study, we found that irradiated cold-drawn linear polyethylene shows double melting peaks and dramatic changes in anisotropy of intracrystal shear movement. These phenomena are much less pronounced in annealed cold-drawn samples, and have not been found in the solid-state extruded, and shear-crystallized samples. The present model suggests that in cold-drawn samples, the combination of nematic-like crystal structure and gamma-ray induced crosslinks results in an internal stress which concentrates around the polyethylene crystals. This internal stress causes the crystal to exp and at high temperature, explaining the observed phenomena.