Rheo-Optical Studies of Drawn Polyethylene Films

Abstract

Previous studies of the rheo-optical properties of polyolefins have shown that the rheo-optical properties of undrawn polyolefins can be well explained by considering three deformation mechanisms. When spherulites in polyolefins are broken either partially or completely, drastic changes in the crystalline structure, and hence in the rheo-optical properties, should occur. In order to make this point clear experimentally, birefringence relaxation and stress relaxation were measured simultaneously for low density polyethylene films drawn to various extents.For undrawn and weakly drawn films, the strain—optical coefficient increased with increasing time, while for a highly drawn film, it decreased with increasing time. This indicates that the highly drawn films do not exhibit the mechanism of crystalline orientation, which causes the birefringence to increase with time.The time curves for the strain-optical coefficient and the relaxation modulus obtained at various temperatures could be superposed by vertical and horizontal shifts. Then, from the horizontal shift factors, activation energies for the rheo-optical and viscoelastic relaxation processes were determined. The activation energies for the highly drawn films are much higher than those for the undrawn and weakly drawn films, suggesting the existence of a fourth deformation mechanism.Relaxation experiments were also carried out for undrawn low density polyethylene at different strain levels ranging from 3% to 12% The effect of strain level appears to be equivalent to the effect of drawing in the sense that the peak of the relaxation spectrum at the shorter relaxation times decreases with increasing strain.