Rheo-Optical Studies of High Polymer X Time Dependent Birefringence for Low Density Polyethylene

Abstract

An equipment has been designed to provide a record of the birefringence change accompanying a stress-strain measurement or a stress relaxation measurement. The equipment consists of Instron type tensile tester and an optical system to measure the birefringence which has been designed to be easily mounted on or removed from the tensile tester. The optical system has a conventional arrangement consisting of a light source, filter, crossed polaroids and so on, and the birefringence can be recorded continuously by the intensity method with a photomultiplier.
The change in the birefringence with time during the stress relaxation has been measured for low density polyethylene with this equipment. The birefringence vs. time curves at different strain levels show a similar shape, being consistent with the data obtained by Strella et al. There can be found straight-line relationship between the birefringence and the strain at any time during the relaxation under the strain level lower than about 5%. The time dependence of the strain-optical coefficient depends highly upon temperature, and a transition of the strain-optical coefficient can be observed at the temperature region covered by this experiment. By shifting the curves at different temperatures horizontally, a single composite curve was obtained. The annealed and quenched films of low density polyethylene show definitely different birefringence, but the temperature dependence of the shift factor log a_T for these films gives the same straight line when it is plotted against reciprocal absolute temperature. This can be understood qualitatively, at least, on the basis that the relaxation mechanism which is responsible for the birefringence of both the annealed and quenched polyethylene films in such a temperature region is the same. The optical relaxation spectrum for the annealed and quenched polyethylene has been obtained by the first approximation method. The longest relaxation time for the annealed material is almost two decades longer than that for the quenched one.