Rheo-optical Properties of Polyvinyl Chloride Films: Unplasticized Homopolymer

Abstract

The rheo-optical properties of PVC homopolymer films, prepared by solvent casting, were studied by measuring length and birefringence changes during creep experiments at constant load. Both temperature and stress level were varied in these experiments; however, the effects of stress level were particularly emphasized. Temperature was varied in the range of 30°–140°C. An unusual phenomenon of stepwise extension was sometimes observed, particularly in the case of unannealed films. There seems to be a unique relation between birefringence and strain over a wide range of experimental conditions, and this relation is significantly different from that corresponding to a rubbery network. Cold drawing is observed at high stress levels. A pattern of curves very similar to that obtained for different temperatures at a constant load, is produced by varying the stress level at a constant temperature. However, the curves at different stress levels and different temperatures do not superpose when shifted vs log time. Optical-creep compliance (birefringence/stress ratio) shows two stages of increase in creep experiments at different stress levels at a single temperature, indicating two birefringence mechanisms. The second mechanism appears to be associated with whitening of the sample. This second-stage process is less clearly visible in the mechanical compliance curves. Annealing conditions seem to have very important effects on the film properties observed. A three-dimensional isothermal surface can be used to describe the relation of strain to stress and log time conveniently. The linear relation between yield stress and temperature, which extrapolates to zero stress near the glass transition temperature, was investigated in more detail by use of these creep experiments. A stress-temperature equivalence of 6.79 kg/cm2 (or 6.66×106 dyn/cm2) tensile stress per degree centigrade was obtained. From the results, it appears that the cold-drawing phenomenon can be described as a stress-induced glass transition. The glass transition must, therefore, be regarded not simply as a temperature, but as a phenomenon which is a function of temperature, stress level, and time.