Crystallization kinetics during hot-drawing of poly(ethylene terephthalate) film: strain-rate/draw-time superposition

Abstract

We have investigated the influence of strain rate during hot-drawing of poly(ethylene terephthalate) on the kinetics of overall crystallization and of crystal growth normal to the 010 and 100 planes. Drawing was at 90°C and at various strain rates between 0.01 s −1 and 2.1 s −1. We found that changing strain rate simply shifts the crystallization rate curves along the log time axis, that the shift factor and the strain rate are related by a power law, and that the value of the exponent reflects the relative influence of two effects of molecular mobility : relaxation of orientation and enhancement of crystallization. The relative importance of these two effects was found to be different in different crystallographic growth directions : normal to the 100 planes, they almost exactly compensate each other; normal to the 010 planes, the relaxation effect dominates. By defining an ‘equivalent time’ for crystallization, degree of crystallinity at any strain rate can be predicted from knowledge of either draw time or draw ratio. A similar treatment permits prediction of crystallite size.