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

Abstract

We have studied the crystallization of poly(ethylene terephthalate) during drawing at temperatures of 83, 90 and 96°C and at strain rates in the range 0.01–2.1 s −1. Decreasing strain rate shifts the onset of crystallization to higher draw ratios and reduces the rate at which crystallinity increases with draw ratio, an effect that becomes more pronounced as draw temperature increases. When the degree of crystallinity is plotted against draw time, it becomes apparent that the effect of changing strain rate is simply to shift the curves along the log-time axis. The shift factor and the strain rate are related by a power law, and the value of the exponent, which reflects the rate of molecular relaxation at the draw temperature, rises with temperature at an increasing rate. Due to strain-rate/draw-time superposition, empirical equations permit prediction of the degree of crystallinity and the crystallization rate at any strain rate or temperature in the range studied from knowledge of draw time or draw ratio. The study also reveals that increasing draw temperature does not necessarily increase the draw ratio for onset of crystallization λ c as previously supposed: at sufficiently high strain rates, λ c decreases with increasing draw temperature.