CRYSTALLIZATION DURING HOT-DRAWING OF POLY(ETHYLENE-TEREPHTHALATE) FILM - INFLUENCE OF TEMPERATURE ON STRAIN-RATE DRAW-TIME SUPERPOSITION

We have studied the crystallization of poly(ethylene terephthalate) du ring drawing at temperatures of 83, 90 and 96-degrees-C and at strain rates in the range 0.01-2.1 s-1. Decreasing strain rate shifts the ons et of crystallization to higher draw ratios and reduces the rate at wh ich crystallinity increases with draw ratio, an effect that becomes mo re pronounced as draw temperature increases. When the degree of crysta llinity is plotted against draw time, it becomes apparent that the eff ect of changing strain rate is simply to shift the curves along the lo g-time axis. The shift factor and the strain rate are related by a pow er law, and the value of the exponent, which reflects the rate of mole cular relaxation at the draw temperature, rises with temperature at an increasing rate. Due to strain-rate/draw-time superposition, empirica l equations permit prediction of the degree of crystallinity and the c rystallization rate at any strain rate or temperature in the range stu died from knowledge of draw time or draw ratio. The study also reveals that increasing draw temperature does not necessarily increase the dr aw ratio for onset of crystallization lambda(c) as previously supposed : at sufficiently high strain rates, lambda(c) decreases with increasi ng draw temperature.