Crystallization kinetics for poly(ethylene terephthalate) oriented by solid‐state coextrusion

A series of uniaxially drawn films has been prepared from isotropic, amorphous poly(ethylene terephthalate) (PET) by using solid‐state coextrusion below its Tg. To evaluating the crystallization behavior of the coextrudates, the cold‐crystallization temperature Tc, Crystallization rate constant k, and activation energy Ea have been studied as a function of extrusion draw temperature and draw ratio. The rapid crystallization of highly oriented amorphous PET in extrudates was followed by a temperature‐programmed DSC technique. This avoids the marked precrystallization often found in samples brought to appropriate isothermal temperatures. With increasing extrusion draw ratio, Tc shifts to lower temperatures, the first‐order crystallization rate constant k increases markedly (by two or three orders of magnitude) and Ea decreases markedly. For undrawn samples annealing at temperatures below or near Tg (50–70°C) does not lead to crystallization, but with increased annealing temperature both Tc and Ea decrease. In contrast, for extrusion‐drawn PET, both Tc and Ea increase with draw temperature. All these features are consistent with our previous correlations with molecular draw ratio and birefringence.