Crystallization orientation and relaxation in uniaxially drawn poly(ethylene terephthalate)

Amorphous polyethylene terephthalate film has been uniaxially drawn below and above the glass transition temperature, T g, and the effect of temperature, strain rate and extent of elongation on the development of crystallinity investigated by differential scanning calorimetry (DSC). The isothermal crystallization time dependence was analysed using the Avrami equation and the mechanism derived was in good agreement with the morphology observed by SEM. Nucleation and growth changed with the degree of strain and there was a change from isotropic spherulites growing uniformly in all directions from randomly oriented nuclei to elliptical spherulites originating from row nuclei and an increased growth rate. The effect of initial orientation on crystallization rate and its dependence on temperature were quantitatively described by Ziabicki's equation. Changes in molecular orientation on drawing have been measured by polarized FT-IR spectroscopy, based on the ratio of the trans to gauche conformers of CH 2 group in glycol segments. A combination of polarized FT-IR spectroscopy and DSC enabled the degree of orientation in the crystalline and non-crystalline phases to be determined separately. Results indicated that orientation and the strain-induced crystallization increased with increasing draw ratio and elongation, but decreased with increasing temperature in the range of 75–85°C due to the relaxation of molecular chains segments. Relaxation of chain segments mainly occurred in non-crystalline region.