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

Amorphous polyethylene terephthalate film has been uniaxially drawn below a nd above the glass transition temperature, T,, and the effect of temperatur e, strain rate and extent of elongation on the development of crystallinity investigated by differential scanning calorimetry (DSC). The isothermal cr ystallization time dependence was analysed using the Avrami equation and th e mechanism derived was in good agreement with the morphology observed by S EM. 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 nu clei and an increased growth rate. The effect of initial orientation on cry stallization rate and its dependence on temperature were quantitatively des cribed 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 o f CH2 group in glycol segments. A combination of polarized FT-IR spectrosco py and DSC enabled the degree of orientation in the crystalline and non-cry stalline phases to be determined separately. Results indicated that orienta tion and the strain-induced crystallization increased with increasing draw ratio and elongation, but decreased with Increasing temperature in the rang e of 75-85 degreesC due to the relaxation of molecular chains segments. Rel axation of chain segments mainly occurred in non-crystalline region. (C) 20 01 Elsevier Science Ltd. All rights reserved.