Effect of draw ratio and temperature on the strain-induced crystallization of poly (ethylene terephthalate) at fast draw rates

Wide angle X-ray scattering (WAXS) data recorded during the drawing of poly(ethylene terephthalate) (PET) under industrial processing conditions was analysed in terms of changes in the degree of polymer orientation and crystallinity over a wide range of draw temperatures and draw ratios. The actual draw rate and draw ratio at the region in the specimen from which X-ray data was recorded was determined independently with high consistency by direct video observation of strain and total X-ray scattering, In contrast to previous claims, the start of strain-induced crystallization coincided with the end of draw within the 40 ms time-resolution of the investigation. Crystallization has been shown to follow first order kinetics and the rate constant was determined over a range of temperatures from 85 degrees C-125 degrees C. Above 125 degrees C little orientation is observed with no evidence of strain-induced crystallization. A detailed determination was made of the critical value of draw ratio below which strain-induced crystallization does not occur and where a relaxation in molecular orientation is observed after the extension process. The implications of these experimental observations for existing theory of strain-induced polymer crystallization are discussed, (C) 1999 Elsevier Science Ltd, All rights reserved.