Influence of temperature and chain orientation on the crystallization of poly(ethylene terephthalate) during fast drawing

Abstract

Three regimes of poly(ethylene terephthalate) (PET) deformation are identified from a detailed analysis of time-resolved wide angle X-ray scattering (WAXS) data recorded during the mechanical deformation of PET at a wide range of draw rates (0.05–12s−1), temperatures (90–120°C) and draw ratios. These are:(i) where the onset of crystallization occurs after the end of deformation with a tendency for preferred alignment of the (100) plane in the plane of the sample;(ii) where the onset of crystallization occurs during deformation and with increasing tilt of the crystal chain axis away from the draw direction; and(iii) where there is no oriented crystallization.A comparison of the crystallization rate with the orientation parameter 〈P2(cosθ)〉 at the onset of crystallization shows a strong dependence of the crystallization rate on both temperature and molecular orientation. It is proposed that a previously observed insensitivity of crystallization rate to temperature was a consequence of fortuitous choice of drawing conditions in which the opposing effects of temperature and molecular orientation produced similar net crystallization rates. The effects of the temperature dependence have been factored out using a WLF shift factor to reveal an overall dependence of the crystallization rate on approximately the fourth power of 〈P2(cosθ)〉. At high draw temperatures where the rate of the chain retraction motion is faster than the draw rate, there is a significant tilt of the crystal chain axis away from the draw direction. During the crystallization process both the degree of crystal tilt and the half-width of the crystal reflections remain essentially unaltered over the whole crystallization process.