Orientation prior to crystallisation during drawing of poly(ethylene terephthalate)

Abstract

Wide angle X-ray scattering data have been recorded during the drawing of poly(ethylene terephthalate) (PET) using a wide range of draw rates (0.05–12s−1), temperatures (90–120°C) and draw ratios. The data were analysed to follow the development of molecular orientation and the onset of crystallisation. The molecular orientation prior to crystallisation has been characterised in terms of the orientation order parameter 〈P2(cosθ)〉. The rate of increase of 〈P2(cosθ)〉 with draw ratio decreases with both increasing temperature and decreasing draw rate. A superposition of all the data to a common reference temperature of 90°C was obtained using a WLF shift factor to provide a master curve showing the dependence of the development of 〈P2(cosθ)〉 on draw rate. A comparison of the known chain relaxation motions of PET with the observed relation between draw rate and the onset of crystallisation provides an explanation of a previous discrepancy in the literature concerning the point of onset of crystallisation. For draw rates faster than the rate of the chain retraction motion, the onset of crystallisation is delayed until the end of the deformation process. For draw rates slower than the chain retraction motion, there is evidence of the onset of crystallisation occurring before the end of the deformation process.