Biaxial hot drawing of poly(ethylene terephthalate): measurements and modelling of strain-stiffening

A previous study of hot-drawing of amorphous films of poly(ethylene terephthalate) under biaxial stress (Polymer 1996;37:2403) was extended to compare constant width (CW) and equal biaxial (EB) strain states, with special emphasis on the pronounced strain-stiffening that begins at draw ratios of approximately 2. Some as-drawn specimens were also studied by density measurement and wide-angle X-ray scattering. Stress-strain data were obtained during drawing at a constant extension-rate of 1/s, across the temperature range between the glass transition and terminal regions. They were interpreted in terms of the multiaxial glass-rubber constitutive model proposed before, and the CW and EB stress-strain relations for the PET rubber-like entanglement network were extracted. The information was then used to quantify the entanglement slippage superposed on deformation of the network at higher temperatures. This revealed anomalous behaviour: entanglement slippage was found to arrest spontaneously at a critical level of orientation, associated with the first appearance of crystallinity in the drawn specimens. The constitutive model was extended to incorporate these features, with the arrest of entanglement slippage entering the model via a viscosity increasing asymptotically at a critical maximum principal network stretch. The model was then able to capture strain-stiffening behaviour through the temperature range. (C) 1999 Elsevier Science Ltd. All rights reserved.