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 terepht halate) under biaxial stress (Polymer 1996;37:2403) was extended to compare constant width (CW) and equal biaxial (EB) strain states, with special emp hasis on the pronounced strain-stiffening that begins at draw ratios of app roximately 2. Some as-drawn specimens were also studied by density measurem ent and wide-angle X-ray scattering. Stress-strain data were obtained durin g drawing at a constant extension-rate of 1/s, across the temperature range between the glass transition and terminal regions. They were interpreted i n terms of the multiaxial glass-rubber constitutive model proposed before, and the CW and EB stress-strain relations for the PET rubber-like entanglem ent network were extracted. The information was then used to quantify the e ntanglement slippage superposed on deformation of the network at higher tem peratures. This revealed anomalous behaviour: entanglement slippage was fou nd to arrest spontaneously at a critical level of orientation, associated w ith the first appearance of crystallinity in the drawn specimens. The const itutive model was extended to incorporate these features, with the arrest o f entanglement slippage entering the model via a viscosity increasing asymp totically at a critical maximum principal network stretch. The model was th en able to capture strain-stiffening behaviour through the temperature rang e. (C) 1999 Elsevier Science Ltd. All rights reserved.