Crystallisation-Enhanced Stress Relaxation After Drawing of PET

The work to be described forms part of a study of the rheology of poly(ethylene terephthalate) (PET), initially amorphous and isotropic, during hot-drawing under biaxial stress. Such conditions are encountered in several processes for the manufacture of PET products containing molecular orientation (notably films and bottles). Results have been interpreted in terms of a constitutive model developed for large deformations of amorphous polymers near the glass transition. In the case of PET, however, it has been necessary to extend the model to include the effects of stress-crystallisation. Specifically, a series of isothermal experiments was carried out at temperatures immediately above the glass transition (75 to 115°C), to study stress-relaxation after the cessation of constant-width drawing of PET film. Results were compared with predictions of the glass-rubber constitutive model used previously to interpret stress-strain curves during draw. Relaxation of the glassy element dominates at low temperatures, with an increasing contribution from slippage of entanglements at higher temperatures, as expected from the model. However, at high strains where strain-stiffening is observed (greater than ca 100%), only very limited stress relaxation is expected, as entanglement slippage is arrested by the onset of crystallisation during draw. By contrast, the magnitude of observed stress-relaxation in this region increases with final draw ratio, and can be as high as 50% of the final draw stress. An extension to the model is proposed which attributes this enhanced stress relaxation to crystallisation of the oriented network. Entanglement movement is assumed to be affine. If part of each chain “crystallises” by orienting preferentially in the draw direction, the remaining segments are left with a reduced stretch and hence carrying a lower force. Such a model suggests a quantitative relation between the magnitude of stress relaxation and the degree of crystallinity developed. Its predictions were checked by density measurements made after quenching to room temperature.