High-temperature viscoelasticity and heat-setting of poly(ethylene terephthalate)

Abstract

Viscoelasticity of semicrystalline drawn filaments of poly(ethylene terephthalate) was studied in the temperature range critical in forming processes: between the glass transition and melting regions. Measurements were made of strain recovery following heat-setting in torsion. Microstructural features were also monitored, by density measurement and X-ray scattering. Heat-treated specimens were found to exhibit two distinct heat-setting processes. One corresponded to the glass transition. The other was situated between the glass transition and melting regions. This second process has been compared with predictions for heat-setting due to (a) melting/recrystallization, (b) viscous flow. The mechanism is deduced as being viscous flow, corresponding to mechanical relaxation — an α′-relaxation — separate from the glass transition, and probably arising from entanglement slippage. On heat-treatment, the dominant α′-relaxation time at the temperature of treatment was found consistently to increase to approximately equal the time allowed for heat-treatment. This indicates a link between the molecular diffusion processes responsible for α′ stress relaxation and those involved in evolution of microstructure and properties during heat-treatment. The magnitude of the α′-relaxation showed increases with increasing time, but decreases with increasing temperature of pre-treatment above 200°C. These are explicable in terms of microstructural changes by using a simple quantitative model for calculating the effective cross-link density of the non-crystalline matrix provided by crystals.