Extrusion drawn amorphous poly(ethylene terephthalate): 4. Irreversible spontaneous elongation

Abstract

Uniaxially-oriented poly(ethylene terephthalate) (PET) films prepared by solid-state co-extrusion exhibit irreversible spontaneous elongation (rather than shrinkage) under specific conditions. Results for these conditions show that marked elongation (up to 20%) can occur during annealing of unconstrained samples. This phenomenon, which is not commonly observed for polymers, depends strongly on the prior conditions of extrusion draw. There is significant increase in length for films prepared with extrusion draw ratio (EDR) at 2.0 in the extrusion draw temperature ( T ext) range 80–100°C. The extrusion rate is also significant. Lower extrusion rates favour spontaneous elongation on subsequent heating. In addition, the annealing temperature ( T a) also affects elongation. Samples extruded at T ext=80°C to EDR of 2.0 show maximum elongation at T a=180–190°C. However at higher temperatures, e.g. at 10°C below the melting temperature and higher, shrinkage occurs instead. Moreover, annealing at T a=90°C for different periods of time ( t a) shows that prior to the elongation a moderate amount of shrinkage occurs ( t a ⩽ 30 s). The results suggest a correlation between spontaneous elongation and crystallization during anealing.