Stress-optical properties of amorphous polyethylene terephthalate fibres

Abstract

At temperatures above the glass transition, amorphous polyethylene terephthalate fibres have been shown to possess optical and mechanical properties identical with those for a rubber. Thus, both the stress and the optical anisotropy may be directly related to strain (conveniently determined by the degree of retraction or shrinkage of pre-oriented fibres). The theoretical treatment of Kuhn and Grün has been employed to calculate the polarizability of the random link and the number of effective cross-links per molecular chain. Additionally, measurements have been made of the decay of stress and optical anisotropy (birefringence) in water at various temperatures and as expected for a rubber-like network both decay at identical rates. Moreover, the decay curves for different temperatures can be superimposed by a simple frequency shift. The shift factors as a function of temperature fit the Williams-Landel-Ferry equation.