The influence of pre-orientation on the optical and mechanical anisotropy of drawn polymers

Expressions have been derived for the optical birefringence and elastic constants of an initially isotropic polymer or fibre subjected to a two-stage stretching process. The first stage is carried out at high temperatures, with the polymer or fibre in its rubbery state, and the second stage is cold drawing. The predicted birefringences and extensional moduli show good agreement with experimental data for polyethylene terephthalate.