Temperature dependence of photoelastic properties of crosslinked amorphous polyethylenes

Crosslinked samples of polyethylene were prepared by electron irradiation of both high‐ and low‐density polymers in the crystalline state. A further crosslinked sample was obtained by curing a high‐density polyethylene by reaction with dicumyl peroxide at 180°C. The stress–strain–birefringenece relations were obtained on specimens cut from these samples at temperatures between 130 and 250°C. All samples showed a substantial decrease in stress‐optical coefficient with increasing degree of crosslinking and with increasing temperature. The stress‐optical properties at each temperature were extrapolated to zero degree of crosslinking to give quantities characteristic of the Gaussian network. Comparison of these properties with the theory of networks of rotational isomeric chains with both independent and interdependent rotation allows estimates to be obtained for ( 1 ) the trans – gauche energy differences in rotation around skeletal bonds and ( 2 ) the difference in principal optical polarizabilities for the CH 2 group in the elastomeric state. This latter quantity is shown to be more nearly given by Denbigh's than by Bunn and Daubeny's bond polarizability values.