Anisotropic hygroelastic behaviour of oriented poly(methyl methacrylate)

A series of samples of oriented poly(methyl methacrylate) was prepared by drawing at a range of temperatures above T_g to a constant extension ratio. The birefringence was used as a measure of orientation. The specimens were immersed in water at 21°C and the coefficients of hygroelasticity were calculated from the dimensional change and weight gain data. Two coefficients, namely the longitudinal and the transverse, were measured for each sample. The longitudinal coefficients of hygroelasticity exhibit a linear decrease and the transverse exhibit an increase as the birefringence increases from zero. The results are analogous to those obtained for thermoelasticity (Wilson, J. D. and Treloar, L. R. G. J. Phys. (D) 1972, 5, 1614).