The elastic constants of oriented glassy polymers

The paper is concerned with the measurement of the anisotropic elastic properties of drawn glassy polymers, namely perspex and polystyrene. The method used involved the measurement of the critical angle for total reflection of an ultrasonic beam incident on the surface of the polymer when immersed in a suitable liquid. Measurements were made for both compressional and shear waves propagated at various angles to the direction of drawing; from the data so obtained the values of the five stiffness constants required to define the properties of an axially symmetric material were calculated. The design of the instrument for the critical-angle measurements is described, and the basic theory required for the calculations presented. Results are given showing the dependence of stiffness constants on degree of orientation (birefringence). Only the axial stiffness constant is found to be markedly dependent on orientation and for this the increase is only about 30%. These conclusions are supported by data on the angular dependence of Young's modulus for the same polymers obtained by a static loading method.