The anisotropy of the elastic constants of transversely isotropic polyethylene terephthalate

Amorphous polyethylene terephthalate was stretched below the glass transition temperature to various degrees of orientation. The resulting material had transverse symmetry and, according to classical elasticity theory, its elastic behaviour could be described by five independent constants. Three could be calculated from Young's modulus experiments and the remaining two from torsional measurements. Such experiments were carried out using a 25 second (square) stress cycle. Increasing orientation (birefringence) caused an increase in Young's modulus in the stretch direction to about five times the isotropic value. The transverse modulus and the 45° modulus were little affected by orientation. The shear modulus involved when twisting about the symmetry direction also showed only small variations while the value of the other shear modulus decreased to about half the isotropic modulus. Calculations of the volume and linear compressibilities showed that oriented polyethylene terephthalate behaves under hydrostatic pressure essentially as the isotropic polymer of the same density. This observation implies two further relations between the elastic constants and thus the number of independent constants for the oriented material reduces from five to three.