Evaluation of free and hydrostatic equibiaxial deformation of poly(ethylene terephthalate) by trirefringence measurement

Understanding the deformation behavior of an amorphous thermoplastic, e.g., poly(ethylene terephthalate) (a‐PET) and one of semicrystallinity, viz., i‐PP, can be is of both fundamental and practical importance. Much has been done to study the influence of temperature and strain, but not the influence of deformation conditions on the three‐dimensional orientation of many thermoplastic. In this study we consider the incremental effect of hydrostatic pressure and process geometry on strain‐induced deformation. a‐PET has been free drawn (i.e., no hydrostatic pressure) isothermally on a tenter frame device. The hydrostatic, equibiaxial deformation was achieved by a forging process involving squeezing between two circular plates. These two processes have been carried out at a series of deformation rates, with the three‐dimensional refractive index measurements having been made on the PET and i‐PP films using a modified Abbe refractometer, as per Samuels. The reported trirefringence measurement provides a sensitive scale for the planar deformation.