EFFECTS OF DISTORTIONAL COMPONENTS IN BIAXIAL STRETCHING OF POLY(ETHYLENE-TEREPHTHALATE) SHEETS ON DIMENSIONAL STABILITY AND STRUCTURE

Thin sheets of poly(ethylene terephthalate) were stretched biaxially o ver a wide range to temperatures below the melting point of the polyme r. The linear shrinkage occurring at temperatures between 85 and 100-d egrees-C decreased with increasing draw temperature and draw ratio. Sp ecimens taken near the edges of the drawn sheets, which had been subje cted to in-plane shear deformations, were found to exhibit linear shri nkage 5-8 times lower than those taken from the middle of the sheet. S ubsequent experiments, using purpose-built clamps to achieve a more un iform state of shear in both directions of the biaxially drawn samples , confirmed the universality of the principle of shrinkage suppression by the superposition of shear deformations. X-ray diffraction studies revealed that the phenomenon was not related to differences in type o f orientation of the crystals. The information from the X-ray diffract ion studies and data from thermal analysis have led to the conclusion that the enhanced dimensional stability of biaxially drawn sheets subj ected to superimposed shear deformations results from a combination of a higher rate of stress-induced crystallization and a reduction in th e level of orientation within the amorphous phase.