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 over a wide range to temperatures below the melting point of the polymer. The linear shrinkage occurring at temperatures between 85 and 100-degrees-C decreased with increasing draw temperature and draw ratio. Specimens taken near the edges of the drawn sheets, which had been subjected to in-plane shear deformations, were found to exhibit linear shrinkage 5-8 times lower than those taken from the middle of the sheet. Subsequent experiments, using purpose-built clamps to achieve a more uniform 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 of orientation of the crystals. The information from the X-ray diffraction studies and data from thermal analysis have led to the conclusion that the enhanced dimensional stability of biaxially drawn sheets subjected to superimposed shear deformations results from a combination of a higher rate of stress-induced crystallization and a reduction in the level of orientation within the amorphous phase.