Orientation and crystallization in poly(ethylene terephthalate) during drawing at high temperatures and strain rates

Abstract

AbstractWe review some recent research developments on structure development during drawing of poly(ethylene terephthalate) film, and we report a study of constant‐load drawing of amorphous PET film at temperatures of 120°C and 132°C, including the effects of redrawing high‐temperature drawn film at lower temperature. To permit constant‐load drawing at high temperature without inducing crystallization in the undrawn specimen, a drawing instrument was built that permits very rapid heating of the sample, and its operation is described. The initial stage of drawing at high temperatures is characterized by polymer flow where, owing to high rates of molecular relaxation, neither molecular orientation nor crystallization occurs. Strain‐rate increases sharply in the course of the deformation, reducing the time available for relaxation, and the chains start to orient at a draw ratio that depends on temperature. Orientation rapidly reaches a saturation level, which is lower at the higher draw temperature. Crystallization onset seems to lag only slightly behind orientation onset because the critical orientation for inducing crystallization is very low at these temperatures. It appears that there is time for crystallization to proceed to pseudo‐equilibrium values corresponding to a particular orientation level, which differs from previous results obtained from constant‐force drawing at lower temperatures, and possible reasons for this are discussed. In two‐stage drawing, where film drawn at 132°C was redrawn along the same axis at 100°C, high draw ratios were obtained despite the high strain rates, and the levels of noncrystalline orientation and crystallinity were similar to the levels expected from single stage drawing at 100°C.