Effects of thermal treatment on biaxially oriented poly(ethylene terephthalate). II. The anisotropic glass temperature

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Abstract
Thermal properties of an anisotropic biaxially oriented Poly(ethylene terephthalate) (PET) were determined before and after further deformation of the Mylar film. Film shrinkage in different planar directions was monitored during and following initial heating. After stabilization for three days at 140°C, glass temperatures Tg were determined from the decrease in length of film strips and were found to vary in the different in‐plane directions. An increase in anisotropy brought about by additional deformation in the direction of the greatest orientation enhanced the Tg difference from 8 to 16°C. Tg is highest in the direction of greatest orientation.