The Effect of Volume Reorganization of Amorphous Poly(ethylene Terephthaiate) on Thermal Properties

When poly(ethylene terephthalate) was quenched from above T_g and then heated, it exhibited a step increase in thickness in the glass transition region at every rate tested. When the polymer was cooled more slowly than it was heated, a higher T_g and a slightly larger step increase in thickness were observed as the cooling rate was reduced. These experimental results appear to be adequately interpreted on the basis of the normal structural changes that occur in a glass as its thermal history is varied. Two observations, however, were not easily included in this view. First, the polymer, on cooling from above T_g, exhibited an abnormally high expansion coefficient over much of the range of temperature in which it exists as a fluid. Second, the polymer exhibited a step increase in thickness when it was heated at the same rate at which it had previously been cooled.