Change of amorphous state of poly(ethylene terephthalate) by heat treatment below the glass transition temperature

Several papers have demonstrated that structural changes in the amorphous regions of semicrystalline polymers can be produced by heat treatment below the glass transition temperature (T_g). In this paper, we report structural change in the amorphous phase of poly(ethylene terephthalate), heat-treated below and above T_g. The density, the T_g, the endothermic peak at T_g and the relative spectral intensity in the 973 cm^?1 band (due to the CO stretching vibration), all increased with heat treatment below T_g, but the specific heat decreased. The stability of the amorphous state was examined by further heat treatment at temperatures above T_g and sufficiently high for crystallization, and it was verified that structural changes in the amorphous regions do not result in acceleration of the rate of crystallization. We therefore suggest that the amorphous region is one phase, rather than two phases consisting of random and regular regions.