A study of enthalpic relaxation of poly(ethylene terephthalate) by conventional and modulated temperature DSC

A standard power-compensation Perkin-Elmer DSC-2 was modified for temperature modulation experiments. Samples of amorphous poly(ethylene terephthalate) were aged in the DSC at temperature, Ta, for time, ta below the amorphous Tg. The progress of enthalpic relaxation was measured using conventional and modulated temperature DSC (MTDSC). By conventional DSC the extent of enthalpic relaxation was found to increase with ageing time accompanied by an apparent shift in the glass transition to a higher temperature. Using MTDSC the non-reversing enthalpic transition could be separated from the reversing glass transition. By MTDSC the extent of enthalpic relaxation was also found to increase with ageing time. However, by studying the reversing component it was found that there is no increase in the glass transition temperature with increased ageing time. It was found that the kinetics of enthalpic relaxation were more accurately determined by conventional DSC than MTDSC.