Evaluation of the structure (or nonlinearity) parameter x by peak-shift method from volumetric heating data of PET Original Research Article

The structure parameter x, defining the temperature and structure relative contribution to the relaxation times, was evaluated applying the peak-shift method (PSM) onto volumetric heating data of poly(ethylene terephthalate) (PET) with content of crystallinity 2.6 wt%. The method of mercury-in-glass (MIG) dilatometry and three-step thermal cycle procedure were used. Two sets of heating scans were measured as a function of the amount of volume isothermal relaxation for two different relaxation temperatures below Tg (60 and 52 °C), whereas other experimental variables were kept constant. The experimentally determined heating isobar inflection temperature is a linear function of the amount of volume relaxation, which satisfies the application of PSM method for structure parameter x determination from volumetric heating data. The value of structure parameter of the used PET is independent of relaxation temperature and equal to 0.50±0.02. Finally, the shift of heating isobar inflection temperature with relaxation temperature (60, 56, 52 and 48 °C) measured for constant amount of volume isothermal relaxation was found to be linear, with the slope of 0.13.