TSDC study of the glass transition: correlation with calorimetric data

The glass transition in amorphous poly(ethylene terephthalate) is studied by thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC). The ability of TSDC to decompose a distributed relaxation, as the glass transition, into its elementary components is demonstrated. Two fractional polarization techniques, windows polarization (WP) and non-isothermal windows polarization (NIW) are employed to assess the influence of thermal history in the results. The Tool-Narayanaswami-Moynihan model has been used to fit the TSDC spectra. The most important contributions to the relaxation comes from modes with a value of the non-linearity parameter (x) around 0.7. Activation energies yield by this model are located around 1 eV (96 kJ mol-1) for polarization temperature (Tp) below 50 deg C and they rise up to values higher than 8 eV (771 kJ mol-1) as Tp increases (up to 80 deg C). There are few differences between results obtained with WP and NIW but, nonetheless, these are discussed. The obtained kinetic parameters are tested against DSC results in several conditions. Calculated DSC curves at several cooling and heating rates can reproduce qualitatively experimental DSC results. These results also demonstrate that modelling of the non-equilibrium kinetics involved in TSDC spectroscopy is a useful experimental tool for glass transition studies in polar polymers.