A dielectric study of molecular mobility at glass transition

Abstract

Molecular mobility at glass transition was studied by means of the thermally stimulated depolarization currents (TSDC) method in several glass-forming systems, including simply constituted liquids (propylene glycol), polymers (poly(propylene glycols)), and block co-polymers (polyurethanes). In all cases the TSDC thermogram was more complex than thermograms obtained with the same materials using other experimental techniques: classical ac dielectric methods, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). A sharp dipolar peak is always present in the TSDC thermograms at the glass transition temperature, as it is determined by DSC. The question is examined as to whether a part of the TSDC thermogram is due to space charge formation related to morphology changes at glass transition. The complex TSDC thermograms were experimentally decomposed into approximately single responses by means of special techniques offered by the TSDC method (thermal sampling techniques) and the spectra of activation energies and entropy factors were determined.