Isothermal and isobaric PVT-measurements of anionic polystyrenes: ‘Relaxation zones’ within the glass transition range

PVT-measurements of anionic polystyrenes on heating have shown that, depending on the mode of operation, specific 'relaxation zones' within the glass transition range are observed. Isothermal PVT curves exhibit always 'relaxation zones' independent whether the pressure is increased or decreased during the scan. The shift of the relaxation zones to higher temperatures is, however, higher for isothermal scans with increasing pressure. These 'relaxation zones' are explained by pressure-dependent changes of the state of the polymeric sample isothermally scanned within the glass transition range. At lower pressures the polymer is actually in the molten state, whereas at higher pressures it may be in the metastable glassy state and the actual state depends on the rate of pressure change. In isobaric PVT curves 'relaxation zones' in heating scans are exhibited only if the pressure applied during glass formation differs from the pressure applied during the heating scan. The observed pressure-dependent shift of the glass temperatures to higher temperatures was higher for the studied polystyrenes of different molecular weight that had a higher glass temperature at normal pressure. But the specific molecular weight influence on the width of the 'relaxation zone' could not be ascertained. An attempt to calculate characteristic volume relaxation times failed because of insufficient precision of the measurements.