Dynamics of water in supercooled aqueous solutions of glucose and poly(ethylene glycol)s as studied by dielectric spectroscopy

摘要

The dielectric behaviour of aqueous solutions of glucose,poly(ethylene glycol)s (PEGs) 200 and 600,and poly( vinyl pyr-rolidone) (PVP) has been examined at different concentrations in the frequency range of 10~6-10~(-6) Hz by dielectric spectroscopy and by using differential scanning calorimetry down to 77 K from room temperature.The shape of the relaxation spectra and the temperature dependence of the relaxation rates have been critically examined along with temperature dependence of dielectric strength.In addition to the so-called primary (alpha-) relaxation process,which is responsible for the glass-transition event at T_g,another relaxation process of comparable magnitude has been found to bifurcate from the main relaxation process on the water-rich side,which continues to the sub-T_g region,exhibiting relaxation at low frequencies.The sub-T_g process dominates the dielectric measurements in aqueous solutions of higher PEGs,and the main relaxation process is seen as a weak process.The sub-T_g process was not observed when water was replaced by methanol in the binary mixtures.These observations suggest that the sub-T_g process in the aqueous mixtures is due to the reorientational motion of the 'confined' water molecules.The corresponding dielectric strength shows a noticeable change at T_g,indicating a hindered rotation of water molecules in the glassy phase.The nature of this confined water appears to be anomalous compared to most other supercooled confined liquids.