Dielectric susceptibility of the supercooled liquids: Glycerol and triphenyl phosphite
We measure the dielectric susceptibility &egr;(ν) of supercooled liquid glycerol and map it out as a function of pressure, temperature and volume. We build a high pressure cell made of beryllium copper to exert high pressure (up to 900 MPa) on glycerol at low temperature (near glass transition temperature). The frequency range is from 0.01 Hz to 100 kHz. We also measure the susceptibility of triphenyl phosphite, TPP, to study its polyamorphic phases.
We find that the glass transition temperature, the fragility, and the width of the dielectric loss spectrum of glycerol increase with pressure. We introduce the notion of the generalized fragility to separate the effects due to volume and temperature on the glass transition and conclude that both volume and temperature are comparably important for glycerol. We point out a connection between the generalized fragility and a recently discovered volume-temperature scaling exponent and find that our conclusion holds for many glass formers. The relation between the traditional fragility and the width is compatible with a correlation found between these quantities at 1 atmosphere. The width, too, changes more quickly as a function of fractional changes in volume than the corresponding change in temperature. We verify the Nernst-Einstein relation between the ionic conductivity and the relaxation frequency at various pressures and find that the effective hydrodynamic radius in this relation decreases with density.
We have measured &egr;(ν) of TPP from 0.01 Hz to 10 kHz. The glacial phase of TPP has a longer relaxation time, wider loss spectrum, and lower dielectric constant compared to the supercooled liquid. The width of &egr;(ν) in the glacial phase increases with temperature. The dielectric function is measured as a function of time while TPP is transforming from the supercooled to the glacial and the glacial to the crystal. We present a differential effective medium calculation to investigate the possibility that the glacial phase is a mixture of liquid and crystal. We have also videotaped, in real time, the phase transformation from the supercooled phase to the glacial. Based on these investigations we conclude that the glacial phase is a new amorphous phase, distinct from the supercooled liquid.