Dynamics of sorbitol at elevated pressure

Dielectric relaxation measurements were carried out on sorbitol as a function of pressure at various temperatures. The almost linear dependence of the structural relaxation times on pressure yields values for the activation volume. In light of results for xylitol and glycerol, the activation volume is found to be an increasing function of molecular size. Because the pressure coefficients of the glass temperature for these polyalcohols are all equal, their fragilities should parallel their respective activation volumes. This expectation is borne out by experimental measurements at ambient pressure. Analysis of the volume dependence of the relaxation. times reveals that temperature, rather than density, dominates the structural relaxation of sorbitol. At frequencies higher than the structural relaxation, a secondary process is observed. The weaker pressure sensitivity of the latter effects better resolution of the two peaks at high pressures. The relaxation time for the secondary process is consistent with a calculation from the coupling model based on identification of the secondary process as the precursor to the highly cooperative structural relaxation.