Effect of pressure on the α relaxation in glycerol and xylitol

The effect of pressure on the dielectric relaxation of two polyhydroxy alcohols is examined by analysis of existing data on glycerol, together with new measurements on xylitol. The fragility, or T-g-normalized temperature dependence, changes with pressure for low pressures, but becomes invariant above 1 GPa. When compared at temperatures for which the alpha-relaxation times are equal, there is no effect of pressure (<1 GPa) on the shape of the alpha dispersion at higher temperatures. However, nearer T-g, pressure broadens the alpha peak, consistent with the expected correlation of fragility with the breadth of the relaxation function. We also observe that the alpha-relaxation peaks for both glycerol and xylitol show an excess intensity at higher frequencies. For xylitol, unlike for glycerol, at lower temperatures this wing disjoins to form a separate peak. For both glass formers, elevated pressure causes the excess wing to become more separated from the peak maximum; that is, the properties of the primary and excess intensities are not correlated. This implies that the excess wing in glycerol is also a distinct secondary process, although it cannot be resolved from the primary peak. (C) 2002 American Institute of Physics.