The importance of the activation volume for the description of the molecular dynamics of glass-forming liquids.

High pressure dielectric measurements were carried out on hydrogen bonded d-glucose and two van der Waals peracetyl saccharides, i.e. α pentaacetyl glucose and α octaacetyl maltose. In this study we found that after removing H bonds, the molecular dynamics of both modified saccharides is very sensitive to pressure, as reflected by a large value of the pressure coefficient of the glass transition temperature, equal to 270 K GPa(-1) and 280 K GPa(-1) for α pentaacetyl glucose and α octaacetyl maltose, respectively. On the other hand, dT(g)/dP for d-glucose is much lower, equal to 67 K GPa(-1). Our result confirms the general rule that the hydrogen bonding glass-forming liquids exhibit much lower values of dT(g)/dP compared to the van der Waals systems. Additionally, on the basis of results reported herein and also recent literature data for polyalcohols, we point out that the activation volume correlates fairly well with the molecular volume in the case of hydrogen bonding liquids. On the other hand, much larger values of the activation volumes at T(g) with respect to the molecular volumes were found for both peracetyl saccharides.