Pressure dependence of the glass transition temperature in molecular liquids and plastic crystals

The effect of pressure on the glass transition temperature of a variety of molecular liquids both hydrogen bonded and non-hydrogen bonded, and their solutions has been investigated. It is found generally that the more extensive the hydrogen bonding in the liquid the smaller is the effect of pressure on T_g. This observation applies also to a binary mixture, in which dT_g/dP is found to change linearly between the end member values. The effect of pressure on the glasslike transition in the cyclohexanol plastic crystal phase is as expected for a liquid alcohol. For the lubricant 5P4E, the data are in accord with predictions based on high-pressure light scattering data. The results are interpreted by using the second Davies-Jones relation, dT_g/dP = VT(Δα/ΔC_p). Some evidence suggesting that the excess entropy frozen in at the glass transition is not quite independent of pressure as often supposed is presented.