The effect of pressure on the structural and secondary relaxations in 1,1′-bis (p-methoxyphenyl) cyclohexane

Abstract

The dielectric spectrum of the organic glass former 1,1′-bis (p-methoxyphenyl) cyclohexane was measured over a range of temperatures and pressures, corresponding to a variation of the structural relaxation time, τα, by 8 decades. The temperature dependence of τα corresponded to a fragility equal to 72, which is consistent with the correlation of same with the shape (breadth) of the relaxation function. The dependence on pressure of τα could be described as a simple activated process, with a pressure-independent activation volume equal to ∼230 cm3/mol, varying inversely with temperature. The pressure coefficient of the glass temperature, Tg, was 240 K/GPa. At frequencies beyond the structural relaxation peak, there is a second, thermally activated process, having an activation energy=74.2 kJ/mol. The relaxation times for this process were invariant to pressure. Extrapolation of these relaxation times (measured below the Tg) intersects the ambient pressure structural relaxation data at a temperature, Tβ=268 K. This is about equal to the temperature, TB, at which the structural relaxation times deviated from a single Vogel–Fulcher–Tamman curve. In this respect, the secondary process exhibits the properties of a Johari–Goldstein relaxation. However, since Tβ varies with pressure, it remains to be seen whether its equivalence to TB is maintained at elevated pressure.