Investigation of the correlation between structural relaxation time and configurational entropy under high pressure in a chlorinated biphenyl

Abstract

Dielectric relaxation measurements on a chlorinated biphenyl (PCB62) were carried out over a broad frequency range, with variation of both temperature and pressure. In combination with calorimetric determinations of the configurational entropy, these data could be described using the Adam–Gibbs model. Specifically, the experimental results were interpreted using a recently introduced equation for both the temperature and pressure dependencies of the structural relaxation time. The $\mathrm{\tau (T,P)}$ data for PCB62 yielded values of the fitting parameters consistent with known physical properties of the material. A change of the dynamics was evident in isobaric measurements at atmospheric pressure, corresponding to a value of the relaxation time ${\tau }_B{\text{∼5×10}}^{\text{−5}}\hspace{0.17em}\mathrm{s}.$ A related change of dynamics was observed in isothermal experiments at varying pressures. It is noteworthy that the latter transpired at a very similar ${\mathrm{\tau \sim \tau }}_B.$ Moreover, the shape of the relaxation function depended only on the value of τ. We believe this is the first reported evidence of such a change of dynamics in experiments using pressure as a variable. These results suggest that this change is governed by the time scale of the relaxation, independently of any particular combination of T and P.