High-pressure specific-heat spectroscopy at the glass transition ino-terphenyl

Abstract

Measurements of the enthalpy relaxations in liquid orthoterphenyl in the supercooled state have been carried out using specific-heat spectroscopy over the frequency range from 2 Hz to 6.3 kHz, as a function of temperature and as a function of pressure. The observed α-relaxation peaks in the phase of the complex specific heat show increasing relaxation times τ with increasing pressure at constant temperature, similar to the divergence of τ when the calorimetric glass temperature ${\mathit{T}}_{\mathit{g}}$ is approached by lowering the temperature at constant pressure. The temperature and pressure dependence of the measured mean relaxation times τ¯ near ${\mathit{T}}_{\mathit{g}}$ are in remarkable agreement with those found by other spectroscopic methods and have been compared with an extended Vogel-Fulcher-Tammann law. However, we find different scaling when the glass transition is approached by cooling or by increasing pressure. This suggests that the assumption of a simple volume-activated process is not adequate.