The dynamics crossover region in phenol- and cresol-phthalein-dimethylethers under different conditions of pressure and temperature

Dielectric relaxation times over a broad range of temperature and pressure for the glass former phenolphthalein-dimethylether (PDE) reveal a change of dynamics at a characteristic relaxation time tau(B). The value of tau(B) was found to be largely insensitive to the particular combination of pressure and temperature of the measurement. Data for a second glass former, cresolphthalein-dimethylether, having a molecular structure very close to that of PDE, were also analysed. In this case, tau(B) is much smaller, so the change of dynamics could not be observed in the elevated pressure experiments. The PDE data were in good agreement with the Adam-Gibbs model near T-g(tau > tau(B)), while deviating for tau < tau(B). Finally, a possible connection between the observed T-B and theoretical models is presented.