Dynamic light scattering studies of supercooled phenylphthalein–dimethylether dynamics under high pressure

The behavior of the α-relaxation process in phenylphthalein–dimethylether (PDE) under high pressure was studied by means of dynamic light scattering–photon correlation spectroscopy (DLS-PCS). The temperature dependence of the relaxation time of the α-process at ambient pressure was found to resemble that obtained from dielectric relaxation (DR) measurements. On the basis of the DLS measurements one can conclude that PDE satisfies the correlation between nonexponentiality and fragility established for low-molecular weight glass forming liquids and polymers. On the other hand, dielectric relaxation studies indicate that PDE is an exception from this correlation. These differences between the DLS and DR studies do not result from the overlap of the α- and β-processes in the DLS experiments but might be due to the different coupling of the probes seen in these experiments (optical anisotropy and dipole moment) to the dynamics of the entire PDE molecule. We also studied the effect of pressure on fragility and the glass transition temperature in PDE and we found that fragility is practically independent of pressure.