Laser light scattering studies of the dynamics of molecular reorientation of a viscoelastic liquid: α-phenyl o-cresol

Abstract

The reorientational motion of α-phenyl o-cresol molecules in the normal liquid and supercooled liquid state in the temperature range between 97 and −49.3 °C has been investigated using the dynamic light scattering technique. Above 5 °C, Fabry–Perot interferometry is used, but the photon correlation spectroscopic technique is employed below −28.9 °C. With the combined use of these two light scattering spectroscopic techniques, the orientational relaxation time over ten decades of change is measured. The shape of the VH time correlation function is nonexponential. The bimodal and the Williams–Watts distribution functions are used to fit the shape of the VH correlation functions at various temperatures. The Williams–Watts function gives the best fit to the observed shape. The correlation time and the shape of the VV time correlation function are found to be similar to that of the VH, suggesting that the reorientational process is coupled to the localized translational motion for molecules in the supercooled state. Angular dependent studies of the VH and VV time correlation functions indicate that in the supercooled state, the shear wave (a collective translational motion) influences the reorientational motion only negligibly. Various theoretical predictions about the profile of the central component of the VH spectrum based on the coupling of the reorientation to the shear wave are not supported by the experiment result. The temperature dependence of the orientational relaxation time does not follow the Arrhenius equation, but it follows the Antoine-WLF equation reasonably well. The Debye–Stokes–Einstein equation is found to be adequate to describe the temperature and viscosity dependence of the orientational relaxation time over ten decades of change.