Dielectric relaxation of some rigid polar molecules in a polystyrene matrix

Abstract

Dielectric absorption studies have been carried out on solutions of twelve polar solutes dispersed in an atactic polystyrene matrix over a range of temperatures in the frequency region 50 to 10 5 Hz and in one case also in the range of 10 4 to 10 7 Hz. Rigid molecules of varying size and molecular dipole moments halobenzenes, p-halotoluenes, p-halobiphenyls, p-nitrobiphenyl and p-bromoethylbenzene - have been examined. All these solute molecules have in common the fact that the molecular dipole moment lies along the main principal axis. The relaxation times and energy barrier parameters for molecular dipole relaxations are determined. The Eyring enthalpy of activation, ΔH E , is found to range from 9 for fluorobenzene to 102 kJ mol −1 for p-iodobiphenyl. Relaxation data of these molecules indicate a linear correlation between enthalpy and entropy of activation, and the former is found to depend upon the volume needed for the reorientation of molecules. Present results for halobenzenes and p-halotoluenes reveal a linear dependence of log (relaxation time) on the mean moment of inertia. The enthalpy data for these rigid molecules are useful in the study of flexible molecules when a decision has to be made as to whether the absorption is to be ascribed to molecular or intramolecular or overlap of the two relaxation processes.