Dielectric studies of molecular motions in some solids

Abstract

Dielectric absorption of a number of nonpolar, weakly polar and fairly polar molecules in the pure solid state are studied. Measurements of these solids using either a General Radio 1621 Precision Capacitance Measurement System or a General Radio 1615-A Capacitance bridge with appropriate temperature-controllable cells are described. The glass transition (Tg) measurements using the Glass Transition Measurement Apparatus are also described. The experimental data as a function of frequency at different temperatures are subjected to analysis by a series of computer programmes written in the APL language. The activation energy barriers for the dielectric relaxation processes were obtained by the application of ;the Eyring rate equation. The dielectric absorption of some spherically shaped nonpolar molecules are observed, the energy barrier values of which agree well with those found from other measurements. It is suggested that this absorption might be due to the interaction of the radiofrequency radiation with an induced moment which results from multiple interaction. Of the spherically-shaped polar molecules examined, both molecular and co-operative relaxations are detected in all the alkylhalides. Dielectric data for molecular relaxations are described by the Cole-Cole plots, whereas those for co-operative motion, in most of the cases, cannot be accurately represented by the Davidson-Cole skewed-arc function. Low temperature molecular relaxations are detected in some nonpolar and weakly polar aromatic hydrocarbons. The results of these molecules are used to suggest a possible correlation between enthalpy and entropy of activation and ionization potential and enthalpy of activation. The dielectric absorption of apparently nonpolar aromatic hydrocarbons are explained on the basis of quadrupole or octupole induced moment which may be of significant magnitude when the internuclear distances are small as in the solid state. Co-operative relaxations of some of the nonpolar and weakly polar hydrocarbons are found above the glass transition temperature (Tg). The variation in the energy barrier values for these relaxations are explained in terms of molecular interaction which is measured either by ionization potential or by the stretching frequency of a suitable solute molecule such as pyrrole (e.g. N-H) with which the aromatic hydrocarbon can form hydrogen bond. The molecular relaxation processes of a number of rigid arylhalides in the pure solid state are found almost in the same temperature and frequency regions as those found in different viscous media. A similar enthalpy of activation values are also observed. For some rigid heterocyclic molecules, except N-methyIpyrrole, no molecular relaxation is observed and this could be accounted for by the crystal structure and the lack of free volume for these molecules to relax.