Some dielectric studies of molecular and intramolecular relaxation processes

Abstract

Dielectric relaxation studies of some potential systems involving molecular interaction particularly intermolecular and/or intramolecular hydrogen bonding as well as of some related molecules have been carried out in which atactic polystyrene and several other glass-forming media, namely, glassy o-terphenyl, bis(m-(m-phenoxy phenoxy)- phenyl) ether (commonly known as Santovac®), cis-decalin and carbontetrachloride were utilized as solvents. Sample preparations and the dielectric measurements by the use of a General Radio 1621 Precision Capacitance Measurement system with appropriate temperature controllable cells have been described. The glass transition temperature (T[subscript g]) measurements using the Glass Transition Temperature Measurement Apparatus have also been described. The experimental data as a function of frequency at different temperatures were subject to analysis by a series of computer programmes written in the APL language. The activation energy barriers opposing the dielectric relaxation processes were obtained by the application of the Eyring rate equation. Different types of polar, fairly spherical, rigid molecules have been studied mainly to provide sources of relaxation data and activation parameters for comparison with those of flexible molecules of analogous size. The molecular relaxation parameters for these rigid molecules were found to depend on the size, shape and volume of the molecules and the nature of the dispersion medium. The solute concentration has a negligible effect on the molecular relaxaton parameters but it influences the dielectric loss factor, e", significantly. At lower .concentration, the dielectric loss factor increases linearly with the solute concentration and at higher concentration after a certain point it begins to decrease towards the value observed for the pure molecule. This is accounted for by intermolecular interactions. Of the flexible molecules, a variety of some simple almost spherical alcohols, and some long-chain aliphatic normal alcohols and thiols have been studied in different g1ass-forming media. In the usual concentration range of polystyrene matrices ('^5% by.wt.) no evidence of intermoleculr hydrogen bonding was found in simple alcohols, long-chain alcohols and thiols. For simple alcohols only molecular relaxation was observed. Long-chain alcohols and thiols exhibited two relaxation processes. The lower temperature processes were attributed to segmental rotation involving CH[subscript 2]X movement while the higher temperatures were respective molecular rotation.. Relaxation due to hydroxyl group rotation was not found in any case. At higher concentration molecular relaxation followed by hydrogen bond breaking and in some cases relaxation for hydrogen bonded species was observed in G.O.T.P., carbontetrachloride and polystyrene. As in the case of rigid molecules, similar effects of solute concentration upon the molecular relaxation parameters and dielectric loss factor, e[symbol], have been observed for simple alcohols in carbontetrachloride. A wide variety of potentially intramolecular hydrogen bonded substituted phenols has been examined in cis-decalin, G.O.T.P. and Santovac® and in most cases, hydroxyl group relaxation was observed. The relaxation parameters for hydroxyl group rotation were found to be significantly influenced by the strength of the intramolecular hydrogen bond but it was virtually independent of the nature of the dispersion medium as well as the nature of the substituent at the para-position of the ring. No evidence for proton tunneling was detected in these molecules.