DIELECTRIC-RELAXATION SPECTROSCOPY OF REACTION-CONTROLLED SLOWING OF MOLECULAR-DIFFUSION IN LIQUIDS

The dielectric relaxation spectra of a liquid in which chemical reactions lead to the growth of a macromolecule have been measured at different instants of reaction and in the kHz and MHz frequency range. The change in the complex permittivity measured for a fixed frequency, as the reaction proceeds, is phenomenologically similar to that observed on varying the frequency for a time-invariant system and obeys a stretched exponential relaxation function. The increase in the relaxation time of the liquid with reaction time is similar to the corresponding increase in the extent of reaction. As the reaction proceeds, a second relaxation, namely the alpha-process, separates and shifts towards the lower frequency side of the spectrum. Concomitantly, its strength decreases as does the strength of the remaining high-frequency process. The manifestation of the negative feedback between chemical reactions and molecular diffusion is remarkably similar to that observed on supercooling a molecular or polymeric liquid. A formalism given here illustrates the general aspects of this similarity.