The effects of pressure and temperature on molecular dynamics during linear-chain polymerization by dielectric measurements

Abstract

Molecular relaxation of the various states formed during the course of growth of a linear-chain polymer by addition reactions of the amine group of cyclohexylamine with the epoxy groups of a diepoxide has been studied at isobaric conditions of hydrostatic pressures up to 206 bar and at several temperatures from 300 K to 314 K, by using dielectric measurements for a fixed frequency of 1 kHz, which are adequate for obtaining information on the relaxation time during the course of polymerization. The reaction occurs faster at high pressures and the curves of permittivity and loss against the polymerization time, which resemble the corresponding spectra, bodily shift to a shorter time. At 206 bar and 307.5 K, the ε″ plot shows contributions from a second, high frequency relaxation. The increase in relaxation time, when the reaction occurred at high pressures, has been discussed in terms of both (a) an increase due to the increase in the rate of chemical reaction and (b) the usual physical effect of pressure on molecular kinetics, and an attempt made to resolve the two effects. The effect of hydrostatic pressure predominates the molecular relaxation dynamics through an increase in the polymerization rate. Formalisms relating the chemical and physical processes are given, but not examined by experiments. The decrease in the configurational entropy is formulated in terms of the polymerization rate and pressure. The increase of the static permittivity of the mixture on compression is marginal. It decreases more rapidly with the progress of polymerization at high pressures. Two issues on obtaining information on molecular dynamics of a time-variant system from single-frequency measurements, raised by others since our earliest studies, have been elaborated and analytically clarified. By using simulated dielectric data it has been shown that the dc conductivity and interfacial polarization alter the shape of the dielectric permittivity and loss plots to make misleadingly alternative parameter fits possible.