Real time dielectric spectroscopy and bond connectivity during polymerization of stoichiometric and amine-rich, diepoxide–diamine compositions

Abstract

The dielectric spectra and thermodynamics of macromolecular growth in the diepoxide–diamine liquid mixtures have been studied simultaneously, and in real time, as the original molecular liquid transformed to a rigid state of a vitrified network structure. Both stoichiometric and the excess amine compositions were studied and the dielectric properties of their final polymerized state investigated. The rate of polymerization is remarkably increased in the diamine-rich composition, and vitrification occurs sooner. A change in the rate of heat release profile is attributed to the onset of another reaction at longer times, which does not require the diamine. The dc conductivity, σ0, decreases progressively more rapidly during the initial stage of polymerization, producing a sigmoid shape when it is plotted against time. This shape is stretched out when plots are made against the number of covalent bonds formed. For a fixed number of covalent bonds, n, formed at 324.5 K, the relaxation time, τ0, for the diamine-rich composition is twice that of the stoichiometric composition. Interpretation in terms of the configurational entropy theory shows that this is due to the lowering of vibrational frequencies. The stretched exponential relaxation parameter. β, increases from 0.25 for the stoichiometric to 0.36 for the diamine-rich composition. The product, τ0σ0, increases with increase in n, thus showing that τ0 and σ0 are not inversely related, and that requires consideration of other mechanisms for dc conduction. A faster dipolar reorientation dynamics evolves on polymerization and, in the fully polymerized state of the amine-rich composition, its peak shifts to a lower frequency.