Relaxation Phenomena In Thermosets

Abstract

<p>Two aspects of the electrical and light scattering properties of various epoxide based thermosets have been studied. In the first, the isothermal curing kinetics of the thermosets have been measured at several temperatures by dielectric spectroscopy and Brillouin scattering measurements. During the process of curing, the dc conductivity decreases according to a scaling law, δ₀ ∝ (tg-t)ˣ, or equivalently to a new equation, σ₀ ∝ exp[-B/(t₀-t)], and approaches zero on gelformation. Concomitantly, the time for the dipolar relaxation process becomes progressively longer and the dielectric permittivity becomes dominated by dipolar relaxation processes. The time dependence of the complex permittivity follows the formalism, ϕ(t)=exp-(t/τcure)ˠ with ɣ<0.4. ɣ decreases as the curing temperature is increased and tends towards a limiting value at a high curing temperature. The relaxation time increases on curing and the rate of its increase with respect to curing time first reaches a maximum and then decreases towards zero. This phenomenon is a manifestation of the rates of chemical processes which control the extent of cure. The initially broad Brillouin peak becomes narrow and shifts towards higher frequencies with the curing. The changes in the hypersonic velocity and attenuation during the curing of a thermoset correspond to the changes observed in the dielectric studies.</p> <p>In the second, both the sub-Tg and main relaxation processes of the thermosets were measured by dielectric spectrometry and their dependence on the curing and ageing were investigated. Amongst the two sub-Tg relaxation processes, the low temperature process is initially prominent and its strength decreases on both curing and ageing of a thermoset. The strength of the high temperature sub-Tg process initially increases, reaches a maximum value and then decreases on further ageing. A concept of accumulated equivalent curing time is introduced and theoretically justified for use in the investigation of the curing of thermosets, and a general method for obtaining the asymmetric distribution of relaxation times parameter from limited relaxation data is developed. For the sub-Tg relaxations, the calculated parameter remains constant during the curing process, but for the main relaxation it monotonically decreases towards a limiting value. The theoretical analysis developed here is generally applicable to phenomena where molecular diffusion allows a chemical reaction to occur, which in turn retards molecular diffusion which slows the chemical reactions, until a material reaches its vitreous state and both the diffusion and chemical reactions cease to occur over ones experimental time scale.</p>