Relaxations of thermosets. IV. A dielectric study of crosslinking of diglycidyl ether of bisphenol‐a by two curing agents

Abstract

AbstractThe dielectric permittivity ϵ′ and loss ϵ″ during the crosslinking or curing of diglycidyl ether of bisphenol‐A with diaminodiphenyl methane and diaminodiphenyl sulfone have been measured from the sol to gel to glass‐formation regions. The ϵ′ monotonically decreases with time and the ϵ″ initially decreases, then increases to a peak value and finally reaches extremely low values characteristic of the glassy state. These features shift to shorter times with an increase in the temperature of curing. Complex‐plane plots of ϵ′ and ϵ″ have the shape of an are skewed at both the short‐time and the long‐time intercepts and resemble Cole–Cole plots of ϵ*. Thus the dielectric consequences of the chemical changes with time during the cross‐linking of a thermoset are analogous to the frequency dependence of ϵ* in an amorphous solid. The time dependence of ϵ* follows a stretched exponential decay, ϕ(t) = exp [− (t/τ)γ] where 0 < γ < 1. The parameter γ is in the range 0.2 to 0.4, and increases with a decrease in the curing temperature. Additional curing at longer times produces polymer segments in the network with a high reorientation rate. This is observed as deviations from the shape of a skewed arc at the limiting long‐time intercept and appears as a secondary relaxation during the curing process, but at very long times. The ϵ′ and ϵ″ have been analyzed and the roles of dc conductivity, Δϵ, and γ in determining the shape of the curing isotherms have been estimated. A representation of the data in the complex electrical modulus, M*, formalism shows the occurrence of two relaxation processes during the period of a typical isothermal cure. The feature observed at the shortest time is due to conductivity relaxation and the subsequent feature is due to the dipolar relaxation processes during the curing. The time of cure at which the relaxation rate reaches a fixed value follows an exponential relation with the reciprocal curing temperature with parameters which are characteristic of a thermoset. The decrease in dc conductivity during sol → gel conversion follows the scaling law used in the description of a critical phenomenon.