Modeling complex permittivity of an epoxy-amine system using simultaneous kinetic and microdielectric studies

A reactive epoxy-amine system based on diglycidyl, ether of bisphenol A (DGEBA) with 4,4 -diaminodiphenylsulfone (DDS) was studied during isothermal curing at, 140degrees and 160 degreesC, using simultaneous. kinetic and micro- dieletric studies to establish simple models to describe the changes in the dipole component of the permittivity, epsilon* , as a function of T-g and reaction advancement x(t). Having found that a simple relationship exists between the logarithm of the, relaxation time tau and the glass transition temperature,T-g, it is shown, that log tau follows the Di Benedetto equation's predicting dependence of T-g on reaction advancement as revisited by Pascault and Williams. Using this equation relating T-g and reaction advancement, the reaction advancement can be monitored directly by dielectric sensing of the changing value of the relaxation time. This equation has advantages, over the WLF relationship; as in place of the fitting parameters C-1 and C-2 there is only one parameter lambda, and it is independently experimentally, determined. The complex permittivity, epsilon*, was fit to the Havriliak-Negami function. The unrelaxed permittivity at high frequency epsilon(u) is assumed to be constant and the skewness parameter beta was found to be independent on the temperature the frequency, and the time while the width of the distribution decreased with time as characterized by alpha. During isothermal cure, the measurements made at different frequencies give the static permittivity epsilon(s) versus curing time t. As the reaction proceeds, the disappearance of epoxy and amine functions as responsible for a decrease in the effective moment of the dipoles as characterized by epsilon(s), and of the diminution of the width of the distribution of relaxation times characterized by an increase in alpha. It is shown that the decrease in epsilon(s) with cure time can also be used to monitor the extent of reaction advancement. Thus, in addition to cure monitoring measurements based on, conductivity, dielectric measurements,of the changes in the: dipolar relaxation time, tau, and the low frequency static dipole polarization epsilon(s) can be quantitatively used to monitor reaction, advancement.