Effect of crosslink length on the enthalpy relaxation of fully cured epoxy-diamine resins

Abstract

Enthalpy relaxation of epoxy-diamine thermosets of different crosslink lengths (CLL) has been studied by DSC. The epoxy resins based on diglycidyl ether of bisphenol A were cured with ethylenediamine (FEDA), and diamines of polyoxypropylene of 2.6 and 5.6 oxypropylene units, named FJ230 and FJ400, respectively. As was expected, increasing the CLL decreases the glass transition temperature T g from 121°C (FEDA) to 47°C (FJ400). Aging experiments at T g - 20 K for each resin permit the determination of the enthalpy loss, the relaxation rate per decade (β H ), and the nonlinearity parameter, x. The apparent activation energy, Δh * , and the nonexponentiality parameter β are found for each resin from intrinsic cycles in which the sample is heated at 10 K min 1 following cooling at various rates through the glass transition region. An increase of CLL is related to an increase of β H , and of the nonlinearity parameter. In agreement with the general trend for thermoplastic polymers, the increase of the parameter x is correlated with a decrease of Δh * and with an increase in the nonexponentiality parameter. Application of the Adam-Gibbs (AG) theory reveals that the parameters B and T f /T 2 increase with CLL, corresponding to a decrease of the nonlinear behavior of the glassy epoxies. However, the T 2 values calculated in this way appear unrealistic, and the alternative assumption that T 2 = T g -51.6 K, making use of the universal WLF constant, leads to a much smaller variation of B, which nevertheless still increases with CLL. From a consideration of the minimum number of configurations required for a cooperative rearrangement, it is argued that the elementary activation energy Δμ increases, and the minimum size of the cooperatively rearranging region decreases as CLL increases. This is consistent with the relaxation process becoming more cooperative as the CLL decreases, as is suggested by the decrease in the value of β.