Enthalpy relaxation of non-stoichiometric epoxy-amine resins

Abstract

The effect of stoichiometry on the enthalpy relaxation of polyethertriamine-cured bisphenol-A-diglycidyl ether epoxies has been investigated by differential scanning calorimetry (DSC). As was expected, the glass transition temperature Tg decreases in the non-stoichiometric epoxies. The enthalpy loss determined by ageing experiments at Tg−20K, for times between 1 and 3000h, allows the calculation of the rate of relaxation per decade (βH) and the non-linearity parameter (x). These parameters have been determined for the stoichiometric resin and also for the non-stoichiometric resin with ratios (r) of amine/epoxy of 0.8 and 1.5. The other key parameters, namely the apparent activation energy (Δh∗) and the non-exponentiality parameter (β), have been determined from intrinsic cycles in which the sample is heated at 10Kmin−1 following cooling at different rates. Results show that βH and x increase and Δh∗ decreases in the non-stoichiometric resins, with no significant change of the non-exponentiality parameter β. These results have been interpreted in terms of the changes of the network structure introduced by the non-stoichiometric ratios.The Adam–Gibbs (AG) theory has also been applied to the enthalpy relaxation of all these systems, from which it is found that the configurational entropy at Tg increases in the non-stoichiometric systems, particularly for r=1.5. By considering that the minimum number of configurations needed for cooperative relaxation (W∗) is constant for all systems, it is also shown that a decrease of the lower limit of cooperatively rearranging regions (z∗) is observed in the non-stoichiometric resins, again particularly for the resin r=1.5. Values of T2, the second-order transition temperature at which the configurational entropy reduces to zero, are derived from the measured values of the non-linearity parameter x, and found to be anomalously low. It is argued that this results from an inadequate extension of the AG theory to the non-equilibrium glassy state, and a recently proposed alternative approach is used to rationalise the observation in terms of processes at the glass transition that favour the freezing-in of either holes or high energy bond conformations.