ENTHALPY RELAXATION IN A PARTIALLY CURED EPOXY-RESIN

The enthalpy relaxation of a partially cured (70%) epoxy resin, derive d from diglycidyl ether of bisphenol-A cured by methyl-tetrahydrophtha lic anhydride with accelerator, has been investigated. The key paramet ers of the structural relaxation (the apparent activation energy Delta h, the nonlinearity parameter x, and the nonexponentiality parameter beta) are compared with those of the fully cured epoxy resin. The agi ng rates, characterized by the dependences of the enthalpy loss and pe ak temperature on log(annealing time), are greater in the partially cu red epoxy than they are in the fully cured resin at an equivalent agin g temperature (T-a = T-g - 20 degrees C). There is a significant reduc tion in Delta h, from 1100 kJ mol(-1) for the fully cured system to 6 15 kJ mol(-1), as the degree of cure is reduced. The parameter x deter mined by the peak-shift method appears essentially independent of the degree of cure (x = 0.41 +/- 0.03 for the partially cured resin compar ed with 0.42 +/- 0.03 obtained previously for the fully cured resin), and does not follow the usually observed correlation of increasing x a s Delta h decreases. This invariability of the parameter x seems to i ndicate that it is determined essentially by the local chemical struct ure of the backbone chain, and rather little by the supramolecular str ucture. On the other hand, the estimated nonexponentiality parameter b eta lies between 0.3 and 0.456, which is significantly lower than in t he fully cured epoxy (beta approximate to 0.5), indicative of a broade ning of the distribution of relaxation times as the degree of cross-li nking is reduced. Like the parameter x, this also does not follow the usual correlation with Delta h. These results are discussed in the fr amework of strong and fragile behavior of glass-forming systems, but i t is difficult to reconcile these results in any simple way with the c oncept of strength and fragility. (C) 1996 John Wiley & Sons, Inc.