Calorimetric studies of isothermal curing of phase separating epoxy networks

To get a better understanding of the curing process of multi-component ther mosets differential scanning calorimetric (DSC) and temperature modulated D SC (TMDSC) measurements were performed during isothermal curing of semi-int erpenetrating polymer networks (semi-IPNs) with amounts of 10 or 20 wt.% of linear polymer and of the corresponding pure networks at temperatures betw een 333 and 393 K. The network component consists of diglycidylether of bis phenol A (DGEBA) cross-linked with diaminodiphenyl methane (DDM) and the li near polymer component is polysulfone (PSn) or polyethersulfone (PES). The curing time dependence of conversion was estimated from time dependent heat flow measurements during isothermal curing. The curing kinetics is discuss ed in the framework of different models taking into account the catalytic e ffects and the influence of diffusion. A lower reaction rate was found in t he semi-IPNs compared with the pure networks which is related to a decrease of the diffusion coefficient and/or the density of reacting units due to t he linear polymer component. The final conversions were found to decrease w ith an increasing amount of the linear polymer component and with decreasin g curing temperature which corresponds to less perfect network structures. The time evolution of the glass transition temperatures during isothermal c uring was determined by the DSC and TMDSC. At the beginning of the reaction only one glass transition-indicating a one phase system-was found whereas at later stages of curing the two phase structure-consisting of a DGEBA/DDM -rich and a PSn-or PES rich phase-was indicated by two glass transition tem peratures. (C) 1999 Elsevier Science Ltd. All rights reserved.