COMPARATIVE RHEOLOGICAL ANALYSIS OF 2 EPOXY-AMINE SYSTEMS NEAR THE GEL POINT

A flexible epoxy-amine system based on diglycidylether of 1,4-butanedi ol (DGEBD) and 4,9-dioxa 1,12-dodecane diamine (4D) and a rigid epoxy- amine system based on diglycidyl ether of bisphenol A (DGEBA) and 4,3' -methylenebis [3-chloro, 2.6-diethylaniline] (MCDEA) were studied by r heological method near the gel point. The maximum glass transition tem perature of the completely cured network, T-g infinity, is low for DGE BD/4D system (T-g infinity = - 12 degrees C). Then, this system presen ts only gelation transition at curing temperatures between 50 degrees C and 70 degrees C. T-g infinity (T-g infinity = 177 degrees C) is hig h for DGEBA/MCDEA system which presents gelation and vitrification bet ween 80 degrees C and 170 degrees C, but only gelation at 180 degrees C and 190 degrees C. At the gel point, it is observed a divergence of the viscosity in steady flow conditions, a crossover of the tan delta curves measured as a function of time at several frequencies, and prop ortionality between G'(omega), G ''(omega) and the power Delta of the pulsation omega(G'(omega)proportional to G ''(omega)proportional to om ega(Delta)). Near the gel point, the viscosity, eta, and the elastic m odulus, G', follows power laws (eta proportional to epsilon(-k) and G' proportional to epsilon(Z) with epsilon = \x - x(gel)\/x(gel), relativ e distance to the gel). At curine temperatures higher than T-g infinit y the critical exponents k, z(0) and Delta remain constant and their v alues are in good agreement with those derived from the percolation th eory with molecular chains corresponding to the Rouse model. Between 1 50 degrees C and 170 degrees C, k and Delta have a constant value for DGEBA/MCDEA system. The exponent z is the most sensitive to vitrificat ion phenomenon. Below 150 degrees C, k and Delta diminish and exponent z no longer exists, which indicates lack of fractality at low curing temperatures. (C) 1998 Elsevier Science Ltd. All rights reserved.