REACTION-INDUCED PHASE-SEPARATION MECHANISMS IN MODIFIED THERMOSETS

The reaction-induced phase separation in amorphous thermoplastic-modif ied epoxy systems was observed in situ using methods of different obse rvation windows: small angle X-ray scattering (SAXS), light transmissi on (LT) and light scattering (LS). The transmission electron microscop y (TEM) technique was concurrently used to get direct representations of morphologies at different levels of the phase separation process. T he selected systems were bisphenol-A diglycidylether cured with either 4,4'-diaminodiphenylsulfone or 4,4'-methylenebis[3-chloro,2,6-diethyl aniline] in the presence of polyetherimide. The phase separation mecha nisms involved were found to be greatly dependent on the initial modif ier concentration and on the ratio of the phase separation rate with r espect to the polymerization rate. Experimental results showed that, f or modifier concentrations close to the critical fraction, the system was directly thrown into the unstable region, even at a low polymeriza tion rate, and phase separation proceeded by spinodal demixing. On the other hand, for off-critical compositions the homogeneous solution de mixed slowly via the nucleation and growth mechanism. In spite of the evolution of the phase diagram with reaction extent, the system remain s in the metastable stare whatever the cure temperature. The cure temp erature has a strong effect on the extent of phase separation, since s ooner or later vitrification of the thermoplastic-rich phase occurs an d stops the evolution of morphologies. A post-cure allows the phase se paration process to go further and sub-particles can be generated depe nding on the precure and post-cure temperatures. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.