Studies on the phase separation of polyetherimide‐modified epoxy resin, 3. Morphology development of the blend during curing

Scanning electron microscopy (SEM) was used to trace the development of the morphology of polyetherimide-modified epoxy resins. On the basis of SEM results and the change of the glass transition temperature of the blends during curing, different phase formation processes were found and discussed. Regarding the low molecular weight polyetherimide-modified system, the phase separation follows a spinodal decomposition mechanism. However, the poor miscibility of high molecular weight polyetherimide with epoxy resin causes the PEl-rich phase to separate in an early stage of curing. As a result, the separated phase, about 10-20 μm in dimension with fine structure in it, is not the same as that observed in ordinary phase separation processes.