Epoxy resins are generally used in advanced composites due to their excellent thermal and dimensional stability, high modulus and good mechanical properties. However, since epoxy resins are inherently brittle, rigid thermoplastics have been used for the toughness improvement of the epoxy resin without lowering the thermal and mechanical properties. The properties of a thermoplastic modified epoxy are so closely related to the morphology that the analysis on the phase separation during cure is needed.
In this work, the phase separation behavior and the morphology of polyetherimide(PEI)-modified diglycidyl ether of bisphenol A (DGEBA) epoxy resins were studied using scanning electron microscopy(SEM) and light scattering. Reaction kinetics, cloud points and onsets of gelation were determined by a differential scanning calorimeter, optical microscope and rheometer, respectively. Rheological and mechanical properties were also studied.
The mixture of partially cured epoxy and PEI showed a bimodal UCST behavior. For PEI content smaller than 10wt%, the mixture exhibited a sea-island morphology formed via nucleation and growth mechanism. Above 25wt% PEI content, the phase separation proceeded via spinodal decomposition mechanism and a nodular structure was formed. With PEI content between 15 and 20wt%, a dual phase morphology was observed. This morphology was formed via primary spinodal decomposition and secondary phase separation within the dispersed and the matrix phases formed by the primary phase separation This morphology was presumed to be formed by the reaction induced phase separation mechanism with the mixture showing a bimodal UCST behavior. The curing temperature and the amount of catalyst had an effect on the final morphology.
The rheological properties of the blend were influenced by the degree of phase separation. The viscosity and moduli curves of the blend showing phase separation during curing showed a maximum peak at the onset of phase separation.
The mechanical property was also dependent on the morphology. The fracture toughness, $k1c$ began to increased abruptly at the phase inversion point.