Polyetherimide (PEI) has been found to be an effective modifier for improving the brittleness of a bisphenol A dicyanate-epoxy novolac resin blend. For example, inclusion of 15 wt-% PEI led to 115% increase in fracture toughness and a modest increase in flexural strength, while retaining the flexural modulus of the unmodified cured blend. The microstructures of the modified resins were examined by scanning electron microscopy and dynamic mechanical thermal analysis. It was observed that the dichloromethane absorption of modified resins having a co-continuous phase structure was significantly higher than that of the unmodified cured blend. These improvements in toughness and changes in solvent resistance were correlated with the morphologies of the modified blends. The dispersed particles of the blends at 10 wt-% PEI loading are comprised of PEI and ultrafine thermosetting resin particles. At lower gel temperature, the extent of the separation is higher than that for the blends gelled at higher temperature. The cure schedule, however, had no practical influence on either the fracture toughness or the flexural properties of blends incorporating 10 wt-% PEI modifier.
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