Effect of Epoxy Resin Structure on Compatibility and Phase Separation of Flexible‐Chain Imide Modifier

ABSTRACT A modifier for epoxy resins with a long‐chain aliphatic structure and imide groups is proposed. The compatibility of the modifier with epoxy resins and the phase‐separated structure of the cured resins are evaluated. The blends of the modifier and the epoxy resin exhibit an upper critical solution temperature‐type phase diagram. In addition, the cloud point for the modifier with biphenyl‐type epoxy resins is lower than that with bisphenol A‐type epoxy resins. The good compatibility between the modifier and the epoxy resin is suggested to be due to charge‐transfer interactions as determined by ultraviolet–visible spectroscopy. The cured epoxy resins containing the modifier form phase‐separated structures. In particular, the cured resin using the biphenyl‐type epoxy resin exhibits submicron island phases of 300 nm or less, as determined by transmission electron microscopy. The addition of the modifier reduces the storage modulus of the cured epoxy resin by approximately 500 MPa while maintaining the glass transition temperature.