Polyimide/dicyanate semi-IPNs and carbon fiber composites with a morphology spectrum

Abstract

The brittleness of thermosetting materials such as epoxy resins and dicyanate resins limited their use in many high-performance applications. In order to improve the fracture toughness without significantly lowering their thermal and mechanical properties, high-performance thermoplastics have been incorporated in the thermosetting materials. The degree of toughening of the thermoplastic modified thermosets is strongly dependent on the phase-separated morphology. Generally, at low thermoplastic contents, sea-island morphology is formed via the nucleation and growth mode of the phase separation mechanism, resulting in a moderate increase in fracture toughness. Above 20 %, a nodular structure is developed via the spinodal decomposition mode of the phase separation mechanism. However, thermoset materials with high thermoplastic content often display poor heat and solvent resistance as well as the processing difficulties caused by the high viscosity of the mixture. Considering the relationship between the morphology and toughness, if concentration gradient is introduced in the specimen, resulting morphology spectrum ranging from the sea-island morphology to the nodular structure could enhance the fracture toughness of the thermoset system or the carbon fiber composite. The semi-interpenetrating polymer networks (semi-IPNs) with the morphology spectrum were prepared either by inserting polyimide (PEI) film or by adding PEI particles into neat dicyanate resin. The relative rate of dissolution and diffusion of the PEI component and the polymerization of dicyanate was controlled by changing the reaction conditions such as the reaction temperature, catalyst concentration, and the degree of precuring before the mixing process. Final morphology of the semi-IPNs exhibited three types of morphology along the thickness direction or the radial direction due to the formation of PEI concentration gradient; (1) nodular spinodal structure where the concentration of PEI was 18 wt ...