Toughening Studies on Cyanate Ester Resins and Composites

Toughening Studies on Cyanate Ester Resins and Composites 

Phase separation in the mixture was the result of a decrease in the entropic contribution due to the increase in the oligomer size, and a decrease in the en-thalpic contribution as a result of the decrease in the cohesive energy density of the CE-oligomer in the course of polymerization. 

Spinodal demixing was excluded as a possible mechanism of phase separation during polymerization in solutions containing less than 12% rubber by volume [250]. 

Whereas high molar mass polyether sulfones are expected to show phase separation, the low molar mass versions can be miscible and can alter the T of the cyanurate network. A recent kinetic study on the reaction of BACY with a low molar mass, cyanated polyether sulfone showed an unexpected S shape for the Tg-composition curve [269]. Studies on model compounds using UV and IR spectroscopy suggested the possible polar interaction between the lone pair of electrons on the N-atoms of the triazine ring and the n orbital of the phenylene rings neighboring the sulfone linkage to be responsible for the miscibility and the peculiar shape of the Tg-composition curve. 

While thermoplastic additives bring about improvement in toughness, it is usually at the expense of increased dielectric properties which is undesirable for certain critical applications as described earlier. A method to introduce toughness and reduce Dk is based on microporous cyanurates prepared via chemically induced phase separation using a solvent such as cyclohexane. The solvent is removed to generate microvoids by heating the matrix in the vicinity of its Tg. Polycyanurates with significantly less density and Dk were thus prepared [279, 280]. 

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