Organic Crystal Engineering of Thermosetting Cyanate Ester Monomers: Influence of Structure on Melting Point
Air Force Research Laboratory (AFMC) AFRL/RQRP Edwards AFB United States
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Key principles needed for the rational design of thermosetting monomer crystals, in order to control the melting point, have been elucidated using both theoretical and experimental investigations of cyanate esters. A determination of the thermodynamic properties associated with melting showed that the substitution of silicon for the central quaternary carbon in the dicyanate ester, 2,2-bis4-cyanatophenylpropane, resulted in an increase in the entropy of melting along with a decrease in the enthalpy of melting, leading to a decrease in the melting temperature of 21.8 or - 0.2 K. In contrast, the analogous silicon substitution in the tricyanate ester, 1,1,1-tris4- cyanatophenylethane, resulted in no significant changes to the enthalpy and entropy of melting, accompanied by a small increase of 1.5 or - 0.3 K in the melting point. The crystal structure of 1,1,1-tris4-cyanatophenylethane was determined via single crystal X-ray diffraction, and the structures of these four dicyanate esters and tricyanate esters were examined. Although both the empirical models of Lian and Yalkowsky, as well as Chickos and Acree, provided reasonable estimates of the entropy of melting of 2,2-bis4-cyanatophenylpropane, they successfully predicted only certain effects of silicon substitution, and did not capture the difference in behavior between the dicyanate esters and the tricyanate esters. Semi-empirical molecular modeling, however, helped to validate an explanation of the mechanism for the increase in the entropy of melting of the silicon-containing dicyanate ester, while providing insight into the reason for the difference in behavior between the dicyanate esters and tricyanate esters. Taken together, the results assist in understanding how freedom of molecular motions in the liquid state may control the entropy of melting, and can be utilized to guide the development of compounds with optimal melting characteristics for high-performance applications.