Polycyanurate Networks with Enhanced Segmental Flexibility and Outstanding Thermochemical Stability

The synthesis and physical properties of cyanurate networks formed from two new tricyanate monomers, 1,3,5-tris[(4-cyanatophenylmethyl]benzene and 3,5-bis[(4-cyanatophenylmethyl)]phenylcyanate, are reported and compared to those of 1,1,1-tris[(4-cyanatophenyl)]ethane (also known as ESR-255). All three networks possessed somewhat different aromatic contents and cross-link densities; however, the thermochemical stability of these networks, as determined by TGA, was outstanding, with that of 1,3,5-tris[(4-cyanatophenylmethyl)]benzene being among the best known for organic cyanate esters despite its comparatively high segmental flexibility. Moreover, the moisture uptake of cured 1,3,5-tris[(4-cyanatophenylmethyl)]benzene, at 2.2% after 96 h immersed in 85 °C water, was comparatively low for a cyanate ester network with a glass transition temperature of 320 °C at full cure. When cured for 24 h at 210 °C, the dry glass transition temperatures of the networks ranged from 245 to 285 °C, while the wet glass transition temperatures ranged from 225 to 240 °C. The similarity in glass transition temperatures resulted from a lower extent of cure in the networks with more rigid segments. In essence, for networks with very high glass transition temperatures at full cure, the process conditions, rather than the rigidity of the network, determined the attainable glass transition temperature. Because networks with a higher extent of cure tend to exhibit slower long-term degradation, in this case, the networks with greater segment flexibility enabled superior performance despite exhibiting a lower glass transition temperature at full cure. These results illustrate that, in contrast to the prevailing heuristics for improving the performance of high-temperature thermosetting polymer networks, a more flexible network with a lower glass transition temperature at full cure can offer an optimal combination of thermomechanical and thermochemical performance.