Synthesis and mesomorphic properties of cyanurates and isocyanurates. Branched mesogens as model crosslinks for liquid crystalline thermosets

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Abstract
A number of new s-triazines (cyanurates and isocyanurates) with diaromatic mesogenic branches have been synthesized and their thermal properties investigated. No liquid crystal phases were found in the series of isocyanurates, while the cyanurates form enantiotropic calamitic mesophases (nematic and in one case smectic). No discotic mesophases could be detected. The mesogenic power of the cyclotrimers is reversed with respect to that of the monomers from which they can be obtained (cyanates and isocyanates). Molecular calculations reveal that the cyanurates can adopt an extended rod-like conformation due to the flexibility introduced by the oxygen bridge between the central ring and the mesogenic branches. The direction of the ester group in the phenyl benzoate mesogenic branches has a tremendous influence on the mesomorphic properties, with the result that mesophases could only be observed if the benzoic acid part was attached to the triazine ring.