Synthesis and photoinitiated polymerization of nematic liquid-crystalline diepoxides

Various liquid crystalline bifunctional symmetrically and unsymmetrically substituted diepoxides based on the 4-(ω-oxiranyl-alkoxy)-benzoic acid 4-(ω-oxiranyl-alkoxy)-phenyl esters were synthesized. By modification of the length of the flexible alkylene chains, the phase transition temperature from the crystalline into the liquid crystalline state could be adjusted between 40°C and 90°C. The phase transition behavior of the monomers was examined by DSC. These diepoxides are capable to undergo photoinduced polymerization in the presence of a cationic photoinitiator with intramolecular photosensitization in the liquid crystalline phase as well as in the isotropic phase. The photoinduced polymerization was monitored by RTIR. For most monomers the rate constants for polymerization are higher in the liquid crystalline state compared to the isotropic melt. A polymer network with liquid crystalline superstructure is formed if the polymerization of the monomers is carried out in the liquid crystalline phase. No glass-transition is measurable for the cross-linked materials, and the gel content is about 96%. Although the polymers are highly cross-linked, they are not brittle at all.