Main-chain lyotropic liquid-crystalline elastomers. 1. Syntheses of cross-linked polyisocyanate gels acquiring liquid-crystalline behavior in the swollen state

Copolymers of n-hexyl and allyl isocyanates were successfully synthesized using an organotitanium(IV) catalyst initiated polymerization. Viscosity measurements indicated that molecular weight increased with decrease in monomer/initiator ratio. Monomer reactivity ratios were calculated using the copolymer composition equation, and the resulting values indicated relative randomness in the copolymer. The liquid-crystalline behavior of these copolymers was studied with regard to their molecular weight, composition, and concentration. Radical cross-linking was extensively studied but yielded only very brittle gels. Consequently, hydrosilation cross-linking was investigated and did yield networks suitable for orientation in a novel technique for increasing the mechanical properties of polymer films. All the hydrosilation cross-linked gels exhibited brilliant mesomorphic birefringence under a cross-polarized microscope at low cross-link density. In addition, the glass transition temperatures of the hydroalation cross-linked networks were found to be lower than those of their constituent polymers.