NOVEL PHOTO-CROSS-LINKABLE LIQUID-CRYSTALLINE POLYMERS - POLY[BIS(BENZYLIDENE)] ESTERS

A new class of photo-cross-linkable main-chain liquid crystalline polymers (PMCLCPs) containing bis(benzylidene)cycloalkone groups have been synthesized and studied for their liquid crystalline and photochemical properties. The bis(benzylidene)cycloalkanone group in the chain functions both as a mesogen and as a photoreactive center. All of the polymers exhibit a nematic mesophase. Two kinds of photoreactions, namely, photoisomerization and photo-cross-linking, operate in these polymers. Above T(g), at the initial stages of irradiation, photoisomerization predominates the cross-linking, which results in the disruption of the chromophore aggregates. Below T(g), because of the restricted mobility of the chains, only cross-linking takes place. Studies on the model compound, bis(benzylidene)cyclopentanone, confirm the above observations and demonstrate further that the cross-linking proceeds by the 2pi + 2pi cycloaddition reaction of the bis(benzylidene)cycloalkanone moieties. The cross-linking rate decreases with increase in the size of the cycloalkanone ring. Heating the solution cast polymer film results in the ordered aggregation of the chromophores just above T(g) and also at the crystal to crystal transition temperature, which facilitates the photo-cross-linking reactions. In the isotropic phase, the random orientation of the chromophores drastically curtails the cross-linking rate.