An Investigation of the Role of Cross-Linking and Photodegradation of Side-Chain Coumarin Polymers in the Photoalignment of Liquid Crystals

A surface anisotropy has been shown previously to be induced in thin films of photoreactive coumarin side-chain polymers by polarized UV illumination. Consequently, the resultant cross-linked polymer layers can be used as photoalignment layers for liquid crystal displays. Homogeneous alignment of a nematic liquid crystal in contact with a layer of a model coumarin side chain polymer is obtained with the director parallel or perpendicular to the UV polarization axis depending on the incident fluence. Spectroscopic analysis of the alignment layer now confirms that both photodegradation and cross-linking occur with different dependencies on fluence. Low UV fluences give parallel photoalignment and high cross-linking reactivity. However, the residual, unreacted polymer side chains show negligible anisotropy because of their freedom to move in an isotropic fashion. Hence, parallel liquid crystal alignment is attributed to a steric interaction between the liquid crystal and syn-dimerized side chains of the cross-linked polymer. A switch of the photoalignment direction accompanies the subsequent development of anisotropy of the intact, unreacted polymer side chains. The side-chain anisotropy and hence perpendicular liquid crystal alignment is ascribed to photodegradation rather than cross-linking.