New Photorotor for the Induction of Anisotropy

摘要

Anisotropic polymer films are of high technical importance as polarization elements[1] in LC displays, such as polarizers and retarders, and as aligning layers.[2,3] Photoinduction of patterned optical anisotropy could be used for polarization gratings, security features and optical data storage. In contrast to the stretching of thick polymer films and the rubbing technique for aligning of liquid crystals (LCs), the photoinduction of anisotropy by polarized light is a contact-free processing technique allowing the preparation of micrometer- and even nanometer-thick anisotropic films. Further advantages are the generation of orientational pixels, defined spatial 3D anisotropy and access to thin film polarization gratings. Generally, the induction of optical anisotropy can be caused by angular dependent photo-bleaching, angular selective formation of rod-like photoproducts such as in the case of the photocycloaddition of cinnamates or coumarines or, most effective, by the repeated photoselection events in the steady state of the photoisomerization of azoben-zenes. The amount of inducible anisotropy and its stability is restricted in amorphous polymers. Higher values of anisotropy can be generated using liquid crystalline materials due to surface'4"6' or bulk'7! alignment. While the first method is caused by an anisotropic interface effect due to polarized exposure of a first photosensitive layer followed by photopolymerization of reactive mesogens (two layer processing), the second one is based on the photoinduction of anisotropy in the bulk of liquid crystalline polymer films and can be amplified by the thermo-tropic self-organization upon subsequent annealing at temperatures of the mesophases (one layer processing). Incorporation of mesogenic groups in the polymers can be achieved introducing mesogens as separate side-groups or by prolonging the photosensitive side group using mesogens.[8,9]