COMMAND SURFACES OF LANGMUIR-BLODGETT-FILMS - PHOTOREGULATIONS OF LIQUID-CRYSTAL ALIGNMENT BY MOLECULARLY TAILORED SURFACE AZOBENZENE LAYERS

Reversible homeotropic reversible planar photochemical alignment controls of a nematic liquid crystal (LC) are investigated using photochromic command layers comprised of Langmuir-Blodgett (LB) films of side chain type azobenzene (Az) amphiphilic polymers. Photoresponses of the LC alignment are examined by changing such details as the molecular structure of the LB films, number of deposited layers, two-dimensional density of Az units, the deposition method, and the method of light irradiation. It is confirmed by this LB study that single Az monolayers are sufficient to induce LC alignment changes, provided that the Az unit is separated from the poly(vinyl alcohol) backbone by a methylene spacer of adequate length. The effect of the spacer length on the commanding ability can be correlated to UV-vis spectral changes of Az monolayers upon contact with LC molecules. As for the packing density of the Az unit on the substrate, photoregulation requires an area of less than 1.0 nm2 per Az unit. Both vertical dipping and horizontal lifting methods are applicable to obtain photoresponsive LC cells; however, preparation by the vertical dipping is more beneficial in that homogeneously aligning LB films are obtained on which LC molecules are oriented parallel to the dipping direction. Irradiation of linearly polarized UV light induces subsequent reorientation of LC molecules directing orthogonal to the polarization plane. This reorientation behavior is strongly dependent on the spacer length of the Az LB films and deposition numbers.