INVESTIGATIONS ON LANGMUIR-BLODGETT-FILMS AS ALIGNMENT LAYERS FOR LIQUID-CRYSTALS

Multilayer polyimide (PI) films were successfully fabricated using the Langmuir-Blodgett (LB) technique. These films were studied in several ways relevant to their use as liquid crystal alignment layers. (1) Th e influence of dipping speed and creep time on the orientational order of PI-LB films was investigated by means of birefringence measurement s. (2) For comparison, we measured the pretilt angles and polar anchor ing strengths of liquid crystal (LC) cells assembled with PI-LB films and rubbed PI-LB films as alignment layers. (3) The anchoring directio n of the liquid crystal was found to be solely determined by the dippi ng direction of uppermost PI-LB layer, regardless of the dipping direc tion of deeper layers. (4) Combining results of the above studies with measurements of pretransitional birefringence (above the isotropic to nematic (I --> N) transition) and observations of the growth of the a lignment texture just below the nematic-isotropic transition, we draw the conclusion that the range of interaction between the PI-LB film an d LC molecules is quite short (similar to 4.5 Angstrom) and that the a lignment mechanism is epitaxial. Therefore, anisotropic short range mo lecular interactions are responsible for the alignment of the first li quid crystal layer. (5) From studies of PI-LB films deposited perpendi cular to the rubbing direction of underlying spin-coated PI films, we also found evidence, as expected, that the grooves induced by the rubb ing process are not decisive for LC alignment on a rubbed polymer surf ace.