Atomic force microscopy characterization and liquid crystal aligning effect of polymerizable diacetylene Langmuir Blodgett films

Abstract

The surface topology of polymerizable diacetylenic lecithin Langmuir Blodgett (LB) films deposited on indium-tin oxide (ITO) coated glass substrates has been investigated using atomic force microscopic (AFM) techniques. The liquid crystal alignment effect of the LB films was compared to that of the random film formed by dipping the ITO glass into chloroform-lecithin solution. The orientation of a nematic monodomain based on these substrates was changed from homeotropic to tilted upon ultraviolet (UV) irradiation, and a planar alignment was achieved if the substrates were UV treated and buffed. AFM pictures reveal that the substrate surface corrugation is decreased by both random coating and LB film coating before photopolymerization. After UV irradiation, some unique star-like features were found on the monolayer LB film. These features provide evidence that the conformation of the diacetylene chains has been changed due to the photopolymerization, thus causing the liquid crystal orientation to undergo a transition from homeotropic to tilted alignment.