A material-independent method for determining liquid-crystal (LC) anchoring energy on rubbed polyimide (PI) surfaces has been devised. This method exploits the changes in the easy axis of rubbed PI film induced by exposure to linearly polarized UV (LPUV) Light. The distribution of PI chains in a rubbed film is approximated by a Gaussian function and its width determined from the measured rotation of the LC easy axis as a function of exposure time. A quasimicroscopic free energy of the LC-substrate interface is used to model LC anchoring properties. The experimental and calculated values of the azimuthal anchoring energy are in good agreement and found to depend inversely on the width of the distribution function. The measurements of the width of the chain distribution function provide a simple LC material-independent method for determining the LC anchoring properties. With this method, it is also possible to calculate the strength of the interaction between PI chains and LC molecules. [S1063-651X(99)04612-7].
Observation of Pattern Evolution During Homeotropic-Focal Conic Transition in Cholesteric Liquid Crystals06/14/1999
We have observed an interesting pattern evolution of cholesteric liquid crystals during the homeotropic-focal conic transition in a dispersed polymer network. From the electrical field induced homeotropic state, if the field is:reduced to an appropriate bias level focal conics grow at a constant speed, being an open or compact structure depending on the field strength. When the field is increased, these patterns change to ramified structures. We phenomenologically explained the observation.