Surface electrical potential of the aligning polymer substrates and dielectric properties of LC layers

UV light influence on the surface potential s of poly(vinylcinnamate) (PVCN) layers widely used for liquid crystal (LC) photoalignment is investigated. In case when PVCN film is spin coated on the ITO electrodes the surface potential changes under irradiation in both PVCN aligning film and ITO coating. The measured increment of the surface potential s=400 mV is mainly caused by the photoconductivity of ITO electrodes, whereas the part caused by PVCN substrate is at least one order of magnitude lower. The relaxation time of s for the ITO-PVCN coatings was estimated to be about 1 h. The induced potential may substantially modify dielectric properties of the aligned LC layers and its anchoring characteristics when the freshly irradiated substrates are used. On the contrary, this influence is weak when the substrates are irradiated several hours before the use for LC alignment.