<title>Orientation of a nematic liquid crystal on a soft photoaligning surface</title>

Orientation of a liquid crystal on an aligning surface is characterized by the direction of an easy axis, e and anchoring energy, W. Traditional rubbed aligning layers provide a rigid surface and a strong anchoring of LC (W>10^-1 erg cm^-2). Therefore, magnetic or electric field usually does not reorient director on the aligning surface, and both of these parameters remain constant during the application of the field to the LC cell. Novel aligning materials, such as photoaligning and brush- like orientants possess very soft surface, containing flexible groups and usually provide much smaller anchoring energy (W<10^-2 erg cm^-3). In this case the reorientation of the director on the aligning surface is essential. In this report we show that the surface director reorientation can result in the realignment of flexible fragments of a weak anchoring photoaligning surface, i.e. in the modification of the aligning surface. Therefore, both the easy axis and the anchoring energy of the soft aligning layer can be changed during the application of external field to LC cell. The effect of the aligning surface modification induced by director reorientation on a soft surface of polyvinylcinnamate (PVCN) was found in a cell filled with LC 5CB. Namely, we observed that application of electric field results in the producing of a transient pretilt angle of 5CB on the PVCN surface, which relaxed to the initial state after switching off the external field during about 10 min. To describe the obtained results we propose to consider the interface containing LC molecules and flexible fragments of the aligning layer as an LC with effectively high orientation viscosity (gamma) _s. Anchoring energy and easy axis of the LC corresponds to its equilibrium state. In the studied case the value (gamma) _s approximately =5 P is appeared to be in fifty times greater than the value (gamma) ₁ of 5CB.