Anchoring and alignment in a liquid crystal cell: self-alignment of homogeneous nematic

Abstract

The rubbing or photo-aligning process is commonly utilized to anchor and also to align nematic liquid crystal (LC) molecules on the polymer-coated glass substrates for the light valve of LC displays. Here, we propose a “self-alignment” concept to anchor and align the nematic LC molecules homogeneously in a cell without the rubbing technique. If the glass-transition temperature of the coated polymer is low enough to the I–N transition temperature of the LC, the director can freely rotate on the wetted polymer liquid layers over the glass substrates at a sufficiently high temperature. Hence, unidirectional director alignment can easily be attained by application of the extremely weak external field, then we simply cooled down the LC cell keeping this orientation. As a result, for the first time, it is experimentally verified that the LC molecules are anchored i.e., fixed, and simultaneously aligned i.e., orient unidirectionally, at the same moment on the quenched polymer layer surfaces. The result clearly pointed out that the “anchoring” force to fix LC molecules on the polymer layers originates from the local molecular interaction between LC molecules and polymers. The result also clearly shows that the “anchoring” force is intrinsically distinguished from the “alignment” force, which orients the director, n, of the LC molecules unidirectionally, resulting from the rotational symmetry breaking of the nematic order. In fact, we confirmed experimentally that the anchoring energy of the polymer surface treated by the self-alignment method is almost equivalent to that obtained by the conventional rubbing process. Furthermore, the rotational symmetry of the polymer surfaces treated by the self-alignment method was found to still preserve the isotropic symmetry in nature, even in the case that unidirectionally aligned LC was anchored on the surface, while the symmetry of the whole LC cell was broken when the surface is integrated with anisotropic LCs by the anchoring force.