Comparative study of the anchoring strength of reactive mesogens and industrial polyimides used for the alignment of liquid crystals.

We measured the Rapini-Papoular polar anchoring strength coefficient W for 4'-pentyl-4-cyanobiphenyl (5CB) on alignment layers formed by the reactive mesogen photopolymers RM 257, RM 82, and RM 84 [4,4'-bis(acryloyl) biphenyl] (by Merck). These materials are commonly used for the photostabilization of the liquid crystal (LC) director in the bulk as well as at the surface of the LC layer via the formation of a loose polymer network that captures the director orientation. We developed a method of fabrication of alignment layers from these polymers, and estimated W from the measurements of the optical retardation as a function of applied voltage in uniformly aligned cells. We found that RM 257 yielded W of about 6 x 10(-4) J/m(2), whereas RM 82 and RM84 provided anchoring strengths of about 2 x 10(-4) and 4 x 10(-4) J/m(2), respectively. Subsequent heating of the sample either destroyed the alignment layer, or substantially decreased W to about 1 x 10(-4) J/m(2), which was comparable to the anchoring strength of weakly rubbed commercial polyimides.