Mesogen-co-polymerized transparent polyimide as a liquid-crystal alignment layer with enhanced anchoring energy

Enhancing the azimuthal anchoring energy of the aligning film is a key requirement to improve the quality of liquid-crystal displays. Based on the assumption of mesogenic side groups being able to align liquid-crystal (LC) molecules, we have designed a novel transparent polyimide bearing cyanobiphenyl mesogenic side chains, which has a similar structure to that of liquid crystal 5CB. A pyrimidyl diamine monomer with a cyanobiphenyl mesogenic unit was first synthesized, and then co-polymerized with 4,4′-diaminodiphenyl ether and 4,4′-oxydiphthalic anhydride. The obtained polyimide (PI) exhibited good thermal stability and mechanical properties, as well as high optical transparency with transmittance of up to 86% in the wavelength range 450–700 nm. By evaluating the adhesion force between PI films and LCs with a super-sensitive microelectro-mechanical balance system, we have verified that appending mesogenic groups on the side chains of PI strengthens intermolecular interactions between the LC and PI surfaces, thereby enhancing the surface anchoring energy. Compared with the unmodified PI, the anchoring energy of the derivatized PI was improved more than 2.2-fold. By virtue of its good transparency and strong anchoring energy, this PI would seem to be an ideal candidate as a LC alignment film, and the concept of mesogens aligning LCs may open a new door in the design of LC alignment films.