Synthesis of a novel polyimide used as liquid crystal vertical alignment layers

Abstract

A novel functional diamine containing a triphenylamine moiety and a biphenyl group, N,N-bis(4-aminophenyl)-4-(biphenyl)-4′-aminophenyl ether (N0), was successfully synthesized and characterized. A series of polyimides (PIs) and poly(amic acid)s (PAAs) were synthesized from cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA), 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFDB), 4-dodecyloxy-phenyl-4′,4′′-diaminotriphenylamine (C12) and the newly synthesised diamine monomer (N0) through a conventional two-step procedure that included a ring-opening polyaddition to give polyamic acids, followed by thermal cyclodehydration. Pretilt angles of a liquid crystal (LC) cell fabricated with the PIs were measured, and the rubbing resistance of PIs containing different sidechain structures was investigated. PI1 with a rigid biphenyl side chain could induce LC parallel alignment after the rubbing process. Although PI3 containing a flexible alkyl side chain could induce LC vertical alignment before the rubbing process, it only induced LC parallel alignment after the rubbing process. However, PI2 containing both rigid biphenyl side chains and flexible alkyl side chains could induce LC vertical alignment before and after the rubbing process. A new method for improving the rubbing resistance of PIs and a generation mechanism of rubbing resistance have been proposed. In addition, all PI films showed high thermal stability and high transmittance in the wavelength range of 400–700 nm.