Synthesis and characterization of novel polyimides with 1‐octadecyl side chains for liquid crystal alignment layers

A series of poly(amic acid)s have been synthesized from cyclobutane‐1,2,3,4‐tetracarboxylic dianhydride (CBDA), a functional diamine, 1‐(3,5‐diaminophenyl)‐3‐(1‐octadecyl)‐succinicimide and 4,4‐diaminodiphenylmethane (DDM) to give liquid crystal alignment layers with high pretilt angle after a rubbing process for vertically aligned liquid crystal display devices. Inherent viscosities of the poly(amic acid)s measured were in the range 0.2–1.56 dl/g, which decreased with an increase of content of 1‐(3,5‐diaminophenyl)‐3‐(1‐octadecyl)‐succinicimide. The cured polyimide films had good transparency, which was above 95% at 400 nm. Surface energies of all the polyimide films prepared from 1‐(3,5‐diaminophenyl)‐3‐(1‐octadecyl)‐succinicimide were below 40.7 dyn/cm before the rubbing process, which resulted in very high pretilt angles above 89°. By a rubbing process, the surface energy of the polyimide prepared from diamine mixture with 10 mol% of 1‐(3,5‐diaminophenyl)‐3‐(1‐octadecyl)‐succinicimide and 90% of DDM increased from 40.7 to 42.7 dyn/cm.