Chemically Induced High Pretilt Angle by CN-Containing Polyimide

New copolyimides containing nitrile side group were synthesized from copolymerization of pyromellitic dianhydride, m-phenylene diamine and 3,5-diaminobenzonitrile and subsequent thermal imidization of the resulting poly(amic acid). Crystallinity, glass transition temperature and initial decomposition temperature of copolyimides were almost identical to those of homo polyimide prepared from PMDA and m-PDA. Change of pretilt angle induced by the orientation layer of resulting copolyimide was investigated by using a nitrile-containg nematic liquid crystal cell after rubbing. As the content of polar nitrile group was increased in the copolymer, pretilt angle was increased from $3.65^{\circ}$ to $6.49^{\circ}$. The mechanism of this was speculated as the dipolar interaction between the liquid crystal and nitrile groups in copolyimide.