Synthesis, characterization and liquid-crystal-aligning properties of novel aromatic polypyromellitimides bearing (n-alkyloxy)biphenyloxy side chains

Novel aromatic polypyromellitimides bearing (n-alkyloxy)biphenyloxy side chains were prepared by two-step polycondensation of 1,4-phenylenediamine (PDA) and biphenyl-4,4′-diamine (BZ) with 3,6-bis[4′-(n-alkyloxy)biphenyl-4-oxy]pyromellitic dianhydrides (C_mB-PMDAs, m = 6, 8, 10, 12), which had been synthesized by the nucleophilic substitution of N,N′-diphenyl-3,6-dibromopyromellitimides with sodium 4-(n-alkyloxy)biphenoxides. Inherent viscosities of the poly(amic acid)s were in the 0.26-0.62 dL/g range. Poly{1,4-phenylene-3,6-bis[4′-(n-alkyloxy)biphenyl-4-oxy]pyromellitimide}s (C_mB-PPIs) and poly{4,4′-biphenyl-3,6-bis[4′-(n-alkyloxy)biphenyl-4-oxy]pyromellitimide}s (C_mB-BPIs) obtained in films by thermal imidization of the corresponding poly(amic acid)s were characterized by FT-IR spectroscopy and elemental analysis, and their crystalline structure and thermal properties were measured and discussed with respect to the side chain length. After the polyimide films were surface-treated by rubbing with velvet fibers, standard liquid crystal (LC) cells containing 4-cyano-4′-n-pentylbiphenyl (5CB) were fabricated and their LC-aligning properties were investigated in terms of pretilt angle. The pretilt angles were remarkably affected by side chain length and on surface of the polyimides with m = 6 and 8 LCs aligned parallel to the rubbing direction while on surface of the polyimides with m = 10 and 12 they aligned nearly or completely vertical to the rubbing direction.