Novel polypyromellitimides and their liquid crystal aligning properties

Various aromatic poly{3,6‐bis(4‐n‐alkyloxy)phenyloxy]‐pyromellitimide}s poly(Ar‐C_m‐PMDA)s, m = 1, 4, 8, 12] were prepared in film form by routine two‐step condensation of 3,6‐bis(4‐n‐alkyloxy)phenyloxy]pyromellitic dianhydrides (C_m‐PMDAs) with various aromatic diamines. After characterization of their chemical structures their solution, thermal and liquid crystal (LC)‐aligning properties in terms of pretilt angle at various rubbing densities were measured and discussed with respect to their backbone structures. All polyimide films showed excellent thermal stability and homogeneous LC alignment, but the poly(p‐phenylene‐C₁₂‐PMDA) exhibited completely homeotropic alignment while the pretilt angle values of poly(p‐phenylene‐C₈‐PMDA) varied with varying rubbing density.