Enhanced thermal conductivity of semi‐aliphatic liquid crystalline polybenzoxazoles using magnetic orientation

Polybenzoxazole (PBO) model compounds consisting of benzoxazole mesogenic units linked with long alkyl chains via ether or ester groups were prepared from flexible dicarboxylic acids with two kinds of bis(o‐aminophenol)s, i.e. 4,4′‐diamino‐3,3′‐dihydroxybiphenyl (p‐HAB) and 3,3′‐diamino‐4,4′‐dihydroxybiphenyl (m‐HAB). The results revealed that the use of p‐HAB completely erased the thermotropic liquid crystallinity and that the m‐HAB‐based ether‐linked model compound was much more advantageous for the formation of a stable liquid crystal phase than the corresponding ester‐linked one. However, the relevant high‐molecular‐weight PBOs showed quite opposite results. Only the m‐HAB‐based ester‐linked C10 PBO system showed a liquid crystal‐like texture among various semi‐aliphatic PBO systems examined. A very high thermal conductivity of 1.79 W m−1 K−1 along the thickness direction for the m‐HAB‐based ester‐linked C10 PBO film was achieved by heat treatment in the liquid crystalline state under a strong magnetic field of 10 T.