Synthesis of liquid-crystalline benzoxazines containing a biphenyl group in the mesogenic moiety

In this work, we describe a process to synthesize novel liquid-crystalline (LC) benzoxazines from a LC phenol that contains biphenyl, ester and azomethine groups in the mesogenic moiety, various alkyl amines and formaldehyde. The LC behavior of the biphenyl-containing benzoxazines was investigated by differential scanning calorimetry and polarized optical microscopy and compared with analogous phenyl-containing LC benzoxazines containing phenyl, ester and azomethine groups in the mesogenic moiety. While the LC benzoxazines containing the phenyl groups only exhibited a nematic phase upon heating, the LC benzoxazines containing biphenyl groups exhibited a smectic phase as well as a nematic phase. Additionally, the LC-isotropic transition temperatures were over 100 °C higher than that for the phenyl-containing LC benzoxazines. A film obtained by curing the biphenyl-containing LC benzoxazine at 180 °C for 1 h exhibited birefringence. Finally, the thermal diffusivity of the cured LC benzoxazines was also investigated.