Synthesis and polymerization behavior of novel liquid-crystalline benzoxazines

Novel liquid-crystalline (LC) benzoxazines were synthesized by using 4(4′-heptoxybenzoyloxy)benzylidene-4″-aminophenol, various alkylamines, and formalin in chloroform under reflux. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) were used to analyze the LC behavior of the synthesized benzoxazines. The novel benzoxazines showed nematic and smectic phases upon cooling cycle from the isotropic liquids. The LC benzoxazines also showed a nematic phase upon heating cycle, indicating that the novel benzoxazines are enantiotropic liquid crystals. The ring-opening polymerization behavior of the LC benzoxazines was investigated by DSC and IR analyses. POM observations showed that a polybenzoxazine film, prepared by thermally curing the LC benzoxazine up to 160 °C for 1 h, exhibited birefringence at room temperature.