Thermosets containing benzoxazole units: Liquid‐crystalline behavior and thermal conductivity

A series of liquid‐crystalline (LC) thermosetting monomers containing benzoxazole (BO) units were synthesized to evaluate the thermal conductivities ( λ ) of their cured resins. A BO‐containing bisnadiimide system showed LC behavior during the heating process. However, the thermal cure of the bisnadiimide provided a film without optical anisotropy; consequently, the cured film exhibited normal levels of thermal diffusivity ( α ) and thermal conductivity ( λ ). The disappearance of the optically anisotropic ordered structures during thermal curing is likely related to the temperature gaps between the cure reaction ranges and LC ranges ( T cure ‐ T LC gap). In addition, epoxy resins consisting of bisepoxides and BO‐containing diamines were investigated because of their high flexibility in terms of molecular design that can be used to reduce the T cure ‐ T LC gap. The combination of a terephthalylidene‐type bisepoxide and BO‐containing diamine with a controlled flexible chain length resulted in the smallest T cure ‐ T LC gap among the epoxy resin systems examined herein. The cured epoxy resin film exhibited an appreciably increased λ value (0.257 W m −1  K −1 ) in the Z direction. This indicated the importance of the T cure ‐ T LC gap for enhancing the α and λ values of the cured films. This epoxy resin system was cured under a continuous DC electric field during polarizing optical microscopy. A prompt response with deformation of the LC domains was observed in harmony with temporal ON/OFF switching of the DC power supply. As expected, the cured film exhibited a significantly enhanced λ value (0.488 W m −1  K −1 ) in the Z direction.