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

Abstract

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 (Tcure-TLC 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 Tcure-TLC gap. The combination of a terephthalylidene-type bisepoxide and BO-containing diamine with a controlled flexible chain length resulted in the smallest Tcure-TLC 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 Tcure-TLC 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.