Infusion of graphene in natural rubber matrix to prepare conductive rubber by ultrasound-assisted supercritical CO2 method

In recent years, graphene has become the most popular conductive filler in conductive polymers which can be used in many practical applications. A proper selection of the mixing method is the decisive factor to maximize the excellent conductivity of graphene in a polymer matrix. In this work, ultrasound-assisted supercritical CO2 (scCO(2)) method was used for the first time to prepare graphene infiltrated conductive natural rubber (NR). Many interesting phenomena were discovered. First, it was found that NR was fully swollen after an ultrasoundassisted scCO(2) penetration and a following rapid decompression process. The degree of expansion of NR in scCO(2) was even larger than in toluene. Secondly, ultrasound-assisted scCO(2) method coupled with a rapid decompression process successfully inserted graphene particles in the swollen NR to prepare conductive NR/graphene materials. The impacts of the decompression method, vulcanization degree, and ultrasonication duration on graphene infusion were studied and analyzed. Under an optimal condition, the surface conductivity of as-prepared unvulcanized graphene/NR film or vulcanized film with a low graphene loading fraction (0.34 vol%) could reach to 0.2 S/m or 0.02 S/m, respectively, taking a leading position in the reported graphene/NR composites. Our conductive NR was shown to be satisfactory as a strain sensor for the measurement of fine movements of human body such as heartbeat, finger bending, and vocalization. Since only scCO(2), one of the most environmentally benign solvents, was used in the whole process, this routine is not only efficient but also environmentally friendly. We believe this method is versatile and can be applied to many other polymers that can be swollen in scCO(2).