Stable graphene oxide-based lyotropic liquid crystals for interfacial lubrication

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Abstract
Lyotropic liquid crystals have lubricating properties due to their ordered assembly and fluidity, whose mesogens are often characterized by amphiphilic properties. Despite the attention that graphene oxide (GO) has been studied as a novel amphiphilic lyotropic mesogen this decade, and GO applied as a lubrication additive has been demonstrated in both oil and water-based systems, little research reveals the interfacial lubrication of GO liquid crystals yet. This work reports that GO aqueous dispersion can form lyotropic liquid crystals above a specific critical concentration of 5.00 mg/mL, providing a form of stable water-based lubricant, which can keep stable for several months and can reduce friction by 37.3% and wear by 25.24%. The liquid crystal phase was verified by polarizing microscope and synchrotron radiation small-angle X-ray scattering, and its rheological properties and viscoelasticity were studied by interfacial rheometer. The formation of lyotropic liquid crystals can enhance the stability of GO aqueous dispersions at high density, simultaneously ensuring friction decrease and anti-wear effect. It is attributed to the stable nematic network by the ordered GO sheets. The ordered assembly structure bears vertical shear force, therefore, reducing the wear. It is also assumed that the wide lateral size of graphene oxide promotes the nematic phase thus smoothes the graphene oxide film composed spontaneously under the coincidence of lamellar liquid crystal and 2D layered material. Through this work, the interlayer lubrication of GO was optimized, and the problem of GO dispersion sedimentation was solved by self-assembly. The range of interfacial lubrication of GO aqueous dispersion has been expanded and the synergistic effect is conducive to the environmentally friendly lubricants.

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