Carbon Nanomaterials in Temperature-Responsive Poly(N-isopropylacrylamide) Microgels for Controlled Release and Friction Reduction

As a type of promising lubricating additive, carbon nanomaterials can realize ultralow friction with diamond-like carbon films in water. However, the long-term dispersion stability of their aqueous solution remains a challenge. Herein, we found that a poly(N-isopropylacrylamide) (PNIPAM) microgel, which has been extensively used in nanodrug delivery, can considerably improve the dispersion stability of graphene oxide and hydroxylated multiwalled carbon nanotubes in aqueous solutions, while achieving an ultralow friction coefficient of 0.02. On the basis of structural characterizations, rheological tests, and molecular dynamics simulations, the mechanism for good collaborative effects is ascribed to the unique hybrid structure formed by the carbon nanomaterials and PNIPAM microgel via the hydrogen bonds between them. Interestingly, on the basis of the temperature sensitivity of the PNIPAM microgel, the hybrid structure can be reversed by varying the temperature, realizing controllable release of carbon nanomaterials. Surface characterizations reveal that a soft polymer tribofilm comprising a microgel and released nanomaterials forms on the wear surface after friction, accounting for the reduced friction and wear. The developed temperature-sensitive nanocomposite microgels are ecofriendly and easy to prepare, exhibiting good application prospects in water lubrication systems.