Poly(N-isopropylacrylamide) Microgel Lubricants with High Friction Sensitivity

Poly(N-isopropylacrylamide) (PNIPAM) microgels with a unique thermosensitive swelling and deswelling behavior in water may be capable of sensing the frictional heat in sliding interfaces and altering the performance of water-lubricated contacts accordingly. However, the rheological properties of PNIPAM microgels and the formation mechanism of tribofilms at the sliding interface during friction are currently unclear. In the present study, the variation in the rheological behaviors and kinetic characteristics of PNIPAM microgels during phase transformation was investigated using a temperature-controlled rheometer and molecular dynamics (MD) simulations. As the temperature increases to the lower critical solution temperature (LCST) of PNIPAM, an atypical increase in the viscosity of the microgel solution is observed due to the increased loss modulus (G″). The interaction between PNIPAM microspheres is enhanced above the LCST, as shown by MD simulations. Tribological evaluations demonstrate that PNIPAM microgels could effectively improve the tribological performance due to the formed gel layer that confines water in its porous structure and changes the friction interface from hard-on-hard to soft-on-soft contact, and the forming ability of the gel layer is related to the aggregation and deposition of PNIPAM microspheres due to the deswelling of PNIPAM microgels triggered by frictional heat.