Mechanism of Superlubricity Conversion with Polyalkylene Glycol Aqueous Solutions.
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In this study, ultralow friction coefficient (COF, < 0.01) was obtained through polyalkylene glycol (PAG) aqueous solutions with different molecular weights (MWs) ranging from 270 to 3930 gmol-1 under ambient conditions. With the increase in the MWs of PAG molecules, the threshold concentration to obtain this type of superlubric behavior gradually changed from 90 to 60 wt %. This phenomenon was closely related to the interaction between PAG chains and water molecules and the state of chemical binding. In the superlubricity system, superior load-bearing capacity was achieved at optimal threshold concentrations of all PAG aqueous solutions wherein multilayered adsorption layers that consisted of fully hydrated PAG molecules were formed on the sliding solid surfaces. With respect to the concentration below the threshold value, the existence of a shearing layer was indicated to play a significant role. Thus, the synergetic effect of sufficient adsorption of molecules and the unique shear rheology of the PAG aqueous solution were essential to achieve superlubricity.