Concerning the interactions between Zdol perfluoropolyether lubricant and an amorphous-nitrogenated carbon surface

Abstract

The interactions that occur between the hydroxyl-terminated perfluoropolyethers Zdol 2000/4000 and an amorphous-nitrogenated carbon surface (CNx) were studied via surface energy measurements, kinetic measurements, and ab initio calculations. The results of these measurements are compared with those of previous studies on the Zdol/amorphous-hydrogenated carbon (CHx) system and the major differences identified. The thickness dependence of the dispersive surface energy for the Zdol/CNx system can be fit using a repulsive van der Waals potential. Effective Hamaker constants determined for both the Zdol/CNx and Zdol/CHx systems demonstrate that Zdol is less effective at covering CNx as compared to CHx due to less favorable interactions between the Zdol backbone and the CNx surface. The Zdol thickness dependence of the polar surface energy for the Zdol/CNx system indicates that very few strong polar interactions are present between the initially applied Zdol and the CNx surface. A substantial decrease in the polar surface energy of the first Zdol monolayer however occurs on a time scale of 1-5 weeks after lubricant application. The attractive well that develops in the free energy versus thickness curve reflects the formation of attractive interactions between the polar hydroxyl end groups of Zdol and the polar entities on the CNx surface. A kinetic analysis of the Zdol + CNx system reveals that the rate at which the adhesive interactions are formed is limited by diffusion of the polar end groups to the surface active sites. Ab initio calculations indicate that attractive hydrogen-bonding interactions between the hydroxyl end groups of Zdol and imine (basic) sites on the CNx surface may be responsible for the Zdol adhesion. These calculations further suggest that the appearance of the diffusion step in the bonding kinetics and the less efficient coverage of Zdol on CNx are manifestations of repulsive interactions that exist between the basic imine surface sites and the basic perfluorinated Zdol backbone.