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

The interactions that occur between the hydroxyl-terminated perfluoropolyet hers Zdol 2000/4000 and an amorphous-nitrogenated carbon surface (CNx) were studied via surface energy measurements, kinetic measurements, and ab init io 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 dispe rsive surface energy for the Zdol/CNx system can be fit using a repulsive v an der Waals potential. Effective Hamaker constants determined for both the Zdol/CNx and Zdol/CHx systems demonstrate that Zdol is less effective at c overing CNx as compared to CHx due to less favorable interactions between t he 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 firs t Zdol monolayer however occurs on a time scale of 1-5 weeks after lubrican t application. The attractive well that develops in the free energy versus thickness curve reflects the formation of attractive interactions between t he polar hydroxyl end groups of Zdol and the polar entities on the CNx surf ace. A kinetic analysis of the Zdol + CNx system reveals that the rate at w hich the adhesive interactions are formed is limited by diffusion of the po lar end groups to the surface active sites. Ab initio calculations indicate that attractive hydrogen-bonding interactions between the hydroxyl end gro ups of Zdol and imine (basic) sites on the CNx surface may be responsible f or the Zdol adhesion. These calculations further suggest that the appearanc e of the diffusion step in the bonding kinetics and the less efficient cove rage of Zdol on CNx are manifestations of repulsive interactions that exist between the basic imine surface sites and the basic perfluorinated Zdol ba ckbone.