Investigation of a new cyclotriphosphazene-terminated perfluoropolyether lubricant. Properties and interactions with a carbon surface

Abstract

A novel perfluoropolyether (PFPE) structure terminated on one chain end by a hydroxyl end group and on the other end by a cyclotriphosphazene derivative was studied. The interactions that developed at the interface between molecularly thin films of this material (A20H) and amorphous nitrogenated carbon surfaces were investigated using contact angle goniometry, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and imaging ellipsometry. The results demonstrate that both the cyclotriphosphazene and the hydroxyl end group of monolayer A20H films are capable of attractive interactions with the carbon surface. These interactions induce a layering in the adsorbed film that is manifested in an oscillatory dependence of the surface free energy on applied A20H film thickness. The thickness of the first A20H monolayer is determined from the first minimum in the surface energy to be 13.0 ± 1.0 Å for the 2400 molecular weight material and 15.0 ± 1.0 Å for the 3100 molecular weight material studied. The second A20H monolayer is not completely wetting on the first monolayer. This results in an autophobic dewetting when A20H films in excess of the monolayer thickness are applied to the carbon surface. The mobilities of A20H monolayer films were also qualitatively compared to that of the hydroxyl-terminated PFPE, Fomblin Zdol. While interaction of the cyclotriphosphazene end group with the carbon surface increases the load-bearing capability of the PFPE film when compared to Zdol, it also has the effect of decreasing the mobility of the adsorbed film.