The Adhesion of Monomolecular Hydroxyl-terminated Perfluoropolyether Liquid Films on the Sputtered Silicon Nitride Surface as a Function of End Group Acidity and Mobility

Abstract

The intermolecular interactions at the interface between monomolecular hydroxyl-terminated perfluoropolyether (PFPE) liquids (Zdol, Zdol-TX, Z-Tetraol, Zdiac) and a sputtered amorphous silicon nitride film (SiN x ) are investigated using contact angle goniometry, Fourier transform infrared spectroscopy, and ab initio computational chemistry. The results demonstrate that the adhesion between the PFPE liquids and the SiN x surface occur via the polar interactions between the PFPE end groups (-OH, -COOH) and the polar sites on the SiN x surface (e.g., SiOH). The attractive interactions lead to a lowering of the polar surface energy with increasing PFPE coverage up to a monolayer. The binding energy is computed to be approximately −4 to −9 kcal/mol, depending upon the polarity of the PFPE end group. Adsorbed water is shown to compete with PFPE for surface bonding sites on SiN x (−4.4 kcal/mol) that can lead to a significantly reduced level of adhesion for some of the hydroxyl-terminated PFPEs. A higher level of adhesion between the PFPEs and SiN x can be attained by increasing the strength of the hydrogen bond and/or increasing the configurational entropy of the PFPE end group to facilitate the hydrogen bonding reaction.