Intermolecular interactions between a monomolecular hydroxyl-terminated perfluoropolyether film and the sputtered SiNx surface

Abstract

The intermolecular interactions at the interface between a monomolecular hydroxyl-terminated perfluoropolyether (PFPE) liquid and an amorphous silicon nitride (SiNx) film were investigated using contact angle goniometry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results demonstrate that the surface of the sputtered SiNx film is highly oxidized (SiO2) and contain hydroxyl groups (SiOH) that are capable of attractive interactions with the hydroxyl end groups of the PFPE. The attractive interactions lead to a lowering of the polar surface energy with increasing PFPE coverage up to a monolayer. Ab initio quantum chemical computations on model dimers provide the energetic details associated with the PFPE/surface interactions. The primary source of the attractive, adhesive interactions in the PFPE/SiNx system stems from hydrogen bonding between the PFPE and SiNx hydroxyl groups. The binding energy is computed to be approximately -4 kcal/mol. Both the SiNx surface nitrogen and oxygen atoms in the Si3N and Si-O-Si local geometries are weakly basic and hence incapable of providing strong adhesive interaction sites for the PFPE hydroxyl end groups. The results on the kinetics of the adhesive interactions between the hydroxyl-terminated PFPE and the SiNx surface demonstrate a nonclassical time-dependent rate coefficient, k(t) ∝ kot-h. With increasing temperature, the temporal dependence is observed to change from h = 0.5 (lower temperature) to h = 1.0 (higher temperature), which is interpreted as a two-dimensional melting transition from a solidlike to a liquidlike confined film. Under ambient conditions, adsorbed water competes with PFPE for surface bonding sites on the polar SiNx surface, leading to a significantly reduced level of adhesion for the hydroxyl-terminated PFPE.