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.

Adhesion Between Surfaces Separated by Molecularly Thin Perfluoropolyether Films

“…In particular, we observe that the dispersive surface energy decreases monotonically with increasing film thickness, asymptotically approaching the value of the bulk PFPE-OH liquid c d bulk = 15.6 mJ/m 2 for the thickest films studied. The functional dependence of the dispersive surface energy on film thickness c d ðhÞ can be expressed as [11],…”

“…The bonded fraction was determined by measurement of the total applied film thickness and the film thickness remaining after solvent extraction of the mobile PFPE-OH component [15]. The bonding ratio used herein is defined as the thickness of the bonded PFPE-OH film divided by the target film thickness [11].…”

“…The dispersive and polar surface energy components of the PFPE films, c d s and c p s , were obtained from contact angle measurements performed using hexadecane (c d L = 27.5 mJ/m 2 ) and water (c d L = 21.8 mJ/m 2 , and c p L = 51.0 mJ/m 2 ) as reference liquids [9,11]. The contact angle was determined and the results were averaged over four measurements.…”

“…For example, the dispersive interactions between a surface and a liquid film will result in a free energy that rapidly increases as the film thickness is decreased [11]. The thickness dependence of the dispersive surface energy c d (h) can be expressed in terms of the free energy of the bulk material c d bulk as…”

“…where Dc d ðhÞis termed the excess surface energy, which scales inversely as the square of the film thickness [3,11]. In addition to the van der Waals interactions, molecularspecific interactions, i.e., hydrogen bonding, dipole-dipole interaction, or covalent bonding, can develop between the applied liquid film and the solid substrate.…”