Tailoring mixed self-assembled monolayers (SAMs) for adhesion studies

Abstract

This thesis deals with the application of self-assembled monolayers (SAMs) to the study of the influence of chemical interactions on adhesion. The basic mechanisms of adhesion, although of considerable scientific and technological interest, are not yet satisfactorily understood, mainly due to limited understanding of the adhesive-surface interactions. The study of such phenomena requires a detailed knowledge of the surfaces to be studied. Selfassembled monolayers and mixed monolayers with different terminal groups allow a convenient and well-established technique to produce thin layers with minimal effort and good reproducibility. These thin layers are excellent model systems with a unprecedented control in surface properties, and their composition of the surface groups can be varied easily. In this thesis, the surface composition and wetting properties of mixed monolayers made of 1-dodecanethiol (DDT) and 1-mercapto-11-undecanol (MUD) are discussed together with the characterisation of the underlaying gold substrate (chapter 2). The surface fraction of MUD (” ‘MUD), as determined by XPS, is below the related fraction in solution O/MUD) if YMUD is below 0.7-0.8, and this is an effect of the used solvent. At fractions of MUD above 0.7-0.8 in the solution, the surface fraction is found to be similar to the fraction in solution (= -MUD = ~Mun). This is believed to be due to hydrogen bonding between adjacent hydroxyl groups of adsorbed MUD molecules. Comparison of XPS measurements with results derived from contact angle measurements and interpreted by the Cassie as well as the Israelachvili and Gee equation suggests that the resulting monolayers are randomly mixed for SAMs adsorbed from solutions with 0.2 I y,,,lO.7. This assumption is based on the better fit of the data by the Israelachvili and Gee equation. For Y~un> 0.7, the system might be separated into domains or patches according to the better fit to the Cassie equation. The MUD monolayer and the mixed monolayers react under mild conditions with 1,4phenylene diisocyanate (PDI) as confirmed by infrared spectroscopy, XPS, ellipsometry and contact angle measurements. The optimal reaction conditions were found at a temperature of 40 “C, a MUD concentration of 0.1 M, and a reaction time of 2 h. Higher reaction