Molecular-level insight into the frictional properties of fluorinated self-assembled monolayers (SAMs) was achieved by combining two recently developed techniques that operate at the subnanometer scale:  control of the interfacial composition through molecular self-assembly and tribological measurements performed with the atomic force microscope. To explore the origin of frictional forces in fluorinated films, the frictional properties of two classes of alkanethiols adsorbed on single crystal gold were measured and compared. In these studies, films of equivalent chain length, packing density and packing energy, but different termination (methyl vs trifluoromethyl), were characterized and investigated. For these films, in which the only detectable difference was the outermost chemical structure/composition, a factor of 3 increase in the frictional response was observed in going from the hydrogenated to the fluorinated film. These results support the conclusion that chemical structure/composition alone plays an integral role in determining the frictional properties of an interface. We propose that the difference in friction arises predominantly from the difference in size of the methyl and trifluoromethyl groups.

Self-assembled monolayers (SAMs) were prepared by the adsorption of a series of 2,2-dialkylpropane-1,3-dithiols (1-5) and 2-pentadecylpropane-1,3-dithiol (6) onto the surface of gold. These SAMs were characterized by ellipsometry, contact angle goniometry, X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and atomic force microscopy (AFM).The studies demonstrate that a systematic variation in the structure of R′RC(CH 2 SH) 2 from symmetrical (1; R′ ) R) to progressively more unsymmetrical (2-6; R′ * R) can be used to provide control over the conformational order and interchain packing of the hydrocarbon tail group assembly. The tail group conformation and packing were found to influence profoundly both the wettability and the tribological properties of the SAMs. Assemblies of well-ordered, well-packed hydrocarbon tail groups yielded interfaces that exhibited low wettabilities and low frictional responses when compared to assemblies of disordered, loosely packed hydrocarbon tail groups. The trends in wettability and friction were rationalized by considering the magnitude of the van der Waals interactions between the hydrocarbon film and the contacting probe liquid and AFM tip, respectively.

Alkanethiols possessing terminal phenyl groups (C6H5(CH2)nSH, n ) 12-15) were adsorbed onto the surface of gold to afford phenyl-terminated self-assembled monolayers (SAMs). The SAMs were characterized by optical ellipsometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), atomic force microscopy (AFM), and contact angle goniometry. The films generated from the phenylterminated alkanethiols exhibited greater thicknesses but similar crystallinities and well-ordered lattice structures when compared to analogous SAMs generated by the adsorption of normal alkanethiols onto gold. Advancing contact angle measurements using water as the test liquid supported the presence of interfacial phenyl moieties. Furthermore, contact angle measurements using the test liquids methylene iodide (MI), dimethyl formamide (DMF), and nitrobenzene (NB) revealed an odd-even effect as a function of the number of methylene units underneath the terminal phenyl groups. The tribological properties of the phenyl-terminated films were characterized by AFM and compared to those of films derived from normal alkanethiols and other materials presenting aromatic hydrocarbon moieties at the interface (i.e., graphite and C60). The phenyl-terminated SAMs exhibited a substantially higher frictional response than graphite, a slightly higher frictional response than normal alkanethiol SAMs, but a much lower frictional response than C60-terminated SAMs.

The origin of frictional forces in self-assembled monolayers (SAMs) was investigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy was used to probe the frictional properties of the SAMs formed by the adsorption of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(111) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiols were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-packed backbone structure. For these films, which differed only in the nature of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluoromethyl-terminal groups and for mixed monolayers containing small concentrations of the trifluoromethyl-terminated component. These results strongly support the model that the difference in friction in such systems arises predominantly from the difference in the size of the terminal groups. Larger terminal groups in films of the same lattice spacing give rise to increased steric interactions that provide pathways for energy dissipation during sliding.

We report a comparative study of the structure and frictional properties of self-assembled monolayers (SAMs) generated by the adsorption of three homologous 17-carbon alkanethiolssheptadecanethiol, 2,2dipentadecyl-1,3-propanedithiol, and 2-pentadecyl-1,3-propanedithiolsonto the surface of Au(111). The structural properties of these SAMs were characterized by atomic force microscopy, surface infrared spectroscopy, X-ray photoelectron spectroscopy, spectral ellipsometry, and wettability by water and hexadecane. The frictional properties of the SAMs were examined by friction force microscopy. The results demonstrate that the packing density and the related crystalline order of the hydrocarbon chains influence the frictional properties of organic thin films. The origins of the frictional differences measured from these films are discussed in terms of the structure of the films.

The shear forces between poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG)-modified SiO2 tribopairs have been measured with colloidal-probe, lateral force microscopy (LFM) and related to the mass of solvent absorbed within the brushlike structure of immobilized PEG chains. The amount of solvent (per unit substrate area) absorbed within the tethered, brushlike polymer, referred to as areal solvation, Ψ, appears to be of importance in determining the lubrication properties of the tethered polymers. In this study, the degree of solvation was varied by choosing different solvents (aqueous buffer solution, methanol, ethanol, and 2-propanol) and was determined by a technique that combines the results of quartz crystal microbalance (QCM-D) experiments and optical waveguide lightmode spectroscopy (OWLS). The highest degree of solvation was measured for aqueous buffer solutions, and a progressive decrease in solvation of PLL-g-PEG was observed in moving from methanol to ethanol to 2-propanol. A concomitant increase in the measured shear force was observed with this decrease in solvation. The lubrication mechanism of the PLL-g-PEG-coated SiO2 tribopair is discussed in terms of solvation and solvent quality and compared with the lubrication mechanism of the corresponding tribopair where only one surface is coated with the polymer brush.

We have investigated the collapse−stretching transition of a surface-bound, brushlike copolymer, poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), and the consequence of such transitions on the frictional properties of this coating. The frictional properties of the interface have been measured by colloidal-probe lateral force microscopy (LFM) in liquid environments on the nanoscale. The collapse−stretching transition has been induced through the systematic variation of the chemical composition of the binary solvent mixture comprised of an aqueous buffer solution and 2-propanol. The influence of solvent composition on the polymer conformation was monitored by comparing measurements conducted with optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microbalance with dissipation monitoring (QCM-D). The combined approach employing QCM-D and OWLS has allowed the quantification of the mass of solvent molecules absorbed in the brushlike structure of PLL-g-PEG and has revealed a significant preferential solvation effect. This study has demonstrated preferential solvation of a surface-bound polymer and the role of such solvation in maintaining the favorable lubricating properties of a PEG brush when exposed to mixtures of good and poor solvents.

We have used optical microscopy, atomic force microscopy, and ellipsometry to study the dewetting of films of a perfluoropolyether polymer on silicon substrates. The disjoining pressure of these films is determined, for the first time for a dewetting system, by using noncontact atomic force microscopy to measure the dimensions of the liquid dewetting droplets. The determined disjoining pressure explains the different dewetting processes observed for different initial film thicknesses and is dominated by structural forces and by the inability of the polymer to spread on its own monolayer.[S0031-9007 (99)09002-X] PACS numbers: 68.15. + e, 68.45.Gd, 68.55. -aThin polymer films have a wide variety of applications including protective coatings, dielectric layers, and lubrication films. As the trend for miniaturizing devices continues, so will the demand for increasingly thinner, more uniform polymer films. Unfortunately, for many combinations of polymer and substrate materials, dewetting is observed when the polymer is in the liquid state. Consequently, the physics and chemistry of wetting and dewetting phenomena are topics of great current interest [1][2][3][4][5][6]. In these previous studies, which primarily use apolar liquid films such as polystyrene on silicon, the dewetting process commonly occurs in three successive phases: rupture of the film to form holes, growth of the holes to form a polygonal network of straight liquid rims, and then decay of the rims via a Rayleigh instability to form droplets. The rupturing process, which initiates dewetting, is believed to result either by the formation of holes nucleated at defects or from spinodal decomposition of the unstable film.The physics of why a liquid film wets or dewets is found in the negative derivative of the free energy with respect to film thickness, called the disjoining pressure, which arises from the interaction energies of molecules in a film being different from that in the bulk. Following Derjaguin and Churaev [7], the disjoining pressure P can be written as P P w 1 P e 1 P s , (1) where P w is the pressure component arising from van der Waals forces acting between the film and the substrate, P e is the component from the electronic or polar interactions, and P s is the component from the molecules in the film having a structure different from the bulk liquid. If the interactions between the molecules in the film and the solid substrate are more attractive than the interactions between molecules in the bulk liquid, P . 0. Consequently, a liquid film with a thickness in a range where dP͞dh . 0 can lower its free energy by becoming thicker in some areas while thinning in others, i.e., by dewetting. When dP͞dh , 0, wetting or spreading occurs.In this Letter, we report the first experimental determination of disjoining pressure for a dewetting system. This is accomplished by using noncontact atomic force microscopy (AFM) to measure the curvature of the liquid surface at the top of the dewetting droplets to determine the capillary (or Laplace) pressure ...