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.

Reduction of the interfacial friction for the contact of a silicon oxide surface with sodium borosilicate in aqueous solutions has been accomplished through the adsorption of poly(L-lysine)-graft-poly(ethylene glycol) on one or both surfaces. Spontaneous polymer adsorption has been achieved via the electrostatic attraction of the cationic polylysine polymer backbone and a net negative surface charge, present for a specific range of solution pH values. Interfacial friction has been measured in aqueous solution, in the absence of wear, and on a microscopic scale with atomic force microscopy. The successful investigation of the polymer-coated interfaces has been aided by the use of sodium borosilicate microspheres (5.1 microm diameter) as the contacting probe tip. Measurements of interfacial friction as a function of applied load reveal a significant reduction in friction upon the adsorption of the polymer, as well as sensitivity to the coated nature of the interface (single-sided versus two-sided) and the composition of the adsorbed polymer. These measurements demonstrate the fundamental opportunity for lubrication in aqueous environments through the selective adsorption of polymer coatings.

The tribological properties of poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG)-coated oxide interfaces have been investigated with atomic force microscopy (AFM) as a function of the molecular structure. Polymer-bearing surfaces were obtained via spontaneous adsorption of the polymer onto the oxide substrate from a buffered solution of physiological pH. Interfacial friction of these PLL-g-PEG-coated surfaces was found to be highly dependent on the duration of deposition and the architecture of PLL-g-PEG. In terms of the architecture, the PEG chain length and the grafting ratio (i.e., the molar ratio of L-lysine monomer to PEG side chain) of adsorbed PLL-g-PEG significantly influence the interfacial friction; specifically, friction is reduced as the PEG chain length increases and as the molar ratio of L-lysine monomer to PEG side chain decreases. The characteristics of the polymer deposition time and the influence of the lysine/PEG grafting ratio are rationalized in terms of spatial packing density considerations.

Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction).

Equilibrium geometries, stabilities, and electronic properties of TinAl (n = 1-13) clusters have been studied by using density-functional theory with local spin density approximation and generalized gradient approximation. The ground-state structures of TinAl clusters have been obtained. The resulting geometries show that the aluminum atom remains on the surface of clusters for n < 9, but is slowly getting trapped beyond n = 9, meanwhile, the Al atom exhibits a valent transition from monovalent to trivalent. The geometric effects and electronic effects clearly demonstrate the Ti4Al cluster to be endowed with special stability. The studies on the bonds indicate the change from ionic to metalliclike.

The stability and structures of titanium-doped gold clusters AunTi (n=2–16) are studied by the relativistic all-electron density-functional calculations. The most stable structures for AunTi clusters with n=2–7 are found to be planar. A structural transition of AunTi clusters from two-dimensional to three-dimensional geometry occurs at n=8, while the AunTi (n=12–16) prefer a gold cage structure with Ti atom locating at the center. Binding energy and second-order energy differences indicate that the Au14Ti has a significantly higher stability than its neighbors. A high ionization potential, low electron affinity, and large energy gap being the typical characters of a magic cluster are found for the Au14Ti. For cluster-cluster interaction between magic transition-metal-doped gold clusters, calculations were performed for cluster dimers, in which the clusters have an icosahedral or nonicosahedral structure. It is concluded that both electronic shell effect and relative orientation of clusters are responsible for the cluster-cluster interaction.

This paper presents the synthesis and characterization of two series of polymeric compounds comprising eight furan‐based polyamides prepared via melt polycondensation at low temperatures using various combinations of five aromatic raw materials. The chemical and physical structures and thermal stabilities of the obtained polyamides were investigated by various characterization methods. In addition, the polyamides were subjected to solubility testing in five common organic solvents. The results showed that the proposed furan‐based polyamides possessed thermal stabilities similar to those of conventional high‐performance aromatic polyamides, but with greatly improved solubility. Accordingly, the introduction of furan groups increased the solubility of the polyamides with respect to the solubility of their individual precursors, which is highly advantageous for subsequent polyamide processing and expanding their range of potential applications.