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Notes: We studied the exchange between the adsorbed state and free solution when polyelectrolyte chains, adsorbed to a solid surface of opposite charge, were displaced by chains of higher charge density. Metastable states of surface composition were extremely long-lived (〉2–3 days). The system was a family of poly(1,4 vinyl)pyridines (PVP) with different fractions of charged segments (14%, 48%, and 98% quaternized and the same degree of polymerization); samples were exposed sequentially from aqueous D2O solution to a single silicon oxide substrate at pH where the surface carried a large negative charge (pH=9.2 or 10.5). Measurements were based on Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). As a first conclusion, we found charge of adsorbed polymer to be conserved during extended exchange times, suggesting that charge at the surface (not mass adsorbed) regulated the dynamics of adsorption and desorption. Except at the highest ionic strength charge of polymer at the surface during the displacement process considerably exceeded that for the initially-adsorbed layer, suggesting an intermediate state in which newly-adsorbed chains were more extended from the surface and not yet equilibrated in their conformations. Second, we concluded that desorption was the rate-limiting step in adsorption–desorption, since the desorption rate responded more to changes of ionic strength than did the adsorption rate onto previously-adsorbed polymer. Ionic strength appeared to modulate the intensity of sticking to the surface. Third, we found that the initial stages of desorption obeyed a simple functional form, exponential in the square root of elapsed time. This is conclusively slower than a first-order kinetic process and suggests that desorption in this polyelectrolyte system was diffusion-controlled during the initial stages. It is the same functional form observed for flexible polymers in nonpolar solvents. Fourth, we concluded that at relatively low concentration of salts desorption proceeded in two stages; one subpopulation of adsorbed chains desorbed relatively quickly, with a rate exponential in the square root of time, and a second subpopulation was so much slower to be desorb that it appeared to be kinetically frozen at the surface. The higher the ionic strength, the less the polymer was kinetically frozen and this effect disappeared entirely for the highest ionic strength. The interpretation that the kinetically-frozen states reflected conformational heterogeneities within the adsorbed layer was supported by direct measurements of the dichroic ratio of adsorbed pyridinium rings. Finally, a new kinetic regime was observed at the highest salt concentrations, in which the exchange was inhibited by worsened solubility of the displaced molecules. It is significant that this regime began at salt concentrations significantly below the point of bulk insolubility. Since most organic polyelectrolytes may be considered to be a copolymer of polar charged units and hydrophobic uncharged units, this effect is expected to be general. © 1998 American Institute of Physics.

Notes: Solid planar plates with area up to several square centimeters can be made parallel at controllable separations from ca. 0.1 (if airborne dust is eliminated) to 〉500 μm. Apart from dust and surface roughness, which set the lower bound of plate-plate separation, there is no other fundamental constraint on the type of surface (metallic or dielectric; opaque or translucent) that can be studied. When conducting plates are employed, it is possible to apply an electric field in the direction normal to the plates and observe the competition between shear fields and electric fields in orthogonal directions. The large surface area should afford sufficient quantity of sample to make possible various spectroscopic and scattering experiments (especially infrared and dielectric spectroscopy in the direction normal to the plates, and x-ray and neutron reflectivity). © 1997 American Institute of Physics.

Notes: We contrast the adsorption of human serum albumin (HSA) onto two solid substrates previously primed with the same polyelectrolyte of net opposite charge to form one of two alternative structures: randomly adsorbed polymer and the "brush" configuration. These structures were formed either by the adsorption of quaternized poly-4-vinylpyridine (QPVP) or by end-grafting QPVP chains of the same chemical makeup and the same molecular weight to surfaces onto which QPVP segments did not adsorb. The adsorption of HSA was quantified by using Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). The two substrates showed striking differences with regard to HSA adsorption. First, the brush substrate induced lesser perturbations in the secondary structure of the adsorbed HSA, reflecting easier conformational adjustment for longer free segments of polyelectrolyte upon binding with the protein. Second, the penetration of HSA into the brush substrate was kinetically retarded relative to the randomly adsorbed polymer, probably due to both pore size restriction and electrostatic sticking between charged groups of HSA and QPVP molecules. Third, release of HSA from the adsorbed layer, as the ionic strength was increased from a low level up to the high level of 1 M NaCl, was largely inhibited for the brush substrate, but occurred easily and rapidly for the substrate with statistically adsorbed QPVP chains. Finally, even after addition of a strong polymeric adsorption competitor (sodium polystyrene sulfonate), HSA remained trapped within a brush substrate though it desorbed slowly from the preadsorbed QPVP layer. This method to produce irreversible trapping of the protein within a brush substrate without major conformational change may find application in biosensor design. © 1999 American Institute of Physics.

Notes: We contrast the adsorption, over a wide range of pH and ionic strength, of polyelectrolyte chains with different fractions of charged segments but similar degree of polymerization. The system was a cationic polymer, poly(1,4 vinyl)pyridine (PVP), with 14%, 48%, and 98% quaternized repeat units, adsorbed from aqueous solution (D2O or H2O) onto a single silicon oxide substrate at 25 °C. Measurements were based on Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). In the first phase of this study, we varied the surface charge density by changing pH and showed that attraction of PVP to the surface was electrostatic. The amount adsorbed of charged (quaternized) PVP segments was nearly the same regardless of the overall fraction of charged segments in the chain. In addition, polymer adsorption appeared to enhance the dissociation of silanol groups on the solid surface. In a second phase of this study, the ionic strength was varied systematically under conditions of high negative surface charge density (high pH), focusing on 98% quaternized PVP. Strong chemical specificity was found; the polyelectrolyte was insoluble in KI above a low salt concentration, but soluble in NaCl, signifying that the anions, Cl− and I−, competed with the negatively-charged surface for association with the polyelectrolyte. At the same time, the cations, Na+ and K+, competed with the polyelectrolyte for access to the limited surface area. The mass adsorbed increased strongly with increasing salt concentration and, for polymer in aqueous NaCl, passed through a maximum with subsequent decrease, reflecting a greater abundance of loops and tails at intermediate ionic strength and ultimately complete desorption of the chains when the salt concentration was very high. The maximum in mass adsorbed occurred at very high ionic strength (1 molar NaCl), indicating competitive adsorption of Na+ with charged segments of the polymer. Direct measurements of the infrared dichroism of pyridinium rings of the adsorbed PVP confirmed the presence of a relatively flattened state at low ionic strength and nearly isotropic orientation otherwise. In the third phase of this study, we studied the competitive adsorption to surfaces of high negative charge of Na+ and a monomeric analog of the PVP repeat unit, the 1,4-dimethylpyridinium ion (P+). A tentative quantitative estimate of the effective surface sticking energy of P+ relative to Na+ ions indicated a decrease from 7kBT in low-ionic-strength buffer solution to 4.5kBT in 0.5 M NaCl. These numbers appear to exceed the weak-adsorption limit in which facile equilibration of the adsorbed layer should be expected. © 1998 American Institute of Physics.

Notes: Abstract We studied the shear response of a confined lubricant layer on approach of the transition to sliding with a surface force apparatus modified for oscillatory shear. In a given experiment, we found that the transition to sliding occurred always around the same deformation amplitude although the shear stress needed to initiate sliding varied up to a factor of two depending on sample history. This suggests the concept of deformation-controlled switching from rest to sliding. The elastic spring-constant, in the stick state, weakened with increasing deformation amplitude. This decrease can be described by a power law when plotted versus the distance to a critical deformation amplitude. The build-up of solid-like behavior after sliding stopped was also gradual and was consistent with a logarithmic time dependence. We suggest a model relating the gradual decrease of stiffness to weakening of the boundary layer, specifically to destruction of some elastic links between molecules or between molecules and the solid surfaces. Static friction (the force that must be overcome at the onset of kinetic motion) is proportional to the number of such links formed during the time of stick.

Notes: Abstract This review article summarizes recent progress in investigation of nano‐rheology and thin film lubrication, as well as their contributions to conventional tribology. As the thickness of a lubricating film becomes comparable to molecular dimensions, a lubricant confined between solid walls undergoes a dramatic transition in its rheological properties and physical state, including the formation of ordered structure, enhanced viscosity and slow relaxation, glass transition or solidification, and consequent stick‐slip motion. As a result, it is recognized that there is special regime between EHL and boundary lubrication, identified as thin film lubrication, where lubricant flow and hydrodynamics are still in action but behave differently from expectations of the classical theory. Generalized theories of thin film lubrication are under development. Microscopic studies of thin film lubrication provide a solid theoretical basis to the development of high‐tech and micro devices, the understanding of lubrication failure, the generalization of classical lubrication theory, and friction control and interface design.

Notes: Abstract Dilute solutions of two polar end‐functionalized linear alkanes (1‐hexadecylamine and palmitic acid), each dissolved in tetradecane, were confined between two mica surfaces and investigated using a surface forces apparatus modified to study shear nanorheology. These two solutions showed similar nanorheological properties that differed from those observed for pure n‐alkanes. In static measurements, a “hard wall”, rather than an oscillatory force, was observed as a function of film thickness. The polar alkane component formed a weakly adsorbed single layer at each mica surface, disrupting the layered structures found in neat n‐tetradecane. In dynamic experiments at low shear amplitude, the storage modulus G' exceeded the loss modulus G" at low frequencies; above some characteristic frequencies G' increased such that g' ≈ G", indicating significantly more energy loss through viscous modes at higher frequency. When the amplitude was varied at fixed frequency, no stick–slip was observed and the limiting value of the shear stress at high effective shear rate was an order of magnitude less than for unfunctionalized n‐alkanes at similar loads. Together, these results show that the addition of a small amount of polar alkane component, by disrupting the layered structures that would have been formed in the neat n‐alkane, is effective in suppressing static friction and reducing kinetic friction in the boundary lubrication regime.

Notes: Abstract The dynamic shear properties of molecularly-thin films of unfunctionalized and end-functionalized (telechelic) Fomblin-Z perfluoropolyalkylether (PFPAE) melts with number-average molecular weight M n≈ 3000−4000 g,mol-1 have been studied at shear rates of 10-2−105 s-1 at normal pressures of 1 and 3 MPa. The shear responses are compared to measurements on end-functionalized polymers of the same chemical composition but lower molecular weight, M n≈ 2000 g,mol-1. The predominantly elastic response and high shear moduli of the confined film of unfunctionalized polymer, Fomblin Z03, suggest that it forms a structure likely to solidify already at low pressure. Its lubricating properties are less favorable than the ones found for hydroxyl- (DOL) and piperonyl-terminated Fomblin-Z (AM2001, AM3001), where associated molecules form a structure less prone to solidification under confinement. The thickness of the compressed films of the end-functionalized polymers increased more strongly with molecular weight than as M n 0.5 . The shear moduli were found to be larger, the higher the molecular weight, indicating slower relaxations. At a normal pressure of 3 MPa, these films solidified and displayed stick–slip as seen already at 1 MPa in the Z03 film. The limiting shear stress of the unfunctionalized Z03, σ 〉 3 MPa, exceeded by an order of magnitude the limiting shear stress of all of the end-functionalized polymers. The limiting shear stress of the hydroxyl-terminated polymer was larger than that of the piperonyl-terminated polymer.