The adhesion and friction of smooth polymer surfaces were studied below the glass transition temperature by use of a surface forces apparatus. The friction force of a crosslinked polymer was orders of magnitude less than that of an uncrosslinked polymer. In contrast, after chain scission of the outermost layers, the adhesion hysteresis and friction forces increase substantially. These results show that polymer-polymer adhesion hysteresis and friction depend on the dynamic rearrangement of the outermost polymer segments at shearing interfaces, and that both increase as a transition is made from crosslinked surfaces to surfaces with long chains to surfaces with quasi-free ends. The results suggest new ways for manipulating the adhesion and friction of polymer surfaces by adjusting the state of the surface chains.
Hyaluronan is believed to have an important function in the boundary biolubrication of articular cartilage. Using a Surface Forces Apparatus, we tested the tribological properties of surface bound, rather than "free" hyaluronan. The grafting process of the polyelectrolyte included either a biological route via an HA-binding protein or a chemical reaction to covalently bind the polymer to a lipid bilayer coated surface. In another reaction, we constructed a surface with covalently grafted hylan (crosslinked hyaluronan). We studied the normal and shear forces between these surfaces. None of the systems demonstrated comparable lubrication to that found between cartilage surfaces except at very low loads. Both grafted hyaluronan and hylan generated coefficients of friction between 0.15 and 0.3. Thus, the polysaccharide, which is a constituent of the lamina splendens (outermost cartilage layer), is not expected to be the responsible molecule for the great lubricity of cartilage; however, it may contribute to the load bearing and wear protection of these surfaces. This was concluded from the results with hylan, where a thin gel layer was sufficient to shield the underlying surfaces from damage even at applied pressures of over 200 atmospheres during shear. Our study shows that a low coefficient of friction is not a requirement for, or necessarily a measure of, wear protection.
Using a surface forces apparatus (SFA), we have studied the adhesion and friction of polystyrene (PS) films on mica having various molecular weights from 580 Da (T g ) -45 °C) to 1300 Da (T g ) +39 °C) at temperatures T from 23 to 65 °C, i.e., below and above T g . The purpose of these studies was to establish how different static and dynamic properties and surface deformations, i.e., the contact and adhesion mechanics, of detaching and shearing polymer films change as one passes through the glass transition temperature, i.e., from solidlike or glassy to liquidlike behavior. At T > T g the polymers behave like liquids, and at low detachment and shearing rates their adhesion and friction forces are accounted by their surface energy and bulk viscosity, although fingering instabilities and cavitation accompany the detachment. For the higher MW polymers (T close to T g ), the adhesion and friction forces increasingly depend on the density of chain ends at the surfaces. The adhesion hysteresis peaked close to the bulk T g but also depended on the load, contact time, and detachment rate. Certain correlations and scaling relations were obtained. We discuss these findings in terms of the bulk and surface molecular properties of interacting polymer surfaces above and below T g .
Using a surface forces apparatus to measure forces, and optical (multiple beam) interferometry to measure surface shapes and separations (to +/-1 A), the normal, viscous, and shear (lubrication) forces between smooth mica surfaces in aqueous hyaluronic acid (HA) solutions were measured. The experimental conditions of loading pressures, pH, and HA concentration were set to closely correspond to physiological human knee-joint conditions. From the force and optical (refractive index) measurements, it was concluded that, like other negatively charged polyelectrolytes, HA does not naturally adsorb on the mica surface which is hydrophilic and weakly negatively charged at physiological conditions: the polymer solution exhibits the bulk viscosity (22.5 +/- 1.5 cP) for films thicker than about 0.4 miccrom of the polymer, whereas for thinner films, the viscosity decreases monotonically toward the value of the pure electrolyte solution (1 cP) as HA is extruded from between the surfaces. This is indicative of a repulsive "depletion" interaction of HA with each mica surface and to a weakly attractive polymer-mediated force between the two surfaces. Thus, free HA in synovial fluid is not expected to act as a good "boundary lubricant." Relaxation measurements on approaching and receding surfaces in HA solutions were also performed, and it is shown that the presence of HA in the bulk solution can improve "hydrodynamic" modes of lubrication, for example, by assuaging the compression stroke. The study includes information that is beneficial to researchers working with biomaterials viscosupplementation devices.
Using a surface forces apparatus we studied the adhesion and friction of glassy polystyrene (PS) and poly(vinylbenzyl chloride) (PVBC) surfaces having various molecular weights (MW) from 2 000 000 to 1240 Da. Cross-linking of high MW polymers leads to lower adhesion and friction relative to the untreated polymers, whereas scission (bond-breaking) leads to higher adhesion and friction. "Asymmetric" bond-broken against cross-linked surfaces behave intermediately between the two "symmetric" systems. Both the friction and adhesion of untreated polymers increase with decreasing MW, often resulting in irreversible damage during sliding or separation. We conclude that the population of the chain "ends" at the surfaces is the most important factor that determines the adhesion, adhesion hysteresis, friction, and wear between two polymer surfaces. Segment polarity and layer thickness play secondary roles.
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