Effect of Solvent Quality on the Friction Forces Between Polymer Brushes
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DD11.6.1/AA10.6.1
Effect of Solvent Quality on the Friction Forces Between Polymer Brushes Aaron M. Forster and S. Michael Kilbey II Department of Chemical Engineering Clemson University Clemson, SC 29634 ABSTRACT We have used the surface forces apparatus to measure the structural and frictional force profiles between opposing, solvated brush layers as a function of temperature. Two different polyvinylpyridine-polystyrene [PVP-PS] diblock copolymers were used to make PS brushes. The molecular weights (in thousands) of these PVP-PS materials were [114/103]k, [30/70]k, respectively. Structural and frictional force profiles in toluene and cyclohexane were measured, and the cyclohexane experiments were conducted at temperatures ranging from the theta-point to 50°C. In toluene the PS brushes needed to be compressed to ~1/5th of their equilibrium height before frictional forces were measured, but this onset of frictional forces was detected at a much lower level of compression in near-theta cyclohexane. In cyclohexane the structural force profiles were basically insensitive to the temperature change, but the frictional forces depended strongly on the solvent temperature. When the cyclohexane temperature was raised, the onset of frictional forces decreased toward the good-solvent onset. We also discuss the dependence of frictional force on shearing parameters. INTRODUCTION A polymer brush is formed when polymer chains are end-tethered to surfaces such that the distance between tethering points is less than the radius-of-gyration, Rg, of the free chain in solution. The untethered chains extend away from the surface to relieve the osmotic pressure created by the crowding of the chains. This stretched, upright structure is the distinguishing characteristic of a polymer brush. Because brushes straddle the solid-fluid interface, they can alter the wetting, wear, or lubricative properties of a surface. The structural aspects of polymer brushes have been thoroughly reported on through both theory [1-3] and experiments [4-6], so few details are necessary here. In terms of the behavior of brushes subjected to shearing, previous research has shown that brush layers in a good solvent must be compressed to ~15% their equilibrium height before shear forces are measured [7-10]; however, when opposing brushes in a near-theta solvent are sheared past one another, frictional forces are measured when the brushes are only mildly compressed [8-10,13,14]. Along these lines, Grest has performed molecular dynamics simulations to explore how the shear forces between polymer brushes depend on solvent quality. Those simulations show the expected decrease in the range of the normal force profiles when the solvent condition is changed from good to near-theta; however the simulations do not show that the onset of frictional forces decreases (to smaller distances) as the solvent quality is improved [15]. We continue the study of how
DD11.6.2/AA10.6.2
solvent quality affects the frictional behavior of brushes by focusing on the role of solvent tempe
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