The shear response of polymer brushes is important in a variety of applications, including colloidal stabilization and lubrication. Polymer brushes consist of flexible polymer chains terminally anchored at surfaces. Klein et al. 1 demonstrated that a pair of opposing brushes, when slid past one another in the presence of a good solvent, experience a repulsive normal force and interpreted this as brush swelling under shear. The origin of this shear induced swelling is not well understood, although several analytic theories based on scaling arguments and hydrodynamic models have been proposed2,3. Based on this type of scaling idea, Sevick and Williams4 proposed a novel design for a `smart' pressure-sensitive flow-control valve made from two opposing polymer brushes. The brush senses the shear force, responds by swelling, and thereby readjusts the shear and flow to maintain a constant flow rate.
To explore the mechanism of shear induced swelling, molecular dynamics simulations of two opposing polymer brushes immersed in a good solvent have been carried out under shear. The system consisted of two atomic walls covered by a layer of polymer chains 100 units long. The chains are firmly bound at their ends to the wall at a surface coverage typical of a polymer brush. Shear was induced by sliding the surfaces in opposite directions at a range of shear rates. The normal forces between the surfaces, calculated as the net force acting on the surface atoms, increases with the shear rate above a certain velocity. This increase in normal force between the brushes is a function of shear rate, seperation between the surfaces, grafting density and the chain length. These initial results are encouraging and suggest design of possible nanoscale flow-valves.
Applications for a `smart' flow control valve would be numerous, including, for example, constant discharge of drugs from an implant and constant solvent flow for chemical analysis under remote conditions. The increase in normal force between sheared brushes may spur speculation on new strategies for producing new lubricants and may shed light on the origin of very low friction biological joints.
The simulations were carried out using the DL_POLY MD code from Daresbury Laboratories, UK, on the CRAY-T3E at the North Carolina Supercomputer Center.
Supported by the NASA-Ames Computational Nanotechnology Program
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J-L Barrat, Macromolecules 25, 832 (1992).
V. Kumaran, Macromolecules 26, 2464 (1993).
E.M. Sevick and D.R.M. Williams, Macromolecules 27, 5285 (1994).
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