The equilibrium structure of polymer brushes (strongly stretched chains terminally attached to a surface) and the interaction of brush-bearing surfaces have been studied extensively and are now well-understood. [1][2][3][4][5][6][7][8][9][10][11][12] Much less is known, however, on the behavior of brushes under shear. [13][14][15][16] Force measurements between brushes in good solvent under lateral oscillatory shear indicate additional repulsion above a certain shear velocity, 17 suggesting possible extension of the chains. Theoretical calculations of the brush layer thickness (L 0 ) under shear, on the other hand, have produced mixed predictions ranging from no significant change 13,18 to substantial increase 14,19-22 or even to a decrease 23,24 of L 0 relative to its equilibrium value. We have used neutron reflectivity to probe directly the density profile of polystyrene brushes in toluene (a good solvent) under steady shear flow. We find that the reflectivity profiles remain insensitive to strong shear, but above certain shear rates substantial chain desorption is abruptly observed. There is no indication of a monotonic increase in L 0 or any evidence of shear thinning as a function of shear rate. Our results demonstrate the remarkable robustness of polymer brushes under high shear and have direct technological implications on dispersion stability and rheology.Experimental Section. The structure of a polymer brush under steady laminar shear flow in a good solvent was investigated using the technique of neutron reflectivity. 25 This technique allows determination of the brush layer thickness (or brush height) and is an effective nanometer-scale probe of the polymer segment density profile normal to the grafting surface. 7 To accomplish this, we have constructed a cell designed to have plane Poiseuille flow, with both walls stationary and the fluid pumped between them. 26 The upper plane is defined by the polished surface of a quartz slab which serves as the adsorbing surface. The lower plane is formed by a shallow trough, precision milled into the top of a Teflon block so as to create a flow chamber beneath the quartz. The depth of the trough can be varied in the range 0.3-2.0 mm depending on the shear rate required. The macromolecule used was a polystyrene (PS)-poly(ethylene oxide) (PEO) diblock copolymer of 184 × 10 3 molecular weight and polydispersity index 1.10, the details of which have been reported before. 9 This polymer is known to adsorb terminally onto quartz or mica from toluene (a good solvent) and to form a welldefined polymer brush. 7,9 The cell was assembled by clamping the quartz crystal against the Teflon block with the aid of a machined sealing lip. Flow was effected by means of a speedadjustable pump, operating in a closed loop between a reservoir and the cell. Shear rates in the range 1 × 10 1 -2 × 10 4 s -1 could be attained under conditions of laminar flow, the highest recorded Reynolds number being ca. 770 at the upper limit of the shear rate range. Extreme care was taken to clean the qu...