Normal and Shear Interactions Between High Grafting Density Polymer Brushes Grown by Atom Transfer Radical Polymerization

The normal and shear interactions in toluene of polystyrene polymer brushes with ultra-high surface coverage ranging from 15 to 70 mg m(-2) formed by atom transfer radical polymerization were measured with a surface force apparatus. Significant hysteresis was observed between compression and separation cycles over the experiment time scale for all surface coverages. The magnitude of the hysteresis increased with increasing film thickness. The experimental relaxation time of the thickest brush layer was at least four orders of magnitude longer than that predicted by the Rouse model. Remarkably, the shear performance of the thickest brushes still demonstrated very good lubricity under compressions down to 35% solvent content. These findings are consistent with a reduction in solvent quality with compression leading to a shrinkage or collapse of the brush under high compression, while still maintaining a region of well solvated chains in the overlap region between the brushes. The results suggest the hysteresis in compression is primarily due to intra-brush entanglements and collapse of the brush layer rather than inter-brush entanglements and brush-brush interpenetration.