In this talk, I will present novel neutron reflectivity measurements of the structure of opposing polymer brushes as a function of confinement. I will compare these results to the structure of polymer brushes at a single interface as well as to high precision surface force apparatus measurements of the force required to compress and shear polymer brushes. Although good agreement can be obtained between physical experiments, simulations and theory under low to moderate grafting densities, ultra-high density brushes offer new challenges and behavior that has not yet been fully addressed. Significantly, the experimental relaxation time of ultra-high density brushes formed by grafting from methods (ATRP) was found to be at least four orders of magnitude longer than that predicted by the Rouse model resulting in enormous hysteresis during compression. Yet, the shear performance of such brushes demonstrated excellent lubricity under compressions down to 35% solvent content. The findings are consistent with a reduction in solvent quality with compression leading to shrinkage or collapse of the brush under high compression (increasing polymer concentration), while still maintaining a region of well solvated chains in the overlap region between opposing brushes. Together, the results suggest that compression hysteresis is primarily due to intra-brush entanglements and collapse of the brush layer rather than inter-brush entanglements and brush–brush interpenetration. The importance of the ultra-high density, ultra-polydisperse brushes yielding favorable lubrication properties is that a highly durable lubricating layer can be formed from a “realistic” system versus the more ideal, monodisperse systems that have been primarily studied in the past.

Polymer brushes in restricted geometries

250th National Meeting (2015)

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