Interactions of grafted polymer brushes measured by atomic force microscopy

The interaction forces exerted by latex particles bearing densely grafted polymer layers of poly(N,N-dimethylacrylamide) (PDMA), poly(methoxyethylacrylamide) (PMEA), and poly(N-isopropylacrylamide) (PNIPAM) in aqueous media (good solvent) were investigated by atomic force microscopy (AFM). The molecular weight (M n), grafting density (sigma), and polydispersity (PDI) of the brushes were determined by gel permeation chromatography (GPC) and multi-angle laser light scattering after cleaving the polymer from the latex surface by hydrolysis. The hydrodynamic thicknesses were measured by dynamic light scattering and were comparable to the equilibrium thicknesses measured by AFM. The values were also similar to those predicted by a numerical self-consistent mean field model after scaling the dimensions and flexibility of the lattice chains using experimentally determined parameters.; Force profiles of PDMA (0.017 nm-2 ≤ sigma ≤ 0.17 nm-2) and PMEA (sigma = 0.054 nm-2 ) brushes were purely repulsive upon compression, with forces increasing with Mn and sigma, as expected, due to excluded volume interactions. At a sufficiently low grafting density (sigma = 0.012 mn-2 ), PDMA exhibited a long-range exponentially increasing attractive force which was attributed to bridging. A bridging force was detected in the PNIPAM brush at sigma = 0.037 nm-2; at this density the PDMA brush force profile was purely repulsive. Bridging was therefore found to depend on grafting density as well as on the nature of the monomer.; The Mn and PDI of the polymer brushes were determined by two independent force techniques, the suitability of which depended on the interaction of the polymer with the AFM tip. For non-adsorbing polymers, the distribution of contour lengths measured by stretching individual chains was compared with GPC distributions for surfaces with different grafting densities. The force technique was appropriate for dense brushes in good solvent but underestimated the Mn for surfaces with low sigma (mushroom regime). A novel technique was developed for determining the molecular weight distribution of an adsorbing polymer brush based on a purely empirical relationship between the decompression force profile and the cumulative weight fraction obtained by GPC.