Direct force measurements at polymer brush surfaces by atomic force microscopy

We present the results of two studies: (1) a comparison of force-distance (F-D) profiles obtained by atomic force microscopy (AFM) and the surface forces apparatus (SFA) for a poly(2-vinylpyridine)-polystyrene (PVP-PS) brush in good solvent; (2) a series of F-D profiles for a poly(4-tert-butylstyrene)-sodium poly(styrene-4-sulfonate) (PtBS-NaPSS) brush as a function of aqueous NaCl concentration. The AFM force profiles of the neutral PVP-PS brush are less steep than the corresponding surface forces data in the regime of high brush compression, in agreement with a recent molecular simulation study that indicated the tip would splay polymer chains and penetrate the brush. We also observe a bimodal distribution of interaction distances for the AFM force profiles of the PVP-PS brush which we ascribe to the tip sampling regions of higher and lower chain density during consecutive force measurements. AFM F-D profiles of the PtBS-NaPSS brush show a strong dependence of interaction distance on NaCl concentration, and a plot of interaction distance vs salt concentration shows predicted power law behavior. Images of both the PVP-PS and PtBS-NaPSS brushes show that the chain density is not uniform which gives rise to variations in the interaction distances measured by AFM. By facilitating measurements of local force profiles, AFM complements SFA measurements of interfacial forces and allows measurement of brush heterogeneity.