Direct Force Measurements at Polymer Brush Surfaces by Atomic Force Microscopy

Abstract

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.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…For a microfabricated tip with its small radius of curvature this is often not fulfilled. Indeed, when comparing steric forces measured with microfabricated tips with results obtained with the SFA the AFM force curves are less steep indicating an apparently softer polymer layer [834,835]. Theory and simulations showed that the polymer chains partially avoid compression by escaping from underneath the AFM tip [838][839][840][841].…”

“…Several diblock copolymers have been analyzed with the AFM. Examples are PEO-PS in xylene, propanol, and dodecane on mica [828], PVP-PS and poly(4-tert-butylstyrene)-sodium poly(styrene-4-sulfonate) in aqueous medium [834], PEO-PMAA on aluminum oxide in aqueous medium [833], and PVP-PS on silicon analyzed in toluene and water [835]. In a similar way the triblock copolymers, PEO-PP-PEO (trade name ''Pluronic''), were studied in aqueous medium adsorbed to hydrophobic surfaces [832,836].…”

Tunable Self-assembled Weak Polyelectrolyte Brushes

“…The concentration of the polyelectrolyte solution was 0.1 mg/mL and left to incubate typically for 12-14 h. It is known from previous studies that the chains attach to the surfaces via the hydrophobic part of the diblock copolymer, which in our polymer is poly(isoprene) [17,18]. No measurable adsorbance detected because of the electrostatic (coulombic) repulsion forces between the polyelectrolyte chains and the surface.…”

Confinement and compression of an oligomer brush

“…19 Experimentally, oligomer molecules are end grafted to a surface: a second surface ͑which may also be covered with a SAM or oligomer brush͒ is then brought in contact with the first, and the resulting compressional force is measured as a function of the separation via the surface force apparatus 20,21 or atomic force microscope ͑AFM͒. 19,22,23 Simple behavior of compressed SAMs can be understood using scaling arguments, 8 creation of gauche defects, 24 geometrical models ͑cooperative tilting͒, 25 or cubic lattice models. 26 However, these models neglect polymer-specific characteristics that can result in unexpected behavior.…”