Indentation of Highly Charged PSPM Brushes Measured by Force Spectroscopy: Application of a Compressible Fluid Model

Abstract

Highly charged dense poly(sulfopropyl methacrylate) polyelectrolyte brushes were indented with an atomic force microscopy (AFM) tip as well as with an 8 μm silica colloidal probe at different ionic strengths ranging from Millipore water to 1 M NaCl. The force response during indentation was fitted to a phenomenological equation analogous to the equation of state of a compressible fluid. In this way, internal energy and brush thickness were obtained as a function of ionic strength. Long-range forces decayed exponentially with distance. The characteristic decay lengths were much larger than the Debye screening lengths at the respective ionic strengths. It was therefore concluded that long-range repulsion was due to compression of a loose corona of polymers in front of the dense part of the brush. The size of the indentor determines which region of the brush can be explored by AFM. The tip probes the denser parts of the brush, while with the colloidal probe the corona of the brush can be investigated. The obtained fits of the experimentally measured force distance curves were used as regularization tools for obtaining the brush swelling pressure or "force per unit area" as a function of brush compression. The swelling pressure as a function of brush thickness, h, followed over a wide range a power law close to ∼h −2 . This approach allowed deriving fundamental brush parameters on a thermodynamical basis like the compressibility as a function of thickness.

Mapping nano-scale mechanical heterogeneity of primary plant cell walls

“…Such dependency is consistent with a mechanism whereby ‘grafting’ density of polymer chains changes with separation. This can be realized either for a polydisperse brush where density increases as the tip ventures deeper into the polymer layer, and starts to probe shorter chains, or through the lateral displacement of polymer molecules originating from the conical shape of an AFM tip that effectively wedges in between the chains ( Balastre et al , 2002 ; Bremmell et al , 2013 ; Cuellar et al , 2013 ). The presence of such a loose polymer layer is consistent with the fact that walls of L. multiflorum SCCs have a relatively low content of cellulose (<15%/dry weight), but are rich in MLG and arabinoxylan ( Table 1 ).…”

Quantitative mechanical analysis of thin compressible polymer monolayers on oxide surfaces

“…9,10 There are several methods to study the mechanical properties of ultrathin polymer lms in the dry state; [11][12][13][14] however, there are currently few reports on the stiffness of thin, uniform polymer monolayers deposited on oxide nanoparticles and/or oxide nanober structures in the liquid state, which is likely rooted in the challenges associated with synthesizing conformal, uniform monolayer lms and the difficulties in accurately quantifying the mechanical properties at the nanometer scale at the same time. [15][16][17][18] Many theories have been proposed to explain the interaction forces involved in nanometer thickness polymer deformations. [19][20][21][22][23][24] For example, Spencer et al reported on the absorption of a copolymer consisting of PEG chains graed on the poly(L-lysine) backbone on niobium oxide substrates, and studied the mechanical properties of different PEG graing densities via colloid-probe atomic force microscopy (AFM) under varying ionic strength.…”

Atomic force microscopy of food assembly: Structural and mechanical insights at the nanoscale and potential opportunities from other fields

“…Among repulsion forces exerted between colloids, that of steric hindrance by polymer brushes has raised interest in the field of AFM force spectroscopy. Mathematical models were developed for the indentation of a polymer brush by a conical or a colloidal AFM tip (Cuellar, Llarena, Moya, & Donath, 2013). A reference force curve is measured onto a bare spot of the sample, made by locally scratching and clearing the polymer brush with the AFM tip, to get to the contact point.…”