Indentation of Highly Charged PSPM Brushes Measured by Force Spectroscopy: Application of a Compressible Fluid Model Jose ́ Luis Cuellar, † Irantzu Llarena, ‡ Sergio Enrique Moya,* ,‡,§ and Edwin Donath † † Institute of Biophysics and Medical Physics, Faculty of Medicine, University of Leipzig, Leipzig, Germany ‡ CIC biomaGUNE, Paseo Miramó n 182 C, 20009 San Sebastian, Spain § Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China * S Supporting Information ABSTRACT: Highly charged dense poly(sulfopropyl methacrylate) polyelectro- lyte 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. ■ INTRODUCTION In a brush, polymer chains are anchored by one end to a surface while the other end extends freely into the solution. 1-3 Among brushes, polyelectrolyte brushes (PE brushes) are of special interest as they observe a particular sensitivity to changes in the ionic strength of the surrounding solution. 4 PE brushes may have attractive electronic, industrial, and biomedical applica- tions because brushes of polyelectrolyte chains respond with a collective change in the conformation of chain molecules as a function of applied external stimuli. 2,3 Increasing the ionic strength causes the chains to acquire more coiled conformations, 5 resulting in a reduction of the entire brush thickness. If the reduction in brush thickness with ionic strength occurs sharply, it is called collapse. Such a collapse of a polyelectrolyte brush can be explained with electrostatic arguments, as the increase in the ionic strength in the bulk reduces internal repulsion in the polyelectrolyte chains. The collapse can be also understood based on osmotic arguments since in conditions of low ionic strength the high density of repeating units in a brush together with their counterions generates an osmotic difference with the bulk that results in brush swelling. When the ionic strength is increased, this osmotic difference is reduced and water is released from the brush. 5-9 As a result of the collapse, the mechanical properties of the brush change from a viscoelastic character toward a more rigid structure as it has been demonstrated by means of quartz crystal microbalance with dissipation measurements and by atomic force microscopy (AFM). 7,9,10 This particular respon- siveness of polyelectrolyte brushes toward electrolyte con- ditions of the environment is the physicochemical basis for the application of polyelectrolyte brushes as chemo-mechano- transducers in the nanoscale. Experiments carried out by the group of Huck probed that microcantilevers coated with a PE brush layer can effectively convert chemical potential changes into mechanical deflection of cantilevers. 11 Experimental and theoretical studies of the responsive properties, the stability, and the reversibility of polyelectrolyte brushes will thus be beneficial for the design of polyelectrolyte brushes with tailored responsiveness. It is expected that a deeper understanding of the brush behavior could facilitate their use in the fabrication of novel coatings with unique properties with regard to abrasion, lubrication, friction, or, for example, hydrophobicity. 11-17 Although brushes have been investigated with the surface force apparatus, 18 the thickness as well as mechanical and electrostatic properties of polyelectrolyte brushes can be conveniently explored with indentation experiments by means of AFM. In such experiments the AFM probe is moved with a given rate toward the brush, and the deflection of the cantilever is recorded. 19-22 Small cross sections as well as large surface areas can be used to explore the interface because indentation can be made with an acute AFM tip or with a micrometer-sized Received: December 13, 2012 Revised: February 25, 2013 Published: March 15, 2013 Article pubs.acs.org/Macromolecules © 2013 American Chemical Society 2323 dx.doi.org/10.1021/ma302562v | Macromolecules 2013, 46, 2323-2330