Article pubs.acs.org/Macromolecules
Indentation of Highly Charged PSPM Brushes Measured by Force Spectroscopy: Application of a Compressible Fluid Model José 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 Miramón 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) 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.
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atomic force microscopy (AFM).7,9,10 This particular responsiveness of polyelectrolyte brushes toward electrolyte conditions of the environment is the physicochemical basis for the application of polyelectrolyte brushes as chemo-mechanotransducers 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
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 applications 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 © 2013 American Chemical Society
Received: December 13, 2012 Revised: February 25, 2013 Published: March 15, 2013 2323
dx.doi.org/10.1021/ma302562v | Macromolecules 2013, 46, 2323−2330
Macromolecules
Article
performed in dry state with large scan areas (40 or 60 μm2) to localize fringes and surface defects as height references for the subsequent characterization of the brush. Imaging in liquids was performed in contact mode. The sample was always imaged first in Millipore water followed by measurements in NaCl solutions. The order of incubation and subsequent force measurements was as follows: Millipore water, 10 mM, 100 mM, and finally 1000 mM NaCl. Indentation experiments on the brushes were performed with a regular silicon nitride tip as well as with an 8 μm in diameter Silica colloid from microparticles GmbH, Berlin. The silica colloid was glued at the end of a tipless cantilever (MikroMasch Spain, Model: CsC12) with UHU PlusEndfest 300 2-komponenten Epoxidharz. The spring constant did not change noticeably after attachment of the colloid. Force−distance curves were converted into force−separation curves by subtracting the cantilever deflection from the distance traveled by the piezo.
colloid attached at the end of a tipless cantilever.23−26 Drechsler et al. investigated the interaction forces and energies between silica colloids and weakly charged polyelectrolyte brushes under different pH and ionic strength conditions.27 The responsive nanomechanical properties of PE brushes are complex. Knowledge on the nanomechanics of brushes is still scarce.28,29 Here we present an AFM indentation study on negatively charged poly(sulfopropyl methacrylate) (PSPM) brushes. Indentations were performed with a regular AFM tip and an 8 μm in diameter silica colloidal probe at different ionic strengths. The combination of the sharp tip and colloidal probe allowed for a more comprehensive description of the nanoscale features of the brush, allowing differentiating between a dense brush zone and a looser polymer layer protruding into solution. The force response curves upon approach were quantitatively analyzed from a phenomenological thermodynamical perspective. This allowed for obtaining quantitative data on the influence of ionic strength on the brush properties such as internal energy, compressibility, and brush thickness from the indentation experiments. First, the indentation data were fitted to an equation analogous to the equation of state of a compressible liquid. Long-range forces followed an exponential decay. These fits were then used as tools for the regularization of the experimental data, which permitted us to numerically obtain the swelling pressure of the brush as a function of thickness. This dependence of the swelling pressure on thickness can be interpreted as the equation of state of the brush, which can be used to obtain intrinsic brush characteristics, such as the differential compressibility. Notably, the swelling pressure increased with brush thickness, h, following a dependency close to h−2.
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RESULTS AND DISCUSSION
Experimental Results. The graf ting f rom technique used in this study yields surfaces entirely covered with brush molecules. Therefore, it is a problem to measure the distance of the tip from the gold substrate, as the substrate in most cases cannot be reached with the tip. To obtain a reference value for the distance between the AFM tip and the substrate the scratching technique with a fine needle was applied.26 The scratches showed irregular variations in depth. There were, however, parallel fringes, which were smooth and regular. The grainy features of the gold substrate could be identified within these fringes. It is likely that these fringes were produced by polymer entanglement of brush molecules with the needle during scratching. The z-position of the fringes was set to zero and subsequently taken as the reference for indentation measurements. Figure 1 shows height images of the PSPM brush with the same fringe in the center obtained in contact mode in dry state and in Millipore water. A remarkable difference in brush thickness between dry and swollen state can be appreciated. The brush swells considerably as can be seen in Figure 1c,
MATERIALS AND METHODS
Materials. Potassium 3-sulfopropyl methacrylate (SPMA), 2,2′bipyridine (bpy), CuICl, CuIICl2, ethyl 2-bromoisobutyrate (2-EBiB), mercaptoundecane or blank thiol, and 1-dodecyl-3-methylimidazolium bromide were purchased from Aldrich. ω-Mercaptoundecyl bromoisobutyrate (thiol initiator) was synthesized for the study. Methods. Brush Synthesis. The “grafting from” technique was applied to fabricate dense brushes by atom transfer radical polymerization (ATRP). The brushes were grown from thiol assembled initiating monolayers onto gold-coated surfaces.30,31 Grafting densities of 0.5−0.8 chains nm−2 have been reported.30 These densities correspond to a distance between the grafting points of about 1.2−2 nm. The formation of poly(potassium 3-sulfopropyl methacrylate) (PSPM) brushes was accomplished by using Arrandee gold-coated preannealed glass substrates as surface platforms for growth. The brush growth was performed as follows.13 The substrates coated with the thiol initiators were immersed for 3 h in a polymerization solution where 1.24 g of SPMA, 0.75 mL of distilled water, and 1.25 mL of DMF were mixed in a Schlenk flask and degassed with nitrogen. A stock solution of Cu−bpy was prepared by addition of 2 mL of degassed water to 39.6 mg of CuCl, 53.8 mg of CuCl2, and 312 mg of bpy. From the stock solution of Cu−bpy, 19.5 mg of 2-EBiB and 0.5 mL of Cu-bpy were added to the SPMA solution to follow polymerization. After the polymerization the substrates were washed repeatedly with ethanol and water. AFM Measurements. Force measurements were made on a Molecular Force Probe 3D (Asylum Research, Goleta, CA) under liquid conditions at room temperature. Silicon nitride (Si3N4) AFM cantilevers (model MSCT-AUHW; Bruker Instruments) with a spring constant of 0.02 N/m and a tip with radii
