ARTICLE pubs.acs.org/Langmuir
AFM Imaging of Adsorbed Nafion Polymer on Mica and Graphite at Molecular Level Roland Koestner,† Yuri Roiter,‡ Irina Kozhinova,§ and Sergiy Minko*,‡ †
Electrochemical Energy Research Laboratory, GM Research and Development, 10 Carriage Street, Honeoye Falls, New York 14472-1039, United States ‡ Department of Chemistry and Biomolecular Science, Clarkson University, 8 Clarkson Avenue Potsdam, New York 13699-5810, United States § Trison Business Solutions, Honeoye Falls, New York 14472-1039, United States ABSTRACT: Perfluorosulfonic acid ionomer (PFSA, specifically Nafion at EW = 975 g/mol) was visualized at the single molecule level using atomic force microscopy (AFM) in liquid. The diluted commercial Nafion dispersion shows an apparent Mw = 1430 kg/mol and Mw/Mn = 3.81, which is assigned to chain aggregation. PFSA aggregates, imaged on mica and HOPG during adsorption from EtOHH2O solvent at pH(e) 3.0 (below isoelectric point), showed a stable, segmented rodlike conformation. This structure is consistent with earlier NMR, SAXS/SANS, and TEM results that support a stiff helical Nafion conformation with long persistence length, a sharp solventpolymer interface, and an extension of the sulfonated side chain into solution. Adsorption of Nafion structures on HOPG was observed at even higher pH(e) from EtOH due to screening of the repulsive electrostatic interaction in lower dielectric constant solvent, while the chain adopted an expanded coil conformation. These measurements provided direct evidence of the chain aggregation in EtOHH2O solution and revealed their equilibrium conformations for adsorption on two model surfaces, highly ordered pyrolitic graphite (HOPG) and mica. The commercial Nafion dispersion was autoclaved at 0.10% w/w in nPrOH/H2O = 4:1 v/v solvent at 230 °C for 6 h to give a single-chain dispersion with Mw = 310 kg/mol and Mw/Mn = 1.60. The autoclaved chains adopt an electrostatically stabilized compact globule conformation as observed by AFM imaging of the single PFSA molecules after rapid deposition on mica and HOPG at a low surface coverage.
’ INTRODUCTION Perfluorosulfonic acid polymer (PFSA) is added to the Pt/C catalyst ink in electrode layer fabrication for proton exchange membrane fuel (PEMFC) cells to provide proton conduction to the dispersed Pt nanoparticle catalyst as well as binding of the porous carbon network.1 Commercial Nafion polymer is widely used in this application and has a random copolymer structure as shown in Figure 1. Due to its hydrophobic backbone, Nafion is not soluble in water but does dissolve in an alcoholwater mixed solvent at higher temperature (230 °C). The solvent dielectric constant (εr) is expected to affect both chain conformation and aggregation in solution.2,3 TEM (transmission electron microscopy) imaging of freeze-dried dispersions at dilute Nafion concentration implies an expanded coil conformation in lower dielectric constant N,N-dimethylformamide (DMF; εr = 36.7) and DMAC (εr = 37.8) solvents where both the hydrophobic backbone and the hydrophilic side-chain segments have a favorable solvent interaction, while rod-shaped aggregates appear for higher dielectric constant NMF (εr = 182.4) solvent and EtOH (εr = 24.5)/H2O (εr = 78.4).4 Because the PFSA location distribution is not currently controlled in the PEMFC electrode coating process, an improvement r 2011 American Chemical Society
in electrode performance and durability is likely possible by manipulating the polymer structure in solution and its interaction with the electrode surfaces in the ink formulation. The goal of this study is to improve our understanding of PFSA adsorption on the graphite surface using atomic force microscopy (AFM) single molecule experiments.58 This paper uses in situ AFM single molecule experiments in liquid914 to directly image adsorbed Nafion chains at a slow and rapid deposition by spin-coating to trace their solution conformation.10,1521 These experiments can be conducted on atomically smooth substrates. Thus, highly ordered pyrolitic graphite (HOPG) was used to study adsorption of a commercial D2020 PFSA dispersion with and without autoclave treatment. Autoclave treatment was used to obtain molecular (nonaggregated) solutions of the ionomer. Adsorption on mica was used in reference experiments. We discuss in the article different conformations of adsorbed PFSA chains: a collapsed sphere (globule), segmented rod, or expanded coil conformation depending on the imposed solution and deposition conditions. Received: April 7, 2011 Revised: June 24, 2011 Published: July 07, 2011 10157
dx.doi.org/10.1021/la201283a | Langmuir 2011, 27, 10157–10166
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Due to the low specific refractive index increment (0.035 mL/g) in this SEC solvent system and the low polymer concentration (
