1. Nafion : Hydration, Microstructure and Schroeder’s paradox Viatcheslav Freger Maria Bass, Amir Berman (BGU) Oleg Konovalov, Amarjeet Singh (ESRF) Technion – Israel Institute of Technology Wolfson Department of Chemical Engineering Haifa, Israel

2. Nafion and Its Uses Fuel Cells Membrane electrolysis Sensors Catalysis An ionomer developed by DuPont in 70s

3. Unique Microstructure: Microphase separation and 2D Micelle Morphology Schmidt-Rohr and Chen, Nat Mater., 2008 Gebel, Diat et al, Macromolecules, 2002, 2004 Gebel, Polymer, 2000 Hsu and Gierke, JMS, 1983

4. 2D Morphology: Transport vs. Hydration Conductivity VF et al., JMS, 1999 Kreuer, JMS, 2001 Water self-diffusion (NMR)

5. Schroeder’s Paradox: Two Isotherms? Bass and Freger, 2008 Li-Nafion Sample Osmotic stressor solution

6. Schroeder’s Paradox and Water Transport If the thermodynamic potential of water is ill-defined, how does one model water transport and “water management”?

7. Schroeder’s paradox explained ?  Choi and Datta (JES, 2003) were first to publish an explanation, but they assumed permanent pores; hydrophobic pore walls (despite ionic groups); stability of surface structure and 3-phase line.

8. Fixing the Model: Structure and Equilibrium  Four terms are the minimal set osmotic “inflation” interface “corona” 1 3 4 5 2 VF, Polymer, 2003; JPC B, 2009  Minimize g = f –  to get 

9. Chemical Equilibrium as Balance of Pressures g  ”” ’’ ” ’ Pressures:  out,  in - osmotic  d - inflation (transient)  s - interfacial-elastic (“Laplace”) VF, JPC B, 2009 The interfacial tension is zero, but the “Laplace” pressure is not unless  = 1.

10. Surface Equilibrium  Two more equilibrium conditions at the surface: Balance of 3 tensions (Neumann construction) Equilibrium between polymer bulk and surface vapor matrix (2) an ionic group liquid (1) 11 22  12 ab c d e VF, JPC B, 2009

11. Surface Equilibrium: Interim Summary  In vapor water gets buried under surface;  s ≥ 0.  In liquid micelles are inverted and  s = 0 (Schroeder’s paradox).  Nafion should dissolve in water, but dissolution never happens (relaxation time ≥ 10 5 s).  However, (quasi-)dissolution may occur at the surface. normal-type micelles (“spaghetti”) surface-aligned bundle (“macaroni”) water

12. Examining the Surface Structure: GISAXS Rubatat and Diat, Macrmolecules, 2007 (bulk SANS)

13. ESRF and ID10B

14. Nafion Surface in Vapor (GISAXS) 100 nm thick Nafion film spin-cast on a Si wafer T = 30 C, RH ~ 97% Beam 8 keV Bass et al., JPC B, 2010

15. GISAXS: Going Under Water water vapor C18-capped Si substrate Nafion film

16. Vapor vs. Liquid: Contact Angle and AFM  CA: Nafion surface is hydrophobic in vapor and hydrophilic in water  AFM: under water the surface gets rougher (surface tension drops). Dry  = 96.4 ± 1.2 hydrophobic Vapor RH=97%  = 94.5 ± 1.1 hydrophobic water Air bubble Water drop Air Water drop Air Liquid water  = 25.4 ± 0.25 hydrophilic

17. Hydrophilic vs. Hydrophobic Substrate OTS on Si:  = -59 mV,  = 130 o (Yang & Abbott, Langmuir, 2010) Dura et al., Macromolecules, 2009 (NR) C18-capped Si substrate Nafion film Native Si substrate (SiO 2 ) Nafion film

18. Micelle Orientation at Interfaces C18-capped Si substrate a micelle bundle Vapor Native Si (SiO 2 ) substrate Water Nafion film Micelle bundles bundles breaking up Bass et al., 2010 Some of these are metastable non- equilibrium structures! (non-relaxed elastic stress, relaxation time >10 5 s) Balsara et al, NanoLett, 2007

19. Summary VaporNafion Liquid  Solid Nafion is a non-equilibrium structure.  Non-relaxed pressures in Nafion result in a non-thermodynamic behavior (Schroeder’s paradox); this needs to be accounted for in transport modeling.  Interfaces affect the morphology and orientation of micelles in thin Nafion films; this could be attractive for developing barriers with enhanced and stable transport characteristics.

20. ISF ESRF Maria Bass Oleg Konovalov, Amarjeet Singh, Jiři Novak (ESRF, ID10B) Amir Berman, Yair Kaufman, Juergen Jopp (BGU) Special thanks: Emmanuel Korngold (BGU), Klaus-Dieter Kreuer, Martin Ise (MPI Stuttgart) Thanks

21. Another old puzzle: microscopic vs. macroscopic swelling  The relative change of Bragg spacing (d-d o )/d (“microscopic swelling”) may be compared with the relative macroscopic linear expansion (1/  p – 1) 1/3 calculated from.  Though for high the relation is as for dilute 2D micelles, for solid Nafion (small and moderate ) it is nearly linear, as if the structure is 1D (lamellae) Gebel, 2000; Fujimura et al., 1981, 1982

22. Microscopic vs. macroscopic swelling  The model shows a good agreement with scattering data, provided a 2D morphology is “plugged in”