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
Nafion and Its Uses
An ionomer developed by
DuPont in 70s
Fuel Cells
Catalysis
Sensors
Membrane electrolysis
Unique Microstructure:
Microphase separation and 2D Micelle Morphology
Hsu and Gierke, JMS, 1983
Schmidt-Rohr and Chen,
Nat Mater., 2008
Gebel, Polymer,
2000
Gebel, Diat et al,
Macromolecules, 2002, 2004
2D Morphology: Transport vs. Hydration
Conductivity
Water self-diffusion (NMR)
1
D/Dw
0.1
0.01
Blum et al.
SPE
PEEKK
Nafion
2D
3D
0.001
0.01
0.1
water volume fraction XV
Kreuer, JMS, 2001
VF et al., JMS, 1999
1
Schroeder’s Paradox: Two Isotherms?
Sample
Osmotic stressor
solution
Sample
30
vapor
liquid
l
20
Li-Nafion
10
0
0
0.2
0.4
0.6
water activity
0.8
1
Bass and Freger, 2008
Schroeder’s Paradox and Water Transport
DwC w
Jw  
  w  J H 
RT
 ~ 1 5
If the thermodynamic potential of water is ill-defined, how
does one model water transport and “water management”?
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.
Fixing the Model: Structure and Equilibrium

Four terms are the minimal set
f  go ( )  Gef (  0 ) 2    BR / 
osmotic “inflation” interface “corona”
R

(ve  v g )
2

1
Minimize g = f – l to get (l)
4
3
5
VF, Polymer, 2003; JPC B, 2009
Chemical Equilibrium as Balance of Pressures
 out   in   d   s
s 

(1   )2
R
R  R 2/3
Pressures:
g
out , in - osmotic
d
- inflation (transient)
s
- interfacial-elastic (“Laplace”)
The interfacial tension is zero, but the
“Laplace” pressure is not unless  = 1.
l’
l”
 ’ ”
VF, JPC B, 2009

Surface Equilibrium

Two more equilibrium conditions at the surface:

Balance of 3 tensions (Neumann construction)

Equilibrium between polymer bulk and surface
a
2
12
1
liquid (1)
b
matrix (2)
vapor
an ionic group
c
d
e
VF, JPC B,
2009
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 ≥ 105 s).

However, (quasi-)dissolution may occur at the surface.
normal-type micelles
(“spaghetti”)
surface-aligned
bundle (“macaroni”)
water

 s  (1   ) 2
R
Examining the Surface Structure: GISAXS
d p ~ 3 nm
c  0.2 for 8 keV
Rubatat and Diat,
Macrmolecules, 2007
(bulk SANS)
ESRF and ID10B
Nafion Surface in Vapor (GISAXS)
1
Qxy *I, A-1 a.u.
100 nm thick Nafion film
spin-cast on a Si wafer
T = 30 C, RH ~ 97%
Beam 8 keV
0.11
0.17
0.2
0.25
0.1
0.01
0.001
0.01
0.1 Qxy , A-1
1
Bass et al., JPC B, 2010
10
GISAXS: Going Under Water
Nafion film
C18-capped Si substrate
water
vapor
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).
Vapor RH=97%
q = 94.5 ± 1.1
hydrophobic
Air
Air
bubble
Liquid water
q = 25.4 ± 0.25
hydrophilic
water
Dry
q = 96.4 ± 1.2
hydrophobic
Air
Water
drop
Water
drop
Hydrophilic vs. Hydrophobic Substrate
Nafion film
Nafion film
Native Si substrate (SiO2)
C18-capped Si substrate
OTS on Si: z = -59 mV, q = 130o
(Yang & Abbott, Langmuir, 2010)
Dura et al., Macromolecules, 2009 (NR)
Micelle Orientation at Interfaces
Water
bundles
breaking
up
Vapor
Nafion film
C18-capped Si substrate
Bass et al., 2010
a micelle
bundle
Micelle
bundles
Native Si (SiO2) substrate
Some of these are metastable nonequilibrium structures!
(non-relaxed elastic stress, relaxation
time >105 s)
Balsara et al, NanoLett, 2007
Summary

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.
Vapor
Nafion
Liquid

 s  (1   ) 2
R
Thanks
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)
Another old puzzle: microscopic vs. macroscopic swelling


The relative change of Bragg spacing (d-do)/d (“microscopic swelling”)
may be compared with the relative macroscopic linear expansion
(1/p – 1)1/3 calculated from l.
Though for high l the relation is as for dilute 2D micelles, for solid
Nafion (small and moderate l) it is nearly linear, as if the structure is
1D (lamellae)
Gebel, 2000; Fujimura et al., 1981, 1982
Microscopic vs. macroscopic swelling
The model shows a good agreement with scattering data, provided a
2D morphology is “plugged in”
Microscopic swelling
6
100
D=3
5
D=2
4
D=2 var
dmax, nm

3
10
2
constant
variable
1
1
0
0
0.5
1
1.5
0.01
0.1
p
Linear expansion
n 1 D

d  d0   
1  1  g 1 
1 D
 
 p  1   n  
  w


d0
D  g
D 
 g 

n  2 3  for D=2 theoretical initial slope is  7 (exp 6)
1