Biomedical Engineering Reference
In-Depth Information
charged ionic groups at the surface so as to repel the doubly charged cations. As a
result, only single charges can pass through the membrane. Large-scale manufac-
turers are Ionics Inc. in the United States and Tokuyma Corporation in Japan.
The bipolar membranes are also fabricated based upon the idea that a membrane
has two layers side by side—cation conducting and anion conducting, respectively. A
good application of such a membrane is water splitting creating H
under a
small applied electric voltage. The fabrication of such a membrane is rather complicated.
Another popular ion-exchange material is perfluorinated alkenes with short side
chains terminated by ionic groups (i.e., Nafion
+
and OH
-
from DuPont, typically sulfonate or
carboxylate [SO
] for cation exchange or ammonium ions for anion exchange).
Such fluorocarbon polymers have linear backbones with no cross-linking and relatively
few fixed ionic groups attached (Yeager, 1982). The large polymer backbones determine
their mechanical strength. Short side chains provide ionic groups that interact with water
and the passage of appropriate ions. When swollen by water, Nafion undergoes phase
separation (“clustering”) (Gierke, Hunn, and Wilson, 1982) on a supermolecular struc-
ture. When they are swollen, hydrophobic zones around the fluorocarbon backbones and
hydrophilic zones around the fixed ionic groups coexist. Therefore, the ionic groups
attract water and can move water under an electric voltage through nanoscale pores and
channels where ions along with water migrate within the polymer matrix.
One interesting feature is the cation dependency of water content in Nafion: in
general, H
3
-
or COO
-
+
> Li
+
> Na
+
> K
+
> Cs
+
(Davis et al., 1997). This can also be interpreted
that their moduli, in general, are H
. Their popular chemical
structure and properties are provided in figure 3.2 and table 3.1. A similar product,
Aciplex-S™, from Asahi Chemical Industry Co., Ltd, of Japan is also a perfluorinated
cation-exchange membrane with a sulfuric acid functional group and its chemical
structure and properties are also included in figure 3.2 and table 3.1.
Although there are several commercial ion-exchange material manufacturers (Davis
et al., 1997), including Aqualitics, Asahi Chemicals, Asahi Glass, Du Pont, W. L. Gore,
Ionics, Solvay, Sybron, and Tokuyama, popular products used as IPMNC materials are
Nafion from DuPont, Neosepta™ from Tokuyama, Aciplex from Asahi Chemical, and
Flemion™ or Selemion™ from Asahi Glass. At the present time, all these products
perform fairly well when the IPMNC chemical-plating technique properly treats them.
Since these materials exhibit ion transport intrinsically, an important process is
electric charge transport through the materials by ions. Therefore, it can be explained
in terms of an ionic flux,
+
< Li
+
< Na
+
< K
+
< Cs
+
∏
,
i
J
zF
E
zFR
Π
i
==
i
D
(3.1)
i
i
A
where
J
is the current density (
I
/
A
)
i
z
is the charge on the transported ion
i
F
is the Faraday constant (96,485 C/mol)
E
and
R
are the electric field across the material and the area resistance of the
material (
D
A
Ω
m
2
), respectively
