Biomedical Engineering Reference
In-Depth Information
The conductance of the material
k
(
Ω
m
) is also conveniently used as
R
=
L
/
k
-1
-1
i
A
i
(where
= material thickness).
Another interesting property of Nafion can be recognized in terms of its func-
tional groups: SO
L
. Table 3.2 compares the properties of the sulfonate
and carboxylate Nafion typically used in the industry.
Figure 3.3 depicts a general illustration of fixed ionic groups, counter-ions, and
co-ions for a typical cation-exchange polymer. In this cation-exchange polymer, the
fixed ionic groups refer to the ion-exchange groups that are covalently bonded to
the polymer molecular network. Typical cation-exchange polymers have carboxylate
or sulfonate groups. Counter-ions are conjugated charges to the fixed ionic groups.
For charge balance, counter-ions must be present within the polymer. They migrate
within the polymer network under an imposed electric field. There also exist co-ions
such as H
and COO
3
-
-
with the same charge as the fixed ionic groups, depending upon the
pretreatment of the polymers. Co-ions arise from the presence of a salt (or salts)
within the polymer.
+
TABLE 3.2
Properties of Sulfonate and Carboxylate Ion-Exchange Material Used
in Industry
Water
content
Current
density
Electric
conductivity
Chemical
stability
pK
A
SO
<1
High
High
High
Good
3
-
COO
3-4
Low
Low
Low
Good
-
Cation
SO
3
Fixed ionic group
SO
3
SO
3
Li
+
Y
−
Co-ions
Li
+
SO
3
SO
3
SO
3
Li
+
Li
+
Li
+
SO
3
Li
+
Y
−
Li
+
Li
+
SO
3
Li
+
SO
3
Y
−
SO
3
Li
+
Li
+
SO
3
Li
+
SO
3
Li
+
Li
+
Li
+
SO
3
SO
3
FIGURE 3.3
Illustration of the cation polymer showing fixed ionic groups (SO
3
-
), counter-
ions (Li
+
), and co-ions (Y
-
), respectively.
