Biomedical Engineering Reference
In-Depth Information
Anode
Cathode
Solution
(NaCl)
C-PAN muscle
4H
+
4e
−
2H
2
O O
2
+
+
2H
2
O
+
2e
−
H
2
+ 2OH
−
FIGURE 4.87
Experimental setup for electrical activation of PAN fibers.
to have been removed from the carboxylic acid groups, giving the group an overall
negative charge. Negative charge repulsion between neighboring acid groups likely
forces the polymer backbone to elongate. Other factors may affect the length of
activated PAN, such as charges on pyridine rings. However, it should be pointed out
that such electrostatic repulsions would prevail if the carboxylic ions were the sole
ions present, but others do exist. Therefore, the effect of fixed charges is expected
to reduce the electrostatic repulsion forces significantly.
Another explanation is based upon the exchange of counter-ions and solvent (in
this case, water) into and out of activated PAN (illustrated in fig. 4.87). Donnan
equilibrium theory may possibly describe the situation properly. If so, the swelling
force may be identified by the net osmotic pressure difference associated with
relevant ions. Also, the columbic force could play a role. The combination of such
effects can describe the situation reasonably well. One key finding was that the
degree of ionization governs the degree of swelling. However, much study is needed
to further clarify the mechanism of PAN contraction/elongation behavior.
If the kinetics of PAN fibers is described by using the diffusion-controlled slab-
type ionic gel model, then the elongation would be
(
)
2
∞
21
n
+
t
∆
∆
l
l
8
21
∑
=−
1
exp
−
(4.15)
(
)
2
τ
n
+
π
2
0
n
=
0
where the characteristic time,
τ
, is given by
4
l
2
(4.16)
τ
=
ch
π
2
D
