Biomedical Engineering Reference
In-Depth Information
4.6.9
C
ONTRACTION
AND
E
LONGATION
M
ECHANISM
According to Schreyer and colleagues (1999), a possible explanation for the
contraction and elongation is based upon the carboxylic acid groups having the
molecular geometry of activated PAN. At high cationic concentration, all carbox-
ylic acid groups on activated PAN are likely to be protonated, thus potentially
contracting the polymer chain through neutral charge of the acid groups and
hydrogen bonding between neighboring carboxylic acid groups. At lower cationic
concentration or higher anionic concentration, protons are likely to have been
removed from the carboxylic acid groups, giving the group an overall negative
charge. Negative charge repulsion between neighboring acid groups forces the
polymer backbone to swell or expand. Other factors may affect the length of
activated PAN, such as charges on pyridine rings. However, such electrostatic
repulsions would prevail if the carboxylic ions are sole ions present but others
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 and is illustrated in figures 4.78
and 4.86. 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 coulombic 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. In the next
section, a mathematical model is presented for such electrochemically ion-induced
contraction and expansion of PAN fibers and C-PAN-N microfibrils.
Solution
Gel
Solvent
Fixed charges
Counter ions
FIGURE 4.86
The exchange of counter-ions and surrounding solvent.
