General properties of polymer networks - Physical Gels from Biological and Synthetic Polymers - page 108

Continuous and discontinuous swelling of a gel, induced by change of solvent quality. Figure 4.4

alone justi es the many synthetic papers which have appeared is less clear. The chemistry

of formation of such PAm gels is straightforward, at least in the laboratory, but it is far
from simple in terms of theories of polymer network formation. In almost all cases, the
proportion of intermolecular cross-links is completely unknown, but is certainly much
lower than the total number of cross-links formed, so the majority are actually

in
various chemical and physical side-reactions. The approach that many authors have
adopted is simply to show that, if they add more cross-linker, the Flory

' wasted '

' molecular

mass between cross-links parameter M c , mentioned above, tends to decrease.

However, the latter is itself a less than useful factor since it has little physical meaning,
except that it naturally decreases when the swelling ratio falls, so this becomes a circular
argument. Added to this is the fact that no correction tends to be made for the wastage, so
quoted M c values are usually far too low to have any signi

'

cance.

4.3.2 Kinetics of swelling

The diagrams above show the equilibrium behaviour on swelling or de-swelling, but do
not, of course, say anything about the (de)swelling rate. In practice this is mainly under
osmotic control so, for a typical gel, it may take hours to occur, even for small

- say

1mm samples. Since for a spherical gel sample the rate of swelling is approximately

inversely proportional to the square of the radius (Tanaka and Fillmore, 1979 ), for
working devices micro- or nano-scale samples are required, so reducing the size of the
gel particles from 1 mm to 10

-

μ

mwill reduce the time from, say, 3 h to around 1 s. In other
words, the rate of swelling can be accelerated simply by chopping the macroscopic gel
into micro-particles.

4.3.3 Smart gels

The exploitation of swelling phenomena owes much to the work of the late Toyoichi
Tanaka (Tanaka, 1981 ; Shibayama and Tanaka, 1993 ). Drawing on, but extending, work
in the review by Dusek and Prins ( 1969 ), he was able to establish the conditions leading