the chain (probably stretches devoid of bulky side groups). This then
prevents the parts of the chains near a double helix to become twisted.
Nevertheless, each junction tends to contain several helices that align to
form a microcrystallite, as depicted in Figure 17.12a. It is often observed
that the helices form in a very short time upon cooling (as inferred by
circular dichroism spectroscopy), whereas the modulus develops at a much
slower rate after helix formation. It may be that the helices are unstable
unless they are part of a microcrystallite.
Figure 17.12 shows two other types of junctions, those involvingtriple
helices(b) as in gelatin, and the so-calledegg-box junctions(c) that appear in
some ionic polysaccharides. Gels with junctions of the latter type do not
melt above a given temperature, as do gels with junctions involving helices.
Presumably, the egg boxes can also form stacks, further enhancing the
stiffness of the gel.
In most polymer gels, the junctions contain a substantial proportion of
the polymer material, say 30%. This implies that the length of the cords
between junctions is limited, and hence these cords do not have a truly
Gaussian conformation. Consequently, the enthalpic contribution to the
modulus tends to be considerable. Moreover, the gels contain numerous
entanglements. These structural aspects also cause the ‘‘linear region’’—i.e.,
the critical strain above which the (apparent) modulus starts to depend
significantly on the strain—to be (much) smaller than for a rubberlike gel.
Table 17.3 gives some examples of critical strain values.
FIGURE17.12 Various types of junctions in polymer gels. (a) Stacked double
helices, e.g., in carrageenans. (b) Partly stacked triple helices in gelatin. (c) ‘‘Egg-
box’’ junction, e.g., in alginate; the dots denote calcium ions. Highly schematic;
helices are indicated by hatching.