junctions are made of quite weak bonds, a significant number of bonds have
to be formed at the same time to become more or less permanent; in other
words, junction formation involves a cooperative transition.
A common type of junction is shown in Figure 17.12a. Many
polysaccharides formdouble helicesbelow a given temperature and at given
physicochemical conditions. Apparently, the helices often involve two
molecules. This may be difficult to achieve because of geometrical
constraints: the parts of a molecule not incorporated in the helix then
would also become twisted to the same extent (but in the opposite sense),
which is largely prevented by the entanglements in the system. However, in
many polysaccharides, complete rotation (i.e., by 360 8 ) about the bonds
between monomers appears to be possible, at least at some positions along
FIGURE17.11 The effect ofn^0 (the number of statistical chain elements in a cord
between cross-links) on the relation between stress and strain of a polymer gel in
elongation.s 0 is the force divided by the original cross-sectional area of a cylindrical
test piece,nis twice the cross-link density,Lis the length, andL 0 the original length
of the test piece. (After calculations by L. R. G. Treloar. The Physics of Rubber
Elasticity. Clarendon, Oxford, 1975.)