Page 270 - Introduction to Colloid and Surface Chemistry

The time taken for the structural units in viscoelastic materials,
such as high polymers, to flow into new positions relative to one
another is within a few decades of 1 s. Polymer molecules are in a
continual state of flexing and twisting, owing to their thermal energy.
The configurations of the polymer chains alter more rapidly on a local
scale than on a long-range scale. Under the influence of an external
stress, the polymer molecules flex and twist into more relaxed
positions, again more rapidly on a local scale. In general, there is a
continuous range on the time scale covering the response of such
systems to external stresses. On this basis, information concerning
the structural nature of viscoelastic materials (particularly high
polymers) can be obtained by measuring the compliance over a wide
range on the time scale 122 (by dynamic methods for times of less than
c, I s, and creep measurements for times of greater than c. 1 s).

Rubber elasticity

Rubber-like materials (elastomers) have a structure based on polymer
chains (e.g. polyisoprene chains, -CH 2—C(CH 3)=CH-CH 2 in
natural rubber) anchored at various points of cross-linkage. The
extent of cross-linking can be increased by vulcanisation. When
stretched, the polymer chains are extended lengthwise and compressed
crosswise (Figure 9.8) from their average configurations, the change
in total volume being inappreciable, and thermal restoring forces are
generated."When the tension is released, the polymer chains return
(usually rapidly) by thermal motion to their original average
configurations.

Figure 9.8 The stretching of rubber