92168.pdf

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