Published online by Cambridge University Press: 05 June 2012
Introduction
Basic concepts
The elastic properties of rubber-like materials are so strikingly unusual that it is essential to begin by defining rubber-like elasticity, and then to discuss what types of materials can exhibit it. Accordingly, this type of elasticity may be operationally defined as very large deformability with essentially complete recoverability. In order for a material to exhibit this type of elasticity, three molecular requirements must be met: (i) the material must consist of polymeric chains, (ii) the chains must have a high degree of flexibility and mobility, and (iii) the chains must be joined into a network structure [1–5].
The first requirement arises from the fact that the molecules in a rubber or elastomeric material must be able to alter their arrangements and extensions in space dramatically in response to an imposed stress, and only a long-chain molecule has the required very large number of spatial arrangements of very different extensions. This versatility is illustrated in Fig. 1.1 [3], which depicts a two-dimensional projection of a random spatial arrangement of a relatively short polyethylene chain in the amorphous state. The spatial configuration shown was computer generated, in as realistic a manner as possible. The correct bond lengths and bond angles were employed, as was the known preference for trans rotational states about the skeletal bonds in any n-alkane molecule. A final feature taken into account is the fact that rotational states are interdependent; what one rotational skeletal bond does depends on what the adjoining skeletal bonds are doing [6–8].
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