Rubber Elasticity of Covalently Crosslinked Elastomers [98–129]

Abstract

Since rubber elastic deformation behavior results, to a first approximation, from reversible change of site processes of chain segments, the first law of thermodynamics can be used as a starting point for a consideration of this phenomenon: The internal energy U of a system can be increased by introducing energy from outside the system, either in the form of thermal energy or as a result of mechanical or electrical work:

$${\text{dU}} = {\text{dQ}}+ {\text{dA}}$$

(183)

In order to thermodynamically describe equilibria, variables of state are introduced. An isothermal, isobaric system will tend towards a minimum Gibbs free energy G:

$${\text G} = {\text{U-TS}} + {\text{pV}}$$

(184)

$${\text{dG }} = {\text{0 with T and p constant at thermal equilibrium }}$$

(185)

An isothermal, isochoral system tends towards a minimum Helmholtz energy F:

$${\text F} = {\text {U-TS}}$$

(186)

$${\text{dF}} = {\text{0 with T and V constant at thermal quilibrium }}$$

(187)

Thus, all processes in an isothermal, isochoral system tend to a minimum internal energy and a maximum entropy.