= Rubber elasticity
In a flexible crosslinked <polymer> network, deformation restricts the configurations of chain segments between <cross-links>. The resulting decrease of <entropy> produces a restoring <entropic force>. For an ideal affine Gaussian network, the <shear modulus> is $G=\nu_s k_BT$, where $\nu_s$ is the <number density> of elastically active strands. A nearly incompressible isotropic rubber has <Young's modulus> $E\simeq3G$. At fixed network density within the rubbery regime, both increase approximately linearly with absolute <temperature>. This statement does not describe crossing the glass transition or chemical degradation.
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