Entropic elasticity

Entropic elasticity is a restoring response arising from the reduction of accessible molecular configurations under deformation. For an ideal freely jointed chain, weak extension has free energy $\Delta \mathcal{F}=3k_{B}T{R}^2/(2N{b}^2)$ and spring constant $k=3k_{B}T/(N{b}^2)$. The proportionality to temperature assumes fixed segment geometry; temperature-dependent Kuhn length modifies it.