The Flory theory of rubber elasticity suggests that rubber elasticity has primarily entropic origins. By using the following basic equations for Helmholtz free energy and its discussion about entropy, the force generated from the deformation of a rubber chain from its original unstretched conformation can be derived. The Ω Ω is the number of conformations of the polymer chain. Since the deformation does not involve enthalpy change, the change in free energy can simply be calculated as the change in entropy − − T Δ Δ S . Note that the force equation resembles the behavior of a spring and follows Hooke's law : F = k x , where F is the force, k is the spring constant and x is the distance. Usually, the neo-Hookean model can be used on cross-linked polymers to predict their stress-strain relations: Ω Ω = C exp ⁡ ⁡ ( − − 3 R → → 2 2 N b 2 ) S = k B ln ⁡ ⁡ Ω Ω ≈ ≈ − − 3 k B R → → 2 2 N b 2 Δ Δ F ( R → → ) ≈ ≈ − − T Δ Δ S d ( R → → 2 ) = C + 3 k B T N b 2 R → → 2 f = d F ( R → → ) d R → → = d d R → → ( 3 k B T R → → 2 2 N b 2 ) = 3 k B T N b 2 R → →