The free energy change per volume is just: Δ Δ f def = Δ Δ F def ( R → → ) V = − − k B T v s β β ( λ λ x 2 + λ λ y 2 + λ λ z 2 − − 3 ) 2 where n s is the number of strands in network, the subscript "def" means "deformation", v s = n s / V , which is the number density per volume of polymer chains, β β = ⟨ ⟨ R 2 ⟩ ⟩ / R 0 2 which is the ratio between the end-to-end distance of the chain and the theoretical distance that obey random walk statistics. If we assume incompressibility, the product of extension ratios is 1, implying no change in the volume: λ λ x λ λ y λ λ z = 1 .