In a uniaxial deformed rubber, because λ λ x λ λ y λ λ z = 1 it is assumed that λ λ x = λ λ y = λ λ z − − 1 / 2 . So the previous free energy per volume equation is: 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 = k B T v s β β 2 ( λ λ z 2 + 2 λ λ z − − 3 )