E = d ( σ σ eng ) d λ λ z = k B T v s β β ( 1 + 2 λ λ z 3 ) | λ λ z = 1 = 3 k B T v s β β = 3 ρ ρ β β R T M s where v s = ρ ρ N a / M s , ρ ρ is the mass density of the chain, M s is the number average molecular weight of a network strand between crosslinks. Here, this type of analysis links the thermodynamic theory of rubber elasticity to experimentally measurable parameters. In addition, it gives insights into the cross-linking condition of the materials.