to crystals. This, and the assumption that monomers and solute molecules are really the same size, are the main geometric approximations in this model. For a real synthetic polymer, there is a statistical distribution of chain lengths, so would be an average. The enthalpy is the internal energy corrected for any pressure-volume work at constant (external) . We are not making any distinction here. This allows the approximation of Helmholtz free energy, which is the natural form of free energy from the Flory–Huggins lattice theory, to Gibbs free energy. In fact, two of the sites adjacent to a polymer segment are occupied by other polymer segments since it is part of a chain; and one more, making three, for branching sites, but only one for terminals. Polymer chemistry Solutions Thermodynamic free energy Statistical mechanics