Combining the FH entropy of mixing and the standard RS enthalpy of mixing yields the FH solution theory for polymers: ∆G

Combining the FH entropy of mixing and the standard RS enthalpy of mixing yields the FH solution theory for polymers: ∆G N = Bφ1φ2 + RT (φ1 lnφ1 + φ2 N lnφ2). As written above, this equation is comparable to the free energy of mixing from the Regular Solution Model. Notice that the expressions differ in two ways: first, FH is cast in volume fractions rather than mole fractions, and second, FH is no longer symmetric in concentration due to the 1/N factor in the entropy term. We need to do one more thing before completing this derivation of FH: insert the Flory-Huggins parameter, χ, which is defined as