The Gibbs free energy of mixing, , must be negative for a blend to be miscible. According to Flory-Huggins theory, a revision of regular solution theory, the entropy change per mole of lattice sites of blending polymer 1 and polymer 2 is
, where ΔS is the change in entropy of mixing, R is the gas constant, Φ is the volume fraction of each polymer, and x is the number of segments of each polymer. x and x increase with higher degrees of polymerization and thus molecular weight. Since most useful polymers are high in molecular weight, the change in entropy experienced from the mixing of two large polymer chains is very low, and typically does not bring the Gibbs free energy low enough to constitute miscibility.