# Flory–Huggins solution theory

Flory–Huggins solution theory' is a mathematical model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecular sizes in adapting the usual expression for the entropy of mixing. The result is an equation for the Gibbs free energy change for mixing a polymer with a solvent. Although it makes simplifying assumptions, it generates useful results for interpreting experiments.

The thermodynamic equation for the Gibbs energy change accompanying mixing at constant temperature and (external) pressure is

A change, denoted by , is the value of a variable for a solution or mixture minus the values for the pure components considered separately. The objective is to find explicit formulas for and , the enthalpy and entropy increments associated with the mixing process.

The result obtained by Flory and Huggins is

The right-hand side is a function of the number of moles and volume fraction of solvent (component ), the number of moles and volume fraction of polymer (component ), with the introduction of a parameter to take account of the energy of interdispersing polymer and solvent molecules. is the gas constant and is the absolute temperature. The volume fraction is analogous to the mole fraction, but is weighted to take account of the relative sizes of the molecules. For a small solute, the mole fractions would appear instead, and this modification is the innovation due to Flory and Huggins. In the most general case the mixing parameter, , is a free energy parameter, thus including an entropic component.[1][2]

## Derivation

We first calculate the entropy of mixing, the increase in the uncertainty about the locations of the molecules when they are interspersed. In the pure condensed phasessolvent and polymer — everywhere we look we find a molecule. Of course, any notion of "finding" a molecule in a given location is a thought experiment since we can't actually examine spatial locations the size of molecules. The expression for the entropy of mixing of small molecules in terms of mole fractions is no longer reasonable when the solute is a macromolecular chain. We take account of this dissymmetry in molecular sizes by assuming that individual polymer segments and individual solvent molecules occupy sites on a lattice. Each site is occupied by exactly one molecule of the solvent or by one monomer of the polymer chain, so the total number of sites is

is the number of solvent molecules and is the number of polymer molecules, each of which has segments.

From statistical mechanics we can calculate the entropy change, the increase in spatial uncertainty, as a result of mixing solute and solvent.

where is Boltzmann's constant. Define the lattice volume fractions and

These are also the probabilities that a given lattice site, chosen at random, is occupied by a solvent molecule or a polymer segment, respectively. Thus

For a small solute whose molecules occupy just one lattice site, equals one, the volume fractions reduce to molecular or mole fractions, and we recover the usual equation from ideal mixing theory.

In addition to the entropic effect, we can expect an enthalpy change. There are three molecular interactions to consider: solvent-solvent , monomer-monomer (not the covalent bonding, but between different chain sections), and monomer-solvent . Each of the last occurs at the expense of the average of the other two, so the energy increment per monomer-solvent contact is

The total number of such contacts is

where is the coordination number, the number of nearest neighbors for a lattice site, each one occupied either by one chain segment or a solvent molecule. That is, is the total number of polymer segments (monomers) in the solution, so is the number of nearest-neighbor sites to all the polymer segments. Multiplying by the probability that any such site is occupied by a solvent molecule, we obtain the total number of polymer-solvent molecular interactions. An approximation following mean field theory is made by following this procedure, thereby reducing the complex problem of many interactions to a simpler problem of one interaction.

The enthalpy change is equal to the energy change per polymer monomer-solvent interaction multiplied by the number of such interactions

The polymer-solvent interaction parameter chi is defined as

It depends on the nature of both the solvent and the solute, and is the only material-specific parameter in the model. The enthalpy change becomes

Assembling terms, the total free energy change is

where we have converted the expression from molecules and to moles and by transferring Avogadro's number to the gas constant .

The value of the interaction parameter can be estimated from the Hildebrand solubility parameters and

where is the actual volume of a polymer segment.

This treatment does not attempt to calculate the conformational entropy of folding for polymer chains. (See the random coil discussion.) The conformations of even amorphous polymers will change when they go into solution, and most thermoplastic polymers also have lamellar crystalline regions which do not persist in solution as the chains separate. These events are accompanied by additional entropy and energy changes.

It should be noted that in the most general case the interaction and the ensuing mixing parameter, , is a free energy parameter, thus including an entropic component.[1][2] This means that aside to the regular mixing entropy there is another entropic contribution from the interaction between solvent and monomer. This contribution is sometimes very important in order to make quantitative predictions of thermodynamic properties.

More advanced solution theories exist, such as the Flory-Krigbaum theory.

## Oligomer-gel solutions theory

Recently a team of scientists from the University of Durham (UK) introduced a new form of the Flory-Huggins free energy for a solution where there is an oligomer with a cross-linked polymer like a reticulated gel. They considered the mixing entropy for the oligomer and the elastic entropy for the reticulated gel keeping the mean field interaction through χ. They found that increasing the elastic modulus of the cross-linked polymer it is possible to increase the critical value of the mean field over which there is phase separation. This theory introduces a new way of controlling the phase separation in such a system. Furthermore they used this theory for studying the wetting and the migration of the oligomer specie to a surface.[3]

## References

1. Burchard, W (1983). "Solution Thermodyanmics of Non-Ionic Water Soluble Polymers.". In Finch, C. Chemistry and Technology of Water-Soluble Polymers. Springer. pp. 125–142. ISBN 978-1-4757-9661-2.
2. Franks, F (1983). "Water Solubility and Sensitivity-Hydration Effects.". In Finch, C. Chemistry and Technology of Water-Soluble Polymers. Springer. pp. 157–178. ISBN 978-1-4757-9661-2.
3. Krawczyk, Jarosław; Croce, Salvatore; McLeish, T. C. B.; Chakrabarti, Buddhapriya (2016-05-18). "Elasticity Dominated Surface Segregation of Small Molecules in Polymer Mixtures". Physical Review Letters. 116 (20): 208301. doi:10.1103/PhysRevLett.116.208301.