Basic concepts

Polymer solutions are complex liquids at any given temperature and require specialized thermodynamic treatment. The phase stability of polymer solutions is a pre-requisite for any potential application. In general, the theoretical calculation of the thermodynamic properties of liquids and solutions involves determination of their configurational properties (those that depend only on intermolecular interaction) ignoring the internal movement of molecules. As a result, we can define configurational or intermolecular energy of a solution as the energy of a liquid minus the energy of the same substance in the state of an ideal gas at the same temperature. Thus, as is evident, configurational thermodynamic properties can have combinatorial and/or non-combinatorial properties. This attribute of polymer solutions has attracted much attention in the past (Flory 1953; Hildebrand 1953; Huggins 1941, 1942).

Thermodynamics demands that entropy be the deciding factor that governs solution stability. Entropy of mixing arising due to the rearrangement of different molecules is called the geometrical or combinatorial entropy of mixing. The non-geometrical (non-combinatorial) contribution of the entropy of mixing results from the energy of interaction between the components present in the solution, resulting in contraction of the solvent and the formation of oriented solvation layers (hydration sheathes). This involves a decrease in entropy of the solvent. The former contribution(∆Scomb > 0) favours dissolution (∆G = ∆H – T∆S becomes more negative), the latter contribution (∆Snon-comb < 0) does not favour dissolution. We find that under specific conditions, in some systems, the first contribution may dominate over the second and then the total entropy of mixing becomes negative. This concept of polymer solutions has been discussed in excellent detail by Flory (1953).