Polymer solutions are solutions containing dissolved polymers. These may be  (e.g. in ), or solid solutions (e.g. a substance which has been plasticized).
The introduction into the polymer of small amounts of a solvent (plasticizer) reduces the temperature of glass transition, the yield temperature, and the viscosity of a melt.
 An understanding of the thermodynamics of a polymer solution is critical to prediction of its behavior in manufacturing processes — for example, its shrinkage or expansion in injection molding processes, or whether pigments and solvents will mix evenly with a polymer in the manufacture of paints and coatings. A recent theory on the viscosity of polymer solutions gives a physical explanation for various well-known empirical relations and numerical values including the Huggins constant, but reveals also novel simple concentration and molar mass dependence.