Thermodynamics of polymer solutions and blends

Polymerization reactions are quite often accompanied by liquid‐liquid phase separations. Processing of polymer blends is commonly also based on the phase behaviour of the system; for some applications homogenous blends are to be obtained while for other two‐phase systems are desired. Experimental methods suitable for gaining insight into the thermodynamic behaviour in such viscous systems will be discussed and some relevant measurements, performed by equilibrium methods, will be shown. Classic thermodynamic equilibrium considerations, based on simple molecular models lead to useful descriptions and predictions of the phase behaviour of partially miscible polymer systems. Depending on the amount of experimental information and on the complexity of the polymeric system, predictions will be more or less quantitative. Solubility parameter and group contribution approaches present the first level, and such predictions indicate whether a system is miscible or not. The second level of description is given by Flory‐Huggins‐Staverman approaches. These models call for some experimental evidence but permit estimation of the location of miscibility gaps and their dependence on the polymer molar mass. The concentration ranges of the demixed region are usually not well presented by these models. The quality of the description is improved considerably when the disparity of size and shape between the repeat units of the polymer and solvent constituents are taken into account. On this third level the influence of pressure can be included in the model and reasonable descriptions of the P‐T‐x space can be obtained. Extensions to copolymer systems are being developed nowadays, but still fail to give quantitative representation of the actual complex phase behaviour of such systems.