ABSTRACT
In order to gain insight into the polymer dissolution process, let us briefly review the dissolution of lowmolecular-weight (simple) substances. We know, for example, that while oil will not mix with water, an oil stain in clothing can be removed rather easily by using hydrocarbon solvents like naphtha. On the other hand, ordinary table salt, or sodium chloride, dissolves readily in water but not in gasoline. As will become evident presently, the physical phenomena associated with the solubilities of various substances in different solvents are intimately tied with the nature of the solutes and solvents. For example, in molecular crystals, the attractive forces are of the dipole-dipole or London dispersion type, which is relatively weak and therefore fairly easy to break apart. Consequently, this type of solid dissolves to an appreciable degree in nonpolar solvents, where the molecules are held together by London-type attractive forces also. However, crystals will not dissolve to any great extent in polar solvents since the strong attraction between the polar solvent molecules cannot be overcome by the much weaker solute-solvent interaction forces. By similar arguments, polar solutes and ionic solids are soluble only in polar solvents. They are insoluble in nonpolar solvents because the weak solute-solvent interaction is not strong enough to overcome the strong attractive forces between the solute molecules and hold them apart. In essence, therefore, when a solute dissolves in a solvent, solute-solute molecular contacts are replaced by solute-solvent contacts. Consequently, for solute particles to enter into solution, the solute-solvent forces of attraction must be sufficient to overcome the forces that hold the solid together.
