Cohesive Properties and Solubility

This chapter deals with the cohesive energy and the solubility parameter, and explains the correlation of these quantities with chemical structure. The cohesive properties of a polymer find direct expression in its solubility in organic liquids. The cohesive properties of a substance are expressed quantitatively in the cohesive energy. This quantity is closely related to the internal pressure, a parameter appearing in the equation of state of the substance. The solubility of a given polymer in various solvents is largely determined by its chemical structure. Besides the chemical structure, also the physical state of a polymer is important for its solubility properties. Crystalline polymers are relatively insoluble and often dissolve only at temperatures slightly below their crystalline melting points. As a general rule, the solubility decreases as the molar mass of the solute increases. This property can be used to fractionate polymers according to molar mass. This chapter begins by pointing out that the order of solubility of a given solute in a series of solvents is determined by the internal pressures of the solvents. Following this, it introduces the concept of “cohesive energy density,” identifying this quantity with the cohesive energy per unit volume. Finally it provides a comprehensive treatment of this concept and verifies the proposition that the square root of the cohesive energy density is a parameter identifying the behavior of specific solvents.