Some considerations on equation of state and phase relations: Polymer solutions and blends

We review briefly the general assumptions underlying hole theories of the configurational thermodynamic functions for single and multiconstituent systems. From the original Simha‐Somcynsky theory several important modifications have recently evolved. First, there is a revision of the combinatorial entropy originating from the mixing of segments and holes in the spirit of Huggins's theory. With consistent additional modifications of the configurational free energy, quantitatively significant consequences for certain aspects of phase equilibria can arise. Finally, allowance for nonrandom mixing of constituents species and holes has been made. We illustrate the theory's potentials first in terms of pressure‐volume‐temperature (PVT) data for high and low molar mass species and their miscible mixtures. The influence of PVT properties on the miscibility behavior of solutions and blends is examined next. Of particular concern are the connections between predictability of lower from upper mixing spinodals, pressure effects, and the binary interaction χ‐function.