For the modeling of phase diagrams and interfacial tensions it is essential
to dispose of closed mathematical expressions describing the Gibbs energies
of mixing as a function of composition, temperature, pressure and the
molar masses of the polymers. Unfortunately theoretical predictions resting
exclusively on the equations of state of the pure components prove inadequate.
For that reason experimental information on the systems of interest are
in many cases simply cast into Flory-Huggins interaction parameters and
their dependence on the variables of state. Two fundamental deficiencies
show up in this context: Firstly, the fact that the segments of macromolecules
cannot be evenly distributed on the system at low polymer concentration
and, secondly, the variable shape of such solutes. The consequences of
the latter reality have already been studied in detailed and led to the
concept of dimensional (conformational) relaxation.