system.
• Variations in the polymer chain length alone (at a fixed chemical composition,
temperature, and pressure) can bring about marked changes in phase behavior,
thus furnishing an additional parameter.
• The phase boundary in polymer solutions is situated at elevated pressures and
temperatures. Hence, experimental measurements are costly and require very
sophisticated apparatuses. Consequently, data on phase behavior of polymer
solutions is limited.
• Concentrated polymer solutions and blends are very viscous even at high tem-
peratures. Kinetic hindrances to the achievement of an equilibrium state may
therefore limit the applicability of the equilibrium models. Moreover, phase
behavior experiments are difficult to carry out in this concentration regime.
Hence, polymer systems are very challenging to model. Nevertheless, there have
been a number of theoretical developments in the field of polymer thermodynamics
that somehow incorporate the molecular description of the polymers. The earliest
of these is the Flory-Huggins theory [9, 10]. In this approach, a molecular lattice
model is proposed with the polymer and solvent occupying all the available sites
of the lattice. Each of the segments of the polymer molecule occupies one lattice
site, and the rest of the sites are filled with the solvent molecules, each of which
also occupies a single lattice site. The lattice is thus incompressible. The entropy