STATISTICAL THERMODYNAMICS OF MACROMOLECULAR LIQUIDS AND SOLUTIONS.

The separability of the configuration partition function for long chain molecules and their solutions into intermolecular and intramolecular factors is discussed. Two consequences are emphasized: (i) the thermodynamic properties of a polymer solution are virtually independent of the structure of the macromolecule and especially of its flexibility, and (ii) the chain configuration does not depend appreciably on concentration, even at the limit of the undiluted polymer. This separation of the partition function holds, however, only if the molecular chains are sufficiently flexible. Above a critical degree of stiffness that depends on the concentration, a state of partial order (e. g. , liquid crystalline) is preferred. Theory for the limiting case of rigid, rod-like particles accounts satisfactorily for the biphasic equilibrium between an isotropic and an anisotropic (nematic) phase above a concentration limit that depends on the axis ratio x. The theory can be adapted to polydisperse systems in which the solute comprises species that differ in x.