The equilibrium phase behavior of the partially miscible system polystyrene-polybutadiene-chloroform was studied at 25Β°C and 1 atm pressure by using gel permeation chromatography to analyze the conjugate phases. Three narrow molecular weight distribution samples of each polymer were used to provide nine examples of the system for study. The Flory-Huggins theory was used to determine the polymer-polymer interaction parameter x 2 3 . When literature values of the two polymer-solvent interaction parameters x1e and xI3 were used, the xa3 values generally were found to be negative and to show no distinct concentration dependence. No significant improvement was found when the literature value of only the polybutadiene-chloroform interaction parameter was used to simultaneously generate values of x 2 3 and xle. Values of xe3 at the critical point, X23,cr, were found to increase with increasing polymer concentration.
' To whom corre~pondrncr \hould be addressed mer science: t h e nature of specific polymer-polymer interactions. Even in the presence of the solvent, phase separation behavior, colligative properties, nonideal solution behavior, etc., are, to the first approximation, linear combinations of the interaction parameters ~1 2 , x13, and x 2 3 . The latter is, of course, t h e quantity needed in order to attack problems of polymer-polymer miscibility. The phenomenon of phase separation in mixtures of two polymers, either in a common solvent or in the solid state, is known as incompatibility and is of considerable practical importance. Limited miscibility plays a role in t h e preparative and analytical fractionation of polymers, in t h e preparation of plastic films, including paint and varnish coatings, and in t h e determination of service properties of polymer blends. An extensive review of compatibility studies previously reported has been given by Krause ( 3 ) .