Theory of the interface between polymers or polymer solutions. I. Two components system

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Abstract
A theory is developed to determine the properties of the interface between two coexisting liquid phases, where one or both of the components comprising the system can be polymeric. The local free energy in the interfacial region of nonuniform composition is expressed in terms of a modified Flory−Huggins free energy of mixing, which was derived earlier and found successful in predicting the extent of adsorption from polymer solutions. The system is defined in terms of the chain lengths of the two components r1 and r2 and the segmental pair interaction parameter χ. When χ − χc is small, where χc is the value of χ for critical phase separation, the interfacial tension γ and the reciprocal of the interface thickness d vary with (χ − χc)α, the value of the positive exponent α being dependent on the chain lengths of the components. Agreement with available, limited experimental data is satisfactory. As an aid for more critical tests against experiment, the values of γ and d for polystyrene−cyclohexane solutions are computed with the use of realistic χ values.