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Differential equations for the surface tension as a function of temperature, pressure, and composition of a two-phase system containing linear macromolecules are derived within the framework of the quasi-solution theory. An alternative version of the theory has been developed for concentrated solutions and melts, in which the units and end groups are considered to be mobile components. Analysis shows that the existence of selective adsorption of chain units or end groups in the surface layer cannot be established simply on the basis of data on the surface tension in an homologous series. The feasibility of observing this hypothetical effect in measurements on the equilibrium elasticity of thick films of liquid polymer homologues is discussed.

On the basis of the concept of a quasisolution and the Bronsted principle of congruence the authors have obtained fundamental equations for the Gibbs energy of a polydisperse polymer in which the units of the chain molecules and their terminal groups act as independent components of the system including the general case of the presence of any other substances. From analysis of the chemical potentials of the units and the terminal groups they infer the equivalence of the congruence principle and the Flory principle on the indistinguishability of these components in the same or different macromolecules. The use of the equations obtained for describing chemical equilibrium on polycondensation of bifunctional monomers is examined. Where necessary it is easy to bring the system investigated into line with the Flory principle if the lower homologues differing in reactivity are brought into the category of extraneous substances-comonomers, the mathematical apparatus of the theory remaining unchanged.

It is proposed that an infinite sequence of linear polymer homologues be described as an open thermodynamic system whose equilibrium state is characterized by an additional parameter, i.e. by the length of macromolecules. This idea has led to the derivation of basic equations for the energy of a system: the repeat-units of chain molecules and the endgroups figure as independent components, like other substances present in the system. An analysis of the phase equilibria led to the derivation of an analogue of the known generalized differential van der Waals equation.