A criterion of solubility is proposed for polymers based on the analysis of forces of interfacial tension and wetting generated by the immersion of polymers in a solvent. The criterion allows prediction of the solubility of polymers in different solvents, taking the chemical structures of the polymer and solvent into account. About 300 polymer-solvents were analysed. The probability of correct predictions of solubility, using the proposed criterion, was found on the basis of our analysis to be ∼85%.

The limiting strength and relaxation (stress relaxation and creep) properties of films based on copolyphenylquinoxaline naphthoylene benzimidazole of regular structure have been studied. A new criterion of solubility is employed to analyse the effect of the quality of the dissolving medium in obtaining the films on their mechanical properties and its is shown that they may change within wide limits for a polymer of the same chemical structure.

The strength, deformation and relaxational properties of films of polyphenyl quinoxaline and PI have been studied as a function of the prehistory of their preparation from solution. Pure solvents and mixtures of them with a small quantity of precipitants were used as dissolving media. Addition of precipitants (∼ 2–5% in relation to the solvent) gave films with raised strength and considerably greated deformability. When the films are obtained from solution the rate of stress relaxation and creep may be regulated.

A physically validated scheme is proposed for calculating the surface tension of organic liquids and polymers from their chemical structure. Satisfactory agreement of the calculated and experimental values is demonstrated for a large number of substances. The proposed calculation scheme may be used to evaluate the contribution of the weak dispersion and strong intermolecular interactions to surface tension.

It has been shown with PS and PMMA as examples that the mechanical relaxation processes and strength properties of glassy polymers depend on the specimens' processing history. The reasons for the effect of the history are connected with differences in the thermodynamic affinity between the polymer and the solvent as well as differences in the microporosity of the structure, which depends on the volume of the solvent molecules.