The method of polarized light scatter has been used to study the structure of dilute and moderately concentrated polyamido imide solutions based on the diaminodiphenyl ether in N-methylpyrrolidone and also films prepared from the polymer in a pure solvent and mixture with a precipitant. For the comparative analysis of the structure the authors also investigated solutions of the high molecular mass fraction of PMMA and a polyamido acid in good solvents. With change in the concentration the correlation radii of the fluctuation in the polarizability of the solutions pass through a maximum explained by association processes. Passage from solution to the solid phase is accompanied by substantial increase in the values of the mean square of fluctuation in polarizability reflecting increase in the heterogeneity of the system and also by increases in the mean square optical anisotropy.

The structure of polyamido-imide copolymers containing different amounts of rigid diamine units was studied by thermomechanical methods, IR spectroscopy, and X-ray diffraction techniques. Geometric correspondence of parameters of the diamine units favours the formation in the polymer of a mesomorphic structure characterized by the maximum number of intermolecular hydrogen bonds. All investigated films exhibit a texture with the macromolecular axis oriented preferentially in parallel with the film surface.

Density data have been analyzed for 74 polyimides, polyamidoimides, polyesterimides, polyesteramidoimides and polyimidazopyrrolones, prepared by thermal imidization. It was shown that the molecular packing coefficient in the amorphous state for the above and all other classes of polymers is constant and equals Ka = 0·683. The packing coefficient in the crystalline state, Kc is constant only for similar classes of polymers. For polyheteroarylenes, Kc = 0·76. The constancy of Ka and Kc permits evaluation of the degree of crystallinity from the sample density and the chemical formula of the polymer.

Isotropic poly-(4,4′-oxydiphenylene)pyromellitamic acid films were kept in mixtures of benzene, acetic anhydride and pyridine, which led to transformation of polyamic acid into cyclochain heterogeneous polymers containing imide and iso-imide units. After being removed from the imidizing mixture the films underwent heat treatment resulting in formation of the final chemical structure of poly-(4,4′-oxydiphenylene)pyromellitimide. A study was made of changes in the strength and the breaking elongations of films during treatment with the imidizing mixture and subsequent heating. An increase in the number of imide units in the cyclochain polymers was accompanied by an improvement in mechanical properties of the polyimides obtained after heat treatment.

The melting temperatures Tm of four basic types of aromatic polyimides, amongst them those melting well above their decomposition temperature, have been determined by calculation and by experiment. The largest Tm (up to 1000°C) was possessed bypoly imides with rod-like macromolecules. The Tm of the various polymide types have been theoretically analysed. The Tm has been found to depend on the entropy of melting in which the factors associated with ring movement play the main part. This is assumed to be applicable also to other thermodynamic functions and properties of polyimides, and to other polymers with rings in the chains.

Several polyhydantions were prepared by interaction of bis-(iminoacetates) with di-isocyanates. It was shown that they are able to form strong films from solutions and compressed materials of high heat stability, from powders. It was established that at increased temperature aromatic polydantoin films can be oxidized to poly-(parabanic acids), strength properties being retained and with a fairly high heat stability.

In the interval —190–450°, 70–2500 c/s, the temperature-frequency dependences of Young's complex dynamic modulus were investigated for a group of aromatic polyimides. The chemical structures were controlled through the arrangement, number and type of “hinge” bands in the monomer units. It was found that there were α-, β- and γ-relaxation transitions: the α-transition was found with group IV polyimides only, and is associated with rotational segmental motion. β-Relexation appeared with all the polyimides, and is associated with the motion of individual groups of the main chain, and with the quasi-equilibrium configuration of the latter. The numerical values of the activation energies for all the transitions do not differ from the corresponding values for carbochain polymers.

A study over a wide range of temperatures of the relations between time and strength, deformation and strength properties in elongation and thermo-mechanical and relaxation properties of isotropic and oriented polyimide films prepared from benzophenone tetracarboxylic acid dianhydride and diaminodiphenyl ester indicates that the temperature and time relations of strength and deformation characteristics of polyimide are determined to a large extent by β type relaxation processes.