Phase equilibria, interdiffusion and structure in the initial uncured mixtures of epoxy oligomer–polysulfone–alkyl glycidyl ether were studied. Binodal curves were constructed on isothermal sections of the ternary phase diagram. Thermodynamic mixing parameters were calculated and spinodal curves were plotted. The interdiffusion coefficients of components, establishing the technological modes of mixing the components, were determined. To validate the phase diagram, the phase structure of mixtures, the composition of which at a temperature of 40 °C corresponds to heterogeneous and homogeneous regions, was studied.

Combining epoxy oligomers with various functionality and thermoplastic modifiers enables targeted regulation of the architecture of polymer networks and the phase structure of composites. A comprehensive interference–microscopic approach is proposed for studying phase equilibria, diffusion characteristics, and structure formation kinetics in the ternary system thermoplastic polyurethane (TPU)–trifunctional epoxy oligomer (Tactix)–difunctional epoxy oligomer (DGEBA) cured with 4,4′-diaminodiphenylsulfone (DDS). It may provide control of the morphology of epoxy compounds in a narrow concentration range without changing the chemical nature of the components. The TPU–Tactix–DGEBA system is found to be thermodynamically compatible over a wide temperature range. A ternary diagram is constructed, serving as a concentration map of the final morphologies. Four topological regions were experimentally identified and described. It has been shown that the addition of Tactix (more than 30 vol.%) accelerates the onset of thermodynamic incompatibility and shifts the point of phase separation to lower conversion levels (from 22% to 15%). It was established that the average domain size of the dispersed phase (from 0.15 to 0.6 µm) is controlled by the relationship between the time to reach thermodynamic incompatibility and the kinetic inhibition of diffusion as conversion increases. The proposed approach demonstrates the possibility of precisely altering the morphology of epoxy compounds within a narrow concentration range (within 10 vol.%) without altering the chemical nature of the initial components.