The transformation of the micro-domain structure of polyimide films during thermally induced chemical conversion from the polyamic acid prepared from 3,3',4,4'-diphenyl tetracarboxylic dianhydride and p-phenylene diamine, also in the presence of triphenyl phosphate, is studied with a transmission electron microscope. The surface morphology of the films is typified by a broad size distribution of micro-domains that depends on the history of the sample treatment. To describe these observations thermodynamics of irreversible processes is used with the special feature that very slowly running stationary nonequilibrium states are considered to operate as temporary states of reference. In terms of an increment model a generalized version of the law of mass action comes out that holds under stationary nonequilibrium conditions. The observed stationary nonequilibrium patterns developed at elevated temperatures under different conditions (also in the presence of a stabilizer) are different due to process controlled temporarily fixed constraints. However, they belong altogether to the same universal class. The crucial point is that these domain ensembles are optimized controlled by the same logistics. One of the central symmetries demanded by the model is thus impressively exemplified this way, demonstrating the quality of the concept. The tenacity of the films is also studied. The results interpreted in terms of the kinetic conception of fracture of solids are shown to depend in a defined manner on the domain structure of the polymers.