Diimides 4,4'-oxydiphenylene diphthalylimide (PT-ODA-PT) and diphenylbiphenyltetracarboxyimide (A-BPDA-A) of an evident affinity to polyimides have been dispersed in a matrix of poly(4,4'-oxydiphenylene biphenyltetracarboxyimide) (BPDA-ODA). Changes in thermal and mechanical properties of BPDA-ODA films modified by diimides have been found to depend on the amount of diimide added and the conditions of film preparation (stretching). It has been established that diimides can play two different roles. When dissolved in polyimide, they act as plasticizers improving the segment mobility of BPDA-ODA chains, which may be critically needed to achieve the better molecular packing. The films plasticized by diimides show lower T-g and the order-disorder transition temperature; the unstretched films exhibit better tensile properties, compared to unmodified films. However, dispersed beyond the limit of solubility in polyimides, diimides have detrimental effects on mechanical properties of the films. Though changes of the thermal properties of compositions derived from BPDA-ODA polyimide and microparticular diimides are less expressed than those of polyimide films plasticized by diimides, the compositions have poorer ultimate strength and shorter elongation at break. The conclusion may be drawn that the mechanical properties of the films modified by microparticular diimide are mainly controlled not by the interfacial interaction matrix-microparticles, but more likely by the much stronger macromolecular chain-chain interaction. (C) Koninklijke Brill NV, Leiden, 2010