A novel ionene elastomer (DPI) consisting of 2,2＇-bipyridinium units and poly(oxytetramethylene) (POTM) segments was synthesized by cationic polymerization of tetrahydrofuran followed by termination using 2,2＇-bipyridine. The molecular mass between the ionic sites was ca. 10 000 g/mol. The DPI film displayed a high tensile strength up to 34 MPa and 540% elongation at break at 25 degrees C. The higher-order structure of the DPI film was investigated by thermal and dynamic mechanical analyses and by wide-angle X-ray diffraction and small-angle X-ray scattering (SAXS) measurements. The DPI film had a microphase-separated structure consisting of three phases of the POTM amorphous matrix, the POTM crystalline domain and the ionic domain at room temperature. The POTM crystalline domain was lamellar, and its spacing was ca. 270 Angstrom. A new model was proposed to successfully account for the upturn of the scattering intensity toward zero angle in the SAXS profile of DPI at 65 degrees C, where two phases of the POTM amorphous matrix and the ionic domain were present. Namely, the cascade model far randomly branched f-functional polycondensates was adapted, where the inhomogeneous ionic domains consist of scattering points in a tree-like network. Each domain is described by the Debye-Bueche random-two-phase model, and the repulsive interaction takes place between the ionic domains. The size of the ionic domains was evaluated to be ca. 12 Angstrom, and the mean interaction distance between the ionic domains was ca. 39 Angstrom from the fitting of the model calculation to the observed SAXS profile. The system was in the vicinity of a gel point in terms of the cascade model, where the ionic domain serves as a physical cross-linking point.