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 °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 Å. 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 °C, where two
phases of the POTM amorphous matrix and the ionic domain were present.
Namely, the cascade model
for 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 Å, and the mean
interaction distance between the ionic
domains was ca. 39 Å 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.