Small-Angle X-ray Scattering Study on Internal Microscopic Structures of Poly(N-isopropylacrylamide-co-tris(2,2′-bipyridyl))ruthenium(II) Complex Microgels

Internal microscopic structures of poly(N-isopropylacrylamide-co-tris(2,2'-bipyridyl))ruthenium(II) complex microgels were investigated using small-angle X-ray scattering (SAXS) in the extended q-range of 0.07 <= q/nm(-1) <= 20. The microgels were prepared by aqueous free-radical precipitation polymerization, resulting in formation of monodispersed, submicrometer-sized microgels, which was confirmed by transmission electron microscopy and dynamic light scattering. To reveal the changes in the microscopic structures of the microgels during swelling/deswelling or dispersing/flocculating oscillation, the redox state of Ru(bpy)(3) complexes was fixed in the microgels using Ce(IV) or Ce(III) ions under high ionic strength (1.5 M) during the SAXS Measurements. The scattering intensity of the microgels manifested five different structural features. In particular, the correlation length (xi), which was obtained from the fitting analysis using the Ornstein-Zernike equation, of the microgels both in the reduced and oxidized Ru(bpy)(3) states exhibited divergent-like behavior. In addition, a low-q peak centered at q approximate to 5 nm(-1) did not appear clearly in both the reduced [Ru(bpy)(3)](2+) and oxidized [Ru(bpy)(3)](3+) states, indicating that the formation of a polymer-rich domain was suppressed; thus, Ru(bpy)(3) complexes can be active even though the microgels are deswollen Or flocculated during the oscillation reaction.