The structure of slide-ring (SR) gels in various types of solvents was investigated by small-angle X-ray scattering (SAXS). The SR gels have a unique characteristic called the “pulley effect” that the cross-links made of α-cyclodextrin molecules in a figure-eight shape can slide along the polymer chain. The SAXS results show that, in a poor solvent, the sliding cross-links form aggregates that prevent the pulley effect, while the polymer chains freely pass through the cross-links acting like pulleys in a good solvent. A vertically elliptic pattern was observed in two-dimensional SAXS profiles for covalent-bonded chemical gels in a good solvent under uniaxial horizontal deformation, while an isotropic profile was observed for the SR gels in a good solvent even under deformation. This difference in the deformation mechanism between the SR gels and the chemical gels supports the pulley effect of the SR gels.

Abstract:The volume phase transition of slide-ring gels with freely-movable cross-linking junctions was investigated. Ionic chemical gels with fixed cross-linking junctions undergo volume phase transitions when they have higher than the critical degree of ionization. However, the experimentally-observed critical ionization value for slide-ring gels is much higher than theoretical values for chemical gels. This difference indicates that the volume phase transition is significantly suppressed in slide-ring gels. The mesoscale structure at various swollen or shrunken states was also investigated by small angle X-ray scattering. Changes in the scattering patterns with shrinking slide-ring gels suggest microphase separation due to the sliding of cyclic molecules threaded along the axis of the polymer chains, which may suppress the volume phase transition. In addition, slide-ring gels absorbed/desorbed greater than equilibrium volumes in the shrinking/swelling processes and showed slow dynamics; these observations are also related to their sliding properties.