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.
Highlights
A slide-ring (SR) gel [1] was obtained by cross-linking cyclic molecules of different polyrotaxanes (PR) [2,3] that have cyclic molecules threaded along the axis of a polymer chain capped by bulky end groups (Figure 1)
We measured the mesoscale structure of the SR gels at each degree of swelling by small angle X-ray scattering (SAXS)
The weakly ionized SR gels did not show discontinuous volume changes, while the strongly ionized SR gels showed a volume phase transition, but had a much higher critical degree of ionization than the theoretical value determined by the Flory–Rehner–Tanaka model
Summary
A slide-ring (SR) gel [1] was obtained by cross-linking cyclic molecules of different polyrotaxanes (PR) [2,3] that have cyclic molecules threaded along the axis of a polymer chain capped by bulky end groups (Figure 1). Internal stress is reduced by exchange between highly-extended and shrunken chains through cross-linking junctions, the so-called “pulley effect” [1]. Another crucial and unique feature of SR gels is the entropy of rings [9,10,11,12]. SR gels have small Young’s moduli that are not proportional to the cross-linking density and are much lower than those of chemical gels with the same density
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