The effect of ring size on the mechanical relaxation dynamics of polyrotaxane gels

Mechanically interlocked molecules have unique intramolecular dynamics owing to the relative motion of different components. Although the characteristic molecular dynamics in solution can be controlled by the design of their components, this generally does not define the macroscopic material properties. We demonstrate that the size of the ring components in polyrotaxanes significantly affects the mechanical relaxation dynamics of their cross-linked gels through the relative translational motion of polymer chains and cross-links. We synthesized a size-mismatched polyrotaxane consisting of polyethylene glycol (PEG) and γ-cyclodextrins (γ-CDs) for comparison with a size-matched polyrotaxane with smaller rings of α-cyclodextrins (α-CDs). Each polyrotaxane was cross-linked in solution to form gels whose networked polymer chains could slide through the cross-links formed by the CDs. Viscoelastic measurements of the gels showed similar stress relaxation behaviors, with relaxation times considerably longer for gels with larger rings. Detailed analyses of the relaxation dynamics revealed that the stress relaxation corresponded to the dynamics of chain sliding through the cross-links and that the difference in dynamics is attributable to the difference in friction in the ring cavity. The increased friction is explainable by enhanced interactions caused by penetration of solvent molecules in the extra cavity of γ-CD, as supported by NMR relaxation measurements and molecular modeling.