Effects of Chemical Modification on the Molecular Dynamics of Complex Polyrotaxanes Investigated by Solid-State NMR

The chemical modification of the pendant hydroxyl functional groups on cyclodextrins (CDs) significantly suppresses the hydrogen-bonding interactions between the cyclodextrin molecules and leads to the unique viscoelastic properties of hydroxypropylated polyrotaxane (HyPR) Inomata et al. Macromolecules 2010, 43, 4660–4666. HyPR consists of poly(ethylene glycol) (PEG) and α-CDs that are partially modified with a hydroxypropyl (Hy) group, setting them apart from other polyrotaxanes (PRs). The molecular dynamics of PR and HyPR with 25% (HyPR25) and 78% (HyPR78) modification ratios were investigated using various solid-state NMR techniques. Two-dimensional 1H–13C wide-line separation (WISE) NMR spectra of three samples demonstrated that the PEG chains provide two components of the restricted and the near-isotropic components in a fast motion limit at 329 K. The fraction of restricted dynamics of the threaded PEG chains was found to depend on the chemical modification ratio. In addition, WISE experiments proved that the CD side chains exhibit enhanced mobility when the modification fraction is increased. Centerband-only detection of exchange (CODEX) NMR was used to characterize the slow dynamics of both CD and PEG molecules with frequencies directly comparable to those used in viscoelastic measurements. The CD molecules undergo slow main-chain dynamics in HyPR78 in the mechanical-relaxation temperature range, whereas the other two systems do not. The temperature dependence of the correlation time ⟨τc⟩ determined by CODEX revealed Arrhenius behavior with a high activation energy (163 ± 16 kJ/mol), which is consistent with the previous viscoelastic result. The high activation energy for the dynamics of the CDs was interpreted in terms of cooperative motions with the threading PEG chains. The dependence of the evolution time of the CODEX data and simulation results indicated that the CD dynamics match random-jump and uniaxial rotation diffusion models. These results indicate that chemical modifications of the side groups can dramatically affect not only the molecular dynamics of both the CD main and side chains but also the threading of PEG chains across wide time scales.