The sol-gel transition of methylated polyrotaxane investigated by small angle neutron scattering

Polyrotaxane (PR) is a kind of supramolecules, consisting of thread-like molecules of poly(ethylene glycol) (PEG) and α-cyclodextrin (CD) molecules. The PR is exclusively soluble in DMSO and alkali solutions due to the aggregation of CD molecules by hydrogen bonding between CDs. In order to avoid the aggregation of CD molecules and develop a new physical property, we synthesized methylated polyrotaxane (MPR) where the hydroxyl groups on CD were substituted by methyl groups. The methylated polyrotaxane is soluble in water, and exhibits a new function, i.e., temperature-sensitivity and sol-gel transition. Small angle neutron scattering (SANS) and dynamic light scattering (DLS) experiments were carried out to investigate the structure change as a function of temperature and the degree of methylation (DM). In low DM (≈ 30%), MPR did not show thermo-sensitivity. On the other hand, In high DM (≈ 80%), MPR showed strong temperature dependence. SANS intensity drastically increased at 60°C, and also a peak appeared. This change is consistent with the results of DLS. The origin of the thermo-sensitivity was found to be the hydrophobic interaction between methyl groups on CDs. j.