Thermal properties and microstructures of methylated polyrotaxane solutions

Aqueous solutions of polyrotaxanes consisting of poly(ethylene glycol) and methylated alpha-cyclodextrins (alpha-CD) were studied by means of differential scanning calorimetry (DSC), dynamic light scattering, and X-ray diffraction in order to investigate the effect of the degree of methylation on thermoresponsive behavior. Polyrotaxanes with a degree of methylation higher than 50% had a lower critical solution temperature (LCST) and showed reversible associations and dissociations in water. In the transmittance measurements, the cloud point of methylated polyrotaxanes (MePR) shifted to a lower temperature with an increase in the degree of methylation. The heating curve obtained by DSC for the nearly permethylated polyrotaxane showed one broad endothermic peak that was associated with the microcrystallization of methylated CDs by hydrophobic interactions. On the other hand, the DSC profiles for partially methylated polyrotaxanes had several endothermic peaks, indicating multiple phase transitions of the MePR solutions. The results imply that the thermal properties of the MePR-water system are significantly affected by not only the methyl groups on alpha-CDs but also by the remaining hydroxyl groups.