Physical Gels Based on Polyrotaxanes: Kinetics of the Gelation, and Relative Contributions of alpha-Cyclodextrin and Poly(ethylene oxide) to the Gel Cohesion
Résumé
Polyrotaxanes (PRs) based on alpha-cyclodextrins (alpha-CDs) threaded onto 22 kg mol(-1) poly(ethylene oxide) (PEO) chains in concentrated solution in dimethyl sulfoxide (DMSO) showed the particularity to form physical gels when resting at 21 degrees C. The wide range of studied degrees of complexation N (from 7 up to 176), where N is the number of threaded alpha-CDs per PEO chain, allowed to better understand the molecular origin of the physical gelation. The non-monotonous evolution of the mechanical properties of the gels with the complexation degree can be attributed to the crystallization of naked PEO segments and the aggregation of alpha-CDs. In fact, differential scanning calorimetry measurements carried out on these physical gels showed two distinct endothermic peaks. The first peak at 29.4 degrees C was attributed to the crystals of naked PEO segments (the parts of the PR molecule not covered by alpha-CDs). The second peak at 32.0 degrees C was attributed to alpha-CD aggregates. The dissolution enthalpy of the low temperature peak decreased monotonously with increasing N and the dissolution enthalpy of the high temperature peak increased monotonously with increasing N. These results were confirmed using X-ray scattering and (1)H NMR spectroscopy measurements showing these two contributions to gelation. These results showed that the cohesion of the physical gel was due to the crystallization of naked PEO segments, on the one hand, and, on the other hand, to the regular aggregation of alpha-CDs driven by intra- and inter-molecular hydrogen bonding interactions of their hydroxyl groups.