Supramolecular Hydrogels Made of End-Functionalized Low-Molecular-Weight PEG and α-Cyclodextrin and Their Hybridization with SiO2 Nanoparticles through Host−Guest Interaction

As quite generally stated in the literature, low-molecular-weight (MW) poly(ethylene glycol) (PEG) (M n less than 2K) and α-cyclodextrin (α-CD) can lead to only crystalline precipitates (not hydrogels). However, in this study we found that: (1) adamantane monoend-functionalized low-MW PEG (Ada-PEG, M n= 1.1 or 2K) and α-CD lead to hydrogels but not to crystalline precipitates and (2) β-cyclodextrin (β-CD) surface-functionalized silica nanoparticles (β-CD−SiO2) can be well dispersed in low-MW Ada-PEG and α-CD aqueous mixtures, resulting in hybrid hydrogels. The hydrophobic aggregation of Ada-PEG in case 1 and the further functionalization of β-CD−SiO2 with PEG chains due to the inclusion complexation between β-CD and the Ada group attached to PEG in case 2 were found to play a key role as a supra-crosslink that promoted the gelation of the inclusion complexes of α-CD and the low-MW Ada-PEG. The resultant native and hybrid hydrogels retained the basic characteristics of the supramolecular physical hydrogels, especially the shear-thinning property. The mechanical strength and the viscosity of the hybrid hydrogel were greatly improved in comparison with those of the corresponding native hydrogels.