Gelation Kinetics and Viscoelastic Properties of Pluronic and α-Cyclodextrin-Based Pseudopolyrotaxane Hydrogels

The results of a systematic investigation into the gelation behavior of alpha-cyclodextrin (alpha-CD) and Pluronic (poly(ethylene oxide). poly(propylene oxide)-poly(ethylene oxide) block copolymers) pseudopolyrotaxane (PPR) hydrogels are reported here in terms of the effects of temperature, alpha-CD concentration, and Pluronic type (Pluronic F68 and Pluronic F127). It was found that alpha-CD significantly modifies the gelation behavior of Pluronic solutions and that the PPR hydrogels are highly sensitive to changes in the alpha-CD concentration. In some cases, the addition of alpha-CD was found to be detrimental to the gelation process, leading to slower gelation kinetics and weaker gels than with Pluronic alone. However, in other cases, the hydrogels formed in the presence of the alpha-CDs reached higher moduli and showed faster gelation kinetics than with Pluronic alone and in some instances alpha-CD allowed the formation of hydrogels from Pluronic solutions that would normally not undergo gelation. Depending on composition and ratio of alpha-CD/Pluronic, these highly viscoelastic hydrogels displayed elastic shear modulus values ranging from 2 kPa to 7 MPa, gelation times ranging from a few seconds to a few hours and self-healing behaviors post failure. Using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), we probed the resident structure of these systems, and from these insights we have proposed a new molecular mechanism that accounts for the macroscopic properties observed.