Study of the Gelation Process of Polyethylene Oxidea–Polypropylene Oxideb–Polyethylene OxideaCopolymer (Poloxamer 407) Aqueous Solutions

The gelation process of polyethylene oxide_a–polypropylene oxide_b–polyethylene oxide_acopolymer (poloxamer 407) aqueous solutions is studied by means of FTIR spectroscopy, rheology, and differential scanning calorimetry (DSC). The C–O–C stretching bands of infrared spectra were found to shift toward higher wavenumbers with sol–gel transition, indicating that hydrogen bonding is not the driving force for gelation. Linear viscoelastic data provide an alternative method for gel point determination of these thermoreversible gels. The results obtained with this method are found to coincide with those obtained using DSC. Gel point temperatures determined by these methods are close to 14°C for 25% (w/w) poloxamer 407 solutions and 10°C for 30% (w/w) solutions. In gel state, the elastic modulus (G′) and dynamic viscosity (η*) are found to be concentration and temperature independent for temperatures ranging from 20 to 35°C. In light of our results, we suggest a mechanism of gelation based on micelles packing and entanglements.