Study of aqueous solutions of poloxamers by rotational viscometry and spin probe method

The aim. To study aqueous solutions of different poloxamers by spin probe method and rotational viscometry depending on the temperature and poloxamer content. Materials and methods. The aqueous solutions of poloxamers 188, 237, 338 and 407 were studied. The solutions were studied by rotational viscometry at different temperatures; the flow behaviour, yield stress (t0), and dynamic or apparent viscosity (η) were determined. Five spin probes differing in molecular structure, solubility, and radical localisation were introduced into the solutions. Electron paramagnetic resonance (EPR) spectra were obtained. The EPR spectra were used to determine their type and to calculate parameters. Results. Three factors are important for gel formation: the poloxamer type, its concentration in aqueous solution, and temperature. As the temperature of aqueous solutions of poloxamers 237, 338, and 407 increases, the rotational correlation times of fatty acid-based spin probes and the order parameters of their EPR spectra decrease. This indicates a decrease in the packing density and orderliness of the polypropylene oxide (PPO) chains in the non-polar part of the poloxamer associates, leading to an increase in the volume fraction of micelles/mesophases and promoting the formation of gels. As the temperature decreases, the opposite processes occur, leading to a gel → sol transition. At 37 °C, non-polar micelle cores could be characterised as two-dimensionally liquid and one-dimensionally solid. The rotational correlation times of the hydrophilic spin probe 4-OXO-TEMPO in 25 % aqueous solutions of poloxamers 338 and 407 are approximately constant or increasing despite an increase in the temperature. This indicates that in the polar part of the poloxamer associates, where this probe is partially localised, structural rearrangements occur with increasing temperature, which probably prevents hydrophobic hydration of the PPO chains. Conclusions. The rheological properties of aqueous solutions of poloxamers depend on their type, concentration, and temperature. According to the parameters of the EPR spectra of fatty acid-based spin probes, it was found that with increasing temperature, the packing density and the orderliness of the PPO chains in the non-polar part of the poloxamer associates decrease, probably leading to an increase in the volume of the micelles and causing a sol → gel transition