Equilibrium and kinetics of drying and swelling of poloxamer hydrogels

Both equilibrium and transport properties are important for the characterization and utilization of Poloxamer, poly(ethylene oxide)-poly(propylene oxide), (PEO-PPO) block copolymers. We report here on equilibrium properties such as drying/ swelling isotherm, osmotic pressure and water activity for representative Poloxamers in the presence of water (selective solvent for PEO). We have found the osmotic pressure to increase exponentially with increasing Poloxamer concentration in water, and the intermolecular interactions in Poloxamer-water systems to occur at different hydration levels: Poloxamer micelles, PEO coil, and PEO segment. The interaction parameter (χ₁₂ between Poloxamer and water (obtained by fitting the water activity data to the FloryHuggins equation) is above 1/2 and increases further with Poloxamer concentration. We also report here on the kinetics of drying and swelling of Poloxamer hydrogels when exposed to air of fixed relative humidity. We have found swelling to be a diffusion-limited process, while drying is mainly evaporation-limited. We have used a diffusion model to fit the water loss or water gain as a function of time and to extract useful parameters such as water diffusion coefficients in the hydrogel.