Micellization and gelation of PEO-PPO-PEO binary mixture with non-identical PPO block lengths in aqueous solution

Sol/Gel behavior of a mixture of two Pluronic species with non-identical polypropylene oxide (PPO) and polyethylene oxide (PEO) block lengths in aqueous medium is explored. Emphasis is placed on the nature of micelles (either mixed or separate micelles), nature of gelation, gel strength and microstructure. Two mixtures are considered: F127 + L64 and F127 + P105. Cooperative aggregation leading to mixed micelles is found for both cases via dynamic light scattering, differential scanning calorimetry and cloud point temperature experiments. From rheometry, F127 + L64 is found to exhibit two hard gel states separated by a soft gel as the temperature increases, although each species alone does not show such behavior. The first hard gel formed at lower temperature appears clear and has a cubic packing structure, while the second hard gel at higher temperature is hexagonally structured and appears hazy. Interestingly, the second hard gel is observed only during the temperature ramp up process, whereas the first gel is seen both in the ramp up and down processes. The aforementioned behavior does not occur for F127 + P105.