Abstract
Micellization and gelation of binary mixtures of EO99PO69EO99 (pluronic(®) F127) and EO80PO27EO80 (pluronic(®) F68) in aqueous solutions were investigated by means of micro-differential scanning calorimetry and rheology and for a total copolymer concentration fixed at 20 wt%. The aim of this investigation is to determine the interplay between micellization and macroscopic gelation of the mixed solutions. Micro-DSC reveals the formation of two distinct populations in F127/F68 mixtures during heating and subsequent cooling of the solutions. The enthalpies of micellization of each copolymer and the respective onset temperatures remained constant after mixing indicating the predominance of two independent processes of micellization in the mixtures. The F127 exhibits a crystallization transition, at a distinct temperature which persists, but increases in the mixtures with concentrations higher than 10 wt%. Rheological measurements were performed during heating ramps or after maturation periods versus frequency. They showed two types of gelation transitions: either a steep increase of the storage and the loss moduli, which corresponds to the crystallization temperature of the F127 micelles or a progressive jamming transition when no crystal can form. Maturation process has a major effect on the rheological properties of the mixed gels, possibly related to local rearrangements of the two micellar phases. This investigation highlights the unique features of the binary pluronic(®) mixtures, compared to dilution effects of single component aqueous solutions.
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