Effect of PPO/PEO Ratio on the Phase Behavior of Reverse Pluronics

Systems 10R5 [(PPO)8 – (PEO)22 – (PPO)8], 17R4 [(PPO)14 – (PEO)24 – (PPO)14] and 31R1 [(PPO)26 – (PEO)7 – (PPO)26], selected based on their different molecular weights and PPO/PEO ratios, were studied in aqueous solution in order to investigate the effect of the variation of the copolymer hydrophobicity on the phase behavior of the system. Measurements and analysis of density, sound velocity, viscosity, and surface tension of the solutions were carried out, as well as confirmatory methods such as direct observation, dynamic light scattering and dynamic and shear rheological properties of the systems. From the density and sound velocity results, molar volume, apparent adiabatic compressibility, and hydration profiles were calculated. The zones of random networks, flower-like micelles and micellar networks were delineated for the three systems and plotted within phase diagrams in the concentration ranges of 0.1 - 90 wt.% and temperatures from 6 to 70°C. The results also show that the 31R1/water system does not form flower-like micelles. In contrast, the 17R4/water and 10R5/water systems form them in a small interval each, which moves at higher concentration and temperature values with the PPO/PEO ratio, as well as the zones where the micellar networks and their transition are located. The study of the physicochemical behavior of the three systems is reported, providing important information for future research to evaluate their use as hydrophobic nanocarriers, either individually or in combination with another copolymer.