SANS study on self-assembled structures of Pluronic F127 triblock copolymer induced by additives and temperature

In the temperature range of 303-333 K, the self-assembled structures of a mixture of Pluronic F127 triblock copolymer [PEO₁₀₆PPO₇₀PEO₁₀₆; PEO is poly(ethylene oxide) and PPO is poly(propylene oxide)] and an organic derivative, 5-methyl salicylic acid (5mS), in aqueous solution have been investigated using small-angle neutron scattering (SANS). Above a 5mS concentration of 1.93 g l^-1, the F127-5mS mixture solution became cloudy with a blue colour arising from the Tyndall effect, indicating that large polymer aggregates had formed in the mixture solution. SANS measurements showed that the self-assembled structure of the F127-5mS mixture transformed from a spherical to a cylindrical micelle with increasing the concentration of 5mS in the temperature range of 303-323 K. When the 5mS concentration was increased to 3.3 g l^-1, the self-assembled structure of the F127-5mS mixture at 333 K underwent an additional phase transition from a cylindrical to a spherical micelle of large size at a 5mS concentration of 2.75 g l^-1, although its self-assembled structure changed from a spherical to a cylindrical micelle at a 5mS concentration of 1.93 g l^-1 as well. The phase transitions are explained by the variation of the mass fraction of the hydrophilic part of F127 and the coupled effect of the limited solubility and the strong tendency to bind with amphiphilic molecules of 5mS. Using a simple material balance equation and the structural information obtained from SANS model analyses, the numbers of D₂O and of 5mS molecules in the core and corona regions are calculated. This result can provide a simple and easy way to prepare various nanostructures using a Pluronic triblock copolymer in aqueous solution and may be very useful for practical applications of a Pluronic polymer such as various nanobuilding blocks or nanotemplates.