FTIR Spectroscopic Study on Effects of Temperature and Polymer Composition on the Structural Properties of PEO−PPO−PEO Block Copolymer Micelles

Abstract

Fourier transform infrared spectroscopy was used to study the effects of temperature and polymer composition on the structural properties of poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) block copolymer micelles in D2O solutions. The intrinsic probe, methyl groups in PPO blocks, is very sensitive to the local environment and used to characterize the aggregation process. When the temperature approaches the critical micellization temperature, the antisymmetric C−H stretching band of methyl groups and the symmetric deformation band of methyl groups shift toward lower frequencies. It indicates that the methyl groups are experiencing a progressively less polar environment and the interaction of methyl groups with water molecules is weakened by heat. At higher temperatures, the symmetric deformation mode of methyl groups is composed of two bands, which are assignable to hydrated (the band at 1378 cm-1) and dehydrated states (the band at 1373 cm-1). Part of methyl groups are dehydrated, and a significantly nonpolar microenvironment is created. The proportion of dehydrated methyl groups in Pluronic micelles increases with an increase of temperature, which means exclusion of water from the micellar core and formation of micelles with a denser PPO core at higher temperatures. The composition of Pluronic polymers also has a strong effect on the structural properties of the micelles. A more hydrophobic Pluronic polymer would form micelles with a higher proportion of the dehydrated methyl groups.