FTIR Spectroscopic Investigation of Effects of Temperature and Concentration on PEO−PPO−PEO Block Copolymer Properties in Aqueous Solutions

The effects of temperature and concentration on poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) block copolymer properties in aqueous solutions were studied by Fourier transform infrared (FTIR) spectroscopy. The temperature-dependent changes of some bands were used to denote the critical micellization temperatures (cmt). It was deduced that the appearance of the symmetric deformation band of anhydrous methyl groups in the spectra of aqueous Pluronic F127 solutions at temperature below the cmt means the existence of a hydrophobic microenvironment. The appearance of the symmetric deformation band of hydrated methyl groups at temperature above the transition region indicates that the micellar core must contain a certain amount of water. The proportion of anhydrous methyl groups was increased during the micellization process with the decrease of the proportion of hydrated methyl groups. The cmt values of Pluronic F127 in aqueous solutions were decreased with an increase of concentration. In the meantime, the water content in the micellar core was also decreased. The effect of gelation on the spectral behavior of Pluronic F127 in aqueous solutions is too weak to be observed by the FTIR technique. It was found that a certain part of the PEO segments of Pluronic F127 in aqueous solutions are in the crystalline state, which is not caused by micellization and gelation. The proportion of PEO segments in the crystalline state was increased with an increase of temperature.