Association Behavior of PEO–PPO–PEO Block Copolymers in Water or Organic Solvent Observed by FTIR Spectroscopy

Abstract

Fourier transform infrared (FTIR) spectroscopy is applied to investigate the association behavior of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers in water or organic solvent as a function of temperature. In aqueous solutions, when the temperature approaches the critical micellization temperature, the antisymmetric C-H stretching vibration of methyl groups shifts toward lower wavenumber. It indicates that the methyl groups are experiencing a progressively less polar environment and the interaction of the methyl groups with water molecules is weakened by heat. At higher temperatures, the symmetric deformation vibration of methyl groups is composed of two bands, which are related to hydrated and dehydrated state. The proportion of the dehydrated methyl groups of Pluronic polymers increases with an increase of temperature, which means the exclusion of water from the micelle cores and formation of micelles with a denser PPO core at higher temperatures. A more hydrophobic Pluronic polymer would form a micelle in water with a higher proportion of dehydrated methyl groups. No changes in FTIR spectra correspond to temperature dependent gelation of Pluronic F127 in water. Based on FTIR spectroscopic results of Pluronic P104, the PPO blocks are hydrated only in a condition of lower temperature and lower polymer concentration. In the hydration process, PEO blocks would be hydrated preferentially where H2O molecules are bound easily to PEO segments. The water-induced reverse micelle formation of Pluronic L92, in p-Xylene solution has also been investigated by FTIR spectroscopic technique. The reason of reverse micelle formation is explained by the strengthening of the intermolecular interactions of copolymers, which was originated from the interactions of solubilized water.