Mechanism of PEO–PPO–PEO micellization in aqueous solutions studied by two-dimensional correlation FTIR spectroscopy

Abstract

The micellization mechanism of PEO–PPO–PEO block copolymer in aqueous solutions was studied by two-dimensional correlation FTIR spectroscopy. The 1400–1000 cm −1 region was investigated, involving the stretching vibrations of ether band, C–H wagging vibrations of EO methylene groups and C–H symmetric deformation vibrations of PO methyl groups. In the 2D correlated spectra, the hydrous and anhydrous state of the ether band, PO methyl groups, and the two conformations of EO methylene groups were observed. Molecules with different PO lengths and increasing molecular weight were investigated to determine the sequence of association of the separate groups. During temperature-induced micellization, the following changes were detected: firstly, EO methylene groups changed from a gauche state to a trans state; secondly, conformational transitions led to the dehydration of hydrated methyl groups; next, the hydrogen bonding between C–O band and water diminished; and finally, dehydrated groups approached to form hydrophobic cores, resulting in micelle formation. From this variation in the sequence of group associations, it is concluded that aggregates of unimers first formed, then hydrophobic cores formed through the hydrophobic interaction from dehydrated PPO blocks, and proper micelles eventually evolved. The temperature-induced conformational changes are suggested the reason for micellization.