Structure and conformation in mixtures of methyl-terminated poly(ethylene oxide) and water. Principal component analysis and band fitting of infrared absorptions.
Infrared absorption spectra of aqueous mixtures of poly(ethylene oxide) dimethyl ether (PEO), with a number average molecular weight of 500 and a water volume fraction between 0 and 0.96 have been recorded at 18, 26, and 40 degrees C. Composition and temperature were found to influence the intensity and wavenumber of PEO-related absorptions. Spectral band fitting of the CH(2) wagging/twisting region of the spectra showed that the proportion of gauche and trans conformers of the C-C and C-O bonds varied with composition. Principal component analysis (PCA) of the PEO-related absorptions showed that the configuration with a gauche conformation about the C-C bond and trans conformation about the C-O bond was favored at all the compositions and temperatures tested but was decreasingly favored as the temperature was increased. PCA also enabled the assignment of spectral features associated with a specific conformation of either the C-C or C-O bond. Water-related O-H stretching absorptions indicated that hydrogen bonds were formed between water and PEO ether oxygen atoms. Variation in the wavenumber and integrated absorbance of band-fitted contributions to the water O-H stretching region suggest that the degree of water-water hydrogen bonding changes with composition. At water volume fractions between 0.2 and 0.6, almost all of the water appeared to be strongly hydrogen bonded. The smallest infrared absorption intensity from weakly hydrogen bonded water was recorded for mixtures with a water volume fraction of around 0.4. Such PEO-water mixtures had the greatest proportion of trans conformers about the C-O bond, and there were unexpectedly large PEO and water absorption intensities. Correlations between the average PEO configuration and the water O-H stretching absorptions suggest that, while the conformation about the C-C bond was mainly determined by the polarity of the mixture, the conformation about the C-O bond was influenced by the formation of a strongly hydrogen bonded PEO-water network.
Shephard
Jacob J
JJ
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand.
Bremer
Phil J
PJ
McQuillan
A James
AJ