Change in Solvation of Poly(N,N-diethylacrylamide) during Phase Transition in Aqueous Solutions As Observed by IR Spectroscopy

Phase transitions of poly(N,N-dimethylacrylamide) (PdEA) in water and methanol/water mixture were investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of the solutions measured as a function of temperature exhibited critical changes upon the phase transition. The amide I band of PdEA consists of three components centered at 1638, 1619, and 1599 cm-1, which can be assigned to free carbonyl groups and those connected to one and two water molecules through hydrogen bonds, respectively. The relative areas of the 1619 and 1599 cm-1 components decrease and that of the 1638 cm-1 component increases above the phase transition temperature (Tp), but the change is relatively small and most of the amide CO groups remain hydrated even above Tp. The C−H stretching bands shift toward lower wavenumbers that are close to those measured in the neat solid state, indicating that the alkyl groups are almost fully dehydrated above Tp. In the methanol/water mixture, Tp is almost independent of methanol concentration up to ca. 20% and increases steeply above ca. 30%. The heat of transition (ΔH) decreased linearly with an increase of methanol concentration up to 30% and is nearly zero above 40%. The areas of the 1599 and 1638 cm-1 bands decrease with an increase in methanol concentration with a slight change in the 1619 cm-1 band. The result suggests that methanol replaces parts of the CO···H−O−H hydrogen bonds to form a CO···H−O−CH3 hydrogen bond, and its bulky methyl group prevents the carbonyl group to form the second hydrogen bond that is responsible for the 1599 cm-1 band.