Change in Hydration State during the Coil−Globule Transition of Aqueous Solutions of Poly(N-isopropylacrylamide) as Evidenced by FTIR Spectroscopy

Abstract

Coil−globule transition of poly(N-isopropylacrylamide) (PIPA), followed by intermolecular association in H2O and D2O, was investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of the solutions were measured as a function of temperature, and spectral changes induced by the transition were observed. The intensities of the difference IR bands due to the vibrational modes of isopropyl and amide groups critically increased at the lower critical solution temperature (LCST). Heating of the PIPA solutions above the LCST led shifts of the amide II, C−H-stretching, and C−H-bending bands to lower wavenumbers during a shift of the amide I band to a higher wavenumber. The amide I band of the PIPA observed below the LCST could be fitted with a single component centered at 1625 cm-1, whereas two components (1625 and 1650 cm-1) were necessary to fit the band above the LCST. These components may be assigned to the CO group which is bound to water molecules as the solvent (1625 cm-1) and to the N−H in the side chain (1650 cm-1) via hydrogen bonding. About 13% of the CO group is estimated to form the intra- or interchain hydrogen bonding, and the remaining CO group forms a hydrogen bond with water in the globule state. Red shifts of the antisymmetric and symmetric C−H-stretching bands for the isopropyl group also indicate dehydration of the hydrophobic moiety during the transition. A mechanistic hypothesis of the coil−globule transition, insisting that above the LCST the polymer chain is dehydrated and hydrophobic interaction between isopropyl groups induces the collapse of the chain, is strongly supported. Though the presence of metal halides (NaCl, KCl, KBr, KI) lowered the LCST, the profiles of IR difference spectra of the PIPA in those solutions were similar to those measured in pure water. Specific interactions between the amide groups on the polymer chain and the ions were unlikely. Importance of the structure and properties of water in the solution to determine the LCST of the polymer solutions is suggested.