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 CO 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 CO group is estimated to form the intra- or interchain hydrogen bonding, and the remaining CO 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.

The phase transitions of copolymers of N-isopropylacrylamide (iPA) with acrylamide (PiPA-AAm) or acrylonitrile (PiPA-AN) in water have been observed by means of Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The effects of both incorporation of these comonomer units and addition of salts to their aqueous solutions on the phase behaviors have been investigated. The positions of IR bands due to iPA and the comonomer units critically shift at lower critical solution temperatures (LCST) of these copolymers. The phase transition leads shifts of the amide II, C−H stretching, and C−H bending bands to lower wavenumbers and a shift of the amide I band to a higher wavenumber. The C⋮N stretching band (ν(C⋮N)) of AN units in PiPA-AN shifts to a lower wavenumber. These shifts of the IR bands indicate that corresponding chemical groups experience dehydration to some extent upon the transition. The ν(C⋮N) band is composed of two components due to the C⋮N groups that form a hydrogen bond with water (2247 cm-1) and dehydrated C⋮N groups (2241 cm-1). Analysis of the band by using a curve fitting method shows that almost all C⋮N groups form hydrogen bonds with water in the coil state and 90% of the C⋮N groups are dehydrated in the globule state. The hydration of the C⋮N group in the globule state is quite different from that of the amide CO group, 85% of which forms hydrogen bonds with water even in the globule state. DSC measurements show that the heat of phase transition (Δ H) linearly decreases with an increase in the LCST of the copolymers, meaning that Δ H is essentially determined by the LCST of the copolymer, which in turn depends on its composition. The structure of water surrounding the polymer chains as well as the balance between hydrophilicity and hydrophobicity of the polymers as a whole has importance to the phase transition behaviors of iPA copolymers.