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.

Phase transitions of poly(N-n-propylacrylamide) (PnPA), poly(N-isopropylacrylamide) (PiPA), and poly(N-cyclopropylacrylamide) (PcPA) in H2O and D2O were investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of these solutions were measured as a function of temperature (T) both in the heating and cooling processes. Subtraction of a spectrum at starting temperature (T 0) from a spectrum at T gives an IR difference spectrum (Δ A T - T 0 ). The magnitudes of Δ A T - T 0 for IR absorption bands attributable to the alkyl and amide groups of these polymers critically increased at their lower critical solution temperatures (LCSTs). Redshifts of the amide II, C−H stretching, and C−H deformation bands were observed during the coil-to-globule transitions of these polymers. The amide I‘ bands of N-deuterated amide groups (−COND−) in PnPA and PiPA measured below their LCSTs could be fitted with single Gaussian components centered at 1630 and 1625 cm-1, respectively. In contrast, two components were observed in the solutions of PnPA (1630 and 1650 cm-1) and PiPA (1625 and 1650 cm-1) above their LCSTs. The low-frequency and high-frequency components of the amide I‘ bands of PnPA and PiPA can be attributed to the carbonyl groups that form hydrogen bonds with water (CO···D−O−D, polymer−water hydrogen bond) and with the amide N−D groups on the polymers (CO···D−N, polymer−polymer hydrogen bond), respectively. The molar fractions of the CO···D−N species in the globule states were estimated to be 0.30 and 0.13 for PnPA and PiPA, respectively. Although the phase transition temperatures of PnPA solutions depend on the polymer concentration, the fractions of the CO···D−N species are independent of the concentration (5−20 wt %), suggesting that the states of globules of PnPA at different polymer concentrations are identical with respect to hydration and water contents. The change in the profiles of the amide I‘ band of PcPA during the phase transition was quite different from those of PnPA and PiPA. Although the amide I‘ band of PcPA observed below the LCST was composed of a single component that could be assigned to the CO···D−O−D species at 1634 cm-1, the IR absorptions at the higher (ca. 1667 cm-1) and lower (ca. 1600 cm-1) wavenumbers increased above the LCST. The hydrogen bonding states about the CO group of PcPA in the globule state are suggested to be different from those of PnPA and PiPA. As for effects of salts on the phase transition, although the LCSTs of the polymers linearly decreased and the heats of transition (Δ H) slightly decreased with an increase in NaCl concentration, NaCl had no influence on the profiles of the IR spectra of these polymers. Moreover, linear relationship between the parameters that represent the strength of anion−water interactions and the LCSTs of the polymer solutions containing one of the potassium halides (KF, KCl, KBr, and KI) was observed. The effects of ions on the LCSTs of the polymers can be explained by the strength of the interactions between the ions ...

Phase transitions of poly(N-vinylcaprolactam) (PVCL) and poly(N-vinylpyrrolidone) (PVPy) in H2O and D2O were investigated by turbidimetry, differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy. The phase diagram of PVCL solution is binodal with a lower critical solution temperature (LCST) of 32.5 °C and a critical concentration of about 5 wt %. PVPy also undergoes a phase separation around 30 °C in aqueous 1.5 M KF solution. The IR spectra of PVCL and PVPy in water critically change at the phase transition temperature (T p). Above T p the C−H stretching and the C−H bending bands shift to lower wavenumbers. The amide I band of PVCL is divided into four components, and the intensities of the 1565, 1588, and 1625 cm-1 components increase and that of the 1610 cm-1 component decreases while the phase transition proceeds. Addition of KCl and KF lower T p of PVCL, whereas KI and KBr raise T p. Addition of methanol gradually raises T p up to ca. 40 vol % and steeply raises T p at > 40 vol %. Although these salts do not alter the profiles of IR spectra of PVCL, methanol induces a change in the amide I band.

The phase transitions of poly(vinyl methyl ether) (PVME) in H2O and D2O have been investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of PVME solution are measured as a function of temperature, and changes in the spectra induced by the phase transition are observed. The IR bands attributable to the vibration modes of backbone and methoxy side group exhibit critical changes at a lower critical solution temperature (LCST). Heating of the solution above the LCST leads shifts of C−H stretching bands toward lower wavenumber during a high-wavenumber shift of C−O stretching band (ν(C−O)). Among the C−H stretching bands, symmetric and antisymmetric C−H stretching bands due to the methyl group (νs(CH3) and νas(CH3)) shift to the largest extent (20 cm-1). The νs(CH3) band is composed of two components centered at 2839 and 2820 cm-1, which are assignable to hydrated and dehydrated methyl species, respectively. The molar fraction of the hydrated methyl species is close to 1 below 36 °C and is nearly 0 above 40 °C, insisting that most of the methyl groups are dehydrated above the LCST, whereas the peak position of the ν(C−O) band indicates only partial dehydration of the ether group above the LCST. Though PVME concentration dependence of the peak positions of the νs(CH3) and the ν(C−O) bands measured below the LCST shows that dehydration of the methyl group and the ether oxygen initiates at different water/monomer ratios (methyl, 5.5; ether, 2.8), the dehydration of these groups simultaneously proceeds in the course of the phase transition.

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 CdO 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 T p. 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 (∆H) 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 CdO‚‚‚H-O-H hydrogen bonds to form a CdO‚‚‚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.