Changes in the Hydration States of Poly(N-alkylacrylamide)s during Their Phase Transitions in Water Observed by FTIR Spectroscopy

Abstract

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 (T0) from a spectrum at T gives an IR difference spectrum (ΔAT-T0). The magnitudes of ΔAT-T0 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 (CO···D−O−D, polymer−water hydrogen bond) and with the amide N−D groups on the polymers (CO···D−N, polymer−polymer hydrogen bond), respectively. The molar fractions of the CO···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 CO···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 CO···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 CO 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 (ΔH) 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 and water.