Changes in the Hydration States of Poly(N-n-propylmethacrylamide) and Poly(N-isopropylmethacrylamide) during Their Phase Transitions in Water Observed by FTIR Spectroscopy

Phase transitions of poly(N-n-propylmethacrylamide) (PnPMA) and poly(N-isopropylmethacrylamide) (PiPMA) in H2O and D2O have been investigated by using Fourier transform infrared (FTIR) spectroscopy. IR spectra of these polymer solutions have been collected as a function of temperature in both heating and cooling processes. The positions of IR bands due to the alkyl and amide groups of these polymers critically shift at their phase transition temperatures (Tp). The amide II, C−H stretching and C−H bending bands undergo red shift, whereas the amide I band undergoes a blue shift upon the coil-to-globule transitions. The amide I bands of PnPMA and PiPMA measured below their Tp consist of three (1642, 1611, and 1586 cm-1) and two (1635 and 1606 cm-1) Gaussian components, respectively. The areas of the 1642 cm-1 component of PnPMA and the 1635 cm-1 component of PiPMA, which can be attributed to the carbonyl group that forms a hydrogen bond with an amide N−D group (CO···D−N), increase with temperature while the phase transitions of the polymer solutions proceed. Molar fractions of the CO···D−N species are estimated to be 0.40 and 0.42 in the globule states of PnPMA and PiPMA, respectively, which are much larger than those of corresponding poly(N-alkylacrylamide)s, i.e., poly(N-n-propylacrylamide) (PnPA) and poly(N-isopropylacrylamide) (PiPA). We attribute greater thermal hystereses of PnPMA and PiPMA than those of PnPA and PiPA, which are characterized by a large difference between Tp for a heating process and Tp for a cooling process, to higher degrees of the polymer−polymer hydrogen bonding in the globule states of PnPMA and PiPMA. Although Tp of each polymer solution greatly depends on polymer concentration, the molar fraction of the CO···D−N species and the wavenumbers of C−H stretching bands at the end point of its phase transition are independent of the concentration (5−20 wt %). The result indicates that the local concentration of polymer chains in each globule is independent of the total polymer concentration as it is expected from its phase diagram.