Conformational Change of Poly(<i>N</i>-isopropylacrylamide) during the Coil−Globule Transition Investigated by Attenuated Total Reflection/Infrared Spectroscopy and Density Functional Theory Calculation

A conformational change in the coil-globule transition of poly(<i>N</i>-isopropylacrylamide) (PNiPA) was investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) method and density functional theory (DFT) calculations. ATR/IR spectra of PNiPA in an aqueous solution change dramatically in the vicinity of the coil-globule transition temperature (θ temperature). Below the θ temperature, unimodal peaks are observed at 1624 cm^-1 in the amide I region and at 1562 cm^-1 in the amide II region, respectively. Above the θ temperature, a new peak appears abruptly near 1653 cm^-1 in the amide I region and the amide II band shifts gradually to a lower frequency by 6 cm^-1. In the amide III region, the relative intensity of a band at 1173 cm^-1 is weaker than that of a band at 1155 cm^-1 at lower temperatures, but it becomes larger during the coil-globule transition of PNiPA. DFT calculation for dimer models of PNiPA suggests that the amide I band at 1624 cm^-1 is assigned mainly to a stretching vibration of the C═O group that forms a strong hydrogen bond with the N-H bond of a neighboring amide group. The band at 1653 cm^-1 observed above the θ temperature may be due to a free C═O group. It is, therefore, suggested that some of the intramolecular hydrogen bonds between neighboring amide groups are broken during the coil-globule transition. Furthermore, it is deduced from the DFT calculation that the relative intensity of the bands at 1173 and 1155 cm^-1 in the amide III region reflects the population change in the gauche and trans conformations in the main chain during the coil-globule transition.