Attenuated total reflectance/Fourier transform infrared studies on the phase-separation process of aqueous solutions of poly(N-isopropylacrylamide)

Temperature-induced phase separation of poly(N-isopropylacrylamide) in aque ous solutions was studied by attenuated total reflectance (ATR)/Fourier tra nsform infrared spectroscopy. The main objectives of the study were to unde rstand, on a molecular level, the role of hydrogen bonding and hydrophobic effects below and above the phase-separation temperature and to derive the scenario leading to this process. Understanding the behavior of this partic ular system could be quite relevant to many biological phenomena, such as p rotein denaturation. The temperature-induced phase transition was easily de tected by the ATR method. A sharp increase in the peaks of both hydrophobic and hydrophilic groups of the polymer and a decrease in the water-related signals could be explained in terms of the formation of a polymer-enriched film near the ATR crystal. Deconvolution of the amide I and amide II peaks and the O-H stretch envelope of water revealed that the phase-separation sc enario could be divided, below the phase-separation temperature, into two s teps. The first step consisted of the breaking of intermolecular hydrogen b onds between the amide groups of the polymer and the solvent and the format ion of free amide groups, and the second step consisted of an increase in i ntramolecular hydrogen bonding, which induced a coil-globule transition. No changes in the hydrophobic signals below the separation temperature could be observed, suggesting that hydrophobic interactions played a dominant rol e during the aggregation of the collapsed chains but not before. (C) 2001 J ohn Wiley & Sons, Inc.