Characterization of the phase transition mechanism of P(NiPAAm-co-AAc) copolymer hydrogel using 2D correlation IR spectroscopy.

Abstract A thermo-responsive polymer, poly(N-isopropylacrylamide) (PNiPAAm), was copolymerized with acrylic acid (AAc) in this study. Its phase transitions during the heating and cooling processes were investigated using IR spectroscopy, principal component analysis (PCA), and two-dimensional correlation spectroscopy (2D-COS). During the heating process, the hydrogen bonding between side chain in P(NiPAAm-co-AAc) copolymer hydrogel and H2O was broken first, and then the formation of the intramolecular interaction in P(NiPAAm-co-AAc) copolymer hydrogel occurred. However, unlike the heating process, intensities of bands in the C H stretching region were changed before those in the C O stretching including the N H bending region during the cooling process. The results indicate that the phase transition of P(NiPAAm-co-AAc) copolymer hydrogel is an irreversible process at the molecular levels.