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

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 CH stretching region were changed before those in the CO stretching including the NH 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.