Understanding Thermal Behavior of Poly(ethylene glycol)-block-poly(N-isopropylacrylamide) Hydrogel Using Two-Dimensional Correlation Infrared Spectroscopy.

In this study, one of the thermoresponsive polymers, block copolymer consisting of poly(ethylene glycol) and poly(N-isopropylacylamide), was investigated using Fourier transform infrared (FT-IR) spectroscopy, principal component analysis (PCA), and two-dimensional correlation spectroscopy (2D-COS). The apparent trend of the spectral changes in the temperature-dependent FT-IR spectra of poly(ethylene glycol)-block-poly(N-isopropylacylamide) (PEG-b-PNiPAAm) hydrogel during the heating process looks similar to that during the cooling process. The results of the PCA and 2D-COS, however, clearly indicate an irreversible phase transition mechanism of PEG-b-PNiPAAm hydrogel during the heating and cooling processes. It has been also shown that PEG affects the phase transition mechanism of PEG-b-PNiPAAm hydrogel, especially during the heating process. Consequently, we can successfully determine the phase transition temperature and the mechanism of PEG-b-PNiPAAm hydrogel during the heating and cooling processes using PCA and 2D-COS, respectively.