Chain Collapse and Revival Thermodynamics of Poly(N-isopropylacrylamide) Hydrogel

Two-dimensional correlation infrared spectroscopy (2DIR) and a newly developed perturbation correlation moving window (PCMW) technique were employed to study the precise chain collapse and revival thermodynamic mechanism of poly(N-isopropylacrylamide) (PNIPAM) hydrogel. Both Boltzmann fitting and PCMW had figured out the volume phase transition temperature for heating and cooling processes to be about 35 and 33.5 degrees C, respectively, close to the results obtained from DSC. Furthermore, determination of the isosbestic points for v(CH3) and v(C=O) overlaid spectra showed that the chain collapse of PNIPAM hydrogel took place along with some intermediate states or a completely continuous phase transition while the chain revival occurred with only conversion between two single states. Finally, 2Dcos discerned all the sequence of group motions of PNIPAM hydrogel, indicating that in the heating process, PNIPAM hydrogel occurred to collapse along the backbone before water molecules were expelled outside the network, while in the sequential cooling process, PNIPAM hydrogel had water molecules diffusing into the network first before the chain revival along the backbone occurred.