Relationship between the coil-globule transition of an aqueous poly(N-isopropylacrylamide) solution and structural changes in local conformations of the polymer

Structural changes in the local conformation of poly(N-isopropylacrylamide) (PNiPA) during the thermally and solvent-induced coil-globule transitions in an aqueous solution were studied by using attenuated total reflection / infrared (ATR/IR) spectroscopy combined with density functional theory (DFT) calculation. DFT calculation makes it possible to connect the spectral changes observed during the transitions with the structural changes of the local conformation of polymer chains. The results suggest that some of the intramolecular C=O...H-N hydrogen bonds of amide groups are broken, and the changes in local conformations occur during the coil-globule transitions of PNiPA. In this paper, an overview of our recent studies on the coil-globule transitions of PNiPA is given for introducing a new idea that may explain the stimulus sensitivities of PNiPA in solutions; the solubility of segments concerning with the local conformation of repeating monomer units is changed by an external perturbation, and then the polymer system shows the coil-globule transition.