NMR Methods to Study Effects of Additives on Phase Separation of Thermoresponsive Polymer

NMR methods including H-1 NMR spectroscopy, 2D NOESY spectroscopy, pulsed-field gradient (PFG) NMR as well as spin-spin relaxation times T-2 measurement were employed to study the effects of one surfactant (sodium n-dodecyl sulfate, SDS) and three inorganic salts (NaCl, Na2SO4, Na3PO4) on the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution. The molecular interactions between SDS and PNIPAM during the coil-to-globule transition of PNIPAM have been accurately observed through 2D NOESY spectra and PFG NMR. Through the combination of H-1 NMR spectra and T-2 measurements, the salts effects have been found not only shift the phase transition of PNIPAM but also lead to difference in the final conformational state of PNIPAM in aqueous solution after phase transition. Diffusion coefficients of Na+ cations support the idea that the major effect of salts on phase transition of PNIPAM in aqueous solution is produced by anions.