NMR studies of water dynamics during sol-to-gel transition of poly (N-isopropylacrylamide) in concentrated aqueous solution

•Change in water dynamics of PNIPAM solution during sol-to-gel transition is studied.•Several states of water dynamics is revealed by 2D 2H T1-T2 NMR relaxation.•An interaction model between water and PNIPAM during the phase transition is proposed.

The focus of this work was on the significant changes in the water dynamics of aqueous poly (N-isopropylacrylamide) (PNIPAM) solution during sol-to-gel transition. Through the use of NMR (particularly two-dimensional 2H NMR T1-T2 relaxation) and rheology, we were able to show that below 34° C fast exchange occurs among free water and water molecules adsorbed on the surface of PNIPAM molecules. At 34° C, PNIPAM becomes aggregated; most of the water molecules are trapped in the PNIPAM aggregates, where water molecules with different dynamics are found. Above 34° C, PNIPAM molecules aggregate further to form a gel network; the free bulk water then becomes dominant at this stage. On the basis of these observations, a model where water molecules interact with PNIPAM in different ways during the transition was proposed. We believe that our experimental approach provides new information and fresh perspectives on the sol-to-gel transition of PNIPAM.

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