Solvent relaxation of swelling PNIPAM microgels by NMR

The dynamics of solvent inside swelling microgel particles is studied using 1H NMR. Solvent relaxation is sensitive to the microgel volume transition after temperature changes, showing the effect of the local environmental conditions on solvent confinement. A microscopic view of the volume phase transition of microgel particles is then shown. NMR measurements indicate that solvent dynamics is controlled only by the swelling degree of the microgel particles. Along the volume phase transition, the collapse of the polymer network restricts the mobility of bound water defining the confinement state. The effect of the amount of microgel polymer on the solvent molecules mobility is also investigated. In the collapsed state, water molecules still exist inside the microgel particles, being strongly confined within the polymer network. Despite that there are not many bound molecules under this circumstance, they contribute so strongly to the total averaged relaxation time that the behaviour is controlled by the presence of those. On the other hand, the solvent molecules were found to be weakly constrained for swollen microgels.