2H and 17O NMR studies of solvent dynamics related to the cononsolvency of poly(N-isopropyl acrylamide) in ethanol–water mixtures

Abstract

Although the thermoresponsive polymer poly(N-isopropylacrylamide) (pNIPAM) is well soluble in both ethanol and water, it shows a miscibility gap in ethanol–water mixtures, an effect termed cononsolvency. We use ²H and ¹⁷O nuclear magnetic resonance (NMR) together with appropriate isotope labelling to selectively study reorientation dynamics of ethanol and water related to the cononsolvency effect over the whole range of solvent compositions from pure ethanol to pure water. At low ethanol concentrations (≤30 vol%), spin–lattice (T₁) and spin–spin (T₂) relaxation times show a step-like decrease when heating across the lower critical solution temperature for the respective solvent composition. However, the drop is notably stronger for ethanol (²H NMR) than for water (¹⁷O NMR) in the solvent mixtures. These observations show that the coil-to-globule transition of pNIPAM is accompanied by a slowdown of average solvent dynamics, which is more prominent for ethanol than for water. The different degree of slowdown of the solvent components implies that preferential interaction with the polymer plays a significant role for cononsolvency. Field-cycling relaxometry reveals a low-frequency T₁ dispersion above the coil-to-globule transition, indicating that the average solvent dynamics is slower because a major free solvent fraction is accompanied by a minor bound solvent fraction, which shows strongly retarded dynamics. From intermediate to high ethanol concentrations (>50 vol%), the T₁ and T₂ relaxation times yield no evidence for significant changes in ethanol and water dynamics when crossing an expected upper critical solution temperature.