Host–Guest Interactions in Poly(N-isopropylacrylamide) Hydrogel Seen by One- and Two-Dimensional 1H CRAMPS Solid-State NMR Spectroscopy

Abstract

Host–guest interactions in poly(N-isopropylacrylamide) (PNIPA) hydrogels with pyrocatechol were investigated by 1H single-pulse as well as one- and two-dimensional combined rotation and multiple-pulse spectroscopy (CRAMPS) 1H solid-state NMR techniques. Differential scanning calorimetry (DSC), swelling measurements, and density functional theory (DFT) calculations were used as supplementary methods. The results are compared with the phenol–PNIPA system to explore possible similarities or differences in the effects of the two phenolic guest molecules on the temperature-induced volume phase transition of the hydrogel. Both small molecules shift the lower critical solution temperature (LCST) of the hydrogel, but with phenol the transition is broader. This observation may reflect changes in the 1H signal from residual water trapped in cavities in the hydrogel above the LCST. Although in the pyrocatechol-containing hydrogel only two signals are observed, the phenol-containing hydrogel displays numerous signals...