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

International audienceHost-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. In the case of phenol, the formation of water cavities is a more complex process, although reaching the equilibrium state requires a very long time (about 40 days). The CRAMPS protocol increases the resolution and makes it possible to perform two-dimensional correlation 1H-1H experiments. The rate matrix calculation yields a host-guest distance that is 0.3 Ǻ shorter for pyrocatechol than for phenol. Independent calculations by DFT indicate that the host-guest distance is 0.2 Ǻ shorter for pyrocatechol. The satisfactory agreement between the results from different methods demonstrates that the power of this NMR method extends not only to crystalline, but also to macroscopically amorphous systems possessing only local order