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 1 H single-pulse as well as one-and twodimensional combined rotation and multiple-pulse spectroscopy (CRAMPS) 1 H 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 1 H 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 1 H− 1 H experiments. The rate matrix calculation yields a host−guest distance that is 0.3 Å ́shorter for pyrocatechol than for phenol. Independent calculations by DFT indicate that the host−guest distance is 0.2 Å ́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.

Search for the Origin of Discrepancies in Osmotic Measurements of the PNIPAM - Water System

“…Such local interactions have already been detected in homopolymer PNIPAM microgels by FTIR and Raman spectroscopy [60], while with foreign probe molecules, such as phenol, they are visualised directly by 1 H CRAMPS solid-state NMR [61].…”

Interactions in aromatic probe molecule loaded poly(N-isopropylacrylamide) hydrogels and implications for drug delivery

“…The gels were studied above LCST conditions, where the otherwise freely moving organic chains form thick hydrophobic walls that still confine the swelling liquid [34]. The single pulse spectra (Fig.4) state NMR measurements [14][15] proved that the phenol molecules are connected by strong H-bonds to the acrylamide side groups. By contrast, interactions between dopamine and the gel molecules could not be detected [16].…”

“…Molecules with phenolic OH can influence the swelling behaviour of the PNIPA in various ways [12][13][14][15][16]. While some molecules have only a slight effect or no effect at all, other guest molecules can change the transition temperature even at low additive concentrations [16,17].…”

“…The temperature shift depends on the concentration and chemical structure of the additive. Several molecules and ions reduce the LCST [12][13][14][15][16].…”

“…The associative behaviour between phenol and PNIPA was also demonstrated by smallangle neutron scattering (SANS) and solid state NMR techniques [12][13][14]. By contrast, 1 H solid-state NMR results showed that dopamine does not associate with the polymer chains [15].…”