Advanced 1H solid-state NMR spectroscopy on hydrogels, 1 -: The effect of hydrogen bonding in the collapse of poly(methacrylic acid) (PMAA) hydrogels

The nature of the pH dependent collapse of poly(methacrylic acid) (PMAA) hydrogels is investigated using recent H-1 solid-state NMR methods. In aqueous solution, PMAA changes from an expanded conformation at high pHs to a compact contracted form at low pHs, where hydrogen bonds play a central role. In solid-state H-1 NMR spectra, recorded under fast magic angle spinning (MAS), dried PMAA samples previously collapsed at low pHs show characteristic signals in the spectral region of the carboxylic acid protons. With the aid of 2D H-1-H-1 double-quantum (DQ) MAS NMR spectra, three signals can be distinguished at 8, 10.5 and 12.5 ppm, which are attributed to free carboxylic groups and two different types of hydrogen bonded forms, respectively. The 12.5 ppm signal arises from the hydrogen bond with the shortest H-H distance, corresponding to the form that is most stable with respect to increasing temperature and pH. The weaker hydrogen-bonded form (with a signal at 10.5 ppm) requires a slightly lower pH, while the free acid signal (at 8 ppm) emerges under the most acidic medium. Moreover, the stabilities of the hydrogen-bonded carboxylic acid dimers can be inferred from the proton-proton distances within the dimers, i.e. (275 +/- 5) pm and (295 +/- 15) pm for the protons at 12.5 and 10.5 ppm, respectively, which are determined by means of DQ MAS sideband patterns. Both the stability of the hydrogen bonds and the acidity of the protons may be related to the stereochemistry and the conformation of the PMAA chains.