Dynamics of 1H-13C cross polarization in nuclear magnetic resonance of poly(3-hydroxybutyrate)

Abstract

Poly(3-hydroxybutyrate) (PHB) is a semicrystalline biodegradable polymer with chains consisting of methyl (CH3), methylene (CH2), methine (CH) and carbonyl (CO) groups. The 1H-13C cross polarization NMR measurements were performed on an as-supplied powder PHB sample at a magic-angle spinning rate of 10 kHz. The measured cross polarization build-up curves and their analysis provided information on the dynamics of 1H-13C NMR cross polarization in functional groups with directly bonded hydrogens. The measurements required setting up the Hartmann-Hahn condition, which was inferred from the Hartmann-Hahn matching profiles measured for each functional group. The cross polarization build-up curves displayed an oscillatory course, which indicates the presence of rigid 1H-13C spin pairs isolated from the lattice. The frequency of the observed oscillations is directly proportional to the 1H-13C dipolar coupling constant, which is related to the C-H distance and its value also reflects the mobility of particular functional groups. The values of dipolar coupling constants were derived from splittings in the Fourier transforms of cross polarization build-up curves. The mobility of particular groups was assessed with the order parameter hSi calculated using experimental and rigid lattice values of dipolar coupling constants.