Observing Apparent Nonuniform Sensitivity Enhancements in Dynamic Nuclear Polarization Solid-State NMR Spectra of Polymers
Résumé
High-field dynamic nuclear polarization (DNP)
may enhance the sensitivity of solid-state NMR experiments
on a wide range of systems, including synthetic polymers,
owing to the transfer of electron spin polarization from radicals
to nuclei upon microwave irradiation (usually at cryogenic
temperatures). Provided that the radicals are homogeneously
dispersed in the sample, a uniform DNP enhancement is
expected for all the signals in the 13C cross-polarization magic
angle spinning (CPMAS) spectrum. Here, we show that, in the
case of methyl group containing polymers, a change in the
cross-polarization (CP) dynamics induced by the moderate increase in sample temperature due to microwave irradiation may
lead to the observation of apparent nonuniform enhancements in the DNP-enhanced 13C CPMAS spectra. This peculiar
behavior should be accounted for when measuring 13C CP DNP enhancements on polymer materials, especially for
heterogeneous polymer samples (for which truly nonuniform DNP enhancements could potentially be detected), or when
quantitative results are sought.