Observing Apparent Nonuniform Sensitivity Enhancements in Dynamic Nuclear Polarization Solid-State NMR Spectra of Polymers.

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.