Structural Investigations of Polymer Materials by Dynamic Nuclear Polarisation Solid-state NMR

Abstract

Solid-state nuclear magnetic resonance (SSNMR) has established itself as an essential structural elucidation technique in polymer science because it can non-destructively provide unique molecular-level information with atomic resolution on complex macromolecular materials, especially when combined with complementary techniques such as scattering and computer simulation. The Achilles' heel of NMR, however, is its low sensitivity due to the weak nuclear spin polarisation at thermal equilibrium, even at very high magnetic fields. One intriguing way to enhance NMR sensitivity is dynamic nuclear polarisation (DNP), which relies on the microwave-driven transfer of thermal electron spin polarisation to nuclei. DNP is attracting scientific attention owing to the availability of both high-power/high-frequency microwave sources and efficient DNP polarising agents (i.e. paramagnetic species used for doping diamagnetic samples to provide the source of electron polarisation for DNP). At moderately high magnetic fields (~10 T), large DNP signal enhancements can now be readily obtained, which has led to unprecedented SSNMR applications. In this review, we describe the requirements for high-field DNP SSNMR and provide examples of its use for the structural analysis of organic polymer materials, so as to highlight its advantages and (current) limitations for the field.

Supercritical CO2 impregnation process applied to polymer samples preparation for dynamic nuclear polarization solid‐state NMR

“…Dynamic nuclear polarization (DNP) has become a technique of choice for improving the sensitivity of solid-state nuclear magnetic resonance (ssNMR) experiments. [1][2][3][4][5][6][7][8][9][10][11] DNP enhances nuclear magnetization through the microwave (MW)-driven transfer at cryogenic temperatures of electron spin polarization to nuclei via paramagnetic centers, which are typically referred to as polarizing agents (PAs). In principle, whereas PAs can either be contained in (endogenous) or added to (exogenous) the material under investigation, the best DNP sensitivity enhancements obtained so far have been observed when using exogenous PAs, although there are some notable exceptions, [12][13][14][15] including recent work where endogenous radicals were generated in the sample by γ-ray irradiation.…”

Monitoring the influence of additives on the crystallization processes of glycine with dynamic nuclear polarization solid-state NMR

“…Inspired by Tycko and co-workers [23][24][25], we have recently proposed a methodology for studying the time-evolution of crystallization processes, in which the crystallizing system was quenched rapidly to low temperature at specific time points during crystallization [26,27]. Combined with dynamic nuclear polarization (DNP) [28][29][30][31][32][33][34][35][36][37][38][39][40][41], this experimental strategy proved useful to enhance NMR signal intensity of transient crystallizing phases (which would not be observable with standard NMR experiments) and allowed these transient forms to be investigated with a range of NMR experiments [26,27]. It should be noted that complementary information could be obtained with dissolution-DNP experiments [42].…”

Investigating the efficiency of silica materials with wall-embedded nitroxide radicals for dynamic nuclear polarisation NMR

“…1,2 In particular, when combined with magic-angle spinning (MAS), DNP can tremendously improve the sensitivity of high-field solid-state NMR (SSNMR) experiments, 3–7 and this has allowed for the detailed structural characterisation of a very broad range of samples, from biomolecular compounds 8–14 to inorganic 15–20 or hybrid materials, 21–24 organic crystals 25–29 and polymers. 30–37 More precisely, DNP enhances nuclear magnetisation through the microwave-driven transfer (usually at cryogenic temperatures) of electron spin polarisation to nuclei via exogenous paramagnetic centres referred to as polarising agents (PA). These polarising agents are stable paramagnetic species ( e.g.…”