Optimizing Sample Preparation Methods for Dynamic Nuclear Polarization Solid-state NMR of Synthetic Polymers

Abstract

International audienceThis work compares the overall sensitivity enhancements provided by dynamic nuclear polarization (DNP) for the solid-state NMR characterization of polymer samples doped with biradicals and prepared either by film casting (FC), or by glass forming (GF) using 1,1,2,2-tetrachloroethane as the solvent. Analysis of amorphous and semicrystalline polymers (polystyrene, poly(ethylene oxide), polylactide, poly(methyl methacrylate)) of varying molecular weights showed that GF provided larger sensitivity enhancements than FC but yielded DNP-enhanced 13C CPMAS spectra of lower resolution for semicrystalline polymers, owing to line-broadening due to conformational distribution of the polymer chains in frozen solution. Moreover, use of deuterated solvents significantly reduced the intensity of the solvent signals in the DNP-enhanced 13C CPMAS spectra of polymers prepared by GF, while preserving the sensitivity enhancement observed for the polymer signals. For the polymers investigated here, both FC and GF performed better than incipient wetness impregnation, yielding overall sensitivity enhancements between 5 and 40

Is solid-state NMR enhanced by dynamic nuclear polarization?

“…However, the situation changes when one is using an organic-solvent-based DNP matrix, such as those already used for a variety of studies [29][30][31][32][33][34][35][36], and/or quadrupolar nuclei [31,[35][36][37][38]. Organic solvents are chosen based on a few criteria: their inertness to the analyte and polarizing agent, the position of their resonances in NMR spectra, their RT vapor pressure, and their physical state at experimental temperatures [39].…”

“…This involves solubilizing both the polarizing agent and the polymer analyte in a co-solvent, with subsequent solvent evaporation. This approach has been revisited recently by Viel and co-workers [72] and also compared to the more common glass-forming approach as well as an impregnation method [33]. It was shown that a consistently higher ASR was returned using a DNP matrix (glass-forming approach) compared to film casting [33].…”

“…This approach has been revisited recently by Viel and co-workers [72] and also compared to the more common glass-forming approach as well as an impregnation method [33]. It was shown that a consistently higher ASR was returned using a DNP matrix (glass-forming approach) compared to film casting [33]. However, for semicrystalline polymers (rather than amorphous) significant spectral line broadening resulted from the glass-forming approach [33].…”

“…It was shown that a consistently higher ASR was returned using a DNP matrix (glass-forming approach) compared to film casting [33]. However, for semicrystalline polymers (rather than amorphous) significant spectral line broadening resulted from the glass-forming approach [33]. Therefore, the optimal sample preparation for the organic polymers depends on whether sensitivity or resolution is preferred.…”

“…Therefore, the optimal sample preparation for the organic polymers depends on whether sensitivity or resolution is preferred. Furthermore, the impregnation method, where a minimal amount of polarizing agent containing solvent is added to the dry powder [73], gave little or no DNP enhancement [33]. The reason for the larger ASR for the glass-forming approach, despite the film-casting approach allowing higher χ weight , was the much larger 1H,CP ε DNP .…”