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Porosity of Molecularly Imprinted Polymers Investigated by ¹²⁹Xe NMR Spectroscopy.

ABSTRACT: Molecularly imprinted polymers (MIPs) display intriguing recognition properties and can be used as sensor recognition elements or in separation. In this work, we investigated the formation of hierarchical porosity of compositionally varied MIPs using ¹²⁹Xe Nuclear Magnetic Resonance (NMR) and ¹H Time Domain Nuclear Magnetic Resonance (TD-NMR). Variable temperature ¹²⁹Xe NMR established the morphological variation with respect to the degree of cross-linking, supported by ¹H TD-NMR determination of polymer chain mobility. Together, the results indicate that a high degree of cross-linking stabilizes the porous structure: highly cross-linked samples display a significant amount of accessible mesopores that instead collapse in less structured polymers. No significant differences can be detected due to the presence of templated pores in molecularly imprinted polymers: in the dry state, these specific shapes are too small to accommodate xenon atoms, which, instead, probe higher levels in the porous structure, allowing their study in detail. Additional resonances at a high chemical shift are detected in the ¹²⁹Xe NMR spectra. Even though their chemical shifts are compatible with xenon dissolved in bulk polymers, variable temperature experiments rule out this possibility. The combination of ¹²⁹Xe and TD-NMR data allows attribution of these resonances to softer superficial regions probed by xenon in the NMR time scale. This can contribute to the understanding of the surface dynamics of polymers.

SUBMITTER: Boventi M

PROVIDER: S-EPMC9745730 | biostudies-literature | 2022 Dec

REPOSITORIES: biostudies-literature

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Porosity of Molecularly Imprinted Polymers Investigated by 129Xe NMR Spectroscopy.

Boventi Matteo M Mauri Michele M Golker Kerstin K Wiklander Jesper G JG Nicholls Ian A IA Simonutti Roberto R

ACS applied polymer materials 20221104 12

Molecularly imprinted polymers (MIPs) display intriguing recognition properties and can be used as sensor recognition elements or in separation. In this work, we investigated the formation of hierarchical porosity of compositionally varied MIPs using 129Xe Nuclear Magnetic Resonance (NMR) and 1H Time Domain Nuclear Magnetic Resonance (TD-NMR). Variable temperature 129Xe NMR established the morphological variation with respect to the degree of cross-linking, suppo ...[more]

PMID: 36532888

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Project description:Flexible metal-organic frameworks (MOFs) are capable of changing their crystal structure as a function of external stimuli such as pressure, temperature, and type of adsorbed guest species. DUT-49 is the first MOF exhibiting structural transitions accompanied by the counterintuitive phenomenon of negative gas adsorption. Here, we present high-pressure in situ 129Xe NMR spectroscopic studies of a novel isoreticular MOF family based on DUT-49. These porous materials differ only in the length of their organic linkers causing changes in pore size and elasticity. The series encompasses both, purely microporous materials as well as materials with both micropores and small mesopores. The chemical shift of the adsorbed xenon depends on xenon-wall interactions and thus on the pore size of the material. The xenon adsorption behavior of different MOFs can be observed over the whole range of relative pressure. Chemical shift adsorption/desorption isotherms closely resembling the conventional, uptake-measurement-based isotherms were obtained at 237 K where all materials are rigid. The comparable chemical environment of the adsorbed xenon in these isoreticular MOFs allows to establish a correlation between the chemical shift at a relative pressure of p/p 0 = 1.0 and the mean pore diameter. Furthermore, the xenon adsorption behavior of MOFs is studied also at 200 K. Here, structural flexibility is found for DUT-50, a material with an even longer linker than that of the previously known DUT-49. Its structural transitions are monitored by 129Xe NMR spectroscopy. This compound is the second known MOF showing the phenomenon of negative gas adsorption. Further increase in the linker length results in DUT-151, a material with an interpenetrated network topology. In situ 129Xe NMR spectroscopy proves that this material exhibits another type of flexibility compared to DUT-49 and DUT-50. Further surprising observations are made for DUT-46. Volumetric xenon adsorption measurements show that this nonflexible microporous material does not exhibit any hysteresis. In contrast, the in situ 129Xe NMR spectroscopically detected xenon chemical shift isotherms exhibit a hysteresis even after longer equilibration times than in the volumetric experiments. This indicates kinetically hindered redistribution processes and long-lived metastable states of adsorbed xenon within the MOF persisting at the time scale of hours or longer.

Dataset Information

Porosity of Molecularly Imprinted Polymers Investigated by ¹²⁹Xe NMR Spectroscopy.

Publications

Porosity of Molecularly Imprinted Polymers Investigated by <sup>129</sup>Xe NMR Spectroscopy.

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