# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Polymers
By performing variable temperature and variable pressure continuous flow hyperpolarized \({}^{129}\)Xe NMR, instead, the authors were able to identify the presence of a nonregular and nonuniform porous structure, spacing from micro to mesopores (room temperature spectra are shown in Figure 7). More specifically, xenon was revealed to be in fast exchange between two different nonuniform regions of porosity in the NMR time scale. This provided a much more in-depth level of detail regarding the porous structure of the materials compared to N\({}_{2}\) adsorption and TEM data. Moreover, by analyzing the functionalized materials, they determined that the hydrocarbon chains had two main effects: first, the accessibility of the silica surface was partially blocked; second, xenon dissolved in the hydrocarbon chains, generating broad resonances at around 200 ppm in some spectra. This paper clearly demonstrated the applicability of \({}^{129}\)Xe NMR in detecting surface effects and different pore size distributions in silica-based materials. Similar results were obtained in a work on mesoporous silica functionalized with various ratios of 3-[2-(2-aminoethylamino)ethylamino] propyltrimethoxysilane (AEPTMS) and 3-cyanopropyltriethoxysilane (CPTES) [97]. In addition to distinguishing the effects of functionalization on the mesoporous structure, 2D EXSY spectra of AEPTMS-functionalized silica were analyzed to gain a deeper insight into the interconnectivity of different domains. It was observed that xenon exchanged rapidly between the mesopores and the free gas signal at around 0 ppm, while a longer mixing time was required to observe the exchange between the organic surface phase and the mesopores and between the free gas and the organic phase. HPLC columns are constituted by silica-based materials, often functionalized to enhance their capability to retain apolar compounds, making separation processes more efficient. Some functionalized HPLC materials have been studied employing \({}^{129}\)Xe NMR by the group of Ba [98]. In their study, xenon was chosen as a probe molecule representative of a possible mobile phase, and the structure of the stationary phase along with its interaction with xenon was analyzed. By performing a series of variable temperature experiments, the authors were able to distinguish between four different xenon populations: xenon solvated in the stationary phase, xenon in the pores of the silica particles, xenon in the interparticle spaces of the material, and xenon outside the column material. In addition to that, 2D EXSY experiments at low temperatures were performed, directly observing the diffusion of xenon between the interparticle spaces and the pores of the silica particles. In a subsequent work, the authors also applied \({}^{1}\)H-\({}^{129}\)Xe CP NMR experiments to confirm the separation and the distinction between xenon inside the pores and xenon in the interparticle space [99]. Overall, these studies allowed determination of the structure of the column materials, as well as the solvation and the dynamics of xenon atoms, gaining information about the interaction of these materials with apolar species. Mesostructured silica-based materials containing metallic species can be prepared by synthesizing metal nanoparticles directly inside the mesoporous structure. The resulting metal-containing mesoporous materials can show widely different porous structures, depending not only on the silica-based constituents but also on the shape and size of the metal particles. \({}^{129}\)Xe NMR was employed to study Ag-functionalized silica synthesized by templating with octadecyltrimethylammonium bromide (C\({}_{18}\)TAB) [100]. First, calcined materials not containing silver were analyzed. Contrary to the results obtained by N\({}_{2}\) adsorption, which showed a bimodal distribution of pores, the Xe NMR spectrum showed three distinct resonances at 74, 44 and 10 ppm, along with the typical peak at 0 ppm assigned to free gas.