# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Polymers
In the former, the signal of the starting pore structure could be detected for up to 50% MCM-41 content, indicating that the original structure was not completely hindered by the presence of MCM-41. Combining these findings with N\({}_{2}\) adsorption data, they determined that, in this case, the transformation proceeded via the formation of distinct MCM-41 domains dispersed over the particles, as illustrated schematically in Figure 9. When CPG was used as the starting material, instead, the signal of the original porosity disappeared even at low conversion values. To better understand this phenomenon, a series of 1D selective EXSY experiments were performed. Even in this case, the signal associated with the starting CPG porosity was not observed, indicating that the MCM-41 component either completely blocked the initial porous structure or significantly slowed down the exchange between the two porous systems. Thus, it was concluded that the transformation preferentially took place in the outer regions of the material, creating a surface layer of MCM-41. The authors attributed the two different mechanisms to the different ability to accommodate the newly formed MCM-41 structure, deriving from the different pore sizes of the starting materials: LiChrospher 60 possesses small mesopores, whereas the studied pore glass had an average pore diameter of 53 nm. Silica aerogels are highly porous and extremely lightweight materials. Their peculiar properties make them suitable for a wide variety of applications, such as thermal and acoustic insulation, energy absorption, catalysis and catalyst support, extraction, fuel storage, and drug delivery [106; 107]. These materials are obtained by conventional sol-gel processes followed by aging of the gel and drying. This last step is conducted in particular conditions, such as with supercritical fluids, to preserve the original gel structure to the greatest possible extent. The porous structure of silica aerogels is generally characterized with conventional N\({}_{2}\) adsorption measurements. However, the possibility to alter the porous structure of these materials along with the typical broad pore size distributions represent serious limitations in this type of approach. HP \({}^{129}\)Xe NMR can provide in-depth information with minimal or no sample disruption and with high sensitivity. In a very recent work, different hydrophobic silica aerogels were synthesized, and their porous structure was characterized using hyperpolarized xenon NMR [108]. Two series of materials  
Figure 9: (**a**) Schematic representation of the transformation of LiChrospher 60 into MCM-41. (**b**) Schematic representation of the transport of \({}^{129}\)Xe in the partially transformed material. Reprinted with permission from reference [105]. Copyright 2017 American Chemical Society. were prepared with different aging times, employing acid (series A) and basic catalysis (series B), respectively. All the materials showed asymmetric broad peaks in the range 100-150 ppm, indicative of inhomogeneous pore size distributions originated by different pore species (Figure 10). The somewhat high chemical shift values were attributed to the high hydrophobicity of the modified silica gels, which strongly enhances the interaction with xenon, affecting its NMR signal. Consequently, quantitative determination of pore sizes was not conducted. Nonetheless, the spectra clearly showed that the porous structure of the A series was different from that of the B series. This was due to the different reaction mechanisms: acid-catalyzed synthesis created linear chains with overall low degree of crosslinking, whereas basic catalysis increased the degree of crosslinking and created highly branched networks. \({}^{129}\)Xe NMR has also been used to characterize the hierarchical structure of dendritic fibrous nanosilica (DFNS) [109], a recently discovered new class of mesoporous silica. A series of variable temperature spectra in the range 173-293 K revealed the existence of two different, non-connected porous structures.