# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Polymers
The resonance at 10 ppm was attributed to xenon weakly confined in the  
Figure 7: Room temperature continuous flow hyperpolarized \({}^{129}\)Xe NMR spectra of ordered mesoporous silica (OMS) materials and their functionalized counterparts (referred to as PNNL) with different pore sizes: 5 nm, 9.3 nm, and 30 nm. (**A**) OMS-5, (**B**) OMS-9.3, (**C**) OMS-30, (**D**) PNNL-5, (**E**) PNNL-9.3, (**F**) PNNL-30. Reprinted with permission from reference [9]. Copyright 2002 American Chemical Society. interparticle clusters formed by sintering processes, also observed in TEM images, while the other two peaks indicated other two different pore environments. Thus, this technique revealed fine structural details not detected by conventional adsorption-desorption methods. Ag-functionalized silica again showed three distinct peaks but with two main differences: resonances were shifted downfield at 81, 63 and 22 ppm, respectively, and the signals were much more intense. Since in a previous work it was observed that the interaction of xenon with Ag\({}^{+}\) cations led to strong upfield shifts, sometimes resulting in negative chemical shift values [101], the authors excluded the presence of this species in the synthesized materials. Instead, they attributed the differences to the presence of Ag nanoparticles which reduced the volume of the pores, increasing xenon confinement and, consequently, its chemical shift. An analogous study was conducted in 2016 on rod-shaped silica particles [102]. First, the authors compared the spectra of these materials with spherical silica particles having similar pore sizes, analyzed both before and after compression. The comparison revealed that the single signal observed at 54 ppm for rod-shaped particles, which shifted at 73 ppm upon compressing the material, was indicative of xenon dynamically exchanging between different environments in the NMR time scale, including inter-particle connections and intra-particle mesopores. This fast exchange could only be possible with cylindrical pores open at both ends. Upon compression, the signal shifted to a higher chemical shift value, very similar to that of spherical particles with pores of analogous sizes. This indicated that compressing the sample blocked most of the inter-particle connections, resulting in a signal mostly indicative of xenon inside the porous structure. The analysis of the corresponding Ag-loaded particles revealed that most of the pore structure was occupied by Ag nanoparticles, even though a very small signal could still be observed at 54 ppm, indicating that a small fraction of the pores remained unoccupied (Figure 8). Given that \({}^{129}\)Xe NMR provides a high level of detail regarding the structure of the analyzed materials, it can also be used to monitor chemically-induced variations in the morphology of the materials and gain insight into the mechanism and the dynamics involved. For example, starting from silica-based materials with a disordered pore structure, it is possible to obtain ordered mesoporous materials such as MCM-41 and MCM-48, with the aid of a surfactant and a structure-directing agent [103]. The degree of transformation can be controlled by the synthetic conditions, allowing the creation of materials with both the original and the newly formed porous structures, and the creation of complex hierarchical  
Figure 8: Comparison of the spectra of rod-shaped mesoporous silica particles and their corresponding Ag-loaded counterparts. Reprinted with permission from reference [102]. Copyright 2006 _Journal of Solid State Chemistry_. architectures with well-defined morphology. This process, called pseudomorphic synthesis, has been studied with HP \({}^{129}\)Xe NMR in the case of the transformation of LiChrospher 60 silica gel and controller pore glasses (CPG) into MCM-41 by analyzing materials with different degrees of transformation [104; 105]. The authors were able to distinguish between two different transformation mechanisms for LiChrospher 60 and CPG, respectively.