# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Polymers
Upon going from ethyl to butyl chains, the increase of the relative aliphatic fraction caused an increase in \(\delta\), while, with octyl chains, the strong decrease in the relative amount of terminal -CH\({}_{3}\) groups prevailed, shifting the signal upfield. This interpretation, combined with the even lower chemical shift for the [Tf\({}_{2}\)N]\({}^{-}\) based membrane, pointed to the existence of different domains whose separation was dependent on the nature of the anion, as demonstrated previously (see Section 5.1). These results were confirmed by \({}^{13}\)C SS-NMR, which indicated a much higher mobility for the alkyl chains of the cations compared to the other components. Xenon NMR spectroscopy has also been used to characterize polymers below the glass transition temperature. For example, this technique coupled with positron annihilation lifetime spectroscopy (PALS) has been used to study the free volume of a series of glassy perfluorinated polymers [86]. Free volumes were obtained by employing Equations (3)-(5), and the chemical shift values extrapolated at zero xenon pressure. Even though the equations are formally valid only for microporous zeolites, reasonable agreement was found between xenon NMR and PALS data. A comparison between PALS and \({}^{129}\)Xe NMR data was also made by [PERSON] et al. on poly(2-hydroxyethyl methacrylate) hydrogels [87]. In these samples, the absence of peaks due to xenon in water led to the conclusion that xenon was located in weakly or non-hydrated hydrophobic domains. Moreover, the analysis  
Figure 5: \({}^{129}\)Xe NMR spectra obtained at 30 \({}^{\circ}\)C of ionic liquid-polyacrylamideic copolymer membranes: (a) poly[P\({}_{222}\)VB][BF\({}_{4}\)]+acrylates, (b) poly[P\({}_{444}\)VB][BF\({}_{4}\)]+acrylates, (c) poly[P\({}_{888}\)VB][BF\({}_{4}\)]+acrylates, (d) poly[P\({}_{888}\)VB][Tf\({}_{2}\)N]+acrylates. The vertical dashed line marks the chemical shift of xenon in water. Reprinted with permission from reference [85]. Copyright 2021 _Journal of Membrane Science_. of samples prepared with different water contents revealed that the \({}^{129}\)Xe chemical shift decreased with increasing water content. This was attributed to an increase in free volume due to the materials progressively passing from a glassy to a rubbery state with increasing water content. [PERSON] et al. studied several other glassy polymeric systems, including poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) [88], PPO/PS blends [89], and tetramethyl bisphenol A polycarbonate (TMPC)/PS blends [90], using \({}^{129}\)Xe NMR and Xe sorption measurements. For these studies, the sorption behavior of xenon in glassy polymers was described using a dual-mode adsorption model. The assumption is that xenon dissolves in the polymer matrix via a Henry's mechanism and is adsorbed in the nanovoids via a Langmuir mechanism. This generates two different contributions to the xenon chemical shift: \(\delta_{\rm D}\) (dissolved xenon, Henry's mechanism) and \(\delta_{\rm H}\) (adsorbed xenon, Langmuir mechanism). The fitting of the xenon adsorption isotherms at various temperatures, along with the analysis of variable temperature and pressure \({}^{129}\)Xe NMR spectra, allowed determination of the relative concentrations of xenon atoms in the two different sites, and the corresponding chemical shift values. Lastly, these values were used along with Equation (3) to determine the effective size of the nanovoids. A widely diffused method for obtaining polymers is emulsion polymerization. This technique allows latexes constituted by homo or copolymer particles to be obtained. The careful selection of the synthetic conditions allows the dimensions and the morphology of the latex particles to be tailored, allowing many different architectures to be obtained [91]. \({}^{129}\)Xe NMR has been used as a technique to evaluate the structure and the morphology of various homo- and copolymer particles synthesized by emulsion polymerization [92].