# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Polymers  
In the late 1980s, researchers began applying \({}^{129}\)Xe NMR to polymeric materials by analyzing the spectra of xenon dissolved in the amorphous region of polymers above the glass transition temperature [12]. Since then, this technique has been applied to study several homopolymers [15; 41; 73], copolymers [13], polymer blends [16; 74; 75; 76; 77], and crosslinked  
Figure 2: \({}^{129}\)Xe NMR spectrum of the cryptophane sensor in the presence of Pb\({}^{2+}\), Zn\({}^{2+}\), and Cd\({}^{2+}\) cations, each with a 48.5 \(\upmu\)M concentration. Reprinted with permission from reference [71]. Copyright 2014 American Chemical Society. polymers [78, 79, 80]. Typical diffusion coefficients of Xe dissolved in polymers result in an average displacement of several tens of nm within the NMR experiment timeframe: as a result, Xe is an ideal probe of morphology, dynamics, phase separation, local order, and effects of crosslinking in polymeric materials. In 2006, continuous flow hyperpolarized xenon NMR was used to study porous PP (polypropylene) and PP/EPR (ethylene propylene rubber) millimeter-sized particles, obtained by polymerization with supported MgCl\({}_{2}\)/TiCl\({}_{4}\) Ziegler-Natta catalysts [81]. For PP particles, the intensity of the dissolved xenon signal relative to the free gas peak was linearly dependent on temperature, and a temperature of 260 K was extrapolated by this dependence as the temperature at which xenon could not be seen in the polymer structure (Figure 3). This temperature corresponded to the \(T_{\text{g}}\) of PP, at which the permeability of xenon in the polymer was too low to detect the signal. Thus, a new way to determine the \(T_{\text{g}}\) via continuous flow hyperpolarized \({}^{129}\)Xe NMR was proposed. The spectra of PP/EPR particles, instead, showed two distinct peaks corresponding to the two phases, whose dimensions were estimated to be larger than 200 nm on average, considering the difference between the respective xenon chemical shifts and assuming a diffusion coefficient of xenon in the material around \(10^{-9}\) m\({}^{2}\) s\({}^{-1}\). Two different glass transition temperatures were obtained by analyzing the relative intensities of the peaks versus the temperature, and their values were very similar to those obtained by differential scanning calorimetry (DSC). 2D EXSY spectra acquired at different mixing times revealed that xenon gas first explored the EPR phase, then the PP phase. From these results, it was concluded that the particles were characterized by a hierarchical structure, with the EPR phase covering the PP phase and preventing direct exchange between the latter and the free gas. An application of xenon NMR has been proposed for the characterization of ultra-high molecular weight polyethylene (UHMWPE) fibers prepared by gel spinning [82]. Equations (3) and (5) were used to estimate the average void size of a series of samples, and this value was found to increase with the tensile modulus of the fibers. Thus, a direct  
Figure 3: Continuous flow hyperpolarized \({}^{129}\)Xe NMR spectra of polypropylene (PP) particles. At the top, the relative intensity of the dissolved xenon signal is reported as a function of temperature. Reprinted with permission from reference [81]. Copyright 2006 American Chemical Society. relationship between the size of the nanovoids in the fibers and their tensile strength was found. Another interesting application for polymer physics has been illustrated in the study of polyisobutylene (PIB), poly(ethylene-_co-_1-butene) (PEB), and their blends [83]. One-dimensional spectra coupled with pulsed field gradient spin echo experiments allowed the determination of the length scale of mixing in the polymer blends. In 1D spectra, one of the blends, PIB/PEB-66, showed a single peak with a chemical shift in-between those of the corresponding pure components.