# Time Domain NMR in Polymer Science: From the Laboratory to the Industry
## 3 Applications
### Discovery and Characterization of New Materials
In reference [46], a carboxylated nitrile rubber (XNBR) was crosslinked with variable amounts of MgO, since carboxyl groups form ionic aggregates with magnesium oxide. The combined use of MQ, in its five-pulse version, and MSE-FID provides evidence of the structure of the elastomer, underlining the existence of two domains of coupled chains: a heavily trapped fraction in the proximity of the ionic aggregatesand a more mobile component of chains between the crosslinks. The increasing addition of MgO increases the conversion of carboxylic groups into carboxylates, consequently enhancing both the crosslink density and the trapped rubber fraction (Figure 8a), which reaches a plateau above the stoichiometric ratio, where all the carboxylic groups are converted. However, crosslinks density further increases because of an additional number of ionic interactions of ionic moieties with the excess of MgO, that derives from a reduction in the aggregation number of ionic clusters. At higher temperatures, the trapped fraction and the crosslinks density decrease, demonstrating the reversibility of the bonding strategy. Alternatively, thermo-reversible covalent linkages can be created though Diels-Alder reactions. Ethylene/propylene rubber (EPM) and ethyl/vinyl acetate rubber (EVM) were chemically modified with the introduction in the backbone of furfurylamine (FFA) and furan that can undergo a Diels-Alder coupling with maleimide. MQ NMR with a low field instrument allowed to determine the Dres distributions for the two polymer classes, which translates into a distribution of crosslinks in the network. There was evidence in the EPM for the formation of a bimodal distribution, attributed to polar clusters, while the EVM presents a monomodal, though large, distribution, because of the absence of phase separation based on polarity (Figure 8b). The difference was confirmed by SAXS. The different mechanical properties between the two materials were interpreted on the basis of the homogeneity (or lack thereof) of the network [47]. Another strategy to allow recyclability and, at the same time, allowing for use in a biomedical application is the use of biocompatible prepolymer. Some biocompatible polyurethanes made of biocompatible diols, such as polyethylene glycol (PEG), hexamethylene diisocyanate (HDI), and calcium glycerol phosphate (GPCA), were characterized in respect to the variation of their dynamic properties with the introduction of new components into the formulation. ILT was employed to obtain the T2 and T1 distributions. It gave evidence of the crosslinking in the material and of the softening effect with the addition of mobile diols as PEG and the hardening effect provided by the GPCA salt [48]. Particular interest has been directed towards the realization of objects with nanometer size which show unique physical properties and can be applied in many fields, from medicine to energy production [49; 50]. Their size makes their characterization complicate and requires novel approaches. For example, complete phase separation or the existence of an interphase between distinct components  
Figure 8: Change of residual dipolar couplings in reversible networks. (**a**) Increasing trapped rubber Dres with increasing MgO content in ionomeric rubbers (Reprinted with permission from reference [46]. Copyright 2014 American Chemical Society), (**b**) different Dres distributions for Diels–Alder cross-linked EPM (red) and EVM (blue) rubbers, where the component at higher Dres values was attributed to polar clusters of crosslinks (Reprinted with permission from reference [47]. Copyright 2017 American Chemical Society). could be assessed, as done for core-shell PBA-PS nanoparticles with baroplastic properties. Low-field NMR is, in fact, able to prove phase separation through MSE.