# Time Domain NMR in Polymer Science: From the Laboratory to the Industry
## 3 Applications
### Discovery and Characterization of New Materials  
In the last decades, materials science had been gaining more and more relevance to the development of materials with new or improved properties [43]. Most materials require a thorough characterization to understand how they behave in defined conditions. Especially polymers, as we said in the introduction, can be quite easily modified in order to access new characteristics, for example, the ability to self-heal after damage or to be biocompatible or to self-assemble in nanostructures or porous materials. Thermoplastics, elastomers, and composites can all be so modified. Some relevant examples, addressing, in particular, the solution to technical and environmental issues, will be reported. A great effort has been deployed in recent years for the synthesis of rubbery materials endowed with self-healing properties because rubbers are often employed in challenging environments, where long-term performances are hoped for. The ability to autonomously recover from damage is therefore very attractive, even from an economic point-of-view, since those materials could potentially be low-maintenance and with longer lifetime. NMR can help to give an insight into the self-healing mechanism and retention of properties after recovering. The mechanism, that is responsible for effective self-healing in a popular self-healing rubber, that associates through reversible hydrogen bonding, was examined. The rubber is composed of fatty acids, amines, such as 1-2-aminoethylimidazololone (UDETA) and diethylenetriamine (DETA), and urea. It is claimed that segregation of polar and apolar moieties occurs. MSE measurements indicate that two phases exist, with a different composition (established through high-field NMR) and different dynamic behavior. MQ was used to further characterize the heterogeneous structure and proved the existence of anisotropic mobility, that is proof of a rubber network. The three-pulse version of BP was used because the temperature for an appropriate determination of Dres would have been too high and would have accelerated the aging, therefore the measures were carried out at a lower temperature, where a large Dres is attained. Two components with different Dres are apparent, suggesting two subcomponents in the dipolar coupled fraction. Therefore, a very complex and heterogeneous structure was speculated. MQ was also employed to assess the aging process of the rubber at high temperature, showing an increase of the Dres with a higher temperature at short aging time, which leads to the formation of irreversible crosslinks, with a detrimental effect on the self-healing properties [44]. A similar rubber made of polyurethane, urea, and UDETA was characterized in respect to its self-healing ability as a function of moisture. MSE and HE experiments, fitted simultaneously, were used to assess the variation of NMR Tg as function of the relative humidity at which the sample was conditioned. The authors used the increase in the lower relaxation time Tg and its exponent in the three components fitting as the main parameters to confirm the increased mobilization of chains with increased humidity and increased amount of UDETA. Matching the variation of a mobile fraction with observations from other optical techniques, they could define a threshold of at least 59% of mobile-like dynamics to achieve self-healing [45]. Another attractive feature for rubbers is the possibility to be easily recycled. In fact, although crosslinking is mandatory to obtain the long-range elasticity that renders rubbers so useful, its irreversibility prevents recycling, with great environmental problems. The creation of reversible crosslinking could help to overcome this issue. One approach to address the problem is to exploit ionic interactions between the polymer chains and highly polar crosslinkers, introducing ionic moieties in common polymers.