## 3 Results
### Evaluation of the Elastocaloric Properties  
Figure 9a-c shows the temperature variation of the surface of the NR samples during the stretching and retraction phases as a function of time and deformation, while Figure 10a-e reports the most important results. Figure 9: Temperature variation of the surface of the samples as a function of the maximum deformation applied. (**a**) NR_1.5S; (**b**) NR_2.5S; (**c**) NR_3.5S. Figure 10: Elastocaloric properties of the produced NR samples. (**a**) Maximum \(\Delta\)T\({}_{\rm heat}\), (**b**) maximum \(\Delta\)T\({}_{\rm cool}\) (**c**) \(Q_{\rm ab}\), (**d**) work of deformation W and (**e**) COP\({}_{\rm mat}\) values as a function of the maximum deformation applied and the crosslinking density. For all the samples, in all testing conditions, the elastocaloric effect is evident. The fast stretching of the material induces a positive temperature variation caused by the orientation of the macromolecules in the impact direction. When the samples are maintained in traction, the temperature progressively drops, returning to ambient temperature and reaching thermal equilibrium. In the retraction phase, the return of the material to the undeformed state, characterized by a high entropy state, causes the rubber to cool down. The deformation strongly affects the surface temperature evolution of the samples, as in all the cases the increase in strain results in a more significant temperature variation, during both the stretching and retraction phases. The higher orientation of the macromolecules achieved with increasing strain levels probably favors strain-induced crystallization phenomena. Such an effect of deformation on temperature variation is in line with the results reported by [PERSON] et al. [20; 40]. The increased thermoelastic and SIC contributions are probably at the basis of this experimental observation. However, in the future, XRD analysis should be performed in situ to confirm this hypothesis. By comparing the temperature responses of the samples in Figures 9a-c and 10a-b, it is clear that the crosslinking density is another parameter that influences the thermal response of NR. Moving from Figure 9a to Figure 9c, a decreasing trend in the temperature variation of the samples can be observed. This experimental observation is probably the result of two contributions. The first is related to the limit of chain extensibility. As was previously explained, when the degree of crosslinking increases, the chain length between crosslinks is reduced and the limit of chain extensibility is reached earlier. Intuitively, this means that the entropy variation from the random coil configuration to the limit of chain extensibility is more pronounced for the rubber showing the lower crosslinking degree, since macromolecular mobility is less restricted. The second aspect concerns strain-induced crystallization. As reported in the literature, by reducing crosslinking density, the ability of natural rubber to crystallize under strain improves [34]. Thus, at a given deformation, the higher strain-induced crystallization phenomena and thermoelastic contribution make NR_1.55 the samples with the higher \(\Delta\)T, during both the stretching and retraction phases. From Figure 10b, it is possible to see that NR_1.5S shows a \(\Delta\)T\({}_{\text{cool}}\) of 2.9 \({}^{\circ}\)C at 300% and 11.0 \({}^{\circ}\)C at 500%. In the literature, the highest \(\Delta\)T\({}_{\text{cool}}\) registered for NR is 12 \({}^{\circ}\)C [9]. For a given deformation and crosslinking degree, \(\Delta\)T\({}_{\text{cool}}\) is similar to or even larger than \(\Delta\)T\({}_{\text{heat}}\) (Figure 10a,b) in almost all the cases. Only for NR_2.5S deformed at 200% and NR_3.5S deformed at 200% and 300%, the temperature increase during stretching is larger than the temperature reduction during unloading. This is an interesting result since, in the literature, \(\Delta\)T\({}_{\text{heat}}\) is very often larger than \(\Delta\)T\({}_{\text{cool}}\)[11], suggesting non-negligible dissipative phenomena such as friction.