## 3 Results
### Mechanical Properties
This makes the behavior of NR interesting and of great potential for solid-state cooling applications. An elastocaloric cooling device requires the repetitive stretching and releasing of rubber. Consequently, it becomes crucial to thoroughly examine the mechanical response of the samples under cyclic loading conditions, particularly considering their susceptibility to the Mullins effect. Figure 5a-c shows the stress-strain response of the NR samples under cyclic tensile loading when maximum strain levels of 200%, 300% and 400% are applied. Analyzing Figure 5a-c, the Mullins effect can be observed in all the samples, particularly in those subjected to a deformation of 300% and 400%. Progressing from the first to subsequent cycles, a noteworthy stress-softening phenomenon becomes evident, together with a reduction in the hysteresis loop, which is representative of the dissipated energy. As the deformation level increases, the prominence of the Mullins effect becomes increasingly apparent. This empirical observation aligns with existing literature on the subject [30]. However, to derive a more precise comparison among the cyclic behaviors of the samples, the stress at the maximum deformation per each cycle (peak stress) and the energy dissipated in the 1 st, 2 nd, 5 th and 10 th cycles have been determined, with the results shown in Figures 6a-c and 7a-c, respectively, while the residual deformation at the end of the 10 th cycle is reported in Figure 8.  
Several considerations can be made. As can be seen from Figure 6, the stress-softening is more pronounced moving from the first to the second cycle. After the second cycle, the maximum stress only slightly decreases, suggesting the mechanical response of the material has stabilized. With increasing strain levels, the difference between the peak stress of the first and second cycles increases as well. The same consideration can be made also for the energy dissipated. The variation of the hysteresis loop from the first and successive cycles is even more pronounced than the stress-softening. For example, NR_2.5S experiences a reduction of dissipated energy from 0.8 J/cm\({}^{3}\) to 0.4 J/cm\({}^{3}\) moving from the first to the second cycle. For maximum deformation levels of 200% and 300%, the samples show quite similar dissipated energy values. In contrast, significant differences can be noticed for a deformation of 400%, in which NR_1.5S presents the highest value, followed by NR_3.5S and NR_2.5S. A larger number of dangling chains and trapped entanglements compared to the other crosslinking levels may explain the higher hysteresis found for the NR_1.5S sample [38]. Similar results were found by [PERSON] et al. [39], who investigated the influence of filler size and crosslinking degree on the Mullins effect in filled NR/butadiene rubber (BR) blends. They observed that by keeping the filler content constant, the dissipated energy increased when reducing crosslinking density. Such a clear trend is not observable in Figure 7c, but still, NR_1.5S was shown to be the sample with the most important hysteresis. However, in the future, it will be essential to conduct further analysis to validate this hypothesis. Finally, the residual deformation values at the end of the 10 th cycle are shown in Figure 8. The values range from 2% to 6% depending on crosslinking degree and applied strain. When holding the degree of crosslinking constant, there is a rise in residual deformation with increasing strain levels. Conversely, maintaining a constant strain level, the residual deformation increases with the crosslinking density, specifically at 200% and 300% strain levels, while at 400%, the values are close to each other. Since the residual deformation was evaluated at the end of the 10 th cycle, its trend as a function of cycle number is not known. However, since both dissipated energy and peak stress show the same behaviors, it could be reasonable to assume that the residual deformation does so as well.