## 3 Results
### Evaluation of the Elastocaloric Properties
In this work, it is plausible that during the maintenance of the samples in traction, crystalline domains progressively develop. As the crystallization of these domains is not instantaneous, the heat generated during their formation may not have been considered in \(\Delta\)T\({}_{\text{heat}}\). Conversely, during unloading, if all crystalline domains, regardless of their formation time, melt, they could contribute to \(\Delta\)T\({}_{\text{cool}}\), potentially resulting in \(\Delta\)T\({}_{\text{cool}}\) values greater than \(\Delta\)T\({}_{\text{heat}}\). However, it is crucial to emphasize that this is still a hypothesis and demands further experimental investigation. Following further analysis, it can be noted that the width of the temperature peaks in Figure 9a-c (i.e., the time required to the sample to return to ambient temperature) is wider in the retraction phase with respect to the stretching stage. The characteristic time of temperature dampening, determined during both heating (stretching phase) and cooling (retraction phase) reported in Table 7, confirms this observation. For a given degree of crosslinking and deformation, the \(\tau_{\text{c}}\) calculated in the retraction phase is always higher than the one calculated in the stretching phase. More specifically, the ratio between \(\tau_{\text{c}}\) determined in the retraction phase and \(\tau_{\text{c}}\) determined in the stretching stage ranges between 1.8 and 3.6. During stretching, the specimen becomes thinner, favoring a more rapid heat transfer. For the heat transfer coefficient, values ranging from 11 to 24 W/(m\({}^{2}\cdot\)K) are obtained. According to the literature, typical ranges of h are 3-20 W/(m\({}^{2}\cdot\)K) under the condition of free convection, and 10-100 W/(m\({}^{2}\cdot\)K) under forced convection [41]. The rapid stretching and unloading of the samples might have triggered air movements, inducing a slight forced convection. Thus, it can be stated the values obtained are in line with data from the literature. Going into details on the elastocaloric effect, Figure 10c-e show Q\({}_{\text{ab}}\) and the COP\({}_{\text{mat}}\) values of the samples as a function of deformation and degree of crosslinking. Since the elastocaloric effect of natural rubber is being studied for solid-state cooling applications, it is fundamental to quantify the capability of the material to extract thermal energy from the environment. For a given crosslinking density, Q\({}_{\text{ab}}\) increases with increasing deformation, whereas, if the deformation is fixed, Q\({}_{\text{ab}}\) decreases with the degree of crosslinking. NR_1.5S is the sample able to extract a higher amount of thermal energy during the unloading phase. In particular, at a deformation of 500% and in one loading/unloading cycle, NR_1.5S absorbs approx. 16.4 J/cm\({}^{3}\). However, to evaluate the performance of the materials, the attention should be focused on the Coefficient of Performance, evaluated by the ratio between the extracted thermal energy and the mechanical work required to deform the samples (W, Figure 10d). The findings clearly show that NR_1.5S is the best formulation among the ones prepared, exhibiting a COP\({}_{\text{mat}}\) ranging from 1.8 to 2.4. The optimal compromise appears to be reached by deforming NR_1.5S at 400%. This not only allows for the highest COP\({}_{\text{mat}}\) value, but also ensures a safer operational condition, as it is further from the strain at break. The COP\({}_{\text{mat}}\) values obtained were found to be higher than those reported by [PERSON] et al., who used the same expression for calculating COP [11]. To conclude, it is worth making some remarks. From a practical point of view, the time required by the sample to return to room temperature, both during the stretching and retraction phases, proves to be rather long. Consequently, the time of the entire refrigeration cycle was set at 480 s to ensure the return of the materials at room temperature in each step and that the same time interval of waiting is applied in the two phases.