## 3 Results
### Mechanical Properties  
Representative stress-strain curves obtained in the uniaxial tensile tests are shown in Figure 4, while the main results are summarized in Table 6.  
The stress-strain curves shown in Figure 4 present the typical hyperelastic behavior, characterized by an initial \"toe region\" in which small deformation results in a linear increase in stress, followed by a \"plateau region\", where stress remains relatively constant despite further increases in strain, and, finally, a stiffening region in which the material experiences a significant increase in stress as a result of material stiffening. According to the literature [34], rubber stiffening can be attributed to two phenomena: the first is related to the achievement of the maximum extensibility of the polymeric chains during stretching, while the second mechanism regards the occurrence of strain-induced crystallization. As the crosslinking density of natural rubber increases, the stress-strain curves in Figure 4 show a progressive stiffening of the samples. This can also be noted when comparing the stress required to deform the materials by 200% and 400% (see Table 6), the values of which increase, moving from NR_1.5S to NR_3.5S. For example, \(\sigma_{400\%}\) is equal \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **E (MPa)** & \(\sigma_{200\%}\) **(MPa)** & \(\sigma_{400\%}\) **(MPa)** & \(\sigma_{\mathbf{b}}\) **(MPa)** & \(\varepsilon_{\mathbf{b}}\) **(\%)** \\ \hline NR\_1.5S & 2.94 \(\pm\) 0.21 & 1.704 \(\pm\) 0.049 & 0.081 \(\pm\) 0.001 & 0.132 \(\pm\) 0.003 & 730 \(\pm\) 14 \\ NR\_2.5S & 4.04 \(\pm\) 0.22 & 1.822 \(\pm\) 0.063 & 0.081 \(\pm\) 0.002 & 0.143 \(\pm\) 0.006 & 573 \(\pm\) 79 \\ NR\_3.5S & 5.24 \(\pm\) 0.51 & 1.775 \(\pm\) 0.044 & 0.082 \(\pm\) 0.001 & 0.143 \(\pm\) 0.004 & 523 \(\pm\) 47 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Results of uniaxial tensile tests at 25 \({}^{\circ}\)C on NR_1.5S, NR_2.5S and NR_3.5S samples. Figure 4: Representative stress–strain curves from quasi-static tensile tests on NR_1.5S, NR_2.5S and NR_3.5S samples. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \begin{tabular}{c} **Crosslinking Density** \\ **(mol**\(\cdot\)**10**\({}^{-}\)**4/cm**\({}^{3}\)**)** \\ \end{tabular}  & \begin{tabular}{c} **Specific Heat Capacity** \\ **(/g**\(\cdot\)**K)** \\ \end{tabular}  & \ \begin{tabular}{c} **Thermal Diffusivity** \\ **(mm**\({}^{2}\)**/s)** \\ \end{tabular}  & \begin{tabular}{c} **Thermal Conductivity** \\ **(W/m**\(\cdot\)**k)** \\ \end{tabular}  \\ \hline NR\_1.5S & 2.94 \(\pm\) 0.21 & 1.704 \(\pm\) 0.049 & 0.081 \(\pm\) 0.001 & 0.132 \(\pm\) 0.003 \\ NR\_2.5S & 4.04 \(\pm\) 0.22 & 1.822 \(\pm\) 0.063 & 0.081 \(\pm\) 0.002 & 0.143 \(\pm\) 0.006 \\ NR\_3.5S & 5.24 \to 2.1 MPa in NR_1.5S and 8.0 MPa in NR_3.5S. This is a reasonable result considering that the chain length between crosslinks is reduced as the degree of crosslinking increases and, consequently, the limit of chain extensibility is reached earlier [35; 36]. Focusing on the stress and strain at break, it is possible to observe that while \(\sigma_{\text{b}}\) is comparable among the samples, \(\epsilon_{\text{b}}\) undergoes a reduction, even if not dramatic, with an increasing degree of crosslinking. To evaluate the elastocaloric properties, the NR samples were deformed up to 500%. It is interesting to note that the stress required to achieve this deformation ranges between 3 MPa (NR_1.5S) and 15 MPa (NR_3.5S), and is close to the stress required to compress a refrigerant fluid in the current air conditioning systems, i.e., approx. 1 MPa [37]. This is a positive aspect that must be underlined since other caloric materials under study for developing solid-state refrigeration systems, such as shape memory alloys or ferroelectric materials, require stresses of at least one or two orders of magnitude higher. For example, for shape memory alloys, several hundreds of MPa are required to exhibit elastocaloric performance [8].