## 2 Materials and Methods
### Characterization
#### 2.3.4 Evaluation of the Elastocaloric Effect  
To evaluate the effect of crosslinking density on the elastocaloric performance of the prepared materials, a high-speed tensile testing machine, STEP Lab XUD05 (STEP Lab S.r.l, Resana TV, Italy), equipped with a 1 kN load cell was used. The tests were carried out at room temperature (approx. 22 \({}^{\circ}\)C) and rectangular specimens 1 mm thick and 15 mm wide were used. The distance between the specimen grips was set at 25 mm (tested volume of the specimens of about approx. 0.4 cm\({}^{3}\)). The following procedure was adopted to evaluate the elastocaloric performance: At first, the specimens were rapidly stretched up to the desired deformation level, setting the crosshead speed at 0.5 m/s. After that, they were maintained in traction for 250 s, to allow the materials to return to room temperature. In this work, the cooling of natural rubber occurred by natural convection. The NR samples were then unloaded, at the same crosshead speed of the stretching phase, followed by a relaxation step of 250 s. For each formulation, three strain levels (i.e., 300%, 400% and 500%) were considered and two specimens were tested per each strain level. The surface temperature variation of the samples was recorded by using an infrared thermal imaging camera IRtech Fotric 348A (E Instrument Group s.r.l., Lesmo MB, Italy), placed at a fixed distance of 50 cm from the surface of the NR strips, having a thermal sensitivity of 0.04 \({}^{\circ}\)C. The acquisition frequency was set at 16 fps and the emissivity parameter employed was 0.9. The AnalyzIR software Version 5.0.7.117 (E Instrument Group s.r.l., Lesmo MB, Italy) was used to measure punctual temperature values along the specimen's longitudinal axis. As the thickness of the samples was 1 mm and the Biot number (i.e., the ratio between the thermal resistance for conduction inside the body and the resistance for convection at the surface of the body [31; 32]) was found to be less than 0.1, the temperature difference between the surface and the core of the specimens could be considered negligible. Thus, the temperature could be deemed uniform throughout the material. During both the stretching and retraction phases, the temperature of the samples was represented as a function of time. The temperature variation of NR_1.5S, deformed during the test at 500%, is reported as an example in Figure 1.  
Figure 1: Representation of the temperature variation of NR_1.5S sample during the stretching and retraction phase (maximum deformation applied = 500%). The numbers 1–5 reported in the IR thermograms correspond to the stages 1–5 highlighted in the graph. 1: Before stretching, 2: just after stretching, 3: end of the stretching phase, 4: Just after the sample unloading, 5: end of the releasing phase. For each specimen and strain level, the temperature increase during the stretching phase (\(\Delta\)T\({}_{\text{heat}}\)) and the temperature reduction during the retraction phase (\(\Delta\)T\({}_{\text{cool}}\)) were registered. In addition, the characteristic time of temperature dampening (\(\tau_{\text{c}}\)), the heat absorbed from the environment per unit volume during the retraction phase (Q\({}_{\text{ab}}\)), the work to deform the sample per unit volume (W) and the coefficient of performance of the material (COP\({}_{\text{mat}}\)) were determined. The \(\Delta\)T\({}_{\text{heat}}\) and \(\Delta\)T\({}_{\text{cool}}\), represented in Figure 1, were, respectively, evaluated as the difference between the temperatures of the NR samples just after the stretching and releasing phase (points 2 and 4 in Figure 1) and the temperature assumed by the material before the mechanical action (points 1 and 3 in Figure 1, assuming the material to be in thermal equilibrium with the environment).