## 1 Introduction
Among elastocaloric polymers, natural rubber (cis 1,4 polyisoprene, NR) seems to be among the most promising ones. The elastocaloric effect of NR was discovered by [PERSON], a natural philosopher in the early nineteenth century, and was then reported for the first time by [PERSON] in 1859 [9]. However, only very recently, the elastocaloric properties of NR have been considered for developing a solid-state cooling technology. NR is a biobased non-toxic material, and its good fatigue properties make it ideal for a future solid-state cooling application [10]. Furthermore, recent research works have brought to light its optimum elastocaloric performance in comparison to other elastomers. [PERSON] et al. studied the elastocaloric properties of five types of rubbers, including natural rubber, cis-butadiene rubber, chlorosulfonated polyethylene, silicon rubber and styrene-butadiene rubber [11]. They observed that natural rubber presented the best cooling performance among the studied elastomers, showing a coefficient of performance (COP), evaluated as the ratio between thermal energy extracted during the stress release phase and the mechanical energy needed to deform it, of more than 2, and a correspondent temperature decrease of 3.5 \({}^{\circ}\)C. However, they observed that the temperature difference (\(\Delta\)T) of the material during the stretching phase was higher than that in the recovery stage, meaning that undesired dissipative phenomena occurred. [PERSON] et al. examined the elastocaloric effect of thermoplastic polyurethane (TPU), noting that while its elastocaloric performance was promising, it was slightly lower than that of natural rubber. Additionally, TPU's inelastic behavior and limited fatigue resistance posed challenges for its use in solid-state cooling applications [12]. Differently from other elastomers, the elastocaloric effect of NR is attributed to two contributions. The first is related to the reversible orientation/disorientation of macromolecules in the strain direction, as described before. The second contribution comes from the phenomenon of strain-induced crystallization (SIC), i.e., a partial crystallization of the elastomer as a result of deformation. When NR is stretched, the molecular orientation in the rubber network favors the nucleation and growth of crystallites, which melt once the NR returns to the initial unstretched state. Since crystallization and melting are exothermic and endothermic phenomena, respectively, the heat developed/released in the stretching/release phases is added to the thermoelastic heat contribution. In the literature, it has been extensively evidenced that the SIC of NR occurs at a strain of approx. 300% [9; 13; 14; 15; 16]. Consequently, to enhance the cooling performance of this material, it should be deformed at a strain level close to or higher than 300%. Recent works have explored the effects of temperature, strain rate, maximum deformation and filler addition on the elastocaloric properties of NR. The temperature dependence was the objective of the study of [PERSON] et al. [17]. They observed that NR exhibited interesting elastocaloric properties over a broad temperature range between 0 and 49 \({}^{\circ}\)C. The maximum temperature change (12 \({}^{\circ}\)C) was observed at 10 \({}^{\circ}\)C and a strain of 600%. In the temperature range 10-49 \({}^{\circ}\)C, the \(\Delta\)T decreased from 12 to 5.8 \({}^{\circ}\)C, while in the range 0-10 \({}^{\circ}\)C, it dropped to 4 \({}^{\circ}\)C. These results suggest that working at a temperature of 10-30 \({}^{\circ}\)C is needed to take full advantage of the cooling performance of NR. The elastocaloric properties of natural rubber and natural/waste rubber blends were also investigated by [PERSON] et al, who discriminated between the contributions of thermoelastic effects and strain-induced crystallization/melting by using a thermodynamic frame [18]. They observed that the elastocaloric properties of the blends increase with a waste rubber content of 20 wt. %.