Unveiling the Origin of the Giant Barocaloric Effect in Natural Rubber

The barocaloric effect (BCE) is characterized as thermal responses (variations of temperature or entropy) in a material resulting from compression. Several materials exhibit a BCE suitable for development of solid-state cooling devices, typically associated with pressure-induced phase transitions. A giant BCE has been observed for natural rubber (NR), which makes it a cheap and environmentally friendly candidate for such a purpose. The reason for the significant BCE in NR is still elusive, considering that there is no evidence of phase transitions in the process. The present study uses a combination of classical molecular dynamics (MD) simulations and a thermodynamic analysis to investigate the origin of the giant BCE in NR. MD simulations of adiabatic compression cycles for NR were carried out under varied applied pressures and initial temperatures and were able to capture the BCE. A detailed analysis of the results helped us to elucidate the structural transformations and resulting energy changes in the material under compression. MD results for isothermal compression along with the thermodynamic analysis showed that the high compressibility of NR combined with an unusual decrease in the potential energy at the molecular level upon compression favors significantly the BCE (quantified by isothermal entropy changes and adiabatic temperature changes in the process), a feature not commonly seen in other materials. These findings can be extended to other polymers and are certainly going to be useful toward the design of materials with an enhanced BCE.