Barocaloric properties of reduced graphene oxide-shape memory polymer nanocomposites

Solid-state refrigeration based on the barocaloric effect—temperature/entropy change resulting from applied hydrostatic pressure—offers a promising alternative to vapor compression systems. The significant barocaloric response of soft materials such as polymers, elastomers and plastic crystals demonstrates their potential for high-efficiency and low-cost refrigeration. Nevertheless, the low thermal conductivity and high compressibility of soft materials pose challenges related to heat transfer and the effective application of pressure within a device. To mitigate this issue, one approach is to incorporate additives into soft materials. This study investigates the barocaloric performance of novel nanocomposites comprising polyurethane-based shape memory polymers (SMPs) with reduced graphene oxide (rGO) additives. We characterized the mechanical, thermal, and barocaloric properties of the SMP and its composites with 2, 4 and 6 wt% rGO, and assessed their viability for barocaloric refrigeration. We observe that while the SMP barocaloric isothermal entropy change under 0.09 GPa pressure decreases by 0.5 × with 6 wt% rGO loading, the thermal conductivity increases by 1.5 × and compressibility decreases by 0.6 ×. The adiabatic temperature change, measured using quasi-adiabatic measurements at room temperature with 0.43 GPa applied pressure, decreased from 14.5 K to 12.6 K with rGO addition. The material COP for these composites, which decreased with rGO addition, ranged between 10 and 14 around room temperature. However, comparison of the heat transfer rate and the normalized barocaloric temperature change—parameters relevant for the operation of a barocaloric device—revealed an optimal rGO loading of 2 wt%. These results underscore the promise of SMPs, as well as the role of additives in optimizing the performance of solid-state refrigerants for barocaloric refrigeration devices.