Elastocaloric effect dependence on pre-elongation in natural rubber

In the context of solid-state-cooling, the elastocaloric effect offers a very large controlled entropy change based in low-cost polymers, especially natural rubber which is environmentally friendly. However, large elastocaloric activity requires large elongation (>5), which makes this material impractical for cooling systems due to the large change in sample's area. By performing a pre-elongation, area change is limited, and beta = -partial derivative gamma/partial derivative lambda (where gamma is the specific entropy and lambda is the elongation) is larger. The highest beta value is obtained when pre-elongation is right before (at the eve) the onset of the strain-induced crystallization, which is also interpreted in the view of molecular conformation. Experimental results obtained on a natural rubber sample showed an adiabatic temperature change of 4.3 degrees C for pre-elongation of 4 with further elongation of 4 (true strain change of 69%). Furthermore, the entropy exhibits a quasi-linear dependence on elongation, and the beta value is found to be 6400 JK(-1) m(-3). (C) 2015 AIP Publishing LLC.