Comparison of direct and indirect measurement of the elastocaloric effect in natural rubber.

The directly measured temperature change ΔT upon deformation (elastocaloric effect) of natural rubber was compared with indirect method, which is deduced from the Clausius-Clapeyron factor (∂σ/∂T)ε, where σ is the stress and ε is the strain. The factor (∂σ/∂T)ε can be measured by two different methods. One is to measure the stress vs. strain behavior at different static temperatures. It is found that the ΔT deduction is underestimated or even of opposite sign compared with the directly measured one. These behaviors are different from elastocaloric effect of shape memory alloys. An interpretation based on strain-induced crystallite is proposed. The other characterization is to measure the stress vs. temperature at constant strain. It results in a prediction, which is in good quantitative agreement with the directly measured one. The stress appears then to be a non-state variable, thus questioning the ergodicity of the material. [ABSTRACT FROM AUTHOR]