Mechanocaloric properties of poly(dimethylsiloxane) and ethylene-propylene rubbers

We measured the temperature change in strips of poly(dimethylsiloxane) (PDMS) and ethylene-propylene rubbers that occurred as they were stretched and allowed to shrink by a factor of 3.5-4.5, along with the tensile force that effected the deformation. Main results obtained are as follows: (1) the temperature change is fully reversible in E-P rubber and slightly but definitely irreversible in PDMS rubber. The temperature rise in the latter on stretching is larger than the fall on shrinking by ca. 20 %. (2) The reversible part of heat that evolves from or is absorbed by PDMS rubber is smaller than, but close to, the mechanical energy expended. For E-P rubber, the heat generated greatly exceeds the expended mechanical energy. (3) The entropy of extension as a function of extension is reproduced well by Wang and Guth calculation for PDMS rubber, but not for E-P rubber.