Elastocaloric properties of polydimethylsiloxane, ethylene-propylene, and polyisoprene rubbers

An apparatus for measuring the elastocaloric effect was constructed to elucidate the thermodynamic origin of rubber elasticity. The apparatus was designed to simultaneously measure the temperature and stress changes during the rapid elongation and contraction of rubber stripes. Compared with the previous instrument, the data quality was considerably improved: the temperature resolution was ~ 1 mK, the data acquisition time interval was ~ 0.05 s, and the elongation distance was controlled to a precision of ± 0.01 mm. Measurements were conducted for three rubbers, polydimethylsiloxane (PDMS), ethylene-propylene (EP), and polyisoprene (PI), and the contributions of entropy and internal energy to the rubber elasticity were evaluated. For the PDMS and PI rubbers, entropic elasticity was dominant, but energetic elasticity was also non-negligible, whereas the EP rubber exhibited a negative energetic contribution to rubber elasticity. For the PDMS rubbers, the extent of the temperature rise during elongation was greater than that of the temperature drop during contraction, indicating that the process was irreversible. After examining several possible reasons such as the heat of friction, creep relaxation while maintaining the stretched state, and relaxation of residual strain after contraction, the main cause was identified as the difference in the thermodynamic path of elongation and contraction.