Strain Effects on Melting of Thermally Induced Crystals in Polyisoprene Rubber

The melting behavior of thermally induced crystals (TICs) of synthetic cis ‐1,4‐polyisoprene rubber (IR) under uniaxial stretching was investigated using high‐precision adiabatic calorimetry. Improved data analysis enabled clear separation of the thermal signals associated with melting and recrystallization. The melting temperature ( T m ) of TICs showed a slight increase (~4 K) upon stretching, which is considerably smaller than that commonly reported for strain‐induced crystals (SICs). In addition, stretching resulted in a pronounced broadening of the melting peak of TICs, indicating a widening of the crystal size distribution. Elastocaloric measurements were further carried out to independently determine the changes in internal energy (Δ str U ) and entropy (Δ str S ) associated with stretching. The results indicate that Δ str U is small and that the dominant thermodynamic effect of stretching is a reduction in the conformational entropy of polymer chains. By combining Δ str U and Δ str S with the calorimetric fusion parameters, the small increase in T m with stretching was quantitatively reproduced without adjustable parameters. This demonstrates that the strain‐induced shift in T m of TICs originates primarily from entropy reduction in the liquid (rubbery) state.