Mechanisms of deformation in crystallizable natural rubber. Part 1: Thermal characterization

This paper investigates the mechanisms of deformation in rubber, especially stress-induced crystallization, using infrared thermography. Temperature variations are measured during cyclic uniaxial mechanical tests at ambient temperature. Results show that natural rubber mainly exhibits entropic behaviour: the material produces (resp. absorbs) heat during loading (resp. unloading). The crystallization of the polymer chains under tension leads to a temperature increase of the order of several degrees Celsius. If crystallization and crystallite melting occur over one mechanical cycle, a hysteresis loop is observed in terms of the strain-stress relationship. Stress relaxation tests show that the thermal signatures of crystallization and of crystallite melting are different. Indeed, if the strain is maintained fixed during loading, the temperature continues to increase for a few seconds before returning to the ambient temperature. This reveals that crystallization continues during relaxation. On the contrary, if the strain is maintained fixed during unloading, the specimen returns instantaneously to the ambient temperature.