ABSTRACT
Rubber ability to crystallize under strain is known to be strongly enhanced-in terms of kinetics and value of crystallinity-when the temperature is cooled down below room temperature (Trabelsi et al. 2004, Andrew et al. 1971). Conversely, when the temperature of a sample maintained in a stretched state is raised above room temperature, its crystalline phase melts (Myamoto et al. 2003, Rault et al. 2006). An increase of the temperature also increase the stretching ratio at SIC onset (λc) and the melting stretching ratio (λm) but also decrease their difference. Such difference may be seen as a “superstraining” effect which refers to the classical and well known supercooling effect met in thermal crystallization. Nevertheless, given the lack of investigation on the effect of the temperature on the morphology of the crystallites induced by strain, or on the effect of a temperature increase on the morphology of crystallites of a constantly stretched rubber, no study proposed a complete thermodynamic description of SIC of NR when the sample is submitted to different thermo-mechanical paths (cyclic deformation at different temperatures or crystallization and melting in the deformed state). In particular, the respective contributions of entropic energy (so-called strain energy), enthalpic energy (temperature dependent) and surface energy (related to crystallite size) on SIC and melting has been scarcely discussed. This study is thus dedicated
2.2 In situ WAXS analysis
The in situ WAXS experiments are carried out on the D2 AM beamline of the European Synchrotron Radiation Facility (ESRF). The X-ray wavelength is 1.54 Å. The tests are performed in a temperature-controlled chamber. The crystallinity index CI is deduced like in Candau et al. (2014), from the intensity of the amorphous phase at the peak top in the unstretched state and the stretched state, respectively. The average crystallite sizes L200, L102 and L002 are estimated from the Scherrer equation with a K parameter of 0,64.
