A stroboscopic X-ray apparatus for the study of the kinetics of strain-induced crystallization in natural rubber

A stroboscopic X-ray diffraction machine is described that allows studying kinetics aspects of strain-induced crystallization in natural rubber. Besides conventional mechanical cycling in extension, samples may be submitted to periodic extension cycling over a 0.05 Hz–30 Hz frequency range and for various amplitudes of elongation. Pertinent parameters as crystalline content, crystallite orientation and segmental order parameter for the amorphous fraction are determined. The existence of a regulation process of the strain experienced by the chains that remain molten due to the phenomenon of strain-induced crystallization is clearly demonstrated during conventional mechanical cycling at low deformation rate. Melting retardation is shown to lead to an equalization of crystalline content in conditions of periodic cycling of moderate amplitude and below some average elongation. On the other hand, crystallization appears to be a rapid process subsisting up to the maximal cycling frequency corresponding to a stretching time of 17 ms. This is confirmed by periodic cycling of higher amplitude that suggests that strain-induced crystallization may appear within millisecond time. An approximate logarithmic dependence of the crystalline content and elongation of the amorphous fraction upon stretching time is further found.