We show in this study the high benefits that can be retrieved from the SAXS technique (Small Angle X-ray Scattering) performed with synchrotron lightsource (beamline cSAXS at PSI-SLS) to follow the initiation and the development of a necking (plastic instability) phenomenon underwent by a semicrystalline polymer under tension. High sensitivity and fast detectors used at this beamline as well as motorized fast displacement systems enable to realize 1D and even 2D scans of the specimen during the test. The integration time (33ms) and read out time (3ms) make a spatial resolution of the scanning of 360microm at the displacement rate of 10mm/s ensured by the displacement OWIS stages. At the same time, the tensile machine proceeds with a displacement rate of 20mm/s which means a negligible displacement of the material points on the specimen during the scan.

The methodology to handle the data is quite tedious also not complicated. In order to study quantitatively the microstructure evolution during tension for different material points, a precise calibration procedure has been set up thanks to 3-D stereo Digital Image Correlation (DIC system ARAMIS from GOM instruments). The same tensile tests have been performed under DIC to provide the relationship between the running time of the test, the material points coordinates in the reference frame, their respective eulerian positions according to time and their measured local true strain. This gives rise to the following Fig.1 linking all of these variables.

The points give the position of the recorded scans along the specimen (Z screaning). The red lines figure out the trajectory of initial material points and allow to find the patterns taken for a same material points at different times. The blue curves are isostrain curves and allow to find all patterns corresponding to a given strain, although for different material points. This figure is the key to be able now to determine whether or not the microstructure state is only governed by the true strain.

As a result of the [0.017-1] nm-1 q-range of our experiments, the microstructure is probed at the length scale of scatterers ranging approximately in the 10-350 nm. Its evolution is monitored through a quantitative observable raised from the SAXS pattern analysis. It is an anisotropy index [1] defined as where correspond to the intensities measured along the horizontal and vertical axis of the image frames, respectively the tensile and transverse directions. The local anisotropy is then measured at different locations for different true strain values ranging from 0 to 2 (about 700% of extension ratio). We cover the range of viscoelasticity followed by a long stage of plastic development and a plastic propagation accompanying the hardening rubbery state. The anisotropy index measured for different material points as a function of true strain produces identical curves (within the uncertainty of the measurements). This new result proves that the microstructure is fully governed by the sole strain. Such results enable to consider that this observable gives a true signature of the kinetics of internal reorganization during the tensile deformation of the polymer. It is worthwhile to mention that next efforts made in the modeling of semicrystalline polymer should pretend to recover such signature.

References

[1] L. Farge, S. André, F. Meneau, J. Dillet, C. Cunat, A common multiscale feature of the deformation mechanisms of a semi-crystalline polymer, Macromolecules, 46(24), (2013), 9659-9668.