Séminaire de Cristian Ovalle (MINES Paris, Centre des Matériaux CMAT)

Nanocavitation size distribution and morphology evolution in deformed High Density PolyEthylene (HDPE)

Ovalle, C.

MINES Paris, PSL University, Centre des Matériaux (CMAT), CNRS UMR 7633, BP 87 91003, Evry, France

Abstract

The observation of cavitation in polymers depends on the resolution of the involved experimental technique. Addressing the void nucleation requires resolution able to reach the nanometric scale: SAXS, WAXS, SEM, TEM were commonly used to obtain the best resolution. 3D observations like tomography is limited to micro-scale. Furthermore, the size and the morphology of voids are dependent on the spherulitic microstructure characteristic length.

This study deals with the examination of the nanocavitation distribution in deformed High Density PolyEthylene (HDPE), for which the average diameter of the spherulite was about 2- 3 μm. The motivation of this work is therefore to show that the cavitation mechanisms are the same as in other semicrystalline polymers but at a lower scale.

A flat notched sample of semicrystalline HDPE was deformed under steady crosshead speed up to the end of the stress softening. The microstructure and the voiding state in the unloaded sample was ex-situ observed in 3D at the European Synchrotron Radiation Facility (ESRF). Two parallel regions were studied: four locations of interest near the notch edge and three locations of interest centred on the longitudinal axis, through the necked zone up to the un- necked zone with a particular focus on the neck shoulder, by classical synchrotron parallel beam microtomography and magnified holotomography with a true spatial resolution of 0.7 μm (HDPE0.7) and 0.05 μm (HDPE0.05), respectively. A qualitative study on the evolution of the distribution and morphology of cavities along the longitudinal axis was first carried out. The results of both HDPE0.7 and HDPE0.05 were compared with the axial evolution of the microstructure in PA6, from previous works [1, 2]. The qualitative study showed that the cavitation mechanisms in HDPE are the same as in the PA6 but at a lower scale. Then, thanks to 3D image segmentation and 2D-Fast Fourier Transform, quantification of void volume fraction and void morphology was performed.

It was found that the nanovoid morphology and spatial distribution in HDPE revealed to be similar to those of other semicrystalline polymers, but at a different scale. While a slight decrease of the void diameter was seen, from the end of the neck shoulder to the extended neck region, a significant increment of the height was evidenced. Furthermore, the results provide compelling evidence of morphology transition of cavities within the test sample from an oblate to a prolate geometry [3].

References

[1] Cayzac, H.-A.; Saï, K.; Laiarinandrasana, L., International Journal of Plasticity 2013, 51, 4372- 7383.

[2] Laiarinandrasana, L.; Klinkova, O.; Nguyen, F.; Proudhon, H.; Morgeneyer, T.F.; Ludwig, W., International Journal of Plasticity 2016, 83, 19-36.

[3] Ovalle, C.; Cloetens, P.; Proudhon, H.; Morgeneyer, T.F.; Laiarinandrasana, L., Polymer 2021, 229, 123959.