PhD student

PhD position available in Computational Mechanics at Arts et Métiers-ParisTech in collaboration with CMMS / Polish Academy of Science in Lodz

Scientific context:

Deformation of a polymer was long used as a mean to obtain thin and strong fibers. However, large stretching is not easily obtained and fibers become thick and not strong enough. The main difficulty arises from entanglements of macromolecules. To get unentangled macromolecules is not easy. There are three ways to achieve disentanglement: (1) by dissolving a polymer at low-concentration and cool slowly until a gel formation, (2) by crystallization of certain polymers under high pressure and (3) by polymerization below melting temperature with simultaneous crystallization. Polymers with unentangled macromolecules can be drawn to a very high degree of deformation up to 100 000 times. At such high deformation polymer nanofibers can be produced with a thickness as low as 10 nm. A new idea to deform crystalline polymer particles made of unentangled macromolecules into nanofibers by shearing via viscous media has been exploited recently in [1,2]. The deformation of the crystalline particles involves crystal plasticity mechanisms without melting. Nanofibers can be generated by shearing of polymer particles dispersed in another molten polymer. The larger the deformation ratios and shear rates, and the longer the shearing times, the thinner and stronger polymer nanofibers are obtained. Shearing during compounding results in generation of nanofibers within polymer matrix and thus, forming all-polymer nanocomposites. An important problem is the stress transfer from molten polymer matrix to crystals. That aspect is related to dynamic viscosity, interfacial tension, wettability and critical shear stresses required for crystal plasticity [3]. Polymer crystal plasticity was studied in the past based on bulk crystallized polymers where crystals were interconnected with fragments of macromolecules. Those macromolecules were entering the crystals only on crystalline lamellae basal planes while none of them were attached to side walls of the crystals. Crystal side walls were in contact with the surrounding matter via Van der Walls interaction. The system with embedded single crystals in a molten polymer which interacts only via Van der Walls forces is a unique opportunity to determine the deformability and true parameters of polymer crystal plasticity.

Main objectives of the PhD work:

Although it was observed experimentally, the phenomenon of crystal plasticity taking place within crystalline should be investigated from a computational viewpoint using finite elements. The role of various microstructural (crystal orientation, elastic anisotropy) and experimental parameters (shear rate, temperature) will be assessed. Full-field simulation of the fibrillation process of crystalline polymer has never been performed. Most of the computational work resides in implementing generalized continua theories able to account for interfacial tension, in a FE large deformation crystal plasticity framework. Crystal plasticity of polymer crystallites interacting with a matrix only through Van der Walls bounds is a new topic both from an experimental and numerical viewpoint. Finally, by furthering our understanding of the mechanisms taking place during plastic shearing of polymer crystals, and validating the approach proposed in this work, we aim at developing a predictive computational tool for nanocomposites processing.

Candidate:

The candidate should have obtained a Master’s degree with a strong background in computational mechanics, numerical methods or any related field, as well as interest in polymer science; although prior knowledge of the French language is not mandatory, spoken and written English proficiency is needed. The employment contract is for 3 years full-time, starting from October 2015. Net salary is about 1400-1800 EUR per month, depending on the will of the candidate to take on teaching responsabilities (64 hours per year). Applications should be sent to by May 15th 2015.
PhD advisors: Justin Dirrenberger (France), Gilles Régnier (France) & Andrzej Galeski (Poland).

References:

[1] K. Jurczuk, A. Galeski, E. Piorkowska, Polymer, 2014, 54, 4617.
[2] K. Jurczuk, A. Galeski, E. Piorkowska, J. Rheology, 2014, 58, 589.
[3] A. Galeski, G. Regnier, In Nano- and Micromechanics of Polymer Blends and Composites, eds. J. Karger-Kocsis, S. Fakirov, HANSER, Cincinatti, (2009) Chapter 1, pp.3-58