PhD 2014-17 (Cédric Sanfourche)

Effects of mechanical and microstructural gradients on the coupling between mechanics and gas diffusion to the surface of a polymer

This thesis focuses on the transfer of heat and material involved to the surface of parts in contact with gas flows. This situation, common in industrial context, is particularly critical for polymers because their mechanical response and damaging are very sensitive to temperature and diffusion of some gases.
As part of this project, we will focus on semi-crystalline thermoplastics.
The complexity of the project is the multiplicity of gradients present in the polymer layer in the vicinity of the interface:

  • microstructuregradientsresulting fromthe shapingand /or appearingwhile in use
  • gradient of damageinduced bydifferentstates of progressofdegradation phenomenadue to the presenceofgasorinducedexternally
  • macroscopic mechanicalgradients in thecaseofgeometriesand/ orcomplex mechanical

Each of these effects is likely to change, individually and coupled, both diffusional properties and the mechanical response.
The general objective of this thesis is to characterize the relative influence of microstructure gradients, and mechanics damaging on the diffuso-mechanical response of a semi-crystalline polymer, clarify their hierarchy and their degree of coupling.
The effects of these gradients will first be apprehended in more decoupled as possible, working :

  1. First with samples of variable microstructural characteristics from one sample to the other but homogeneous inside one particular sample,
  2. Then on more or less damaged sample, chemically or mechanically.

The section on microstructural features is a priority to start due to their importance on the diffuso-mechanical response. A study on the microstructure coupled to the mechanical characteristics will also be implemented.

Microstructure visualisation of polypropylene after annealing

Microstructure visualisation of polypropylene after annealing

Secondly, samples gradients will be studied to better understand how the diffuso-mechanical properties homogenize.
The study includes an important experimental component, based on experiences developed in laboratory to characterize the mechanical and diffusive aspects (hydrogen, carbon dioxide), in coupled conditions or not. The experimental part is inseparable from numerical simulations needed to find by reverse analysis the mechanical gradients and gas concentration in the sample.

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