PhD defence of Tianqi Huang

Characterization and modeling of the mechanical behavior of PE-vitrimers.

 

M. Xavier COLIN, Laboratoire Procédé et Ingénierie en Mécanique et Matériaux, Rapporteur

M. Stéphane ANDRE, Laboratoire Énergies & Mécanique Théorique et Appliquée, Université de Lorraine, Rapporteur

M. Renaud NICOLAÿ, Laboratoire Matière Molle et Chimie, CNRS, ESPCI Paris

Mme Sandrine HOPPE, Laboratoire Réactions et Génie des Procédés, Université de Lorraine

Mme Julie ALVES, Aliaxis Research & Technology

M. Yannick TILLIER, Centre de mise en formes des matériaux, Mines Paris PSL université

M. Jean-Luc BOUVARD, Centre de mise en formes des matériaux, Mines Paris PSL université

 

 

Abstract:

 

This Ph.D. thesis focuses on the characterization of the behavior of several polyethylene-based polymers. In particular, to evaluate their recycling properties, the behavior of several vitrimers (with different cross-linking degrees) is compared, before and after aging, with that of a thermoplastic (HDPE) and a thermoset (PEXb). To better understand the relationship between the microstructure and the properties of these materials, a physical model was also proposed to model these behaviors. The parameters of this model were identified thanks to numerous experimental observations performed at different scales. The crystalline structure (at the microstructural scale) was characterized using DSC and X-ray. The dynamic properties (at the mesoscopic scale) were characterized thanks to DMTA tests and rheological analysis. The mechanical behavior (at the macroscopic scale) was characterized using tensile and creep tests. The test conditions used to characterize the mechanical behavior at large deformation were chosen according to a methodology based on the "equivalent strain rate at a reference temperature" (at the α-transition temperature). Therefore, the double effect of temperature and strain rate is taken into account. For the initial unaged state, the degree of crystallinity changes little for the different types of polymers. However, the thickness of the crystal lamellae and the viscoelastic properties show a strong dependence on the type polymer studied. The application of the time-temperature equivalence below the melting temperature, validated here at small strain and also at large deformation, results in a unique master curve for the different polymers used in this study. However, this is not the case above this temperature where only vitrimers and PEXb show a rubbery plateau. For HDPE and vitrimers, the aging protocol causes chain scission, resulting in a decrease in molecular weight (Mw). This has a direct effect on the properties observed using DMTA and on the mechanical behavior at large deformation. For aged vitrimers and PEXb, the same true strain levels as HDPE are observed during a creep test at a longer time scale. Unlike PEXb, the effect of aging on vitrimers can be erased by heating above the α'-transition temperature. Finally, the model used in this study allows to reproduce the mechanical behavior observed experimentally for each type of PE. This model also shows its ability to take into account the specificities of the different chain networks that characterize these materials. To conclude, the vitrimers show thermomechanical properties that are globally close to those of HDPE and PEXb, but with a higher recycling potential compared to the latter.

 

Keywords: Vitrimer, Microstructure, Time-temperature equivalence, Mechanical behavior, Aging, Modeling