Titre de la dissertation: Investigation of the microstructural parameters and deformation mechanisms until yield in Ziegler-Natta and Metallocene catalyst-based iPP beyond glass transition.

Promoteurs de thèse: Monsieur Jean-Marie Raquez, Monsieur Oliver Lhost, Monsieur Pierre Lambert

Résumé de la dissertation: This thesis deals with the behaviour of isotactic polypropylene under its a-crystalline form in the linear visco-elastic and in the yield domain, the two opposite limits of the small deformation range. Six homopolymers produced with two catalytic systems have been analysed.The purpose is to establish the relation between these two limits, to understand the various processes at work and to establish the relation with the microstructure that is itself dependent on the catalytic system and on the thermo-mechanical history, highlighting discrepancies and similarities.More specifically, two aspects have been addressed: the influence of the catalytic system on the yield behaviour measured with quasi-static tensile tests, especially the presence of a double yield and its impact; and the relationship between the visco-elastic relaxations – measured with dynamic mechanical and thermal analysis (DMTA) – and the yield.The results obtained in the range of the yielding behaviour have been modelled thanks to the Ree-Eyring approach and have shown that a double yielding phenomenon was indeed present, though it could not be ascribed clearly to the well-known couple « heterogeneous/homogeneous » mechanism. By contrast, we suggest that it could result from the more less strong competition between yield and cavitation leading to two mechanisms of heterogeneous yield, likely to be due to the multiphasic composition of the products.The results obtained in the visco-elastic range have been modelled with a thermorheological approach usually carried out in the melt. This was compulsory because the strong overlap of the viscoelastic curves prevented a relevant exploitation of the time-temperature superposition principle and this type of model was particularly relevant in our case, being very sensitive to any small differences. Three to four mechanisms could be identified and their activation energy was put in relation with the activation energy found in the quasi-static tests, ranging between 100kJ/mol to 200kJ/mol. The DMTA tests unveiled more activated processes than those measured in quasi-static tests.The influence of the catalytic system on the activation volume, energy and time constant was analysed from the viewpoint of the underlying diffusion of screw dislocations throughout the interfaces of the different phases, especially thanks to the helical jumps. The same mechanism was also used to explain the behaviour in the viscoelastic range.Some preliminary structure/properties relationships were presented with respect to the crystalline thickness of lamellae and amorphous phase, to the isotactic length as well as to the yield and the viscoelastic properties. It was found that, comparing two of the three products exhibiting the same mechanism of deformation at yield, the flexibility of the amorphous phase due to the difference of catalyst was critical to explain the differences in the activation volumes, energies and time constants. It was also found that the lower a-transitions of the three products were occurring in a very narrow time-temperature frame, and seemed to correspond to the transition of the rigid amorphous fraction or of the conformationally disordered crystals, around 50°C-60°C.Several Matlab® programs were implemented to handle and analyse the data coming from the quasi-static and dynamic tests.To our knowledge, it was the first time that the thermorheological approach was applied to the solid state. It was also the first time that a set of isotactic polypropylene products was systematically tested at low to very low strain rates, and that the link and interpretation of the activation constants was discussed in relation with its specific structure and not in general.