Évènements

30 Sep
30/09/2019 14:00

Sciences & Société

Soutenance de thèse : Zengqiang ZHAI

Molecular dynamics simulation of uni- and bi-modal semicrystalline polymers: nucleation, chain topology and microstructure

Doctorant : Zengqiang ZHAI

Laboratoire INSA : MATEIS
Ecole doctorale : ED162 : Mécanique, Energétique, Génie Civil, Acoustique de Lyon

The crystallization behavior and the molecular topology of bimodal molecular weight distribution polymers are studied using a coarse-grained molecular dynamics model. Extensive simulations of isothermal crystallization have been performed, the nucleation behavior and crystal growth have been investigated, and the final influence on the molecular topology of the obtained semi-crystalline polymer have been addressed. A Growing Probability Method has been proposed to determine the critical nuclei, which is used to calculate the free energy barrier of forming nuclei. The activation energy of diffusion has also been obtained through the temperature dependence of diffusion coefficient. The nucleation rate has been calculated by two ways: classical nucleation theory and directly from MD simulations. The fitting of these two curves confirms the validity of classical nucleation theory to homogeneous nucleation of polymers. Primitive path analysis has also been performed, the entanglement evolution has been continuously monitored during the process of crystallization. We have discovered a quantitative correlation between the degree of disentanglement and crystallinity, indicating that chain disentanglement drives the process of crystallization.  Such a quantitative correlation is the first to the best of the authors’ knowledge. In addition, lamellar thickness also displays a linear relation with the degree of disentanglement, and this rule maintains even the thickness at different temperatures. Based on the observation in our simulations, we are able to elaborate the scenario of the whole process of chain disentangling and lamellar thickening in the manner of chain sliding diffusion.