Soutenance de thèse : Junxiong WANG

Molecular dynamics simulation of semicrystalline polymers: from molecular topology to mechanical properties

Doctorant : Junxiong WANG

Laboratoire INSA : MateiS

École doctorale : ED34 : Matériaux de Lyon

Semi-crystalline polymers have attracted widespread attention due to their wide range of industrial applications, attractive mechanical properties, and good chemical resistance. Semi-crystalline polymers exhibit excellent mechanical properties due to their unique molecular structure (crystalline and amorphous phases overlapping each other). Topological molecules, like ties, loops, … and entanglements in amorphous phase, serve as stress transmitters and can be crucial to mechanical properties. However, these microstructures cannot be studied quantitatively experimentally, and the nonequilibrium process of crystallization and how the microstructure affects mechanical properties cannot be studied at the nanoscale. The dependence of the mechanical properties of semi-crystalline polymers on topology and entanglement has been explored using a coarse-grained model through molecular dynamics simulations. From cooling a melt, and after isothermal treatment, semi-crystalline polymers with lamellar structures were obtained with different entanglement densities and topologies. The strongest mechanical properties are shown when the tensile direction is highly consistent with the crystal chain orientation. And the system with a higher entanglement density has a smaller yield stress but a significant stress-hardening regime, indicating that high entanglement density effectively increases the stress-hardening effect. Additionally, the effect of different topologies on mechanical properties has been explored. Uniaxial tensile test results show that cilia have little effect on mechanical properties. The yield stress increases with the number of loops, showing that not only the loops but also the number of topologies has a strong influence on the mechanical properties. The tie molecules appear to have a slightly greater impact on the mechanical properties than the loops, manifesting in a slight strain softening effect. These results will enhance the understanding of the relationship between microstructure and mechanical properties of semicrystalline polymers.