Numerical Analysis Of Microscopic Deformation Mechanism Of Polyethylene And Its Composites

Molecular dynamics (MD) simulations were used to investigate the tensile behavior of polyethylene. The initial chain structures of a united atom (UA) model and a coarse grained (CG) model were created using a method similar to Monte Carlo self-avoiding random walks. United-atom and coarse-grained potentials were derived from detailed all-atom simulations. Uniaxial tensile tests of these initial structures after an equilibration sequence were simulated by different MD methods. The results suggest that the stress-strain curves from the united atom model clearly comprise elastic, yield, strain softening and strain hardening regions that were all qualitatively in agreement with all-atom simulations and experimental results. The coarse grained model is suitable to study polymer crystallization. The elastic modulus and yield strength of the resulting semi-crystalline structure are larger than those of amorphous polyethylene due to the orientation of the crystallization region.Reverse mapping method lead crystallinity of semi-crystalline polyethylene to be greatly reduced. To get the mechanical behavior of semi-crystalline polyethylene with a higher crystallinity, the coarse-grained potential still needs to be optimized to drop down the high pressure in coarse-grained model system. In addition, coarse-grained method will make the molecular chains become softer and smoother, which result in lower strength and modulus than the one in UA model. So the Dissipative Particle Dynamics method need to be introduced to slow down the dynamics of the CG model, making the molecular chain stiffer without altering the conservative potential.UA model and CG model were respectively used for studying CNT/PE composite material. We found that the composites in UA model have no obvious enhancement under uniaxial tensile. Because the strain rate in MD simulation is so high that yield strength is far greater than the experimental value and that the load on polyethylene matrix cannot transfer to CNT. However, the composites in CG model have the enhancement, due to the reduced strength with the softer CG potential.