A molecular dynamics study on the size effects of Fe3O4 nanoparticles on the mechanical characteristics of polypyrrole/Fe3O4 nanocomposite

Nanoparticle size effects on elastic properties of polymeric bio-nanocomposites were studied using atomistic molecular dynamics (MD) simulations. Spherical magnetite nanoparticles (MNPs) were placed at the centre of amorphous polypyrrole (PPy) polymer in three models with different sizes. Three models of nanocomposites with the same particle volume fractions with different sizes were considered. The Lennard-Jones 6-12 potential modelled the interface interaction between polymer and particles. Initially, the obtained mechanical properties for pristine PPy and magnetite in the bulk state showed an acceptable agreement with experimental data. Subsequently, simulation results illustrated that incorporating MNP into the PPy matrix improves the elastic modulus of the matrix by 28-58%, and more importantly, decreasing the size of the nanoparticle from 2.40 to 1.80 nm in the system leads to increasing Young's modulus of the nanocomposite from a 3.20 to 3.94 GPa. Furthermore, the atomic investigation demonstrated that this change in elastic modulus is due to the change in interatomic interaction between nanoparticle and polymer when the nanoparticle size changes. Finally, the comparison between the results of MD and micromechanical analysis shows that as the size of the nanoparticle in nanocomposites increases, the elastic properties of nanocomposites converge with the results obtained by the micromechanical approach.