Molecular dynamics simulation study of the fracture properties of polymer nanocomposites filled with grafted nanoparticles

By employing coarse-grained molecular dynamics simulations, we investigated the fracture behavior of polymer nanocomposites (PNCs) filled with polymer-grafted nanoparticles (NPs) in detail by particularly regulating the grafting density and the length of the grafted chain. By calculating their fracture energy, we observed that their rupture properties first increase and then decrease with the increase of the grafting density or the length of the grafted chains. Their bond orientation degree and their van der Waals energy change are characterized to understand their fracture behavior. To further explain it, we analyzed the contributions of the matrix chains, grafted chains, and NPs to the total stress. It is interesting to find that the stress borne by one bead of matrix chains or NPs gradually increases with the grafting density, while the stress borne by the grafted chains first increases and then decreases. In addition, the stress borne by one bead of matrix chains or grafted chains gradually increases with the length of the grafted chains, while the stress borne by NPs remains nearly unchanged. As a result of these contributions, the optimal fracture properties appear at the moderate grafting density or length of the grafted chain. Then, the number of voids is quantified, which first increases and then decreases with strain because of the coalescence of small voids into large ones. Accompanying this, the maximum void size increases significantly. Furthermore, the maximum number of voids increases with the grafting density, while it is nearly independent of the length of the grafted chain. In particular, the voids are preferably generated at the end beads of the chains or at the surfaces of the NPs. In summary, this work could provide some further understanding of how the grafted chains affect the fracture properties of the PNCs.