Molecular dynamics simulation of fracture and energy dissipation in polymer/clay nanocomposites

Molecular dynamics simulation method was used to investigate the effect of nanofillers on fracture strength and energy dissipation of polymers, including nanofillers contents, interaction strength between the nanofillers and polymer chains, relaxation time and geometry of the nanofillers. Molecular dynamics simulation results revealed that the addition of layered silicate can improve the fracture strength of polymers. The interactions between the surface of layered silicate and polymer chains, and the difference between the relaxation times of layered silicate and that f polymer chains have significant influences on the fracture strength and energy dissipation of polymers. For these polymers, which Tgs are lower than room temperature, such as polyurethane, or nearby (or equal to) room temperature, such as Nylons, the nanoplatelets can always enhance the mechanical properties. However, for these polymers, which Tgs are higher than room temperature, such as epoxy and polystyrene, the addition of the nanoplatelets does not work well for toughening these polymers. If one wants the nanoplatelets to be working for toughening these polymers, it is necessary to build up a stress relaxation interface between the polymer matrix and the nanoplatelets, such as the modification of the surfaces of nanofillers using coupling agents. When the relaxation time of the polymer is long enough, the incorporation of nanofillers into the polymer will cause the polymer to become more brittle. This result explains why the toughness of epoxy/ clay nanocomposites becomes poor. The simulation results clearly revealed that' the orientation of nanoplatelets is reversible at low strain of 50% suggesting that additional energy dissipation only results from the frictional sliding at the interface, whereas the orientatiqn of nanoplatelets at large strain of 200% showed more irreversibility suggesting that the additional energy loss results from both the interfacial frictional sliding and the orientation of the nanoplatelets. The additional dissipated energy was also influenced by the strength of interactions between polymer chains and clay platelets. The stronger interactions the more energy dissipated. Molecular dynamics simulation results revealed that the geometry of nanofillers also affect the mechanical properties of polymer nanocomposites. The enhancement if carbon nanotubes on the mechanical properties of the polymers are enhanced the greatest by carbon nanotubes.