Simulation study of hardness via nanoindentation of polymer nanocomposites through molecular dynamics

Polyethylene (PE) and polylactic acid (PLA) are two of the most widely used polymers. It is known that their mechanical properties, such as hardness, are poor. In order to enhance the mechanical properties of those polymers, reinforcements have to be incorporated. Carbon nanotubes (CNTs) have proven to be an excellent choice for reinforcement. However, due to the π-π interactions, the nanotubes tend to agglomerate. One of the strategies to avoid agglomerations is chemical functionalization. The 3-amino-propyl tri-ethoxy silane (APTES) is a suitable option for functionalization. In this work, three different polymeric configurations were analyzed to verify their effect on hardness: linear, hyperbranched, and star-like. Further, the configuration with the highest hardness was reinforced with functionalized CNTs with APTES groups. Results indicate that a linear configuration, both of PE and PLA, generates greater hardness due to better structural arrangement. The percentage of functionalization of CNTs that generates a better interaction with PLA is 3%, which corresponds to five anchored groups. The addition of CNTs increases the hardness 14 times with respect to that of PLA without reinforcement.

Molecular models were built and visualized using MedeA and OVITO software programs. All simulations were run using the LAMMPS software. The force fields utilized were PCFF for polyethylene and COMPASS for PLA and carbon nanotubes. A van der Waals model was used to consider the non-bonding interactions between the indenter and substrate. An NVT ensemble was used to construct the substrates, and the indentation procedure involved iterative cycles of minimization and displacement of the indenter.