A microscopic model to investigate stress-strain response for polymer nanocomposites containing rod-like particles

Abstract

Polymer nanocomposites are an important class of materials that exhibit distinct physicochemical properties that are inaccessible with individual components acting alone. These nanocomposites have recently attracted intensive research owing to their potential industrial interests. Polymers nanocomposites consist of nanofillers dispersed in the polymer matrix. Applications of these composites are enhanced by making them porous. This project is an endeavor to study the mechanical properties of such composite containing the porous percolating structure of the nanoparticle-polymer mesh surrounding micron-sized inter-connected voids. In experiments, such porous structures are realized by the ice-templating method such that nanoparticles(NPs) are trapped in polymer mesh, which forms the walls around large inter-connected open voids. The nanoparticles are held together by polymers and cross-linkers. We approximate the interaction between them by a simple coarse-grained description which consists of springs. The mechanical response arises primarily from the network of cross-linked polymer and not from the nanoparticles. These nanocomposites with 90 − 95% porosity are potential interests.They retain their original shape and size after the release of uni-axial compression up to 20% of their original volume. The stress response of NP-polymer mesh containing spherical particles has been microscopically modeled and the plot/data matches the experiment. We use non-equilibrium molecular dynamics as implemented in LAMMPS for our studies. First we benchmark the previous results for the coarse-grained model. We replace the spherical voids in this model with cubical voids and note the change in response. We obtain the functional dependence of Young’s Modulus on the nominal density ρ0 of the scaffold containing spherical particles. Our focus will be on nanocomposites that incorporate anisotropic rod particles. We also derive the functional dependence of Y on nominal density ρ0 for scaffolds incorporating rod particles. This increases the complexity as non-spherical particles have an additional orientation degree of freedom. The aspect ratio of rod-like fillers strongly influences the mechanical response of hybrid nanocomposites. Experimentally mechanical response of nanocomposites prepared from ellipsoidal fillers has been shown to have complex non-trivial behavior whose microscopic detail is not understood. Elastic modulus (Y) shows nontrivial dependence on the nominal density ρ0 of the sponge. In experiments, we observe a scaling in ρ0 dependence of G from linear to quadratic as the aspect ratio increases from 1 to 4 arising due to a change in the mechanism of sponge response. Understanding the interplay between filler aspect ratio and filler matrix interaction has important implications for the control of nanocomposite properties.