ATOMISTIC SIMULATIONS TO STUDY PROPERTIES OF 2 DIMENSIONAL NANOCOMPOSITES

Abstract

Owing to their exceptional mechanical and thermal properties, graphene and hexagonal boron nitride (h-BN) nanofillers are emerging as potential candidates for reinforcing the polymer based nanocomposites. Due to challenges associated with the experimental characterization of these nanomaterials, their capabilitites are still unexplored to its full potential. Quantum calculations are accurate but highly intensive in terms of computational efficiency, whereas the experimental characterization at these spatial and temporal scales is not feasible. Reactive empirical force fields are viable alternatives to quantum calculation and experimental techniques, and are accurate enough to predict the mechanical properties, fracture toughness and thermal conductivities of graphene and corresponding nanocomposites. Pristine graphene is a hypothetical concept; synthesizing techniques inadvertently introduce several topological defects such as vacancy, dislocations and Stone-Thrower-Wales (STW) defects. Among them STW defects are generated without deleting any atom from the lattice position, but are introduced by rotating single C-C bond. STW defects helps in generating out of plane displacement in conjunction with redistribution of stress around the rotated bond that can used to improve the fracture toughness of brittle graphene or can also be used to heal crack separation. An overall improvement in the fracture toughness of pristine graphene as well as graphene containing hydrogen at the crack edges was predicted. Also, molecular dynamics based simulations were performed to study the effects of functional groups such as hydroxyl and epoxide on the mechanical strength and failure morphology of graphene oxide. Atomistic simulations predicted a transition in the failure morphology of hydroxyl functionalised graphene from brittle to ductile as a function of its spatial distribution on graphene. This transition in failure morphology from brittle to ductile was gradual in nature and was observed at lower percentage coverage of graphene. Failure morphologies depict that hydroxyl groups tend to boost the ductility through chains and elongated rings formation at lower percentage coverage. Also, the electrostatic charge redistribution and epoxide to ether transformation were found to be the decisive mechanisms behind the ductile response shown by hydroxyl and epoxide groups, respectively. Same study was extended to observe the effect of these functional groups on the fracture toughness of graphene. Similar kind of studies was also performed to study the mechanical, fracture and thermal behavior of bicrystalline graphene oxide. It was seen that the epoxide functionalisation helps in transforming the catastrophic brittle behaviour into ductile. Failure morphologies depict that epoxide groups tend to boost the ductility through altering the fracture path and not affecting the grain boundaries either. Also, the epoxide to ether transformations was found to be the decisive mechanism behind the plastic response shown by epoxide groups. In addition to mechanical and fracture behaviour of pristine and bi crysatlline graphene, non-equilibrium molecular dynamics simulations were performed to investigate the effect of chemical functionalisation on the thermal conductivity of graphene. Hydroxyl and epoxide functionalisation of graphene has deteriorating effect on the thermal conductivity of graphene. Functionalisation of graphene alters the local structural deformation, phonon transmittance and vibrational frequency of the graphene lattice network. Spatial distribution of hydroxyl group in conjunction with tensile strain engineering plays a significant role in tailoring the thermal conductivity of functionalised graphene. Atomistic alterations such as stable flattening, formation of atomic chains network and epoxide-to-ether chemical transformation upon stretching resulted in an enhanced thermal conductivity. It was concluded that oxidation can be used to tailor the mechanical and fracture properties of prsitine and bicrystalline graphene. Simulations were also performed to study the reinforcing capabilitites of bicrystalline as compared to pristine graphene for reinforcing polymer based nanocomposites. Atoms at the higher energy state in bi-crystalline graphene helps in improving the interaction at the nanocomposite interphase. Geometrical imperfections such as wrinkles and ripples helps the bicrystalline graphene in increasing the number of adhesion points between the nanofiller and matrix, which eventually improves the strength and toughness of nanocomposite. These outcomes will help in opening new opportunities for defective nanofillers in the development of nanocomposites for future applications. The same parameters were used to elaborate the influence of grain boundaries on the interfacial thermal conductance between bi-crystalline graphene and polyethylene nanocomposite. Reverse non-equilibrium molecular dynamics simulations were implemented in combination with the Lennard-Jones and reactive force field interatomic potential parameters. According to the simulations results, high-energy grain boundary atoms in bi-crystalline graphene played a substantial role in enhancing the interfacial thermal conductance values. To further illuminate the mechanisms of enhanced graphene polyethylene interfacial thermal conductance in the presence of grain boundaries, a systematic study on the vibrational density of states and structural evolution was also performed. Thus, bi crystalline graphene can be considered as a superior potential reinforcement for nanocomposites as compared to pristine configuration for the thermoelectric and thermal interface material applications.