Self-assembling tetrakis-Schiff base compounds for CNT reinforced composites: combined MD, DFT & charge transfer study

Abstract

Development of polymer composites by addition of nano-sized inclusions to matrix attracts increasing attention in the last decades as a potential way to prepare functional materials, the properties of which could be fine-tuned by varying the nanoadditives' concentration and alignment inside polymer. Carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) are widely used as highly promising additives when high mechanical strength and good electrical conductivity are desired. However their poor interaction with surrounding matrix and tendency to form agglomerates lead to decline of material properties' improvement with increasing concentration. To tackle this problem surface functionalization is applied. However, as chemical bonding interrupts continuous sp2-bonded outer layer, the electrical properties could be severely damaged by grafting of functional groups. For this reason, there is a high demand for non-bonding functional molecules. This work is focused on the interaction between CNT/GNR and tetrakis-Schiff base compounds. As it was shown previously, the later can form continuous networks of interconnected micrometer sized rings and rods with rim thickness of a few nanometers both on the surface and in the bulk of polymer. It was also noticed, that CNTs could be incorporated into these molecular networks and aligned along the rings and rods. Here we report the investigation of the interaction between tetrakis-Schiff bases and the surface of carbon nanotubes and graphene nanoribbons by means of density functional theory and molecular dynamics and its influence on the composite electrical properties studied by meso-scale Monte Carlo modeling. Using the ONETEP and SIESTA density functional packages the relaxed structures of molecular complexes, consisting of CNT/GNR fragment and tetrakis-Schiff base molecule, were obtained both with and without van der Waals correction and the orientation dependant interaction energy were studied. A special attention is devoted to the frontier orbitals of the molecular complexes. It is shown, that HOMO and LUMO orbitals of the neighboring molecules and CNT/GNR surface level can overlap in the proximity of frontier orbitals of CNT/GNR thus facilitating charge transfer. For single base tetrakis-Schiff molecules with flat geometry a ballistic conduction channel appears for stacked molecular wire. For wedge-like double base molecules, forming the mentioned above ring-rod networks, with smaller overlap of frontier orbitals and more complex mutual arrangement, the electron hopping model is proposed to investigate the effect of this peculiar interaction on the entire composite electric properties. Final impact of CNT/GNR incorporation into self-assembled rings-and-rods molecular networks on the percolation threshold and conductivity of polymer composite is assessed by evaluation of resistivity of a model sample with molecular network, simulated with Monte Carlo method using the experimental data on rings distribution, sizes and connectivity. The resistivity of CNTs uniformly distributed, agglomerated and arranged in ring-rod network is compared.