Analysis of mechanical properties of nanocomposite materials reinforced with functionalized carbon nanotubes based on molecular dynamics simulations

In the last decades, nanocomposite materials have attracted the attention of the research community and private companies for their wide range of applicability and their exceptional mechanical, thermal and electrical properties, making them the best candidates for new innovative applications in automotive, aeronautical and energy industry. This work takes part in the European project MODCOMP (Modified cost effective fibre based structures with improved multi-functionality and performance), in collaboration with ITAINNOVA (Aragaon Institute of Technology), Spain, and the Politecnico of Turin, which purpose is to study, develop and propose fiber-based materials for high value technical applications. The aim of this study is to analyze, through classical molecular dynamics simulations, the effect of the carbon nanotube functionalization with azomethine molecules on mechanical and interfacial properties of nanocomposite materials based on epoxy resin. Concretely the parameters of interest are the density profile, the elastic properties and the interfacial shear strength (IFSS). To generalize as much as possible the problem, two extreme situations, with and without covalent bond interactions between matrix and reinforce, are studied, for a total of six models proposed: two for periodic finite systems with diameter fiber of 20,3 Å, two for periodic infinite systems with diameter fiber of 101,7 Å and two non-periodic systems for the pull-out test. The first part of the work is dedicated to the development of the models, executing firstly the functionalization of the carbon nanotubes and then the cross-linking process, by forcing the creation of some bonds and breaking some others. Lastly, the simulation cell is equilibrated at 1 atm and 300 K, obtaining a system with zero residual stress. In the second part the elastic properties are evaluated imposing finite displacements to the cell and, once the stress is obtained applying the Virial theorem, defining the stiffness matrix. The density profile derives by a binning procedure: the atoms are firstly classified into bins depending on the distance from the axis; then the density of each bin is calculated and plotted. Now, trough the analysis of the density profile it is possible to identify the Van der Waals gap between resin and filler and the interface thickness. With a pull-out test, where the nanotube is extracted from the matrix with subsequent finite displacement of 0,2 Å, the interfacial shear stress (IFSS) is calculated. The results are finally compared with the data of pristine nanotubes, demonstrating that the functionalization plays an important role on the interface region, with a particular reference to the systems characterized by covalent bonds between resin and fillers, where the effects are more significant.