Study of the mechanical properties of interfaces in nanocomposites with molecular simulations

Abstract

Computational physics is an essential domain of scientific research around the world, as it bridges experimental and theoretical findings. It is a tool which offers the possibility to address a certain problem from perspectives and under conditions that are not always accessible through experimental approaches, because of physical or financial constraints. Especially in the field of materials science, where the atomistic or molecular detail is often crucially important, computational methods can assist or guide experimental efforts and stimulate the development of better macroscopic models. The pressing need for new industrial materials with tailored properties for use in everyday life applications renders the computational modeling of complex systems, such as polymers and nanocomposites, necessary. Modeling helps in elucidating structure-property-processing-performance relations and in developing better, less expensive, environmentally friendlier, and energetically more economical materials. A very important class of nanocomposites is characterized by the presence of interfaces between a polymeric matrix and an inclusion that is distributed in the form of particles which are nanoscopic in at least one dimension. The interactions between matrix and nanofiller can impart to the final product a set of desired properties, which none of the individual materials can exhibit on its own. Interfaces in these systems present fundamentally interesting and practically very relevant phenomena, which are difficult to describe. Such systems require accurate and efficient computational tools in order to understand their nature, and therefore to predict their macroscopic behavior from the atomistic level, e.g., from their chemical constitution and molecular architecture. Atomistic molecular dynamics simulations are used to study the thermodynamic, rheological and mechanical properties of such materials. Combined with a firm knowledge of statistical mechanics, these simulation techniques can shed light on the connection between the microscopic structure of the nanocomposite material and its macroscopic properties.

The subject of the present doctoral thesis is the detailed investigation of structural, mechanical, topological and local-segmental properties of epoxy networks of different crosslinking degrees, as well as of interfaces between these networks and graphenic (inclusion) materials. Using atomistic simulations as our principal tool, we first study an epoxy in the bulk. Following that, we investigate the thermodynamics, structure, and interfacial kinetics of a nanocomposite consisting of parallel single layers of pristine, defective, or chemically modified graphene enclosed in the epoxy. Our study of the nanocomposite interface is targeted at the failure of adhesion brought about by imposition of shear deformation. We develop a new methodology which extends the Boltzmann-Arrhenius-Zhurkov model and applies it in the prediction of the mechanical response of the material under shear stress. Testing our methodology for the three types of graphenic inclusions over a wide variety of temperatures and stresses, we observe favorable agreement with experimental studies and elucidate the factors controlling adhesion at the interface.