Ductility, toughness and strain recovery in self-healing dual cross-linked nanoparticle networks studied by computer simulations

One of the challenges in formulating useful nanocomposites is creating materials that are both tough and strong. Here, we review results of computational studies on a new class of nanocomposites that exhibit these desirable properties. The fundamental unit in these materials is a polymer grafted nanoparticle (PGN), which encompasses a rigid core and a corona of end-grafted polymers. We focus on a concentrated solution of these PGNs; the solution is assumed to be a good solvent for the grafted chains, which are in the semi-dilute regime. The free ends of the grafted chains encompass chemically reactive groups. Hence, with the overlap of the coronas on neighboring nanoparticles, the reactive end groups can form labile or more stable (permanent) bonds, leading to the creation of a dual cross-linked network. To predict the mechanical properties of these dual cross-linked PGN networks, we developed a multi-scale model that captures interactions occurring over the range of length and time scales that characterize the performance of the system. Namely, the model integrates the essential structural features of the polymer grafted nanoparticles, the interactions between the overlapping coronas, the kinetics of bond formation and rupture between the reactive end-groups and the response of the entire sample to mechanical deformation. Using this computational approach, we determined the effect of the labile bond energy, the fraction of permanent bonds and the introduction of high-strength bonds on the ductility and toughness of the PGN network. Furthermore, we determined the strain recovery and self-healing behavior of the material after it was allowed to relax from an applied tensile force. Through these studies, we isolated critical parameters that control the mechanical response and rejuvenation of dual cross-linked PGN networks. Our findings allow researchers to understand how variations in these key parameters can lead to changes in the materials' mechanical behavior and thus, can facilitate the fabrication of the next generation of nanocomposites with novel and technologically useful properties. (C) 2014 Elsevier Ltd. All rights reserved.