Rheology of Covalent Adaptive Networks Using a Hybrid Molecular Dynamics-Monte Carlo Technique

We propose a new model capable of describing the complex bonds dynamic as well as to simulate the viscoelastic behavior of a particular class of associative covalent adaptive networks (ACANs) known as vitrimers.  In contrast to conventional thermosets, vitrimers represent a novel class of plastics in which covalent chemical bonds can be efficiently and reliably exchanged between different positions of the polymer network without risking structural damage or permanent loss of material properties.  This unique property allows vitrimers to behave like thermoset resins (show creep and stress-cracking resistance) at low temperatures and a thermoplastic resin (malleability, flow, and plasticity) at high temperatures. However, due to their novelty, there is an urgent need to provide robust models and simulation tools that provide a molecular-level understanding and help to quantify the characteristics that lead to the observed thermophysical properties of these materials.  In this study, a combination of a coarse-grained molecular dynamics (MD) and Monte-Carlo (MC) simulations is proposed to mimic the thermodynamic, microstructural, and rheological properties of vitrimers.  Volumetric results show the ability of the model to capture the characteristic topological freezing temperature T_v of vitrimers derived from the network crosslink exchange reactions. Results for the statistics of exchanged bonds and their lifetime in the glassy and rubbery regimes will be presented.  Furthermore,  nonequilibrium MD (NEMD) simulations will be used to study the rheology of these networks in small to moderate oscillatory shear deformations. With the aid of the time-temperature superposition principle, the rheological measurements will be collapsed onto a master curve, thus allowing to capture of the main feature of experimental observation, which is the terminal regime of the elastic modulus. The zero-shear viscosity of the model vitrimer will also be presented, showing an Arrhenius-like temperature dependence at temperatures above the topology freezing point that is consistent with the temperature dependence of the shift factors obtained from collapsing the rheological measurements onto the master curves. Finally, the lifetime of bonds and deformation timescale will be utilized as the main factors to establish a linkage between the rheological response of the vitrimer and its microstructure at different temperatures. The coarse-grained nature of this model provides universality in the results while still being versatile in allowing the change and optimization of parameters such as kinetic of reaction, network architecture, and mesh size. The results of the proposed work not only will provide valuable insight concerning the reprocessing and recycling of thermosets, but they will also help with the rational design of shape-memory and self-healing polymeric materials.