Investigating the Effect of Network Architecture and Topography in tuning Vitrimer Behavior through Coarse-Grained Molecular Dynamics Simulations

Vitrimers are dynamically cross-linked polymers that undergo associative bond exchange at high temperatures (above the topological transition temperature, T_v) allowing material reprocessability, while retaining the mechanical strength of thermosets. Optimal design of vitrimers with the desired operating temperature window for reprocessability can be achieved by tuning T_v. Our vitrimer architecture consists of linear polymer chains and star cross-linkers modeled by Lennard-Jones beads. A combined molecular dynamics (MD)/Monte Carlo (MC) approach is used to model associative bond exchange with Arrhenius type temperature (T) dependence. We investigate the effect of vitrimer architecture and topography on shifting T_v by varying chemical building blocks: length of polymer chain beads (N_w,p = 5, 10, 20, and 40), number of arms per cross-linker (f=3, 4), and activation energy barrier for bond exchange (E_a,BE = 40, 53, 60, 67, and 74 kJ/mol). T_v is predicted through shear viscosity (h) calculations from Non-Equilibrium MD simulations. From the h -T curves, we observe that an increasing E_a,BE produces a pronounced shift in the vitreous region to higher temperatures and increases the width of the vitreous region in comparison to f, while varying N_w,p does not produce any noticeable trends. Hence, an increased energy barrier to swapping and increased stiffness of the network (f=4) can be used to tune the vitreous region. The trends obtained from viscosity curves and the underlying molecular mechanisms provide insight for optimizing design of experiments to obtain vitrimer behavior at service temperatures.