(585f) Effect of Reaction Kinetics on Mechanical and Rheological Properties of Vitrimers

Plastic pollution is one most pressing issues of this decade. About 300 million tons of plastic are produced each year; of that, less than 20% are successfully recycled. From a scientific and technological standpoint, the recycling of thermoplastic materials can be performed in a relatively straightforward process. On the other hand, thermosets (18% of the worldwide polymer production) are classified amongst the most challenging materials to recycle due to the permanent covalent crosslinks in their structure that aim to increase the strength and stiffness of the network compared to their thermoplastic counterparts. Currently, most of the thermoset plastic waste is being managed by landfilling, which is, according to the Environmental Protection Agency, the least preferred waste management approach. A more promising route to achieve recyclable thermosets can be done by introducing dynamic covalent bonds within their polymer network. In these systems, the dynamic response of the crosslink bonds is usually triggered by an external stimulus, which can be of thermal, chemical, or optical origin. Vitrimers represent a new class of polymer materials that can exchange covalent bonds and adjust their topology without risking structural damage or permanent loss of material properties above a characteristic topology freezing temperature. This unique feature makes vitrimers perfect candidates to design self-healing polymer materials and improve the lifetime and circularity of plastics. The main challenge is that the self-healing process should occur at service conditions, but the material should still show stable mechanical properties at the same conditions. In this regard, dynamics simulations can provide detailed molecular mechanisms that control the macroscopic rheology and mechanics of vitrimers. This study shows how coarse-grained molecular dynamics, in conjunction with a Monte Carlo step, can be used as a simulation technique to model the bond exchanges in these networks accurately. The linear and nonlinear rheological and mechanical properties of model vitrimers will be studied at different service conditions. The hybrid MD-MC technique, which incorporates the temperature role on the bond exchange, in conjunction with a coarse-grained model for the network, will be used to study the impact of the strain and frequency magnitude on the rheology and mechanics of vitrimers. This framework also shows flexibility in accommodating different rate constants of bond exchange reaction that is consistent with the catalyst effect in the reaction kinetics during experiments. We will show how the dynamic, rheological, and mechanical properties of the vitrimer are affected by the kinetics of the bond exchange reaction, thus allowing for an in-depth study of the bond lifetime and microstructure relaxation time of vitrimers. The results of this work will help with the rational design of shape-memory and self-healing polymeric materials.