Understanding the viscoelasticity of vitrimers: from metathesis to steric factor

With growing production, plastic waste continues to plague our environment. Polymers with dynamic (reversible) bonds might provide a solution for the design of recyclable polymers. Additionally, dynamic bonds enable many other unique properties such as self-healing, super extensibility, shape memory, and superb toughness. However, understanding the microscopic mechanisms underlying the viscoelasticity of polymers with dynamic bonds remains a challenge. In this work, we analyze the dynamic behavior of model polymer systems functionalized with either boronic ester or imine functionalities. We highlight the importance of free OH or amine groups to facilitate network rearrangement, and our results strongly question the possibility of a metathesis in these systems. Most interestingly, our analysis of the temperature dependence of terminal relaxation times revealed an incredibly large pre-factor for the bond exchange time. We attribute this behavior to the chemical steric factor, which slows down this exchange time by many decades and is usually not considered in analysis of vitrimers viscoelasticity. Based on our results, we propose a general description of the viscoelastic properties in vitrimers which includes the steric factor in addition to the traditionally considered activation energy and matrix relaxation. This work provides deep insight into the fundamental dynamics of vitrimers and will guide intelligent design of vitrimers with desired properties.