(531i) Linear Viscoelasticity of Vitrimer Melts: A Theoretical Understanding of Their Peculiar Rheological Behavior

The rheological behavior and processability of a polymer melt are contingent on the microscopic chain topology. Thermoplastics, in which the polymer chains are discrete units, can be injected or extruded because they flow when heated above a particular temperature. They are, however, typically inappropriate for thermally demanding applications since their melt viscosity decreases significantly at elevated temperatures. Thermosets, where the chains are covalently crosslinked, have improved thermal, chemical, and mechanical stability compared to thermoplastics, but cannot be reprocessed or recycled. Vitrimers, a novel class of polymers invented by Leibler et al. in 2011, are insoluble, yet malleable just like glass. These materials have dynamic links and/or crosslinks that undergo an exchange reaction. The dynamic crosslinks conserve network connectivity and also allow chain topology fluctuations. Consequently, these materials exhibit enhanced mechanical properties while still maintaining processability. While significant research effort has focused on expanding the library of dynamic exchange reactions, the unique viscoelasticity of vitrimer melts remains poorly understood. For example, literature data of the complex moduli of vitrimers exhibit multiple plateaus and peaks that are not predicted by conventional bead-spring or reptation models. The scarcity of systematic studies and rigorous analyses of vitrimer melt rheology have stifled molecular understanding of these phenomena and passage from lab bench to production line. To clear this ambiguity, we attempt to model the linear viscoelasticity of vitrimers possessing exchange reactions that do not require catalyst. We calculate frequency sweep curves as a function of various molecular parameters, such as molecular weight and dynamic crosslink density. We compare the simulated frequency sweep curves to experimental data on a model vitrimer and discuss the physical origin of the multiple plateaus and peaks. The findings from this study will elucidate critical structure-property relationships needed to help present vitrimer development and guide future vitrimer material design.