The Role of Catalyst Mobility and Concentration on the Linear Viscoelasticity of Associative Dynamic Covalent Networks

Associative dynamic covalent networks combine the elastic properties of cross-linked thermosets with the viscous flow of thermoplastics, thus enabling reprocessability while maintaining network connectivity. In this work, we investigate the effect of catalyst mobility and concentration on the linear viscoelasticity of associative thiol-thioester dynamic covalent networks. We present a simple method to covalently tether nucleophiles capable of catalyzing thiol-thioester exchange reactions to polymer networks and compare their viscoelastic behavior to their analogous untethered catalyst. We first show that the cross-linking density and bond dynamics can be independently tuned using precise stoichiometric adjustments for both catalyst systems. Notably, we find that the mobility of the catalysts does not affect the bond dynamics, as validated by similar relaxation times between the tethered and untethered catalysts for a given temperature and catalyst loading. Creep experiments show that the dynamic bonds are continuously exchanging in the polymer networks and can be well described by a single-mode Maxwell model. We find that a single set of horizontal and vertical shift factors was sufficient to construct time–temperature superposition (TTS) master curves and was directly related to the relaxation time and the cross-linking density, respectively, further validating the structure–property relations. Activation energies calculated from the Arrhenius relationship with the flow viscosity are systematically higher for the tethered catalysts, which we hypothesize is due to steric effects in the polymer networks. Finally, we propose the use of time–temperature-catalyst superposition (TTCS) as a framework to probe the bond dynamics spanning substantially longer time scales than TTS as a function of temperature and catalyst loading.