(83g) Unentangled Vitrimer Melts: Generalized Rouse Theory Reveals Impact of Cross-Link and Backbone Chemistry on Linear Viscoelasticity

Vitrimers are polymer networks that engage in dynamic associative exchange reactions. Their covalent cross-links preserve network connectivity but permit topology fluctuations, rendering the networks to be both insoluble and processable. In this work, we employ a generalized inhomogeneous Rouse model (IHR) to investigate structure-viscoelasticity relationships for unentangled vitrimer melts with cross-link densities well beyond the gel point. This model accounts for interactions between relaxation modes of regular monomers and those that are “sticky” (i.e., cross-links). We use the IHR to explore the influence of structure and temperature on the zero-shear viscosity (η₀) and characteristic relaxation time (Ï„_η). Vitrimers with uniform and random cross-link distributions exhibit larger η₀ and Ï„_η than gradient and blocky types. Polydimethylsiloxane vitrimer (which has a flexible backbone) shows an Arrhenius temperature dependence for η₀, while polystyrene and poly(methyl methacrylate) vitrimers (which have rigid backbones) are only Arrhenius at high temperatures. During stress relaxation, the short time dynamics represent monomer friction, while the long time dynamics encompass a combination of network strand relaxation and cross-link exchange. Because of the different temperature dependences of the processes, time-temperature superposition fails. The effective rheological activation energy can be estimated a priori from the cross-link exchange activation energy and the backbone Williams-Landel-Ferry parameters. Based on these findings, we discuss the utility of the IHR for understanding vitrimer rheology, and best practices for characterizing η₀ and Ï„_η.