Vitrimers are polymer networks that are insoluble in a good solvent yet are still processable. The network strands bear covalent cross-links that preserve network connectivity but enable topology fluctuations. Here, we use a generalized inhomogeneous Rouse model (IHR) to study structure-viscoelasticity relationships for unentangled vitrimer melts with cross-link densities well beyond the gel point. The model accounts for interactions between relaxation modes of regular monomers and those that are “sticky” (i.e., cross-links). We use the IHR to investigate the influence of vitrimer chemistry and temperature on the zero-shear viscosity (η0) and characteristic relaxation time (τη). Vitrimers with uniform and random cross-link distributions demonstrate larger η0 and τη than gradient and blocky types. Polydimethylsiloxane vitrimer (which has a flexible backbone) exhibits 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 under small strains, the short time dynamics represent monomer relaxation, while the long time dynamics encompass a combination of network strand relaxation and cross-link exchange. Due to 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 τη.

Unentangled vitrimer melts: Inhomogeneous rouse theory reveals structure-viscoelasticity relationships

ACS Fall 2021

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