Unentangled Vitrimer Melts: Interplay between Chain Relaxation and Cross-link Exchange Controls Linear Rheology

Vitrimers are polymer networks that engage in dynamic associative exchange reactions. Their covalent cross-links preserve network connectivity but permit topology fluctuations, making them both insoluble and processable. Here, we use a sticky Rouse model approach to elucidate structure-viscoelasticity relationships for unentangled vitrimer melts. Two different versions of the sticky Rouse model are explored; the simplified sticky Rouse (SSR) and the inhomogeneous Rouse (IHR). Unlike the SSR, the IHR model accounts for interactions between slow modes that arise due to cross-linking and fast Rouse modes of the underlying polymer chain. First, we identify the conditions where the SSR sufficiently approximates the IHR. Then, we use the IHR to explore the influence of the structure and temperature on the zero-shear viscosity (eta(0)) and characteristic relaxation time (tau*). Vitrimers with uniform and random cross-link distributions exhibit larger.0 and t* than gradient and blocky types. Polydimethylsiloxane vitrimer (which has a flexible backbone) shows an Arrhenius temperature dependence for eta(0), while polystyrene vitrimer (which has a rigid backbone) is only Arrhenius at high temperatures. For stress relaxation measurements, the short time dynamics represent monomer friction, 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 backbone Williams-Landel-Ferry parameters. Finally, we discuss the utility and limitations of the sticky Rouse approach for studying vitrimer viscoelasticity and best practices for measuring eta(0) and tau*.