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

Using this theoretical approach, we explore the influence of molecular structure and temperature on vitrimer linear viscoelasticity. We observe that vitrimers with uniform and random cross-link distributions exhibit larger viscosities and relaxation times than gradient and blocky types. Polydimethylsiloxane vitrimer (which has a flexible backbone) shows an Arrhenius temperature dependence for viscosity, while polystyrene vitrimers (which has 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. We also show that the effective rheological activation energy can be estimated a priori using only the cross-link exchange activation energy and the backbone Williams-Landel-Ferry parameters. (Submitted to Macromolecules)