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

Abstract

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 structure and temperature on the zeroshear viscosity (η 0 ) and characteristic relaxation time (τ * ). Vitrimers with uniform and random cross-link distributions exhibit larger η 0 and τ * than gradient and blocky types. Polydimethylsiloxane vitrimer (which has a flexible backbone) shows an Arrhenius temperature dependence for η 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, timetemperature 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 η 0 and τ * .

Boronic Acid Esters and Anhydrates as Dynamic Cross-Links in Vitrimers

“…With a further temperature increase the exchange kinetics change to an exchange reaction controlled regime, and in consequence, the flow follows the Arrhenius law ( Figure 1 ). The complexity of the vitrimers flow and relaxation behavior, which cannot be precisely described with basic models, has given rise to extensive theoretical studies [ 65 , 66 , 67 , 68 , 69 , 70 , 71 ]. Dynamics simulations proved to be suitable to elucidate the complex structure and dynamics of vitrimers systems, as well as to predict their rheological behavior [ 67 ].…”

Effect of bond exchange rate on dynamics and mechanics of vitrimers

“…45 The most recent model was proposed by Ricarte and Shanbhag, where two different types of the sticky Rouse model were used to describe the linear viscoelasticity of monodisperse unentangled vitrimer melts. 46 In the latter study, the influence of the polymer backbone structure (rigid vs. flexible) and temperature on the zero-shear viscosity and characteristic relaxation time of vitrimers were determined. 46 It was shown that vitrimers with uniform and random crosslinks exhibit large viscosity and relaxation time.…”

Impact of Telechelic Polymer Precursors on the Viscoelastic Properties of Vitrimers

“…The activation energies of TN were also estimated by the time‐temperature superposition of the relaxation curves (Figure S18, Supporting Information), while the applicability of the superposition to vitrimer is discussed in recent papers. [ 49,50 ] The plot between E a and M n showed the similar tendency, while there was a significant difference between the activation energies determined in different method. Although the reason is unclear, the detail relaxation mechanism might be related.…”