Structure, Dynamics, and Rheology of Vitrimers

Abstract

Vitrimers are associative covalent adaptable networks that undergo reversible bond-exchange reactions while maintaining a fixed cross-linking density with changing temperature. To date, experimental studies that rely on macroscopic rheology have not been able to reveal topological changes and microscopic dynamics in these materials. Here, coarse-grained molecular dynamics simulations combined with a Monte Carlo method are implemented to investigate the topological structural changes, microscopic dynamics, and linear rheology of unentangled side-chain-linked vitrimers in conjunction with the sticky Rouse model (SRM). We find that there is a minor variation in the topological structure with temperature. The dynamic heterogeneities of the bond-exchange behavior and the system dynamics increase remarkably when approaching the topological freezing transition temperature T v . Quantitative agreement between the simulation results and the SRM predictions is observed for the stress relaxation, elastic and loss moduli, and the relative mean-squared displacement, especially at the intermediate- and long-time or low-frequency regimes, where the time–temperature superposition principle is satisfied. We obtain a scaling collapse curve for the dynamic bond relaxation time, the zero-shear viscosity, and the horizontal shift factors without introducing any parameters, suggesting that the microscopic and macroscopic dynamics exhibit a similar relaxation behavior even in the presence of loop defects. Moreover, these results are in good agreement with those predicted by the SRM, indicating that the linear rheology of unentangled vitrimers with a fast bond-exchange rate can be analyzed via a single-chain approach based on the SRM.

Self-Consistent Theory for Structural Relaxation, Dynamic Bond Exchange Times, and the Glass Transition in Polymeric Vitrimers

“…We note that a very recent simulation study at a single cross-link density found such a bending-up behavior of the bond exchange time . There, the strong upturn behavior was interpreted as beginning at the so-called topological freezing transition temperature, T v .…”

“…The prediction of even stronger upturns at lower temperatures than probed experimentally is a target for future laboratory studies. As discussed above, a recent simulation also found a crossover of the exchange time from an Arrhenius law regime to the bending up regime which was attributed to topological freezing of bond exchange. This interpretation is different from our conclusion here that the crossover is attributed to the importance of activated glassy segmental relaxation physics ( t p τ α,K ) on bond exchange.…”

“…Due to the unique self-healing properties, reprocessability and recyclability, and the numerous dynamic covalent chemistries that can be used to tailor these aspects, vitrimers have received increasing attention from the experimental polymer research community. − Moreover the basic physics of such systems is a frontier fundamental problem in statistical mechanical theory. Vitrimers are a subcategory of dynamic covalent networks where the polymeric backbones are cross-linked by associative exchangeable bonds. − Unlike supramolecular polymers and dissociative dynamic covalent networks − that undergo breaking and reforming reactions of physical bonds, vitrimers − undergo exchange reactions that do not involve bond breaking. These dynamical aspects may provide new opportunities for creating separation membranes, especially for larger molecules .…”

Covalent Adaptable Networks Made by Reactive Processing of Highly Entangled Polymer: Synthesis-Structure-Thermomechanical Property-Reprocessing Relationship in Covalent Adaptable Networks

“…We hypothesize that the molecular picture of our CANs and their stress relaxation could be modeled by sticky reptation theory, initially described by Leibler, Rubinstein, and Colby . Although sticky reptation theory has been commonly employed to explain the behavior of entangled multisticker polymers, − it has seldom been discussed in the context of associative CANs (vitrimers) or dissociative CANs. , Discussions of sticky theories in the context of CANs have largely been limited to unentangled CANs modeled by sticky Rouse theory. − In sticky reptation theory, entangled polymers containing reversible junctions (known as “stickers”), such as hydrogen bonds, metal–ligand interactions, or dynamic covalent cross-links, may exhibit an additional relaxation process dictated by the slow dissociation of stickers and their subsequent diffusion to find other sticker partners. This additional relaxation process occurs following an initial relaxation attributed to the Rouse motions of sol chains and reptation of some entanglements (at a length scale below the size of network strands) .…”

Trapping bond exchange phenomenon revealed for off-stoichiometry cross-linking of phase-separated vitrimer-like materials

“…The relaxation dynamics are known to be dominated by the bond exchange rate in a given system. [32][33][34] In the present system, the dynamics should be determined by the frequency of interdomain bond exchange. Thus, if the bond exchange is trapped in the matrix, the relaxation rate is naturally slowed.…”