Understanding the Reshaping of Fluorinated Polyester Vitrimers by Kinetic and DFT Studies of the Transesterification Reaction

Vitrimers are a third class of polymers gathering the mechanical properties and solvent resistance of 3D thermosets and the reprocessability of thermoplastics. This unique behaviour is due to the triggering of certain covalent exchange reactions that allow the network to rearrange upon application of a stimulus. The constitutive feature of vitrimers is the adoption of a glass-like viscosity during the rearrangement of the network, often due to an associative mechanism for the exchange reaction. Transesterification networks are one of the most studied type of vitrimers that usually require the incorporation of a catalyst, implying the associated drawbacks. Following up on a recent report on catalyst-free transesterification vitrimers in which the ester functions are particularly reactive thanks to the presence of fluorine atoms in α- or β-position, parallel DFT calculations and an experimental kinetic study on model molecules are presented in order to quantitatively assess the effect of neighbouring fluorinated groups on the transesterification reaction rate.     

Dry Processing and Recycling of Thick Nacre–Mimetic Nanocomposites

Bioinspired nanocomposites with high levels of reinforcement hold great promise for future, green lightweight, and functional engineering materials, but they suffer from slow, tedious, and nonscalable preparation routes, that typically only lead to very thin films. A rapid and facile dry powder processing technique is introduced to generate bioinspired nanocomposite materials at high fractions of reinforcements (50 wt%) and with millimeter scale thickness. The process uses powder drying of vitrimer-coated nanoplatelets (nanoclay and MXene) from aqueous solution and subsequent hot-pressing. As a method of choice in industrial lightweight composite materials engineering, hot-pressing underscores a high potential to translate this approach to actual products. The use of the vitrimer chemistry with temperature-activated bond shuffling is important to facilitate smooth integration into the nanocomposite design, leading to layered nacre-inspired nanocomposites with nanoscale hard/soft order traced by X-ray diffraction and excellent mechanical properties investigated using flexural tests. Recycling by grinding and hot-pressing is possible without property loss. The compatibility with existing composite processing techniques, scalable thickness and dimensions, and recyclability open considerable opportunities for translating bioinspired nanocomposites to real-life applications.     

On the Welding of Vitrimers: Chemistry,Mechanics and Applications

Linear chains in thermoplastics make them relatively weak in performance but inherently weldable and recyclable. By contrast, thermosets with permanently crosslinked networks possess outstanding mechanical performance, thermal, and chemical stability, but are unweldable and unrecyclable. In the last decade, a kind of thermoplastics-like thermosets termed as vitrimer has been developed with extensive applications, in which welding of vitrimers plays a central and fundamental role. Herein, we present the current state of the art of the welding of vitrimers and discuss the welding of vitrimers from a broad picture of chemistry, physics, and mechanics: i) chemistry and mechanics of the welding of vitrimers; ii) applicability of the mechanical assessment methods for the welding of vitrimers; iii) design principles and implement strategies to the welding of vitrimers; iv) effects of welding conditions on the welding strength and toughness; and v) applications to the adhesion of chemically inert materials. Finally, advantages, challenges, and open questions to the welding of vitrimers are highlighted, and future opportunities in chemistry, mechanics, design of tough welding, artificial intelligence aided programming of welding technology, mechanical assessment standard, and so on are discussed. The development of vitrimer welding would fuse disciplines and make transformative impact in polymer industry.     

Vulcanization of polypropylene

Dynamic covalent crosslinking of commodity thermoplastics is a desirable target in material development, as it promises to combine the enhanced mechanical properties and thermal/solvent stability of thermosets with reprocessability and plastic flow under certain conditions activating the bond exchange. Many attempts of this development suffer from the same two problems: enhanced cost due to complex and often toxic chemicals, and the effective melt-flow index being too low for practical use. Here we return to the origins of polymer networks, and mimic the vulcanization of natural rubber in the commodity polypropylene using elemental sulfur initiated by peroxide. Forming sulfur bridges allows easy catalyst-free reprocessability based on the disulfide bond exchange. We study a broad range of compositions and reaction conditions, finding optimal balance between the crosslinking and chain scission in the melt compounder, and demonstrating much enhanced characteristics of the resulting materials. We specifically discuss and evaluate the balance between the rubber-elastic network response at high temperatures and the plastic flow enabled by disulfide exchange, responsible for the reprocessing of our vitrimers.     

Conformational Entropy as a Means to Control the Behavior of Poly(diketoenamine) Vitrimers In and Out of Equilibrium

Control of equilibrium and non‐equilibrium thermomechanical behavior of poly(diketoenamine) vitrimers is shown by incorporating linear polymer segments varying in molecular weight (MW) and conformational degrees of freedom into the dynamic covalent network. While increasing MW of linear segments yields a lower storage modulus at the rubbery plateau after softening above the glass transition (T_g), both T_g and the characteristic time of stress relaxation are independently governed by the conformational entropy of the embodied linear segments. Activation energies for bond exchange in the solid state are lower for networks incorporating flexible chains; the network topology freezing temperature decreases with increasing MW of flexible linear segments but increases with increasing MW of stiff segments. Vitrimer reconfigurability is therefore influenced not only by the energetics of bond exchange for a given network density, but also the entropy of polymer chains within the network.     

Coordination Adaptable Networks: Zirconium(IV) Carboxylates

Vitrimers are 3D “covalent adaptable networks” (CANs) with flow properties thanks to thermally activated associative exchange reactions. This contribution introduces coordination adaptable networks, or CooANs, that are topologically related to metal–organic frameworks with octahedral Zr₆ clusters as secondary building units in a carboxylic acid-functionalized acrylate-methacrylate copolymer matrix. A series of Zr-CooAN-x materials (x=percent of Zr₆ loading relative to maximum capacity) was synthesized with x=5, 10, 15, 20, 25, 50 and 100. The mechanical and rheological investigations demonstrate vitrimer-like properties for x up to 50, the crosslink migration being ensured by carboxylate ligand exchange, with relaxation becoming slower as the Zr₆ content is increased. The flow activation energy of Zr-CooAN-10 is 92.9±3.6 kJ mol⁻¹. Rapid (30 min) hot-press reshaping occurs at temperatures in the 50–100 °C range under a 3-ton pressure and does not significantly alter the material properties.     

Tuning the Viscosity Profile of Ionic Vitrimers Incorporating 1,2,3‐Triazolium Cross‐Links

Vitrimers are dynamic polymer networks with unique viscoelastic behavior combining the best attributes of thermosets and thermoplastics. Ionic vitrimers are a recent class of dynamic materials, where 1,2,3‐triazolium cross‐links are reshuffled by trans‐N‐alkylation exchange reactions. Comparison of dynamic properties with a selection of vitrimers relying on different exchange reactions highlights the particularly high viscous flow activation energies of trans‐N‐alkylation reactions, thus providing an enhanced compromise between fast reprocessing at moderately high temperatures and low creep at service temperature. Varying the [monomer]/[cross‐linker] ratio in the initial formulation of these 1,2,3‐triazolium‐based networks affords a fine tuning of their viscosity profiles. Confrontation of rheometry and X‐ray photoelectron spectroscopy data allows the correlation of variations in chemical composition with changes in the covalent exchange dynamics. This unprecedented approach enables the proposition of a dissociative two‐step mechanism for the trans‐N‐alkylation of 1,2,3‐triazoliums initiated by a nucleophilic attack of the 1,2,3‐triazolium cross‐links by the iodide counteranion, yielding uncrosslinking by de‐N‐alkylation. Subsequent rapid re‐N‐alkylation of the formed 1,2,3‐triazole by surrounding iodide‐functionalized dangling chains affords exchange of the cross‐link position. This study highlights that strictly associative exchange reactions are not compulsory to induce vitrimer behavior, and may pave the way to a much wider variety of vitrimers relying on conventional reversible covalent reactions.     

Computational study of the transamination reaction in vinylogous acyls: Paving the way to design vitrimers with controlled exchange kinetics

One of the key points in the design of vitrimers is controlling the associative exchange kinetics. One common chemistry used in vitrimers is based on the dynamic amine exchange reaction of vinylogous acyl compounds in presence of free amine. Understanding the reaction mechanism is essential to assist the optimization of the reaction conditions as well as the molecular structure of the reactant compounds in the pursuit of new materials. In this work, a computational study has been performed to explore different reaction mechanisms in neutral, acidic and in basic conditions or in the presence of Lewis acids, as well as the effect of chemical modifications in the exchange reaction. The results reveal that the formation of hydrogen bonds are a key feature and that the vinylogous urea improves the transamination compared to vinylogous urethane. The esteric hindrance of the amino group in the vinylogous compound also plays an important role. Finally, the nature of the free amine can improve the reactivity by equilibrating two contrary effects: the basicity favors the nucleophilic attack and the conjugated acidity favors the protonation. The findings of this theoretical work shed light in the design of new vitrimers with controlled exchange kinetics by chemical modifications.