"Cloth-to-Clothes-Like" Fabrication of Soft Actuators.

Inspired by adaptive natural organisms and living matter, soft actuators appeal to a variety of innovative applications such as soft grippers, artificial muscles, wearable electronics, and biomedical devices. However, their fabrication is typically limited in laboratories or a few enterprises since specific instruments, strong stimuli, or specialized operation skills are inevitably involved. Here a straightforward "cloth-to-clothes-like" method to prepare soft actuators with a low threshold by combining the hysteretic behavior of liquid crystal elastomers (LCEs) with the exchange reaction of dynamic covalent bonds, is proposed. Due to the hysteretic behavior, the LCEs (resemble "cloth") effectively retain predefined shapes after stretching and releasing for extended periods. Subsequently, the samples naturally become soft actuators (resemble "clothes") via the exchange reaction at ambient temperatures. As a post-synthesis method, this strategy effectively separates the production of LCEs and soft actuators. LCEs can be mass-produced in bulk by factories or producers and stored as prepared, much like rolls of cloth. When required, these LCEs can be customized into soft actuators as needed. This strategy provides a robust, flexible, and scalable solution to engineer soft actuators, holding great promise for mass production and universal applications.

Recycling of Polydicyclopentadiene Enabled with N-Coordinated Boronic Ester Bonds.

In this contribution, the transformation of polydicyclopentadiene (PDCPD) from thermoset into vitrimer is introduced. First, two N-coordinated diboronic diols are successfully synthesized via the reaction of N,N,N-tri(2-hydroxyethyl)amine and/or N,N,N",N"-tetrakis(2-hydroxyethyl)ethylene diamine with 4-(hydroxymethyl) phenylboronic acid and then they are transformed into two N-coordinated cyclic boronic diacrylates. The latter two dienes carrying electron-withdrawing substituents are used for the ring opening insertion metathesis copolymerization (ROIMP) of dicyclopentadiene to afford the crosslinked PDCPD. In the crosslinked PDCPD networks, N-coordinated cyclic boronic ester bonds are integrated. It is found that the as-obtained PDCPD networks displayed the excellent reprocessing properties. In the meantime, the fracture toughness is significantly improved. Owing to the inclusion of N-coordinated cyclic boronic ester bonds, the modified PDCPDs have the thermal stability much superior to plain PDCPD. The results reported in this work demonstrate that PDCPD can successfully be transformed into the vitrimers via the introduction of N-coordinated cyclic boronic ester bonds.

Molecular Simulation of Covalent Adaptable Networks and Vitrimers: A Review.

Covalent adaptable networks and vitrimers are novel polymers with dynamic reversible bond exchange reactions for crosslinks, enabling them to modulate their properties between those of thermoplastics and thermosets. They have been gathering interest as materials for their recycling and self-healing properties. In this review, we discuss different molecular simulation efforts that have been used over the last decade to investigate and understand the nanoscale and molecular behaviors of covalent adaptable networks and vitrimers. In particular, molecular dynamics, Monte Carlo, and a hybrid of molecular dynamics and Monte Carlo approaches have been used to model the dynamic bond exchange reaction, which is the main mechanism of interest since it controls both the mechanical and rheological behaviors. The molecular simulation techniques presented yield sufficient results to investigate the structure and dynamics as well as the mechanical and rheological responses of such dynamic networks. The benefits of each method have been highlighted. The use of other tools such as theoretical models and machine learning has been included. We noticed, amongst the most prominent results, that stress relaxes as the bond exchange reaction happens, and that at temperatures higher than the glass transition temperature, the self-healing properties are better since more bond BERs are observed. The lifetime of dynamic covalent crosslinks follows, at moderate to high temperatures, an Arrhenius-like temperature dependence. We note the modeling of certain properties like the melt viscosity with glass transition temperature and the topology freezing transition temperature according to a behavior ruled by either the Williams-Landel-Ferry equation or the Arrhenius equation. Discrepancies between the behavior in dissociative and associative covalent adaptable networks are discussed. We conclude by stating which material parameters and atomistic factors, at the nanoscale, have not yet been taken into account and are lacking in the current literature.

Adaptive and Robust Vitrimers Fabricated by Synergy of Traditional and Supramolecular Polymers.

Vitrimers offer a unique combination of mechanical performance, reprocessability, and recyclability that makes them highly promising for a wide range of applications. However, achieving dynamic behavior in vitrimeric materials at their intended usage temperatures, thus combining reprocessability with adaptivity through associative dynamic covalent bonds, represents an attractive but formidable objective. Herein, we couple boron-nitrogen (B-N) dative bonds and B-O covalent bonds to generate a new class of vitrimers, boron-nitrogen vitrimers (BNVs), to endow them with dynamic features at usage temperatures. Compared with boron-ester vitrimers (BEVs) without B-N dative bonds, the BNVs with B-N dative bonds showcase enhanced mechanical performance. The excellent mechanical properties come from the synergistic effect of the dative B-N supramolecular polymer and covalent boron-ester networks. Moreover, benefiting from the associative exchange of B-O dynamic covalent bonds above their topological freezing temperature (Tv), the resultant BNVs also possess the processability. This study leveraged the structural characteristics of a boron-based vitrimer to achieve material reinforcement and toughness enhancement, simultaneously providing novel design concepts for the construction of new vitrimeric materials.

Ultrastrong and Reusable Solar‒Thermal‒Electric Generators by Economical Starch Vitrimers.

In frigid regions, it is imperative to possess functionality materials that are ultrastrong, reusable, and economical, providing self-generated heat and electricity. One promising solution is a solar‒thermal‒electric (STE) generator, composed of solar‒thermal conversion phase change composites (PCCs) and temperature-difference power-generation-sheets. However, the existing PCCs face challenges with conflicting requirements for solar‒thermal conversion efficiency and mechanical robustness, mainly due to monotonous functionalized aerogel framework. Herein, a novel starch vitrimer aerogel is proposed that incorporates orientational distributed carboxylated carbon nanotubes (CCNT) to create PCC. This innovative design integrates large through-holes, mechanical robustness, and superior solar‒thermal conversion. Remarkably, PCC with only 0.8 wt.% CCNT loading achieves 85.8 MPa compressive strength, 102.4 °C at 200 mW cm-2 irradiation with an impressive 92.9% solar-thermal conversion efficiency. Noteworthy, the STE generator assembled with PCC harvests 99.1 W m-2 output power density, surpassing other reported STE generators. Strikingly, even under harsh conditions of -10 °C and 10 mW cm‒2 irradiation, the STE generator maintains 20 °C for PCC with 325 mV output voltage and 45 mA current, showcasing enhanced electricity generation in colder environments. This study introduces a groundbreaking STE generator, paving the way for self-sufficient heat and electricity supply in cold regions.

Catalyst-Free α-Acetyl Cinnamate/Acetoacetate Exchange to Enable High Creep-Resistant Vitrimers.

Vitrimers represent an emerging class of polymeric materials that combine the desirable characteristics of both thermoplastics and thermosets achieved through the design of dynamic covalent bonds within the polymer networks. However, these materials are prone to creep due to the inherent instability of dynamic covalent bonds. Consequently, there are pressing demands for the development of robust and stable dynamic covalent chemistries. Here, we report a catalyst-free α-acetyl cinnamate/acetoacetate (α-AC/A) exchange reaction to develop vitrimers with remarkable creep resistance. Small-molecule model studies revealed that the α-AC/A exchange occurred at temperatures above 140 °C in bulk, whereas at 120 °C, this reaction was absent. For demonstration in the case of polymers, copolymers derived from common vinyl monomers were crosslinked with terephthalaldehyde to produce α-AC/A vitrimers with tunable thermal and mechanical performance. All resulting α-AC/A vitrimers exhibited high stability, especially in terms of creep resistance at 120 °C, while retaining commendable reprocessability when subjected to high temperatures. This work showcases the α-AC/A exchange reaction as a novel and robust dynamic covalent chemistry capable of imparting both reprocessability and high stability to cross-linked networks.

Catalyst-Free Cardanol-Based Epoxy Vitrimers for Self-Healing, Shape Memory, and Recyclable Materials.

Bio-based vitrimers present a promising solution to the issues associated with non-renewable and non-recyclable attributes of traditional thermosetting resins, showcasing extensive potential for diverse applications. However, their broader adoption has been hindered by the requirement for catalyst inclusion during the synthesis process. In this study, a cardanol-based curing agent with poly-hydroxy and tertiary amine structures was prepared by a clean synthetic method under the theory of click chemistry. The reaction of a cardanol-based curing agent with diglycidyl ether of bisphenol A formed catalyst-free, self-healing, and recyclable bio-based vitrimers. The poly-hydroxy and tertiary amine structures in the vitrimers promoted the curing of epoxy-carboxylic acid in the cross-linked network and served as internal catalysts of dynamic transesterification. In the absence of catalysts, the vitrimers network can achieve topological network rearrangement through dynamic transesterification, exhibiting excellent reprocessing performance. Moreover, the vitrimers exhibited faster stress relaxation (1500 s at 180 °C), lower activation energy (92.29 kJ·mol-1) and the tensile strength of the recycled material reached almost 100% of the original sample. This work offers a new method for preparing cardanol-based epoxy vitrimers that be used to make coatings, hydrogels, biomaterials, adhesives, and commodity plastics in the future.

The Kinetic Study of the Influence of Common Modifiers on the Curing Process of Epoxy Vitrimers.

An analysis of the influence of common modifiers on the kinetics of the curing process of epoxy-anhydride vitrimers was carried out. As common modifiers to enhance the "vitrimeric" nature of the material, zinc acetylacetonate as a transesterification catalyst and glycerol as a modifier of hydroxyl group content were chosen. The curing process of all obtained compositions was studied by differential scanning calorimetry (DSC) followed by the application of the isoconversional approach. It was shown that additives significantly affect the curing process. The resulting cured polymers were shown to be chemically recyclable by dissolution in the mixture of ethylene glycol and N-methylpirrolidone in a volume ratio of nine to one. The introduction of both zinc acethylacetonate and glycerol to the neat formulation led to a decrease in the dissolution time by 85.7% (from 35 h for the neat epoxy-anhydride formulation to 5 h for the modified formulation). In order to show the opportunity of the secondary use of recyclates, the mixtures based on the basic composition containing 10 wt. % of secondary polymers were also studied. The introduction of a recycled material to neat composition led to the same curing behavior as glycerol-containing systems.

Vinylogous Urea-Urethane Vitrimers: Accelerating and Inhibiting Network Dynamics through Hydrogen Bonding.

Vinylogous urethane (VUO ) based polymer networks are widely used as catalyst-free vitrimers that show rapid covalent bond exchange at elevated temperatures. In solution, vinylogous ureas (VUN ) undergo much faster bond exchange than VUO and are highly dynamic at room temperature. However, this difference in reactivity is not observed in their respective dynamic polymer networks, as VUO and VUN vitrimers prepared herein with very similar macromolecular architectures show comparable stress relaxation and creep behavior. However, by using mixtures of VUO and VUN linkages within the same network, the dynamic reactions can be accelerated by an order of magnitude. The results can be rationalized by the effect of intermolecular hydrogen bonding, which is absent in VUO vitrimers, but is very pronounced for vinylogous urea moieties. At low concentrations of VUN , these hydrogen bonds act as catalysts for covalent bond exchange, while at high concentration, they provide a pervasive vinylogous urea - urethane (VU) network of strong non-covalent interactions, giving rise to phase separation and inhibiting polymer chain dynamics. This offers a straightforward design principle for dynamic polymer materials, showing at the same time the possible additive and synergistic effects of supramolecular and dynamic covalent polymer networks.

Programmable and Shape-Color Synchronous Dual-Response Wood with Thermal Stimulus.

Stimuli-responsive materials exhibit huge potential in sensors, actuators, and electronics; however, their further development for reinforcement, visualization, and biomass-incorporation remains challenging. Herein, based on the impregnation of thermochromic microcapsule (TCM)-doped dynamic covalent vitrimers, a programmable shape-color dual-responsive wood (SRW-TC) was demonstrated with robust anisotropic structures and exchangeable covalent adaptable networks. Under mild conditions, the resultant SRW-TC displays feasible shape memorability and programmability, resulting from the rigidity-flexibility shift induced by the glass-transition temperature (34.99 °C) and transesterification reaction triggered by the topology freezing transition temperature (149.62 °C). Furthermore, the obtained SRW-TC possesses satisfactory mechanical performance (tensile strength of 45.70 MPa), thermal insulation (thermal conductivity of 0.27 W/m K), anisotropic light management, and benign optical properties (transmittance of 51.73% and haze of 99.67% at 800 nm). Importantly, the incorporation of compatible TCM enables SRW-TC to visualize shape memory feasibility and rigidity/flexibility switching and respond to the external thermal stimulus through the thermal-induced shape-color synchronous dual-responsiveness, which successfully demonstrates the applications of sensing temperature, grasping objects, encrypting/decoding icon messages, and so on. The proposed facile and highly effective strategy could serve as a guideline for developing high-performance multifunctional wood composite with promising intelligent applications in performance visualization, environmental sensing, materials interactivity, information dual-encryption, local precision shape and color regulation, etc.

3D-Printed Architectured Silicones with Autonomic Self-Healing and Creep-Resistant Behavior.

Self-healing silicones that are able to restore functionalities and extend the lifetime of soft devices hold great potential in many applications. However, currently available silicones need to be triggered to self-heal or suffer from creep-induced irreversible deformation during use. Here, a platform is proposed to design and print silicone objects that are programmed at the molecular and architecture levels to achieve self-healing at room temperature while simultaneously resisting creep. At the molecular scale, dioxaborolanes moieties are incorporated into silicones to synthesize self-healing vitrimers, whereas conventional covalent bonds are exploited to make creep-resistant elastomers. When combined into architectured printed parts at a coarser length scale, the layered materials exhibit fast healing at room temperature without compromising the elastic recovery obtained from covalent polymer networks. A patient-specific vascular phantom and fluidic chambers are printed to demonstrate the potential of architectured silicones in creating damage-resilient functional devices using molecularly designed elastomer materials.

Next-Generation Vitrimers Design through Theoretical Understanding and Computational Simulations.

Vitrimers are an innovative class of polymers that boast a remarkable fusion of mechanical and dynamic features, complemented by the added benefit of end-of-life recyclability. This extraordinary blend of properties makes them highly attractive for a variety of applications, such as the automotive sector, soft robotics, and the aerospace industry. At their core, vitrimer materials consist of crosslinked covalent networks that have the ability to dynamically reorganize in response to external factors, including temperature changes, pressure variations, or shifts in pH levels. In this review, the aim is to delve into the latest advancements in the theoretical understanding and computational design of vitrimers. The review begins by offering an overview of the fundamental principles that underlie the behavior of these materials, encompassing their structures, dynamic behavior, and reaction mechanisms. Subsequently, recent progress in the computational design of vitrimers is explored, with a focus on the employment of molecular dynamics (MD)/Monte Carlo (MC) simulations and density functional theory (DFT) calculations. Last, the existing challenges and prospective directions for this field are critically analyzed, emphasizing the necessity for additional theoretical and computational advancements, coupled with experimental validation.

Hyperbranched Dynamic Crosslinking Networks Enable Degradable, Reconfigurable, and Multifunctional Epoxy Vitrimer.

Degradation and reprocessing of thermoset polymers have long been intractable challenges to meet a sustainable future. Star strategies via dynamic cross-linking hydrogen bonds and/or covalent bonds can afford reprocessable thermosets, but often at the cost of properties or even their functions. Herein, a simple strategy coined as hyperbranched dynamic crosslinking networks (HDCNs) toward in-practice engineering a petroleum-based epoxy thermoset into degradable, reconfigurable, and multifunctional vitrimer is provided. The special characteristics of HDCNs involve spatially topological crosslinks for solvent adaption and multi-dynamic linkages for reversible behaviors. The resulting vitrimer displays mild room-temperature degradation to dimethylacetamide and can realize the cycling of carbon fiber and epoxy powder from composite. Besides, they have supra toughness and high flexural modulus, high transparency as well as fire-retardancy surpassing their original thermoset. Notably, it is noted in a chance-following that ethanol molecule can induce the reconstruction of vitrimer network by ester-exchange, converting a stiff vitrimer into elastomeric feature, and such material records an ultrahigh modulus (5.45 GPa) at -150 °C for their ultralow-temperature condition uses. This is shaping up to be a potentially sustainable advanced material to address the post-consumer thermoset waste, and also provide a newly crosslinked mode for the designs of high-performance polymer.

Fully Recyclable Cured Polymers for Sustainable 3D Printing.

The most prevalent materials used in the Additive Manufacturing era are polymers and plastics. Unfortunately, these materials are recognized for their negative environmental impact as they are primarily nonrecyclable, resulting in environmental pollution. In recent years, a new sustainable alternative to these materials has been emerging: Reversible Covalent Bond-Containing Polymers (RCBPs). These materials can be recycled, reprocessed, and reused multiple times without losing their properties. Nonetheless, they have two significant drawbacks when used in 3D printing. First, some require adding new materials every reprinting cycle, and second, others require high temperatures for (re)printing, limiting recyclability, and increasing energy consumption. This study, thus, introduces fully recyclable RCBPs as a sustainable approach for radiation-based printing technologies. This approach enables multiple (re)printing cycles at low temperatures (50 °C lower than the lowest reported) without adding new materials. It involves purposefully synthesized polymers that undergo reversible photopolymerization, composed of a tin-based catalyst. An everyday microwave oven quickly depolymerized these polymers, obtaining complete reversibility.

Does the Varying Reactivity in the Transient Polymer Network through Dynamic Exchange Regulate the Closed-Loop Circularity in Polyolefin Vitrimers?

According to current projections, of the 400 mega tons of plastic produced globally, 70% is waste and of that only 16% is recycled and the rest is incinerated. This is estimated to contribute to ca. 16% of the net carbon emission by 2050. Such a massive amount of unmanaged plastic waste and the associated huge carbon footprint sets a significant challenge to tackle in the coming decades. To achieve net-zero carbon emission, closed-loop circular economy in plastics is crucial but collection, sorting and processing the postconsumer recycled (PCR) plastics poses humongous challenge in achieving this circularity, unless an effective strategy is designed. In a first of its kind, a designer biobased molecule was synthesized (here maleated castor oil, mCO) that is steric and thermally stable and forms in situ "homo-cross-linking" in the melt post grafting onto PCR-PP. This designer molecule, besides offering a transient network, helps bridge the fragmented PP chains which is usually not amenable from the traditional grafting (like maleic anhydride), thereby addressing a long-standing challenge of retaining the properties post grafting due to chain scission in the melt. The resulting maleated (m) PCR-PP now offers abundant functionality which helped us design single and dual covalent adaptable network (CANs) and evaluate their consequences on the structure-property correlation. The PCR-PP Vitrimers demonstrate a distinct rubbery plateau in the melt and reprocessability with >90% recovery in mechanical properties even after the fifth sequence of recycling. We propose here for the first time how the varying reactivity (single or dual) in the transient polymer network, through dynamic exchange, regulates the closed-loop circularity in PP Vitrimers. Our results begin to suggest that the varying reactivity should be taken into account as an additional design parameter, as it influences both the stress relaxation rates and the flow activation energy. We now understand that the topology reconfiguration is strongly dependent on this varying reactivity, which also controls the overall crystalline morphology and the structural properties in the Vitrimers. This study, in addition to opening new avenues for recycling PP, will help guide researchers working in this field from both academia and industry.

Highly Recyclable and Tough Elastic Vitrimers from a Defined Polydimethylsiloxane Network.

Despite intensive research on sustainable elastomers, achieving elastic vitrimers with significantly improved mechanical properties and recyclability remains a scientific challenge. Herein, inspired by the classical elasticity theory, we present a design principle for ultra-tough and highly recyclable elastic vitrimers with a defined network constructed by chemically crosslinking the pre-synthesized disulfide-containing polydimethylsiloxane (PDMS) chains with tetra-arm polyethylene glycol (PEG). The defined network is achieved by the reduced dangling short chains and the relatively uniform molecular weight of network strands. Such elastic vitrimers with the defined network, i.e., PDMS-disulfide-D, exhibit significantly improved mechanical performance than random analogous, previously reported PDMS vitrimers, and even commercial silicone-based thermosets. Moreover, unlike the vitrimers with random network that show obvious loss in mechanical properties after recycling, those with the defined network enable excellent thermal recyclability. The PDMS-disulfide-D also deliver comparable electrochemical signals if utilized as substrates for electromyography sensors after the recycling. The multiple relaxation processes are revealed via a unique physical approach. Multiple techniques are also applied to unravel the microscopic mechanism of the excellent mechanical performance and recyclability of such defined network.

Analysis of the Effect of Network Structure and Disulfide Concentration on Vitrimer Properties.

A set of five vitrimers with glass transition temperatures in the range of 80-90 °C were designed to assess the effect of the network structure and disulfide concentration on their dynamic and mechanical properties, and to find the best performing system overall compared to the commercial Araldite LY1564/Aradur 3486 commercial thermoset system. Vitrimer networks were prepared by incorporating mono- and bifunctional epoxy reactive diluents and an amine chain extender into the Araldite LY1564/4-aminophenyldisulfide system.

Bio-Based Vitrimeric Silicone Materials with High-Strength, Reprocessable, Healing, and Transparent Properties.

Developing reprocessable polymeric materials from earth-abundant elements and renewable biomass is attractive for dealing with fossil resource crisis and achieving sustainable development. Based on the unique reactivity of biomass-derived gluconolactone, polydimethylsiloxane (PDMS) terminated with glucosamide groups is synthesized and used for preparing a series of silicone boronic ester based vitrimers. The whole preparation process is quite straightforward without any purification required and highly efficient with water as the only byproduct. The mechanical properties of obtained vitrimers can be precisely controlled by adjusting the content of 1,4-benzenediboronic acid or the molecular weight of PDMS precursor, producing boronic ester based vitrimers ranging from soft elastomers to rigid plastics. The obtained vitrimers exhibit excellent thermal stability, robust reprocessability, and efficient healing capacity. By encapsulating green-emitting CsPbBr3 nanocrystals, these materials are fabricated into hydrophobic, transparent, and luminescent coatings, promising for applications in flexible optical devices.

Exploring the Limits of High-Tg Epoxy Vitrimers Produced through Resin-Transfer Molding.

Over the past few years, scientists have developed new ways to overcome the recycling issues of conventional thermosets with the introduction of associative covalent adaptable networks (i.e., vitrimers) in polymer materials. Even though various end-use vitrimers have already been reported, just a few of them have targeted high-performance industrial applications. Herein, we develop a promising high-performance epoxy vitrimer based on a commercially available resin widely used in aeronautics with the highest glass transition temperature (Tg) of 233 °C ever reported for a vitrimer. A complete study of its physicochemical properties and cure kinetics was conducted, enabling the construction of the first time-temperature-transformation (TTT) diagram reported in the literature. This diagram allows a full determination of the processing and curing parameters leading to the manufacturing of vitrimer samples by the resin-transfer molding (RTM) process. The reshapability and limits therefrom of this high-Tg vitrimer were evaluated by three successful thermoforming cycles without degradation.

From Lignins to Renewable Aromatic Vitrimers based on Vinylogous Urethane.

During the two last decades, covalent adaptable networks (CANs) have proven to be an important new class of polymer materials combining the main advantages of thermoplastics and thermosets. For instance, materials can undergo reprocessing cycles by incorporating dynamic covalent bonds within a cross-linked network. Due to their versatility, renewable resources can be easily integrated into these innovative systems to develop sustainable materials, which can be related to the context of the recent development of a circular bioeconomy. Lignins, the main renewable sources of aromatic structures, are major candidates in the design of novel and biobased stimuli-responsive materials such as vitrimers due to their high functionality and specific chemical architectures. In the aim of developing recyclable lignin-based vinylogous urethane (VU) networks, an innovative strategy was elaborated in which lignin was first modified into liquid polyols and then into polyacetoacetates. Resulting macromonomers were integrated into aromatic VU networks and fully characterized through thermal, mechanical, and rheological experiments. Viscoelastic behaviors of the different aromatic vitrimers exhibited fast stress-relaxations (e. g., 39 s at 130 °C) allowing easy and fast mechanical reprocessing. A thermomechanical recycling study was successfully performed. Then, the developed strategy enabled the fabrication of healable biobased aromatic vitrimers with tunable structures and properties.