Covalent Adaptable Networks Research Articles

Tailoring dynamic mechanism of covalent adaptable polyurea networks by varying the category of isocyanates

Hindered urea bonds (HUBs), a type of dynamic covalent motifs, have attracted much attention due to their facile dynamic features. However, when varying HUBs are synthesized by altering isocyanate compounds the dynamic mechanism of HUBs is still puzzled. To systematically investigate the dynamic mechanism of HUBs, a series of model reactions was performed firstly. It was found that HUBs deriving from the aromatic isocyanate displayed a dissociated mechanism, whereas those synthesized by aliphatic isocyanate presented an associated pathway. Based on the difference in dynamic mechanisms, varying covalent adaptable polyurethane networks (CANs) were synthesized by tuning the type of isocyanate monomers. Owing to the dissociated mechanism, the CANs prepared by aromatic isocyanate exhibited a lower activated energy accompanying with a shorter stress relaxation time, compared with the CANs composed of aliphatic urea dynamic motifs. Furthermore, the CANs consist of aromatic urea moiety exhibited rapidly self-healing properties and temperature dependence of moduli in thermal measurement. The gradual decrease of modulus with temperature further verifies the dissociated mechanism of aromatic urea bonds, which accelerates the self-healing properties of CANs. Based on the dissociated mechanism of aromatic HUBs, PU-M composited of aromatic HUBs enabled be remodelled by 3D-printer and applied for the preparation of dynamic crosslinked elastomers. This work provides a systematic insight into the dynamic mechanism of HUBs and a novel application pathway in fibrous materials.

Mechanical recycling of epoxy composites reinforced with short-cut aramid fibers: Surface functionalization – The missing piece of the puzzle

Fiber reinforced thermosetting composites are essential materials for present and future technologies. However, their lack of reprocessability puts them far behind in achieving current sustainability targets. The recent development of covalent adaptable networks represents a promising approach to mitigate this. Here, mechanical recycling of short-cut aramid fiber reinforced epoxy composites was evaluated with a special focus on the impact of fiber surface functionality. The introduction of aramid fibers with four different surface chemistries to a covalent adaptable network was shown to influence the mechanical properties before and after reprocessing. Epoxy functionalized fibers enabled 52 % and 36 % retention of the original tensile stress and elongation at break after two recycling cycles. In comparison, the unmodified reference samples could only retain 19 % and 12 % of the original tensile stress and elongation at break. Fiber functionalization was thus proven to be a key piece in the development of more sustainable solutions for short-cut fiber reinforced polymer composites (SCFRPC).

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.

Catalyst-free readily dual-recyclable acetal-based covalent adaptable cellulose networks

Despite covalent adaptable networks (CANs) imparting the favorable features of crosslinked polymers, as well as the functionality of reprocessing, reshaping and welding, due to exchange reaction enabled topology changes; it is still a huge challenge to design catalyst-free, fast reprocessing, controlled degradation and polymer recyclable biomass base CANs. Herein, for the first time, acetal-based covalent adaptable cellulose networks (ACCs) were utilized to synthesize readily reconstructable cellulose-based thermosets with mechanical tunability. ACCs were synthesized via catalyst-free “click” addition of cellulose and divinyl ether without releasing small molecule byproducts. Different crosslinking densities and crosslinkers were used to explore the structure-property relationship, the mechanical and thermal properties of the ACCs were strongly influenced by these factors. ACCs can obtain enhanced tensile strength or elongation at break by changing the structure of the crosslinker. Furthermore, the reworking, welding and shape memory properties of these ACCs, based on the dynamic exchange reaction of acetal bonds, were investigated. In addition, these ACCs can be degraded under acidic conditions, and closed-loop utilization of polymer was possible. Thus, ACCs can be mechanically and chemically double-cycled, which will contribute to solving the white pollution problem and resource waste as a new class of sustainable plastics.

Eugenol-based dual-cured materials with multiple dynamic exchangeable bonds

In the present work, the preparation of sustainable thermosets has been approached simultaneously from three different points of view: a) the use of bio-based monomers chemically modified through green methodologies, b) the adoption of dual curing through click-type reactions to implement more efficient manufacturing processes, and c) inclusion of interchangeable groups in the network, to enable the reuse and recycling of the material at the end of its useful life and avoid waste generation.The first goal has been approached by synthesizing in a greener way an acrylate-epoxy derivative of eugenol (AEEU) and a glycerol triacrylate (GTA), both biobased resources. Then, the second approach was addressed by using biobased cystamine as a crosslinker to obtain materials through a dual-curing procedure based on a first “click” aza-Michael reaction and a second “click” epoxy-amine reaction. Intermediate and final materials could be prepared with different tailorable properties by controlling the molar ratio of the AEEU and GTA. By using DSC and rheology, we could evaluate the sequentiality and the gelation of the curing process. Finally, the covalent adaptable networks (CANs) prepared contained three different types of dynamic bonds (disulfide, esters, and β-aminoesters) and their thermomechanical properties were tested by DMA revealing Tgs above room temperature from 47 to 70 °C. Bending tests at break were performed to evaluate the mechanical properties reaching values up to 90 MPa of stress at break and 7 % of deformation. Stress relaxation tests showed that all materials could relax the stress at relatively low temperatures (120 °C) in less than 21 min. The associative and dissociative behavior of these materials was investigated through rheology revealing a clear drop of the modulus at high temperatures and frequencies when cystamine was used as a crosslinker. Moreover, their reprocessability was tested obtaining homogeneous samples with no significant changes in their chemical and thermal properties highlighting the great potential and wide range of possibilities in many different fields of these CANs.

Self‐healing and reprocessing of saturated hydroxyl‐terminated polybutadiene‐based networks enabled by dynamic covalent chemistry

AbstractHydroxyl‐terminated polybutadiene (HTPB) is found in many applications due to its ease of manufacturing, useful mechanical properties over a wide temperature range, and reactive hydroxyl chain ends. Typically, HTPB is crosslinked with isocyanates to form polyurethane thermosets. Limitations of this approach include the use of toxic isocyanates and the oxidative instability of backbone alkenes. In this work, saturated HTPB is used to form reprocessable covalent adaptable networks that are capable of stress relaxation and reprocessing, without relying on isocyanates or unstable alkenes. This approach introduces dynamic chemistry to the HTPB network via chain extension and subsequent crosslinking with 4‐methyl caprolactone (4mCL) and a novel bislactone crosslinker. Using benzenesulfonic acid (BSA) as a transesterification catalyst, stress relaxation times range from 150 to 8 min at temperatures of 70 to 100 °C. Despite crosslinking, these networks behave elastically, as evidenced by strain‐at‐break values of 93% for pristine samples, and dynamically, as shown by a strain‐at‐break of 72% after reprocessing the damaged samples. Shape reprogramming is also demonstrated by straining the crosslinked networks and heating to elevated temperatures where bond exchange occurs. These findings illustrate the advantageous properties that can be achieved by using cheap commodity building blocks to achieve dynamic properties. We anticipate that valorizing commodity polymers into reprocessable thermosets will be of utility in applications that lack other viable recycling pathways.

Self-Healing Activation by Conventional Resistive Heating through the Addition of Carbon Nanotubes in Epoxy Systems Based on Covalent Adaptable Networks

Self-Healing Activation by Conventional Resistive Heating through the Addition of Carbon Nanotubes in Epoxy Systems Based on Covalent Adaptable Networks

Understanding the application of covalent adaptable networks in self-repair materials based on molecular simulation.

Covalent adaptable networks (CANs) are widely used in the field of self-repair materials. They are a group of covalently cross-linked associative polymers that undergo reversible chemical reactions, and can be further divided into dissociative CANs (Diss-CANs) and associative CANs (Asso-CANs). Self-repair refers to the ability of a material to repair itself without external intervention, and can be classified into self-adhesion and self-healing according to the utilization of open stickers. Unlike conventional materials, the viscoelastic properties of CANs are influenced by both the molecular structure and reaction kinetics, ultimately affecting their repair performance. To gain deeper insight into the repair mechanism of CANs, we conducted simulations by using the hybrid MC/MD algorithm, as previously proposed in our research. Interestingly, we observed a significant correlation between reaction kinetics and repair behavior. Asso-CANs exhibited strong mechanical strength and high creep resistance, rendering them suitable as self-adhesion materials. On the other hand, Diss-CANs formed open stickers that facilitated local relaxation, aligning perfectly with self-healing processes. Moreover, the introduction of crosslinkers in the form of small molecules enhanced the repair efficiency. Theoretically, it was found that the repair timescale of Asso-CANs is slower than that of Diss-CANs with identical molecular structures. Our study not only clarifies the similarities and differences between Diss-CANs and Asso-CANs in terms of their self-repairing capabilities, but more importantly, it provides valuable insights guiding the effective utilization of CANs in the development of self-repair materials.

Reprocessable and chemically recyclable poly(acylhydrazone–imine) covalent adaptable networks with enhanced mechanical strength and creep resistance

A room temperature chemically recyclable poly(acylhydrazone–imine) covalent adaptable network with high mechanical strength and creep resistance was constructed by designing synergetic hydrogen bonds and acylhydrazone bonds in a single polymer network.

Photocurable acylhydrazone covalent adaptable networks with fast dynamic exchange and tunable viscoelastic properties

Photo-curable vanillin-derived acylhydrazone-based covalent adaptable networks (CANs) were designed with tunable acylhydrazone exchange reactions and viscoelastic behavior.

A tutorial review of linear rheology for polymer chemists: basics and best practices for covalent adaptable networks

This tutorial review of linear viscoelasticity is targeted at polymer chemists working with covalent adaptable networks. It provides an overview of concepts, analysis, misconceptions, and best practices for rheological experiments.

Itaconic acid-based sustainable polyurethane covalent adaptable networks with robust mechanical, reshaping and UV-resistant properties based on reversible disulfide bond

Thermoset polymer materials are widely used in various fields due to their excellent performance brought by their permanently cross-linked structures. However, their inability to be recycled also leads to a serious waste of resources. The emergence of covalent adaptable networks (CANs) has solved the problem perfectly that thermoset materials are difficult to recycle. Meanwhile, with the increasing concern for resources and the environment, bio-based materials have sparked a research boom. Itaconic acid is a biomass feedstock with great prospects for development. In this study, itaconic acid, 1,6-hexanediol, 6-mercapto-1-hexanol, isocyanate, and 4,4′-dithiodiphenylamine were used as raw materials. Itaconic acid-based polyols were first synthesized by thiol-ene click reaction, and a series of itaconic acid-based polyurethane (PU) materials containing dynamic covalent networks were synthesized by varying the ratios between the amino and hydroxyl groups. The obtained material has good mechanical properties with a maximum tensile strength of 31.5 MPa and a modulus of 717 MPa. The presence of disulfide bonds allows the material to be recycled by remodeling, and the recovery efficiency of tensile strength after three remodeling cycles is 85.1 %. The material also has excellent solvent resistance and UV-blocking resistance. In conclusion, this work provides possible ideas for the high-value utilization of itaconic acid in polyurethanes.

Unprecedented associative exchange in CO2-sourced cyclic S,O-acetal-based covalent adaptable networks

An unexplored associative dynamic exchange reaction between CO2-sourced cyclic S,O-acetals and thiolates was investigated and utilized to create covalent adaptable networks suitable for the production of healable coatings.

Recyclable and elastic highly thermally conductive epoxy-based composites with covalent-noncovalent interpenetrating networks.

High-power electronic architectures and devices require elastic thermally conductive materials. The use of epoxy resin in thermal management is limited due to its rigidity. Here, based on epoxy vitrimer, flexible polyethylene glycol (PEG) chains are introduced into covalent adaptable networks to construct covalent-noncovalent interpenetrating networks, enabling the elasticity of epoxy resins. Compared to traditional silicone-based thermal interface materials, the newly developed elastic epoxy resin shows the advantages of reprocessability, self-healing, and no small molecule release. Results show that, even after being filled with boron nitride and liquid metal, the material maintains its resilience, reprocessability and self-healing properties. Leveraging these characteristics, the composite can be further processed into thin films through a repeated pressing-rolling technique that facilitates the forced orientation of the fillers. Subsequently, the bulk composites are reconstructed using a film-stacking method. The results indicate that the thermal conductivity of the reconstructed bulk composite reaches 3.66 W m-1 K-1, achieving a 68% increase compared to the composite prepared through blending. Due to the existence of covalent adaptable networks, the inorganic and inorganic components of the composite prepared in this work can be completely separated under mild conditions, realizing closed-loop recycling.

Graphene oxide offers precise molecular sieving, structural integrity, microplastic removal, and closed-loop circularity in water-remediating membranes through a covalent adaptable network

CAN-enabled membranes promote effective end-use management and circular economy.

BiTEMPS methacrylate dynamic covalent cross-linker providing rapid reprocessability and extrudability of covalent adaptable networks: high-yield synthesis with strong selectivity for disulfide linkages

A dialkylamino disulfide-based dynamic covalent cross-linker (BTMA) was synthesized with high purity and selectivity for disulfides and was used to produce rapidly reprocessable and extrudable covalent adaptable networks with n-hexyl methacrylate.

β-Amino amide based covalent adaptable networks with high dimensional stability

Catalyst-free reversible β-amino amides in covalent adaptable networks with reprocessability and high dimensional stability.

Tough, recyclable and degradable plastics with multiple functions based on supramolecular covalent adaptive networks

The PPVA-BOL plastic based on supramolecular covalent adaptive networks is tough, recyclable and degradable, and is expected to solve plastic pollution.

Polypropylene-based transesterification covalent adaptable networks with internal catalysis

With neighboring group participation, polypropylene-based transesterification covalent adaptable networks were facilely prepared via the reaction of maleic anhydride grafted polypropylene and diol without using external catalysts.

Closed-loop chemically recyclable covalent adaptive networks derived from elementary sulfur.

The development of sulfur-rich polymers derived from elementary sulfur provides an innovative approach to industrial waste valorization. Despite significant advancements in polymerization techniques and promising applications beyond traditional polymers, polysulfide networks are still primarily stabilized by diene crosslinkers, forming robust C-S bonds that hinder the degradation of sulfur-based polymers. In this study, the anionic ring-opening copolymerization of chemically homologous S8 and cyclic disulfides was explored to yield robust sulfur-rich copolymers with high molecular weight. The incorporation of polysulfide segments not only efficiently activated the crosslinked networks for excellent reprocessability and mechanical adaptability but also endowed the resulting copolymer with high optical transparency in the near-infrared region. More importantly, the dynamic disulfide crosslinking sites promoted the chemical closed-loop recyclability of the polysulfide networks via reversible S-S cleavage. This innovative inverse vulcanization strategy utilizing dynamic disulfide crosslinkers offers a promising pathway for the advanced applications and upcycling of high-performance sulfur-rich polymers.