Synergistic Dual Dynamic Bonds in Covalent Adaptable Networks

Covalent adaptable networks(CANs),comprising polymer networks crosslinked by dynamic covalent bonds(DCBs),have garnered considerable attention as sustainable materials.Mastering the stress relaxation of CANs is essential for controlling their viscoelastic properties.An unexpected acceleration of stress relaxation has been observed in CANs containing dual dynamic bonds.The dynamic behavior of the second dynamic bonds can accelerate stress relaxation and lower the relaxation activation energy of dual dynamic CANs compared to analogous CANs that rely on only one type of DCB.These findings complement current approaches that utilize catalysts or adjust network parameters.In this minireview,we summarize the synergistic acceleration effects in various CANs containing dual dynamic bonds.We classify these effects based on the second dynamic bonds,including noncovalent bonds,mechanical bonds,and the second DCBs.We also discuss the mechanisms behind this synergy.Finally,we highlight the challenges and offer perspectives on harnessing the synergistic effects of these dual dynamic systems to expand the chemistry and applications of CANs.

Poly(imine-amide)Hybrid Covalent Adaptable Networks via in situ Oxidation Polymerization

Polyimine represents a rapidly emerging class of readily accessible and affordable covalent adaptable networks(CANs)that have been extensively studied in the past few years.While being highly malleable and recyclable,the pioneering polyimine materials are relatively soft and not suitable for certain applications that require high mechanical performance.Recent studies have demonstrated the possibility of significantly improving polyimine properties by varying its monomer building blocks,but such component variations are usually not straightforward and can be potentially challenging and costly.Herein,we report an in situ oxidation polymerization strategy for preparation of mechanically strong poly(imine-amide)(PIA)hybrid CANs from simple amine and aldehyde monomers.By converting a portion of reversible imine bonds into high-strength amide linkages in situ,the obtained hybrid materials exhibit gradually improved Young’s modulus and ultimate tensile strength as the oxidation level increased.Meanwhile,the PIAs remain reprocessable and can be depolymerized into small molecules and oligomers similar as polyimine.This work demonstrates the great potential of the in situ transformation strategy as a new approach for development of various mechanically tunable CANs from the same starting building blocks.

Recent advances in recyclable thermosets and thermoset composites based on covalent adaptable networks

Recyclable thermosets and thermoset composites with covalent adaptable networks(CANs,or dynamic covalent networks) have attracted considerable attention in recent years due to the combined merits of excellent mechanical and thermal properties,and chemical stabilities of traditional thermosets and recyclable,remoldable,and reprocessable attributes of thermoplastics.In this paper,we present an overview of the current strategies for synthesizing recyclable thermosets based on CANs,which involve recyclability,reprocessability,and possible rehealability.The recent literature examples are categorized based on the underlying controlled-cleavable linkages such as transesterification,DA/retro-DA chemistry,imine bonds,disulfide metathesis,dynamic B-O bonds,hemiaminals/hexahydrotriazines,and acetal linkages.Various degradation and malleability methods and resulting mechanical properties of the recycled thermosets and thermoset composites are presented.The emerging applications of recyclable thermosets and thermoset composites,with emphasis on their usage in adhesives,biomedical materials,wearable devices,coatings,and 3D printing materials,are also illustrated.Finally,a perspective on the challenges and future perspectives is briefly summarized.

Reshapeable,rehealable and recyclable sensor fabricated by direct ink writing of conductive composites based on covalent adaptable network polymers

Covalent adaptable network(CAN)polymers doped with conductive nanoparticles are an ideal candidate to create reshapeable,rehealable,and fully recyclable electronics.On the other hand,3D printing as a deterministic manufacturing method has a significant potential to fabricate electronics with low cost and high design freedom.In this paper,we incorporate a conductive composite consisting of polyimine CAN and multi-wall carbon nanotubes into direct-ink-writing 3D printing to create polymeric sensors with outstanding reshaping,repairing,and recycling capabilities.The developed printable ink exhibits good printability,conductivity,and recyclability.The conductivity of printed polyimine composites is investigated at different temperatures and deformation strain levels.Their shape-reforming and Joule heating-induced interfacial welding effects are demonstrated and characterized.Finally,a temperature sensor is 3D printed with defined patterns of conductive pathways,which can be easily mounted onto 3D surfaces,repaired after damage,and recycled using solvents.The sensing capability of printed sensors is maintained after the repairing and recycling.Overall,the 3D printed reshapeable,rehealable,and recyclable sensors possess complex geometry and extend service life,which assist in the development of polymer-based electronics toward broad and sustainable applications.

Cooperative Chemical Coupling and Physical Lubrication Effects Construct Highly Dynamic Ionic Covalent Adaptable Network for High-Performance Wearable Electronics

Covalent adaptable networks(CANs),which combine the benefits of traditional thermosets and thermoplastics,have attracted considerable attention.The dynamics of reversible covalent bonds and mobility of polymer chains in CANs determine the topological rearrangement of the polymeric network,which is critical to their superior features,such as self-healing and reprocessing.Herein,we introduce an ionic liquid to dimethylglyoximeurethane(DOU)-based CANs to regulate both reversible bond dynamics and polymer chain mobility by cooperative chemical coupling and physical lubrication.Small-molecule model experiments demonstrated that ionic liquids can catalyze dynamic DOU bond exchange.Ionic liquid also breaks the hydrogen bonds between polymeric chains,thereby increasing their mobility.As a combined result,the activation energy of the dissociation of the dynamic network decreased from 110 to 85 kJ mol^(−1).Furthermore,as a functional moiety,the ionic liquid imparts new properties to CANs and will greatly expand their applications.For example,the consequent conductivity of resultant ionic CAN(iCAN)has demonstrated a great power to build high-performance multifunctional wearable electronics responsive to multiple stimulations including temperature,strain,and humidity.This study provides a new design principle that simultaneously uses the chemical and physical effects of two structural components to regulate material properties enabling novel applications.

A weak but inert hindered urethane bond for high-performance dynamic polyurethane polymers

Covalent adaptable networks(CANs),which share the properties of both thermosets and thermoplastics at the same time,are desirable for many applications.Introducing bulky substituents is a feasible way to design dynamic covalent bonds for constructing CANs,as evidenced by the successful implementation in CANs based on hindered urea bonds(HUBs).However,the dynamicity induced by introducing bulky substituents always come with low bond energy,resulting in low mechanical strength and poor stability of the CANs.Herein,we designed a novel hindered urethane bond,which is weak in thermodynamic(K_(eq)=1701.23 L/mol at 25℃)and inert in kinetic at low temperature,but stable in thermodynamic(K_(eq)=1.54×10^(4) L/mol at 100℃)and active in kinetic at high temperature(k_(-1)=0.105 h^(-1) at 80℃ and 0.315 h^(-1) at 120℃).As a result,the polyurethane based on it exhibits high mechanical properties(with Youngs’modulus of 1011±29MPa and flexible modulus reached 1833±50MPa)and excellent reversibility(can be reprocessed at 60℃ under 100 kPa in 30 min and completely healed at 40℃ in 10 min).Moreover,unlike to many CANs based on hindered urea bonds,our dynamic polyurethanes are highly stable in humid environment or even water solutions due to the slow hydrolysis kinetics.Such highperformance dynamic polyurethane polymers are attractive for many applications.

A Strong and Rigid Coordination Adaptable Network that Can Be Reprocessed and Recycled at Mild Conditions

The investigation of covalent adaptable networks(CANs)is expanding rapidly due to the growing demand for sustainable materials,as CANs show thermoset-like behavior and yet can be reprocessed,recycled,and healed.However,most of the CANs reported so far have a trade-off between mechanical strength and reversible properties and often show performance reduction after reprocessing and/or recycling.Herein,we designed and synthesized a coordination adaptable network(CoAN)by crosslinking low-molecular-weight monomers with abundant coordination bonds.Owning to its excellent variable-stiffness property,leading to high stiffness at ambient conditions and low viscosity at elevated temperature,the as-prepared CoAN showed high mechanical rigidity but could be reprocessed rapidly and recycled at mild conditions.After reprocessing or recycling,the mechanical properties of the samples showed no performance reduction,compared with a pristine sample.Density functional theory calculations showed that free thiol ligands played a key role in reducing the activation energy for bond exchange.When used as binders for composites,the embedded carbon fibers could be recycled rapidly and still maintain the original microstructure.The material also showed temperature-sensitive dielectric and conductive properties due to the release of metal ions upon heating.Overall,such performances are superior among the CANs reported previously.

Thiol-acrylate Catalyst Enabled Post-Synthesis Fabrication of Liquid Crystal Actuators

Synthesizing orientated liquid crystal elastomers(LCEs)via the two-stage thiol-acrylate Michael addition and photopolymerization(TAMAP)reaction is extensively used.However,excess acrylates,initiators,and strong stimuli are inevitably involved in the second stage crosslinking.Herein,we simplify the strategy through taking advantage of a volatile alkaline(originally added to catalyze the thiol-acrylate addition in the first crosslinking stage).Without excess functional groups,the residual catalyst after annealing is still enough to trigger reactions of dynamic covalent bonds at a relatively mild temperature(80℃)to program the alignment of LCEs.The reversible reaction switches off by itself after this process since the catalyst gradually but totally evaporates upon heating.The obtained soft actuators exhibit robust actuation during repeated deformation(over 1000 times).Many shape-morphing modes can be achieved by rationally designing orientation patterns.This strategy not only facilitates the practical synthesis of LCE actuators,but also balances the intrinsic conflict between stability and reprogrammability of exchangeable LCEs.Moreover,the method of applying volatile catalysts has the potential to be extended to other dynamic covalent bonds(DCBs)applied to crosslinked polymer systems.

Dynamic Oxime-Urethane Bonds, a Versatile Unit of High Performance Self-healing Polymers for Diverse Applications

Oxime-urethane bond featuring with high reversibility even at room temperature and multiple reactivity is an emerging dynamic covalent bond,and has shown great potential for self-healing polymers,which are one of the most attractive development directions for next generation of polymeric materials.In this review,recent progresses on the oxime-urethane-based self-healing polymers,including their designs and applications in diverse fields such as biomedicine,flexible electronics,soft robots,3D printing,protective materials,and adhesives,are summarized,and outlooks on the future development of this field are discussed.