Tuning the Mechanical and Dynamic Properties of Elastic Vitrimers by Tailoring the Substituents of Boronic Ester.

Elastic vitrimers, i.e., elastic polymers with associative dynamic covalent bonds, can afford elastomers with recyclability while maintaining their thermal and chemical stability. Herein, we report a series of boronic ester-based vitrimers with tunable mechanical properties and recyclability by varying the substitute groups of boronic acid in polymer networks. The dynamic polymer networks are formed by reacting diol-containing tetra-arm poly(amidoamine) with boronic acid-terminated tetra-arm poly(ethylene glycol), which possesses different substituents adjacent to boronic acid moieties. Varying the substituent adjacent to the boronic ester unit will significantly affect the binding strength of the boronic ester, therefore affecting their dynamics and mechanical performance. The electron-withdrawing substituents noticeably suppress the dynamics of boronic ester exchange and increase the activation energy and relaxation time while enhancing the mechanical strength of the resulting elastic vitrimers. On the other hand, the presence of electron-rich substituent affords relatively reduced glass transition temperature (Tg), faster relaxation, and prominent recyclability and malleability at lower temperatures. The developed pathway will guide the rational design of elastomers with well-tunable dynamics and processabilities.

Hybrid Dynamic Covalent Network-Based Protecting Layer for Stable Li-Metal Batteries.

Metallic lithium (Li) is considered as the "Holy Grail" anode material for next-generation energy storage systems due to its extremely high theoretical capacity and low electrochemical potential. Before the commercialization of the Li electrode, dendritic Li growth and the unstable solid electrolyte interphase layer should be conquered. Herein, a hybrid covalent adaptable polymer network (HCAPN) is prepared via the random copolymerization of poly(ethylene glycol) methyl ether methacrylate and -acetoacetoxyethyl methacrylate, followed by chemical cross-linking with polyethylenimine (PEI) and amine-modified silicon dioxide (SiO2). Such a hybrid network, where PEI and amine-modified SiO2 formed a vinylogous urethane-based dynamic covalent bond with the copolymer, respectively, shows improved mechanical properties, solvent resistance, and excellent healability/recyclability. As the protecting layer on the Li electrode, the assembled HCAPN@Li||HCAPN@Li symmetric cell shows a long cycle life of 800 h with low overpotential at a current density of 1 mA cm-2, and superior electrochemical performance can be achieved in the HCAPN@Li||LiFePO4 full cell (capacity retention of 77% over 400 cycles at 1.5 C) and HCAPN@Li||NCM811 cell (capacity retention of 79% after 300 cycles). Surface morphology analysis is also performed for physical insight into their role as protecting layer. This work provides a new perspective for constructing a hybrid dynamic covalent network-based polymer protecting layer for inhibiting Li dendrite growth.

Ductile adhesive elastomers with force-triggered ultra-high adhesion strength.

Elastomers play a vital role in many forthcoming advanced technologies in which their adhesive properties determine materials' interface performance. Despite great success in improving the adhesive properties of elastomers, permanent adhesives tend to stick to the surfaces prematurely or result in poor contact depending on the installation method. Thus, elastomers with on-demand adhesion that is not limited to being triggered by UV light or heat, which may not be practical for scenarios that do not allow an additional external source, provide a solution to various challenges in conventional adhesive elastomers. Herein, we report a novel, ready-to-use, ultra high-strength, ductile adhesive elastomer with an on-demand adhesion feature that can be easily triggered by a compression force. The precursor is mainly composed of a capsule-separated, two-component curing system. After a force-trigger and curing process, the ductile adhesive elastomer exhibits a peel strength and a lap shear strength of 1.2 × 104 N m-1 and 7.8 × 103 kPa, respectively, which exceed the reported values for advanced ductile adhesive elastomers. The ultra-high adhesion force is attributed to the excellent surface contact of the liquid-like precursor and to the high elastic modulus of the cured elastomer that is reinforced by a two-phase design. Incorporation of such on-demand adhesion into an elastomer enables a controlled delay between installation and curing so that these can take place under their individual ideal conditions, effectively reducing the energy cost, preventing failures, and improving installation processes.

Superstretchable Liquid-Metal Electrodes for Dielectric Elastomer Transducers and Flexible Circuits.

Dielectric elastomer transducers (DETs), with a dielectric elastomer (DE) film sandwiched between two compliant electrodes, are highly sought after in the fields of soft robotics, energy harvesting, and human-machine interaction. To achieve a high-performance DET, it is essential to develop electrodes with high conductivity, strain-insensitive resistance, and adaptability. Herein, we design an electrode (Supra-LMNs) based on multiple dynamic bond cross-linked supramolecular networks (Ns) and liquid metal (LM), which realizes high conductivity (up to 16,000 S cm-1), negligible resistance changes at high strain (1.3-fold increase at 1000% strain), instantaneous self-healability at ambient temperature, and rapid recycling. The conductive pathway can be activated through simple friction by transmitting stress through the silver nanowires (AgNWs) and cross-linking sites of LM particles. This method is especially attractive for printing circuits on flexible substrates, especially DE films. Utilized as dielectric elastomer generator (DEG) electrodes, it reduces the charge loss by 3 orders of magnitude and achieves high generating energy density and energy conversion efficiency on a low-resistance load. Additionally, serving as sensor (DES) and actuator (DEA) electrodes, it enables a highly sensitive sensing capability and complex interaction.

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.

Selective deconstruction of mixed plastics by a tailored organocatalyst.

Plastic represents an essential material in our society; however, a major imbalance between their high production and end-of-life management is leading to unrecovered energy, economic hardship, and a high carbon footprint. The adoption of plastic recycling has been limited, mainly due to the difficulty of recycling mixed plastics. Here, we report a versatile organocatalyst for selective glycolysis of diverse consumer plastics and their mixed waste streams into valuable chemicals. The developed organocatalyst selectively deconstructs condensation polymers at a specific temperature, and additives or other polymers such as polyolefin or cellulose can be readily separated from the mixed plastics, providing a chemical recycling path for many existing mixed plastics today. The Life Cycle Assessment indicates that the production of various condensation polymers from the deconstructed monomers will result in a significant reduction in greenhouse gas emissions and energy input, opening a new paradigm of plastic circularity toward a net-zero carbon society.

Puncture-resistant self-healing polymers with multi-cycle adhesion and rapid healability.

The structural design of self-healing materials determines the ultimate performance of the product that can be used in a wide range of applications. Incorporating intrinsic self-healing moieties into puncture-resistant materials could significantly improve the failure resistance and product longevity, since their rapidly rebuilt bonds will provide additional recovery force to resist the external force. Herein, we present a series of tailored urea-modified poly(dimethylsiloxane)-based self-healing polymers (U-PDMS-SPs) that exhibit excellent puncture-resistant properties, fast autonomous self-healing, multi-cycle adhesion capabilities, and well-tunable mechanical properties. Controlling the composition of chemical and physical cross-links enables the U-PDMS-SPs to have an extensibility of 528% and a toughness of 0.6 MJ m-3. U-PDMS-SPs exhibit fast autonomous self-healability with 25% strain recovery within 2 minutes of healing, and over 90% toughness recovery after 16 hours. We further demonstrate its puncture-resistant properties under the ASTM D5748 standard with an unbreakable feature. Furthermore, the multi-cycle adhesive properties of U-PDMS-SPs are also revealed. High puncture resistance (>327 mJ) and facile adhesion with rapid autonomous self-healability will have a broad impact on the design of adhesives, roofing materials, and many other functional materials with enhanced longevity.

Hybrid Dynamic Covalent Network as a Protective Layer and Solid-State Electrolyte for Stable Lithium-Metal Batteries.

Lithium (Li) metal is a highly promising anode material for next-generation high-energy-density batteries, while Li dendrite growth and the unstable solid electrolyte interphase layer inhibit its commercialization. Herein, a chemically grafted hybrid dynamic network (CHDN) is rationally designed and synthesized by the 4,4'-thiobisbenzenamine cross-linked poly(poly(ethylene glycol) methyl ether methacrylate-r-glycidyl methacrylate) and (3-glycidyloxypropyl) trimethoxysilane-functionalized SiO2 nanoparticles, which is utilized as a protective layer and hybrid solid-state electrolyte (HSE) for stable Li-metal batteries. The presence of a dynamic exchangeable disulfide affords self-heability and recyclability, and the chemical attachment between SiO2 nanoparticles and the polymer matrix enables the homogeneous distribution of inorganic fillers and mechanical robustness. With integrated flexibility, fast segmental dynamics, and autonomous adaptability, the as-prepared CHDN-based protective layer enables superior electrochemical performance in half cells and full cells (capacity retention of 83.7% over 400 cycles for the CHDN@Li/LiFePO4 cell at 1 C). Furthermore, benefiting from intimate electrode/electrolyte interfacial contact, CHDN-based solid-state cells deliver excellent electrochemical performance (capacity retention of 89.5% over 500 cycles for the Li/HSE/LiFePO4 cell at 0.5 C). In addition, the Li/HSE/LiFePO4 pouch cell exhibits superior safety, even exposing various physical damage conditions. This work thereby provides a fresh insight into a rational design principle for dynamic network-based protective layers and solid-state electrolytes for battery applications.

Intrinsically Self-Healing Polymers: From Mechanistic Insight to Current Challenges.

Self-healing materials open new prospects for more sustainable technologies with improved material performance and devices' longevity. We present an overview of the recent developments in the field of intrinsically self-healing polymers, the broad class of materials based mostly on polymers with dynamic covalent and noncovalent bonds. We describe the current models of self-healing mechanisms and discuss several examples of systems with different types of dynamic bonds, from various hydrogen bonds to dynamic covalent bonds. The recent advances indicate that the most intriguing results are obtained on the systems that have combined different types of dynamic bonds. These materials demonstrate high toughness along with a relatively fast self-healing rate. There is a clear trade-off relationship between the rate of self-healing and mechanical modulus of the materials, and we propose design principles of polymers toward surpassing this trade-off. We also discuss various applications of intrinsically self-healing polymers in different technologies and summarize the current challenges in the field. This review intends to provide guidance for the design of intrinsic self-healing polymers with required properties.

Single-Ion Conducting Polymeric Protective Interlayer for Stable Solid Lithium-Metal Batteries.

With many reported attempts on fabricating single-ion conducting polymer electrolytes, they still suffer from low ionic conductivity, narrow voltage window, and high cost. Herein, we report an unprecedented approach on improving the cationic transport number (tLi+) of the polymer electrolyte, i.e., single-ion conducting polymeric protective interlayer (SIPPI), which is designed between the conventional polymer electrolyte (PVEC) and Li-metal electrode. Satisfied ionic conductivity (1 mS cm-1, 30 °C), high tLi+ (0.79), and wide-area voltage stability are realized by coupling the SIPPI with the PVEC electrolyte. Benefiting from this unique design, the Li symmetrical cell with the SIPPI shows stable cycling over 6000 h at 3 mA cm-2, and the full cell with the SIPPI exhibits stable cycling performance with a capacity retention of 86% over 1000 cycles at 1 C and 25 °C. This incorporated SIPPI on the Li anode presents an alternative strategy for enabling high-energy density, long cycling lifetime, and safe and cost-effective solid-state batteries.

Unravelling the Influence of Surface Modification on the Ultimate Performance of Carbon Fiber/Epoxy Composites.

The overall performance of polymer composites depends on not only the intrinsic properties of the polymer matrix and inorganic filler but also the quality of interfacial adhesion. Although many reported approaches have been focused on the chemical treatment for improving interfacial adhesion, the examination of ultimate mechanical performance and long-term properties of polymer composites has been rarely investigated. Herein, we report carbon fiber (CF)/epoxy composites with improved interfacial adhesion by covalent bonding between CFs and the epoxy matrix. This leads to the improved ultimate mechanical properties and enhanced thermal aging performance. Raman mapping demonstrates the formation of an interphase region derived from the covalent bonding between CFs and the epoxy matrix, which enables the uniform fiber distribution and eliminates phase separation during thermal cycling. The covalent attachment of the CF to the epoxy matrix suppresses its migration during temperature fluctuations, preserving the mechanical performance of resulting composites under the thermal aging process. Furthermore, the finite elemental analysis reveals the effectiveness of the chemical treatment of CFs in improving the interfacial strength and toughness of silane-treated CF/epoxy composites. The insight into the mechanical improvement of CF/epoxy composites suggests the high potential of surface modification of inorganic fillers toward polymer composites with tunable properties for different applications.

On the Interfacial Behavior of Catenated Poly(l-lactide) at the Air-Water Interface.

Interfacial properties of polymeric materials are significantly influenced by their architectural structures and spatial features, while such a study of topologically interesting macromolecules is rarely reported. In this work, we reported, for the first time, the interfacial behavior of catenated poly(l-lactide) (C-PLA) at the air-water interface and compared it with its linear analogue (L-PLA). The isotherms of surface pressure-area per repeating unit showed significant interfacial behavioral differences between the two polymers with different topologies. Isobaric creep experiments and compression-expansion cycles also showed that C-PLA demonstrated higher stability at the air-water interface. Interestingly, when the films at different surface pressures were transferred via the Langmuir-Blodgett method, successive atomic force microscopy imaging displayed distinct nanomorphologies, in which the surface of C-PLA exhibited nanofibrous structures, while that of the L-PLA revealed a smoother topology with less fiber-like structures.

Self-Healable, Highly Stretchable, Ionic Conducting Polymers as Efficient Protecting Layers for Stable Lithium-Metal Electrodes.

Although numerous studies on polymeric protective films to stabilize lithium (Li)-metal electrodes have been reported, the construction of self-healing polymers that enables the long-term operation of Li-metal batteries (LMBs) at relatively low temperatures has rarely been demonstrated. Herein, a highly stretchable, autonomous self-healable, and ionic-conducting polymer network (SHIPN) is synthesized as an efficient protective film for LMBs. The network backbone, synthesized from copolymerization of poly(ethylene glycol)-mono-methacrylate (PEGMMA) and 2-[[(butylamino)carbonyl]oxy]ethyl acrylate (BCOE), is chemically cross-linked via diisocyanate. With SHIPN-modified electrodes, enhanced electrochemical performance can be achieved in Li/Cu, Li/Li, and Li/LiFePO4 (Li/LFP) cells. The SHIPN@Li/LFP cell delivers a capacity retention of 85.6% after 500 cycles at 5 °C, resulting from the low-temperature self-healability of SHIPN. In full cells with a high-mass-loading LFP cathode (∼17 mg cm-2), the capacity retention is at least 300% higher than that with a bare Li electrode. Further physical characterizations of electrodes confirm the effect of SHIPN in enhancing the interfacial stability and suppressing Li dendrite growth. Our results will provide insights into rationally designing soft and hybrid materials toward stable LMBs at different temperatures.

Unravelling the Mechanism of Viscoelasticity in Polymers with Phase-Separated Dynamic Bonds.

Incorporation of dynamic (reversible) bonds within polymer structure enables properties such as self-healing, shape transformation, and recyclability. These dynamic bonds, sometimes refer as stickers, can form clusters by phase-segregation from the polymer matrix. These systems can exhibit interesting viscoelastic properties with an unusually high and extremely long rubbery plateau. Understanding how viscoelastic properties of these materials are controlled by the hierarchical structure is crucial for engineering of recyclable materials for various future applications. Here we studied such systems made from short telechelic polydimethylsiloxane chains by employing a broad range of experimental techniques. We demonstrate that formation of a percolated network of interfacial layers surrounding clusters enhances mechanical modulus in these phase-separated systems, whereas single chain hopping between the clusters results in macroscopic flow. On the basis of the results, we formulated a general scenario describing viscoelastic properties of phase-separated dynamic polymers, which will foster development of recyclable materials with tunable rheological properties.

Unraveling the Role of Neutral Units for Single-Ion Conducting Polymer Electrolytes.

With the cationic transference number close to unity, single-ion conducting polymer electrolytes (SICPEs) are recognized as an advanced electrolyte system with improved energy efficiency for battery application. The relatively low ionic conductivity for most of the SICPEs in comparison with liquid electrolytes remains the major "bottleneck" for their practical applications. Polyethylene oxide (PEO) has been recognized as a benchmark for solid polymer electrolytes due to its high salt solubility and reasonable ionic conductivity. PEO has two advantages: (i) the polar ether groups coordinate well with lithium ions (Li+) providing good dissociation from anions, and (ii) the low Tg provides fast segmental dynamics at ambient temperature and assists rapid charge transport. These properties lead to active use of PEO as neutral plasticizing units in SICPEs. Herein, we present a detailed comparison of new SICPEs copolymerized with PEO units vs SICPEs copolymerized with other types of neutral units possessing either flexible or polar structures. The presented analysis revealed that the polarity of side chains has a limited influence on ion dissociation for copolymer-type SICPEs. The Li+-ion dissociation seems to be controlled by the charge delocalization on the polymerized anion. With good miscibility between plasticizing neutral units and ionic conductive units, the ambient ionic conductivity of synthesized SICPEs is still mainly controlled by the Tg of the copolymer. This work sheds light on the dominating role of PEO in SICPE systems and provides helpful guidance for designing polymer electrolytes with new functionalities and structures. Furthermore, based on the presented results, we propose that designing polyanions with a highly delocalized charge may be another promising route for achieving sufficient lithium ionic conductivity in solvent-free SICPEs.

Turning Rubber into a Glass: Mechanical Reinforcement by Microphase Separation.

Supramolecular associations provide a promising route to functional materials with properties such as self-healing, easy recyclability or extraordinary mechanical strength and toughness. The latter benefit especially from the transient character of the formed network, which enables dissipation of energy as well as regeneration of the internal structures. However, recent investigations revealed intrinsic limitations in the achievable mechanical enhancement. This manuscript presents studies of a set of telechelic polymers with hydrogen-bonding chain ends exhibiting an extraordinarily high, almost glass-like, rubbery plateau. This is ascribed to the segregation of the associative ends into clusters and formation of an interfacial layer surrounding these clusters. An approach adopted from the field of polymer nanocomposites provides a quantitative description of the data and reveals the strongly altered mechanical properties of the polymer in the interfacial layer. These results demonstrate how employing phase separating dynamic bonds can lead to the creation of high-performance materials.

Viscoelasticity in associating oligomers and polymers: experimental test of the bond lifetime renormalization model.

Recent findings that the association bond lifetimes τα* in associating polymers diverge from their supramolecular network relaxation times τc challenge past theories. The bond lifetime renormalization proposed by Rubinstein and coworkers [Stukalin et al., Macromolecules, 2013, 46, 7525] provides a promising explanation. To examine systematically its applicability, we employ shear rheology and dielectric spectroscopy to study telechelic associating polymers with different main chain (polypropylene glycol and polydimethylsiloxane), molecular weight (below entanglement molecular weight) and end groups (amide, and carboxylic acid) which form dimeric associations by hydrogen bonding. The separation between τc (probed by rheology) and τα* (probed by dielectric spectroscopy) strongly increases with chain length as qualitatively predicted by the model. However, to describe the increase quantitatively, a transition from Rouse to reptation dynamics must be assumed. This suggests that dynamics of super-chains must be considered to properly describe the transient network.

Polymer Binders Constructed through Dynamic Noncovalent Bonds for High-Capacity Silicon-Based Anodes.

Silicon (Si) is a promising candidate for high-capacity anode materials owing to its high theoretical capacity (3579 mAh g-1 ), low working voltage, and wide natural abundance, although its huge volume variation during charge/discharge processes always results in a short cycling life. Polymer binders play a vital role in improving the cycling performance of Si-based anodes, although traditional polyvinylidene difluoride cannot fulfil the requirements owing to its weak van der Waals forces with the Si surface. Recently, polymer binders constructed by dynamic bonds have been developed, which are reported to allow high-energy-density electrodes with improved electrochemical performance. With dynamic bonds including hydrogen bonding, ionic bonding, and host-guest interactions, these polymer binders possess self-healing capabilities and enhanced mechanical performance, achieving a tremendous advance in addressing the capacity fading of Si-based anodes. In this review, we will summarize the research progress of polymer binders constructed with dynamic bonds, and the challenges for their real applications in advanced Li-ion batteries will also be discussed.

Plasmonic Retrofitting of Membrane Materials: Shifting from Self-Regulation to On-Command Control of Fluid Flow.

This work calls for a paradigm shift in order to change the operational patterns of self-regulated membranes in response to chemical signals. To this end, the fabrication of a retrofitting material is introduced aimed at developing an innovative generation of porous substrates endowed with symbiotic but fully independent sensing and actuating capabilities. This is accomplished by transferring carefully engineered plasmonic architectures onto commercial microfiltration membranes lacking of such features. The integration of these materials leads to the formation of a coating surface proficient for ultrasensitive detection and "on-command" gating. Both functionalities can be synergistically modulated by the spatial and temporal distribution of an impinging light beam offering an unprecedented control over the membrane performance in terms of permeability. The implementation of these hybrid nanocomposites in conventional polymeric porous materials holds great potential in applications ranging from intelligent fluid management to advanced filtration technologies and controlled release.

Polymer Nanosheet Containing Star-Like Copolymers: A Novel Scalable Controlled Release System.

Poly(ε-caprolactone) (PCL)-based nanomaterials, such as nanoparticles and liposomes, have exhibited great potential as controlled release systems, but the difficulties in large-scale fabrication limit their practical applications. Among the various methods being developed to fabricate polymer nanosheets (PNSs) for different applications, such as Langmuir-Blodgett technique and layer-by-layer assembly, are very effort consuming, and only a few PNSs can be obtained. In this paper, poly(ε-caprolactone)-based PNSs with adjustable thickness are obtained in large quantity by simple water exposure of multilayer polymer films, which are fabricated via a layer multiplying coextrusion method. The PNS is also demonstrated as a novel controlled guest release system, in which release kinetics are adjustable by the nanosheet thickness, pH values of the media, and the presence of protecting layers. Theoretical simulations, including Korsmeyer-Peppas model and Finite-element analysis, are also employed to discern the observed guest-release mechanisms.