Toughening a Self-Healable Supramolecular Polymer by Ionic Cluster-Enhanced Iron-Carboxylate Complexes

Supramolecular polymers that can heal themselves automatically usually exhibit weakness in mechanical toughness and stretchability. Here we exploit a toughening strategy for a dynamic dry supramolecular network by introducing ionic cluster-enhanced iron-carboxylate complexes. The resulting dry supramolecular network simultaneous exhibits tough mechanical strength, high stretchability, self-healing ability, and processability at room temperature. The excellent performance of these distinct supramolecular polymers is attributed to the hierarchical existence of four types of dynamic combinations in the high-density dry network, including dynamic covalent disulfide bonds, noncovalent H-bonds, iron-carboxylate complexes and ionic clustering interactions. The extremely facile preparation method of this self-healing polymer offers prospects for high-performance low-cost material among others for coatings and wearable devices.     

Strong,Self‐Healable,and Recyclable Visible‐Light‐Responsive Hydrogel Actuators

The most pressing challenges for light‐driven hydrogel actuators include reliance on UV light, slow response, poor mechanical properties, and limited functionalities. Now, a supramolecular design strategy is used to address these issues. Key is the use of a benzylimine‐functionalized anthracene group, which red‐shifts the absorption into the visible region and also stabilizes the supramolecular network through π–π interactions. Acid–ether hydrogen bonds are incorporated for energy dissipation under mechanical deformation and maintaining hydrophilicity of the network. This double‐crosslinked supramolecular hydrogel developed via a simple synthesis exhibits a unique combination of high strength, rapid self‐healing, and fast visible‐light‐driven shape morphing both in the wet and dry state. As all of the interactions are dynamic, the design enables the structures to be recycled and reprogrammed into different 3D objects.     

An Intrinsically Self‐Healing NiCo||Zn Rechargeable Battery with a Self‐Healable Ferric‐Ion‐Crosslinking Sodium Polyacrylate Hydrogel Electrolyte

Self‐healing solid‐state aqueous rechargeable NiCo||Zn batteries are inherently safe and have a high energy density and mechanical robustness. However, the self‐healability of solid‐state batteries has only been realized by a few studies in which electron/ion‐inactive self‐healable substrates are utilized. This arises from the lack of self‐healable electrolytes. Now an intrinsically self‐healing battery has been designed that utilizes a new electrolyte that is intrinsically self‐healable. Sodium polyacrylate hydrogel chains are crosslinked by ferric ions to promote dynamic reconstruction of an integral network. These non‐covalent crosslinkers can form ionic bonds to reconnect damaged surfaces when the hydrogel is cut off, providing an ultimate solution to the intrinsic self‐healability problem of batteries. As a result, this NiCo||Zn battery with this hydrogel electrolyte can be autonomically self‐healed with over 87 % of capacity retained after 4 cycles of breaking/healing.     

Facile Synthesis of Supramolecular Ionic Polymers That Combine Unique Rheological,Ionic Conductivity,and Self‐Healing Properties

A new family of supramolecular ionic polymers is synthesized by a simple method using (di‐/tri‐)carboxylic acids and (di‐/tri‐)alkyl amines. These polymers are formed by carboxylate and ammonium molecules that are weakly bonded together by a combination of ionic and hydrogen bonds, becoming solid at room temperature. The supramolecular ionic polymers show a sharp rheological transition from a viscoelastic gel to a viscous liquid between 30 and 80 °C. This sharp viscosity decrease is responsible for an unprecedented jump in ionic conductivity of four orders of magnitude in that temperature range. As a potential application, this chemistry can be used to develop polymeric materials with self‐healing properties, since it combines properties from supramolecular polymers and ionomers into the same material.     

Self‐Healing Polymers via Supramolecular Forces

As polymers and polymeric materials are “the” smart invention and technological driving force of the 20th century, the quest for self‐healing or self‐repairing polymers is strong. The concept of supramolecular self‐healing materials relies on the use of noncovalent, transient bonds to generate networks, which are able to heal the damaged site, putting aspects of reversibility and dynamics of a network as crucial factors for the understanding and design of such self‐healing materials. This Review describes recent examples and concepts of supramolecular polymers based on hydrogen bonding, π–π interactions, ionomers, and coordinative bonds, thus convincingly discussing the advantages and versatility of these supramolecular forces for the design and realization of self‐healing polymers.     

Formation and Properties of Self‐Assembly‐Driven Fluorescent Nanoparticle Sensors

Fluorescent nanoparticles (FNPs) are obtained in water by self‐assembly from a polymeric ionic liquid, fluorescent carboxylate moiety, and a surfactant through two main supramolecular interactions, that is, ionic bonds and hydrophobic/hydrophilic interactions. The hydrophobicity of the surfactant is tunable and a highly hydrophobic surfactant increases the fluorescence intensity and stability of the FNPs. The fluorescence of the FNPs is sensitive to a quenching effect by various ions with high selectivity, and consequently, they may be used as sensors. The self‐assembly approach used to generate the FNPs is considerably simpler than other methods based on more challenging synthetic methods and the flexibility of the approach should allow a wide and diverse range of FNPs to be prepared with specific sensor applications.     

Synergistic Covalent and Supramolecular Polymers for Mechanically Robust but Dynamic Materials

Nature has engineered delicate synergistic covalent and supramolecular polymers (CSPs) to achieve advanced life functions, such as the thin filaments that assist in muscle contraction. Constructing artificial synergistic CSP materials with bioinspired mechanically adaptive features, however, represents a challenging goal. Here, we report an artificial CSP system to illustrate the integration of a covalent polymer (CP) and a supramolecular polymer (SP) in a synergistic fashion, along with the emergence of notable mechanical and dynamic properties which are unattainable when the two polymers are formed individually. The synergistic effect relies on the peculiar network structures of the SP and CPs, which endow the resultant CSPs with overall improved mechanical performance in terms of the stiffness, strength, stretchability, toughness, and elastic recovery. Moreover, the dynamic properties of the SP, including self‐healing, stimuli‐responsiveness, and reprocessing, are also retained in the CSPs, thus leading to their application as programmable and tunable materials.     

Self‐Healing Materials from V‐ and H‐Shaped Supramolecular Architectures

Integrating self‐healing capability into supramolecular architectures is an interesting strategy, and can considerably enhance the performance and broaden the scope of applications for this important class of polymers. Herein we report the rational design of novel V‐shaped barbiturate (Ba) functionalized soft–hard–soft triblock copolymers with a reversible supramolecular healing motif (Ba) in the central part of the hard block, which undergoes autonomic repair at 30 °C. The designed synthesis also offers a suitable macromolecular building block to further self‐assemble with heterocomplementary α,ω‐Hamilton wedge (HW) functionalized polyisoprene (PI; HW‐PI‐HW), resulting in an H‐shaped supramolecular architecture with efficient self‐healing capabilities that can recover up to around 95 % of the original mechanical performance at 30 °C within 24 h.     

Dynamic cross‐links to facilitate recyclable polybutadiene elastomer with excellent toughness and stretchability

A facile cross‐linking strategy of using small molecules as physcial crosslinkers to facilitate recyclable polybutadiene (PB) elastomer with excellent toughness and stretchability is demonstrated. Carboxylic acid groups were incorporated along the PB backbone via thiol‐ene reaction, and then the polymer can be cross‐linked by ionic hydrogen bonds between the carboxylic acid groups from PB and the amine groups of the cross‐linkers. The ionic hydrogen bonds can dynamiclly break and reconstruct upon deformation, thus endowing the resultant polymer with not only high toughness and stretchability (～1800%), but also good self‐recovery and enhanced damping properties. Remarkably, the dynamically cross‐linked PB elastomer can be thermally recycled owing to the thermal reversibility of the ionic hydrogen bonds and the mechanical properties can be largely recovered after reprocessing. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1357‐1366     

Self‐Healing,Expansion–Contraction,and Shape‐Memory Properties of a Preorganized Supramolecular Hydrogel through Host–Guest Interactions

Supramolecular materials cross‐linked between polymer chains by noncovalent bonds have the potential to provide dynamic functions that are not produced by covalently cross‐linked polymeric materials. We focused on the formation of supramolecular polymeric materials through host–guest interactions: a powerful method for the creation of nonconventional materials. We employed two different kinds of host–guest inclusion complexes of β‐cyclodextrin (βCD) with adamantane (Ad) and ferrocene (Fc) to bind polymers together to form a supramolecular hydrogel (βCD‐Ad‐Fc gel). The βCD‐Ad‐Fc gel showed self‐healing ability when damaged and responded to redox stimuli by expansion or contraction. Moreover, the βCD‐Ad‐Fc gel showed a redox‐responsive shape‐morphing effect. We thus succeeded in deriving three functions from the introduction of two kinds of functional units into a supramolecular material.     

Self‐Healing,Expansion–Contraction,and Shape‐Memory Properties of a Preorganized Supramolecular Hydrogel through Host–Guest Interactions

Supramolecular materials cross‐linked between polymer chains by noncovalent bonds have the potential to provide dynamic functions that are not produced by covalently cross‐linked polymeric materials. We focused on the formation of supramolecular polymeric materials through host–guest interactions: a powerful method for the creation of nonconventional materials. We employed two different kinds of host–guest inclusion complexes of β‐cyclodextrin (βCD) with adamantane (Ad) and ferrocene (Fc) to bind polymers together to form a supramolecular hydrogel (βCD‐Ad‐Fc gel). The βCD‐Ad‐Fc gel showed self‐healing ability when damaged and responded to redox stimuli by expansion or contraction. Moreover, the βCD‐Ad‐Fc gel showed a redox‐responsive shape‐morphing effect. We thus succeeded in deriving three functions from the introduction of two kinds of functional units into a supramolecular material.     

Light‐Powered Self‐Healable Metallosupramolecular Soft Actuators

Supramolecular functional materials able to respond to external stimuli have several advantages over their classical covalent counterparts. The preparation of soft actuators with the ability to respond to external stimuli in a spatiotemporal fashion, to self‐repair, and to show directional motion, is currently one of the most challenging research goals. Herein, we report a series of metallopolymers based on zinc(II)–terpyridine coordination nodes and bearing photoisomerizable diazobenzene units and/or solubilizing luminescent phenylene–ethynylene moieties. These supramolecular polymers act as powerful gelating agents at low critical gelation concentrations. The resulting multiresponsive organogels display light‐triggered mechanical actuation and luminescent properties. Furthermore, owing to the presence of dynamic coordinating bonds, they show self‐healing abilities.     

CO2‐Cross‐Linked Frustrated Lewis Networks as Gas‐Regulated Dynamic Covalent Materials

The design of structurally dynamic molecular networks can offer strategies for fabricating stimuli‐responsive adaptive materials. Herein we first report a gas‐responsive dynamic gel system based on frustrated Lewis pair (FLP) chemistry. Two trefoil‐like molecules with bulky triphenylborane and triphenylphosphine groups are synthesized as complementary Lewis acid and base with trivalent sites. They can together bind CO₂ gas molecules and further form a cross‐linked network via the bonding interactions between FLPs and CO₂. Such CO₂‐bridged dative linkages are shown to be dynamic covalent bonds, which endow the frustrated Lewis network with adaptable behaviors and unprecedented gas‐regulated viscoelastic, mechanical, and self‐healing performance. This study is an initial attempt to apply the FLP concept in materials chemistry, but we believe that this strategy will open a promising future for gas‐sensitive smart materials.     

Self‐Assembly of Giant Spherical Liquid‐Crystalline Complexes and Formation of Nanostructured Dynamic Gels that Exhibit Self‐Healing Properties

Supramolecular self‐assembly of 24 forklike mesogenic ligands and 12 transition metal ions led to the formation of giant spherical coordination complexes that exhibit liquid‐crystalline (LC) phases. Self‐healing LC supramolecular gels were also obtained through the introduction of these LC nanostructured supramolecular giant spherical complexes into dynamic covalent networks formed by cross‐linkers and bifunctional polymers. The giant spherical structures of the Pd^II complexes with 72 rodlike moieties on the periphery were characterized by NMR, diffusion‐ordered NMR spectroscopy, and mass spectrometry. These complexes are stable and exhibit lyotropic LC behavior, while the mesogenic ligands show thermotropic LC properties. The self‐assembled LC structures of the spherical complexes can be tuned by the length of the rodlike moieties.     

The Tris‐Urea Motif and Its Incorporation into Polydimethylsiloxane‐Based Supramolecular Materials Presenting Self‐Healing Features

Materials of supramolecular nature have attracted much attention owing to their interesting features, such as self‐reparability and material robustness, that are imparted by noncovalent interactions to synthetic materials. Among the various structures and synthetic methodologies that may be considered for this purpose, the introduction of extensive arrays of multiple hydrogen bonds allows for the formation of supramolecular materials that may, in principle, present self‐healing behavior. Hydrogen bonded networks implement dynamic noncovalent interactions. Suitable selection of structural units gives access to novel dynamic self‐repairing materials by incrementing the number of hydrogen‐bonding sites present within a molecular framework. Herein, we describe the formation of a tris‐urea based motif giving access to six hydrogen‐bonding sites, easily accessible through reaction of carbohydrazide with an isocyanate derivative. Extension towards the synthesis of multiply hydrogen‐bonded supramolecular materials has been achieved by polycondensation of carbohydrazide with a bis‐isocyanate component derived from poly‐dimethylsiloxane chains. Such materials underwent self‐repair at a mechanically cut surface. This approach gives access to a broad spectrum of materials of varying flexibility by appropriate selection of the bis‐isocyanate component that forms the polymer backbone.     

Polymeric Halogen‐Bond‐Based Donor Systems Showing Self‐Healing Behavior in Thin Films

The synthesis and comprehensive characterization of a systematic series of cleft‐type anion receptors imbedded into a polymeric architecture is presented. For the first time, isothermal calorimetric titrations on polymeric halogen‐bond‐based donors were exploited to evaluate the dependence of the anion affinity on different key parameters (i.e. monomeric versus polymeric receptor, halogen versus hydrogen bonding, charge assistance). The combination of these donor systems with a copolymer bearing accepting carboxylate groups led to supramolecular cross‐linked polymer networks showing excellent intrinsic self‐healing behavior. FT‐Raman spectroscopy and nano‐indentation measurements were utilized to clarify the thermally induced self‐healing mechanism based on the formation of halogen bonds. These first self‐healing materials based on halogen bonds pave the way for new applications of halogen‐bond donors in polymer and material science.     

Multiple Hydrogen Bonding Enables the Self‐Healing of Sensors for Human–Machine Interactions

Despite its widespread use in signal collection, flexible sensors have been rarely used in human–machine interactions owing to its indistinguishable signal, poor reliability, and poor stability when inflicted with unavoidable scratches and/or mechanical cuts. A highly sensitive and self‐healing sensor enabled by multiple hydrogen bonding network and nanostructured conductive network is demonstrated. The nanostructured supramolecular sensor displays extremely fast (ca. 15 s) and repeatable self‐healing ability with high healing efficiency (93 % after the third healing process). It can precisely detect tiny human motions, demonstrating highly distinguishable and reliable signals even after cutting–healing and bending over 20 000 cycles. Furthermore, a human–machine interaction system is integrated to develop a facial expression control system and an electronic larynx, aiming to control the robot to assist the patient's daily life and help the mute to realize real‐time speaking.     

Mechanical Robust and Self‐Healable Supramolecular Hydrogel

Development of self‐healing polymers with spontaneous self‐healing capability and good mechanical performance is highly desired and remains a great challenge. Here, mechanical robust and self‐healable supramolecular hydrogels have been fabricated by using poly(2‐dimethylaminoethyl methacrylate) brushes modified silica nanoparticles (SiO₂@PDMAEMA) as multifunctional macrocrosslinkers in a poly(acrylic acid) (PAA) network structure. The SiO₂ nanoparticles serve as noncovalent crosslinkers, dissipating energy, whereas the electrostatic interactions between cationic PDMAEMA and anionic PAA render the hydrogel self‐healing property. This process provides a simple and broadly applicable strategy to produce mechanical strong and self‐healable materials.

     

Self‐Healing Behavior in a Thermo‐Mechanically Responsive Cocrystal during a Reversible Phase Transition

The molecular‐level motions of a coronene‐based supramolecular rotator are amplified into macroscopic changes of crystals by co‐assembly of coronene and TCNB (1,2,4,5‐tetracyanobenzene) into a charge‐transfer complex. The as‐prepared cocrystals show remarkable self‐healing behavior and thermo‐mechanical responses during thermally‐induced reversible single‐crystal‐to‐single‐crystal (SCSC) phase transitions. Comprehensive analysis of the microscopic observations as well as differential scanning calorimetry (DSC) measurements and crystal habits reveal that a thermally‐reduced‐rate‐dependent dynamic character exists in the phase transition. The crystallographic studies show that the global similarity of the packing patterns of both phases with local differences, such as molecular stacking sequence and orientations, should be the origin of the self‐healing behavior of these crystals.     

A facile and controllable synthesis of dual‐crosslinked elastomers based on linear bifunctional polydimethylsiloxane oligomers

Dual‐crosslinked supramolecular elastomers with the hybrid network consisting of hydrogen bonds and covalent bonds combine the reversibility of hydrogen bond and mechanical properties of covalent crosslinking network. In this article, isocyanate mixture is used as curing agent to prepare dual‐crosslinked elastomer based on bifunctional polydimethylsiloxane under mild condition. This method can effectively build up a hybrid network with the designed structure. A series of elastomers with same hydrogen bond density and variable covalent crosslinking degree are obtained. Swelling measurements and ¹H‐NMR spectra confirm the feasibility and controllability of curing method, the increasing of bifunctional isocyanate give rise to higher covalent crosslinking degree, improving the solvent resistance. The studies on viscoelastic property show that the introduction of an irreversible covalent crosslinking network stabilize the hybrid network, restrain the chain movement. The mechanical and self‐healing property studies reveal that the covalent crosslink significantly reinforce the whole network, while the reparable strength seems to mainly depend on the hydrogen bond density. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3760–3768