Supramolecular polymers can be random and entangled coils with the mechanical properties of plastics and elastomers, but with great capacity for processability, recycling, and self-healing due to their reversible monomer-to-polymer transitions. At the other extreme, supramolecular polymers can be formed by self-assembly among designed subunits to yield shape-persistent and highly ordered filaments. The use of strong and directional interactions among molecular subunits can achieve not only rich dynamic behavior but also high degrees of internal order that are not known in ordinary polymers. They can resemble, for example, the ordered and dynamic one-dimensional supramolecular assemblies of the cell cytoskeleton and possess useful biological and electronic functions.

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Functional Supramolecular Polymers

Smart materials with an in-built ability to repair damage caused by normal wear and tear could prove useful in a wide range of applications. Most healable polymer-based materials so far developed require heating of the damaged area. But Burnworth et al. have now produced materials — in the form of polymer strands linked through metal complexes — that can be mended through exposure to light. The metal complexes in these materials can absorb ultraviolet light that is then converted into heat, which temporarily unlinks the polymer strands for quick and efficient defect healing. In principle, healing can take place in situ and while under load. Polymers with the ability to repair themselves after sustaining damage could extend the lifetimes of materials used in many applications1. Most approaches to healable materials require heating the damaged area2,3,4. Here we present metallosupramolecular polymers that can be mended through exposure to light. They consist of telechelic, rubbery, low-molecular-mass polymers with ligand end groups that are non-covalently linked through metal-ion binding. On exposure to ultraviolet light, the metal–ligand motifs are electronically excited and the absorbed energy is converted into heat. This causes temporary disengagement of the metal–ligand motifs and a concomitant reversible decrease in the polymers’ molecular mass and viscosity5, thereby allowing quick and efficient defect healing. Light can be applied locally to a damage site, so objects can in principle be healed under load. We anticipate that this approach to healable materials, based on supramolecular polymers and a light–heat conversion step, can be applied to a wide range of supramolecular materials that use different chemistries.

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Optically healable supramolecular polymers

Supramolecular materials, dynamic materials by nature, are defined as materials whose components are bridged via reversible connections and undergo spontaneous and continuous assembly/disassembly processes under specific conditions. On account of the dynamic and reversible nature of noncovalent interactions, supramolecular polymers have the ability to adapt to their environment and possess a wide range of intriguing properties, such as degradability, shape-memory, and self-healing, making them unique candidates for supramolecular materials. In this critical review, we address recent developments in supramolecular polymeric materials, which can respond to appropriate external stimuli at the fundamental level due to the existence of noncovalent interactions of the building blocks.

Stimuli-responsive supramolecular polymeric materials.

Pressure sensitivity and mechanical self-healing are two vital functions of the human skin. A flexible and electrically conducting material that can sense mechanical forces and yet be able to self-heal repeatably can be of use in emerging fields such as soft robotics and biomimetic prostheses, but combining all these properties together remains a challenging task. Here, we describe a composite material composed of a supramolecular organic polymer with embedded nickel nanostructured microparticles, which shows mechanical and electrical self-healing properties at ambient conditions. We also show that our material is pressure- and flexion-sensitive, and therefore suitable for electronic skin applications. The electrical conductivity can be tuned by varying the amount of nickel particles and can reach values as high as 40 S cm−1. On rupture, the initial conductivity is repeatably restored with ∼90% efficiency after 15 s healing time, and the mechanical properties are completely restored after ∼10 min. The composite resistance varies inversely with applied flexion and tactile forces. These results demonstrate that natural skin's repeatable self-healing capability can be mimicked in conductive and piezoresistive materials, thus potentially expanding the scope of applications of current electronic skin systems. A supramolecular polymer with embedded nanostructured Ni particles shows mechanical and electrical self-healing capabilities as well as piezoresistive properties, making it a good candidate for electronic skin applications.

An electrically and mechanically self-healing composite with pressure- and flexion-sensitive properties for electronic skin applications

In chemistry, some dynamic bonds can be selectively and reversibly broken and reformed in response to an environmental stimulus. This Review article discusses the incorporation of dynamic bonds, or interactions, in polymeric materials and the structural changes and macroscopic responses observed in the presence of different stimuli.

Using the dynamic bond to access macroscopically responsive structurally dynamic polymers

Films exhibiting multiresponsive shape-memory properties have been accessed using covalently cross-linked metallo-supramolecular polymers. Low molecular weight poly(butadiene) was end-capped with 4-oxy-2,6-bis(N-methylbenzimidazolyl)pyridine (−OMebip) ligands that upon addition of metal salts spontaneously formed high molecular weight metallo-supramolecular polymers. The addition of a tetra-functional thiol along with a photoinitiator results in mechanically stable films via solution-casting. These films consist of a soft poly(butadiene) phase and a hard metal–ligand phase. Photo-cross-linking of the poly(butadiene) soft phase, via the thiol–ene reaction, upon exposure to relatively low intensity light, allows access to a diverse range of permanent shapes. Investigations into the temporary shape fixing and recovery of these materials were undertaken to determine the effects of cross-link density and the nature of the metal salts. The key component in fixing and releasing the temporary shape is the metal–l...

Thermo-, Photo-, and Chemo-Responsive Shape-Memory Properties from Photo-Cross-Linked Metallo-Supramolecular Polymers

A series of stimuli-responsive films based on metallo-supramolecular polymers have been prepared and studied. The metallo-supramolecular polymers are comprised of a 4-oxy-2,6-bis-(1′-methylbenzimidazolyl)pyridine ditopic endcapped poly(tetrahydrofuran) with different ratios of Zn2+ and Eu3+. A combination of the optical properties of the Eu3+ complex along with its more labile nature results in the formation of highly stimuli-responsive materials. The films of the Eu3+-containing metallo-supramolecular polymers show a pronounced optical response to an increase in temperature or upon exposure to chemicals such as triethyl phosphate, which was used as a mimic for organophosphate pesticides and nerve gas agents.

Stimuli-responsive europium-containing metallo-supramolecular polymers

Square-planar platinum(II) complexes of the 4-dodecyloxy-2,6-bis(N-methylbenzimidazol-2′-yl)pyridine ligand have been blended into a series of methacrylate polymers. Each of the polymer films displayed vapochromic (yellow to red) and vapoluminescent behaviour as a result of the vapour induced change in the Pt–Pt interactions. The solid-state absorption and emission properties of the films were characterized, and the influence of the polymer matrix on the vapochromic response was investigated. Notably, the rate of recovery of the yellow colour after vapour-exposure was found to be dependent on the Tg of the matrices, and the response can be effectively erased and the materials restored to the original yellow colour within minutes by simply heating above their Tg. The polymer–complex blends also displayed interesting mechanochromic and mechanoluminescent properties upon deformation either by compression, scratching, or stretching. The accumulated data are consistent with a solvato- or mechano-induced structural rearrangement that results in a shortening of the Pt–Pt distances.

Vapochromic and mechanochromic films from square-planar platinum complexes in polymethacrylates

Detailed rheological studies of metallosupramolecular polymer films in the melt were performed to elucidate the influence of the metal ion and polymer components on their mechanical and structural ...

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Justin R. Kumpfer

Papers

Optically healable supramolecular polymers

Thermo-, Photo-, and Chemo-Responsive Shape-Memory Properties from Photo-Cross-Linked Metallo-Supramolecular Polymers

Stimuli-responsive europium-containing metallo-supramolecular polymers

Vapochromic and mechanochromic films from square-planar platinum complexes in polymethacrylates

Influence of Metal Ion and Polymer Core on the Melt Rheology of Metallosupramolecular Films