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.

/pdf/functional-supramolecular-polymers-3wt4hbf4y0.pdf

Functional Supramolecular Polymers

The adhesive strategy of the gecko relies on foot pads composed of specialized keratinous foot-hairs called setae, which are subdivided into terminal spatulae of approximately 200 nm (ref. 1). Contact between the gecko foot and an opposing surface generates adhesive forces that are sufficient to allow the gecko to cling onto vertical and even inverted surfaces. Although strong, the adhesion is temporary, permitting rapid detachment and reattachment of the gecko foot during locomotion. Researchers have attempted to capture these properties of gecko adhesive in synthetic mimics with nanoscale surface features reminiscent of setae; however, maintenance of adhesive performance over many cycles has been elusive, and gecko adhesion is greatly diminished upon full immersion in water. Here we report a hybrid biologically inspired adhesive consisting of an array of nanofabricated polymer pillars coated with a thin layer of a synthetic polymer that mimics the wet adhesive proteins found in mussel holdfasts. Wet adhesion of the nanostructured polymer pillar arrays increased nearly 15-fold when coated with mussel-mimetic polymer. The system maintains its adhesive performance for over a thousand contact cycles in both dry and wet environments. This hybrid adhesive, which combines the salient design elements of both gecko and mussel adhesives, should be useful for reversible attachment to a variety of surfaces in any environment.

A reversible wet/dry adhesive inspired by mussels and geckos

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 this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers’ complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradat...

New Insights into Perfluorinated Sulfonic-Acid Ionomers

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.

/pdf/optically-healable-supramolecular-polymers-58jhlfa1ij.pdf

Optically healable supramolecular polymers

Topographical patterns are used to selectively tune the adhesion of polymers. Although nature has provided guidance, relatively little is known of how topographic patterns can be intelligently used not only to enhance adhesion but, more importantly, to tune adhesion. We demonstrate that properly designed, low-aspect-ratio posts can alter adhesion from 20% to 400% the value of conventional adhesion descriptors for nonpatterned interfaces. This control is not related to the magnitude of interfacial area but, rather, to altering the local separation processes at an interface by geometry. We establish general relationships that govern the interaction between material properties, pattern length scales, and the control of adhesion. These relationships provide insight into the mechanisms of adhesion for examples in nature, such as the gecko, while also providing concrete guidance for the future design of “smart” adhesives and coatings with nanoscale patterns.

Controlling polymer adhesion with "pancakes".

Ionic polymer transducers (IPT) based on electroactive polymers (EAP) display electromechanical coupling that enables breakthroughs in the design of high-performance actuators and sensors The ion-exchange membrane Nafion remains as the benchmark for a majority of research and development in IPT technology Advances in IPT performance, elemental understanding of transduction mechanisms, and the development of future technologies (eg, artificial muscles) are largely due to significant efforts to tailor complementary transducer compositions relative to commercially available Nafion Current fundamental advances in the synthesis, characterization, and application of tailored ion-containing polymers are reviewed for increased performance, decreased cost, and improved processing of IPTs Recent attention to the correlation of morphological structure to ion conduction mechanisms has led to new paradigms for performance Tailoring of ionomeric membranes to isolate material interactions within the composite IPT

Beyond Nafion: Charged Macromolecules Tailored for Performance as Ionic Polymer Transducers

Zwitterionomers containing less than 13 mol % zwitterion functionality were synthesized using free radical copolymerization of n-butyl acrylate (nBA) and sulfobetaine monomers. X-ray scattering res...

Effect of Ionic Liquid on Mechanical Properties and Morphology of Zwitterionic Copolymer Membranes

Dynamic density functional theory calculations were performed for thermoplastic elastomer gels composed of an ABA triblock copolymer immersed in a B-attractive solvent. The triblock copolymer model was parameterized for poly(sty- rene-b-(ethylene-co-butylene)-b-styrene) (SEBS), while the sol- vent model was parameterized for the hydrocarbon oil tetradecane. The effect of the solvent concentration and S-EB interaction on the morphology was investigated, where com- plementary experimental data was used to validate results at vABN � 100. Agreement was observed at solvent volume frac- tions of 0.2, 0.4, and 0.6, which correspond to the cylindrical, spherical, and spherical phases, respectively. Qualitative agree- ment was observed for 0.8 volume fraction solvent, where a core-shell spherical micelle morphology was found. For a 50/50 vol % mixture of polymer/solvent, the effect of solvent molecu- lar weight on the morphology was considered, where a transi- tion between micro and macrophase separation was predicted at a critical solvent molecular weight. V C 2011 Wiley Periodicals, Inc. § J Polym Sci Part B: Polym Phys 49: 1479-1491, 2011

Andrew J. Duncan

Papers

Optically healable supramolecular polymers

Controlling polymer adhesion with "pancakes".

Beyond Nafion: Charged Macromolecules Tailored for Performance as Ionic Polymer Transducers

Effect of Ionic Liquid on Mechanical Properties and Morphology of Zwitterionic Copolymer Membranes

Phase behavior of SEBS triblock copolymer gels