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.

As representative soft materials with widespread applications, gels with various functions have been developed. However, traditional gels are vulnerable to stress-induced formation of cracks. The propagation of these cracks may affect the integrity of network structures of gels, resulting in the loss of functionality and limiting the service life of the gels. To address this challenge, self-healing gels that can restore their functionalities and structures after damage have been developed as “smart” soft materials. In this paper, we present an overview of the current strategies for synthesizing self-healing gels based on the concept of constitutional dynamic chemistry, which involves molecular structures capable of establishing dynamic networks based upon physical interactions or chemical reactions. The characterization methods of self-healing gels and the key factors that affect self-healing properties are analyzed. We also illustrate the emerging applications of self-healing gels, with emphasis on their usage in industry (coatings, sealants) and biomedicine (tissue adhesives, agents for drug or cell delivery). We conclude with a perspective on challenges facing the field, along with prospects for future development.

Self-healing gels based on constitutional dynamic chemistry and their potential applications

Low molecular weight supramolecular gels consist of small molecules (gelators) that in an appropriate solvent self-assemble into nano- or micro-scale network structures resulting in the formation of a gel. Most supramolecular gels consist of two parts, namely the solvent and the gelator. However, the concept of multi-component supramolecular gels, in which more than one compound is added to the solvent, offers a facile way (e.g. by changing the ratio of the different components) to tailor the properties of the gel. The simplest multi-component gels consist of two components added to the solvent and are the most widely studied to date. There are three general classes of such multi-component gels that have been investigated. The first class requires all the added components to access the gel; that is, no component forms a gel on its own. A second class uses two (or more) gelators which can either co-assemble or self-sort into distinct assemblies and the final class consists of one (or more) gelator and one (or more) non-gelling additive which can impact the assembly process of the gelator and therefore the gel's properties.

Supramolecular gels formed from multi-component low molecular weight species

Supramolecular gels are a fascinating class of soft materials. Their gelators can self-assemble into nano- or micro-scale superstructures, such as fibers, ribbons, sheets and spheres in an appropriate solvent, thereby resulting in the formation of 3D networks. The dynamic and reversible nature of the non-covalent interactions that contribute to the formation of these network structures together gives these supramolecular gels the inherent ability to respond to external stimuli. However, the dynamic nature of supramolecular gels, which endows them with unique properties, makes their characterization diversified at the same time. Therefore, we present here a review summarizing various methods for characterizing supramolecular gels, including nuclear magnetic resonance spectroscopy, computational techniques, X-ray techniques, microscopy techniques, dynamic light scattering, thermal analysis, and rheology. Based on the gelation mechanisms and influencing factors of supramolecular gels, suitable and sufficient characterization methods should be carefully employed to make full use of their respective advantages to better investigate these materials.

Characterization of supramolecular gels

Advances in the field of supramolecular chemistry have made it possible, in many situations, to reliably engineer soft materials to address a specific technological problem. Particularly exciting are “smart” gels that undergo reversible physical changes on exposure to remote, non-invasive environmental stimuli. This review explores the development of gels which are transformed by heat, light and ultrasound, as well as other mechanical inputs, applied voltages and magnetic fields. Focusing on small-molecule gelators, but with reference to organic polymers and metal–organic systems, we examine how the structures of gelator assemblies influence the physical and chemical mechanisms leading to thermo-, photo- and mechano-switchable behaviour. In addition, we evaluate how the unique and versatile properties of smart materials may be exploited in a wide range of applications, including catalysis, crystal growth, ion sensing, drug delivery, data storage and biomaterial replacement.

/pdf/gels-with-sense-supramolecular-materials-that-respond-to-3h0irg5zdc.pdf

Gels with sense: supramolecular materials that respond to heat, light and sound

Stimulus-responsive gels have recently attracted widespread attention as new functional materials for potential applications in sensors, actuators, shape memories, drug delivery devices, and displays. One of the promising properties that organogels based on low molecular mass organic gelators (LMOGs) can offer is their reversible sol–gel phase transition as a result of external stimuli. As far as we know, redoxresponsive organogels from LMOGs, however, are limited. Shinkai and coworkers reported the first example of organogels of this kind, which contains a redox-active Cu/ Cu center. Besides, they also synthesized a series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl moieties at the a-position. It was found that a sol–gel phase transition can be implemented by addition of oxidizing and reducing reagents. Zhu and colleagues prepared an electro-active LMOG containing a tetrathiafulvalene (TTF) entity. The gel formation can be tuned by means of oxidation/ reduction of the TTF group chemically or electrochemically. Although these gel systems are redox responsive, their properties, such as mechanical strength, flexibility, and sensitivity to external stimulus, are far from those required for practical uses. Therefore, creating instant, reversible, redox-responsive, and mechanically flexible organogels still remains a challenge. As a remarkable organometallic compound, ferrocene (Fc) contains an oxidizable metal ion, Fe, and is a nonpolar compound in the neutral state, and thereby it dissolves readily in hydrocarbon solvents. This property, however, can be easily reversed by simple oxidation of the central ion. Our interest in stimulus-responsive supramolecular gel systems led us to consider the compound as a neutral–cation redox pair that may be employed to tune the gelling ability of a gelator containing it. Actually, the same idea has been adopted by a number of groups for studies of switchable complexation and molecular aggregation in micelles and vesicles. However, all com-

An Organometallic Super‐Gelator with Multiple‐Stimulus Responsive Properties

A novel low-molecular-mass gelator with a redox active ferrocenyl group: tuning gel formation by oxidation.

Ionic liquids are solvents of future. One of the promising methods to boost their uses is to solidify them in a physical way but with little interruption of their properties. It is therefore of interest to create gels of ionic liquids by using low-molecular mass compounds as gelators (LMMGs). Herein, we report a number of ionic liquid gels (ionogels) of which specially designed and synthesized cholesteryl derivatives were employed as gelators. The ionogels as obtained are thermo-reversible. In particular, the one with 1-butyl-3-methylimidazolium tetrafluoroborate (IL2) as solvent and a cholesteryl derivative containing a D-phenylalanine residue (1D) as a gelator is very stable both in neutral and acidic mediums as demonstrated by a yield stress of 76 Pa for a self-standing cylinder of the ionogel. Furthermore, the ionogel can be easily converted into a hydrogel via simple replacement of the solvent with water in situ. More interestingly, the conversion is reversible, a phenomenon never reported before. At the same time, the critical gelation concentration (CGC) of 1D for IL2 is only 0.06%, w/w, which is almost the lowest value reported for ionogels till now, and falls into the category of “super-gelator”. Magnetization of the ionogel has been realized by introduction of micro-/nano-Fe3O4 particles. As expected, the magnetic gel as obtained is responding to external magnetic field. Specifically, it changes into fluid with the presence of a magnetic field exceeding certain strength, and retains to gel upon removing the magnetic field and with a treatment of sonication and heating–cooling cycle. SEM and TEM observations revealed the continuous fibrous network structures of the molecules of the gelator in the ionogels. To the best of our knowledge, this is the first report on ionogels possessing stimulus-responsive properties, good mechanical strength, and super-gelation talent.

Cholesterol-based low-molecular mass gelators towards smart ionogels

Ultra-low density porous polystyrene monoliths have been prepared via using water/styrene gel-emulsions, which were stabilized by a low-molecular mass gelator (LMMG), as templates It was demonstrated that the porous materials as prepared possess a number of superior features such as highly adjustable internal structure and density, the lowest density could be lower than 001 g cm−3 Moreover, such materials can be used as excellent absorbents for many organic liquids including viscous oils like mineral oil and high-density oils such as dichloromethane In particular, the absorption process is fast, selective and efficient when they are used in the purification of oil contaminated water Importantly, the oils absorbed in the absorbent can be easily squeezed out, and furthermore the wet absorbent as obtained can be reused by suitable washing and natural drying More importantly, the LMMG-based gel-emulsions employed as templates are simple in preparation, only mixing and agitation at ambient temperature are required Moreover, further treatment of the wet porous materials as prepared from the templates is just washing and natural drying, which is completely different from those such as freeze-drying or supercritical drying widely adopted by others in the preparation of low density materials via routine approaches It is believed that the LMMG-based gel-emulsion template method developed in the present work may have created a novel and facile way for preparing low density materials with ideal properties

/pdf/ultra-low-density-porous-polystyrene-monolith-facile-1t09o28bgi.pdf

Ultra-low density porous polystyrene monolith: facile preparation and superior application

Three novel LS2-type dimeric-cholesteryl derivatives (1–3), where S is a steroidal residue and L stands for a linker connecting the two S residues and contains three benzene rings and two amide and...

Novel dimeric cholesteryl derivatives and their smart thixotropic gels.

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