The fascinating properties of single‐layer graphene isolated by mechanical exfoliation have inspired extensive research efforts toward two‐dimensional (2D) materials. Layered compounds serve as precursors for atomically thin 2D materials (briefly, 2D nanomaterials) owing to their strong intraplane chemical bonding but weak interplane van der Waals interactions. There are newly emerging 2D materials beyond graphene, and it is becoming increasingly important to develop cost‐effective, scalable methods for producing 2D nanomaterials with controlled microstructures and properties. The variety of developed synthetic techniques can be categorized into two classes: bottom‐up and top‐down approaches. Of top‐down approaches, the exfoliation of bulk 2D materials into single or few layers is the most common. This review highlights chemical and physical exfoliation methods that allow for the production of 2D nanomaterials in large quantities. In addition, remarkable examples of utilizing exfoliated 2D nanomaterials in energy and environmental applications are introduced.

This study demonstrates the preparation of dual cross-linked hydrogels capable of selective, spontaneous de-cross-linking behavior triggered by head-to-tail depolymerization. A primary covalent structure of the materials was established on a thermoresponsive network containing pendant functional moieties that could induce noncovalent interaction when combined with a macro-cross-linker. The incorporation of the macro-cross-linker not only reinforced the entire structure while providing secondary physical cross-linking but also caused the resulting materials to show autonomous responses via depolymerization. As a result, transformation of the material was achieved without structural collapse, as the noncovalent network in the materials was rapidly, selectively, and completely removed through the depolymerization reaction of the polymer cross-linker when initiated by a trace (<0.01 wt %) stimulus. Thus, the macroscopic changes in the physical and chemical properties of the hydrogels were investigated. Furthermore, we used the proposed strategy to design actuating materials that exhibit reversible, programmed, large-scale behavior and finally demonstrated re-cross-linking by the addition of an extra macro-cross-linker after the de-cross-linking reaction.

We demonstrated the modular synthesis of polymer thermosets exhibiting programmed responses that arise from two predesigned functional units: a reversible cross-linker and a selfimmolative linker. The former unit contains a 1,2,3-triazolium moiety that provides the cross-linked network but undergoes dynamic covalent bond exchange via transalkylation, while the latter phenyl-based unit generates a signal-specific elimination reaction that promotes selective de-cross-linking of the entire network. The two units form a functional, dual cross-linked structure after thiol−ene click polymerization followed by a thermal curing reaction, allowing large-scale synthesis. By changing the ratio of functional units, the physicochemical properties of the materials during polymerization can be finely tuned, and adequate combinations result in tailored thermosets that are reversible yet removable. Thus, we were able to reuse the thermosets in bulk, solid state, or restructure without overall structural collapse. Furthermore, by exploiting the designed nature, the materials can be used as a stimuli-responsive adhesive. We observed a high adhesive strength when gluing glass pieces and rejoined them upon heating after they were detached by force. However, exposure to a molecular signal significantly diminished the adhesion under benign conditions, and the joint was easily and irreversibly separated. The design concept and materials presented herein elucidate or advance the chemical concepts for the recycling and disposal of commercial thermosetting plastics when they are no longer needed.

This paper describes the multifunctional effect of molybdenum disulfide (MoS2) that enables the rapid and accessible preparation of nanocomposite hydrogels via a bottom-up design. The MoS2 nanoplatelet forms radical species through a redox reaction with persulfate under aqueous conditions while initiating the polymerization of acrylic monomers and providing noncovalent cross-linking points without requiring external stimuli or extra cross-linkers, leading to the formation of hydrogels that are in situ embedded with inorganic flakes. Furthermore, the addition of MoS2 could induce more rigid and elastic networks compared to those in control hydrogels using a typical cross-linker at the same level; for example, 0.08 wt % MoS2 resulted in a composite hydrogel of which the elastic modulus was 2.5 times greater than that from a hydrogel using N,N′-methylenebis­(acrylamide) as the showing phase transition during polymerization. The composite hydrogels are self-healable, taking advantage of reversible physical cross-links. Thus, two cut hydrogel strips could be readily rejoined by heating at 70 °C, and the resulting whole strip showed mechanical strength similar to that of the pristine sample before it was cut. This synthetic approach would give way to the modular design of MoS2-containing composite hydrogels.

A sustainable biobased thermoset exhibiting shape-memory behavior and modular recycling capabilities has been developed herein. The prepared thermoset consists of naringenin and biocompatible polymer components. Naringenin, which has three phenolic moieties, has been converted to a multifunctional monomer containing glycidyl groups and readily formed a thermosetting network via epoxide ring opening reaction with a poly­(ethylene glycol) diacid under solvent-free conditions. The resulting material is malleable yet as strong as articular cartilage and selectively absorbs water when compared with n-dodecane oil. Moreover, the thermoset can be physically reused. After being crumpled, stretched, or coiled, the initial shape of the material is restored in response to heat or water. Furthermore, the material is amenable to chemical recycling in a bulk state via transesterification, and its components can be recovered on a molecular level after degradation under benign conditions, as was confirmed using a model compound.

Superabsorbent hydrogels are significant not only in materials science but also in industries and daily life, being used in diapers or soil conditioners as typical examples. The main feature of these materials is their capacity to hold considerable amount of water, which is strongly dependent on the cross-linking density. This study focuses on the preparation of hydrogels by reweighing the effect of cross-linking density on physical properties, which provides green fabrication of bilayered hydrogels that consist of homogeneous structural motifs but show programmed responses via sequential radical polymerization. In particular, when two hydrogel layers containing different cross-linking densities are joined together, an integrated linear bilayer shows heterogeneous deformation triggered by water. We monitor the linear hydrogel bilayer bending into a circle and engineer it by incorporating disperse dyes, changing colors as well as physical properties. In addition, we demonstrate an electric circuit switch using a patterned hydrogel. Anisotropic shape change of the polyelectrolyte switch closes an open circuit and lights a light-emitting diode in red. This proposed fabrication and engineering can be expanded to other superabsorbent systems and create smart responses in cross-linked systems for biomedical or environmental applications.

A bottom-up design enables the preparation of a mesoporous, compressible, polymer monolith that shows rapid separation of oil–water mixture.

Compressible, microporous polymers have been prepared as a monolithic sponge and further regulated macroscopic conductivity when combined with carbon materials.