Closed-Loop Recycling of Vinylogous Urethane Vitrimers.

Devising energy-efficient strategies for the depolymerization of plastics and the recovery of their structural components in high yield and purity is key to a circular plastics economy. Here, we report a case study in which we demonstrate that vinylogous urethane (VU) vitrimers synthesized from bis-polyethylene glycol acetoacetates (aPEG) and tris(2-aminoethyl)amine can be degraded by water at moderate temperature with almost quantitative recovery (≈98 %) of aPEG. The rate of depolymerization can be controlled by the temperature, amount of water, molecular weight of aPEG, and composition of the starting material. These last two parameters also allow one to tailor the mechanical properties of the final materials, and this was used to access soft, tough, and brittle vitrimers, respectively. The straightforward preparation and depolymerization of the aPEG-based VU vitrimers are interesting elements for the design of polymer materials with enhanced closed-loop recycling characteristics.

Skin-Inspired Tough Elastomer with Moisture-Triggered Switchable Mechanical Properties.

Biological tissue usually exhibits good water adaptive mechanical properties, which can maintain high strength and toughness in both wet and dry states. However, synthetic tissue like hydrogel usually becomes hard and brittle in its dry state. Herein, this challenge is met by exploring iron-catechol complex (TA-Fe3+ ) as a great platform combining extremely different polymers (elastomer and hydrogel) to construct new tissue-like soft composite materials with two different continuous phases, which have not yet been reported. In its dry state, the xerogel phase becomes a reinforced segment to increase the strength of PB without losing its toughness. In its wet state, this soft material becomes high performance hydrogel, where hydrogel phase absorbs high water and elastomer phase can sustain high loading. Such heterogeneous phase structures provide a good idea for designing the soft materials, exhibiting a trade-off between its high strength and toughness in both wet and dry states. Furthermore, its shape memory behaviors in both its wet and dry state, which shows a great potential application for complex adaptive shape transformation and engineering application like lifting of heavy objects under remote control due to high photo-thermal transition of TA-Fe3+ is explored.

Chemical Upcycling of Conventional Polyureas into Dynamic Covalent Poly(aminoketoenamide)s.

The chemical upcycling of polymers is an emerging strategy to transform post-consumer waste into higher-value chemicals and materials. However, on account of the high stability of the chemical bonds that constitute their main chains, the chemical modification of many polymers proves to be difficult. Here, we report a versatile approach for the upcycling of linear and cross-linked polyureas, which are widely used because of their high chemical stability. The treatment of these polymers or their composites with acetylacetone affords di-vinylogous amide-terminated compounds in good yield. These products can be reacted with aromatic isocyanates, and the resulting aminoketoenamide bonds are highly dynamic at elevated temperatures. We show here that this conversion scheme can be exploited for the preparation of dynamic covalent poly(aminoketoenamide) networks, which are healable and reprocessable through thermal treatment without any catalyst.

Dynamic Covalent Structure for the Design of Recyclable Polyurethane Based on the Diketone Chemistry.

Herein, the rational design of an enaminone compound is reported which can be easily and conveniently designed and obtained via the reaction of amine and diketone. The dynamic enaminoneamide structure is formed via the reaction between isocyanate and enaminone in fabricating a novel dynamic crosslinked polyurethane. The new kind of polyurethane can be efficiently recycled via the shifting between crosslinked structure and oligomer for the reversible dissociation of the enaminoneamide structure. Besides remolding itself via the thermal treatment, oligomers liberated from recycled-polyurethane can directly take part in constructing a new polyurethane without further purification. Therefore, the recyclability of this polyurethane shows high-value characteristics. The ease with which polyurethane can be produced, used, recycled, and reused without losing value offers a new green solution in designing sustainable polymer materials with a high economic value and a minimal environmental burden.

Rapid Stress Relaxation, Multistimuli-Responsive Elastomer Based on Dual-Dynamic Covalent Bonds and Aniline Trimer.

Covalent adaptable networks (CANs) are an emerging kind of smart materials in which cross-links are reversible upon some stimuli and then provide malleability and a stimuli-responsive ability to the materials. There is a trend to endow CANs with multistimuli-responsive capabilities and rapid stress relaxation to pursue more advanced applications. To integrate these two features into one material, here, dual-dynamic covalent bonds (imines and boronic esters) and aniline trimer (ACAT) were incorporated into the styrene butadiene elastomer as dynamic cross-links. The obtained CANs were demonstrated with rapid stress relaxation and a relatively low activation energy of 36 ± 1 kJ mol-1, resulting from the synergistic effect of dual-dynamic covalent bonds to rearrange the network at a faster rate than for either imines or boronic esters. Because of the dynamic nature of imines or boronic esters, the elastomer can be recycled upon heat. Moreover, the appearance and configuration of the elastomer could also be manipulated by pH and light because of the inclusion of ACAT. All in all, the coupled multistimuli-responsive behavior and rapid stress relaxation in one single elastomer would potentially be applicable for sensors and actuators with good recyclability.

Integrating Boronic Esters and Anthracene into Covalent Adaptable Networks toward Stimuli-Responsive Elastomers.

Stimuli-responsive polymer materials have a promising potential application in many areas. However, integrating multi-stimuli into one elastomer is still a challenge. Here, we utilized boronic esters and anthracene to prepare a cross-linked poly(styrene-butadiene-styrene) (SBS) which was endowed with responsiveness to three stimuli (light, heat, and alcohols). SBS was first functionalized with a certain amount of dihydroxyl groups via a thiol-ene "click" reaction between unsaturated double bonds in PB block and thioglycerol. Then, 9-anthraceneboronic acid was applied to form a cross-linked SBS network upon heat and ultraviolet radiation (λ = 365 nm). The prepared elastomer was demonstrated to be stimuli-responsive based on the dynamic nature of boronic esters and the reversible dimerization of anthracene. In addition, the mechanical properties of the elastomer could be regulated continuously owing to the stimulus responsiveness to ultraviolet or heat.

Wavelength-Selective Photo-Cycloadditions of Styryl-Anthracene.

Light-tunable covalent chemistry is highly urgent in the fields of chemistry, biology, and materials science, especially for the smart materials and surface, due to the spatiotemporal control and feasible operation. Here, a new type of wavelength-selective photo-cycloaddition of styryl-anthracene carboxylic acid (SACA) is reported. Upon the irradiation of 450 nm visible light or 365 nm UV light, SACA can undergo [2+2] or [2+4] photocycloaddition, respectively. Furthermore, the [2+2] photocycloaddition induced by vis-light of 450 nm is reversible and can be disrupted by 365 nm UV light to form dimer-24 which cannot be photo-cleavable. Owing to the feasibility and spatiotemporal characteristics of UV-vis light-controlled photocycloaddition, the SACA possesses potential applications in various areas such as self-assembly, dynamic wrinkles, and fluorescence patterns, which are also explored and exhibited in this work.

Stress Communication between the Chain Movement and the Shape Transformation from 2D to 3D.

Shape memory polymers can change their initial shape under the stimulation of the external environment, but most of the stimulations require not only an external force but also a high temperature, which limits their application to a certain extent. Inspired by the unmatched elongation of cells on both sides of the mimosa petiole in nature, which leads to leaf closure, we designed a new type of shape transformation polymer, which can transform between 2D and 3D by simple stretching and releasing steps at room temperature. Surface patterning on one side of the sample film was realized via a coordination network of Fe3+-COOH to achieve different coordination gradients along its thickness. By this way, different movements of polymer chains along the thickness would lead to 2D-3D transformation upon releasing the stretched sample. Using this method, we obtained a series of transformations from customized 2D materials to complex 3D shapes and explored their potential application in information encryption transmission.

Aminoesterenamide Achieved by Three-Component Reaction Heading toward Tailoring Covalent Adaptable Network with Great Freedom.

Covalent adaptable networks (CANs) have recently received extensive interests due to their reprocessability and repairability. Rethinking the libraries of the published CANs, most of them are fabricated by one/two-component reactions and few cases utilize multi-component reactions to construct CANs while multi-component reactions are conductive to tailoring the properties of polymers due to their structural designability and flexible choice of raw materials. A novel kind of dynamic covalent bond named aminoesterenamide is presented through three-component reaction between acetoacetyl, amine and isocyanate. Aminoesterenamide exhibits thermal reversibility through dissociating into vinylogous urethane and isocyanate. When it is used to prepare CANs, the synthesized polymer networks can be reprocessed many times via the exchange reaction between aminoesterenamides. Moreover, the forming of aminoesterenamide involving three starting components imparts CANs with great freedom to tailor their properties. Therefore, the authors believe this method that utilizes three-component reaction to fabricate CANs would bring new stories and perspectives to the exploration of new types of CANs.

Metal-Organic Frameworks Corset with a Thermosetting Polymer for Improved Molecular-Sieving Property of Mixed-Matrix Membranes.

Metal-organic frameworks (MOFs) are promising materials for gas separation membranes. However, the framework flexibility affects their molecular-sieving properties. Herein, we restrict the flexibility of zeolitic imidazolate framework-7 (ZIF-7) by controlling its phase transition in mixed-matrix membranes (MMMs), relying on the so-called "space-confinement effect" of a novel thermosetting polymer, poly 2,2'-(p-oxydiphenyl)-5,5'-bibenzimidazole (OPBI) polymer. Compared with the pure OPBI membrane, the optimized membranes containing 30 wt % ZIF-7 with a narrow-pore (np) phase (ZIF-7-II) exhibited a significant improvement in H2/CO2 separation, e.g., the H2/CO2 ideal selectivity increased ∼2.8 times, surpassing the state-of-the-art upper bound of polymeric membranes and exhibited excellent stability at increased pressure and temperature (8 bar, 180 °C).

Revisiting Acetoacetyl Chemistry to Build Malleable Cross-Linked Polymer Networks via Transamidation.

Covalent adaptable networks (CANs) have various potential applications for their dynamic features benefiting from the existence of dynamic covalent bonds (DCBs). Here we exploit acetoacetyl chemistry to design CANs in which acetoacetyl formed amides (AFAs) act as a type of DCBs. We first illuminate that the transamidation of AFAs is ascribed to a "proton-switch" mechanism via model study and DFT calculations. When such AFA linkages are incorporated into cross-linked polymer networks, the malleability and recyclability are demonstrated. After recycling the polymer networks for three times, there are no significant mechanical changes or degradations observed. The study reveals that the transamidation is an economic and efficient exchange reaction in the preparation of CANs.

Light-Written Reversible 3D Fluorescence and Topography Dual-Pattern with Memory and Self-Healing Abilities.

To achieve the dynamical dual-pattern with multiplex information of complex topography and 3D fluorescence is challenging yet promising for wide applications ranging from visual bioassays, memory, smart devices to smart display. Here, we develop a convenient, reliable, and versatile method to realize the well-ordered dual-pattern with reversible topography and 3D fluorescence via a light direct-writing approach based on the wrinkle mechanism. By introducing the charge transfer (CT) interaction between π-electron-rich anthracene (AN) and π-electron-poor naphthalene diimide (NDI) into the polymer system, both modulus and fluorescence of the polymer films can be spatially regulated through the photodimerization of AN, which is controlled in-plane by photomasks, and becomes gradient in the vertical direction due to the filter effect of light. Therefore, the exposed sample displays a well-ordered complex pattern with the same topography as the applied photomask and 3D gradient change of fluorescence from red to green laterally across the layers simultaneously. The spatial cross-linking and CT interaction of the gradient layer can be controlled independently, which not only provides the reliability and reversibility of the topographical and fluorescence dual-pattern but also endows the possibility for tailoring the pattern with memory and self-healing. These characters of the dual-pattern with reversible topography and 3D fluorescence declare the clear applications in smart multiplex displays, memory, anticounterfeiting, visual detections, and so on.

Versatile Approach to Building Dynamic Covalent Polymer Networks by Stimulating the Dormant Groups.

Despite many efforts, there is no versatile way to realize reversible cross-linking for most polymers. Inspired by the abstraction of hydrogen and the iniferter polymerization of benzophenone (BP), we report a versatile approach for building dynamic covalent networks for polymers containing C-H bonds. Under ultraviolet irradiation, BP can effectively abstract the hydrogen from polymers to form dormant diarylsemipinacol (DASP) groups on the polymer chains. Then, the dormant DASP-based linkages can be homolytically cleaved upon heating, after which they generate carbon-centered aliphatic radicals and DASP-based radicals. Therefore, the cross-linked polymer network can rearrange its topology through the dissociation and association of DASP-based linkages, which endow polymer networks with remodeling and self-healing abilities. Given that most commercially available polymers contain aliphatic C-H bonds, this provides a general method for forming thermal reversible cross-linked networks.

Dynamically Cross-linked Elastomer Hybrids with Light-Induced Rapid and Efficient Self-Healing Ability and Reprogrammable Shape Memory Behavior.

Pristine carbon nanotubes (CNTs) were activated to exhibit Diels-Alder (DA) reactivity in a polymer matrix, which was modified with monomers containing furan groups. The DA-active polymer matrix was transferred into a dynamic reversible cross-linked inorganic-organic network via a Diels-Alder reaction with CNTs, where pristine CNTs were used as dienophile chemicals and furan-modified SBS acted as the macromolecular diene. In this system, the mechanical properties as well as resilience and solvent resistance were greatly improved even with the presence of only 1 wt % CNTs. Meanwhile, the hybrids retained recyclability and exhibited some smart behaviors, including self-healing and reprogrammable shape memory properties. Furthermore, due to the photothermal effect of CNTs, a retro-Diels-Alder (rDA) reaction was activated under laser irradiation, and healing of a crack on the hybrid surface was demonstrated in approximately 10 s with almost complete recovery of the mechanical properties. Such fast and efficient self-healing performance provides a new concept in designing self-healing nanocomposites with tunable structures and mechanical properties. Furthermore, the DA and rDA reactions could be combined to reprogram the shape memory behavior under laser irradiation or thermal treatment, wherein the temporary shape of the sample could be transferred to a permanent shape via the rDA reaction at high temperature.

Shape Memory: An Efficient Method to Develop the Latent Photopatterned Morphology for Elastomer in Two/Three Dimension.

Shape memory behavior was applied here as a new approach for developing the latent photopatterned morphologies in two/three dimension (2D/3D) on the modified poly(styrene-block-butadiene-block-styrene) (SBS). By attaching anthracene groups onto the SBS chains, the elastomer frozen in the deformed state was photopatterned via the photodimerization of anthracene. Upon thermal treatment, shape memory process could effectively develop the latent photopatterning induced 2D-2D and 2D-3D shape transformation. Due to the reversible dimerization of anthracene, the photoinduced patterns and the shape conformation could be erased and redeveloped for multiple times.

Selective Adsorption and Separation through Molecular Filtration by Hyperbranched Poly(ether amine)/Carbon Nanotube Ultrathin Membranes.

In response to the increasing public awareness of serious dye-contained wastewater contamination, we herein fabricated a novel anthracene-containing hyperbranched poly(ether amine) (hPEA-AN)/carbon nanotube (CNT) ultrathin membrane (UTM), which combined both the merits of the conventional dye adsorption strategy and membrane filtration process, to implement efficient selective adsorption of dye molecules and also the separation of dye mixtures by molecular filtration. Taking advantage of the π-π stacking interactions between anthracene and CNT sidewalls and hydrophobic interactions, CNTs were coated tightly with hPEA-AN to form the hPEA-AN@CNT complex, which can be well-dispersed very stably in water. The formation of the hPEA-AN@CNT complex was confirmed using X-ray photoelectron spectroscopy, Raman spectra, fluorescence spectra, and thermogravimetric analysis. Meanwhile, a simple filtration process was applied to prepare hPEA-AN@CNT UTMs with a thickness of 1.5 µm, which can be further cross-linked through photodimerization of anthracene moieties. The UTMs represented selective adsorption behaviors toward hydrophilic dyes even with similar backbones and the same charge states, namely, they showed high adsorption capacities (Qeq) toward eosin B, erythrosin B (ETB), 4',5'-dibromofluorescein, and Evans blue (EVB) dyes up to 300 µmol/g while showing low adsorption capacities toward calcein (Cal), methyl red, and Ponceau S dyes. On the basis of this unique selective adsorption, molecular filtration was then realized toward mixed ETB/Cal and EVB/Cal dyes, with a separation efficiency of up to 100% and regeneration without an obvious efficiency decrease.

High performance carbon molecular sieving membranes derived from pyrolysis of metal-organic framework ZIF-108 doped polyimide matrices.

Carbon molecular sieve membranes (CMSMs) were fabricated by pyrolysis of MOF-doped polyimide mixed matrix membranes. ZIF-108 (Zn(2-nitroimidazolate)2) was used as a dopant to tailor the micropores of the as-prepared CMSMs into narrow ultramicropores, providing a remarkable combination of permeability and selectivity of membranes in CO2/CH4, O2/N2 and N2/CH4 separation.

Self-Assembly of Amphiphilic Anthracene-Functionalized ß-Cyclodextrin (CD-AN) through Multi-Micelle Aggregation.

Multi-micelle aggregation (MMA) mechanism is widely acknowledged to explicate large spherical micelles self-assembly, but the process of MMA during self-assembly is hard to observe. Herein, a novel kind of strong, regular microspheres fabricated from self-assembly of amphiphilic anthracene-functionalized ß-cyclodextrin (CD-AN) via Cu(I)-catalyzed azide-alkyne click reactions is reported. The obtained CD-AN amphiphiles can self-assemble in water from primary core-shell micelles to secondary aggregates with the diameter changing from several tens nm to around 600-700 nm via MMA process according to the images of scanning electron microscopy, transmission electron microscopy, and atomic force microscopy as well as the dynamic light scattering measurements, followed by further crosslinking through photo-dimerization of anthracene. What merits special attention is that such photo-crosslinked self-assemblies are able to disaggregate reversibly into primary nanoparticles when changing the solution conditions, which is benefited from the designed regular structure of CD-AN and the rigid ranging of anthracene during assembly, thus confirming the process of MMA.

Revisiting the mechanism of redox-polymerization to build the hydrogel with excellent properties using a novel initiator.

In this paper, polyetheramine, a kind of linear epoxy polymer, has been used as both an initiator and a cross-linker for the synthesis of polyacrylamide hydrogel by means of a one pot method, which is a simple, time saving, facile and easily controlled process and the obtained hydrogel showed super stretchable and highly elastic properties. More impressively, only very low content of polyetheramine (0.1 wt%) could exhibit greatly enhanced mechanical properties toward polyacrylamide and the properties of hydrogel could be easily tailored by the weight ratio of polyetheramine to acrylamide. These hydrogels could be stretched up to 2000% with a maximal fracture energy up to 3.2 MJ m(-3). These types of hydrogels could be recovered immediately with low residual strains upon unloading even after 1000% strain. The presence of polyetheramine could create a more homogeneous distribution of crosslinking points and can dissipate the energy effectively, which can be responsible for the improved mechanical properties.

Unzipped multiwalled carbon nanotube oxide/multiwalled carbon nanotube hybrids for polymer reinforcement.

Multiwalled carbon nanotubes (MWNTs) have been widely used as nanofillers for polymer reinforcement. However, it has been restricted by the limited available interface area of MWNTs in the polymer matrices. Oxidation unzipping of MWNTs is an effective way to solve this problem. The unzipped multiwalled carbon nanotube oxides (UMCNOs) exhibit excellent enhancement effect with low weight fractions, but agglomeration of UMCNOs at a relatively higher loading still hampered the mechanical reinforcement of polymer composites. In this paper, we interestingly found that the dispersion of UMCNOs in polymer matrices can be significantly improved with the combination of pristine MWNTs. The hybrids of MWNTs and UMCNOs (U/Ms) can be easily obtained by adding the pristine MWNTs into the UMCNOs aqueous dispersion, followed by sonication. With a π-stacking interaction, the UMCNOs were attached onto the outwalls of MWNTs. The morphologies and structure of the U/Ms were characterized by several measurements. The mechanical testing of the resultant poly(vinyl alcohol) (PVA)-based composites demonstrated that the U/Ms can be used as ideal reinforcing fillers. Compared to PVA, the yield strength and Young's modulus of U/M-PVA composites with a loading of 0.7 wt % of the U/Ms approached ∼145.8 MPa and 6.9 GPa, respectively, which are increases of ∼107.4% and ∼122.5%, respectively. The results of tensile tests demonstrated that the reinforcement effect of U/Ms is superior to the individual UMCNOs and MWNTs, because of the synergistic interaction of UMCNOs and MWNTs.