Reversible covalent polymers are able to change their bond arrangement and structure via reversible reaction triggered by external stimuli including heating, light and pH, while retaining the ...stability of irreversible covalent polymers in the absence of the stimuli. In recent years, more and more research has been devoted to utilization of reversible covalent bonds in synthesizing new materials, which not only overcomes disadvantages of permanent covalent polymers, but also brings in new functionalities. More importantly, a series of novel techniques dedicated to polymerized products with features such as properties regulation, self-healing, reprocessing, solid state recycling, and controllable degradation are developed, heralding the opportunity of upgrading of traditional polymer engineering. Although the exploration of this emerging topic is still in its infancy, the advances so far are encouraging and clearly directed to large scale applications. This review systematically outlines this promising trend, following a bottom-up strategy, taking into account both theoretical and experimental achievements. It mainly consists of four parts, involving design and preparation: (i) the basis of reversible covalent chemistry, (ii) rheology of reversible covalent polymers, (iii) methods of construction of reversible covalent polymers, and (iv) smart, adaptive properties offered by reversible covalent chemistry. The key elements for realizing reorganization of polymers containing reversible covalent bonds are covered. The advantages and weaknesses of representative reaction systems are analyzed, while the challenges and opportunities to engineering application of the equilibrium control based on reversible covalent chemistry for producing end-use polymers are summarized. In this way, the readers may grasp both the overall situation as well as insight into future work.
Developing low-cost electrocatalysts to replace precious Ir-based materials is key for oxygen evolution reaction (OER). Here, we report atomically dispersed nickel coordinated with nitrogen and ...sulfur species in porous carbon nanosheets as an electrocatalyst exhibiting excellent activity and durability for OER with a low overpotential of 1.51 V at 10 mA cm
and a small Tafel slope of 45 mV dec
in alkaline media. Such electrocatalyst represents the best among all reported transition metal- and/or heteroatom-doped carbon electrocatalysts and is even superior to benchmark Ir/C. Theoretical and experimental results demonstrate that the well-dispersed molecular S|NiN
species act as active sites for catalyzing OER. The atomic structure of S|NiN
centers in the carbon matrix is clearly disclosed by aberration-corrected scanning transmission electron microscopy and synchrotron radiation X-ray absorption spectroscopy together with computational simulations. An integrated photoanode of nanocarbon on a Fe
O
nanosheet array enables highly active solar-driven oxygen production.
Stimuli‐responsive polymers built by reversible covalent bonds used to possess unbalanced mechanical properties. Here, a crosslinked polyurethane containing aromatic pinacol as a novel reversible CC ...bond provider is synthesized, whose tensile strength and failure strain are tunable from 27.3 MPa to as high as 115.2 MPa and from 324% to 1501%, respectively, owing to the relatively high bond energy of the CC bond of pinacol as well as the hydrogen bond between hard segments and semicrystalline soft segments. Moreover, the dynamic equilibrium of pinacol enables self‐healing and recycling of the polymer. Interestingly, the dynamic exchange among macromolecules, for the first time, successfully cooperates with solid‐state drawing that applies to thermoplastics, realizing strengthening of thermoset. Meanwhile, the radicals derived from homolysis of pinacol can repeatedly initiate polymerization of vinyl monomers. The fruitful outcomes of this work may create a series of promising new techniques.
The CC bond of pinacol is dynamically reversible at moderate temperature. A crosslinked polyurethane carrying pinacol unit is synthesized, which can be self‐healed, reprocessed, and recycled via the catalyst‐free reversible homolysis/radicals recombination of the CC bond. Moreover, its strength can be greatly improved by solid‐state drawing. The radicals created during homolysis are enabled to repeatedly initiate polymerization of vinyl monomers.
Inspired by naturally occurring species that allow for self-healing of nonfatal harm, self-healing polymeric materials have been prepared and represent a component of the intelligent materials ...family. These materials possess the inherent ability to rehabilitate damage produced during manufacturing and/or usage. The self-healing methodologies developed to date can be classified as intrinsic or extrinsic according to the method used to deliver the healing components to the target site in the material. Intrinsic self-healing operates through inter- or intra-macromolecular interactions, whereas extrinsic self-healing makes use of a pre-embedded healing agent. Extrinsic self-healing can be more easily realized in commercially available polymers because no structural modification of the matrix molecules is required. In recent years, extrinsic self-healing based on microencapsulated healing agents has attracted growing interest. Extrinsic self-healing in a variety of materials (including thermosets, thermoplastics, rigid, and elastomeric materials) has been demonstrated and offers recovery of both mechanical and non-structural functional properties. Self-healing based on microcapsules can deliver further results if combined with intrinsic self-healing. Using a bottom-up perspective, the current article presents a comprehensive review of recent progress in this field from the viewpoint of material design and preparation. The topics presented include (i) a basic overview of self-healing systems, (ii) microencapsulation techniques (e.g., in situ polymerization, interfacial polymerization, Pickering emulsion templating, miniemulsion polymerization, solvent evaporation/solvent extraction, sol–gel reaction, etc.), (iii) crack response of microcapsules, and (iv) healing chemistries (e.g., ring-opening metathesis polymerization, polycondensation, anionic ring opening polymerization, cationic polymerization, free radical polymerization, addition reaction, etc.). The strengths and weaknesses of each microencapsulation technique and type of healing chemistry are analyzed and compared. Additionally, formulation optimization (including species of healing agent and wall substance of capsules), processing, structure and property relationship, healing mechanisms, and stability are discussed. Trends and challenges are summarized at the end of the review. The scope of this review is to provide the reader with an overview of achievements to date and insight into future development for engineering applications.
One great challenge in understanding the history of life is resolving the influence of environmental change on biodiversity. Simulated annealing and genetic algorithms were used to synthesize data ...from 11,000 marine fossil species, collected from more than 3000 stratigraphic sections, to generate a new Cambrian to Triassic biodiversity curve with an imputed temporal resolution of 26 ± 14.9 thousand years. This increased resolution clarifies the timing of known diversification and extinction events. Comparative analysis suggests that partial pressure of carbon dioxide (
co
) is the only environmental factor that seems to display a secular pattern similar to that of biodiversity, but this similarity was not confirmed when autocorrelation within that time series was analyzed by detrending. These results demonstrate that fossil data can provide the temporal and taxonomic resolutions necessary to test (paleo)biological hypotheses at a level of detail approaching those of long-term ecological analyses.
...two covalent adaptive networks containing orthogonal reversible covalent bonds are synthesized. ...the interlaced networks with relative independence (i.e. RILNs) are constructed through ...topological rearrangement during the removal of the stimulus and co-solvent. ...the freedom of choosing raw materials for making polymer blends is greatly increased, and versatile multicomponent polymer systems with high performance and novel functionalities can be developed.
To provide self-healing ability to polyurethane elastomer, alkoxyamine-based diol was synthesized and reacted with tri-functional homopolymer of hexamethylene diisocyanate (tri-HDI) and polyethylene ...glycol (PEG). Because alkoxyamines acted as crosslinkers of the resultant polyurethane, the thermally reversible fission/recombination of C–ON bonds in alkoxyamine moieties enabled repeated crosslinking and de-crosslinking of polyurethane chains at certain temperature and hence crack healing. Both qualitative and quantitative characterizations demonstrated that the reversibly crosslinked polyurethane was capable of re-bonding ruptured parts and restoring mechanical strength. The remendability was a function of molecular structure and compositions of the components, which can be purposely tuned according to different application demands.
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A crosslinked silicone elastomer is prepared by utilizing polydimethylsiloxane as the host material and disulfide bond-containing silane as the crosslinking agent. By taking advantage of the sunlight ...activated metathesis of the included disulfide bonds, the crosslinked silicone elastomer can be self-healed and recycled for multiple times without any catalyst under sunshine. A series of verification experiments are carried out to reveal the underlying mechanism and the dynamicity of the silicone. Because the key raw materials are commercially available and the manufacturing follows common industrial procedures, the outcomes of this proof-of-concept study may be easily applied in practice. The simple and eco-friendly method would help to increase reliability and durability of silicone products and minimize wasting of resources.
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•Crosslinked polydimethylsiloxane exhibits sunlight induced dynamicity.•Catalyst-free disulfide metathesis in silicone elastomer is activated by sunlight.•Silicone elastomer can be repeatedly self-healed and recycled under sunshine.
To help inform decision making in the clinical setting, we carried out a systematic review and meta-analysis to estimate the association of thyroid disease risks with obesity.
Pubmed, Embase, Web of ...Science, Cochrane database and Google Scholar electronic databases were searched from inception to October 31, 2018 without language restrictions to explore the relationship between thyroid disorders and obesity. The relative risk (RR) or odds risk (OR) for thyroid disorders were pooled using the SPSS and STATA software.
A total of 22 studies were included in the study. (1) Meta-analysis showed that obesity was significantly associated with an increased risk of hypothyroidism (RR = 1.86, 95% CI 1.63-2.11,
< 0.001). Meta-analyses after stratification further showed that obese population had increased risks of overt hypothyroidism (RR = 3.21, 95% CI 2.12-4.86,
< 0.001) and subclinical hypothyroidism (RR = 1.70, 95% CI 1.42-2.03,
< 0.001). (2) Further meta-analysis also showed obesity was clearly associated with Hashimoto's thyroiditis (RR = 1.91, 95% CI 1.10-3.32,
= 0.022), but not with Graves' disease. (3) In the meta-analysis of antibodies, obesity was correlated with positive thyroid peroxidase antibody (TPOAb) (RR = 1.93, 95% CI 1.31-2.85,
= 0.001), but not with positive thyroglobulin antibody (TGAb).
Obesity was significantly related to hypothyroidism, HT, and TPOAb, implying that prevention of obesity is crucial for thyroid disorders.
PROSPERO: CRD42018096897.
Aimed at the safety and stability problems of intelligent vehicles under extreme conditions such as low adhesion road surface and emergency lane change and obstacle avoidance, this article designs a ...lane change and obstacle avoidance controller based on road adhesion coefficient. Using the nonlinear vehicle dynamics model as the prediction model, using the recursive least squares method to identify the road adhesion coefficient, considering the road adhesion coefficient to plan and adjust in the obstacle avoidance path as well as limit constraint conditions of the model predictive control controller, using model predictive control method for the expectation of intelligent vehicle trajectory tracking, travels tremendously guarantee the security and stability of driving. The joint CarSim–Simulink simulations results show that under poor road conditions, the trajectory tracking accuracy after optimization is higher and the vehicle is less prone to sideslip and instability. The lane change controller designed in this article has strong adaptability to different road surface adhesion coefficient, and all parameters can be controlled within a reasonable safety range at different speeds, with good robustness.
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