Benzene Tetraamide: A Covalent Supramolecular Dual Motif in Dynamic Covalent Polymer Networks.

In dynamic polyamide networks, 1,2,4,5-benzene tetraamide (B4A) units act simultaneously as a dynamic covalent cross-linker and as supramolecular stacking motif. This results in materials with a rubbery plateau modulus that is about 20 times higher than that of a corresponding reference network in which the supramolecular interaction is suppressed. In branched polyamides with the same B4A dynamic motif, hydrogen bonding and stacking lead to strong and reversible supramolecular networks, whereas a branched polyamide with the nonstacking reference linker is a viscous liquid under the same conditions. Wide-angle X-ray scattering and variable-temperature infrared experiments confirm that covalent cross-linking and stacking cooperatively contribute to the dynamics of the network. Stress relaxation in the reference network is dominated by a single mode related to the dynamic covalent chemistry, whereas relaxation in the B4A network has additional modes assigned to the stacking dynamics.

Visual Detection of COVID-19 from Materials Aspect.

Abstract: In the recent COVID-19 pandemic, World Health Organization emphasized that early detection is an effective strategy to reduce the spread of SARS-CoV-2 viruses. Several diagnostic methods, such as reverse transcription-polymerase chain reaction (RT-PCR) and lateral flow immunoassay (LFIA), have been applied based on the mechanism of specific recognition and binding of the probes to viruses or viral antigens. Although the remarkable progress, these methods still suffer from inadequate cellular materials or errors in the detection and sampling procedure of nasopharyngeal/oropharyngeal swab collection. Therefore, developing accurate, ultrafast, and visualized detection calls for more advanced materials and technology urgently to fight against the epidemic. In this review, we first summarize the current methodologies for SARS-CoV-2 diagnosis. Then, recent representative examples are introduced based on various output signals (e.g., colorimetric, fluorometric, electronic, acoustic). Finally, we discuss the limitations of the methods and provide our perspectives on priorities for future test development.

Significant loss of soil inorganic carbon at the continental scale.

Widespread soil acidification due to atmospheric acid deposition and agricultural fertilization may greatly accelerate soil carbonate dissolution and CO2 release. However, to date, few studies have addressed these processes. Here, we use meta-analysis and nationwide-survey datasets to investigate changes in soil inorganic carbon (SIC) stocks in China. We observe an overall decrease in SIC stocks in topsoil (0-30 cm) (11.33 g C m-2 yr-1) from the 1980s to the 2010s. Total SIC stocks have decreased by ∼8.99 ± 2.24% (1.37 ± 0.37 Pg C). The average SIC losses across China (0.046 Pg C yr-1) and in cropland (0.016 Pg C yr-1) account for ∼17.6%-24.0% of the terrestrial C sink and 57.1% of the soil organic carbon sink in cropland, respectively. Nitrogen deposition and climate change have profound influences on SIC cycling. We estimate that ∼19.12%-19.47% of SIC stocks will be further lost by 2100. The consumption of SIC may offset a large portion of global efforts aimed at ecosystem carbon sequestration, which emphasizes the importance of achieving a better understanding of the indirect coupling mechanisms of nitrogen and carbon cycling and of effective countermeasures to minimize SIC loss.

Dynamic Polyamide Networks via Amide-Imide Exchange.

The diamide-imide equilibrium was successfully exploited for the synthesis of dynamic covalent polymer networks in which a dissociative bond exchange mechanism leads to high processibility at temperatures above ≈110 °C. Dynamic covalent networks bridge the gap between thermosets and thermoplastic polymers. At the operating temperature, when the network is fixed, dynamic covalent networks are elastic solids, while at high temperatures, chemical exchange reactions turn the network into a processible viscoelastic material. Upon heating a dissociative network, the viscosity may also decrease due to a shift of the chemical equilibrium; in such materials, the balance between processibility and excessive flow is important. In this study, a network is prepared that upon heating to above ≈110 °C dissociates to a significant extent due to a shift in the amide-imide equilibrium of a bisimide, pyromellitic diimide, in combination with poly(tetrahydrofuran) diamines. At room temperature, the resulting materials are elastic rubbers with a tensile modulus of 2-10 MPa, and they become predominantly viscous above a temperature of approximately 110 °C, which is dependent on the stoichiometry of the components. The diamide-imide equilibrium was studied in model reactions with NMR, and variable temperature infrared (IR) spectroscopy was used to investigate the temperature dependence of the equilibrium in the network. The temperature-dependent mechanical properties of the networks were found to be fully reversible, and the material could be reprocessed several times without loss of properties such as modulus or strain at break. The high processibility of these networks at elevated temperatures creates opportunities in additive manufacturing applications such as selective laser sintering.

Mechanochemistry unveils stress transfer during sacrificial bond fracture of tough multiple network elastomers.

The molecular level transfer of stress from a stiff percolating filler to a stretchable matrix is a crucial and generic mechanism of toughening in soft materials. Yet the molecular details of how this transfer occurs have so far been experimentally unreachable. Model multiple network elastomers containing spiropyran (SP) force sensors incorporated into the stiff filler network or into the stretchable matrix network are used here to detect and investigate the mechanism of stress transfer between distinct populations of polymer strands. We find that as the filler network progressively breaks by random bond scission, there is a critical stress where cooperative bond scission occurs and the macroscopic stretch increases discontinuously by necking. Surprisingly, SP molecules reveal that even in the necked region both filler and matrix chains share the load, with roughly 90% of the SPs force-activated in the filler chains before necking still being loaded in the necked region where significant activation of the SP incorporated into the matrix chains occurs. This result, where both networks remain loaded upon necking, is qualitatively consistent with the model proposed by Brown, where holes or microcracks are formed in the stiff regions and are bridged by stretched matrix chains. Detection of merocyanine (i.e. activated SP) fluorescence by confocal microscopy shows that such microcrack formation is also active at the crack tip even for materials that do not exhibit macroscopic necking. Additionally, we demonstrate that when the ethyl acrylate monomer is replaced by hexyl methacrylate in the first network, preventing molecular connections between the two networks, the stress transmission is less efficient. This study outlines the different roles played by these multiple networks in the onset of fracture and provides molecular insights for the construction of molecular models of fracture of elastomers.

Correction: Mechanochemical tools for polymer materials.

Correction for 'Mechanochemical tools for polymer materials' by Yinjun Chen et al., Chem. Soc. Rev., 2021, 50, 4100-4140, DOI: 10.1039/D0CS00940G.

Mechanochemical tools for polymer materials.

Mechanochemistry provides a unique approach to investigate macroscopic deformation, failure and healing of polymer materials. The development of mechanophores - molecular units that respond to mechanical force - has been instrumental in the success of this endeavor. This review aims to provide a critical evaluation of the large variety of mechanophores reported in literature, and to assess the molecular and macroscopic factors that determine their activation. Applications in materials science are highlighted, and challenges in polymer mechanochemistry are discussed.

Ruthenium-Catalyzed PIII-Directed Remote ε-C-H Alkylation of Phosphines.

The ruthenium-catalyzed remote ε-C-H alkylation of phosphines with tertiary alkyl halides has been developed. This novel PIII-directed C-H activation strategy tolerated various functional groups and delivered a wide variety of modified phosphines with excellent meta-site selectivity. Preliminary mechanistic studies indicated that a PIII-assisted ortho-cyclometalation/remote σ-activation pathway might be involved in this methodology.

From force-responsive molecules to quantifying and mapping stresses in soft materials.

Directly quantifying a spatially varying stress in soft materials is currently a great challenge. We propose a method to do that by detecting a change in visible light absorption. We incorporate a spiropyran (SP) force-activated mechanophore cross-linker in multiple-network elastomers. The random nature of the network structure of the polymer causes a progressive activation of the SP force probe with load, detectable by the change in color of the material. We first calibrate precisely the chromatic change in uniaxial tension. We then demonstrate that the nominal stress around a loaded crack can be detected for each pixel and that the measured values match quantitatively finite element simulations. This direct method to quantify stresses in soft materials with an internal force probe is an innovative tool that holds great potential to compare quantitatively stresses in different materials with simple optical observations.

Fast reversible isomerization of merocyanine as a tool to quantify stress history in elastomers.

A mechanochemistry based approach is proposed to detect and map stress history during dynamic processes. Spiropyran (SP), a force sensitive molecular probe, was incorporated as a crosslinker into multiple network elastomers (MNE). When these mechanochromic MNEs are loaded, SP undergoes a well-known force-activated reaction to merocyanine (MC) changing its absorption in the visible range (visible blue color). This SP to MC transition is not reversible within the time frame of the experiment and the color change reports the concentration of activated molecules. During subsequent loading-unloading cycles the MC undergoes a fast and reversible isomerization resulting in a slight shift of absorption spectrum and results in a second color change (blue to purple color corresponding to the loading-unloading cycles). Quantification of the color changes by using chromaticity shows that the exact color observed upon unloading is characteristic not only of the current stress (reported by the shift in color due to MC isomerization), but of the maximum stress that the material has seen during the loading cycle (reported by the shift in color due to the change in MC concentration). We show that these two color changes can be separated unambiguously and we use them to map the stress history in the loading and unloading process occurring as a crack opens up and propagates, breaking the material. Color maps on fractured samples are compared with finite element simulations and the agreement is excellent.

Mechanochromism and optical remodeling of multi-network elastomers containing anthracene dimers.

Multi-network elastomers are both stiff and tough by virtue of containing a pre-stretched stiff network that can rupture and dissipate energy under load. However, the rupture of this sacrificial network in all described covalent multi-network elastomers is irreversible. Herein, we describe the first example of multi-network elastomers with a reformable sacrificial network containing mechanochemically sensitive anthracene-dimer cross-links. These cross-links also make our elastomers mechanochromic, with coloration that is both persistent and reversible, because the fluorogenic moiety (anthracene dimer) is regenerated upon irradiation of the material. In proof-of-concept experiments we demonstrate the utility of incorporating anthracene dimers in the backbone of the sacrificial network for monitoring mechanochemical remodeling of multi-network elastomers under cycling mechanical load. Stretching or compressing these elastomers makes them fluorescent and irradiating them eliminates the fluorescence by regenerating anthracene dimers. Reformable mechanochromic cross-links, exemplified by anthracene dimers, hold potential for enabling detailed studies of the molecular origin of the unique mechanical properties of multi-network elastomers.

Mechanical Activation of Mechanophore Enhanced by Strong Hydrogen Bonding Interactions.

A mechanically active spiropyran (SP) mechanophore is incorporated into the backbone of prepolymer which is further end-capped with ureidopyrimidinone (UPy) or urethane. Strong mechanochromic reaction of SP arises in the bulk films of UPy containing materials whereas much weaker activation occurs in urethane containing counterparts, coincident with their stress-strain responses. The difference in the magnitudes of supramolecular interactions leads to different degrees of chain orientation and strain induced crystallization (SIC) in the bulk and consequently distinct capabilities to transfer the load to the mechanophores. This study may aid the design of novel mechanoresponsive materials whose mechanoresponsiveness can be tailored by tuning supramolecular interactions.

Using metal-ligand interactions to access biomimetic supramolecular polymers with adaptive and superb mechanical properties.

The development of polymer materials that exhibit excellent mechanical properties and can respond to environmental stimuli is of great scientific and commercial interest. In this work, we report a series of biomimetic supramolecular polymers using a ligand macromolecule carrying multiple tridentate ligand 2,6-bis(1,2,3-triazol-4-yl)pyridine (BTP) units synthesized via CuAAC in the polymer backbone together with transition and/or lanthanide metal salts. The metal-ligand complexes phase separate from soft linker segments, acting as physical crosslinking points in the materials. The metallo-supramolecular films exhibit superb mechanical properties, i.e., high tensile strength (up to 18 MPa), large strain at break (>1000%) and exceptionally high toughness (up to 70 MPa), which are much higher than those of the ligand macromolecule and are tunable by adjusting the stoichiometric ratio of Zn2+ to Eu3+ and the stoichiometry of metal ion to ligand. The metal-ligand hard phase domains are demonstrated to be thermally stable but mechanically labile, similar to the behaviors of covalent mechanophores. The thermal stability and mechanical responsiveness are also dependent on the compositions of metal ions. The disruption of the hard phase domains and the dissociation of metal-ligand complexes under stretching are similar to the unfolding of modular domains in modular biomacromolecules and are responsible for the superb mechanical properties. In addition, the biomimetic metallo-supramolecular materials display promising responsive properties to UV irradiation and chemicals. These well designed, created and characterized robust structures will inspire further accurate tailoring of biomimetic responsive materials at the molecular level and/or nanoscale.