Nonmetallic-Bonding Fe-Mn Diatomic Pairs Anchored on Hollow Carbonaceous Nanodisks for High-Performance Li-S Battery.

The issues of polysulfide shuttling and lethargic sulfur redox reaction (SROR) kinetics are the toughest obstacles of lithium-sulfur (Li-S) battery. Herein, integrating the merits of increased density of metal sites and synergistic catalytic effect, a unique single-atom catalyst (SAC) with nonmetallic-bonding Fe-Mn diatomic pairs anchored on hollow nitrogen-doped carbonaceous nanodisk (denoted as FeMnDA@NC) is successfully constructed and well characterized by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, X-ray absorption spectroscopy, etc. Density functional theory calculation indicates that the Fe-Mn diatomic pairs can effectively inhibit the shuttle effect by enhancing the adsorption ability retarding the polysulfide migration and accelerate the SROR kinetics. As a result, the Li-S battery assembled with FeMnDA@NC modified separator possesses an excellent electrochemical performance with ultrahigh specific capacities of 1419 mAh g-1 at 0.1 C and 885 mAh g-1 at 3.0 C, respectively. An outstanding specific capacity of 1165 mAh g-1 is achieved at 1.0 C and maintains at 731 mAh g-1 after 700 cycles. Notably, the assembled Li-S battery with a high sulfur loading of 5.35 mg cm-2 harvests a practical areal capacity of 5.70 mAh cm-2 at 0.2 C. A new perspective is offered here to construct advanced SACs suitable for the Li-S battery.

Unsynchronous conformational transitions induced by the asymmetric adsorption-response of an active diblock copolymer in an inert brush.

To exploit the chemical asymmetry of diblock copolymer chains on the design of high-performance switch sensors, we propose an analytically tractable model system which contains an adsorption-responsive diblock copolymer in an otherwise inert brush, and study its phase transitions by using both analytical theory and self-consistent field calculations. The copolymer chain is chemically asymmetric in the sense that the two blocks assume different adsorption strengths, which is characterized by the defined adsorption ratio. We found that the conformation states, the number of stable phases, and transition types are mainly controlled by the length of each block and the adsorption ratio. In particular, when the length of the ungrafted block is longer than the brush chains, and the adsorption ratio is smaller than a critical value, the copolymer chain shows three thermodynamically stable states, and undergoes two unsynchronous transitions, where the two blocks respond to the adsorption in a different manner, when the adsorption changes from weak to sufficiently strong. For this kind of three-state transition, the transition point, transition barrier, and transition width are evaluated by using the self-consistent field method, and their scaling relationship with respect to the system parameters is extracted, which matches reasonably well with the predictions from the analytical theory. The self-consistent field calculations also indicate that the conformational transitions involved in the three-state transition process are sharp with a low energy barrier, and interestingly, barrier-free transitions are observed. Our finding shows that the three-state transitions not only specify a region where high performance unsynchronous switch sensors can be exploited, but may also provide a useful model understanding the unsynchronous biological processes.

Microphase behaviors and shear moduli of double-network gels: The effect of crosslinking constraints and chain uncrossability.

By performing coarse-grained molecular dynamics simulations, we study the effect of crosslinking and chain uncrossability on the microphase behaviors and mechanical properties of the double-network gels. The double-network systems can be viewed as two separate networks interpenetrating each other uniformly, and the crosslinks in each network are generated, forming a regular cubic lattice. The chain uncrossability is confirmed by appropriately choosing the bonded and nonbonded interaction potentials. Our simulations reveal a close relation between the phase and mechanical properties of the double-network systems and their network topological structures. Depending on the lattice size and the solvent affinity, we have observed two different microphases: one is the aggregation of solvophobic beads around the crosslinking points, which leads to locally polymer-rich domains, and the other is the bunching of polymer strands, which thickens the network edges and thus changes the network periodicity. The former is a representation of the interfacial effect, while the latter is determined by the chain uncrossability. The coalescence of network edges is demonstrated to be responsible for the large relative increase in the shear modulus. Compressing and stretching induced phase transitions are observed in the current double-network systems, and the sharp discontinuous change in the stress that appears at the transition point is found to be related to the bunching or debunching of the network edges. The results suggest that the regulation of network edges has a strong influence on the network mechanical properties.

Bio-Inspired Active Self-Cleaning Surfaces via Filament-Like Sweepers Array.

Hydrodynamic forces from moving fluids can be utilized to remove contaminants which is an ideal fouling-release strategy for underwater surfaces. However, the hydrodynamic forces in the viscous sublayer are greatly reduced owing to the no-slip condition, which restricts their practical applications. Here, inspired by sweeper tentacles of corals, an active self-cleaning surface with flexible filament-like sweepers are reported. The sweepers can penetrate the viscous sublayer by utilizing energy from outer turbulent flows and remove contaminants with adhesion strength of >30 kPa. Under an oscillating flow, the removal rate of the single sweeper can reach up to 99.5% due to dynamic buckling movements. In addition, the sweepers array can completely clean its coverage area within 10 s through coordinated movements as symplectic waves. The active self-cleaning surface depends on the fluid-structure coupling between sweepers and flows, which breaks the concept of conventional self-cleaning.

Reversible cross-linking facilitates the formation of critical nucleus in binary polymer blends.

Using self-consistent field theory, we study the effect of reversible cross-linking on the nucleation behavior of a binary polymer blend where only one of the components is able to form cross-links. To control the total number of cross-links and their distribution, we introduce a position-dependent cross-linking probability function that is characterized mainly by two parameters, the magnitude and the width. In the weakly cross-linked region, where the product of the magnitude and width, I, is small, the nucleation behavior is classical-like and the profile of the free energy excess is unimodal. In contrast, in the strongly cross-linked region, the profile of the free energy excess becomes bimodal, and the free energy minimum specifies a metastable nucleus. In a certain I, the free energy barrier for the metastable nucleus turns to be negative, which means it becomes more stable. In both cases, the free energy barrier of the critical nucleus is lower than that without cross-linking, indicating that cross-linking always facilitates nucleation although the dynamic behavior may be different when a metastable nucleus is involved during the nucleation process. The free energy analysis demonstrates that the interaction energy rather than the entropy is responsible for the properties of the critical nucleus. Our study provides an easy alternative way for the control of the nucleation behavior and may attract practical interest.

Ultrahigh energy-dissipation elastomers by precisely tailoring the relaxation of confined polymer fluids.

Energy-dissipation elastomers relying on their viscoelastic behavior of chain segments in the glass transition region can effectively suppress vibrations and noises in various fields, yet the operating frequency of those elastomers is difficult to control precisely and its range is narrow. Here, we report a synergistic strategy for constructing polymer-fluid-gels that provide controllable ultrahigh energy dissipation over a broad frequency range, which is difficult by traditional means. This is realized by precisely tailoring the relaxation of confined polymer fluids in the elastic networks. The symbiosis of this combination involves: elastic networks forming an elastic matrix that displays reversible deformation and polymer fluids reptating back and forth to dissipate mechanical energy. Using prototypical poly (n-butyl acrylate) elastomers, we demonstrate that the polymer-fluid-gels exhibit a controllable ultrahigh energy-dissipation property (loss factor larger than 0.5) with a broad frequency range (10-2 ~ 108 Hz). Energy absorption of the polymer-fluid-gels is over 200 times higher than that of commercial damping materials under the same dynamic stress. Moreover, their modulus is quasi-stable in the operating frequency range.

Conformational transitions of adsorption-responsive single diblock copolymers in homopolymer brushes.

Using self-consistent field calculations, we examine the effect of brush polydispersity on conformational transitions of single adsorption-active diblock copolymer chains embedded in inert polydisperse brushes. To represent the polydispersity, we adopt the continuous Schulz-Zimm chain length distributions, and three typical distributions are chosen such that a wide range of polydispersity is covered. A phase diagram of the diblock copolymer switches has been constructed showing that the first order phase transitions occupy a larger space in the case of polydisperse brushes. We further characterize these first order phase transitions by specifying their transition points, transition widths and transition barriers, where the latter two are particularly important as they determine the performance of the polymer switches. Our calculation indicates that polydispersity has different effect on the switching behavior depending on the lengths of both the active block and the inert block of the copolymer switch chain. In general, polydispersity improves the switching performance in the case of short active blocks, i.e. shorter or not very longer than the average length of the brush chains, and the corresponding energy barrier is smaller than a few kBT. In contrast, monodisperse brushes have the advantages when these two blocks are particularly long, i.e., lower transition barriers and fast switching. Notably, when the inert block approaches the average length of the brush chains, the transition barrier becomes almost zero in any case for monodisperse brushes, while a large finite value is still observed for that in polydisperse brushes. The complex interplay between the brush polydispersity and the switch behavior is attributed to the wide-range repulsions generated by the polydisperse brushes.

Author Correction: Layered nanocomposites by shear-flow-induced alignment of nanosheets.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Influence of Small-Scale Correlation on the Interface Evolution of Semiflexible Homopolymer Blends.

Within the framework of a dynamic self-consistent field theory, we study the effect of the correlations in a small scale on polymer dynamics, adopting the semiflexible homopolymer blends as the model system. This is accomplished by taking the pair correlation function of ideal semiflexible chains as the Onsager coefficient and the Debye function as an approximation to the Onsager coefficient. Relying on the difference of the two pair correlation functions in the small-scale region, we can identify the effect of small-scale correlations. In the equilibrium state, with the chain length growing, the interface width has a continuous transition from the contour length to radius of gyration. The investigation of interfacial evolution and chain orientation reveals that strong small-scale correlations would accelerate the small-scale dynamic process. We also expect that such a small-scale effect should be highlighted in the process where microscopic phase separation happens.

Layered nanocomposites by shear-flow-induced alignment of nanosheets.

Biological materials, such as bones, teeth and mollusc shells, are well known for their excellent strength, modulus and toughness1-3. Such properties are attributed to the elaborate layered microstructure of inorganic reinforcing nanofillers, especially two-dimensional nanosheets or nanoplatelets, within a ductile organic matrix4-6. Inspired by these biological structures, several assembly strategies-including layer-by-layer4,7,8, casting9,10, vacuum filtration11-13 and use of magnetic fields14,15-have been used to develop layered nanocomposites. However, how to produce ultrastrong layered nanocomposites in a universal, viable and scalable manner remains an open issue. Here we present a strategy to produce nanocomposites with highly ordered layered structures using shear-flow-induced alignment of two-dimensional nanosheets at an immiscible hydrogel/oil interface. For example, nanocomposites based on nanosheets of graphene oxide and clay exhibit a tensile strength of up to 1,215 ± 80 megapascals and a Young's modulus of 198.8 ± 6.5 gigapascals, which are 9.0 and 2.8 times higher, respectively, than those of natural nacre (mother of pearl). When nanosheets of clay are used, the toughness of the resulting nanocomposite can reach 36.7 ± 3.0 megajoules per cubic metre, which is 20.4 times higher than that of natural nacre; meanwhile, the tensile strength is 1,195 ± 60 megapascals. Quantitative analysis indicates that the well aligned nanosheets form a critical interphase, and this results in the observed mechanical properties. We consider that our strategy, which could be readily extended to align a variety of two-dimensional nanofillers, could be applied to a wide range of structural composites and lead to the development of high-performance composites.

Diffusion-Freezing-Induced Microphase Separation for Constructing Large-Area Multiscale Structures on Hydrogel Surfaces.

Hydrogels with multiscale structured surface have attracted significant attention for their valuable applications in diverse areas. However, current strategies for the design and fabrication of structured hydrogel surfaces, which suffer from complicated manufacturing processes and specific material modeling, are not efficient to produce structured hydrogel surfaces in large area, and therefore restrict their practical applications. To address this problem, a general and reliable method is reported, which relies on the interplay between polymer chain diffusion and the subsequent freezing-induced gelation and microphase separation processes. The basic idea is systematically analyzed and further exploited to manufacture gel surfaces with gradient structures and patterns through the introduction of temperature gradient and shape control of the contact area. Moreover, the formed micro/nanostructured surfaces are exemplified to work as capillary systems and thus can uplift the liquid spontaneously indicating the potential application for anti-dehydration. It is believed that the proposed facile and large-area fabrication method can inspire the design of materials with various functionalized surfaces.

Phase transitions in single macromolecules: Loop-stretch transition versus loop adsorption transition in end-grafted polymer chains.

We use Brownian dynamics simulations and analytical theory to compare two prominent types of single molecule transitions. One is the adsorption transition of a loop (a chain with two ends bound to an attractive substrate) driven by an attraction parameter ε and the other is the loop-stretch transition in a chain with one end attached to a repulsive substrate, driven by an external end-force F applied to the free end. Specifically, we compare the behavior of the respective order parameters of the transitions, i.e., the mean number of surface contacts in the case of the adsorption transition and the mean position of the chain end in the case of the loop-stretch transition. Close to the transition points, both the static behavior and the dynamic behavior of chains with different length N are very well described by a scaling ansatz with the scaling parameters (ε - ε*)Nϕ (adsorption transition) and (F - F*)Nν (loop-stretch transition), respectively, where ϕ is the crossover exponent of the adsorption transition and ν is the Flory exponent. We show that both the loop-stretch and the loop adsorption transitions provide an exceptional opportunity to construct explicit analytical expressions for the crossover functions which perfectly describe all simulation results on static properties in the finite-size scaling regime. Explicit crossover functions are based on the ansatz for the analytical form of the order parameter distributions at the respective transition points. In contrast to the close similarity in equilibrium static behavior, the dynamic relaxation at the two transitions shows qualitative differences, especially in the strongly ordered regimes. This is attributed to the fact that the surface contact dynamics in a strongly adsorbed chain is governed by local processes, whereas the end height relaxation of a strongly stretched chain involves the full spectrum of Rouse modes.

Hybrid particle-continuum simulations coupling Brownian dynamics and local dynamic density functional theory.

We present a multiscale hybrid particle-field scheme for the simulation of relaxation and diffusion behavior of soft condensed matter systems. It combines particle-based Brownian dynamics and field-based local dynamics in an adaptive sense such that particles can switch their level of resolution on the fly. The switching of resolution is controlled by a tuning function which can be chosen at will according to the geometry of the system. As an application, the hybrid scheme is used to study the kinetics of interfacial broadening of a polymer blend, and is validated by comparing the results to the predictions from pure Brownian dynamics and pure local dynamics calculations.

Anomalous critical slowdown at a first order phase transition in single polymer chains.

Using Brownian dynamics, we study the dynamical behavior of a polymer grafted onto an adhesive surface close to the mechanically induced adsorption-stretching transition. Even though the transition is first order (in the infinite chain length limit, the stretching degree of the chain jumps discontinuously), the characteristic relaxation time is found to grow according to a power law as the transition point is approached. We present a dynamic effective interface model which reproduces these observations and provides an excellent quantitative description of the simulation data. The generic nature of the theoretical model suggests that the unconventional mixing of features that are characteristic for first-order transitions (a jump in an order parameter) and features that are characteristic of critical points (an anomalous slowdown) may be a common phenomenon in force-driven phase transitions of macromolecules.

Solvent determines nature of effective interactions between nanoparticles in polymer brushes.

We study the effective interaction between two parallel rod-like nanoparticles in swollen and collapsed polymer brushes as a function of penetration depth by 2D self-consistent field calculations. In vertical direction, the interaction is always attractive. In lateral direction, the behavior under good and poor solvent conditions is qualitatively different. In swollen brushes (good solvent), nanoparticles always repel each other. In collapsed brushes (poor solvent), we identify two different regimes: an immersed regime, where the nanoparticles are fully surrounded by the brush, and an interfacial regime, where they are located in the interface between brush and solvent. In the immersed regime, the lateral interactions are repulsive, in agreement with previous theoretical predictions. In the interfacial regime, they are governed by the deformations of the interface and tend to be attractive. This implies that the nature of nanoparticle interactions can be manipulated by changing the solvent condition. The influence of particle size and grafting density are also briefly discussed.

Sharp and fast: sensors and switches based on polymer brushes with adsorption-active minority chains.

We propose a design for polymer-based sensors and switches with sharp switching transition and fast response time. The switching mechanism involves a radical change in the conformations of adsorption-active minority chains in a brush. Such transitions can be induced by a temperature change of only about ten degrees, and the characteristic time of the conformational change is less than a second. We present an analytical theory for these switches and support it by self-consistent field calculations and Brownian dynamics simulations.

Spinodal assisted growing dynamics of critical nucleus in polymer blends.

In metastable polymer blends, nonclassical critical nucleus is not a drop of stable phase in core wrapped with a sharp interface, but a diffuse structure depending on the metastability. Thus, forming a critical nucleus does not mean the birth of a new phase. In the present work, the nonclassical growing dynamics of the critical nucleus is addressed in the metastable polymer blends by incorporating self-consistent field theory and external potential dynamics theory, which leads to an intuitionistic description for the scattering experiments. The results suggest that the growth of nonclassical critical nucleus is controlled by the spinodal-decomposition which happens in the region surrounding the nucleus. This leads to forming the shell structures around the nucleus.

External potential dynamic studies on the formation of interface in polydisperse polymer blends.

The formation of interface from an initial sharp interface in polydisperse A/B blends is studied using the external potential dynamic method. The present model is a nonlocal coupling model as we take into account the correlation between segments in a single chain. The correlation is approximately expressed by Debye function and the diffusion dynamics are based on the Rouse chain model. The chain length distribution is described by the continuous Schulz distribution. Our numerical calculation indicates that for a wide range of the Flory-Huggins parameter the broadening of interface with respect to time obeys a power law at early time, and the power indices are the same for both monodisperse and polydisperse blends. The power index is larger than that in the local coupling model. However, there is no unified scaling form of the broadening of the interface width if only the interfacial width at equilibrium is taken into account as the characteristic length of the system, because the correlation makes an extra characteristic length in the system, and the polydispersity is related to this length.

Nucleation in polydisperse polymer mixtures.

The effect of polydispersity on nucleation in a metastable mixture of polydisperse polymer A and monodisperse polymer B is studied using self-consistent field theory. We adopt the continuous Schulz chain length distribution to describe the polydispersity of species A. The results show that the free energy barrier, as well as many other properties of the critical nucleus, is sensitive to the polydispersity, especially in the highly polydisperse case. This should be attributed to the fact that longer chains have stronger tendencies toward nucleation. As a result, the distribution of the volume fraction as a function of chain length in the nucleus becomes different from that in the bulk. The chain length, which corresponds to the maximum contribution to the volume fraction in the nucleus, becomes larger than the number-average chain length. Meanwhile, the interface between the critical nucleus and the parent metastable bulk phase broadens. This phenomenon is obvious when the polydispersity is high.