Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

The evolution of self-healing polymers has resulted in a myriad of healing designs that have given way to complex systems capable of supporting multiple cycles, among other features. This progression enables us to propose the implementation of a timeline that classifies self-healing polymers in generations based on the healing mechanism, and correlated with historical development. The first generation employed the encapsulation of external healing agents; the second one, based on intrinsic mechanisms, applied reversible chemistries; and the third generation was inspired by natural examples such as plants and human skin, in which the healing agent is embedded in vascular networks. Despite great efforts and, with a few exceptions, polymers with high healing efficiency and high mechanical performance are not common. To improve this situation, a combination of different mechanisms is currently emerging, giving birth to the fourth generation of self-healing materials. This article, focused on self-healing elastomers, provides a rigorous overview of this new generation, in which the combination of covalent bonds and non-covalent interactions provides an optimal balance between mechanical performance and repairability. The implementation of this concept leads to materials with real commercial potential in functional applications, such as coatings, sensors, actuators, controlled release of drugs, seals, gaskets, hoses, and even high-performance applications such as tires and railway components.

/pdf/evolution-of-self-healing-elastomers-from-extrinsic-to-2ng2zi4az8.pdf

Evolution of self-healing elastomers, from extrinsic to combined intrinsic mechanisms: a review

Polymer adhesion is ubiquitous in both the natural world and human technology. It is also a complex multiscale phenomenon, such that the solution of adhesion problems requires a convergence of chem...

Polymer Adhesion: Seeking New Solutions for an Old Problem#

The vulcanization of rubber is a chemical process to improve the mechanical properties by cross-linking unsaturated polymer chains. Zinc oxide (ZnO) acts as an activator, boosting the rubbers' sulfur vulcanization. Maintaining the level of ZnO content in the rubber compounds as low as possible is desirable, not only for economic reasons but also to reduce the environmental footprint of the process. In this contribution, octylamine (OA) capped ZnO nanoparticles (5 nm diameter), prepared through a thermal decomposition method, were demonstrated to be efficient activators for the sulfur vulcanization of natural rubber, enabling the reduction of the required amount of ZnO as compared to commercial systems. The effect of different ZnO activators (OA capped ZnO/commercial indirect process ZnO) on the curing characteristics, cross-linking densities, and mechanical performance, as well as the thermal behavior of rubber compounds, were investigated. Compared to the commercial indirect process ZnO, OA capped ZnO nanoparticles not only effectively enhanced the curing efficiency of natural rubber but also improved the mechanical performance of the composites after vulcanization. This was interpreted as, by applying the OA capped ZnO nanoparticles, the ZnO levels in rubber compounding were significantly reduced under the industrial vulcanization condition (151 °C, 30 min).

Enhancing the Performance of Rubber with Nano ZnO as Activators.

Elastomers presenting good elasticity, ductility, and chemical resistance at low temperatures can serve as superior performers for explorations in extremely cold environments. However, no commercially available elastomer to date can comprehensively fulfill those demands. Here, a perfluoropolyether (PFPE)-based network crosslinked by dynamic urethane chemistry is demonstrated, which may satisfy the demands of application in ultracold environments. As the crucial constitute in such a crosslinked network, PFPE provides the elastomer with excellent elasticity at a temperature down to -110 °C and outstanding ductility within the cryogenic temperature range. Importantly, the high proportion of fluorocarbon segment also provides wonderful compatibility to most organic solvents, accounting for the low-swelling characteristics of the elastomer in sealing applications. Furthermore, the dynamic crosslinking feature allows the cured elastomer to be reprocessed like thermoplastic polymers, which affords great promise to recycle and reuse the elastomer after its disposal. Inherently, this elastomer would inspire a worldwide interest in the design of elastic devices that are adaptable to extremely low temperature.

An Ultra-Low-Temperature Elastomer with Excellent Mechanical Performance and Solvent Resistance.

Understanding the molecular mechanism of mechanical response under a cyclic loading is of fundamental importance in designing high-performance elastomers with a long service life. Herein, we investigated the mechanical response and accompanying microstructural evolution of elastomers under a long-term cyclic loading by using coarse-grained molecular dynamics simulation. The typical characteristics of the Mullins effect are successfully reproduced to confirm the validation of our simulation. Through a systematical analysis for the evolution of an intrinsic structure for a pure elastomer, we find that the Mullins effect results from chain extension and chain slippage at a low cross-linking density, whereas at a high cross-linking density, it is mainly caused by bond rupture. Particularly, the optimized cross-linking density of a pure system can lead to optimum mechanical strength and fatigue resistance because of much greater energy dissipation by polymer chains with high mobility. The filled elastomer indi...

Chemical Bond Scission and Physical Slippage in the Mullins Effect and Fatigue Behavior of Elastomers

Designing sacrificial bonds or networks has emerged as a popular strategy to fabricate high-performance elastomeric materials. Herein, we consider the properties of a special kind of double sacrifi...

Constructing Sacrificial Multiple Networks To Toughen Elastomer

Self-Assembly Strategy for Double Network Elastomer Nanocomposites with Ultralow Energy Consumption and Ultrahigh Wear Resistance

Topological copolymers with a comblike structure have been found to have the ability to recover without the need for extraneous material components and the use of external stimuli; however, a systematic and fundamental understanding at the molecular level is still lacking. In this work, coarse-grained molecular dynamics (CGMD) simulation and noncovalent interaction (NCI) analysis were employed to investigate a model system of self-healing comblike copolymers consisting of methyl methacrylate (MMA) and n-butyl acrylate (n-BA). Specifically, the different interactions of neighboring macromolecules were analyzed by NCI. Then, the influence of the following three parameters on the mechanical properties and interpenetration of side chains was probed: (1) a spacer between side chains lg, (2) average molecular weight per backbone bond mb, and (3) the flexibility of side chains kₛ. Based on the optimum of the three parameters, self-healing simulations were performed to examine the mechanical behavior of the system at different healing temperatures and healing times. The NCI results reveal that the sectional backbone chains are forced to stretch due to the steric repulsion and the side chains of neighboring molecules are interpenetrated due to the attraction of van der Waals force. Favorable steric repulsion and a spacer between side chains allow for interpenetration of side chains of neighboring macromolecules, leading to optimal mechanical properties with the graft polymer composition φ = lg/(lg + Rₛc) ≅ 3/7, where Rₛc and lg are the length of side chains and the spacer separating two consecutive side chains along the polymer backbone, respectively. Interpenetration of side chains is negatively correlated with the average molecular weight per backbone bond (mb) for graft chains. Comblike copolymers with stiff side chains tend to form lock-and-key structures, which are conducive to closer interpenetration and better mechanical properties.

Molecular Dynamics Simulations of Self-Healing Topological Copolymers with a Comblike Structure

Elastomers inevitably suffer scratches and damage during the application; thus, the design and fabrication of self-healing elastomers with covalent adaptive networks is a meaningful strategy to extend the service life of materials. In this study, a facile two-step approach was proposed to synthesize self-healing elastomers based on the dynamic oxime–carbamate bonds. Hydroxyl-terminated polybutadiene was first reacted with isophorone diisocyanate to synthesize the prepolymer with isocyanate groups terminated, followed by further reaction with dimethylglyoxime as a chain extender to obtain self-healing elastomers. Specially, all-atom molecular dynamics simulations were used to construct the same model as the experiments. Together with the experimental characterization of FTIR and 1H NMR, all-atom molecular dynamics simulations can further verify the formation of hydrogen bonds and dynamic oxime–carbamate bonds. By fixing the ratio of hydroxyl to isocyanate constant, we found that the mechanical strength increased with the increase of hard segment content. At the same time, the loss factor decreased in the glass transition region and at room temperature. Finally, the self-healing behavior of the elastomer was verified at a certain temperature. The corresponding mechanism is explained by means of molecular dynamics simulations, where dynamic oxime–carbamate bonds play more important roles than hydrogen bonds. The combined simulation and experimental studies provided a reasonable approach for the subsequent self-healing system. 

Mechanically Robust and Self-Healing Elastomers Based on Dynamic Oxime–Carbamate Bonds: A Combined Experiment and All-Atom Simulation Study

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