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

Engineering applications of synthetic polymers are widespread due to their availability, processability, low density, and diversity of mechanical properties (Figure 1a). Despite their ubiquitous nature, modern polymers are evolving into multifunctional systems with highly sophisticated behavior. These emergent functions are commonly described as “smart” characteristics whereby “intelligence” is rooted in a specific response elicited from a particular stimulus. Materials that exhibit stimuli-responsive functions thus achieve a desired output (O, the response) upon being subjected to a specific input (I, the stimulus). Given that mechanical loading is inevitable, coupled with the wide range of mechanical properties for synthetic polymers, it is not surprising that mechanoresponsive polymers are an especially attractive class of smart materials. To design materials with stimuli-responsive functions, it is helpful to consider the I-O relationship as an energy transduction process. Achieving the desired I-O linkage thus becomes a problem in finding how to transform energy from the stimulus into energy that executes the desired response. The underlying mechanism that forms this I-O coupling need not be a direct, one-step transduction event; rather, the overall process may proceed through a sequence of energy transduction steps. In this regard, the network of energy transduction pathways is a useful roadmap for designing stimuli-responsive materials (Figure 1b). It is the purpose of this review to broadly survey the mechanical to chemical * To whom correspondence should be addressed. Phone: 217-244-4024. Fax: 217-244-8024. E-mail: jsmoore@illinois.edu. † Department of Chemistry and Beckman Institute. ‡ Department of Materials Science and Engineering and Beckman Institute. § Department of Aerospace Engineering and Beckman Institute. Chem. Rev. XXXX, xxx, 000–000 A

Mechanically-induced chemical changes in polymeric materials

Wormlike micelles are elongated flexible self-assembly structures formed by the aggregation of amphiphiles. Above a threshold concentration, they entangle into a dynamic network, reminiscent of polymer solutions, and display remarkable visco-elastic properties, which have been exploited in numerous industrial and technological fields. Relating the microstructure of these intricate structures with their bulk properties is still an ongoing quest. In this review, we present a classification of wormlike micelles, with a focus on novel systems and applications. We describe the current state of understanding of their linear rheology and give a detailed account of recent progress in small-angle neutron scattering, a particularly powerful technique to elucidate their microstructure on a wide range of length-scales.

Wormlike micelles: where do we stand? Recent developments, linear rheology and scattering techniques

ESPResSo - an extensible simulation package for research on soft matter systems

Mechanical deformability underpins many of the advantages of organic semiconductors. The mechanical properties of these materials are, however, diverse, and the molecular characteristics that permit charge transport can render the materials stiff and brittle. This review is a comprehensive description of the molecular and morphological parameters that govern the mechanical properties of organic semiconductors. Particular attention is paid to ways in which mechanical deformability and electronic performance can coexist. The review begins with a discussion of flexible and stretchable devices of all types, and in particular the unique characteristics of organic semiconductors. It then discusses the mechanical properties most relevant to deformable devices. In particular, it describes how low modulus, good adhesion, and absolute extensibility prior to fracture enable robust performance, along with mechanical “imperceptibility” if worn on the skin. A description of techniques of metrology precedes a discussion...

Mechanical Properties of Organic Semiconductors for Stretchable, Highly Flexible, and Mechanically Robust Electronics

The viscoelastic properties of high molecular weight polymeric liquids are dominated by topological constraints on a molecular scale. In a manner similar to that of entangled ropes, polymer chains can slide past but not through each other. Tube models of polymer dynamics and rheology are based on the idea that entanglements confine a chain to small fluctuations around a primitive path that follows the coarse-grained chain contour. Here we provide a microscopic foundation for these highly successful phenomenological models. We analyze the topological state of polymeric liquids in terms of primitive paths and obtain parameter-free, quantitative predictions for the plateau modulus, which agree with experiment for all major classes of synthetic polymers.

https://science.sciencemag.org/content/sci/303/5659/823.full.pdf

Rheology and microscopic topology of entangled polymeric liquids.

(LAMMPS) was used to obtain atomistic insights in slip mechanisms , their dependence on crystal orientation, and the differences in the frictional behavior between low stacking fault energy face-centered-cubic (FCC) copper and hexagonal-close-packed (HCP) magnesium single crystals. Simulations performed as a function of applied sliding velocity revealed that the coefficient of friction (COF) increased with increasing sliding velocity, contact area, and dislocation density for both Mg and Cu.

Large-scale Atomic/Molecular Massively Parallel Simulator

Small-angle neutron and X-ray scattering data have been obtained for micelles of d-polystyrene-polyisoprene (d-PS-PI) of relatively high molecular weight in n-decane. Contrast variation was performed using mixtures of hydrogenated and deuterated decane. Three samples were investigated with d-polystyrene and polyisoprene molar masses of, respectively, 12 000 and 48 000, 40 000 and 40 000, and 40 000 and 80 000. For the two latter samples, the concentration of the polymer was also varied. The data obtained at relatively high resolution were analyzed together with small-angle X-ray scattering data using scattering functions recently derived from Monte Carlo simulations for a model with a spherical core and a corona of semiflexible chains interacting with a hard-core potential. The scattering from the model can be generated by assuming an analytical form of the radial distribution of the corona and an effective single chain form factor of the random-phase approximation type. In the analysis of the experimental scattering data intermicellar interactions were modeled by an effective hard-sphere model. The analysis of the experimental data provides information on shape, aggregation number, polydispersity, core size, core solvation, corona shape/size, and the interactions between the chains in the corona, which are significant for these micelles. The shape of the corona profile depends on the surface coverage of the micelles as well as the curvature of the core-corona interface. For high curvatures the profile is in agreement with a power-law behavior as predicted by scaling theory. For low curvatures the profile is more compact. The profiles and scattering curves are very well reproduced by Monte Carlo simulations based on the parameters for the structures determined in the analysis of the experimental scattering data. The study shows that two parameters are decisive for the profile shape and internal correlations, namely the reduced surface coverage and the curvature.

http://wwwcourses.sens.buffalo.edu/ce435/PedersenEtAl03.pdf

A Small-Angle Neutron and X-ray Contrast Variation Scattering Study of the Structure of Block Copolymer Micelles: Corona Shape and Excluded Volume Interactions

Small-angle neutron and X-ray scattering are techniques, which are frequently used for studying the structure and interactions of block copolymer micelles. Recent developments of models for the analysis of small-angle scattering data are reviewed. The most recent models, based on Monte Carlo simulations, are able to provide information on shape, aggregation number, polydispersity, core size, core solvation, corona shape/size, and on the interactions between the chains in the corona.

Scattering from block copolymer micelles

La relation entre les proprietes viscoelastiques complexes de liquides polymeres et leur structure microscopique et la dynamique est une question cle dans la science des materiaux et de la biophysique. Les theories modernes de la dynamique des polymeres et la rheologie decrivent les aspects universels du comportement viscoelastique sur la base de l'idee que les enchevetrements moleculaires confinent filaments individuels a une dimension, la dynamique diffusifs (reptation) dans le tube-comme les regions dans l'espace. Alors que le modele de tube est valide par son succes, ses elements constitutifs (les statistiques et la dynamique de l'axe du tube ou des chemins primitifs et de l'confinement "cage" de chaines voisines) ne sont pas directement observables. (1) Nous presentons un vaste ensemble de resultats de simulation pour la relaxation des contraintes a l'equilibre et l'etape-tendues perles printemps polymeres fondus (En collaboration avec: C. Svaneborg et GS Grest). Les donnees nous permettent d'explorer la dynamique de la chaine et le module de la relaxation de cisaillement dans le regime plateau pour les chaines avec Z~40 enchevetrements et dans le regime de relaxation terminale pour Z~10. Nous avons effectue des tests sans parametres de plusieurs modeles differents de tubes a l'aide de (Rouse) la mobilite connue des chaines unentangled et la longueur d'enchevetrement de fusion determine par l'analyse du chemin primitif de l'etat microscopique topologique de nos systemes. (2) Nous presentons une comprehension complete pour la detente des empetre polymere lineaire fond que les liens de la dynamique et la theorie de Rouse tube par une interpretation dynamique qui s’appellel’analyse du chemin primitive. La chaine primitive, qui est la moyenne d'ensemble des conformations de la chaine, se retrecit strictement d’apres la dynamique Rouse jusqu'a ce qu'il renconte les obstacles formes par d'autres chaines primitives. Le temps d'arret de la diminution peut etre determinee par l'argument que la zone balayee par la chaine primitive sur une longueur de propagation de tension qui est egale a la taille du maille de filet du travail forme par les chaines de primitives. Le processus physique avant l'heure d'arret est assez presente par l'analyse du chemin primitif. Apres le temps d'arret, les longueurs primitives seront retrecites par la reptation et la fluctuation de la longueur de contour .Cette procedure peut etre decrite comme la modele du tube, par exemple, Likhtman-McLeish (LM) la theorie. (3) Nous constatons que la theorie sous-estime la relaxation LM module de cisaillement du a un double comptage de l'effet de courte longueur d'onde (p> Z) dans les modes partie de relaxation de Rouse et en fonction de la trompe de memoire μ (t). LM extrapole μ (t) a la limite du continuum, ce qui entraine une decroissance sur des echelles de temps inferieur au temps de l'intrication, ou le mouvement de la chaine primitive devrait etre negligeable. Pour corriger cela, nous avons retire de la partie de fluctuation contour longueur de μ (t) la contribution des modes avec un temps de relaxation plus court que le temps d'enchevetrement. Nous trouvons un excellent accord entre nos donnees de simulation et la theorie LM modifiee en utilisant l'approximation reptation double pour la liberation de contrainte, ce qui demonte que l'analyse du chemin primitif de la structure microscopique apporte du modele de tube avec une puissance predictive des processus dynamiques. L'utilisation de systemes plus complexes pour le traitement de la liberation de contrainte devrait conduire a un accord encore mieux.

Carsten Svaneborg

Papers

Rheology and microscopic topology of entangled polymeric liquids.

Large-scale Atomic/Molecular Massively Parallel Simulator

A Small-Angle Neutron and X-ray Contrast Variation Scattering Study of the Structure of Block Copolymer Micelles: Corona Shape and Excluded Volume Interactions

Scattering from block copolymer micelles

Stress relaxation in entangled polymer melts.