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We apply the dissipative particle dynamics strategy proposed by Hijón et al. [Faraday Discuss. 144, 301-322 (2010)] and based on an exact derivation of the generalized Langevin equation to cis- and trans-1,4-polybutadiene. We prove that it is able to reproduce not only the structural but also the dynamical properties of these polymers without any fitting parameter. A systematic study of the effect of the level of coarse-graining is done on cis-1,4-polybutadiene. We show that as the level of coarsegraining increases, the dynamical properties are better and better reproduced while the structural properties deviate more and more from those calculated in molecular dynamics (MD) simulations. We suggest two reasons for this behavior: the Markovian approximation is better satisfied as the level of coarse-graining increases, while the pair-wise approximation neglects important contributions due to the relative orientation of the beads at large levels of coarse-graining. Finally, we highlight a possible limit of the Markovian approximation: the fact that in constrained simulations, in which the centers-of-mass of the beads are kept constant, the bead rotational dynamics become extremely slow. [ABSTRACT FROM AUTHOR]

Describes ab initio calculations of the geometry and vibrational frequencies of the triplet state of tungsten pentacarbonyl amine. Consideration as a model for the unification of the preresonance Raman and the time-resolved infrared experiments; Factors accounting for the unexpected simultaneous lengthening of W-Ceq and C-Oeq bonds.

Presents a study on the transitional metal polyhydride complexes. Parameters of the cyclopentadienyl complex; Analyses of structures using variable nuclear magnetic resonance (NMR); Use of core potentials for osmium and carbon.

Herein, the synthesis of a novel nitroxide‐mediated polymerization (NMP) initiator bearing a photolabile ortho‐nitrobenzyl (oNB) group allowing surface‐initiated NMP preparation of well‐defined photoresponsive polystyrene grafted on silica nanoparticles is described. The photocleavable and photoresponsive properties of the prepared materials are demonstrated using small angle X‐ray scattering (SAXS) characterization. [ABSTRACT FROM AUTHOR]

We apply operational procedures available in the literature to the construction of coarse-grained conservative and friction forces for use in dissipative particle dynamics (DPD) simulations. The full procedure rely on a bottom-up approach: large molecular dynamics trajectories of n-pentane and n-decane modeled with an anisotropic united atom model serve as input for the force field generation. As a consequence, the coarse-grained model is expected to reproduce at least semi-quantitatively structural and dynamical properties of the underlying atomistic model. Two different coarse-graining levels are studied, corresponding to five and ten carbon atoms per DPD bead. The influence of the coarse-graining level on the generated force fields contributions, namely, the conservative and the friction part, is discussed. It is shown that the coarse-grained model of n-pentane correctly reproduces self-diffusion and viscosity coefficients of real n-pentane, while the fully coarse-grained model for n-decane at ambient temperature over-predicts diffusion by a factor of 2. However, when the npentane coarse-grained model is used as a building block for larger molecule (e.g., n-decane as a two blobs model), a much better agreement with experimental data is obtained, suggesting that the force field constructed is transferable to large macro-molecular systems. [ABSTRACT FROM AUTHOR]

Molecular force probes that generate optical responses to critical levels of mechanical stress (mechanochromophores) are increasingly attractive tools for identifying molecular sites that are most prone to failure. Here, a coumarin dimer mechanophore whose mechanical strength is comparable to that of the sulfur–sulfur bonds found in vulcanized rubbers is reported. It is further shown that the strain‐induced scission of the coumarin dimer within the matrix of a particle‐reinforced polybutadiene‐based co‐polymer can be detected and quantified by fluorescence spectroscopy, when cylinders of the nanocomposite are subjected to unconstrained uniaxial stress. The extent of the scission suggests that the coumarin dimers are molecular "weak links" within the matrix, and, by analogy, sulfur bridges are likely to be the same in vulcanized rubbers. The mechanophore is embedded in polymer main chains, grafting agent, and cross‐linker positions in a polymer composite in order to generate experimental data to understand how macroscopic mechanical stress is transferred at the molecular scale especially in highly entangled cross‐linked polymer nanocomposite. Finally, the extent of activation is enhanced by approximately an order of magnitude by changing the regiochemistry and stereochemistry of the coumarin dimer and embedding the mechanophore at the heterointerface of the particle‐reinforced elastomer. [ABSTRACT FROM AUTHOR]

We present a numerical study of the slip link model introduced by Likhtman for describing the dynamics of dense polymer melts. After reviewing the technical aspects associated with the implementation of the model, we extend previous work in several directions. The dependence of the relaxation modulus with the slip link density and the slip link stiffness is reported. Then the nonlinear rheological properties of the model, for a particular set of parameters, are explored. Finally, we introduce excluded volume interactions in a mean field such as manner in order to describe inhomogeneous systems, and we apply this description to a simple nanocomposite model. With this extension, the slip link model appears as a simple and generic model of a polymer melt, that can be used as an alternative to molecular dynamics for coarse grained simulations of complex polymeric systems. [ABSTRACT FROM AUTHOR]

The dynamics of polymer and filler in simplified industrial silica–styrene–butadiene nanocomposites (silica Zeosil 1165 MP, volume fraction 0–21%v) have been studied with broadband dielectric spectroscopy (BDS) and nuclear magnetic resonance (NMR). The fraction of graftable matrix chains was varied from 0 to 100%D3. The introduction of silica nanoparticles is shown to leave the segmental relaxation unaffected, an observation confirmed by the measurement of only a thin (some Angstroms thick) immobilized layer by NMR. The low-frequency measurements are resolved in two distinct dielectric Maxwell–Wagner–Sillars (MWS) processes of different behavior with respect to changes of large-scale silica structures induced by variations of filler fraction and grafting. It is found that increasing grafting leaves the first MWS-process unaffected, while it decreases the strength of the (slower) second MWS by about a decade. At constant silica volume fraction, this indicates depercolation of the filler, thereby providing a microscopic explanation of the evolution of rheological reinforcement. The sensitivity to large-scale reorganizations together with a characterization of local polymer dynamics provides insight over many length- and time-scales into structure and dynamics of nanocomposites, and thus the physical origin of the reinforcement effect. [ABSTRACT FROM AUTHOR]

Rubber-filler composites are a key component in the manufacture of tyres. The filler provides mechanical reinforcement and additional wear resistance to the rubber, but it in turn introduces non-linear mechanical behaviour to the material which most likely arises from interactions between the filler particles, mediated by the rubber matrix. While various studies have been made on the bulk mechanical properties and of the filler network structure (both imaging and by simulations), there presently does not exist any work directly linking filler particle spacing and mechanical properties. Here we show that using STEM tomography, aided by a machine learning image analysis procedure, to measure silica particle spacings provides a direct link between the inter-particle spacing and the reduction in shear modulus as a function of strain (the Payne effect), measured using dynamic mechanical analysis. Simulations of filler network formation using attractive, repulsive and non-interacting potentials were processed using the same method and compared with the experimental data, with the net result being that an attractive inter-particle potential is the most accurate way of modelling styrene-butadiene rubber-silica composite formation. [ABSTRACT FROM AUTHOR]

We report a complete analysis ofmodel silica/styrene–butadienerubber (SBR) nanocomposites including a direct and quantitative correlationbetween the filler structure and the mechanical reinforcement. Wecompared two different ways of sample processing: a solvent castingroute with well-defined colloidal silica and the manufacturing processof internal mixing with industrial silica powder. The multiscale fillerdispersion was characterized with a combination of SAXS/TEM in bothreciprocal and direct space. The mechanical properties were determinedwith oscillatory shear measurements. We evaluated the influence oftwo polymer-filler interfacial additives on the filler dispersion:a coating agent and a coupling agent for different particle concentrations.Using simple analytical functions, we succeed in modeling the fillerdispersion. We obtained surprisingly the same general trend whateverthe sample processing, solvent casting, or internal mixing. The primaryparticles form fractal primary aggregates inside the matrix as a resultof a diffusion-limited aggregation process driven by the interfacialadditive. The coupling agent, which can form covalent bonds with thematrix chains, leads to smaller and denser primary aggregates whilethe coating one gives rise to larger and more ramified objects. Thiscan be explained by the restriction of nanoparticle diffusion dueto covalent bonds. The primary aggregates arrange into a secondarylarge scale structure, agglomerates, or by a branched network. Thespatial correlations between the primary aggregates follow a Percus–Yevickfunction allowing us to distinguish between more or less interpenetratednetworks in a situation of percolation. The viscoelastic behaviorof the composites has been analyzed quantitatively with a percolationmodel. Below the percolation threshold, the reinforcement is mostlydriven by the cluster compactness. We highlight a mechanical percolationwhose threshold is dependent on the interfacial additive, but noton the material fabrication process, arising at lower filler volumefraction than the structural percolation. Above the percolation threshold,the network modulus varies as the power three of filler network densitywhich is determined from geometrical assumptions. The filler networkdensity, traducing the degree of interpenetration of the aggregatesinside the network, is driven by both the interfacial additive andthe samples preparation: the coupling agent as well as the internalmixing process gives rise to a denser network with a resulting improvedmodulus. [ABSTRACT FROM AUTHOR]

Simplified silica (Zeosil 1165 MP) and SBR (140k carryingsilanol end-groups) nanocomposites have been formulated by mixingof a reduced number of ingredients with respect to industrial applications.The thermo-mechanical history of the samples during the mixing processwas monitored and adjusted to identical final temperatures. The fillerstructure on large scales up to micrometers was studied by transmissionelectron microscopy (TEM) and very small-angle X-ray scattering (SAXS).A complete quantitative model extending from the primary silica nanoparticle(of radius ≈10 nm), to nanoparticle aggregates, up to micrometer-sizedbranches with typical lateral dimension of 150 nm is proposed. Imageanalysis of the TEM-pictures yields the fraction of zones of purepolymer, which extend between the branches of a large-scale fillernetwork. This network is compatible with a fractal of average dimension2.4 as measured by scattering. On smaller length scales, inside thebranches, small silica aggregates are present. Their average radiushas been deduced from a Kratky analysis, and it ranges between 35and 40 nm for all silica fractions investigated here (Φsi= 8–21% vol.). A central piece of our analysis isthe description of the interaggregate interaction by a simulated structurefactor for polydisperse spheres representing aggregates. A polydispersityof 30% in aggregate size is assumed, and interactions between theseaggregates are described with a hard core repulsive potential. Thesame distribution in size is used to evaluate the polydisperse formfactor. Comparison with the experimental intensity leads to the determinationof the average aggregate compacity (assumed identical for all aggregatesin the distribution, between 31% and 38% depending on Φsi), and thus aggregation number (ca. 45, with a large spread).Because of the effect of aggregate compacity and of pure polymer zones,the volume fraction of aggregates is higher in the branches than Φsi. The repulsion between aggregates has a strong effect onthe apparent isothermal compressibility: it leads to a characteristiclow-qdepression, which cannot be interpreted asaggregate mass decrease in our data. In addition, the reinforcementeffect of these silica structures in the SBR-matrix is characterizedwith oscillatory shear anddescribed with a model basedon the same aggregatecompacity. Finally, our results show that it is possibleto analyze the complex structure of interacting aggregates in nanocompositesof industrial origin in a self-consistent and quantitative manner. [ABSTRACT FROM AUTHOR]

A correction to the article "Conservative and dissipative force field for simulation of coarse-grained alkane molecules: A bottom-up approach" that was published in the 2014 issue is presented.

We present a new application of nitroxide mediated polymerization (NMP) for grafting of various elastomers onto the surfaces of silica nanoparticles (NPs) while ensuring colloidal stability and thus avoiding particle aggregation. While NMP has been used to graft chains from NP surfaces, our novel contributions are thus two-fold. We show for the first time that this methodology, initially demonstrated for styrene polymerization, can be extended to a variety of other relevant polymer chemistries, i.e., to isoprene, butadiene and styrene-butadiene rubber. We optimize the synthesis conditions for the different monomers under critical pressure conditions and characterize the grafted chains in solution by a combination of small angle x-ray and neutron scattering (with deuterated solvent labelling) to quantitatively describe the grafted chain composition. Second, crucially, we characterized the colloidal stability of the particles during the different steps of the grafting process. We have found that particle aggregation can (and does) occur at a variety of points during synthesis, and hence we have performed a detailed refined characterization of NP agglomeration and then found methods to mitigate it at each stage. These steps ensure that we can begin the preparation of composite materials from a well-defined starting point. Image 1 [ABSTRACT FROM AUTHOR]