1,720,973 research outputs found
Combination of Hybrid Particle-Field Molecular Dynamics and Slip-Springs for the Efficient Simulation of Coarse-Grained Polymer Models: Static and Dynamic Properties of Polystyrene Melts
No full textA quantitative prediction of polymer-entangled dynamics based on molecular simulation is a grand challenge in contemporary computational material science. The drastic increase of relaxation time and viscosity in high-molecular-weight polymeric fluids essentially limits the usage of classic molecular dynamics simulation. Here, we demonstrate a systematic coarse-graining approach for modeling entangled polymers under the slip-spring particle-field scheme. Specifically, a frequency-controlled slip-spring model, a hybrid particle-field model, and a coarse-grained model of polystyrene melts are combined into a hybrid simulation technique. Via a rigorous parameterization strategy to determine the parameters in slip-springs from existing experimental or simulation data, we show that the reptation behavior is clearly observed in multiple characteristics of polymer dynamics, mean-square displacements, diffusion coefficients, reorientational relaxation, and Rouse mode analysis, consistent with the predictions of the tube theory. All dynamical properties of the slip-spring particle-field models are in good agreement with classic molecular dynamics models. Our work provides an efficient and practical approach to establish chemical-specific coarse-grained models for predicting polymer-entangled dynamics
From Mesoscale Back to Atomistic Models: A Fast Reverse-Mapping Procedure for Vinyl Polymer Chains
No full textThis paper introduces a systematic procedure to obtain well-relaxed atomistic melt structures from mesocale models of vinyl polymers based on sequences of diads. Following the methodology introduced by Milano and Müller-Plathe [J. Phys. Chem. B. 2005, 109, 18609], coarse-grain models consisting of sequences of superatoms of two different types meso and racemo have been used to relax mesocale melts of atactic and syndiotactic polystyrene. The proposed method, based on a fully geometrical approach, does not involve expensive potential energy and force evaluations and allows a very fast and efficient reconstruction of the atomistic detail. The method, successfully tested against experimental data, allows us to obtain all atom models of both stereoregular and stereoirregular polymers and opens the possibility of relaxing large molecular weight melts of vinyl chains
Fast relaxation of coarse-grained models of polymer interphases by hybrid particle-field molecular dynamics: Polystyrene-silica nanocomposites as an example
No full textPolymer composites attract large attention for their industrial use because of their unique features. The preparation of equilibrated melts of long entangled chains in the presence of a solid nanoparticle in molecular dynamics simulations is a very difficult task due to the slow relaxation time. We present a coarse-grained (CG) model suitable for polymer nanocomposites which combines Iterative-Boltzmann-Inversion derived polymer models, the hybrid particle-field representation of non-bonded interactions, and a convenient description of a solid nanoparticle suitable for hybrid particle-field models. The proposed approach is applied to test simulations of well characterized polystyrene-silica nanocomposites models. Finally, procedures for an efficient relaxation of pure polymer melts and interphase structures of large molecular weight nanocomposites are proposed
Backmapping coarse-grained polymer models under sheared nonequilibrium conditions
No full textThe method of re-introducing atomistic detail into a coarse-grained polymer structure, so-called backmapping, is extended to a nonequilibrium situation. Problems in backmapping coarse-grained polymer models, on which a nonequilibrium shear flow has been imposed, are discussed. A backmapping protocol, where the globally deformed conformations are maintained during backmapping by applying position restraints, is proposed. The local optimization of the atomistic structure is performed in the presence of these restraints. The artifact of segment isolation introduced by position restraints is minimized by applying different restraint patterns iteratively. The procedure is demonstrated on the test case of atactic polystyrene under a steady shear flow. © the Owner Societies 2009
Coarse-Grained and Reverse-Mapped United-Atom Simulations of Long-Chain Atactic Polystyrene Melts: Structure, Thermodynamic Properties, Chain Conformation, and Entanglements
No full textA coarse-grained model of atactic polystyrene, in which meso and racemo diads are represented as single '' superatoms,'' parametrized using Iterative Boltzmann Inversion, has been subjected to connectivity-altering Monte Carlo simulations in order to simulate monodisperse atactic polystyrene melts of molar mass up to 210000 g mol(-1) at 500 or 413 K and 1 bar. Analysis of the Monte Carlo results reveals excellent equilibration of chain conformations at all length scales. Chain dimensions, as determined from the mean square end-to-end distance, the mean square radius of gyration, and simulated Kratky plots of the single-chain scattering function, are in excellent agreement with experiment. The equilibrated long-chain configurations are reduced to entanglement networks via topological analysis with the CReTA algorithm. The resulting Kuhn length of primitive paths provides an excellent estimate of the molar mass between entanglements and of the entanglement tube diameter extracted from plateau modulus measurements. The distribution of strand lengths between entanglements, when appropriately reduced, follows the same master curve as previously determined distributions of polyethylene, cis-1,4 polybutadiene, and poly(ethylene terephthalate). A new strategy is introduced for reverse mapping the long-chain coarse-grained configurations into detailed united-atom configurations in a manner that preserves the sequence of diad types along the chains. This strategy employs local '' flip '' Monte Carlo moves to relax the reverse-mapped configurations. Relaxation starts using bonded interactions only, and proceeds by gradually introducing nonbonded interactions. Final relaxation is achieved via short-time canonical molecular dynamics simulation. Predicted wide-angle X-ray diffraction patterns from reverse-mapped configurations are indistinguishable from those of short-chain melts equilibrated directly in the united atom representation using molecular dynamics, and in favorable agreement with experiment. Distributions of torsion angles and pairs of successive torsion angles in the reverse-mapped configurations exhibit some deviations from the corresponding distributions of directly equilibrated short-chain united atom melts and from experimental NMR measurements
Influence of Polymer Bidispersity on the Effective Particle-Particle Interactions in Polymer Nanocomposites
No full textWe investigate the role played by the bidispersity of polymer chains on the local structure and the potential of mean force (PMF) between silica nanoparticles (NPs) in a polystyrene melt. We use the hybrid particle-field molecular dynamics technique which allows us to efficiently relax polymer nanocomposites even with high molecular weights. The NPs we investigate are either bare or grafted with polystyrene chains immersed in a melt of free polystyrene chains, whereas the grafted and the free polystyrene chains are either monodisperse or bidisperse. The two-body PMF shows that a bidisperse distribution of free polymer chains increases the strength of attraction between a pair of ungrafted NPs. If the NPs are grafted by polymer chains, the effective interaction crucially depends on the bidispersity and grafting density of the polymer chains: for low grafting densities, the bidispersity of both free and grafted chains increases the repulsion between the NPs, whereas for high grafting densities we observe two different effects. An increase of bidispersity in free chains causes the rise of the repulsion between the NPs, while an increase of bidispersity in grafted chains promotes the rise of attraction. Additionally, a proper treatment of multibody interactions improves the simpler two-body PMF calculations, in both unimodal and bimodal cases. We found that, by properly tuning the bidispersity of both free and grafted chains, we can control the structure of the composite materials, which can be confirmed by experimental observations. As a result, the hybrid particle-field approach is confirmed to be a valid tool for reproducing and predicting microscopic interactions, which determine the stability of the microscopic structure of the composite in a wide range of conditions
Molecular structure and multi-body potential of mean force in silica-polystyrene nanocomposites
No full textWe perform a systematic application of the hybrid particle-field molecular dynamics technique [Milano, et al., J. Chem. Phys., 2009, 130, 214106] to study interfacial properties and potential of mean force (PMF) for separating nanoparticles (NPs) in a melt. Specifically, we consider Silica NPs bare or grafted with Polystyrene chains, aiming to shed light on the interactions among free and grafted chains affecting the dispersion of NPs in the nanocomposite. The proposed hybrid models show good performances in catching the local structure of the chains, and in particular their density profiles, documenting the existence of the "wet-brush-to-dry-brush" transition. By using these models, the PMF between pairs of ungrafted and grafted NPs in Polystyrene matrix are calculated. Moreover, we estimate the three-particle contribution to the total PMF and its role in regulating the phase separation on the nanometer scale. In particular, the multi-particle contribution to the PMF is able to give an explanation of the complex experimental morphologies observed at low grafting densities. More in general, we propose this approach and the models utilized here for a molecular understanding of specific systems and the impact of the chemical nature of the systems on the composite final properties
Atomistic hybrid particle-field molecular dynamics combined with slip-springs: Restoring entangled dynamics to simulations of polymer melts
No full textIn hybrid particle-field (hPF) simulations (J. Chem. Phys., 2009 130, 214106), the entangled dynamics of polymer melts is lost due to chain crossability. Chains cross, because the field-treatment of the nonbonded interactions makes them effectively soft-core. We introduce a multi-chain slip-spring model (J. Chem. Phys., 2013 138, 104907) into the hPF scheme to mimic the topological constraints of entanglements. The structure of the polymer chains is consistent with that of regular molecular dynamics simulations and is not affected by the introduction of slip-springs. Although slight deviations are seen at short times, dynamical properties such as mean-square displacements and reorientational relaxation times are in good agreement with traditional molecular dynamics simulations and theoretical predictions at long times
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