The effect of bond length on the structure of dense bead–spring polymer melts

Crystallization of finite-extensible nonlinear elastic Lennard-Jones coarse-grained polymers

The ability of a simple coarse-grained finite-extensible nonlinear elastic (FENE) Lennard-Jones (LJ) polymer model to be crystallized is investigated by molecular dynamics simulations. The optimal FENE Lennard-Jones parameter combinations (ratio between FENE and LJ equilibrium distances) and the optimal lattice parameters are calculated for five different perfect crystallite structures: simple tetragonal, body-centered tetragonal, body-centered orthorhombic, hexagonal primitive, and hexagonal close packed. It was found that the most energetically favorable structure is the body-centered orthorhombic. Starting with an equilibrated polymer liquid and with the optimal parameters found for the body-centered orthorhombic, an isothermal treatment led to the formation of large lamellar crystallites with a typical chain topology: folded, loop, and tie chains, and with a crystallinity of about 60%-70%, similar to real semicrystalline polymers. This simple coarse-grained Lennard-Jones model provides a qualitative tool to study semicrystalline microstructures for polymers.

Effect of chain stiffness and temperature on the dynamics and microstructure of crystallizable bead-spring polymer melts

We contrast the dynamics in model unentangled polymer melts of chains of three different stiffnesses: flexible, intermediate, and rodlike. Flexible and rodlike chains, which readily solidify into close-packed crystals (respectively, with randomly oriented and nematically aligned chains), display simple melt dynamics with Arrhenius temperature dependence and a discontinuous change upon solidification. Intermediate-stiffness chains, however, are fragile glass-formers displaying Vogel-Fulcher dynamical arrest, despite the fact that they also possess a nematic-close-packed crystalline ground state. To connect this difference in dynamics to the differing microstructure of the melts, we examine how various measures of structure, including cluster-level metrics recently introduced in studies of colloidal systems, vary with chain stiffness and temperature. No clear static-structural cause of the dynamical arrest is found. However, we find that the intermediate-stiffness chains display qualitatively different dynamical heterogeneity. Specifically, their stringlike motion (cooperative rearrangement) is correlated along chain backbones in a way not found for either flexible or rodlike chains. This activated "crawling" motion is clearly associated with the dynamical arrest observed in these systems, and illustrates one way in which factors controlling the crystallization versus glass formation competition in polymers can depend nonmonotonically on chain stiffness.

Effect of chain stiffness on the competition between crystallization and glass-formation in model unentangled polymers

We map out the solid-state morphologies formed by model soft-pearl-necklace polymers as a function of chain stiffness, spanning the range from fully flexible to rodlike chains. The ratio of Kuhn length to bead diameter (lK/r0) increases monotonically with increasing bending stiffness kb and yields a one-parameter model that relates chain shape to bulk morphology. In the flexible limit, monomers occupy the sites of close-packed crystallites while chains retain random-walk-like order. In the rodlike limit, nematic chain ordering typical of lamellar precursors coexists with close-packing. At intermediate values of bending stiffness, the competition between random-walk-like and nematic chain ordering produces glass-formation; the range of kb over which this occurs increases with the thermal cooling rate |Ṫ| implemented in our molecular dynamics simulations. Finally, values of kb between the glass-forming and rodlike ranges produce complex ordered phases such as close-packed spirals. Our results should provide a useful initial step in a coarse-grained modeling approach to systematically determining the effect of chain stiffness on the crystallization-vs-glass-formation competition in both synthetic and colloidal polymers.

Dynamic structure of unentangled polymer chains in the vicinity of non-attractive nanoparticles

Using coarse-grained molecular dynamics simulation, we study the motion of unentangled polymer chains dynamically confined by non-attractive nanoparticles (NPs). Both normal mode and dynamic structure factor S(q, t) analysis are adopted to analyze chain's dynamics. Relaxation behaviors of chains are found to be significantly slowed down by NPs. The relaxation times of chain's normal modes are monotonically increasing with the NP volume fraction ϕ. At the same time, chains' dynamics are becoming non-Gaussian. Inspection of S(q, t) reveals that chain's dynamics can be attributed to two 'phases', a bulk polymer phase and a confined polymer phase between NPs. The dynamics of a confined polymer is slower than that of a bulk polymer, while still exhibiting high mobility. The amount of the bulk polymer phase is found to exponentially decay with increasing ϕ. With this figure at hand, we establish a simple relationship between NP and confined/interphase polymer volume fractions. This work seems to provide the first quantitative prediction on the relationship between NP and confined/interphase polymer volume fractions.

A general method for spatially coarse-graining Metropolis Monte Carlo simulations onto a lattice

A recently introduced method for coarse-graining standard continuous Metropolis Monte Carlo simulations of atomic or molecular fluids onto a rigid lattice of variable scale [X. Liu, W. D. Seider, and T. Sinno, Phys. Rev. E 86, 026708 (2012)] is further analyzed and extended. The coarse-grained Metropolis Monte Carlo technique is demonstrated to be highly consistent with the underlying full-resolution problem using a series of detailed comparisons, including vapor-liquid equilibrium phase envelopes and spatial density distributions for the Lennard-Jones argon and simple point charge water models. In addition, the principal computational bottleneck associated with computing a coarse-grained interaction function for evolving particle positions on the discretized domain is addressed by the introduction of new closure approximations. In particular, it is shown that the coarse-grained potential, which is generally a function of temperature and coarse-graining level, can be computed at multiple temperatures and scales using a single set of free energy calculations. The computational performance of the method relative to standard Monte Carlo simulation is also discussed.

Coarse-grained lattice Monte Carlo simulations with continuous interaction potentials

A coarse-grained lattice Metropolis Monte Carlo (CG-MMC) method is presented for simulating fluid systems described by standard molecular force fields. First, a thermodynamically consistent coarse-grained interaction potential is obtained numerically and automatically from a continuous force field such as Lennard-Jones. The coarse-grained potential then is used to drive CG-MMC simulations of vapor-liquid equilibrium in Lennard-Jones, square-well, and simple point charge water systems. The CG-MMC predicts vapor-liquid phase envelopes, as well as the particle density distributions in both the liquid and vapor phases, in excellent agreement with full-resolution Monte Carlo simulations, at a fraction of the computational cost.

Calculation of local mechanical properties of filled polymers

A study is presented on the effects of smooth nanoparticles on the structure and elastic moduli of a polymer matrix. Structural changes between the unfilled polymer matrix and the nanocomposite give rise to the formation of a glassy layer that surrounds the nanoparticles. Results for the effects of particle size and concentration on the local and overall mechanical properties of the polymer are consistent with experimental macroscopic observations. At the molecular level, it is found that dispersed, attractive nanoparticles alter the nonaffine displacement fields that arise in the polymer glass upon deformation, thereby rendering the nanocomposite glass less fragile.

Coarse-Grained Molecular Dynamics Modeling of Associating Fluids: Thermodynamics, Liquid Structure, and Dynamics in the Limit of Zero Association Strength

A continuous coarse-grained potential model for associating fluids, consisting of an off-center specific site bonded with a harmonic potential to a center particle, has been developed and used in canonical molecular dynamics simulations. The thermodynamic, structural, and dynamic properties of the limiting nonassociating reference coarse-grained fluid are investigated as functions of the mass distribution and bond strength between center and site particles. It is theoretically shown and confirmed by simulation that in this limit variations in these potential parameters do not alter the equation of state of the reference coarse-grained fluid but that they have profound influences on both the translational and the rotational dynamics. From the simulation results we arrive at some guidelines that should be kept in mind in the selection of appropriate values for the model parameters. This work provides the precursory knowledge for the study of coarse-grained associating fluids using the conventional molecular dynamics method.

Coarse-Grained Molecular Modeling of Composite Interfaces

The interface of fiber and matrix strongly influences the performance and strength of fiber-reinforced composite materials. Due to the limitations of continuum mechanics at the nanometer length scale, atomistic level computer simulation has started to play an important role in the understanding of such interfacial systems. Our study focuses on a typical crosslinked interfacial system of glass-epoxy composite with the presence of silanes. To explore the mechanical properties of the interfacial network system, Coarse-grained Molecular Dynamics is used. Currently it is not possible to study mechanical properties of interfacial systems purely through ab initio molecular dynamics simulations because of the huge computational resources required. Although pure atomistic classical molecular dynamics simulations have been used to study systems comprising billions of atoms, classical MD simulation do not take into account the effects of crosslinking of molecular chains. A new force field, which combines the Lennard-Jones potential and a finiteextensible nonlinear elastic attractive potential, is proposed and incorporated in a bead-spring model to simulate glass/epoxy interfacial system with the crosslinked structure of silanes. The finite-extensible nonlinear elastic attractive potential is included to control the motion and breakage of polymer chains. Interfacial adhesion and mechanical properties were studied through the simulation of mechanically separating the interfacial system.

Local mechanical properties of polymeric nanocomposites

The inclusion of a nanoparticle into a polymer matrix is studied by efficient Monte Carlo simulations. The resulting structural changes in the melt and glass exhibit a strong dependence on the strength of the polymer attraction to the surface of the filler. The mechanical properties of the nanocomposite are analyzed in detail through a formalism that permits calculation of local elastic constants. The average shear and Young's modulus of the nanocomposite are higher than those of the pure polymer for neutral or attractive nanoparticles. For repulsive particles, these moduli are lower. Simulation of local properties reveals that a glassy layer is formed in the vicinity of the attractive filler, contributing to the increased strength of the composite material. In contrast, a region of negative moduli emerges around repulsive fillers, which provides a mechanism for frustration relief and a lowering of the glass transition temperature.

Molecular dynamics study of the thermal and the density effects on the local and the large-scale motion of polymer melts: scaling properties and dielectric relaxation

Results from a molecular dynamics simulation of a melt of unentangled polymers are presented. The translational motion, the large-scale and the local reorientation processes of the chains, as well as their relations with the so-called "normal" and "segmental" dielectric relaxation modes are thoroughly investigated in wide temperature and pressure ranges. The thermodynamic states are well fitted by the phenomenological Tait equation of state. A global time-temperature-pressure superposition principle of both the translational and the rotational dynamics is evidenced. The scaling is more robust than the usual Rouse model. The latter provides insight but accurate comparison with the simulation calls for modifications to account for both the local chain stiffness and the nonexponential relaxation. The study addresses the issue whether the temperature or the density is a dominant control parameter of the dynamics or the two quantities give rise to comparable effects. By examining the ratio /alphatau//alphaP between the isochronic and isobaric expansivities, one finds that the temperature is dominant when the dynamics is fast. If the relaxation slows down, the fluctuations of the free volume increase their role and become comparable to those of the thermal energy. Detectable cross-correlation between the "normal-mode" and the "segmental" dielectric relaxations is found and contrasted with the usual assumption of independent modes.

Kinetics of a polysoap collapse

We study the dynamics of collapse of a polysoap by means of large-scale molecular dynamics simulation and scaling arguments. A polysoap consists of a hydrophilic backbone and hydrophobic side chains attached at regular intervals along the backbone. In selective solvent conditions, the hydrophobic components aggregate, forcing the hydrophilic backbone to form loops anchored at the surface of the core, ultimately forming a micelle. The kinetics of polysoap collapse includes two major mechanisms: (1) early aggregation of the hydrophobic side chains controlled by first-order kinetics whose rate constant is given by a contact probability and (2) coalescence into larger clusters which requires activation to overcome energy barriers due to excluded volume repulsions between intermediate micelle coronas. In the late stage, the energy barrier is increasing as p(3/2), with p the number of aggregated side chains in an intermediate micelle. The corresponding late-stage rate constant decays exponentially as approximately exp(-p(3/2)).

Anomalous pressure dependence of the structure factor in 1,4-polybutadiene melts: a molecular dynamics simulation study

Neutron scattering has shown the first diffraction peak in the structure factor of a 1,4-polybutadiene melt under compression to move to larger q values as expected but to decrease significantly in intensity. Simulations reveal that this behavior does not result from loss of structure in the polymer melt upon compression but rather from the generic effects of differences in the pressure dependence of the intermolecular and intramolecular contributions to the melt structure factor and differences in the pressure dependence of the partial structure factors for carbon-carbon and carbon-deuterium intermolecular correlations. This anomalous pressure dependence is not seen for protonated melts.

BPA-PC on a Ni111 surface: the interplay between adsorption energy and conformational entropy for different chain-end modifications

We extend a previous dual scale modeling approach for the behavior of polymers near a metal surface to a variety of end groups. Our approach combines a coarse-grained polymer model with ab initio DFT calculations. Such a procedure was applied to a melt of phenolic-like terminated Bisphenol A-polycarbonate (BPA-PC) interacting with a (111) nickel surface (Delle Site, L.; Abrams, C. F.; Alavi, A.; Kremer, K. Phys. Rev. Lett. 2002, 89, 156103. Abrams, C. F.; Delle Site, L.; Kremer, K. Phys. Rev. E 2003, 67, 021807). This work extends this study to different chain-end modifications of BPA-PC, p-tert-butylphenolic, p-tetramethylpropylphenolic, and p-cumylphenolic. We show how the interplay between adsorption energies and conformational entropy selects different morphologies for the various melts at the interface. Implications of these results for realistic technical materials are finally discussed.

Arrested swelling of highly entangled polymer globules

Upon aging, a collapsed long chain evolves from a crumpled state to a self-entangled globule which can be thought of as a large knot. Swelling of an equilibrium globule in good solvent is a two-step process: (i) fast swelling into an arrested stretched structure with conserved entanglement topology followed by (ii) slow disentanglement. Using computer simulation, we found both mass-mass (m-m) and entanglement-entanglement (e-e) power law correlations inside the swollen globule. The m-m correlations are characterized by a set of two exponents in agreement with a Flory-type argument. The e-e correlations are also characterized by two exponents, both of them larger (by approximately 0.3) than the related m-m exponents. We interpret this difference as evidence of distance-dependent repulsion E=-0.3ln((rho)k(B)T between entanglements sliding along the polymer chain.

Dual-resolution coarse-grained simulation of the bisphenol-A-polycarbonate/nickel interface

We present a dual-resolution coarse-graining scheme for efficient molecular dynamics simulations of bisphenol-A-polycarbonate (BP-A-PC) liquids in contact with a (111) nickel surface. The essential feature of this model is the strong adsorption of phenoxy chain ends, and the absence of adsorption by other parts of the chains. Details of how phenoxy chain ends interact with the nickel surface were extracted from Car-Parrinello molecular dynamics calculations of adsorption of phenol on nickel. These calculations show that phenol adsorption on nickel is short ranged (<3 A) and strongly dependent on the C1-C4 orientation of the ring. The structure of BP-A-PC prevents internal phenylene groups from interacting with the surface, due to steric hindrances from the noninteracting isopropylidenes. These dependencies are incorporated in the coarse-grained model of the BP-A-PC chain by resolving chain-terminating carbonate groups with atomistic detail, while the rest of the chain is represented by coarsened "beads." This allows specification of the C1-C4 orientation of the terminal phenoxy groups, while overall allowing for system equilibration with reasonable computer time. We simulate liquids of up to 240 chains of ten chemical repeat units, confined in a slit pore formed by two frozen (111) planes of atoms with the lattice spacing of nickel. We find that the strong adsorption of chain ends has a large effect on the liquid structure through a distance of more than two bulk radii of gyration from the surface. These effects are explained by a competition among single- and double-end adsorption, and dense packing. The structure of the interface less than 10 A from the wall is greatly sensitive to the orientational dependence of the phenoxy adsorption.

Polymers near metal surfaces: selective adsorption and global conformations

We study the properties of a polycarbonate melt near a nickel surface as a model system for the interaction of polymers with metal surfaces by employing a multiscale modeling approach. For bulk properties, a suitably coarse-grained bead-spring model is simulated by molecular dynamics methods with model parameters directly derived from quantum chemical calculations. The surface interactions are parametrized and incorporated by extensive quantum mechanical density functional calculations using the Car-Parrinello method. We find strong chemisorption of chain ends, resulting in significant modifications of the melt composition when compared to an inert wall.