G Georgios Vogiatzis

Structure of Polymer Layers Grafted to Nanoparticles in Silica–Polystyrene Nanocomposites

The structural features of polystyrene brushes grafted on spherical silica nanoparticles immersed in polystyrene are investigated by means of a Monte Carlo methodology based on polymer mean field theory. The nanoparticle radii (either 8 or 13 nm) are held constant, while the grafting density and the lengths of grafted and matrix chains are varied systematically in a series of simulations. The primary objective of this work is to simulate realistic nanocomposite systems of specific chemistry at experimentally accessible length scales and study the structure and scaling of the grafted brush. The profiles of polymer density around the particles are examined; based on them, the brush thickness of grafted chains is estimated and its scaling behavior is compared against theoretical models and experimental findings. Then, neutron scattering spectra are predicted both from single grafted chains and from the entire grafted corona. It is found that increasing both the grafting density and the grafted chain molar ma...

Local Segmental Dynamics and Stresses in Polystyrene–C60 Mixtures

The polymer dynamics of homogeneous C60–polystyrene mixtures in the molten state are studied via molecular simulations using two interconnected levels of representation for polystyrene nanocomposites: (a) A coarse-grained representation, in which each polystyrene repeat unit is mapped into a single “superatom” and each fullerene is viewed as a spherical shell. Equilibration of coarse-grained polymer–nanoparticle systems at all length scales is achieved via connectivity-altering Monte Carlo simulations. (b) An atomistic representation, where both nanoparticles and polymer chains are represented in terms of united-atom forcefields. Initial configurations for atomistic molecular dynamics (MD) simulations are obtained by reverse mapping well-equilibrated coarse-grained configurations. By analyzing MD trajectories under constant energy, the segmental dynamics of polystyrene (for neat and filled systems) is characterized in terms of bond orientation time autocorrelation functions. Nanocomposite systems are foun...

Application of bi-Helmholtz nonlocal elasticity and molecular simulations to the dynamical response of carbon nanotubes

The nonlocal theory of elasticity is employed for the study of the free vibrations of carbon nanotubes (CNT). For the first time, a bi-Helmholtz operator has been used instead of the standard Helmholtz operator in a nonlocal beam model. Alongside the continuum formulation and its numerical solution, atomistic Molecular Dynamics (MD) simulations have been conducted in order to directly evaluate the eigenfrequencies of vibrating CNTs with a minimum of adjustable parameters. Our results show that the bi-Helmholtz operator is the most appropriate one to fit MD simulation results. However, the estimation of vibration eigenfrequencies from molecular simulations still remains an open (albeit well-posed) problem.

Multiscale Molecular Simulations of Polymer-Matrix Nanocomposites

Following the substantial progress in molecular simulations of polymer-matrix nanocomposites, now is the time to reconsider this topic from a critical point of view. A comprehensive survey is reported herein providing an overview of classical molecular simulations, reviewing their major achievements in modeling polymer matrix nanocomposites, and identifying several open challenges. Molecular simulations at multiple length and time scales, working hand-in-hand with sensitive experiments, have enhanced our understanding of how nanofillers alter the structure, dynamics, thermodynamics, rheology and mechanical properties of the surrounding polymer matrices.

Tracking a glassy polymer on its energy landscape in the course of elastic deformation

The response to deformation of a detailed computer model of glassy atactic polystyrene, represented as a collection of basins on its potential energy landscape, has been investigated. The volumetric behaviour of the polymer is calculated via ‘brute force’ molecular dynamics quenching simulations. Results are compared with corresponding estimates obtained by invoking the quasi-harmonic approximation (QHA) for a variety of temperatures below the glass temperature and with experimental data. The stress-controlled uniaxial deformations fall in the linear elastic regime and the resulting strains are calculated as ensemble averages of QHA estimates over 200 uncorrelated inherent structures of the potential energy landscape. The elastic constants (Youngs modulus and Poisson ratio) and their temperature dependence are in very good agreement with experiments for glassy atactic polystyrene. Additionally, a classification of the deformed inherent structures in respect to the geometry and general shape of their energy minima is undertaken. A distortion of the potential energy basins upon mechanical deformation in the elastic regime is observed in all cases.

Equation of State Based Slip Spring Model for Entangled Polymer Dynamics

A mesoscopic, mixed particle- and field-based Brownian dynamics methodology for the simulation of entangled polymer melts has been developed. Polymeric beads consist of several Kuhn segments, and their motion is dictated by the Helmholtz energy of the sample, which is a sum of the entropic elasticity of chain strands between beads, slip springs, and nonbonded interactions. Following earlier works in the field [Phys. Rev. Lett. 2012, 109, 148302], the entanglement effect is introduced by the slip springs, which are springs connecting either nonsuccessive beads on the same chain or beads on different polymer chains. The terminal positions of slip springs are altered during the simulation through a kinetic Monte Carlo hopping scheme, with rate-controlled creation/destruction processes for the slip springs at chain ends. The rate constants are consistent with the free energy function employed and satisfy microscopic reversibility at equilibrium. The free energy of nonbonded interactions is derived from an app...

Self-consistent-field study of adsorption and desorption kinetics of polyethylene melts on graphite and comparison with atomistic simulations

A method is formulated, based on combining self-consistent field theory with dynamically corrected transition state theory, for estimating the rates of adsorption and desorption of end-constrained chains (e.g., by cross-links or entanglements) from a polymer melt onto a solid substrate. This approach is tested on a polyethylene/graphite system, where the whole methodology is parametrized by atomistically detailed molecular simulations. For short-chain melts, which can still be addressed by molecular dynamics simulations with reasonable computational resources, the self-consistent field approach gives predictions of the adsorption and desorption rate constants which are gratifyingly close to molecular dynamics estimates.

Molecular Modeling and Simulation of Polymer Nanocomposites at Multiple Length Scales

The complexity of intermolecular interactions and confinement in polymer-nanoparticle systems leads to spatial variations in structure and dynamics at both the meso and nanoscale. Molecular simulation holds great promise as a means of predicting these effects and understanding their microscopic origin. In order to shed some light onto local structure and segmental dynamics of atactic polystyrene/silica (PS/SiO2) and atactic polystyrene/fullerene (PS/C60) melt systems, molecular simulations have been conducted using two interconnected levels of representation: 1) A coarse-grained representation. Equilibration of coarse-grained polymer-nanoparticle systems at all length scales is achieved via connectivity-altering Monte Carlo simulations. 2) An atomistic representation. Initial configurations for atomistic molecular dynamics (MD) simulations are obtained by reverse mapping well-equilibrated coarse-grained configurations. The local structure around a silica nanoparticle immersed in the PS matrix, PS segmental, and local dynamics in both composites and mechanical properties and entanglements in PS/SiO2 are studied.

Slip Spring-Based Mesoscopic Simulations of Polymer Networks: Methodology and the Corresponding Computational Code

In previous work by the authors, a new methodology was developed for Brownian dynamics/kinetic Monte Carlo (BD/kMC) simulations of polymer melts. In this study, this methodology is extended for dynamical simulations of crosslinked polymer networks in a coarse-grained representation, wherein chains are modeled as sequences of beads, each bead encompassing a few Kuhn segments. In addition, the C++ code embodying these simulations, entitled Engine for Mesoscopic Simulations for Polymer Networks (EMSIPON) is described in detail. A crosslinked network of cis-1,4-polyisoprene is chosen as a test system. From the thermodynamic point of view, the system is fully described by a Helmholtz energy consisting of three explicit contributions: entropic springs, slip springs and non-bonded interactions. Entanglements between subchains in the network are represented by slip springs. The ends of the slip springs undergo thermally activated hops between adjacent beads along the chain backbones, which are tracked by kinetic Monte Carlo simulation. In addition, creation/destruction processes are included for the slip springs at dangling subchain ends. The Helmholtz energy of non-bonded interactions is derived from the Sanchez–Lacombe equation of state. The isothermal compressibility of the polymer network is predicted from equilibrium density fluctuations in very good agreement with the underlying equation of state and with experiment. Moreover, the methodology and the corresponding C++ code are applied to simulate elongational deformations of polymer rubbers. The shear stress relaxation modulus is predicted from equilibrium simulations of several microseconds of physical time in the undeformed state, as well as from stress-strain curves of the crosslinked polymer networks under deformation.

Molecular modeling and simulation of atactic polystyrene/amorphous silica nanocomposites

The local structure, segmental dynamics, topological analysis of entanglement networks and mechanical properties of atactic polystyrene - amorphous silica nanocomposites are studied via molecular simulations using two interconnected levels of representation: (a) A coarse - grained level. Equilibration at all length scales at this level is achieved via connectivity - altering Monte Carlo simulations. (b) An atomistic level. Initial configurations for atomistic Molecular Dynamics (MD) simulations are obtained by reverse mapping well- equilibrated coarse-grained configurations. By analyzing atomistic MD trajectories, the polymer density profile is found to exhibit layering in the vicinity of the nanoparticle surface. The dynamics of polystyrene (in neat and filled melt systems) is characterized in terms of bond orientation. Well-equilibrated coarse-grained long-chain configurations are reduced to entanglement networks via topological analysis with the CReTA algorithm. Atomistic simulation results for the mechanical properties are compared to the experimental measurements and other computational works.