Papers by Doros Theodorou
Macromolecules, 2020
A simulation strategy encompassing different scales was applied to the systematic study of the ef... more A simulation strategy encompassing different scales was applied to the systematic study of the effects of CO 2 uptake on the properties of atactic polystyrene (aPS) melts. The analysis accounted for the influence of temperature between 450 and 550 K, polymer molecular weights (M w) between 2100 and 31000 g/mol, and CO 2 pressures up to 20 MPa on the volumetric, swelling, structural, and dynamic properties of the polymer as well as on the CO 2 solubility and diffusivity by performing molecular dynamics (MD) simulations of the system in a fully atomistic representation. A hierarchical scheme was used for the generation of the higher M w polymer systems, which consisted of equilibration at a coarse-grained level of representation through efficient connectivity-altering Monte Carlo simulations, and reverse-mapping back to the atomistic representation, obtaining the configurations used for subsequent MD simulations. Sorption isotherms and associated swelling effects were determined by using an iterative procedure that incorporated a series of MD simulations in the NPT ensemble and the Widom test particle insertion method, while CO 2 diffusion coefficients were extracted from long MD runs in the NVE ensemble. Solubility and diffusivity compared favorably with experimental results and with predictions of the Sanchez−Lacombe equation of state, which was reparametrized to capture the M w dependence of polymer properties with greater accuracy. Structural features of the polymer matrix were correctly reproduced by the simulations, and the effects of gas concentration and M w on structure and local dynamics were thoroughly investigated. In the presence of CO 2 , a significant acceleration of the segmental dynamics of the polymer occurred, more pronouncedly at low M w. The speed-up effect caused by the swelling agent was not limited to the chain ends but affected the whole chain in a similar fashion.
INTERNATIONAL CONFERENCE OF COMPUTATIONAL METHODS IN SCIENCES AND ENGINEERING ICCMSE 2020, 2021
This paper focuses on coarse-grained (or mesoscopic) simulations of bidisperse cis-1,4-polyisopre... more This paper focuses on coarse-grained (or mesoscopic) simulations of bidisperse cis-1,4-polyisoprene (cis-PI) melts with the EMSIPON (Engine for Mescoscopic Simulations for Polymer Networks) code that has been developed in the Computational Materials Science and Engineering group at the National Technical University of Athens. The code implements a hybrid particle-field Brownian dynamics/kinetic Monte Carlo method advanced in the course of the last four years. Structural, thermodynamic and rheological properties have been computed and are compared with the corresponding properties of monodisperse cis-PI melts at the same temperature. Moreover, the algorithm employed for creating initial configurations of bidisperse as well as polydisperse polymer melts is explained.
The network of inherent structures visited by a glassy atactic PS specimen during physical aging ... more The network of inherent structures visited by a glassy atactic PS specimen during physical aging is examined by Molecular Dynamics (MD) simulations. However, MD cannot sufficiently sample the long relaxation times in the glassy state. Thus, a novel methodology of efficient sampling of a continuously expanding network of states is developed by combining Transition State Theory (TST) and kinetic Monte Carlo (kMC).
KAERGER:DIFFUSION 2V O-BK, 2012
Diffusion in Nanoporous Materials, 2012
Nucleation and Atmospheric Aerosols, 2015
The nonlocal theory of elasticity is employed for the study of the free vibrations of carbon nano... more 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.
Membranes, Aug 8, 2019
With a wide range of applications, from energy and environmental engineering, such as in gas sepa... more With a wide range of applications, from energy and environmental engineering, such as in gas separations and water purification, to biomedical engineering and packaging, glassy polymeric materials remain in the core of novel membrane and state-of the art barrier technologies. This review focuses on molecular simulation methodologies implemented for the study of sorption and diffusion of small molecules in dense glassy polymeric systems. Basic concepts are introduced and systematic methods for the generation of realistic polymer configurations are briefly presented. Challenges related to the long length and time scale phenomena that govern the permeation process in the glassy polymer matrix are described and molecular simulation approaches developed to address the multiscale problem at hand are discussed.
Macromolecules, Feb 13, 2023
Materials Today: Proceedings, 2018
Discotic polyaromatic hydrocarbons constitute a very promising materials family for organic elect... more Discotic polyaromatic hydrocarbons constitute a very promising materials family for organic electronic applications. Its members can be considered as finite graphene flakes of nanometric dimensions. In this work, a plethora of multiscale simulations are employed for the determination of electronic, structural and dynamical properties of nanographene molecules that bear around their periphery flexible functional groups and tend to form well-organized molecular wires and crystals. Charge transfer properties are examined at the molecular dimer level and relationships between internal molecular wire structure and material properties are elucidated.
Macromolecules, May 24, 2022
Molecular Physics, Jan 6, 2020
ABSTRACT The current article presents results from MD simulations of high molar mass polyethylene... more ABSTRACT The current article presents results from MD simulations of high molar mass polyethylene melts with the scope to investigate the structure and dynamics at the polymer/solid interphase, and to assess their dependence on strong Couette flows. The density profiles are decomposed into the contributions of specific types of segments such as tails, loops and trains in order to arrive at a detailed description of the structure under various flow conditions. The size and orientation of chain segments is quantified to assess the reorganisation of the chains at the interface leading to possible shear-thinning effects. The segmental velocity profiles and the layer- and direction-resolved mean square displacement of the chain segments are extracted from the simulations so as to compute the effective shear rate, slippage, and the emergence of possible interfacial failure mechanisms. Through representative snapshots of chain trajectories we unveil the dominant mechanisms dictating the chain reorganisation in the interfaces and the overall adsorption–desorption processes. Our findings suggest the manifestation of a hybrid boundary condition attributed mainly to interfacial failure and partly to cohesive failure. GRAPHICAL ABSTRACT
Soft Matter, 2021
Polymer/matrix nanocomposites (PNCs) are materials with exceptional properties. They offer a plet... more Polymer/matrix nanocomposites (PNCs) are materials with exceptional properties. They offer a plethora of promising applications in key industrial sectors. In most cases, it is preferable to disperse the nanoparticles (NPs) homogeneously across the matrix phase. However, under certain conditions NPs might lump together and lead to a composite material with undesirable properties. A common strategy to stabilize the NPs is to graft on their surface polymer chains of the same chemical constitution as the matrix chains. There are several unresolved issues concerning the optimal molar mass and areal density of grafted chains that would ensure best dispersion, given the nanoparticles and the polymer matrix. We propose a model for the prediction of key structural and thermodynamic properties of PNC and apply it to a single spherical silica (SiO 2) nanoparticle or planar surface grafted with polystyrene chains embedded at low concentration in a matrix phase of the same chemical constitution. Our model is based on self-consistent field theory, formulated in terms of the Edwards diffusion equation. The properties of the PNC are explored across a broad parameter space, spanning the mushroom regime (low grafting densities, small NPs and chain lengths), the dense brush regime, and the crowding regime (large grafting densities, NP diameters, and chain lengths). We extract several key quantities regarding the distributions and the configurations of the polymer chains, such as the radial density profiles and their decomposition into contributions of adsorbed and free chains, the chains/area profiles, and the tendency of end segments to segregate at the interfaces. Based on our predictions concerning the brush thickness, we revisit the scaling behaviors proposed in the literature and we compare our findings with experiment, relevant simulations, and analytic models, such as Alexander's model for incompressible brushes.
Computer Physics Communications, Apr 1, 2020
DOI to the publisher's website. • The final author version and the galley proof are versions of t... more DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:
Journal of Physical Chemistry B, Dec 5, 2018
A complete thermodynamic analysis of mixtures consisting of molecules with complex chemical const... more A complete thermodynamic analysis of mixtures consisting of molecules with complex chemical constitution can be rather demanding. The Kirkwood-Buff theory of solutions allows the estimation of thermodynamic properties, which cannot be directly extracted from atomistic simulations, such as the Gibbs energy of mixing (Δmix G). In this work, we perform molecular dynamics simulations of n-hexane-ethanol binary mixtures in the liquid state under two temperature-pressure conditions and at various mole fractions. On the basis of the recently published methodology of Galata [ Fluid Phase Equilib. 2018 , 470 , 25 - 37 ] , we first calculate the Kirkwood-Buff integrals in the isothermal-isobaric ( NpT) ensemble, identifying how system size affects their estimation. We then extract the activity coefficients, excess Gibbs energy, excess enthalpy, and excess entropy for the n-hexane-ethanol binary mixtures we simulate. We employ two approaches for quantifying composition fluctuations: one based on counting molecular centers of mass and a second one based on counting molecular segments. Results from the two approaches are practically indistinguishable. We compare our results against predictions of vapor-liquid equilibria obtained in a previous simulation work using the same force field, as well as with experimental data, and find very good agreement. In addition, we develop a simple methodology to identify the hydrogen bonds between ethanol molecules and analyze their effects on mixing properties.
Elsevier eBooks, 1993
ABSTRACT A molecular model has been developed and used to calculate thermodynamic properties of a... more ABSTRACT A molecular model has been developed and used to calculate thermodynamic properties of aromatics in silicalite at low occupancy. The molecules studied are benzene, toluene, p-xylene, m-xylene, and o-xylene. Henry's constants, isosteric heats of adsorption, and siting preferences within the zeolite lattice have been computed. The results are in good agreement with experimental data from the literature.
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Papers by Doros Theodorou