Vlasis Mavrantzas | University of Patras (original) (raw)

Papers by Vlasis Mavrantzas

Journal of Non-Equilibrium Thermodynamics

In “soft–soft nanocomposites” based on film formation of latexes with structured particles, the c... more In “soft–soft nanocomposites” based on film formation of latexes with structured particles, the combination of particle structure and interparticle crosslinking leads to materials that behave as nonlinear viscoelastic fluids at small strains and as highly elastic networks at larger strains. Similarly, in studies of flow-induced crystallization in polymers, a two-phase model is often invoked in which a soft viscoelastic component is coupled with a rigid semi-crystalline phase providing stiffness. In the present work, we use the framework of non-equilibrium thermodynamics (NET) to develop stress-strain relationships for such two-phase systems characterized by a viscoelastic and an elastic component by making use of two conformation tensors: the first describes the microstructure of the viscoelastic phase while the second is related to the elastic Finger strain tensor quantifying the deformation of the elastic phase due to strain and is responsible for strain-hardening. The final trans...

Journal of Rheology, 2020

We show how two-species models, already proposed for the rheology of networks of associative poly... more We show how two-species models, already proposed for the rheology of networks of associative polymer solutions, can be derived from nonequilibrium thermodynamics using the generalized bracket formalism. The two species refer to bridges and (temporary) dangling chains, both of which are represented as dumbbells. Creation and destruction of bridges in our model are accommodated self-consistently by assuming a two-way reaction characterized by a forward and a reverse rate constant. Although the final set of evolution equations for the microstructure of the two species and the expression for the stress tensor are similar to those of earlier models based on network kinetic theory, nonequilibrium thermodynamics sets specific constraints on the form of the attachment/detachment rates appearing in these equations, which, in some cases, deviate significantly from previously reported ones. We also carry out a detailed analysis demonstrating the capability of the new model to describe various sets of rheological data for solutions of associative polymers.

Physics of Fluids, 2018

Red blood cells tend to aggregate in the presence of plasma proteins, forming structures known as... more Red blood cells tend to aggregate in the presence of plasma proteins, forming structures known as rouleaux. Here, we derive a constitutive rheological model for human blood which accounts for the formation and dissociation of rouleaux using the generalized bracket formulation of nonequilibrium thermodynamics. Similar to the model derived by Owens and co-workers [“A non-homogeneous constitutive model for human blood. Part 1. Model derivation and steady flow,” J. Fluid Mech. 617, 327–354 (2008)] through polymer network theory, each rouleau in our model is represented as a dumbbell; the corresponding structural variable is the conformation tensor of the dumbbell. The kinetics of rouleau formation and dissociation is treated as in the work of Germann et al. [“Nonequilibrium thermodynamic modeling of the structure and rheology of concentrated wormlike micellar solutions,” J. Non-Newton. Fluid Mech. 196, 51–57 (2013)] by assuming a set of reversible reactions, each characterized by a forw...

Journal of Physical Chemistry B, Dec 14, 2022

To deal with divergences of functional integrals in field-theoretic simulations (FTS) of complex ... more To deal with divergences of functional integrals in field-theoretic simulations (FTS) of complex fluids, the microscopic density is often smeared by being replaced by a convoluted one, typically using a Gaussian masking function. The smearing changes radically the nature of nonbonded interactions of the original microscopic density and results in a regularized model that is free of ultraviolet (UV) divergences. In this work, we first resolve a few fundamental issues related with the use of masking functions for δ-interactions in FTS and then we detail a new methodology that builds on the concept of multiconvoluted inverse potentials and a principle of model equivalence for statistical weights to accommodate more physically relevant interactions in FTS. The capabilities of the new approach are highlighted by examining the Gaussian-regularized Edwards model (GREM) and the Yukawa potential. A successful test calculation of the excess chemical potential of a polymer chain in a good solvent with the GREM illustrates the power of the new theoretical framework.

Soft Matter, 2018

Phase diagram of α-unsubstituted sexithiophene is driven by six different chain conformational gr... more Phase diagram of α-unsubstituted sexithiophene is driven by six different chain conformational groups.

Macromolecular Theory and Simulations, Nov 15, 2016

Macromolecules, Mar 9, 2017

We present results from very long (on the order of several microseconds) atomistic molecular dyna... more We present results from very long (on the order of several microseconds) atomistic molecular dynamics (MD) simulations for the density, microscopic structure, conformation, and local and segmental dynamics of pure, strictly monodisperse ring and linear poly(ethylene oxide) (PEO) melts, ranging in molar mass from ∼5300 to ∼20 000 g/mol. The MD results are compared with recent experimental data for the chain center-of-mass self-diffusion coefficient and the normalized single-chain dynamic structure factor obtained from small-angle neutron scattering, neutron spin echo, and pulse-field gradient NMR, and remarkable qualitative and quantitative agreement is observed, despite certain subtle disagreements in important details regarding mainly internal ring motion (loop dynamics). A detailed normal-mode analysis allowed us to check the degree of consistency of ring PEO melt dynamics with the ring Rouse model and indicated a strong reduction of the normalized mode amplitudes for the smaller mode numbers (compared to the Rouse model scaling), combined with an undisturbed spectrum of Rouse relaxation rates. We have further measured the zero-shear rate viscosity η 0 of the PEO-5k and PEO-10k rings at several temperatures and extracted their activation energies. These were compared with the activation energies extracted from the MD simulations via analysis of the temperature dependence of the corresponding Rouse relaxation times of the two rings in the same temperature range.

Macromolecules, Jun 1, 2005

Molecular dynamics (MD) simulations have been performed on a dense polymer melt adsorbed on a sol... more Molecular dynamics (MD) simulations have been performed on a dense polymer melt adsorbed on a solid substrate on the one side and exposed to vacuum on the other. As a model system, a thin film of polyethylene (PE) melt supported by a crystalline graphite phase on its one side (the other surface of the film is free) has been examined. Most simulations have been carried out with unentangled PE melt systems, such as C 78 and C156, in the NPT statistical ensemble at T) 450 K and P) 0 atm for times up to 100 ns, using a multiple-time step MD algorithm and by incorporating the correct dependence of the long-range contribution to the energy and stress tensor on the density profile. To increase the statistical accuracy of the results, large systems have been employed in the MD simulations, such as a 200-chain C 78 melt consisting of 15 600 carbon atoms. The MD simulation data have been analyzed to provide information about the spatial dependence of the short-time dynamical properties (conformational relaxation) of the melt and the long-time segmental motion and mobility in the film (transport and diffusion). Local mobility near the graphite phase is predicted to be highly anisotropic: although it remains practically unaltered in the directions x and y parallel to the surface, it is dramatically reduced in the direction z perpendicular to it. To calculate the long time self-diffusion coefficient of adsorbed segments in the direction perpendicular to the graphite plane, MD trajectories have been mapped onto the (numerical) solution of a macroscopic, continuum diffusion equation describing the temporal and spatial evolution of the concentration of adsorbed atoms in the polymer matrix. Our calculations prove that the diffusive motion of segments remains inhomogeneous along the z direction of the adsorbed film for distances up to approximately 5-6 times the root-mean-square of the radius of gyration, R g, of the bulk, unconstrained melt.

Polymers, Aug 16, 2021

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Macromolecules, Jan 17, 2020

Long molecular dynamics simulations are performed for dilute solutions of ring poly(ethylene oxid... more Long molecular dynamics simulations are performed for dilute solutions of ring poly(ethylene oxide) (PEO) molecules in matrices of linear PEO chains where we systematically vary the molecular length of the ring and host chains. Our focus is on the effect of linear chain size on microscopic structure, conformation, and dynamics of the guest ring molecules, and how these properties vary with the corresponding ones in the pure ring melts. Ring molecules are found to be significantly swollen in all ring−linear blends simulated. Ring swelling is more pronounced in matrices of very short linear chains (molecular weights less than about 1.5 kg/mol) due to excess, chain-end free-volume effects. In these very short linear hosts, all PEO rings simulated (molecular weights between 2 and 10 kg/mol) diffuse faster than in their own melts. However, as the size of the host linear chains increases above the entanglement molecular weight, the diffusivity of rings decreases considerably. Interestingly enough, for the shorter PEO rings simulated (molecular weight equal to 2 kg/mol), the diffusion coefficient in long, entangled matrices approaches a constant value independent of the molecular weight of the matrix, whereas that of longer rings (molecular weight equal to 5k and 10k g/mol) decreases continuously (at least for the linear matrix molecular weights examined here). Our simulation predictions for the diffusion coefficient of PEO rings in the linear PEO matrices compare remarkably well with the recent pulse-field gradient NMR measurements of Kruteva et al. [Macromolecules 2017, 50, 9482−9493]. A detailed topological analysis reveals that long ring molecules are heavily threaded by the host linear chains. Their segmental and diffusive dynamics is therefore governed by the rate with which threadings are created and released. Threadings, which are quantified in detail in our analysis, are also seen to cause strong fluctuations in the instantaneous conformation of the host linear chains, thus influencing their average dimensions. Our work provides strong evidence that ring−linear threadings is the key mechanism governing the size, the conformation, and the dynamic behavior of ring−linear polymer blends.

ACS Macro Letters, Jul 13, 2018

We present results from detailed, atomistic molecular dynamics (MD) simulations of pure, strictly... more We present results from detailed, atomistic molecular dynamics (MD) simulations of pure, strictly monodisperse linear and ring poly(ethylene oxide) (PEO) melts under equilibrium and nonequilibrium (shear flow) conditions. The systems examined span the regime of molecular weights (M w) from sub-Rouse (M w < M e) to reptation (M w ∼ 10 M e), where M e denotes the characteristic entanglement molecular weight of linear PEO. For both PEO architectures (ring and linear), the predicted chain center-of-mass self-diffusion coefficients D G as a function of PEO M w are in remarkable agreement with experimental data. From the flow simulations under shear, we have extracted and analyzed the zero-shear viscosity of ring and linear PEO melts as a function of M w .

World Conference on Photovoltaic Energy Conversion, Oct 26, 2012

Macromolecular Theory and Simulations, May 19, 2020

Macromolecules, Oct 16, 2018

We introduce a powerful Monte Carlo (MC) algorithm for the atomistic simulation of bulk models of... more We introduce a powerful Monte Carlo (MC) algorithm for the atomistic simulation of bulk models of oligo-and poly-thiophenes by redesigning MC moves originally developed for considerably simpler polymer structures and architectures, such as linear and branched polyethylene, to account for the ring structure of the thiophene monomer. Elementary MC moves implemented include bias reptation of an end thiophene ring, flip of an internal thiophene ring, rotation of an end thiophene ring, concerted rotation of three thiophene rings, rigid translation of an entire molecule, rotation of an entire molecule and volume fluctuation. In the implementation of all moves we assume that thiophene ring atoms remain rigid and strictly co-planar; on the other hand, interring torsion and bond bending angles remain fully flexible subject to suitable potential energy functions. Test simulations with the new algorithm of an important thiophene oligomer, α-sexithiophene (α-6T), at a high enough temperature (above its isotropic-to-nematic phase transition) using a new united atom 2 model specifically developed for the purpose of this work provide predictions for the volumetric, conformational and structural properties that are remarkably close to those obtained from detailed atomistic Molecular Dynamics (MD) simulations using an all-atom model. The new algorithm is particularly promising for exploring the rich (and largely unexplored) phase behavior and nanoscale ordering of very long (also more complex) thiophene-based polymers which cannot be addressed by conventional MD methods due to the extremely long relaxation times characterizing chain dynamics in these systems.

Macromolecules, Aug 31, 2020

The intensive applications of polymer in many engineering composites have imposed an urgent need ... more The intensive applications of polymer in many engineering composites have imposed an urgent need on the understanding of the mechanical behavior and deformation mechanism of the polymer under cyclic loading. This paper presents the results of a numerical study on the behavior of amorphous polyethylene (PE) subjected to cyclic tensile and compressive loads using molecular dynamics (MD) simulations, based on a united-atom approach. The effects of polymer chain length, the number of chains and strain rates are studied at first. Hysteresis loops, as well as visco-elastoplastic of PE under cyclic loading predicted by MD simulations are qualitatively in agreement with previous experiments. Three distinct hysteresis loops observed in successive loadingunloading reveal the contribution of elasticity, viscosity and plasticity under different loading strains, respectively. The rubber-like recovery behavior of PE at low temperature is attributed to that the mobility of molecular chains is constrained at low temperature. Energy analysis shows that the van der Waals energy and dihedral angle energy are considered to be the primary factors that affects the cyclic behavior of PE.

Bulletin of the American Physical Society, Mar 15, 2017

Submitted for the MAR17 Meeting of The American Physical Society Threading events in ring polymer... more Submitted for the MAR17 Meeting of The American Physical Society Threading events in ring polymer melts: a detailed geometric analysis and their connection with the slow relaxation modes VLASIS MAVRANTZAS, DIMITRIOS TSALIKIS, University of Patras, DIMITRIS VLAS-SOPOULOS, University of Crete FORTH-IESL-We will present results from a detailed geometric analysis of atomistic configurations of ring polyethylene oxide melts accumulated in the course of very long molecular dynamics (MD) simulations at T =413K and P =1atm which allowed us to locate ring-ring threading events and quantify their strengths and survival times. We have identified a variety of threading situations and studied their dependence on ring molecular weight. We have found that threadings can last up to several times the corresponding ring relaxation time, which can explain (at least in part) the appearance of slow relaxation modes observed experimentally in entangled polymer rings [2]. We confirm this by proposing a new expression for the stress relaxation modulus of entangled polymer rings that is found to provide excellent fits to experimentally measured curves. [1]

Bulletin of the American Physical Society, Mar 7, 2007

Journal of Chemical Physics, Jul 9, 2021

Modern field-theoretic simulations of complex fluids and polymers are constructed around a partic... more Modern field-theoretic simulations of complex fluids and polymers are constructed around a particle-to-field transformation that brings an inverse potential u −1 in the model equations. This has restricted the application of the framework to systems characterized by relatively simple pairwise interatomic interactions; for example, excluded volume effects are treated through the use of δ-function interactions. In this study, we first review available nonbonded pair interactions in field-theoretic models and propose a classification. Then, we outline the inverse potential problem and present an alternative approach on the basis of a saddle-point approximation, enabling the use of a richer set of pair interaction functions. We test our approach by using as an example the Morse potential, which finds extensive applications in particle-based simulations, and we calibrate u −1 with results from a molecular dynamics simulation. The u −1 thus obtained is consistent with the field-theoretic model equations, and when used in stand-alone self-consistent field simulations, it produces the correct fluid structure starting from a random initial state of the density field.

Linköping Electronic Press Workshop and Conference Collection

We have designed and implemented a hierarchical simulation methodology capable of addressing the ... more We have designed and implemented a hierarchical simulation methodology capable of addressing the growth rate and microstructural features of thin silicon films deposited through PECVD (Plasma Enhanced Chemical Vapor Deposition). Our main objective is to elucidate the microscopic mechanisms as well as the interplay between atomic level and macroscopic design parameters associated with the development of nano- or micro-scale crystalline regions in the grown film. The ultimate goal is to use multi-scale modeling as a design tool for tackling the issue of local crystallization and its dependence on operating variables. At the heart of our simulation approach is a very efficient, large-scale kinetic Monte Carlo (kMC) algorithm which allows generating samples of representative Si films based on a validated chemistry model. In a second step, the generated film is subjected to an atomistic simulation study which restores the molecular details lost or ignored in the kMC model. The atomistic ...