Vienna Computational Materials Laboratory

Vienna, Austria

Vienna Computational Materials Laboratory

Vienna, Austria

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Fytas N.G.,Complutense University of Madrid | Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory | Georgiou I.,Vienna University of Technology
European Physical Journal B | Year: 2012

We investigate the critical properties of the d = 3 random-field Ising model with an equal-weight trimodal distribution at zero temperature. By implementing suitable graph-theoretical algorithms, we compute large ensembles of ground states for several values of the disorder strength h and system sizes up to N = 128 3. Using a new approach based on the sample-to-sample fluctuations of the order parameter of the system and proper finite-size scaling techniques we estimate the critical disorder strength h c = 2.747(3) and the critical exponents of the correlation length ν = 1.34(6) and order parameter β = 0.016(4). These estimates place the model into the universality class of the corresponding Gaussian random-field Ising model. © 2012 EDP Sciences, Società Italiana di Fisica, Springer-Verlag.


Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory | Fytas N.G.,Complutense University of Madrid
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2012

The effects of bond randomness on the universality aspects of a two-dimensional (d=2) Blume-Capel model embedded in the triangular lattice are discussed. The system is studied numerically in both its first- and second-order phase-transition regimes by a comprehensive finite-size scaling analysis for a particularly suitable value of the disorder strength. We find that our data for the second-order phase transition, emerging under random bonds from the second-order regime of the pure model, are compatible with the universality class of the two-dimensional (2D) random Ising model. Furthermore, we find evidence that, the second-order transition emerging under bond randomness from the first-order regime of the pure model, belongs again to the same universality class. Although the first finding reinforces the scenario of strong universality in the 2D Ising model with quenched disorder, the second is in difference from the critical behavior, emerging under randomness, in the cases of the ex-first-order transitions of the Potts model. Finally, our results verify previous renormalization-group calculations on the Blume-Capel model with disorder in the crystal-field coupling. © 2012 American Physical Society.


Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory | Fytas N.G.,Complutense University of Madrid
Journal of Chemical Physics | Year: 2012

We use a standard bead-spring model and molecular dynamics simulations to study the static properties of symmetric linear multiblock copolymer chains and their blocks under poor solvent conditions in a dilute solution from the regime close to theta conditions, where the chains adopt a coil-like formation, to the poorer solvent regime where the chains collapse obtaining a globular formation and phase separation between the blocks occurs. We choose interaction parameters as is done for a standard model, i.e., the Lennard-Jones fluid and we consider symmetric chains, i.e., the multiblock copolymer consists of an even number n of alternating chemically different A and B blocks of the same length N A N B N. We show how usual static properties of the individual blocks and the whole multiblock chain can reflect the phase behavior of such macromolecules. Also, how parameters, such as the number of blocks n can affect properties of the individual blocks, when chains are in a poor solvent for a certain range of n. A detailed discussion of the static properties of these symmetric multiblock copolymers is also given. Our results in combination with recent simulation results on the behavior of multiblock copolymer chains provide a complete picture for the behavior of these macromolecules under poor solvent conditions, at least for this most symmetrical case. Due to the standard choice of our parameters, our system can be used as a benchmark for related models, which aim at capturing the basic aspects of the behavior of various biological systems. © 2012 American Institute of Physics.


Maleki H.,Johannes Gutenberg University Mainz | Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory
Journal of Physics Condensed Matter | Year: 2011

We report a simulation study for bottle-brush polymers grafted on a rigid backbone. Using a standard coarse-grained beadspring model extensive molecular dynamics simulations for such macromolecules under good solvent conditions are performed. We consider a broad range of parameters and present numerical results for the monomer density profile, density of the untethered ends of the grafted flexible backbones and the correlation function describing the range that neighboring grafted bottle-brushes are affected by the presence of the others due to the excluded-volume interactions. The end beads of the flexible backbones of the grafted bottle-brushes do not access the region close to the rigid backbone due to the presence of the side chains of the grafted bottle-brush polymers, which stretch the chains further in the radial directions. Although a number of different correlation lengths exist as a result of the complex structure of these macromolecules, their properties can be tuned with high accuracy in good solvents. Moreover, qualitative differences with typical bottle-brushes are discussed. Our results provide a first approach to characterizing such complex macromolecules with a standard beadspring model. © 2011 IOP Publishing Ltd.


Theodorakis P.E.,Johannes Gutenberg University Mainz | Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory | And 3 more authors.
Journal of Chemical Physics | Year: 2011

By molecular dynamics simulation of a coarse-grained bead-spring-type model for a cylindrical molecular brush with a backbone chain of Nb effective monomers to which with grafting density side chains with N effective monomers are tethered, several characteristic length scales are studied for variable solvent quality. Side chain lengths are in the range 5 N 40, backbone chain lengths are in the range 50 Nb 200, and we perform a comparison to results for the bond fluctuation model on the simple cubic lattice (for which much longer chains are accessible, Nb 1027, and which corresponds to an athermal, very good, solvent). We obtain linear dimensions of the side chains and the backbone chain and discuss their N-dependence in terms of power laws and the associated effective exponents. We show that even at the theta point the side chains are considerably stretched, their linear dimension depending on the solvent quality only weakly. Effective persistence lengths are extracted both from the orientational correlations and from the backbone end-to-end distance; it is shown that different measures of the persistence length (which would all agree for Gaussian chains) are not mutually consistent with each other and depend distinctly both on Nb and the solvent quality. A brief discussion of pertinent experiments is given. © 2011 American Institute of Physics.


Fytas N.G.,University of Patras | Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory
Journal of Physics Condensed Matter | Year: 2011

We use molecular dynamics simulations to study the static properties of a single linear multiblock copolymer chain under poor solvent conditions varying the block length N, the number of blocks n, and the solvent quality by variation of the temperature T. We study the most symmetrical case, where the number of blocks of monomers of type A, n A, equals that of monomers B, n B (n A = n B = n/2), the length of all blocks is the same irrespective of their type, and the potential parameters are also chosen symmetrically, as for a standard Lennard-Jones fluid. Under poor solvent conditions the chains collapse and blocks with monomers of the same type form clusters, which are phase separated from the clusters with monomers of the other type. We study the dependence of the size of the clusters formed on n, N and T. Furthermore, we discuss our results with respect to recent simulation data on the phase behaviour of such macromolecules, providing a complete picture for the cluster formations in single multiblock copolymer chains under poor solvent conditions. © 2011 IOP Publishing Ltd.


Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory | Fytas N.G.,University of Patras
European Physical Journal B | Year: 2011

We implement a two-stage approach of the Wang-Landau algorithm to investigate the critical properties of the 3D Ising model with quenched bond randomness. In particular, we consider the case where disorder couples to the nearest-neighbor ferromagnetic interaction, in terms of a bimodal distribution of strong versus weak bonds. Our simulations are carried out for large ensembles of disorder realizations and lattices with linear sizes L in the range $L=8{-}64$. We apply well-established finite-size scaling techniques and concepts from the scaling theory of disordered systems to describe the nature of the phase transition of the disordered model, departing gradually from the fixed point of the pure system. Our analysis (based on the determination of the critical exponents) shows that the 3D random-bond Ising model belongs to the same universality class with the site- and bond-dilution models, providing a single universality class for the 3D Ising model with these three types of quenched uncorrelated disorder. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Theodorakis P.E.,Johannes Gutenberg University Mainz | Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory | And 2 more authors.
European Physical Journal E | Year: 2011

Two-component bottle-brush polymers, where flexible side chains containing N = 20, 35 and 50 effective monomers are grafted alternatingly to a rigid backbone, are studied by Molecular Dynamics simulations, varying the grafting density σand the solvent quality. Whereas for poor solvents and large enough σthe molecular brush is a cylindrical object with monomers of different type occupying locally the two different halves of the cylinder, for intermediate values of σan axially inhomogeneous structure of "pearl-necklace" type is formed, where microphase separation between monomers of different type within a cluster takes place. These "pearls" have a strongly non-spherical ellipsoidal shape, due to the fact that several side chains cluster together in one "pearl". We discuss the resulting structures in detail and we present a comparison with the single-component bottle-brush case. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Theodorakis P.E.,University of Vienna | Theodorakis P.E.,Vienna University of Technology | Theodorakis P.E.,Vienna Computational Materials Laboratory | Fytas N.G.,University of Patras
EPL | Year: 2011

Molecular-dynamics simulations are used to study the phase behavior of a single linear multiblock copolymer with blocks of A- and B-type monomers under poor solvent conditions, varying the block length N, number of blocks n, and the solvent quality (by variation of the temperature T). The fraction f of A-type monomers is kept constant and equal to 0.5, and always the lengths of A and B blocks are equal (NA=NB=N), as well as the number of blocks (nA=nB). We identify the three following regimes where: i) full microphase separation between blocks of different type occurs (all blocks of A-type monomers form a single cluster, while all blocks of B-type monomers form another); ii) full microphase separation is observed with a certain probability; and iii) full microphase separation cannot take place. For a very high number of blocks n and very high N (not accessible to our simulations) further investigation is needed. Copyright © EPLA, 2011.

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