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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. Source


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. Source


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. Source


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. Source


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. Source

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