Ioannis Bitsanis - Academia.edu (original) (raw)
Papers by Ioannis Bitsanis
Submitted for the MAR09 Meeting of The American Physical Society Collapse Transitions in Thermose... more Submitted for the MAR09 Meeting of The American Physical Society Collapse Transitions in Thermosensitive Alternating Copolymers: A Monte Carlo Study IOANNIS BITSANIS, ANASTASIA RISSANOU, FORTHIESL, Heraklion, Greece, STANISLAV BUROV, St. Petersburgh Univ., Russia, EVEANGELOS MANIAS, Penn State Univ., PA USA — Alternating copolymers are expected to exhibit a rich transition behavior in selective solvents with implications in biology and the design of thermo and pH-sensitive materials. We studied transitions of model alternating copolymers of the type (AAA...)n1(BBB...)n2, in selective solvents by MC simulations. Results showed that the eminent factor, controlling response to external stimuli, is co-polymer’s chemical composition.. We focused on the extreme case of a single polymer chain of N = 1000 units, distributed equally in alternate blocks of n1= n2 =100 units (Aand Bblocks). The solvent was quite selective, i.e. good for 5 100-A-blocks, whereas the 5 100 B-blocks were quite ins...
Bulletin of the American Physical Society, 2009
Alternating copolymers are expected to exhibit a rich transition behavior in selective solvents w... more Alternating copolymers are expected to exhibit a rich transition behavior in selective solvents with implications in biology and the design of thermo and pH-sensitive materials. We studied transitions of model alternating copolymers of the type (AAA...)n1(BBB...)n2, in selective solvents by MC simulations. Results showed that the eminent factor, controlling response to external stimuli, is co-polymer's chemical composition.. We focused on
Submitted for the MAR09 Meeting of The American Physical Society Phase Equilibrium of Size-Disper... more Submitted for the MAR09 Meeting of The American Physical Society Phase Equilibrium of Size-Dispersed Colloid Systems with soft pair interactions: A Monte Carlo Study IOANNIS BITSANIS, FORTH-IESL, Heraklion, Greece, IOANNIS ECONOMOY, MARIANNA YIANNOURAKOU, NRCPS “Demokritos”, Ag. Paraskevi, Greece — We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interact via power-law potentials. We employed isobaric semi-grand ensemble simulations. Gibbs-Duhem integration traced the coexistence pressure as a function of the variance of the imposed activity distribution. Fluid-solid coexistence densities were determined to be monotonically increasing functions of the breadth of particle size dispersity. Our simulations testified to the existence of terminal diameter dispersity, i.e a dispersity above which there can be no amorphous-crystalline phase coexistence. At the terminus, size-dispersity increases from 5.8% to 6.7% for the crystalline phase as pair in...
Macromolecules, 1990
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
Aps Meeting Abstracts, Mar 1, 2007
ABSTRACT We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk so... more ABSTRACT We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk solution^*, or adsorbed onto a surface, under poor solvent conditions. Our simulations spanned a wide range of chain lengths (N=20-10000) and cohesive energies. The chain length dependence of the chain size in poor solvents was characterized by a wide plateau of almost null growth. This plateau was related with the development of the incipient constant density core. The ``volume approximation'' regime and genuine power law dependence (1/3) was not reached even for the longest chains and poorest solvents studied. Sufficiently long chains became more but not fully spherical and underwent a 2^nd order phase transition. Conformations of the adsorbed chains onto attractive surfaces are not controlled by the bulk theta- temperature, but by a new temperature theta' which depends strongly on the interactions with the surface. The adsorption-desorption transition width is determined by the N-dependence of the bulk radius of gyration, for every solvent quality. In poor solvents and strongly attractive surfaces, the coil-to-globule transition turns into a coil to ``pancake'' transition. ^* Rissanou et al. J. Polymer Sci. :Part B : Polymer Phys., 44, p.3651 (2006)
Aps Meeting Abstracts, Mar 1, 2004
Monte Carlo computer simulations of end-tethered chains grafted onto a hard wall have been perfor... more Monte Carlo computer simulations of end-tethered chains grafted onto a hard wall have been performed. The chains were modeled as self-avoiding chains on a cubic lattice at athermal solvent conditions. The simulations spanned a wide range of chain lengths, N (100-1000, i.e., up to molecular weights of a few hundred thousands), and anchoring densities, r (2 Â 10 À4 to 0.4), to properly chart the relevant parameter space. It is shown that the reduced surface coverage r* ¼ rpR 2 g is the most appropriate variable that quantitatively determines the mushroom, overlapping mushroom and brush regimes, where R g is the radius of gyration of a free chain in solution. The simulation data are analyzed to determine the conformational characteristics and shape of the anchored chains and to compare them with the predictions of the analytical self consistent field theory. The strong stretching limit of the theoretical predictions is obtained only for r* [ 8. V
Thesis University of Minnesota 1989 Source Dissertation Abstracts International Volume 50 07 Section B Page 3038, 1989
The coil to globule transition of polymer chains in bulk solution or attached onto a surface is i... more The coil to globule transition of polymer chains in bulk solution or attached onto a surface is investigated using Monte Carlo simulations on a cubic lattice as a function of MW, solvent quality and interfacial energy. Monte Carlo results for an isolated chain in bulk solution show an anomalous dependence of RG on the MW as the solvent becomes poorer: it exhibits a N-independent regime for intermediate MWs where there is a qualitative change in the form of the density distribution in the inner part of the chain. The coil to globule transition is in accord with Khokhlov's theoretical predictions. The coupling between the coil to globule transition and the adsorption of the chain onto attractive surfaces is investigated as a function of the surface energy. The adsorbed chain in poor solvent tends to a pancake-like conformation for very attractive surfaces. The whole behavior is not controlled by the bulk Theta temperature but by a new temperature Theta^', which depends strongly on the surface energy and tends to the value of the Theta temperature in two-dimensions.
Macromolecules, 1988
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
Macromolecules, 1990
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
Aps Meeting Abstracts, Mar 1, 2001
ABSTRACT A new computer simulation methodology is developed which determines the structure factor... more ABSTRACT A new computer simulation methodology is developed which determines the structure factor of concentrated suspensions accurately, and subsequently calculates the structure factor from the wavelength dependence of the "transport mean free path", l(lambda), for the same systems. Therefore our method can test directly the validity of approximations involved in the analysis of multiple scattering data. The simulation results for l^* agree closely with experimental data and explain the substantial overestimation of l^* by the "photon diffusion" formula observed in some experiments.
AIP Conference Proceedings, 2007
We study the conformational transitions of polymer chains near attractive surfaces using Monte Ca... more We study the conformational transitions of polymer chains near attractive surfaces using Monte Carlo simulations on a simple cubic lattice. The shape of the adsorbed chain is studied as a function of the molecular weight, solvent quality and surface energy. The competition between two transitions induced by varying solvent conditions and the existence of an attractive wall results in various shapes of the adsorbate.
AIP Conference Proceedings, 2007
AIP Conference Proceedings, 2007
We have studied the solid-fluid coexistence for systems of polydisperse soft spheres with near-Ga... more We have studied the solid-fluid coexistence for systems of polydisperse soft spheres with near-Gaussian diameter distributions that interact via an inverse power potential, the softness of which is being tuned. The study employs simulation in the isobaric semigrand ensemble. Gibbs-Duhem integration is used to trace the coexistence pressure as a function of the variance of the imposed activity distribution. Both
Macromolecules, 1988
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
Adsorption, 1996
In this paper we present the findings of our investigations using molecular dynamics, on molecula... more In this paper we present the findings of our investigations using molecular dynamics, on molecularly thin films of n-octane confined between topographically smooth solid surfaces. We focus on the effect of increasing solid surface-methylene unit energetic affinity and the effect of increasing pressure (normal load) of the film in inducing liquid-solid phase transitions. We observed an abrupt transition in the
The Journal of Chemical Physics, Dec 14, 2010
We present a semigrand ensemble Monte Carlo and Brownian dynamics simulation study of structural ... more We present a semigrand ensemble Monte Carlo and Brownian dynamics simulation study of structural and dynamical properties of polydisperse soft spheres interacting via purely repulsive power-law potentials with a varying degree of "softness." Comparisons focus on crystal and amorphous phases at their coexistence points. It is shown through detailed structural analysis that as potential interactions soften, the "quality of crystallinity" of both monodisperse and polydisperse systems deteriorates. In general, polydisperse crystalline phases are characterized by a more ordered structure than the corresponding monodisperse ones (i.e., for the same potential softness). This counter-intuitive feature originates partly from the fact that particles of different sizes may be accommodated more flexibly in a crystal structure and from the reality that coexistence (osmotic) pressure is substantially higher for polydisperse systems. These trends diminish for softer potentials. Potential softness eventually produces substitutionally disordered crystals. However, substitutional order is apparent for the hard-spherelike interactions. Diffusionwise, crystals appear quite robust with a slight difference in the vibrational amplitudes of small and large particles. This difference, again, diminishes with potential softness. Overcrowding in amorphous polydisperse suspensions causes "delayed" diffusion at intermediate times.
Aps Meeting Abstracts, Jan 4, 2007
In dense suspensions of multi-arm star polymers a ``reversible thermal vitrification'' was observ... more In dense suspensions of multi-arm star polymers a ``reversible thermal vitrification'' was observed experimentally under ``marginal'' solvent conditions. We have investigated the origin of this phenomenon via MD simulations at the mesoscopic scale^1,2,. We reported the emergence of an amorphous solid state, upon heating of the ``soft spheres''. This transient glassy state resulted from star swelling, ``free volume'' deprivation and ``dynamical arrest'' of ``soft-spheres''. We monitored the ageing of the amorphous stage towards more crystalline FCC structures. The effects of size-dispersity and arm MW on crystallization were studied qualitatively. The overall picture revealed the existence of new ``dynamically arrested'' states, all of which could be termed ``crystalline'' but differed as to the ``degree of crystallinity''. Quantitative analysis of particle trajectories supplied mean square displacement curves which at the higher temperatures are typical of ``delayed'' Fickian diffusion. Even the aged crystalline states exhibited weak diffusion in contrast with the null diffusion of the crystals resulting from a FCC initial configuration. ^1 Rissanou et al., Phys. Rev. E 71 011402-1 :12 (2005) ^2 Rissanou et al., J. Chem. Phys. 124 044905-1 :11 (2006)
The Journal of Chemical Physics, 1990
Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong dens... more Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong density variations across the pore render the usual dependence of the local viscosity on local density inappropriate. At separations greater than four molecular diameters flow can be described by a simple redefinition of local viscosity. In narrower pores a dramatic increase of effective viscosities is observed
Submitted for the MAR09 Meeting of The American Physical Society Collapse Transitions in Thermose... more Submitted for the MAR09 Meeting of The American Physical Society Collapse Transitions in Thermosensitive Alternating Copolymers: A Monte Carlo Study IOANNIS BITSANIS, ANASTASIA RISSANOU, FORTHIESL, Heraklion, Greece, STANISLAV BUROV, St. Petersburgh Univ., Russia, EVEANGELOS MANIAS, Penn State Univ., PA USA — Alternating copolymers are expected to exhibit a rich transition behavior in selective solvents with implications in biology and the design of thermo and pH-sensitive materials. We studied transitions of model alternating copolymers of the type (AAA...)n1(BBB...)n2, in selective solvents by MC simulations. Results showed that the eminent factor, controlling response to external stimuli, is co-polymer’s chemical composition.. We focused on the extreme case of a single polymer chain of N = 1000 units, distributed equally in alternate blocks of n1= n2 =100 units (Aand Bblocks). The solvent was quite selective, i.e. good for 5 100-A-blocks, whereas the 5 100 B-blocks were quite ins...
Bulletin of the American Physical Society, 2009
Alternating copolymers are expected to exhibit a rich transition behavior in selective solvents w... more Alternating copolymers are expected to exhibit a rich transition behavior in selective solvents with implications in biology and the design of thermo and pH-sensitive materials. We studied transitions of model alternating copolymers of the type (AAA...)n1(BBB...)n2, in selective solvents by MC simulations. Results showed that the eminent factor, controlling response to external stimuli, is co-polymer's chemical composition.. We focused on
Submitted for the MAR09 Meeting of The American Physical Society Phase Equilibrium of Size-Disper... more Submitted for the MAR09 Meeting of The American Physical Society Phase Equilibrium of Size-Dispersed Colloid Systems with soft pair interactions: A Monte Carlo Study IOANNIS BITSANIS, FORTH-IESL, Heraklion, Greece, IOANNIS ECONOMOY, MARIANNA YIANNOURAKOU, NRCPS “Demokritos”, Ag. Paraskevi, Greece — We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interact via power-law potentials. We employed isobaric semi-grand ensemble simulations. Gibbs-Duhem integration traced the coexistence pressure as a function of the variance of the imposed activity distribution. Fluid-solid coexistence densities were determined to be monotonically increasing functions of the breadth of particle size dispersity. Our simulations testified to the existence of terminal diameter dispersity, i.e a dispersity above which there can be no amorphous-crystalline phase coexistence. At the terminus, size-dispersity increases from 5.8% to 6.7% for the crystalline phase as pair in...
Macromolecules, 1990
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
Aps Meeting Abstracts, Mar 1, 2007
ABSTRACT We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk so... more ABSTRACT We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk solution^*, or adsorbed onto a surface, under poor solvent conditions. Our simulations spanned a wide range of chain lengths (N=20-10000) and cohesive energies. The chain length dependence of the chain size in poor solvents was characterized by a wide plateau of almost null growth. This plateau was related with the development of the incipient constant density core. The ``volume approximation'' regime and genuine power law dependence (1/3) was not reached even for the longest chains and poorest solvents studied. Sufficiently long chains became more but not fully spherical and underwent a 2^nd order phase transition. Conformations of the adsorbed chains onto attractive surfaces are not controlled by the bulk theta- temperature, but by a new temperature theta' which depends strongly on the interactions with the surface. The adsorption-desorption transition width is determined by the N-dependence of the bulk radius of gyration, for every solvent quality. In poor solvents and strongly attractive surfaces, the coil-to-globule transition turns into a coil to ``pancake'' transition. ^* Rissanou et al. J. Polymer Sci. :Part B : Polymer Phys., 44, p.3651 (2006)
Aps Meeting Abstracts, Mar 1, 2004
Monte Carlo computer simulations of end-tethered chains grafted onto a hard wall have been perfor... more Monte Carlo computer simulations of end-tethered chains grafted onto a hard wall have been performed. The chains were modeled as self-avoiding chains on a cubic lattice at athermal solvent conditions. The simulations spanned a wide range of chain lengths, N (100-1000, i.e., up to molecular weights of a few hundred thousands), and anchoring densities, r (2 Â 10 À4 to 0.4), to properly chart the relevant parameter space. It is shown that the reduced surface coverage r* ¼ rpR 2 g is the most appropriate variable that quantitatively determines the mushroom, overlapping mushroom and brush regimes, where R g is the radius of gyration of a free chain in solution. The simulation data are analyzed to determine the conformational characteristics and shape of the anchored chains and to compare them with the predictions of the analytical self consistent field theory. The strong stretching limit of the theoretical predictions is obtained only for r* [ 8. V
Thesis University of Minnesota 1989 Source Dissertation Abstracts International Volume 50 07 Section B Page 3038, 1989
The coil to globule transition of polymer chains in bulk solution or attached onto a surface is i... more The coil to globule transition of polymer chains in bulk solution or attached onto a surface is investigated using Monte Carlo simulations on a cubic lattice as a function of MW, solvent quality and interfacial energy. Monte Carlo results for an isolated chain in bulk solution show an anomalous dependence of RG on the MW as the solvent becomes poorer: it exhibits a N-independent regime for intermediate MWs where there is a qualitative change in the form of the density distribution in the inner part of the chain. The coil to globule transition is in accord with Khokhlov's theoretical predictions. The coupling between the coil to globule transition and the adsorption of the chain onto attractive surfaces is investigated as a function of the surface energy. The adsorbed chain in poor solvent tends to a pancake-like conformation for very attractive surfaces. The whole behavior is not controlled by the bulk Theta temperature but by a new temperature Theta^', which depends strongly on the surface energy and tends to the value of the Theta temperature in two-dimensions.
Macromolecules, 1988
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
Macromolecules, 1990
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
Aps Meeting Abstracts, Mar 1, 2001
ABSTRACT A new computer simulation methodology is developed which determines the structure factor... more ABSTRACT A new computer simulation methodology is developed which determines the structure factor of concentrated suspensions accurately, and subsequently calculates the structure factor from the wavelength dependence of the "transport mean free path", l(lambda), for the same systems. Therefore our method can test directly the validity of approximations involved in the analysis of multiple scattering data. The simulation results for l^* agree closely with experimental data and explain the substantial overestimation of l^* by the "photon diffusion" formula observed in some experiments.
AIP Conference Proceedings, 2007
We study the conformational transitions of polymer chains near attractive surfaces using Monte Ca... more We study the conformational transitions of polymer chains near attractive surfaces using Monte Carlo simulations on a simple cubic lattice. The shape of the adsorbed chain is studied as a function of the molecular weight, solvent quality and surface energy. The competition between two transitions induced by varying solvent conditions and the existence of an attractive wall results in various shapes of the adsorbate.
AIP Conference Proceedings, 2007
AIP Conference Proceedings, 2007
We have studied the solid-fluid coexistence for systems of polydisperse soft spheres with near-Ga... more We have studied the solid-fluid coexistence for systems of polydisperse soft spheres with near-Gaussian diameter distributions that interact via an inverse power potential, the softness of which is being tuned. The study employs simulation in the isobaric semigrand ensemble. Gibbs-Duhem integration is used to trace the coexistence pressure as a function of the variance of the imposed activity distribution. Both
Macromolecules, 1988
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
Adsorption, 1996
In this paper we present the findings of our investigations using molecular dynamics, on molecula... more In this paper we present the findings of our investigations using molecular dynamics, on molecularly thin films of n-octane confined between topographically smooth solid surfaces. We focus on the effect of increasing solid surface-methylene unit energetic affinity and the effect of increasing pressure (normal load) of the film in inducing liquid-solid phase transitions. We observed an abrupt transition in the
The Journal of Chemical Physics, Dec 14, 2010
We present a semigrand ensemble Monte Carlo and Brownian dynamics simulation study of structural ... more We present a semigrand ensemble Monte Carlo and Brownian dynamics simulation study of structural and dynamical properties of polydisperse soft spheres interacting via purely repulsive power-law potentials with a varying degree of "softness." Comparisons focus on crystal and amorphous phases at their coexistence points. It is shown through detailed structural analysis that as potential interactions soften, the "quality of crystallinity" of both monodisperse and polydisperse systems deteriorates. In general, polydisperse crystalline phases are characterized by a more ordered structure than the corresponding monodisperse ones (i.e., for the same potential softness). This counter-intuitive feature originates partly from the fact that particles of different sizes may be accommodated more flexibly in a crystal structure and from the reality that coexistence (osmotic) pressure is substantially higher for polydisperse systems. These trends diminish for softer potentials. Potential softness eventually produces substitutionally disordered crystals. However, substitutional order is apparent for the hard-spherelike interactions. Diffusionwise, crystals appear quite robust with a slight difference in the vibrational amplitudes of small and large particles. This difference, again, diminishes with potential softness. Overcrowding in amorphous polydisperse suspensions causes "delayed" diffusion at intermediate times.
Aps Meeting Abstracts, Jan 4, 2007
In dense suspensions of multi-arm star polymers a ``reversible thermal vitrification'' was observ... more In dense suspensions of multi-arm star polymers a ``reversible thermal vitrification'' was observed experimentally under ``marginal'' solvent conditions. We have investigated the origin of this phenomenon via MD simulations at the mesoscopic scale^1,2,. We reported the emergence of an amorphous solid state, upon heating of the ``soft spheres''. This transient glassy state resulted from star swelling, ``free volume'' deprivation and ``dynamical arrest'' of ``soft-spheres''. We monitored the ageing of the amorphous stage towards more crystalline FCC structures. The effects of size-dispersity and arm MW on crystallization were studied qualitatively. The overall picture revealed the existence of new ``dynamically arrested'' states, all of which could be termed ``crystalline'' but differed as to the ``degree of crystallinity''. Quantitative analysis of particle trajectories supplied mean square displacement curves which at the higher temperatures are typical of ``delayed'' Fickian diffusion. Even the aged crystalline states exhibited weak diffusion in contrast with the null diffusion of the crystals resulting from a FCC initial configuration. ^1 Rissanou et al., Phys. Rev. E 71 011402-1 :12 (2005) ^2 Rissanou et al., J. Chem. Phys. 124 044905-1 :11 (2006)
The Journal of Chemical Physics, 1990
Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong dens... more Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong density variations across the pore render the usual dependence of the local viscosity on local density inappropriate. At separations greater than four molecular diameters flow can be described by a simple redefinition of local viscosity. In narrower pores a dramatic increase of effective viscosities is observed