gaurav arya - Academia.edu (original) (raw)
Papers by gaurav arya
We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and m... more We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and molecular theory. In the Monte Carlo calculations we explicitly take into account counterions and polymer configurations and calculate electrostatic interaction using Ewald summation. Rosenbluth biasing, distance biasing, and the use of a lattice are all used to speed up Monte Carlo calculation, enabling the efficient simulation of the polyelectrolyte layer. The molecular theory explicitly incorporates the chain conformations and the possibility of counterion condensation. Using both Monte Carlo simulation and theory, we examine the effect of grafting density, surface charge density, charge strength, and polymer chain length on the distribution of the polyelectrolyte monomers and counterions. For all grafting densities examined, a sharp decrease in brush height is observed in the strongly charged regime using both Monte Carlo simulation and theory. The decrease in layer thickness is due to counterion condensation within the layer. The height of the polymer layer increases slightly upon charging the grafting surface. The molecular theory describes the structure of the polyelectrolyte layer well in all the different regimes that we have studied.
Journal of Physical Chemistry B, 2004
The effect of confinement on the phase behavior of lattice homopolymers has been studied using gr... more The effect of confinement on the phase behavior of lattice homopolymers has been studied using grand canonical Monte Carlo simulations in conjunction with multihistogram reweighting. The scaling of critical parameters and chain dimensions with chain length was determined for lattice homopolymers of up to 1024 beads in strictly 2D and quasi-2D (slab) geometries. The inverse critical temperature scales linearly with the Shultz-Flory parameter for quasi-2D geometries, as it does for the bulk system. The critical volume fraction scales as a power law for all systems, with exponents 0.110 ( 0.024 and 0.129 ( 0.004 for the strictly 2D and slab geometries, respectively. The influence of confinement on critical behavior persists even in a thick slab due to the diverging correlation length of density fluctuations. The scaling of the radius of gyration with chain length in the quasi-2D system increasingly resembles the scaling in the strictly 2D system as the chain length increases. At the extrapolated infinite chain critical temperature, the radius of gyration of the 2D system scales with chain length with exponent 0.56 ( 0.01 = (4/7), in agreement with theoretical predictions.
Chemical Physics, 2007
We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and m... more We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and molecular theory. In the Monte Carlo calculations we explicitly take into account counterions and polymer configurations and calculate electrostatic interaction using Ewald summation. Rosenbluth biasing, distance biasing, and the use of a lattice are all used to speed up Monte Carlo calculation, enabling the efficient simulation of the polyelectrolyte layer. The molecular theory explicitly incorporates the chain conformations and the possibility of counterion condensation. Using both Monte Carlo simulation and theory, we examine the effect of grafting density, surface charge density, charge strength, and polymer chain length on the distribution of the polyelectrolyte monomers and counterions. For all grafting densities examined, a sharp decrease in brush height is observed in the strongly charged regime using both Monte Carlo simulation and theory. The decrease in layer thickness is due to counterion condensation within the layer. The height of the polymer layer increases slightly upon charging the grafting surface. The molecular theory describes the structure of the polyelectrolyte layer well in all the different regimes that we have studied.
Macromolecules, 2005
We have studied the effect of branching on the solution phase behavior of branched homopolymers u... more We have studied the effect of branching on the solution phase behavior of branched homopolymers using grand canonical Monte Carlo (GCMC) simulations in conjunction with multihistogram reweighting and finite-size scaling analysis. The critical temperature (Tc) and the Θ temperature (Θ) decrease as polymer branching is increased, but the drop in Θ is less pronounced than that of Tc. The critical volume fraction (φc) rises with the degree of branching. Branched polymers are found to obey the Shultz-Flory relationship and exhibit a power-law behavior in φc vs chain length, with similar scaling exponents as those for their linear counterparts. Comparisons of the GCMC results are made to results of the lattice cluster theory (LCT). It is observed that the LCT significantly underestimates the impact of polymer branching on the critical behavior of polymers. We speculate this discrepancy between the two formulations to be due to an inadequate representation of the variation of polymer conformations with branching and neglect of fluctuations in the LCT theory.
Langmuir, 2005
We have investigated shear-induced alignment of a bilayer of spherical diblock copolymer micelles... more We have investigated shear-induced alignment of a bilayer of spherical diblock copolymer micelles within thin films using molecular dynamics simulations at two different levels of coarse-graining. At the microscopic level, the copolymers are modeled as bead and spring chains with specific interaction potentials which produce strongly segregated spherical micelles. The simulations qualitatively reveal that longrange shear-induced ordering of hexagonally arranged micelles arises because of the tendency of micelles to pursue trajectories of minimum frictional resistance against micelles in the opposing layer. This influences their alignment in the direction of shear without them breaking apart and reforming within the time scale of the simulations. As observed in experiments, the ordering is shown to be very sensitive to the film thickness and shearing rates. To access larger lengths and longer time scales, we further coarse-grain our system to a mesoscopic level where an individual micelle is represented by a spherical particle, which interacts with other micelles through an effective potential obtained from the microscopic simulations. This approach enables us to follow the time evolution of global order from locally ordered domains. An exponentially fast growth of the orientational correlation length of the hexagonal pattern at early times, followed by a crossover to linear growth, is found in the presence of shear, in contrast to the much slower power-law scalings observed in experiments without shear.
Computer Physics Communications, 2005
This paper presents results from Monte Carlo (MC) and molecular dynamics (MD) simulations on the ... more This paper presents results from Monte Carlo (MC) and molecular dynamics (MD) simulations on the shear-induced longranged alignment of cylindrical micelles in thin films. The surfactant is represented on a lattice and the shear flow is simulated via incorporation of a shear-induced potential energy term within the acceptance criteria in the MC simulations. The MD simulations are conducted on a coarse-grained, off-lattice surfactant while the shear flow is imposed in thin films by sliding confining walls in opposite directions. It is shown that the two methods lead to different steady state orientations of micelles. We also discuss several problematic issues concerned with incorporating shear or dynamics within MC schemes.
Physical Review Letters, 2005
Using molecular dynamics simulations, we show that sheared solutions of cylindrical micelle-formi... more Using molecular dynamics simulations, we show that sheared solutions of cylindrical micelle-forming amphiphiles behave very differently under extreme confinement as compared to the bulk. When confined to ultrathin films, the self-assembled cylindrical micelles roll along the shearing direction and align parallel to each other with their axes along the vorticity direction, as opposed to aligning parallel to the shearing direction in the bulk. It is shown that this new ''log-rolling'' phase arises due to a strong coupling between the rotational degree of freedom of the micelles and the steady sliding motion of the confining surfaces. We examine the microscopic mechanism of the log-rolling phenomenon and also discuss its dependence on the segregation strength and length of the amphiphile, the shear rate, and the film thickness.
Physical Review E, 2004
The behavior of confined cylindrical micelle-forming surfactants under the influence of shear has... more The behavior of confined cylindrical micelle-forming surfactants under the influence of shear has been investigated using Monte Carlo simulations. The surfactants are modeled as coarse-grained lattice polymers, while the Monte Carlo shear flow is implemented with an externally imposed potential energy field which induces a linear drag velocity on the surfactants. It is shown that in the absence of shear, cylindrical micelles confined within a monolayer coarsen gradually with Monte Carlo "time" t, the persistence length of the micelles scaling as t^{0.24}, in agreement with the scaling obtained experimentally. Under the imposition of shear, the micelles within a monolayer align parallel to the direction of shear, as observed experimentally. Micelles confined within thicker films also align parallel to each other with a hexagonal packing under shear, but assume a finite tilt with respect to the velocity vector within the velocity-velocity gradient plane. We propose a novel mechanism for this shear-induced alignment of micelles based on breaking up of micelles aligned perpendicular to shear and their reformation and subsequent growth in the shear direction. It is observed that there exists a "window" of shear rates within which such alignment occurs. A phenomenological theory proposed to explain the above behavior is in good agreement with simulation results. A comparison of simulated and experimental self-diffusivities yields a physical timescale for Monte Carlo moves, which enables an assessment of the physical shear rates employed in our Monte Carlo simulations.
We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and m... more We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and molecular theory. In the Monte Carlo calculations we explicitly take into account counterions and polymer configurations and calculate electrostatic interaction using Ewald summation. Rosenbluth biasing, distance biasing, and the use of a lattice are all used to speed up Monte Carlo calculation, enabling the efficient simulation of the polyelectrolyte layer. The molecular theory explicitly incorporates the chain conformations and the possibility of counterion condensation. Using both Monte Carlo simulation and theory, we examine the effect of grafting density, surface charge density, charge strength, and polymer chain length on the distribution of the polyelectrolyte monomers and counterions. For all grafting densities examined, a sharp decrease in brush height is observed in the strongly charged regime using both Monte Carlo simulation and theory. The decrease in layer thickness is due to counterion condensation within the layer. The height of the polymer layer increases slightly upon charging the grafting surface. The molecular theory describes the structure of the polyelectrolyte layer well in all the different regimes that we have studied.
Journal of Physical Chemistry B, 2004
The effect of confinement on the phase behavior of lattice homopolymers has been studied using gr... more The effect of confinement on the phase behavior of lattice homopolymers has been studied using grand canonical Monte Carlo simulations in conjunction with multihistogram reweighting. The scaling of critical parameters and chain dimensions with chain length was determined for lattice homopolymers of up to 1024 beads in strictly 2D and quasi-2D (slab) geometries. The inverse critical temperature scales linearly with the Shultz-Flory parameter for quasi-2D geometries, as it does for the bulk system. The critical volume fraction scales as a power law for all systems, with exponents 0.110 ( 0.024 and 0.129 ( 0.004 for the strictly 2D and slab geometries, respectively. The influence of confinement on critical behavior persists even in a thick slab due to the diverging correlation length of density fluctuations. The scaling of the radius of gyration with chain length in the quasi-2D system increasingly resembles the scaling in the strictly 2D system as the chain length increases. At the extrapolated infinite chain critical temperature, the radius of gyration of the 2D system scales with chain length with exponent 0.56 ( 0.01 = (4/7), in agreement with theoretical predictions.
Chemical Physics, 2007
We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and m... more We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and molecular theory. In the Monte Carlo calculations we explicitly take into account counterions and polymer configurations and calculate electrostatic interaction using Ewald summation. Rosenbluth biasing, distance biasing, and the use of a lattice are all used to speed up Monte Carlo calculation, enabling the efficient simulation of the polyelectrolyte layer. The molecular theory explicitly incorporates the chain conformations and the possibility of counterion condensation. Using both Monte Carlo simulation and theory, we examine the effect of grafting density, surface charge density, charge strength, and polymer chain length on the distribution of the polyelectrolyte monomers and counterions. For all grafting densities examined, a sharp decrease in brush height is observed in the strongly charged regime using both Monte Carlo simulation and theory. The decrease in layer thickness is due to counterion condensation within the layer. The height of the polymer layer increases slightly upon charging the grafting surface. The molecular theory describes the structure of the polyelectrolyte layer well in all the different regimes that we have studied.
Macromolecules, 2005
We have studied the effect of branching on the solution phase behavior of branched homopolymers u... more We have studied the effect of branching on the solution phase behavior of branched homopolymers using grand canonical Monte Carlo (GCMC) simulations in conjunction with multihistogram reweighting and finite-size scaling analysis. The critical temperature (Tc) and the Θ temperature (Θ) decrease as polymer branching is increased, but the drop in Θ is less pronounced than that of Tc. The critical volume fraction (φc) rises with the degree of branching. Branched polymers are found to obey the Shultz-Flory relationship and exhibit a power-law behavior in φc vs chain length, with similar scaling exponents as those for their linear counterparts. Comparisons of the GCMC results are made to results of the lattice cluster theory (LCT). It is observed that the LCT significantly underestimates the impact of polymer branching on the critical behavior of polymers. We speculate this discrepancy between the two formulations to be due to an inadequate representation of the variation of polymer conformations with branching and neglect of fluctuations in the LCT theory.
Langmuir, 2005
We have investigated shear-induced alignment of a bilayer of spherical diblock copolymer micelles... more We have investigated shear-induced alignment of a bilayer of spherical diblock copolymer micelles within thin films using molecular dynamics simulations at two different levels of coarse-graining. At the microscopic level, the copolymers are modeled as bead and spring chains with specific interaction potentials which produce strongly segregated spherical micelles. The simulations qualitatively reveal that longrange shear-induced ordering of hexagonally arranged micelles arises because of the tendency of micelles to pursue trajectories of minimum frictional resistance against micelles in the opposing layer. This influences their alignment in the direction of shear without them breaking apart and reforming within the time scale of the simulations. As observed in experiments, the ordering is shown to be very sensitive to the film thickness and shearing rates. To access larger lengths and longer time scales, we further coarse-grain our system to a mesoscopic level where an individual micelle is represented by a spherical particle, which interacts with other micelles through an effective potential obtained from the microscopic simulations. This approach enables us to follow the time evolution of global order from locally ordered domains. An exponentially fast growth of the orientational correlation length of the hexagonal pattern at early times, followed by a crossover to linear growth, is found in the presence of shear, in contrast to the much slower power-law scalings observed in experiments without shear.
Computer Physics Communications, 2005
This paper presents results from Monte Carlo (MC) and molecular dynamics (MD) simulations on the ... more This paper presents results from Monte Carlo (MC) and molecular dynamics (MD) simulations on the shear-induced longranged alignment of cylindrical micelles in thin films. The surfactant is represented on a lattice and the shear flow is simulated via incorporation of a shear-induced potential energy term within the acceptance criteria in the MC simulations. The MD simulations are conducted on a coarse-grained, off-lattice surfactant while the shear flow is imposed in thin films by sliding confining walls in opposite directions. It is shown that the two methods lead to different steady state orientations of micelles. We also discuss several problematic issues concerned with incorporating shear or dynamics within MC schemes.
Physical Review Letters, 2005
Using molecular dynamics simulations, we show that sheared solutions of cylindrical micelle-formi... more Using molecular dynamics simulations, we show that sheared solutions of cylindrical micelle-forming amphiphiles behave very differently under extreme confinement as compared to the bulk. When confined to ultrathin films, the self-assembled cylindrical micelles roll along the shearing direction and align parallel to each other with their axes along the vorticity direction, as opposed to aligning parallel to the shearing direction in the bulk. It is shown that this new ''log-rolling'' phase arises due to a strong coupling between the rotational degree of freedom of the micelles and the steady sliding motion of the confining surfaces. We examine the microscopic mechanism of the log-rolling phenomenon and also discuss its dependence on the segregation strength and length of the amphiphile, the shear rate, and the film thickness.
Physical Review E, 2004
The behavior of confined cylindrical micelle-forming surfactants under the influence of shear has... more The behavior of confined cylindrical micelle-forming surfactants under the influence of shear has been investigated using Monte Carlo simulations. The surfactants are modeled as coarse-grained lattice polymers, while the Monte Carlo shear flow is implemented with an externally imposed potential energy field which induces a linear drag velocity on the surfactants. It is shown that in the absence of shear, cylindrical micelles confined within a monolayer coarsen gradually with Monte Carlo "time" t, the persistence length of the micelles scaling as t^{0.24}, in agreement with the scaling obtained experimentally. Under the imposition of shear, the micelles within a monolayer align parallel to the direction of shear, as observed experimentally. Micelles confined within thicker films also align parallel to each other with a hexagonal packing under shear, but assume a finite tilt with respect to the velocity vector within the velocity-velocity gradient plane. We propose a novel mechanism for this shear-induced alignment of micelles based on breaking up of micelles aligned perpendicular to shear and their reformation and subsequent growth in the shear direction. It is observed that there exists a "window" of shear rates within which such alignment occurs. A phenomenological theory proposed to explain the above behavior is in good agreement with simulation results. A comparison of simulated and experimental self-diffusivities yields a physical timescale for Monte Carlo moves, which enables an assessment of the physical shear rates employed in our Monte Carlo simulations.