Shyamal K. Nath

On the simulation of vapor–liquid equilibria for alkanes

A Monte Carlo simulation study has been conducted to assess the ability of recently proposed force fields to predict orthobaric densities, second virial coefficients, and P-V-T data for short and long alkanes. A new, modified force field is proposed that provides good agreement with experimental phase equilibrium and second virial coefficient data over wide ranges of temperature and chain length.

Symmetric diblock copolymer thin films confined between homogeneous and patterned surfaces: Simulations and theory

We have investigated the ability of a simple phenomenological theory to describe the behavior of symmetric diblock copolymer thin films confined between two hard surfaces. Prior knowledge of the morphology in the confined films is crucial for applying this theory to predict the phase diagram of such systems. Taking advantage of our observations in Monte Carlo simulations, we use the theory to construct phase diagrams for thin films confined between patterned-homogeneous surfaces, and obtain good agreement with our results of simulations. Two conditions are essential for obtaining long-range ordered perpendicular lamellae: a lower stripe-patterned surface with the surface pattern period Ls comparable to the bulk lamellar period L0, and an upper neutral or weakly preferential surface. We have also examined the undulation of perpendicular lamellae between two hard surfaces. For the cases of two homogeneous (preferential) surfaces and patterned-preferential surfaces, our calculations using the phenomenologica...

A new united atom force field for α-olefins

A new united-atom force field is proposed for α-olefins. The proposed force field is used to study vapor–liquid equilibria for pure components and binary mixtures. Results of phase equilibria simulations are found to be in good agreement with available experimental data.

Simulation of vapour-liquid equilibria for branched alkanes

A new united-atom force field is proposed for branched and linear alkanes. The proposed force field is used to study vapour-liquid equilibria for pure components and binary mixtures. Results of phase equilibria simulations are found to be in good agreement with available experimental data.

DENSITY FUNCTIONAL THEORY OF MOLECULAR STRUCTURE FOR THIN DIBLOCK COPOLYMER FILMS ON CHEMICALLY HETEROGENEOUS SURFACES

A density functional theoretical formalism is applied to study the ordering of block copolymers near patterned surfaces. This formalism is capable of resolving with molecular-level detail the structure of the system in the directions parallel and perpendicular to an interface. The results of theoretical predictions are in good agreement with those of molecular simulations. This formalism permits a systematic study of the effects of temperature, chain architecture, surface–polymer interaction energy, and pattern shape and size on the surface-induced ordering that occurs in systems consisting of block copolymers deposited on chemically heterogeneous surfaces.

New forcefield parameters for branched hydrocarbons

A new set of united-atom force field parameters is proposed for simulating the phase equilibria of branched alkanes. These parameters complement the already existing set of Nath, Escobedo, and de Pablo revised (NERD) [Nath et al., J. Chem. Phys. 105, 4391 (1998); Nath and de Pablo, Mol. Phys. 98, 231 (2000)] force field parameters. The proposed force field is used to study vapor–liquid equilibria for various isomers of alkanes up to C8. Results of simulations are found to be in good agreement with available experimental data.

Thermodynamic and transport properties of nitrogen and butane mixtures

A force field has been developed to describe the phase behaviour, interfacial, and transport properties of nitrogen and hydrocarbon mixtures under conditions relevant to those found in the high pressure extraction of oil from underground reservoirs. A Gibbs ensemble Monte Carlo method is used to parametrize intermolecular potentials for the pure components by matching experimental and simulated liquid and vapour coexisting densities. Also the surface tension, diffusion coefficient and shear viscosity of nitrogen and its mixtures with butane have been determined. The latter properties were obtained by canonical molecular dynamics simulations. The diffusion coefficient and shear viscosity were calculated by a Green-Kubo method. Results for pure nitrogen are given for temperatures ranging from 70 K to 110K. For mixtures of nitrogen with butane, results are presented at 339.4 K and 380.2 K. Good agreement is found between the results of simulations and available experimental data.

The ordering of symmetric diblock copolymers: A comparison of self-consistent-field and density functional approaches

Polymer reference interaction site model (PRISM) based density functional (DF) theory is used to evaluate the structure and thermodynamics of structurally symmetric, freely jointed, diblock chains with 0.50 volume fraction. These results are compared to the results of self-consistent-field (SCF) theory. Agreement between the predictions of the SCF and DF theories is found for the lamella spacing well above the order–disorder transition (ODT) and for the qualitative behavior of the interfacial thickness as a function of both chain length and Flory–Huggins χ parameter. Disagreement is found for the magnitude of the interfacial thickness where DF theory indicates that the thickness is 1.7±0.2 times larger than that predicted by SCF theory. It appears that behavior on the monomer length scale is sensitive to system specific details which are neglected by SCF theory.

THE STRUCTURE OF A ROTATIONAL ISOMERIC STATE ALKANE MELT NEAR A HARD WALL : COMPARISON OF DENSITY FUNCTIONAL THEORY WITH RELATED THEORIES

Density functional theory is applied to inhomogeneous, rotational isomeric state polymer melts. In particular, a melt of tridecane near a hard wall is investigated, and the variation of polymer–wall correlation functions as a function of packing fraction is of primary interest. In addition to the evaluation of the wall–polymer density profile and the fractional distribution of sites, we use the relation between pressure and contact density to calculate the equation of state of the bulk. Agreement with the generalized Flory dimer equation of state is excellent, and this, in conjunction with our earlier comparison [Sen et al. J. Chem. Phys. 101, 9010 (1994)] with full, multichain simulation, indicates that the density functional theory gives an accurate description of inhomogeneous polymer melts.

The interfacial thickness of symmetric diblock copolymers: Theory and experiment

A recent application of density functional theory to the structure and thermodynamics of the ordering of symmetric, tangent hard site, diblock copolymers [S. K. Nath et al., J. Chem. Phys. 106, 1950 (1997)] predicted an interfacial thickness larger than would be expected from previous self-consistent-field studies of thread chains. Here we compare the theoretical predictions with the few experimental measurements of interfacial thickness in symmetric diblocks. It is observed that predictions of the thickness of the interface are sensitive to the details of the monomer structure included in the underlying model, and that the range of the experimental measurements is spanned by the two theoretical models.