Search results for "molecule"

Theoretical modeling of Langmuir monolayers

1999

Abstract We study coarse-grained continuum models for Langmuir monolayers by self-consistent field theory and by Monte Carlo simulations. Amphiphilic molecules are represented by stiff chains of monomers with one end grafted to a planar surface. In particular, we discuss the origin of successive fluid–fluid transitions, the possible origin of tilt order and the factors which determine the direction of tilt.

Dilute solution rheology of flexible macromolecules (bead–rod model)

1974

The rheological behavior of dilute solutions of flexible macromolecules is studied by means of a freely jointed multiple bead–rod model. The solution of the equations describing the mechanics of the system is obtained by means of a numerical procedure, which applies to arbitrary flow conditions. The case of the transient stress in uniaxial elongational flow is developed in some detail. A comparison with bead–spring models shows both quantitative and qualitative differences which are briefly discussed.

Core-melted clusters

1999

The possibility of the existence of a core-melted cluster is investigated. To this end, a pair potential is introduced, with the property that the solid state of the cluster is less dense than the liquid state. With this kind of potential, the cluster exhibits a quite unusual behavior. In addition to the known states, solid, liquid, and surface-melted, it can also be found in a “dense-liquid” phase (a disordered state appearing at low temperatures), a “core-melted” phase, and a “core-surface-melted” phase. In the core-melted phase, the external part of the cluster consists of atoms that are vibrating around regular crystalline sites, while the core atoms have much bigger mobility, sometimes…

Monte Carlo Simulations in Polymer Science

2012

Monte Carlo methods are useful for computing the statistical properties of both single macromolecules of various chemical architectures and systems containing many polymers (solutions, melts, blends, etc.). Starting with simple models (lattice models such as the self-avoiding walk or the bond fluctuation model, as well as coarse-grained or chemically realistic models in the continuum) various algorithms exist to generate conformations typical for thermal equilibrium, but dynamic Monte Carlo methods can also model diffusion and relaxation processes (as described by the Rouse and the reptation models for polymer melt dynamics). Limitations of the method are explained, and also the measures to…

Shear Modulus of an Irreversible Diblock Copolymer Network from Self-Consistent Field Theory

2019

Using self-consistent field theory, we investigate the stretching-induced microphase separation in an irreversibly cross-linked polymer network composed of diblock copolymer chains and estimate its...

Symmetric diblock copolymers confined into thin films: A Monte Carlo investigation on the CRAY T3E

2000

We present the results of large scale computer simulations targeted at investigating the phase stability and the structure of symmetric AB diblock copolymers in thin films. The connectivity of the two different monomer species A and B in the diblock copolymer prevents macrophage separation and the molecules assemble into A-rich and B-rich domains on the scale of the molecule’s extension. This large length scale of the ordering phenomena makes these polymeric systems a promising candidate for revealing the universal features of self-assembling in amphiphilic molecules. However, the widely spread length and time scales impart protracted long relaxation times to the systems and pose a challeng…

Computer Simulations for Polymer Dynamics

1991

In this paper we review recent work on the dynamics of polymeric systems using computer simulation methods. For a two-dimensional polymer melt, we show that the chains segregate and the dynamics can be described very well by the Rouse model. This simulation was carried out using the bond fluctuation Monte Carlo method. For three-dimensional (3d) melts and for the study of hydrodynamic effects, we use a molecular dynamics simulation. For 3d melts our results strongly support the concept of reptation. A detailed comparison to experiment shows that we can predict the time and length scales for the onset of reptation for a variety of polymeric liquids. For a single chain, we find the expected h…

Anomalous diffusion of polymers in supercooled melts near the glass transition

2007

Two coarse-grained models for polymer chains in dense melts near the glass transition are investigated: the bond fluctuation lattice model, where long bonds are energetically favored, is studied by dynamic Monte Carlo simulation, and an off-lattice bead-spring model with Lennard-Jones forces between the beads is treated by Molecular Dynamics. We compare the time-dependence of the mean square displacements of both models, and show that they become very similar on mesoscopic scales (i.e., displacements larger than a bond length). The slowing down of motions near the glass transition is discussed in terms of the mode coupling theory and other concepts.

Structure of diblock copolymers in supercritical carbon dioxide and critical micellization pressure

1999

This paper reports a small angle neutron scattering investigation of micelle formation by fluorocarbon-hydrocarbon block copolymers in supercritical ${\mathrm{CO}}_{2}{(\mathrm{s}\mathrm{c}\ensuremath{-}\mathrm{C}\mathrm{O}}_{2})$ at 65 \ifmmode^\circ\else\textdegree\fi{}C. A sharp unimer-micelle transition is obtained due to the tuning of the solvating ability of ${\mathrm{s}\mathrm{c}\ensuremath{-}\mathrm{C}\mathrm{O}}_{2}$ by profiling pressure, so that the block copolymer, in a semidilute solution, finds ${\mathrm{s}\mathrm{c}\ensuremath{-}\mathrm{C}\mathrm{O}}_{2}$ a good solvent at high pressure and a poor solvent at low pressure. At high pressure the copolymer is in a monomeric state…

1986

An osmotic pressure equation proposed over 50 years ago is found to be consistent with the des Cloizeaux scaling relation for semi-dilute polymer solutions in good solvents. With a physically plausible modification, the equation can also give a satisfactory representation of dilute solutions and of the cross-over to the semi-dilute regime.