Neutral Walls Research Articles

Monte‐Carlo Study of Triblock Copolymer/Homopolymer Blend Films

AbstractThe morphologies of triblock copolymer/homopolymer blend films confined between two neutral hard walls were studied via MC simulations on a simple cubic lattice. For ABA/A and ABA/B blend films, the effects of φh (the volume fraction of the homopolymer) and Mh/Mb (the ratio of the molecular mass of the homopolymer to that of the corresponding blocks) on the morphologies were investigated in detail. For both ABA/A and ABA/B blend films, a higher φh or Mh/Mb would result in stronger macrophase separation between the triblock copolymer and homopolymer. For ABA/C blend films, Mh/Mb hardly influences the morphologies of homopolymer domains regardless of whether the homopolymer C is more compatible with block A or with block B. Compared to AB/A and AB/C blend films, the morphologies of ABA/A (or ABA/B) and ABA/C blend films are much more irregular. The simulated results in this work show good consistency with experiments and other simulations.magnified image

Fluctuational-electromagnetic interaction of a moving nanoparticle with walls of a flat dielectric gap

General expressions for the tangential and normal components of the force of interaction and the rate of heat exchange between a neutral spherical nanoparticle (atom) and dielectric walls of a flat vacuum gap are obtained for the first time in a nonrelativistic nonretarded approximation of the fluctuational electromagnetic field theory. In the general case, the walls have a temperature different from that of the particle and possess arbitrary permittivities.

Conformational Study on Thin Films of Symmetric AnB2nAn Triblock Copolymer

AbstractSummary: The behavior of symmetric AnB2nAn triblock copolymer films confined between two hard neutral walls was explored by Monte Carlo simulation. The thicknesses of the films were between ≈1Rg0 and ≈7Rg0, where Rg0 is the unperturbed radius of gyration in the bulk. The confinement leads to a lamellar structure normal to the wall and the order‐disorder transition (ODT) temperature was found to be a function of film thickness. When the film thickness (D) was less than a critical value, DC, which is between 3Rg0 and 4Rg0, the ODT temperature (T*ODT) reduced by chain length N (T*ODT/N) decreased with decreasing film thickness. However, T*ODT/N was nearly independent of the film thickness when it was greater than DC. In the case of strong confinement (D < DC), the B block shrinks along the direction perpendicular to the wall and stretches along the direction parallel to the wall with decreasing film thickness, and the volume occupied by the B block shrinks. Under weak confinement conditions (D > DC), the volume of the B block is nearly independent of film thickness. The conformations of the B block in the disordered state are quite different from those in the lamellae. If the film is thick enough, the volume of the B block approaches its value in the unperturbed state, regardless of the morphology. When temperature decreases, the B block stretches in the direction perpendicular to the A/B interface and shrinks in the other two directions. In addition, decreasing the temperature leads to the chains adopting two main extreme conformations, coiling or stretching as much as they can. The scaling behavior of the fraction of bridge chains vs. the temperature obtained in the weak segregation limit was different from that predicted in the strong segregation limit.Schematic diagram of the X, Y and Z axis definition.magnified imageSchematic diagram of the X, Y and Z axis definition.

Measurement of Vapor Pressures and Saturated Liquid Densities of Pure Fluids with a New Apparatus

A new apparatus has been constructed to measure vapor−liquid equilibria and saturated liquid densities of pure and mixed fluids. The density measurement is based on the buoyancy method using a single sinker and a magnetic suspension balance. The permanent magnet of the magnetic suspension balance carries the sinker with a load-coupling device. Both are completely submerged in the liquid phase. The electromagnet is placed outside the magnetically neutral cell walls. Both magnets transfer the buoyancy force of the sinker through the walls to a microbalance. The apparatus is limited to the temperature range from (−60 to 250) °C, pressures up to 200 bar, and densities in the range from (10 to 2000) kg·m-3. Here, the first tests have been done with pure fluids at temperatures from (−40 to 90) °C and pressures up to 60 bar. Measurements of the compressed liquid density of water and gas density of nitrogen, carbon dioxide, and R134a confirm the accuracy of the density measurement. Also, measurements of the vapor...

Gaussian ellipsoid model for confined polymer systems

Polymer systems in slab geometries are studied on the basis of the recently presented Gaussian ellipsoid model [F. Eurich and P. Maass, J. Chem. Phys. 114, 7655 (2001)]. The potential of the confining walls has an exponential shape. For homogeneous systems in thermodynamic equilibrium we discuss density, orientation, and deformation profiles of the polymers close to the walls. For strongly segregated mixtures of polymer components A and B equilibrium profiles are studied near a planar interface separating A and B rich regions. Spinodal decomposition processes of the mixtures in the presence of neutral walls show upon strong confinement an increase of the lateral size of A and B rich domains and a slowing down of the demixing kinetics. These findings are in agreement with predictions from time dependent Ginzburg–Landau theory. In the case, where one wall periodically favors one of the two mixture components over the other, different equilibrium structures emerge and lead to different kinetic pathways of spinodal decomposition processes in such systems.

Lamellar phases confined in quasicylindrical pores: lattice model results.

A two-dimensional (2D) vector lattice model of microemulsions is applied to study the structure of lamellar phases confined in long rectangular pores. One-point distribution functions are calculated within mean field approximation. The effects of pore geometry and surface fields are considered. A 2D analog of an onion phase is favored by a pore with strongly hydrophilic walls. For neutral walls, far from the phase boundaries, the lamellar phase is stable inside the pore. By contrast, close to the lamellar-tubular phase boundary a pore with neutral walls favors a 2D tubular phase. This is the analog of capillary condensation. In all cases the excess pressure is calculated as a function of the pore geometry.

Influence of pH and nature of the anion on the synthesis of pure and iron-containing mesoporous silica

Different salts are employed in the syntheses of pure and iron-containing mesoporous silica. Fluoride ions buffer the pH of the synthesis mixture and favour introduction of iron in the final product. Samples synthesized in neutral conditions present thick pore walls and relatively low pore volume, with excellent stability upon activation. Extraction of surfactants upon washing and shrinking of pore walls upon calcination are responsible for severe modifications of the textural properties of samples synthesized in acidic conditions.

Stokesian dynamics study of quasi-two-dimensional suspensions confined between two parallel walls

We present a Stokesian dynamics (SD) computer simulation study of the static and dynamical properties of a monolayer of spherical colloidal particles restricted to diffuse in the midplane between two parallel walls. SD simulations account for hydrodynamic interactions (HI's) among the particles, and between particles and walls. Three different types of systems are studied: first, a monolayer of neutral spheres and neutral hard walls; second, particles interacting by a repulsive Yukawa-type potential of range depending on the wall separation. As a third system, the interesting case of charged particles between charged parallel walls with a longer-range attractive part in the pair potential is investigated, using the experimentally determined effective pair potential of Acuna-Campa et al. [Phys. Rev. Lett. 80, 5802 (1998)]. Various measurable quantities are calculated in dependence of the particle concentration and the wall distance: short- and long-time self-diffusion coefficients, radial distribution functions and static structure factors, hydrodynamic functions, mean squared displacements, and van Hove real-space correlation functions. We assess the importance of HI's by comparing our results with simulation results where HI's are fully or partially disregarded. Some of our results are also compared with experimental data, and good agreement is found. Remarkable effects are investigated, like the hydrodynamic enhancement of self-diffusion for the case of strongly charged particles, and the strong increase of the hydrodynamic function at small wave numbers.

Monte Carlo Simulation of Amphiphile Self-Assembly during Dip Coating

AbstractFascinating nanostructured porous materials have recently been synthesized by evaporation-driven coassembly of ceramic precursors and amphiphiles. To expand our understanding of this process, we examine the influence of interfaces on self-assembly process using equilibrium lattice-based Monte Carlo simulations of ternary amphiphile-solvent mixtures. The simulations are able to predict the existence of all significant lyotropic mesophases, including lamellae, hexagonal closepacked cylinders, and cubic phases such as the gyroid. In the presence of walls that attract the majority solvent, the amphiphiles are confined to a smaller region of space, and experience a higher local concentration than the bulk concentration. This can lead to early transitions between mesophases. On the other hand, when the surface repels the majority solvent, amphiphiles tend to adsorb at the walls, and the local effective concentration in solution is lower. This can delay mesophase formation. When the amphiphile concentration is high enough that mesophases form in the bulk solution, however, either type of strongly attracting wall will align the mesophase (lamellae or hexagonal channels) parallel to the walls. In contrast, neutral walls (with no preferential interaction with either component) align mesophases perpendicular to themselves, which could be an interesting route to pores aligned normal to a film.

Block copolymer films between neutral walls: A Monte Carlo study

We study thin, symmetric AB-copolymer films between neutral walls using the bond fluctuation model. Near the surface, the monomer distribution with respect to the chains’ center of mass is nonisotropic: The chains flatten on the surface. The resulting A- and B-lamellae have interfaces practically perpendicular to the walls, as also observed in recent experiments. Our simulations indicate the onset of long-range surface ordering before the bulk orders. The strongest effects take place between the spinodal and the order–disorder transition point of the diblock copolymer melt.

Rheology of dilute polyelectrolyte solutions in narrow channels

The shear flow of dilute polyelectrolyte solutions bounded by either neutral or repulsive walls is modeled using a nonlinear dumbbell with conformation-dependent friction. Assuming that the configurational probability density function depends on the internal coordinates (r) and the distance of the center of mass of the molecule to the walls, coupled differential equations for the tensor moments are obtained. Coulombic repulsion between beads is considered to simulate the charge repulsion between ionized sites distributed along the backbone of a real polyelectrolyte. The repulsive interaction between the polyelectrolyte molecule and the charged walls is that of the DLVO model and the molecule is considered to be a charged sphere. Numerical solutions for the components of the tensor are worked out with the preaverage approach, and only when neutral walls considered are exact solutions obtained. Viscosity results show that in the limit of very wide channels, the corresponding viscosity in the bulk is obtained. The wall repulsion on the charged molecules produces migration of molecules towards the center of the channel resulting in a depleted layer with lower viscosity next to the walls. The calculated slip phenomenon using the method employed by Grisafi and Brunn is dependent on the beads repulsion and the shear rate. The slip velocity obtained with the Mooney method shows similarities with available experimental results for polyelectrolyte solutions. Birefringence calculations are performed in narrow and wide channels for different bead repulsions, with interesting results for both flexible and rigid molecules.

Electro-deposition of polymer chains on an adsorbing wall: Density profiles and wall coverage

Growth of polymer density in an electro-deposition model of polymer chains on an impenetrable wall is studied on a two dimensional discrete lattice using a Monte Carlo simulation. Polymer-polymer repulsion and polymer-wall attraction for the adsorbing wall (along with the neutral and repulsive interactions) are considered in an external field. Effects of the field strength (B), temperature (T), and chain length (Lc) on the density profile of the polymer chains and wall coverage are investigated. The spatial density profile shows onset of oscillation near the wall at a characteristic field (Bc) which depends on chain length and temperature. In low field, adsorption-to-desorption transition at the wall appear on increasing the temperature (unlike neutral and repulsive walls). In high field regime, on the other hand, a non-monotonic dependence of coverage on temperature is observed with a maximum at a temperature (Tm) which increases on increasing B. The equilibrium value of the polymer density (pd) shows a power-law decay with the chain length, pd∼Lc−α, at the wall and in the bulk with corresponding values of the exponent αW and αB; these exponents differ substantially and depend on B, T, and Lc. The coverage decays monotonically with the chain length at a constant temperature and field.

Static Properties of Homopolymer Melts in Confined Geometries Determined by Monte Carlo Simulation

The static properties of a homopolymer melt between energetically neutral walls are studied as a function of the degree of confinement (plate spacing) and polymer molecular weight. The cooperative motion lattice model is used and allows all calculations to be performed at full occupancy (i.e., at a volume fraction of 1); chain lengths varying from 384 to 24 are examined. Properties investigated include the magnitude of the end-to-end vector, the radius of gyration, and components of the end-to-end vector. In addition, orientation effects and the distribution of chain ends are also calculated. As the chains are confined, the end-to-end vectors align parallel to the walls beginning at a value of about twice the unconstrained value of the radius of gyration, regardless of the molecular weight. For values less than this, chain conformations are distorted and the magnitude of the average end-to-end vector and radius of gyration increase. The components of the end-to-end vector parallel and perpendicular to the...

Rheology of Colloidal Particles in a Confined Channel under Shear Flow by Brownian Dynamic Simulations

A colloidal system of strongly charged particles, confined between two charged walls, is studied under static conditions and in the presence of shear flows. Brownian Dynamics simulations (BD) are used to determine the concentration profiles for several separations of charged walls. Results show good agreement with those obtained under static conditions using the Hypernetted Chain Approximation (HNC) and Monte Carlo simulations. Results obtained with BD and HNC for neutral walls show more similarities than those between BD and Monte Carlo simulations depending on the initial state of the colloidal particles. The presence of a shear flow field perturbs the equilibrium concentration profiles and the distribution function in the flow direction, generating a structureless system, as confirmed by the absence of peaks in the radial distribution function. The mobility of the particles in the transversal direction decreases rapidly and becomes practically frozen. The flow exhibits a non-Newtonian behavior with shear-thinning viscosity. Due to the interparticle interactions and particle–wall interactions, the viscosity is lower as the wall separation decreases, giving rise to an apparent slip in the colloidal suspension. The slip velocity for repulsive walls is higher than that obtained with neutral walls and increases with the shear stress according to a power law, as observed in polymer solutions. The shear viscosity and the normal stress differences depend strongly on the combined effect of confinement, concentration of particles, magnitudes of inter-particle interactions and wall–particle repulsion.

Equilibrium Orientation of Confined Diblock Copolymer Films

Using a two-dimensional Scheutjens and Fleer self-consistent field calculation, we determine the equilibrium morphology of thin films of symmetric AB diblock copolymers confined between hard, smooth plates. A lamellar phase is established with the stripes either perpendicular or parallel to the walls. With neutral walls, the perpendicular orientation is stabilized by the nematic ordering of the monomers, which arises from the orientational constraint imposed by the walls. When the substrates are composed of pure A monomer (thus repulsive to B), there are transitions from strained parallel conformations, which wet the substrates with A polymer, to distorted perpendicular configurations as the film thickness is varied. It is possible that the removal of one of the walls (the usual experimental scenario of thin films spun cast onto a substrate) can still lead to spontaneous and robust pattern formation on the scale of tens of nanometers.

Polymer melt near a solid wall

We develop a theory for the equilibrium concentration profile formed by a compressible polymer melt near a solid wall (or the mathematically equivalent incompressible polymer solution near a solid wall). The theory uses a Landau–Ginzburg free energy functional with a concentration dependent square gradient coefficient and an additional local contribution characterizing the influence of the wall. We introduce a mean-field algorithm for constructing the surface free energy contribution from the expression for the bulk free energy of lattice polymers. This algorithm automatically includes both energetic and entropic contributions with no adjustable parameters for lattice systems and can be applied for branched polymers as well. Approximate analytical solutions are provided for one-phase polymer density profiles at neutral, repulsive, and attractive walls. The approximate solutions reflect the behavior of the numerical solutions and display (only at lower polymer densities) a near wall linear variation of the polymer density which crosses over to an exponential approach to the bulk concentration. The numerically evaluated profiles for both athermal and nonathermal melts compare well with available Monte Carlo simulation data for a neutral wall. Physical arguments are presented which anticipate the existence of deviations between theory and simulations at higher densities. The use of lattice cluster theory free energy functions enables us for the first time to investigate the dependence of the density profile on the polymer architecture.

Contact theorems for rough interfaces

Contact theorems for rough surfaces are discussed. Simple relations for the average contact density are obtained for neutral and charged walls. When the walls are not planar there are new contributions proportional to the field gradients near the charged wall.

Light-induced isomerization and dimerization of cinnamic acid derivatives in cell walls

The role of light in the isomerization and cyclodimerization of the cinnamic acid component of plant cell walls has been examined in a number of ways. Cinnamic acid derivatives were extracted from neutral detergent fibre cell walls by alkaline hydrolysis and quantified by GC. Amounts of these acids in leaf and stem tissues of barley straw which had naturally been exposed to very different levels of radiation were compared. Leaf cell walls contained less p-coumaric acid than stem cell walls, but a higher proportion of this was found as the ( Z)-isomer and cyclodimer (4,4′-dihydroxy-α-truxillic acid). Samples of wheat straw from a sequence of 10 consecutive years, grown under conditions where meteorological conditions were the major variable, showed considerable year-to-year differences; the ( Z)-/( E)-ratio and dimer/monomer ratio varied three-fold. Additional studies compared the direct effect of light on the cinnamic acid derivatives in growing barley seedlings (live), freeze-dried seedlings (dead) and freshly harvested straw samples (dead). There were no significant changes in the cinnamic acid derivatives extracted from the cell walls of dead tissue which had been stored in the dark. Storage in light resulted in increased ( Z)-isomer and cyclodimer contents. These changes occurred whether the tissue was alive or dead. They can, therefore, be directly attributed to the effect of light, and do not appear to be mediated by any enzymic reactions.

Simulations of solvent effects on confined electrolytes

Monte Carlo simulations are reported for the solvent primitive model (SPM) of the electrical double layer at electrically charged and neutral surfaces. Both solvent and ions are modeled as hard spheres with interactions governed by hard sphere and Coulomb interactions. Density profiles of solvent and ions and electrostatic potential profiles are presented for 1:1 electrolyte concentrations of 1 and 2 M at 300 K. Comparison of results at charged and neutral walls indicates that the density distribution of a dense solvent near a wall induces significant layering of ions and decrease of potentials in the double layer. This layering effect cannot be captured in simulations of the conventional primitive model (PM) of the double layer. The solvent induced steric ordering of electrolyte ions at a neutral wall can create a space charge layer.

Computer simulation of polymers in thin layers. I. Polymer melt between neutral walls – static properties

Systems of linear chains on the face centered cubic lattice with all lattice sites occupied are simulated by the cooperative motion method. Static properties of chains are analyzed and various statistical models of chain shapes are presented, differing in details of individual chain states taken into account. The structure of thin layers of a polymer melt confined between two parallel neutral walls is compared with the melt structure in bulk. It is shown that the effect of neutral walls consists mainly in reorientation of cigar-shaped chains in the vicinity of walls, but not in changes of dimensions and shapes of chains.