Magnetic particle‐loaded polymer brushes induced by external magnetic field: A Monte Carlo simulation

The effects of applied magnetic field on the system composed of polymer brushes and magnetic particles are studied by means of Monte Carlo simulation. The direction of the applied magnetic field is chosen to be perpendicular to the substrate plane. Polymer brushes and magnetic particles are attracted to each other. The average heights of polymer brushes depend not only on the strength of applied magnetic field (H) but also on the brush grafting density (σ) and the chain length (N). The applied magnetic field influences the arrangement of magnetic particles, and in turn, the arrangement of magnetic particles affects the spatial distribution of polymer brush monomers. When the strength of the magnetic field is increased, the average height of polymer brushes will be increased accordingly. The reason is that the orientations of the magnetic moments of particles must be along the magnetic field direction, and this leads polymer brushes to rearrange along the magnetic field direction. At the same time, the chain length and the grafting density of polymer brushes have also effects on the properties of the magnetic particles, such as the magnetic particles distribution, magnetic susceptibility, and pair correlation functions. Comparisons with the experimental ones are also made, and this investigation can provide some insights into statistical properties of magnetic particle‐loaded brushes induced by external magnetic field. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1873–1881, 2010     

The effect of magnetic field on conductive films

Conductive paints consisting of nickel powder (conductive material), solvent, and binder polymer were treated in a magnetic field. The efficiency of magnetic treatments on conductivity of coating films was evaluated. The effect of the following factors on conductivity of composite films by magnetic treatment was studied: metal powder concentration, magnetic flow density, time difference between film preparation and magnetic treatment, drying time of paint films, and effect of distance between terminals. Results showed that the volume resistivity of paint films treated magnetically was lower than that for untreated films at each nickel content. Magnetic treatment provided high conductivity even at low magnetic flow density, and conductivity increased with magnetic flow density.     

Estimation of magnetic particle clustering in magnetic fluids from static magnetization experiments

The volume fraction dependence of the static magnetization of two magnetic fluids with different degrees of steric stabilization was measured at low field values (0-10 kA/m) and it was found to be nonlinear for both magnetic fluids. The nonlinearity is more pronounced in the case of the less stabilized magnetic fluid. The experimental data were processed by nonlinear regression using an analytical model for the formation of chain-like magnetic particle aggregates in magnetic fluids. The calculated dependence on the degree of steric stabilization, magnetic field, and sample concentration of the mean number of particles per chain was in the range (1-1.04).     

Periodic deformations induced by a magnetic field in planar twisted nematic layers

Magnetic field-induced spatially periodic deformations of planar nematic layers twisted by an angle Φ were investigated numerically. Chiral nematics with pitches compatible with the twist angle and non-chiral nematics twisted by Φ ≤π/2 were considered. Two different modes of deformation, taking the form of stripes, were found: the so called Mode X, with periodicity parallel to the mid-plane director in the undisturbed structure, and Mode Y with periodicity perpendicular to the mid-plane director. The static director distributions were calculated for various magnetic field strengths, twist angles and elastic parameters. The influence of surface tilt was also investigated. Mode X appeared for sufficiently large Φ and was possible in nematics with typical elastic properties. Mode Y appeared provided that the k₂₂/k₁₁ elastic constant ratio and the twist angle Φ were sufficiently small. Both modes arose from the undistorted state when the magnetic field exceeded a threshold value. The spatial period of the patterns increased with field strength. At high field, regions with almost homogeneous deformation arose in the two halves of each stripe. Their width and, simultaneously, the spatial period diverged to infinity at some critical field. This divergence corresponds to the transition to a homogeneously deformed state. Diagrams were constructed showing the ranges of parameters favouring the periodic distortions.     

Nonlinear deformation of a ferrofluid droplet in a uniform magnetic field

This paper reports experimental and numerical results of the deformation of a ferrofluid droplet on a superhydrophobic surface under the effect of a uniform magnetic field. A water-based ferrofluid droplet surrounded by immiscible mineral oil was stretched by a magnetic field parallel to the substrate surface. The results show that an increasing flux density increases the droplet width and decreases the droplet height. A numerical model was established to study the equilibrium shape of the ferrofluid droplet. The governing equations for physical fields, including the magnetic field, are solved by the finite volume method. The interface between the two immiscible liquids was tracked by the level-set method. Nonlinear magnetization was implemented in the model. Comparison between experimental and numerical results shows that the numerical model can predict well the nonlinear deformation of a ferrofluid droplet in a uniform magnetic field.     

Enhancement of polarographic reduction currents by a static magnetic field

The effect of a magnetic field on d.c. polarographic reduction currents was studied with a static magnetic field applied perpendicularly to the dropping mercury electrode. In the presence of the magnetic field, diffusion or migration currents show a slight but distinct increase. The factors which can influence this effect have been examined experimentally. The effect is interpreted in terms of suppression of transfer of concentration polarization from one drop to the next. It is shown that certain types of maxima are enhanced by application of a magnetic field.     

磁场影响聚合物本体结晶动力学的机理分析

在经典的热力学理论基础上,探讨了磁场对聚合物本体结晶过程的成核与生长的影响,建立了相关结晶动力学理论方程.初步认为,磁场产生的＂磁结晶效应＂可能是由于晶相与非晶相之间磁化率差异导致了两相之间磁化能的差异,也可能由于聚合物体系在结晶前会形成一种有序相,减小了体系的熵值,进而改变了结晶过程中的体系自由能,影响其成核与晶体生长,乃至整个结晶动力学方程.利用Matlab软件结合PLLA的各结晶参数值,绘制了结晶自由能与各成核临界参数之间的函数图像.结果表明,在低过冷度下,较小的自由能扰动可能导致较大的晶核临界参数变化.     

Anomalous relaxation of a twisted cell under an external magnetic field: The nearly homogeneous relaxation

We report experimental and theoretical investigations of the dynamic behaviour of a π/2 twisted NLC layer in a magnetic field. When a magnetic field H is applied in the layer plane at a suitable angle β with respect to the easy axis on the first surface, the relaxation towards the equilibrium texture occurs through a slow decay of unstable textures. Depending on the values of H and β, the relaxation of the system can be nearly homogeneous or strongly inhomogeneous. In this paper we restrict our attention to the case where the relaxation occurs in a nearly homogeneous way. The theoretical relaxation time τw of the unstable textures is found to depend strongly on the angle β and on the amplitude of the magnetic field. The experimental dependence of τw on H and β is found to be in good agreement with the theoretical predictions. The relaxation process is extremely sensitive to small dishomogeneities of the director easy alignment on the surfaces. From the measured relaxation we are able to estimate a spread of 0.3° on the surface easy axes at a planar anchored SiO surface.     

Effect of a weak magnetic field on hematite sol in stationary and flowing systems

The effect of a weak magnetic field on the aggregation state and electrophoretic mobility of hematite sol was studied in flowing (dynamic) systems as a function of time and electrolyte concentration (0–60 mmol/dm³ KCl) and compared with the effect of the field in stationary (static) systems and flow in the absence of the field. During the entire treatment period, the pH remained almost constant (4.06–4.24). Conductance varied with KCl concentration, but except for minor fluctuations appeared to be unaffected by any form of treatment. While aggregation of hematite was observed during dynamic magnetic treatment (change in turbidity, scattered light intensity, and photon correlation spectroscopy), little effect on aggregation state was observed for the static systems or for the flowing systems in the absence of the field. Mobility also increased during the first 30 min of static and dynamic magnetic treatment. After longer treatment periods (90–120 min), the mobility decreased, but in almost all cases remained larger than in the case of untreated systems. Changes in both mobility and particle aggregation state also showed a significant dependence on electrolyte concentration. These effects are discussed in terms of a magnetohydrodynamic interaction between the magnetic field and the charged colloidal particles, which results only when the particles are made to pass rapidly through the field.     

Layer distortions induced by a magnetic field in planar samples of smectic C liquid crystals

This article derives theoretical results for the onset of the Helfrich-Hurault transition in smectic C liquid crystals induced by a magnetic field applied parallel to the smectic layers. A suitable quadratic energy in terms of the smectic layer displacement u is derived from the nonlinear version of the smectic C energy. This energy is minimized via averaging to enable the calculation of a critical field strength H_c for the onset of layer distortions. Comparisons are made with known results for the corresponding geometry in the smectic A case. An estimate for the value of the smectic C elastic constant A₁₂ can also be made by considering characteristic length scales.     

Self-consistent field theory and its applications in polymer systems

This review article addresses the widely used self-consistent field theory (SCFT) in interacting polymer systems. The theoretical framework and numerical method of solving the self-consistent equations are presented. In this paper, different structures of polymer can be considered, such as homopolymer, block copolymer, polydisperse polymer and charged polymer. Several systems, micro/macro phase separation, interface, self-assembly, are presented as examples to demonstrate its applications in details. Besides, the fluctuation effects are considered. The first order is Gaussian fluctuation theory, which can be used to determine the stability of the mean-field solution and predict the kinetics of unstable structure. The derivation and applications of Gaussian fluctuation theory are presented as well.     

Statistics of polymer chains in orienting fields

The theory of a freely jointed polymer chain is modified by introduction of interactions between dipole chain segments and an orienting field. Such a field results either from external forces (e.g. external electric or magnetic fields) or represents interactions between dipole segments of chains (molecular mean-field). The distribution of orientations of chain segments and the free energy of a chain in such orienting fields are calculated and discussed.     

Destabilizing effect of time-dependent oblique magnetic field on magnetic fluids streaming in porous media

The present work studies Kelvin-Helmholtz waves propagating between two magnetic fluids. The system is composed of two semi-infinite magnetic fluids streaming throughout porous media. The system is influenced by an oblique magnetic field. The solution of the linearized equations of motion under the boundary conditions leads to deriving the Mathieu equation governing the interfacial displacement and having complex coefficients. The stability criteria are discussed theoretically and numerically, from which stability diagrams are obtained. Regions of stability and instability are identified for the magnetic fields versus the wavenumber. It is found that the increase of the fluid density ratio, the fluid velocity ratio, the upper viscosity, and the lower porous permeability play a stabilizing role in the stability behavior in the presence of an oscillating vertical magnetic field or in the presence of an oscillating tangential magnetic field. The increase of the fluid viscosity plays a stabilizing role and can be used to retard the destabilizing influence for the vertical magnetic field. Dual roles are observed for the fluid velocity in the stability criteria. It is found that the field frequency plays against the constant part for the magnetic field.     

Orientation of semiflexible biopolymers in a magnetic field

A theory is presented to explain the stretching and orientation of a semiflexible chain such as the biopolymeric filamentous actin and deoxyribonucleic acid in a magnetic field. Both dilute and concentrated solutions are considered. The theory predicts the requirement of a high magnetic field to achieve a high degree of orientation in a dilute solution. The experimental value of the order parameter for a dilute, polydisperse F‐actin solution has been compared with the theoretical value. On the other hand, a molecular field (nematic liquid crystal) which exists in a concentrated solution can achieve the same effects of stretching and orientation and can be assisted by a magnetic field. Sources of diamagnetic anisotropy in biopolymers have been cited. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3256–3270, 2000     

The isotropic-nematic interface of colloidal goethite in an external magnetic field

Polarization microscopy was used to study the behavior around the isotropic-nematic interface of colloidal goethite dispersions in a magnetic field. It has been found before that the nematic phase is favored in an external field. In the case of goethite this was also observed; nematic droplets formed inside the isotropic phase and coalesced with the nematic phase. However, the behavior was found to be much richer because of the particle rotation around a certain critical field strength. The simultaneous occurrence of (parallel)nematic-(perpendicular)nematic phase separation under the influence of a magnetic field also plays a role here.     

Static and dynamic coupled perturbed Hartree-Fock vibrational (hyper)polarizabilities of polyacetylene calculated by the finite field nuclear relaxation method

The vibrational contribution to static and dynamic (hyper)polarizability tensors of polyacetylene are theoretically investigated. Calculations were carried out by the finite field nuclear relaxation (FF-NR) method for periodic systems, newly implemented in the CRYSTAL code, using the coupled perturbed Hartree-Fock scheme for the required electronic properties. The effect of the basis set is also explored, being particularly important for the non-periodic direction perpendicular to the polymer plane. Components requiring a finite (static) field in the longitudinal direction for evaluation by the FF-NR method were not evaluated. The extension to that case is currently being pursued. Whereas the effect on polarizabilities is relatively small, in most cases the vibrational hyperpolarizability tensor component is comparable to, or larger than the corresponding static electronic contribution.     

Analysis of lipid hydroperoxides and long-chain conjugated keto acids by negative ion electrospray mass spectrometry

Lipid hydroperoxides are important products of enzymatic processes and autooxidation products of polyunsaturated fatty acids. Analysis of such compounds has proved difficult in the past, but negative ion electrospray ionization mass spectrometry was found to be suitable for direct analysis. Abundant [M - H]⁻ ions were observed in full scan mode for hydroperoxyeicosatetraenoic (HPETE), hydroperoxyoctadecenoic acid isomers, and 5,12-diHPETE. Loss of water was observed for all species. Collisional activation and tandem mass spectrometry generated unique and characteristic spectra that shared some common features such as loss of small neutral molecules. More importantly, fragment ions that were indicative of the position of the hydroperoxide were observed. Collision-induced decomposition (CID) of [M - H₂O]⁻ for the HPETE isomers was found to be virtually identical to the CID mass spectra of the [M - H]⁻ anions from corresponding keto-eicosatetraenoic acids, which suggests that the hydroperoxide anions decompose via a dehydration intermediate that resembles the keto acid molecular anion. Cleavage of the double bond allylic to the hydroperoxide formed structurally characteristic ions at m/z 129 from 5-HPETE, m/z 153 from 12-HPETE, and m/z 113 from 15-HPETE. Charge-driven allylic fragmentation led to formation of m/z 203 from 5-HPETE, m/z 179 from 12-HPETE, and m/z 219 from 15-HPETE. Mechanisms consistent with the decomposition of stable isotope analogues are proposed for the formation of these and other characteristic ions. These specific decompositions can be used in multiple reaction monitoring to measure picomolar concentrations of hydroperoxides by direct high performance liquid chromatography tandem mass spectrometry.     

Superfluorescent transitions in an external magnetic field

Polarisation properties of the superfluorescence in the near-infrared regime have been investigated between high-lying levels of Sr and Ba under the influence of a static homogeneous external magnetic field. In some transitions the time-resolved measurements show a change of the polarisation of the superfluorescence depending on the magnetic field strength. In suitable experimental conditions intensity modulations were observed. These were assigned as Zeeman quantum beats or indirectly observed Zeeman superfluorescent beats. The experimental findings of superfluorescence in two-level, three-level, or multi-level configurations in dependence on the magnetic field strength can be explained well in a semiclassical model of multi-level superfluorescence.     

Interface effects in nematic liquid-crystalline structures of polymer solutions

A solution of long semirigid linear macromolecules was considered. The liquid-crystalline nematic ordering in the solution was analyzed theoretically using an Onsager-type approach. The orientation entropy was calculated in the frameworks of Lifshits' approach, successfully developed for this system originally by Khokhlov and Semenov. For homogeneous liquid-crystalline phase using the third virial approximation for intersegmental steric interaction the orientation distribution function, the free energy density, the isotropic-nematic coexistence and the spinodal conditions were computed numerically for two types of polymer flexibility mechanism: persistent chains and chains of freely joint segments. For the asymptotically exact second virial approximation the applicability region was analyzed. We considered the general equations, which describe the concentration and orientational segment distribution for a semirigid persistent polymer chain at a surface (or interface) of any shape and orientation. These equations were numerically solved for the case when the nematic director axis was perpendicular to a planar interface boundary between the real coexisting nematic and isotropic phases. The coordinate-dependencies of the polymer concentration and of the order-parameter take the smooth two-steps form in the interface region.     

Polymer conformation at the polymer melt surface

Due to the symmetry which characterizes the surroundings of each chain segment in the amorphous melt all (but end) segments are equivalent in their embedment in space. Energetic interactions between segments are confined to segments in contact. Hence, in the presence of a surface only the segments actually in the surface layer will be affected and will have a statistical weight and Boltzmann factor different from segments in the bulk. The number of segments so involved is fixed, since the extent of the surface is controlled. The partition function is comprised of factors which depend neither upon the order of the segments in the chain nor upon the conformation of any particular chain. Only segments finding themselves in the surface contribute to the surface tension, with the largest contribution deriving from the density transition in the interface. This, at most temperatures, is confined to a lamella one segment layer, or less, thick. For polymer of high enough molecular weight end-effects can be neglected and no allowance for the presence of ends is made here. It is then possible to reach a representation of surface tension in reduced coordinates, as shown already by Posner and Sanchez. Changes in chain conformation near the surface are calculated.