Transient electric birefringence of wormlike macromolecules in electric fields of arbitrary strength: a computer simulation study

We have studied the birefringence decay of linear models of macromolecules for two different types of flexibility, the broken-rod chain and the wormlike chain, using a computer simulation of a transient electric birefringence experiment. We have paid particular attention to the influence of the intensity of the orienting field, including two orienting mechanisms, the induced dipole, and the permanent dipole. We have compared wormlike and broken-rod models of the same radius of gyration, finding that they present a different decay curve under the influence of the same intensity of the field. We have seen that these differences are due to the faster relaxation times (smaller in the wormlike chain model) and amplitudes, because, regardless of the type of flexibility, the overall size of a molecule (measured by the radius of gyration) essentially determines the longest relaxation time. We have also analyzed how the relaxation process is affected by the degree of flexibility, the orientation mechanisms, and the intensity of the field. Studying a different aspect, we have paid attention to the deformation of a molecule in a transient electric birefringence experiment as a source of information. In this work we have developed equations to characterize this deformation in terms of one of the components of the gyration tensor, if a dynamic light scattering experiment under the influence of an electric field could be performed. To develop this work we have simulated the Brownian dynamics of the different models, relaxing after the removal of an orienting external electric field of arbitrary strength. A comparison with other methods such a the rigid body treatment or the correlation analysis of Brownian trajectories has also been included. We have seen that differences between the two Brownian dynamics methods are small and that the rigid-body treatment is only an acceptable approximation to obtain the longest relaxation time.     

Orientational ordering of a polymer chain in a strong dipole field. Tetrahedral lattice model

Orientational and conformational properties have been investigated of a rotational isomeric model of a polar polymer chain on a tetrahedral lattice in a strong electric or mechanical field of dipole symmetry. Two types of dipole moment distribution along the chain are discussed: (A) constant signs of longitudinal components of the dipole moments, and (B) alternating signs of the longitudinal components of the dipole moments. The second case represents polymer chains such as $ \rlap{--} ({\rm CH}_{\rm 2} \hbox{---} {\rm CR}_{\rm 2} {\rm \rlap{--} )}_n $>/UEQN> when the dipole moments are oriented along the bisector of the CR₂-angle, i.e., normal to the extended trans-chain conformation. It is shown that only a discrete most probable orientation of the lattice relative to the field should be considered, namely that coinciding with one of the symmetry axes of the lattices. The average dipole order parameter and dichroic functions (quadrupole order parameter) are calculated in a strong external dipole field for unit vectors with different orientation relative to the chain backbone. The quadrupole order parameter for different unit vectors is obtained also as a function of chain elongation. The polarizability induced by an additional weak dipole field is calculated as a function of the magnitude of the strong external dipole field. For the model considered here the order parameters are more strongly influenced by the external field than those for the freely jointed chain (FJC) model having the same distribution of dipole moments along the chain. The orientational ordering of the chain in a dipole field is higher than in a quadrupole field of the same magnitude.     

The Influence of External Electric Field on Local Orientations and Phase Transitions in Polymer Liquid Crystals (PLCs)

The starting point is our previous study of influence of the internal molecular mean field of dipole‐dipole interactions on local orientation and phase transitions in polymer liquid crystal (PLC) systems of longitudinal chains.^{[1, 2]} Electric dipoles are created by LC mesogen moieties. The longitudinal PLC is a macromolecule of consecutively copolymerized LC and flexible polymer sequences. We now amplify the model by inclusion of dipole‐external electric field interactions. We find that the external fields can seriously modify the local orientational order of the system and affect phase transition parameters dependent on that order. In particular, the external fields induce the formation of disoriented nematic phases with negative values of the second order orientation parameter 〈P₂〉 for LC sequences in the longitudinal PLCs while the first order parameter 〈P₁〉 is positive. However, some rapid decreases in 〈P₁〉 are observed at points of positive‐to‐negative transitions of 〈P₂〉; thus the LC disorientation manifests itself. The limiting case of the monomer liquid crystal (MLC) systems is included also.     

A comprehensive approach to electro-orientation, electrodeformation, dielectrophoresis, and electrorotation of ellipsoidal particles and biological cells

Suspended cells may respond to AC polarization by orienting, deforming, moving or rotating. For modeling of ellipsoidal cells, a new dipole approach is proposed. Along each of the principal axis of the model, three finite elements of arbitrary but equal cross-sectional area for the interior, low conductive membrane shell and exterior are assumed. The length of the external medium elements is defined by influential radii which are related to the depolarizing factors. The model predicts the potential at the ellipsoid's surface leading to the induced dipole moment. The moment obtained is identical to the Laplace approach for homogeneous ellipsoids; in the single-shell case, it is slightly different. The reason is the constant shell thickness which overcomes the confocal thickness necessary for the Laplace solution. Expressions for electro-orientation, deformation, dielectrophoresis, and electrorotation are derived. In linearly and circularly polarized fields, different orientation spectra are predicted to occur. While in linearly polarized AC fields, particles are oriented along their axis of highest polarizability, in circularly polarized fields, the axis of lowest polarizability is oriented perpendicular to the plane of field rotation. Based on this finding, a new electro-orientation method is proposed. In dielectrophoresis and electrorotation, reorientations are predicted which lead to discontinuous spectra.     

外部电场下水分子间静电相互作用的分子动力学研究

本文利用分子动力学模拟探讨了不同外电场下,液态水的分子间作用及分子排布的变化. 在不同外电场下,O…O原子间的径向分布函数差别很小,但是单个水分子的偶极矩的取向变化却很大. 当外电场为0时,单个水分子偶极取向的范围很宽（30-150度）. 与此同时,本文给出了局域诱导电场随着位置的变化关系图. 当外加电场增强时,局域的诱导电场强度也随之增加. 由于电场下偶极矩有序性的增加,局域诱导的静电相互作用能显著增加. 计算结果表明,相对介电常数随着电场强度的增加而呈现指数衰减的变化形式. 这一变化趋势可以用来理解不同电化学环境下,静电相互作用和局域诱导电场的变化.     

13C NMR of molecules partially aligned by an electric field: A new method for determining the orientation of the dipole moment

Effects of external electric fields on ¹³C NMR spectra have been measured to determine the direction of the electric dipole moment in two asymmetric molecules, i.e. 1-fluoro, 2,4-dinitrobenzene and 4-chloro, 3-nitrotoluene. This new method is based on ¹³C¹H dipolar interactions induced by the electric field. The couplings depend on the angle between a CH-bond and the dipole moment of the molecule, so that the direction of the latter can be determined. The experimental alignments are slightly smaller than predicted on the basis of Onsager's theory.     

Translocation of polymers with folded configurations across nanopores

The transport of polymers with folded configurations across membrane pores is investigated theoretically by analyzing simple discrete stochastic models. The translocation dynamics is viewed as a sequence of two events: motion of the folded segment through the channel followed by the linear part of the polymer. The transition rates vary for the folded and linear segments because of different interactions between the polymer molecule and the pore. It is shown that the translocation time depends nonmonotonously on the length of the folded segment for short polymers and weak external fields, while it becomes monotonous for long molecules and large fields. Also, there is a critical interaction between the polymers and the pore that separates two dynamic regimes. For stronger interactions, the folded polymer moves slower, while for weaker interactions, the linear chain translocation is the fastest. In addition, our calculations show that the folding does not change the translocation scaling properties of the polymer. These phenomena can be explained by the interplay between translocation distances and transition rates for the folded and linear segments of the polymer. Our theoretical results are applied for analysis of experimental translocations through solid-state nanopores.     

A statistical-mechanical model of polymer liquid crystals subjected to external deformations

We consider the nematiclike system of polymer liquid crystal (PLC) macromolecules represented by the Flory semiflexible chain model. Segments of the flexible (F) spacers are shorter than the LC hard-rod sequences. We investigate effects of imposition of external deformations. The behavior of LC sequences is largely governed by orienting interactions while for F spacers the short-range interactions determined by the chemical structure are the most important. The stress-strain relation is obtained in addition to the orientation-deformation relations. Orientational order phase transitions caused by the external deformations of the system are recognized and described.     

The effect of external orienting electric field on dielectric relaxation of segmented polyesters in dilute solution

A comparative study of dipole polarization relaxation in the absence and in the presence of an external orienting electric field was performed for linear segmented polyesters with alternating rigid (oxyfumaroylbis-4-oxybenzoates) and flexible (methylene-CH₂-, ethylene oxide-CH₂CH₂-O-, and dimethylsiloxane-Si(CH₃)₂-O-Si(CH₃)₂-) fragments in dilute solutions. Polyesters that do not display mesomorphic properties in the bulk show several regions of dielectric absorption with relaxation character. These regions are associated with the motions via the local mobility mechanisms of different polar fragments of the macromolecule. In solutions of polyesters that possess LC properties in the bulk, large-scale dipole polarization relaxation with long relaxation times and high activation energies was revealed along with local dielectric relaxation transitions. This process is associated with the cooperative motion of mesogenic fragments in their associates. In an external orienting electric field, the intensity of dielectric absorption usually increases for all types of dielectric transition; relaxation times and activation energies experience changes only for large-scale processes.     

Dynamic uniaxiality in strained networks: A deuterium NMR investigation

The orientational order generated in a polymer network by a uniaxial stress is probed using deuterium NMR. The experiments are performed on end-linked polydimethylsiloxane networks. When a uniaxial force is applied, the observed NMR spectra show that the chain segments reorient uniaxially around the external force direction. This experimental fact appears as new evidence for cooperative orientational interactions between chain segments in the deformed networks.     

A Constant Potential Molecular Dynamics Simulation Study of the Atomic‐Scale Structure of Water Surfaces Near Electrodes

Novel and technologically important processes and phenomena arise at water surfaces in the presence of electric fields. However, experimental measurements on water surfaces are challenging, and the results are scarce and inconclusive. In this work, the constant potential molecular dynamics method, in which the electrode charges are allowed to fluctuate to keep the electric potential fixed, was implemented in the study of a near‐electrode water surface systems. This simulation system was set up with a vapor/liquid‐water/vapor slab and two electrodes under different sets of applied electrostatic potential, yielding i) a detailed characterization of the external E‐field dependent electrostatic potential/density/dipole moment density profiles, and ii) the relationship between the water surface width and the applied electrode voltage differences which has been rarely reported. The adjustments in the number density profiles in the vicinity of water surfaces due to external E‐fields were observed, while the capillary interfacial widths for the surfaces near both cathode and anode were found with different increment rates under increasing E‐fields. By examining dipole density profiles across the water surfaces, we found that external E‐field induced polarization occurs in both bulk and surface regimes, yet the surface polarization densities vary asymmetrically with respect to the increasing E‐fields. Detailed discussions were carried out to explain the correlation between water surface tension and surface widths, as well as the interplay between the surface polarization densities and the hydrogen bond network structure. We conclude that the mechanical and structural properties of the water surfaces could be tuned by both magnitude and direction of the strong external E‐fields. We also recognize that more surface properties with application value, such as dielectric permittivity tensor or surface potential, could also be regulated by the external E‐fields.     

电场对(4, 0)Zigzag模型单壁碳纳米管的影响

The structural and electronic properties of a (4, 0) zigzag single-walled carbon nanotube (SWCNT) under parallel and transverse electric fields with strengths of 0-1.4×10~(-2) a.u. Were studied using the density functional theory (DFT) B3LYP/6-31G~* method. Results show that the properties of the SWCNT are dependent on the external electric field. The applied external electric field strongly affects the molecular dipole moments. The induced dipole moments increase linearly with increase in the electrical field intensities. This study shows that the application of parallel and transverse electric fields results in changes in the occupied and virtual molecular orbitals (Mos) but the energy gap between the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of this SWCNT is less sensitive to the electric field strength. The electronic spatial extent (ESE) and length of the SWCNT show small changes over the entire range of the applied electric field strengths. The natural bond orbital (NBO) electric charges on the atoms of the SWCNT show that increase in the external electric field strength increases the separation of the center of the positive and negative electric charges of the carbon nanotube.     

Density-functional study of model bidisperse ferrocolloids in an external magnetic field

We present phase diagrams of a model bidisperse ferrocolloid consisting of a binary mixture of dipolar hard spheres (DHSs) under the influence of an external magnetic field. The dipole moments of the particles are chosen proportional to the particle volume to mimic real ferrocolloids, and we focus on dipole-dominated systems where isotropic attractive interactions are absent. Our results are based on density-functional theory in the modified mean-field (MMF) approximation. For one-component DHS fluids in external fields, and for corresponding mixtures dominated by one of the components, MMF theory predicts the tricritical point of the transition between an isotropic gas and a ferromagnetic liquid occurring at zero field to be changed into a critical point separating two magnetically ordered phases of different density. The corresponding critical temperature displays a nonmonotonic dependence on the field strength. Completely different behavior is found for the critical temperature related to the demixing phase transitions appearing in strongly asymmetric mixtures [G. M. Range and S. H. L. Klapp, Phys. Rev. E 70, 061407 (2004)]. For such systems, we find a monotonic decrease of the demixing critical temperature with increasing field. The field strength dependence of the critical temperature can therefore be tuned between nonmonotonic and monotonic behaviors just by changing the composition of the mixture--e.g., by adjusting the chemical potentials. This allows us to efficiently control the influence of external magnetic fields on the phase behavior over a large temperature interval.     

TITAN: A Code for Modeling and Generating Electric Fields—Features and Applications to Enzymatic Reactivity

We present here a versatile computational code named “elecTric fIeld generaTion And maNipulation (TITAN),” capable of generating various types of external electric fields, as well as quantifying the local (or intrinsic) electric fields present in proteins and other biological systems according to Coulomb's Law. The generated electric fields can be coupled with quantum mechanics (QM), molecular mechanics (MM), QM/MM, and molecular dynamics calculations in most available software packages. The capabilities of the TITAN code are illustrated throughout the text with the help of examples. We end by presenting an application, in which the effects of the local electric field on the hydrogen transfer reaction in cytochrome P450 OleT_JE enzyme and the modifications induced by the application of an oriented external electric field are examined. We find that the protein matrix in P450 OleT_JE acts as a moderate catalyst and that orienting an external electric field along the Fe─O bond of compound I has the biggest impact on the reaction barrier. The induced catalysis/inhibition correlates with the calculated spin density on the O-atom. © 2019 Wiley Periodicals, Inc.     

Monte Carlo simulation of polarization reversal of ferroelectric polymer polyvinylidene fluoride

We consider a system composed of planar zigzag chain molecules of ferroelectric polymer PVDF. Taking the Lennard–Jones potential and the dipole–dipole interaction into consideration and assuming restrictions on molecular degrees of freedom, we have performed a Monte Carlo simulation which enables us to discuss the polarization reversal of PVDF under constant external electric field. Our simulation shows that the phenomenon is accompanied by nucleation and expansion of reversed domains. It also indicates that the dipole–dipole interaction between molecules causes growth anisotropy of the reversed domains.     

Effect of Electrode Pattern on the Column Structure and Yield Stress of Electrorheological Fluids

For electrorheological (ER) suspensions, the aggregate structures of particles were observed in electric fields by the use of transparent cells with different electrode patterns. Although the suspension is dispersed to noninteracting particles without electric fields, many aggregates are formed on the electrode surface in electric fields. Since the dipole–dipole interactions cause chain structures of particles and equilibrium conformations of chains are always aligned with electric field, the aggregates indicate the presence of columns spanning the electrode gap. The particle concentration in columns which are developed between parallel-plate electrodes is about 22 vol %. In striped electrodes, the particles construct striped aggregates along the electrodes and no particles remain in the insulating region. The particle concentration in striped aggregates is about 35 vol %. The nonuniformity of electric field is responsible for the high particle concentration. The increase in particle concentration of column lead to the high yield stress of electrified suspension. Therefore, the ER performance of suspension as an overall response can be improved by the electrode design.