Orientational Correlation Research Articles

Unconventional Molecular Weight Dependence of Charge Transport in the High Mobility n‐type Semiconducting Polymer P(NDI2OD‐T2)

The charge transport and microstructural properties of five different molecular weight (MW) batches of the naphthalenediimide‐thiophene copolymer P(NDI2OD‐T2) are investigated. In particular, the field‐effect transistor (FET) performance and thin‐film microstructure of samples with MW varying from Mn = 10 to 41 kDa are studied. Unlike conventional semiconducting polymers such as poly(3‐hexylthiophene) where FET mobility dramatically drops with decreasing molecular weight, the FET mobility of P(NDI2OD‐T2)‐based transistors processed from 1,2‐dichlorobenzene is found to increase with decreasing MW. Using a combination of grazing‐incidence wide‐angle X‐ray scattering, near‐edge X‐ray absorption fine‐structure spectroscopy, atomic force microscopy, and resonant soft X‐ray scattering, the increase in FET mobility with decreasing MW is attributed to the pronounced increase in the orientational correlation length (OCL) with decreasing MW. In particular, the OCL is observed to systematically increase from <100 nm for the highest MW samples to ≈1 µm for the lowest MW samples. The improvement in OCL and hence mobility for low MW samples is attributed to the lack of aggregation of low MW chains in solution promoting backbone ordering, with the pre‐aggregation of chains in 1,2‐dichlorobenzene found to suppress longer‐range liquid crystalline order.

Coupling between intra- and inter-chain orderings in flow-induced crystallization of polyethylene: A non-equilibrium molecular dynamics simulation study.

Non-equilibrium molecular dynamics simulations have been performed to study the molecular mechanism of flow-induced crystallization (FIC) of polyethylene (PE). The end-to-end distance of chain Rete and the content of trans conformation Ctrans are extracted out to represent intra-chain conformation ordering at whole chain and segment levels, respectively, while orientation correlation function P, density ρ, and bond orientational order parameter Q4 are taken to depict inter-chain orders. Imposing the extension induces the intra-chain conformational ordering to occur first, which further couples with the inter-chain order and results in the formation of hexagonal packing. Further increasing strain leads to the appearance of orthorhombic order. The results demonstrate that the FIC of PE proceeds via a multi-stage ordering process, during which coupling occurs among stress, intra-chain conformation, and inter-chain orientation and density orderings. Analyzing the flow-induced energy evolution unveils that not only entropy but also energy plays an important role in the FIC.

Steering patchy particles using multivalent electrolytes.

Proteins and many recently designed colloids can be regarded as patchy particles where directional interactions strongly influence and govern assembly behavior. Using explicit ion implicit solvent Metropolis Monte Carlo simulations, we investigate spherical model particles, carrying both charge and electric patches, in dilute aqueous 1 : 1, 1 : 3, and 3 : 1 electrolyte solutions. Striking differences in pair interaction free energies and orientational correlations are induced by three different salts which are discussed and rationalized in terms of ion-binding to surface groups, ion-ion correlations, and double layer forces. These findings suggest a general strategy where directional, intermolecular interactions can be invoked and tuned via small amounts of a carefully chosen electrolyte.

Evaluation of the Impact of Surface Reconstruction on Rough Electrodeposited Copper‐Based Catalysts for Carbon Dioxide Electroreduction

AbstractCarbon dioxide electroreduction (CO2ELR) can combine the use of a gaseous waste byproduct (CO2) with electricity from renewable sources to produce low‐carbon fuels and useful chemicals. Recent reports pointed out the superior performance of rough copper polycrystalline catalysts; however, reconstruction that takes place on such surfaces is under investigation. This report highlights the importance of copper surface reconstruction on rough surfaces during CO2ELR. Electrodeposited copper‐based catalysts composed primarily of Cu(111) were examined under CO2ELR conditions, and it was found that they underwent reconstruction towards Cu(200). The main hydrocarbon product of electrodeposited copper‐based catalysts with Cu(200) was CH4. The Cu/Cu catalyst achieved up to 40 % Faradaic efficiency (FE) to CH4 and low H2 efficiency (16 % FE) at −1.07 V versus the reversible hydrogen electrode (RHE), at a current density of −86 mA cm−2. This study provides new insights regarding the correlation of copper crystal orientation and selectivity on rough copper surfaces and the impact of copper reconstruction on the reaction.

Reproducible Surface Contrasting and Orientation Correlation of Low-Carbon Steels by Time-Resolved Beraha Color Etching

Abstract Color etchants have a huge potential in industrial applications and numerous investigations have been made to obtain a better understanding of the etching process. However, reproducibility and a solid knowledge of the ongoing processes are still missing. The work presented is a systematic study of the reproducibility of the Beraha color etching with potassium metabisulfite. By in situ observation of the etching process, the different states of the Beraha etching could be followed. It was shown that, at the beginning of the etching of dual-phase steels, the contrasting of the second phase by dissolving the less noble part of the second phase is the predominant contrast. Uncommonly, grain boundaries are hardly attacked. The color development of matrix grains is orientation sensitive and it was shown by comparison with electron backscatter diffraction (EBSD) measurements, that similar orientated grains have similar colors.

Ion pairing and phase behaviour of an asymmetric restricted primitive model of ionic liquids.

An asymmetric restricted primitive model (ARPM) of electrolytes is proposed as a simple three parameter (charge q, diameter d, and charge displacement b) model of ionic liquids and solutions. Charge displacement allows electrostatic and steric interactions to operate between different centres, so that orientational correlations arise in ion-ion interactions. In this way the ionic system may have partly the character of a simple ionic fluid/solid and of a polar fluid formed from ion pairs. The present exploration of the system focuses on the ion pair formation mechanism, the relative concentration of paired and free ions and the consequences for the cohesive energy, and the tendency to form fluid or solid phase. In contrast to studies of similar (though not identical) models in the past, we focus on behaviours at room temperature. By MC and MD simulations of such systems composed of monovalent ions of hard-sphere (or essentially hard-sphere) diameter equal to 5 Å and a charge displacement ranging from 0 to 2 Å from the hard-sphere origin, we find that ion pairing dominates for b larger than 1 Å. When b exceeds about 1.5 Å, the system is essentially a liquid of dipolar ion pairs with a small presence of free ions. We also investigate dielectric behaviours of corresponding liquids, composed of purely dipolar species. Many basic features of ionic liquids appear to be remarkably consistent with those of our ARPM at ambient conditions, when b is around 1 Å. However, the rate of self-diffusion and, to a lesser extent, conductivity is overestimated, presumably due to the simple spherical shape of our ions in the ARPM. The relative simplicity of our ARPM in relation to the rich variety of new mechanisms and properties it introduces, and to the numerical simplicity of its exploration by theory or simulation, makes it an essential step on the way towards representation of the full complexity of ionic liquids.

Conformational and hydrodynamic parameters of hyperbranched pyridylphenylene polymers

AbstractHydrodynamic and optical methods were applied to study conformational and physical properties of hyperbranched pyridylphenylene polymers in dilute solutions. The samples were synthesized using Diels–Alder polycyclocondensation. It was demonstrated that hydrodynamic properties of the studied macromolecules were typical for compact non‐percolated spheres. Optical and electro‐optical methods revealed information regarding the shape and asymmetry of the macromolecules (p ≈ 1.4). The contributions of optical shape effect to the observed flow birefringence of polypyridylphenylene solutions and intrinsic anisotropy of polarizability were evaluated and analysed. It was shown that varying the polymer composition (i.e. the degree of branching) caused considerable changes in the anisotropy of optical polarizability of the polymers. Dramatic difference of the electro‐optical properties in non‐polar (toluene) and polar (tetrachloroethane) solvents was found; this difference was related to the orientational correlation of polar solvent molecules with respect to the macromolecules. Dynamic properties were studied by non‐equilibrium electric birefringence which had a reasonable agreement with the dimensions estimated from hydrodynamic data. © 2016 Society of Chemical Industry

A n-vector model for charge transport in molecular semiconductors.

We develop a lattice model utilizing coarse-grained molecular sites to study charge transport in molecular semiconducting materials. The model bridges atomistic descriptions and structureless lattice models by mapping molecular structure onto sets of spatial vectors isomorphic with spin vectors in a classical n-vector Heisenberg model. Specifically, this model incorporates molecular topology-dependent orientational and intermolecular coupling preferences, including the direct inclusion of spatially correlated transfer integrals and site energy disorder. This model contains the essential physics required to explicitly simulate the interplay of molecular topology and correlated structural disorder, and their effect on charge transport. As a demonstration of its utility, we apply this model to analyze the effects of long-range orientational correlations, molecular topology, and intermolecular interaction strength on charge motion in bulk molecular semiconductors.

Intermolecular orientations in liquid acetonitrile: New insights based on diffraction measurements and all-atom simulations

Intermolecular correlations in liquid acetonitrile (CH3CN) have been revisited by calculating orientational correlation functions. In the present approach, hydrogen atoms are included, so that a concept applicable for molecules of (nearly) tetrahedral shape can be exploited. In this way molecular arrangements are elucidated not only for closest neighbours but also extending well beyond the first coordination sphere. Thus a complementary viewpoint is provided to the more popular dipole-dipole correlations. Our calculations are based on large structural models that were obtained by applying diffraction data and partial radial distribution functions from potential-based (all-atom) molecular dynamics simulation simultaneously, within the framework of the Reverse Monte Carlo method.

A stochastic micro-machine inspired by bacterial chemotaxis

Inspired by bacterial chemotaxis, we propose and study the dynamics of a 3D hydrodynamical search-machine at micrometer scale. Chemotactic memory of the proposed system that is borrowed from bacteria, allows it to search the fluid medium and find the required target. As the motion in micron size length scale is dominated by random forces, we analyze the statistical properties of the model. Mean square displacements, orientational correlation functions and also the chemotactic index (CI) of the system are investigated in detail. Because of the chemotactic memory, the system shows superdiffusing displacements in all directions and the diffusion exponents are anisotropic for the directions along or perpendicular to a preferred direction given by the gradient of attractant molecules transmitted by the target.

Maier-Saupe model of polymer nematics: Comparing free energies calculated with Self Consistent Field theory and Monte Carlo simulations

Self Consistent Field (SCF) theory serves as an efficient tool for studying mesoscale structure and thermodynamics of polymeric liquid crystals (LC). We investigate how some of the intrinsic approximations of SCF affect the description of the thermodynamics of polymeric LC, using a coarse-grained model. Polymer nematics are represented as discrete worm-like chains (WLC) where non-bonded interactions are defined combining an isotropic repulsive and an anisotropic attractive Maier-Saupe (MS) potential. The range of the potentials, σ, controls the strength of correlations due to non-bonded interactions. Increasing σ (which can be seen as an increase of coarse-graining) while preserving the integrated strength of the potentials reduces correlations. The model is studied with particle-based Monte Carlo (MC) simulations and SCF theory which uses partial enumeration to describe discrete WLC. In MC simulations the Helmholtz free energy is calculated as a function of strength of MS interactions to obtain reference thermodynamic data. To calculate the free energy of the nematic branch with respect to the disordered melt, we employ a special thermodynamic integration (TI) scheme invoking an external field to bypass the first-order isotropic-nematic transition. Methodological aspects which have not been discussed in earlier implementations of the TI to LC are considered. Special attention is given to the rotational Goldstone mode. The free-energy landscape in MC and SCF is directly compared. For moderate σ the differences highlight the importance of local non-bonded orientation correlations between segments, which SCF neglects. Simple renormalization of parameters in SCF cannot compensate the missing correlations. Increasing σ reduces correlations and SCF reproduces well the free energy in MC simulations.

Topological defect dynamics of vortex lattices in Bose-Einstein condensates

Vortex lattices in rapidly rotating Bose--Einstein condensates are systems of topological excitations that arrange themselves into periodic patterns. Here we show how phase-imprinting techniques can be used to create a controllable number of defects in these lattices and examine the resulting dynamics. Even though we describe our system using the mean-field Gross--Pitaevskii theory, the full range of many particle effects among the vortices can be studied. In particular we find the existence of localized vacancies that are quasi-stable over long periods of time, and characterize the effects on the background lattice through use of the orientational correlation function, and Delaunay triangulation.

Orientational Dynamics of a Functionalized Alkyl Planar Monolayer Probed by Polarization-Selective Angle-Resolved Infrared Pump-Probe Spectroscopy.

Polarization-selective angle-resolved infrared pump-probe spectroscopy was developed and used to study the orientational dynamics of a planar alkylsiloxane monolayer functionalized with a rhenium metal carbonyl headgroup on an SiO2 surface. The technique, together with a time-averaged infrared linear dichroism measurement, characterized picosecond orientational relaxation of the headgroup occurring at the monolayer-air interface by employing several sets of incident angles of the infrared pulses relative to the sample surface. By application of this method and using a recently developed theory, it was possible to extract both the out-of-plane and "mainly"-in-plane orientational correlation functions in a model-independent manner. The observed correlation functions were compared with theoretically derived correlation functions based on several dynamical models. The out-of-plane correlation function reveals the highly restricted out-of-plane motions of the head groups and also suggests that the angular distribution of the transition dipole moments is bimodal. The mainly-in-plane correlation function, for the sample studied here with the strongly restricted out-of-plane motions, essentially arises from the purely in-plane dynamics. In contrast to the out-of-plane dynamics, significant in-plane motions occurring over various time scales were observed including an inertial motion, a restricted wobbling motion of ∼3 ps, and complete randomization occurring in ∼25 ps.

Microstructural characterization of random packings of cubic particles.

Understanding the properties of random packings of solid objects is of critical importance to a wide variety of fundamental scientific and practical problems. The great majority of the previous works focused, however, on packings of spherical and sphere-like particles. We report the first detailed simulation and characterization of packings of non-overlapping cubic particles. Such packings arise in a variety of problems, ranging from biological materials, to colloids and fabrication of porous scaffolds using salt powders. In addition, packing of cubic salt crystals arise in various problems involving preservation of pavements, paintings, and historical monuments, mineral-fluid interactions, CO2 sequestration in rock, and intrusion of groundwater aquifers by saline water. Not much is known, however, about the structure and statistical descriptors of such packings. We have developed a version of the random sequential addition algorithm to generate such packings, and have computed a variety of microstructural descriptors, including the radial distribution function, two-point probability function, orientational correlation function, specific surface, and mean chord length, and have studied the effect of finite system size and porosity on such characteristics. The results indicate the existence of both spatial and orientational long-range order in the packing, which is more distinctive for higher packing densities. The maximum packing fraction is about 0.57.

Impact of diffusion limited aggregates of impurities on nematic ordering

Abstract We study the impact of random bond-type disorder on two-dimensional (2D) orientational ordering of nematic liquid crystal (LC) configurations. The lattice Lebwohl–Lasher pseudospin model is used to model orientational ordering perturbed by frozen-in rod-like impurities of concentration p exhibiting the isotropic orientational probability distribution. The impurities are either (i) randomly spatially distributed or (ii) form diffusion limited aggregation (DLA)-type patterns characterized by the fractal dimensions d f , where we consider cases d f ∼ 1.7 and d f ∼ 1.9 . The degree of orientational ordering is quantified in terms of the orientational pair correlation function G ( r ) . Simulations reveal that the DLA pattern imposed disorder has a significantly weaker impact for a given concentration of impurities. Furthermore, if samples are quenched from the isotropic LC phase, then the fractal dimension is relatively strongly imprinted on quantitative characteristics of G ( r ) .

Shape and Displacement Fluctuations in Soft Vesicles Filled by Active Particles.

We investigate numerically the dynamics of shape and displacement fluctuations of two-dimensional flexible vesicles filled with active particles. At low concentration most of the active particles accumulate at the boundary of the vesicle where positive particle number fluctuations are amplified by trapping, leading to the formation of pinched spots of high density, curvature and pressure. At high concentration the active particles cover the vesicle boundary almost uniformly, resulting in fairly homogeneous pressure and curvature, and nearly circular vesicle shape. The change between polarized and spherical shapes is driven by the number of active particles. The center-of-mass of the vesicle performs a persistent random walk with a long time diffusivity that is strongly enhanced for elongated active particles due to orientational correlations in their direction of propulsive motion. In our model shape-shifting induces directional sensing and the cell spontaneously migrate along the polarization direction.

Molecular Simulations of Hydrogen Bond Cluster Size and Reorientation Dynamics in Liquid and Glassy Azole Systems.

We simulated the dynamics of azole groups (pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, and tetrazole) as neat liquids and tethered via linkers to aliphatic backbones to determine how tethering and varying functional groups affect hydrogen bond networks and reorientation dynamics, both factors which are thought to influence proton conduction. We used the DL_Poly_2 molecular dynamics code with the GAFF force field to simulate tethered systems over the temperature range 200-900 K and the corresponding neat liquids under liquid state temperatures at standard pressure. We computed hydrogen bond cluster sizes; orientational order parameters; orientational correlation functions associated with functional groups, linkers, and backbones; time scales; and activation energies associated with orientational randomization. All tethered systems exhibit a liquid to glassy-solid transition upon cooling from 600 to 500 K, as evidenced by orientational order parameters and correlation functions. Tethering the azoles was generally found to produce hydrogen bond cluster sizes similar to those in untethered liquids and hydrogen bond lifetimes longer than those in liquids. The simulated rates of functional group reorientation decreased dramatically upon tethering. The activation energies associated with orientational randomization agree well with NMR data for tethered imidazole systems at lower temperatures and for tethered 1,2,3-triazole systems at both low- and high-temperature ranges. Overall, our simulations corroborate the notion that tethering functional groups dramatically slows the process of reorientation. We found a linear correlation between gas-phase hydrogen bond energies and tethered functional group reorientation barriers for all azoles except for imidazole, which acts as an outlier because of both atomic charges and molecular structure.

Shear-Aligned Block Copolymer Monolayers as Seeds To Control the Orientational Order in Cylinder-Forming Block Copolymer Thin Films

We study the dynamics of coarsening of a cylinder-forming block copolymer thin film deposited on a prepatterned substrate made of a well-ordered block copolymer monolayer. During thermal annealing the shear-aligned bottom layer drives extinction of the disclinations and promotes a strong orientational correlation, disturbed only by dislocations and undulations along the cylinders of the minority phase. The thin film bilayer system remains stable during annealing, in agreement with self-consistent field theory results that indicate that although the thickness of a stack of two monolayers is not at the optimum thickness condition, it is very close to equilibrium. Phase field simulations indicate that the bottom layer remains undisturbed during annealing and acts as a periodic external field that stabilizes the orientation of those domains that share its orientation. Misoriented cylinders are rapidly reorganized through an instability mechanism that drives the fragmentation of the cylinders. As annealing pro...

Variations in ice velocities of Pine Island Glacier Ice Shelf evaluated using multispectral image matching of Landsat time series data

Abstract Pine Island Glacier (PIG) in West Antarctica drains out to the Amundsen Sea through Pine Island Glacier Ice Shelf (PIGIS). As changes in ice velocities on PIGIS are strongly linked to changes in ice mass discharge from the West Antarctic Ice Sheet, it is very important to evaluate the spatiotemporal variations in ice velocities on the ice shelf. This research estimated ice velocities of PIGIS from an ensemble of image matching of Landsat time series multispectral data obtained from 2000 to 2014. Orientation correlation was adopted for the image matching of blue, green, red, near infrared, panchromatic, and the first principal image of the Landsat multispectral data, from which the results were combined and averaged. The multispectral image matching proposed in this research produced ~ 35% more ice velocity vectors than the use of a single band (i.e., panchromatic band) image matching. The erroneous matches were filtered through simple but rigorous statistical evaluations. The ice velocity of PIGIS accelerated by ~ 55% during 2000–2010, and the acceleration of the image northing velocity component (~ 1.3 km a − 1 ) was higher than that of the image easting velocity component (~ 0.8 km a − 1 ). During 2010–2012, the ice velocity of PIGIS slowed down by ~ 10%. The ice velocity in 2014 increased by 5% from 2012, but was still lower than the peak values observed in 2010. The surface strain rate fields of PIGIS were derived from the ice velocity fields. The longitudinal ice compression was observed near the grounding line and the geographically northern part of the central ice shelf of PIGIS, while a fast-flowing band was observed near the southern margin of the ice shelf. The transverse strain rate showed that ice divergence in the hinge zone of the central ice shelf has increased since 2000. The width of the shear margins of the central ice shelf of PIGIS is ~ 25 km, which has been stable since 2000.

Applicability of effective pair potentials for active Brownian particles.

We have performed a case study investigating a recently proposed scheme to obtain an effective pair potential for active Brownian particles (Farage et al., Phys. Rev. E 91, 042310 (2015)). Applying this scheme to the Lennard-Jones potential, numerical simulations of active Brownian particles are compared to simulations of passive Brownian particles interacting by the effective pair potential. Analyzing the static pair correlations, our results indicate a limited range of activity parameters (speed and orientational correlation time) for which we obtain quantitative, or even qualitative, agreement. Moreover, we find a qualitatively different behavior for the virial pressure even for small propulsion speeds. Combining these findings we conclude that beyond linear response active particles exhibit genuine non-equilibrium properties that cannot be captured by effective pair interaction alone.