Changes In Orientational Order Research Articles

Dynamic analysis of poured packing process of ellipsoidal particles

Packing properties are determined by the dynamic deposition history and affected by many variables. In this work, DEM is used to study the effects of dropping height, deposition intensity and friction coefficient on the dynamic response and stable-state packing properties of ellipsoidal particles. The results demonstrate that with dropping height increasing, packing density increases but the bed becomes less ordered. Dropping height affects the densification process of ellipsoids more significantly than spheres. At high dropping height, the transition period of the orientation angle in the densification process almost disappears for oblate particles, while it becomes longer for prolate particles. With the increase of deposition intensity, packing density decreases significantly, but orientational order changes slightly. With the increase of sliding friction coefficient, packing density and orientational order decrease dramatically. The difference in packing density is mainly caused by dynamic pouring process rather than the densification process.

Modeling out-of-plane actuation in thin-film nematic polymer networks: From chiral ribbons to auto-origami boxes via twist and topology

Various experimental and theoretical studies demonstrate that complex stimulus-responsive out-of-plane distortions such as twist of different chirality, emergence of cones, simple and anticlastic bending can be engineered and pre-programmed in a liquid crystalline rubbery material given a well-controlled director microstructure. Via 3-d finite element simulation studies, we demonstrate director-encoded chiral shape actuation in thin-film nematic polymer networks under external stimulus. Furthermore, we design two complex director fields with twisted nematic domains and nematic disclinations that encode a pattern of folds for an auto-origami box. This actuator will be flat at a reference nematic state and form four well-controlled bend distortions as orientational order changes. Device fabrication is applicable via current experimental techniques. These results are in qualitative agreement with theoretical predictions, provide insight into experimental observations, and demonstrate the value of finite element methods at the continuum level for designing and engineering liquid crystal polymeric devices.

Thermal response of cholesteric liquid crystal elastomers.

The effects of temperature variation on photonic properties of cholesteric liquid crystal elastomers (CLCEs) are investigated in mechanically unconstrained and constrained geometries. In the unconstrained geometry, cooling in the cholesteric state induces both a considerable shift of the selective reflection band to shorter wavelengths and a finite degree of macroscopic expansion in the two directions normal to the axis of the helical director configuration. The thermal deformation is driven by a change in orientational order of the underlying nematic structure S and the relation between the macroscopic strain and S is explained on the basis of the anisotropic Gaussian chain network model. The helical pitch varies with the film thickness in an affine manner under temperature variation. The CLCEs under the constrained geometry where thermal deformation is strictly prohibited show no shift of the reflection bands when subjected to temperature variation. This also reveals the strong correlation between the macroscopic dimensions and the pitch of the helical director configuration.

Probing Orientational Behavior of MHC Class I Protein and Lipid Probes in Cell Membranes by Fluorescence Polarization-Resolved Imaging

Steady-state polarization-resolved fluorescence imaging is used to analyze the molecular orientational order behavior of rigidly labeled major histocompatibility complex class I (MHC I) proteins and lipid probes in cell membranes of living cells. These fluorescent probes report the orientational properties of proteins and their surrounding lipid environment. We present a statistical study of the molecular orientational order, modeled as the width of the angular distribution of the molecules, for the proteins in the cell endomembrane and plasma membrane, as well as for the lipid probes in the plasma membrane. We apply this methodology on cells after treatments affecting the actin and microtubule networks. We find in particular opposite orientational order changes of proteins and lipid probes in the plasma membrane as a response to the cytoskeleton disruption. This suggests that MHC I orientational order is governed by its interaction with the cytoskeleton, whereas the plasma membrane lipid order is governed by the local cell membrane morphology.

Chemically Responsive Gels Prepared from Microspheres Dispersed in Liquid Crystals

Liquid-crystalline materials are a promising class of stimuli-responsive materials that have been demonstrated to undergo surface-induced orientational ordering transitions that can be highly sensitive and specific to chemical species. However, past studies demonstrating surface-induced transitions in liquid crystals (LCs) have employed thin films of low-molecular-weight LCs that are difficult to stabilize (due to dewetting of the LC on a surface). Here, it is reported that it is possible to prepare liquid-crystalline gels using a mixture of polystyrene microspheres and nematic LCs that undergo changes in orientational order, and thus optical appearance, in response to exposure to specific chemical compounds. These colloid-in-liquid-crystal (CLC) gels are mechanically stable and can be molded on chemically functionalized surfaces into thin films containing micrometer-sized LC-rich domains that span the two interfaces of the gels. In contrast to other reports of LC gels, where the presence of a polymeric or self-assembled small-molecule gelator network within a nematic LC frustrates ordering transitions from propagating through the gels over distances, it is demonstrated that thin films of CLC gels, when supported on chemically functionalized surfaces, do undergo easily visualized ordering transitions upon exposure to organophosphonate compounds. Because these optically responsive CLC gels are mechanically robust and can be molded, this class of composite LC material may be broadly useful for the design of chemically responsive LC devices.

Study on Morphology Evolution, Orientational Behavior, and Anisotropic Phase Formation of Highly Filled Polymer-Layered Silicate Nanocomposites

Morphology evolution, anisotropic phase behavior, and orientational behaviors of the highly filled polymer-organically modified layered silicate nanocomposites are investigated by using synchrotron small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and polarized light microscopy (PLM). Two kinds of polymers, maleated polypropylene and maleated polyethylene were used as a matrix polymer. The final morphology of both polymer-organically modified layered silicate nanocomposites evolves via four stages; intercalation, dual states of intercalation and exfoliation, ordered exfoliation, and disordered exfoliation in sequence as the concentration of silicate decreases in the nanocomposites. The morphology evolution of nanocomposites is determined by balancing an interaction between polymer and silicate against interactions between silicate platelets such as a steric interaction and an attractive interaction, which depends on the concentration of silicates. Both the nanocomposites show the optical anisotropy above 12 vol % clay originating from the ordering of silicate layers. The layered silicates subjected to shear flow show the orientation behavior that the silicate platelet's normals are perpendicular to the flow direction. Degree of orientational order changes with the concentration as well as shear rate in a complex manner. Up to an intermediate concentration, the extent of order is enhanced with the clay concentration. In contrast, at high concentrations, the extent of order decreases or increases according to the type of nanocomposite. The differences are discussed through the consideration of the morphology, the particle-particle interaction, and the rheological property of the nanocomposites.

Molecular dynamics simulations of a ferroelectric nematic liquid under shear flow

We perform nonequilibrium molecular dynamics simulations to examine the change in orientational order of a ferroelectric nematic liquid crystal under shear flow, specifically planar Couette flow. The orientational order is found to decrease with increasing shear rate. This drop in orientational order is not due to structural changes but is rather a result of the director dynamics being flow unstable. Employing both nonequilibrium and equilibrium molecular dynamics with the director orientation fixed, we determine the shear and twist viscosities which relate the pressure tensor to the shear or strain rate tensor in a nematic liquid crystal under flow. The Miesowicz viscosities are then obtained from linear combinations of the shear and twist viscosities. The short-range spatial structure of the ferroelectric nematic liquid crystal is similar to that of a ferroelectric tetragonal I lattice. The relative magnitudes of the Miesowicz viscosities can be understood by examining the shear stress response of this lattice to a shear deformation.

Pressure-induced change in orientational order of solutes in liquid crystals

The 1H and 2H NMR spectroscopy of the nematic liquid crystal ZLI 1132 containing small amounts of the solutes 1,3,5-trichlorobenzene, p-dichlorobenzene and acetone was investigated as a function of the pressure of a mixture of molecular hydrogen and deuterium gases above the sample. A general increase of solute orientational order with pressure was observed. Differences in behavior of the orientational order of the hydrogens compared to the other solutes are discussed.

On the Transition State between the Oil−Water and Air−Water Interface

Insoluble monolayers at the air-water interface are exposed to a gas phase containing organic hydrocarbons. The hydrocarbons are partly incorporated within the monolayer which leads to changes in orientational order and the formation of new phases of different morphology. The transition state resembles features of the air-water and oil-water interface and the control of the hydrocarbon partial pressure allows continous tuning between both interfaces. The phospholipid D,L-α-dipalmitoylphosphatidylethanolamine, DPPE, and an esterdiol hexadecanoic acid, 2,3-dihydroxypropyl ester, ESD-16, are used as amphiphiles, and pentane, n-hexane, cyclohexane, 2,2-dimethylbutane, n-heptane, n-decane, and n-dodecane are used as hydrocarbons. Both amphiphiles differ in their headgroup size. In DPPE the aliphatic tail determines the packing within the monolayer, but in the case of ESD-16 it is the headgroups. The structural changes are monitiored by surface pressure-area (π,A) isotherms and imaging ellipsometry. The influence of the chemical nature of the hydrocarbon and the effect of the partial pressure of the hydrocarbon on the monolayer structure are assessed.

Pathways to Macroscale Order in Nanostructured Block Copolymers

Polymeric materials undergo dramatic changes in orientational order in response to dynamic processes, such as flow. Their rich cascade of dynamics presents opportunities to create and combine distinct alignments of polymeric nanostructures through processing. In situ rheo-optical measurements complemented by ex situ x-ray scattering reveal the physics of three different trajectories to macroscopic alignment of lamellar diblock copolymers during oscillatory shearing. At the highest frequencies, symmetry arguments explain the transient development of a bimodal texture en route to the alignment of layers parallel to the planes of shear. At lower frequencies, larger-scale relaxations introduce rearrangements out of the deformation plane that permit the formation of lamellae perpendicular to the shear plane. These explain the change in the character of the pathway to parallel alignment and the emergence of perpendicular alignment as the frequency decreases.

Ferroelectric VDF/TrFE copolymers studied by nonlinear dielectric spectroscopy

Abstract Ferroelectric copolymers of vinylidene fluoride with trifluoroethylene have been investigated by means of nonlinear dielectric measurements conducted on both time and frequency domains. Two kinds of nonlinear responses were separately observed; one associated with a local change in orientational order of crystalline dipoles constituting remanent polarization P r and the other with ferroelectric polarization reversal from—P r to P r. The frequency-domain measurement using a relatively low sinusoidal field characterized the local dipole motion in terms of the frequency spectra of nonlinear complex permittivity e* ′′ = e′ ′′. It is shown that ϵ'2 is negative and ϵ'3 is positive being consistent with cooperative dipoles. The time-domain measurement using a high step-field provided dynamical information about polarization reversal in terms of the field-dependent switching time. The analysis of the switching profile revealed that a change in orientational order of crystalline dipoles precedes polarizat...

Modulation of the orientational order profile of the lipid acyl chain in the L? phase

The orientational order profile along the lipid acyl chain has been characterized under several different conditions of polar headgroup composition, temperature, and cholesterol content. Despite the different nature of these factors, the variation of the order is governed by two common trends. First, the relative change of order induced by the variation of these factors is always more pronounced towards the end of the chain than for the methylene groups near the interface. Second, there is, to a first approximation, a distinct correlation between the magnitude of the order parameters and the shape of the order profile. For example when the chain is highly ordered, the relative width of the order distribution is narrow indicating that the plateau region is longer. These conclusions suggest that the orientational order profile depends on only a small number of parameters and demonstrate clearly that the correlation length for changes in orientational order is much greater than one C-C bond length. Our results also show that the reduced temperature is not related in simple terms to orientational order and probably has little theoretical significance. The orientational order profiles of POPC and POPE bilayers are significantly different even when expressed in terms of reduced temperature. The behavior of POPC/cholesterol systems also indicates that the orientational order of the lipid chain and the gel-to-liquid crystalline phase transition temperature are not related in a straightforward manner.

Calorimetric and nuclear magnetic resonance study of the phase behaviour of dilauroylphosphatidylcholine/water

The phase behaviour of 1,2- dilauroyl-sn- glycero-3- phosphocholine (DLPC) has been investigated by high sensitivity differential scanning calorimetry, 2H nuclear magnetic resonance and X-ray diffraction. Depending on the thermal history of the sample, as many as three specific heat maxima are observed by DSC. Only two of these occur after prolonged incubation at temperatures below −6° C or after repeated temperature cycling. The total enthalpy of these two peaks, 6.75 kcal/mol DLPC, and the changes in orientational order observed by 2H-NMR, suggest that both of these peaks involve hydrocarbon chain melting and the structural reorganization that occurs in the sub-transition of longer chain diacylPC.

A comparison of the headgroup conformation and dynamics in synthetic analogs of dipalmitoylphosphatidylcholine

14N-NMR spectra and relaxation times for dipalmitoylphosphatidylcholine and three analogs were obtained in both the liquid crystal and gel phases. The analogs either changed the PO 4 − to N + (CH 3) 3 distance (P-N) within the headgroup by increasing the number of CH 2 groups from two in the phosphocholine headgroup (PN-2) to six in the phospho-( N′, N′, N′-trimethyl)hexanolamine headgroup (PN-6), or replaced the ester linkages to the hydrocarbon chains with ether linkages. 31P-NMR spectra were obtained for the four samples in the liquid-crystal phase. (1) The 14N- and 31P-NMR spectra and 14N relaxation times all indicate that increasing the P-N distance within the headgroup causes changes in both the average orientation of the C-N bond and its dynamics. (2) The 14N-NMR spectra provide evidence for a change in orientational order of the headgroup as a result of changing the linkage to the acyl chains. On the other hand, the relaxation time measurements indicate that the molecular motion for the headgroup is independent of the type of linkage. (3) The thermal behaviour of the four samples is clearly reflected in the 14N-NMR spectra. The second moments of the spectra show distinct changes at each of the phase tansitions. (4) The 14N-NMR spectra show that the average conformation of the headgroups is not significantly altered by the main phase transition. For the PN-2 samples, T 2e, the decay of the quadrupolar echo, decreases discontinuously in the P β, phase, which is evidence for a possible exchange process between two molecular states within this phase.

Model of the structural phase transformation in sodium azidea)

A microscopic model for the intermolecular interactions is applied to discuss the nature of the α–β structural transformation in sodium azide. NaN3 is an example of an ionic molecular crystal where changes in molecular orientations in the crystal lead to anomalous thermoelastic behavior. Sodium azide differs from other molecular crystals with elastic anomalies such as KCN in that the associated change in orientational order at the transition seems to be quite small. The model, a mean field treatment of the coupling of the rotational motions of the N3− ions to the lattice strains, is shown to lead to indirect N3−–N3− interactions mediated by the lattice. The apparent second-order nature of the transition is examined. Predictions regarding the temperature dependence of the isothermal elastic constants c44 and 1/2(c11−c12) are made.