Anisotropic Aggregates Research Articles

Structure of amorphous alumina tubes studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) is used to investigate the detailed structure of amorphous alumina tubes prepared via thermohydrolysis of anhydrous AlCl3 . The results demonstrate that PALS is a sensitive probe of the structure of amorphous materials, capable of distinguishing between the structures of amorphous alumina tubes and amorphous alumina prepared by a standard procedure, which are shown to differ slightly in the concentration and size of free-volume elements. The observed distinctions can be understood in terms of the anisotropic aggregation of primary alumina particles during hydrolysis in aqueous media or thermohydrolysis in water vapor.

Magnetic‐Field‐Directed Growth of CoPt3 Nanocrystal Microwires

External magnetic fields direct the destabilization and anisotropic aggregation of CoPt3 nanocrystal dispersions, producing one-dimensional nanocrystal microwires. These microwires comprise disordered assemblies of discrete nanocrystals (see Figure). The observed activated-charge-transport properties of the microwires are determined by charging of individual nanocrystals.

Poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)-g-poly(vinylpyrrolidone): Association behavior in aqueous solution and interaction with anionic surfactants

In this work, we aimed to study the association and interaction behavior of poly(ethylene oxide)–b–poly(propylene oxide)–b–poly(ethylene oxide) block copolymers grafted with poly(vinylpyrrolidone). Critical micellization concentrations were determined using fluorescent probes (pyrene) and critical micellization temperatures characterizing temperature-dependent transitions from monomers to multimolecular micelles were measured. The thermal responsiveness of the copolymer is not affected by the grafting. The hydrodynamic radius of the graft copolymer micelles is found to be greater than that of the original copolymer micelles. The graft copolymer is found to form anisotropic aggregates. The structure of the graft copolymer micelles is less disrupted by the anionic surfactant sodium dodecyl sulfate, compared to the ungraft copolymer.

Molar mass distributions and viscosity behavior of perfluorinated sulfonic acid polyelectrolyte aqueous dispersions

AbstractPerfluorinated sulfonic acid polyelectrolyte aqueous dispersions originating from similar polymer feed stocks and having similar compositions can have order‐of‐magnitude viscosity differences that are dependent on the manufacturing process. To better understand this phenomenon at the molecular level, a size exclusion chromatography method incorporating static light scattering detection was developed. The initial apparent mass distributions were broad and bimodal for all dispersions. A high‐molar‐mass shoulder was consistent with a previously postulated aggregate structure, and the evidence suggested that molecular aggregation accounted for viscosity variability. The apparent weight‐average molar masses ranged from 1.3 × 106 to 3.9 × 106 g mol−1. Upon the heating of the dispersions at or above 230 °C, the aggregate structure was broken down, and this resulted in similar low‐viscosity dispersions that had monomodal mass distributions. The weight‐average molar masses were reduced to approximately 2.5 × 105 g mol−1, and the polydispersities were approximately 1.7–1.8. Shear thinning with higher viscosities and apparent molar masses was rationalized with intrinsic viscosity and other measurements, which supported an anisotropic aggregate structure, with particles that could be significantly overlapped at nominal 11% concentrations. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 421–428, 2005

Formation of BaSO4 Whiskers in Microemulsion

Several shapes of BaSO4 particles were synthesized by the reaction of Ba(AOT)2 microemulsion with NaAOT microemulsion containing SO42– at differing [Ba2+]/[SO42–] mole ratios. BaSO4 nanofilaments were obtained under an excess of Ba2+ with the same mole ratio to that of Li and Mann (2000). A plasma replica SEM revealed that the nanofilaments were formed exactly in the microemulsion. Further excess Ba2+ resulted in the formation of short rags and thick spirals rather than nanofilaments. In contrast, fine particles were formed under a shortage of Ba2+. By addition of excess Ba2+ to the microemulsion after the generation of the fine particles under a shortage of Ba2+, whisker-like rods were obtained. These results suggest a new mechanism for formation of the whisker-like rods through an anisotropic aggregation of the fine particles.

Competitive Etching and Oxidation of Vicinal Si(100) Surfaces

ABSTRACTExposure of a vicinal Si(100) surface to oxygen at around 550 C produces etching-mediated step recession. In addition, some oxide islands are formed which locally pin receding steps. We develop an atomistic lattice-gas model for this process which accounts for the interplay between oxygen surface chemistry (adsorption, diffusion, oxide formation, and etching via SiO desorption) and the silicon surface and step dynamics (anisotropic diffusion and aggregation of di-vacancies formed by etching, and ad-dimer attachment-detachment dynamics at steps incorporating anisotropic energetics). Kinetic Monte Carlo simulation of this model produces step morphologies retaining some qualitative but not quantitative features of their equilibrium structure (alternating rough SB steps and smooth SA steps), except for pinning which produces protruding “fingers”. These features are seen in Scanning Tunneling Microscopy studies.

Field-induced structures and rheology of a magnetorheological suspensionconfined between two walls

We have determined experimentally the parameters characterizing the structuresformed in a magnetic colloidal suspension subjected to a unidirectionalmagnetic field and a rotating field for different cell thicknesses. In this lattercase one observes the formation of a periodic structure in parallel sheetssituated in the plane of rotation of the field. A theoretical model based onminimization of the energy allows one to find quantitatively the observationsobtained in a unidirectional field. On the other hand, in a rotating field, theagreement is quantitative only if we take the surface energy as an adjustableparameter. We explain this fact on the basis of the existence of a substructuremade of discs of particles. In a second part we show the influence of thestructures on the rheological properties of the suspension by measuringthe shear moduli for different kinds of structure. We find that for thesame magnetic field, the shear moduli depend strongly on the structureand can be quite well predicted by a mean field theory; also the criticalshear strain is determined and is in agreement with the model. Finally weshow that, in a regime with a unidirectional oscillating field without shearflow, a new phenomenon appears if the confining walls are not parallel.In this case we observe the formation of anisotropic aggregates whichundergo a collective chaotic rotation around the axis of the aggregates.

Mechanical response of zirconium—I. Derivation of a polycrystal constitutive law and finite element analysis

Simulating the forming of anisotropic polycrystals, such as zirconium, requires a description of the anisotropy of the aggregate and the single crystal, and also of their evolution with deformation (texture development and hardening). Introducing the anisotropy of the single crystal requires the use of polycrystal models that account for inhomogeneous deformation depending on grain orientation. In particular, visco-plastic self-consistent models have been successfully used for describing strongly anisotropic aggregates. As a consequence, using a polycrystal constitutive law inside finite element (FE) codes represents a considerable improvement over using empirical constitutive laws, since the former provides a physically based description of anisotropy and its evolution. In this work we develop a polycrystal constitutive description for pure Zr deforming under quasi-static conditions at room and liquid nitrogen temperatures. We use tensile and compressive experimental data obtained from a clock-rolled Zr sheet to adjust the constitutive parameters of the polycrystal model. Twinning is accounted for in the description. The polycrystal model is implemented into an explicit FE code, assuming a full polycrystal at the position of each integration point. The orientation and hardening of the individual grains associated with each element is updated as deformation proceeds. We report preliminary results of this methodology applied to simulate the three-dimensional deformation of zirconium bars deforming under four-point bend conditions to maximum strains of about 20%. A critical comparison between experiments and predictions is done in a second paper (Kaschner et al., Acta mater. 2001, 49(15), 3097–3107).

Particle spin in anisotropic granular materials

We adopt the simplest theory for the description of the mechanical behavior of a random array of elastic spheres and show that it is necessary that the average rotation of the spheres about their centers be distinguished from the average rotation of the aggregate. Without such a distinction, the stress in an anisotropic aggregate, subjected to a deformation that involves an average rotation, would not be symmetric. We illustrate this by calculating the incremental stress–strain relation for an orthotropic aggregate.

Structural Anisotropy of Fractal Aggregates and Its Exhibition in Electrooptical Effects

The first experimental examination of orientational turbidimetric effect [differential orientational spectrum of optical density δD(λ) in suspensions of fractal clusters was presented. Experiments were performed for aqueous suspensions of powdered Aerosil OX-50 aggregates ordered by the alternating electric field oriented along or across the unpolarized light beam. Experimental differential spectra δD(λ) are essentially different from the known spectra for bacterial and yeast suspensions. In particular, the points of zero effect lie in the red (but not in the blue) region of spectrum, and the |δD(λ)| dependence is a decreasing function in the range 300–800 nm. To explain these results, a theory was developed based on the three-dimensional lattice model of cluster-cluster aggregation and the optical model of interacting dipoles. This theory confirms main conclusions obtained previously in the Rayleigh–Debye–Hans approximation (Khlebtsov, N.G. and Mel'nikov, A.G., Kolloidn. Zh., 1998, vol. 60, no. 6, p. 843) and agrees with experimental δD(λ) spectra. It was shown experimentally and theoretically that the structural anisotropy of aggregates exhibited scale invariance, and the differential spectra for the aggregates with various numbers of particles are described by a single universal curve δD(kRg), where k= 2π/λ and Rgis the mean radius of gyration of aggregates.

An analytical approach to deformation of anisotropic ice-crystal aggregates

AbstractDeformation rates of single hexagonal crystals, deforming by glide on the basal plane, are described as a function of stress state and crystal orientation. These results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous. Analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric. This type of fabric is approximated by a uniform distribution of c-axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis. For simple shear stress acting on a single-maximum fabric there is a slight de-enhancement for cone angles of 60–90°. In uniaxial compression a maximum enhancement of ∼1.7 occurs at a cone angle of 57°. A pure shear stress state has similar features, with the additional complication that it causes a non-zero transverse strain rate, except for perfect vertical alignment of crystals and isotropic fabric. In combined states of stress the contribution of each stress component to the strain rate depends on fabric. A single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states.

1,1′-Disubstituted ferrocene containing hexacatenar thermotropic liquid crystals

The preparation and characterization of ferrocene containing hexacatenar metallomesogens 4a–h based on alkoxy terminal groups with hexyloxy (4a), octyloxy (4b), decyloxy (4c), dodecyloxy (4d), tetradecyloxy (4e), hexadecyloxy (4f), octadecyloxy (4g) and eicosyloxy (4h) chains are described. The introduction of alkyl chains of different lengths induces a rich variety of self-assembled liquid crystalline structures. In the crystalline state, the ferrocene containing hexacatenar metallomesogens 4a–h organize into a microphase-separated monolayer lamellar structure. In contrast, a dramatic phase change after crystalline melting of the metallomesogens is observed with variation of the chain length. The ferrocene containing hexacatenar metallomesogens 4b and c display a bicontinuous cubic mesophase with Ia3d symmetry, while the ferrocene containing hexacatenar metallomesogens 4d–g exhibit a hexagonal columnar mesophase. Further increasing the length of the alkyl chain as in the case of 4h suppresses liquid crystallinity and induces only a crystalline phase. This unique behavior in the ferrocene containing metallomesogenic molecules can be understood to originate from the anisotropic aggregation of ferrocene containing aromatic segments and consequent entropic penalties associated with chain stretching.

Estimation of the dynamic size of fractal aggregates

A model is presented for an aggregation act occurring between two aggregates of any mass and fractal dimension. The kinetics of aggregation is also analyzed, as well as some previous works concerning the structure of fractal aggregates. As a result, a generalized curve is derived describing the normalized dynamic radius of clusters of spherical character as dependent on both the aggregate fractal dimension and the space dimension. It is shown how the curve may be utilized to determine the dynamic size of anisotropic aggregates. The obtained dependence can be used to estimate the dynamic size of fractal aggregates, to evaluate the prefactor in mass–radius relation and to model the aggregation kinetics.

Dynamics of Hairy-Rod Polymer Solutions in Simple Shear Flow: Aging Effects

We investigated the dynamics of solutions of a model hairy-rod polyester using optical rheometry and find that in the concentrated regime significant time effects dominate its response in simple shear flow. They are characterized by an enhanced viscosity and shear thinning behavior, accompanied by a corresponding increase of the absolute value of the induced birefringence and the relevant relaxation time upon cessation of shear. However, the most dramatic effect is the change of sign and large increase of the absolute value of the birefringence, signifying the dominance of form effects, associated with the formation of large anisotropic aggregates at longer times, which are oriented in the vorticity direction. This observation of anomalous birefringence is in qualitative agreement with the theoretical approach of Cates (J. Phys. II (Fr.) 1992, 2, 1109) who considered a presmectic local (anisotropic) ordering resulting from the interplay of particle interactions and the external field.

Internal Dynamics and Order Parameters in Surfactant Aggregates: A 2H NMR Study of Adsorption Layers and Bulk Phases

Nonionic surfactant adsorption layers are investigated by applying 2H NMR to selectively deuterated dodecyl penta(ethylene oxide), C12E5, adsorbed on colloidal silica. Results for surface aggregates are compared with different types of bulk aggregates such as micelles, which show isotropically averaged Lorentzian line shapes, and with liquid crystalline phases, which show a Pake pattern spectrum. Surface aggregate spectra are isotropically averaged, with relaxation rates R2 of several kilohertz, indicating a slow motion correlation time of the order of microseconds. Possible mechanisms of isotropic averaging of the quadrupolar interaction in surface aggregates and the implications for the structural arrangement are discussed. To compare results from solid and liquid spectra, we introduce the concept of a relative order parameter profile Srel of the C-2H bond. Adsorption layers can thus be directly compared with micelles, and with lamellar and hexagonal phases with respect to their internal mobility. An increase of Srel with distance of the 2H label position from the headgroup is found for all bulk aggregate types, with the slope depending on aggregate curvature. The result is interpreted in terms of packing constraints and chain interaction. Comparison of the results for surface aggregates with these profiles gives an indication of their curvature and structure. Srel profiles of adsorption samples do not depend on water content or surface coverage. All results, that is, the structural implications from Srel profiles as well as time scales of surfactant motional modes, are consistent with the formation of large anisotropic surface aggregates, which can be described as a disrupted bilayer. The motions causing isotropic averaging probably are lateral diffusion in combination with surfactant exchange processes.

Biomimetic Morphogenesis of Fluorapatite-Gelatin Composites: Fractal Growth, the Question of Intrinsic Electric Fields, Core/Shell Assemblies, Hollow Spheres and Reorganization of Denatured Collagen

The biomimetic growth of fluorapatite in gelatin matrices at ambient temperature (double-diffusion technique) starts with elongated hexagonal-prismatic seeds followed by self-similar branching (fractal growth) and ends up with anisotropic spherical aggregates. The chemical system fluorapatite/gelatin is closely related to in vivo conditions for bone or tooth formation and is well suited to a detailed investigation of the formation of an inorganic solid with complex morphology (morphogenesis). The fractal stage of the morphogenesis leads to the formation of closed spheres with diameters of up to 150 μm. The self-assembled hierarchical growth thereby shows immediate parallels to the topological branching criteria of the macromolecular starburst dendrimers. A second growth stage around the closed spheres of the first stage is characterized by the formation of concentric shells consisting of elongated prismatic fluorapatite units with nearly parallel orientation (maximum diameter of the complete core/shell spheres of 1 mm). The specific structure of the core/shell assembly is similar to the dentin/enamel structure in teeth. Together with the idea of the biological significance of electric fields (pyro-, piezoelectricity) during apatite formation under in vivo or biomimetic conditions the present paper considers the composite character of the material and the mechanisms of fractal growth (branching criteria and architecture, the influence of intrinsic electric fields etc.).

Aggregation of polyA–HIV-1 nucleocapsid protein NCp7 complexes and properties of the aggregates

We have previously shown that in solution, depending on solute and solution conditions, an ordered aggregation of polyA–NCp7 complexes takes place. The kinetic behaviour of the polyA(homoribopolynucleotide)–NCp7(human immunodeficiency virus-I nucleocapsid protein) aggregating system was studied at physiological NaCl concentration (150 mM) using quasielastic light scattering measurements. The kinetic curves were fitted to a power law equation. For the aggregate size growth with time at initial concentrations of NCp7=0.3 μM and nucleotide-to-protein molar ratios of 17 and 50 we obtained for power exponents of 0.15 and 0.23, respectively. On the basis of the aggregate size at which maxima are observed on the light scattering versus time curves we suggested a method for determining the mechanism of aggregation. Thus we found that in the process of polya–NCp7 aggregation, the growth proceeds by fusion of commensurable in size aggregates. The aggregate size dependence on NCp7 concentration showed critical behaviour. The calculated parameters of the power law (rate of growth n and a, related to the aggregate properties) showed in the critical region (0.2–0.3 μM NCp7) a marked change. Electro-optic measurements (electric birefringence and electric light scattering) gave no evidence for the existence of optically anisotropic and/or anisodiametric suspended aggregates under the experimental conditions accessible for the electro-optic instruments used. Quasielastic light scattering and electron microscopy studies showed that at all stages of aggregate growth, under a broad range of experimental conditions, the aggregates were spherical with a unimodal, relatively narrow size distribution. We consider that combining the method suggested by us for studying the mechanism of aggregation with electro-optic and electron microscopy studies might be helpful for modelling the HIV-1 nucleocapsid (composed of genomic RNA–NCp7 complexes) assembly as well as transfectious particles for gene transfer (vectors) or non-infectious particles as candidate vaccines.

From Beads-on-a-String to Colloidal Aggregation: Novel Crystallization Phenomena in the PEO−SDS System

The anionic surfactant sodium dodecyl sulfate (SDS) interacts strongly with poly(ethylene oxide) (PEO) in aqueous solution, forming micellar aggregates attached to the polymer chains above the so-called critical association concentration (cac). By adding lead and sulfide ions to the PEO/SDS solution, we form lead-sulfide-containing crystallites directly on the polymer-bound micelles. We can then, through this indirect staining technique, visualize polymer chains with polymer-bound micelles using TEM. Subsequent aggregation of the particles adsorbed on the polymer chains through a bridging flocculation mechanism leads to the production of novel interesting organic/inorganic nanocomposites, in the form of long (0.5−5 μm) and thin (100 nm) rodlike colloidal aggregates. XRD analysis of these anisotropic aggregates reveals a well-defined layered structure with a spacing of 31 Å. Combination of elemental analysis, FTIR, XPS, and XRD measurements suggests that the aggregates consist of mixed dodecyl sulfate/acetate layers, which sandwich a layer of lead and sulfide ions. Similar colloidal aggregation phenomena have been observed with other cations as well, suggesting that polymer/surfactant complexes are useful novel templates for the preparation of organic/inorganic nanocomposite materials.

Smectic liquid-crystalline physical gels. Anisotropic self-aggregation of hydrogen-bonded molecules in layered structures

Smectic liquid-crystalline states have been successfully gelled by low molecular weight hydrogen-bonded molecules and anisotropic fibrous aggregates of the gelling agents are obtained when the sol–gel transition occurs within the smectic temperature range.

STM study of preferential growth of one-dimensional nickel islands on a Cu(100)-(2 [formula omitted]) R45°-O surface

Aggregation of Cu and Ni adatoms on oxygen preadsorbed Cu(100) was studied by STM. Cu deposited on a Cu(100) c(2×2)-O surface agglomerate into anisotropic islands with a (2 2 × 2 ) R45°-O structure, while Ni deposited on a c(2×2)-O surface forms isotropic Ni islands with c(2×2)-O structure. On the other hand, Ni on a (2 2 × 2 ) R45°-O structure undergoes extremely anisotropic aggregation, which forms Ni-wires with a c(2×2)-O structure. Formation of Ni-wire on a (2 2 × 2 ) R45°-O structure has been inferred by a preferential nucleation along the Cu atom missing trough inlaid with Ni.