Twinning and growth kinetics of lamellar grains in a diblock copolymer solution

The initial stages of growth of the lamellar phase in a block copolymer solution were observed with polarizing optical microscopy (POM). Measurements were made on a poly(styrene‐b‐isoprene) diblock copolymer with block molecular weights of 15 and 13 kg/mol, respectively, dissolved in dioctyl phthalate with 70% polymer by volume. Upon cooling from above the order–disorder transition temperature, 89.5 °C, to temperatures from 87.5 to 88.5 °C, four distinct types of grain were observed: ellipsoidal single grains, twinned ellipsoidal grains, 2‐fold twinned grains, and spherulites. The relative populations were distributed as 50% single ellipsoids, 25% twinned ellipsoids, 10% 2‐fold twinned grains, and 15% spherulites. These grain types cover a range of lamellae orientation. For example, the surface of a 2‐fold twinned grain is composed of lamellar edges, whereas the spherulite surface is composed of lamellar planes. The specific grain types that arise give insight into the thermodynamic and kinetic forces governing lamellae ordering. Furthermore, growth front velocities of individual grains were measured after rapid quenches from above T_ODT. These results were compared to the predictions of Goveas and Milner. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 405–412, 2005     

Multiscale effects in crystal grain growth and physical properties of metals

A multiscale approach for the simulation of physical properties of metals is suggested and discussed. Grain growth in metals involves interactions at three distinct scale levels: the atomic scale (length of crystal lattice spacing), microscale (grain length) and macroscale. A simulation scheme should include these three levels. The crystalline microstructure is simulated by the Monte Carlo method. The average grain radius is then used to determine the yield strength from the Hall-Petch relationship for large grains and the inverse Hall-Petch relationship for submicron-sized grains. The yield strength is then supplied to a continuum macroscale model. The normal grain growth, which has a theoretical solution for grain size as a function of time, is discussed in detail as a case study and conclusions are drawn with regard to more complex situations such as dynamic recrystallization.     

Self-assembly of rod-coil molecules into molecular length-dependent organization

A series of rod-coil molecules (n-x, where n represents the number of repeating units in a PPO coil and x the number of phenyl groups in a rod segment) with variation in the molecular length, but an identical rod to coil volume ratio was synthesized, and their self-assembling behavior was investigated by using DSC and X-ray scatterings. The molecule with a short rod-coil molecule (16-4) shows a 3-D tetragonal structure based on a body-centered symmetry of the discrete bundles in addition to a lamellar structure. This 3-D lattice, on heating, collapses to generate a disordered micellar structure. Remarkably, the molecules based on longer molecular length (21-5 and 24-6) were observed to self-organize into, on heating, lamellar, tetagonally perforated lamellar, 2-D hexagonal columnar and finally disordered micellar structures. Further increase in the molecular length as in the case of 29-7 and 32-8 induces a 3-D hexagonally perforated lamellar structure as an intermediate structure between the lamellar and tetragonally perforated lamellar structures. Consequently, these systems demonstrate the ability to regulate the domain nanostructure, from 2-dimensionally continuous layers, long strips to discrete bundles via periodic perforated layers by small changes in the molecular length, at an identical rod-to-coil volume fraction.     

The mechanism of silver granular electrodeposits formation

Granules as a possible form of metal electrodeposit can be formed during deposition of metals, such deposition processes being characterized by large exchange current density values. Because of this, zero nucleation zones around growing grains are formed, permitting granular metal growth. In some cases of prolonged deposition, macro-crystalline deposits can be formed as well as granular ones, e.g. in the case of silver deposition at overpotentials lower than the critical value for dendrite growth initiation. The mechanism of granular deposit growth as a final form of metal electrocrystallization is proposed. Silver boulders were deposited on␣platinum and silver substrates. At low deposition potentials, various crystallographic forms, some of them ideal or derived from cube-octahedron-type morphology, were obtained as a result of independent grain growth inside zones of zero nucleation. In addition to cube-octahedra, twinned and multiply twinned silver particles were also observed. The nucleation density was found (1) to increase with increasing deposition overpotential, (2)␣to decrease with increasing silver concentration, and (3) to be greater on Ag than on Pt for the same deposition overpotential and dendrite precursors. Increasing overpotential leads to increase of density of twinned grains. The grain growth at greater overpotentials from more concentrated solution is less ideal, producing a granular deposit on prolonged deposition. Received: 21 April 1997 / Accepted: 18 September 1997     

Time-resolved small-angle neutron scattering study of polyethylene crystallization from solution

With time-resolved small-angle neutron scattering (TR-SANS), the crystallization kinetics of polyethylene from deuterated o-xylene solutions upon a temperature jump have been investigated. On the basis of a morphological model of coexisting lamellar stacks and coil chains in solution, experimental data have been quantitatively analyzed to provide structural information, such as the lamellar long period, the lamellar crystal thickness, the thickness of the amorphous layers between lamellae, the degree of crystallinity, and the crystal growth rate at various degrees of undercooling. The viability of TR-SANS for studying polymer crystallization is demonstrated through the consistency of these measurements and well-established knowledge of polyethylene crystallization from xylene solutions. One unique feature of this experimentation is that both the growth of lamellar crystals and the condensation of coil chains from solution are monitored simultaneously. The ratio of the crystal growth to the chain consumption rate decreases rapidly with a decreasing degree of undercooling. The Avrami analysis suggests that the growth mechanism approaches two-dimensional behavior at higher temperatures, and this is consistent with the observation of an increasing ratio of the sharp-surface area to the bulk crystal growth rate with temperature. The limitations, possible remedies, and potentials of TR-SANS for studying polymer crystallization are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3133–3147, 2004     

On the origin of mesoscopic inhomogeneity of conducting polymers

The mesoscopic inhomogeneity of conducting polymer films obtained by electropolymerization and spin-coating was studied using Kelvin probe force microscopy (KFM) and current-sensing atomic-force microscopy (CS-AFM). A well-pronounced correlation was established between the polymer morphology, on the one hand, and its local work function (which is related to the polymer oxidation degree) as well as polymer conductivity, on the other. The most conducting regions were associated with the tops of the polymer grains and showed Ohmic behavior. They were surrounded first by semiconducting and then by insulating polymer. The conductivity of the grain periphery could be lower by as much as 2 orders of magnitude. The grain cores also showed consistently higher values of the local work function as compared to the grain periphery. This fact suggested that the grain cores were more oxidized and/or more ordered as compared to the grain periphery, which is in good agreement with the local conductivity data. More uniform morphology corresponded to less variability in the other properties of the polymer. A model is proposed that relates the observed inhomogeneity to preferential deposition of polymer molecules with higher molecular weight at the early stages of the polymer phase formation. The polymer deposition in either electropolymerization or various solution-casting techniques involves the nucleation of a new phase from a solution containing polymer fractions of different molecular weights. The driving force of the nucleation process depends on the solubility of the polymer fractions, which decreases with an increase in the molecular weight. This gives rise to preferential deposition of more crystalline, higher molecular weight polymer at the early stages of the polymer deposition to form the cores of the polymer grains. The fractions with lower molecular weights are deposited later and form less ordered/less conducting grain periphery. On the basis of this model, we conclude that, to ensure the formation of materials with low inhomogeneity and high quality, one should use the starting polymer with as narrow molecular weight distribution as possible. Yet another possibility is to use solvents which would reduce the differences in the solubilities of polymer fractions with different molecular weight.     

The Stokes–Brinkman Flow Field and Diffusion Deposition of Nanoparticles in a Layer of Hollow Permeable Grains

The diffusion deposition of point particles from a Stokes–Brinkman transverse stationary flow in a model monolayer membrane composed of contacting spherical hollow grains (capsules) with porous permeable shells formed from nanoparticles, is calculated. Monolayers with square and hexagonal packings of the grains are considered. Approximation equations are constructed for the dependences of grain drag force on shell thickness, Brinkman permeability parameter S, and internal shell radius ξ. Efficiencies of point particle collection on the grains are calculated as depending on the Peclet number, S, and ξ, and it is shown that layers of hollow permeable grains possess the highest filtration performance criterion among layers of impermeable and permeable uniform porous grains provided that the zero-concentration boundary condition is fulfilled at the outer radius of the grain.     

Crystallization and chain conformation of semicrystalline and amorphous polymer blends studied by wide‐angle and small‐angle scattering

We examine the crystallization and chain conformation behavior of semicrystalline poly(ethylene oxide) (PEO) and amorphous poly(vinyl acetate) (PVAc) mixtures with wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering (SAXS), and small‐angle neutron scattering (SANS) experiments. For blends with PEO weight fractions (wt_PEO) greater than or equal to 0.3, below the melting point of PEO, the WAXD patterns reveal that crystalline PEO belongs to the monoclinic system. The unit‐cell parameters are independent of wt_PEO. However, the bulk crystallinity determined from WAXD decreases as wt_PEO decreases. The scattered intensities from SAXS experiments show that the systems form an ordered crystalline/amorphous lamellar structure. In a combination of WAXD and SAXS analysis, the related morphological parameters are assigned correctly. With the addition of amorphous PVAc, both the average amorphous layer thickness and long spacing increase, whereas the average crystalline layer thickness decreases. We find that a two‐phase analysis of the correlation function from SAXS, in which the scattering invariant is linearly proportional to the volume fraction of lamellar stacks, describes quantitatively the crystallization behavior of PEO in the presence of PVAc. When wt_PEO is close to 1, the samples are fully spaced‐filled with lamellar stacks. As wt_PEO decreases from 1.0 to 0.3, more PVAc chains are excluded from the interlamellar region into the interfibrillar region. The fraction outside the lamellar stacks, which is completely occupied with PVAc chains, increases from 0 to 58%. Because the radius of gyration of PVAc with a random‐coil configuration determined from SANS is smaller than the average amorphous layer thickness from SAXS, we believe that the amorphous PVAc chains still persist with a random‐coil configuration even when the blends form an ordered structure. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2705–2715, 2001     

Investigation of divergent beam X-ray reflex sections especially indication by computer simulation and assignment to the grains in polycrystalline samples

The divergent beam X-ray interference (DBI) method is commonly used to determine crystal systems, crystallographic orientation and lattice constants of single crystals and large individual grains and to get information on the real structure. Here back reflection divergent beam X-ray interference patterns (Pseudo-KOSSEL technique) were taken from polycrystalline specimens in the SEM and compared with corresponding lattice source interference (LSI) patterns (KOSSEL technique). A simulation program (for DBI) was developed allowing the calculation and indication of the complete reflex system as well as reflex sections. Reflex interruptions were obtained and could be explained by grain boundary effects. The influence of different important parameters such as the distance beween the target and the specimen, the target material, the orientation, the position, the shape and the size of the grains were discussed. Conclusions with respect to a local assignment of interruptions to grain boundaries are drawn so that now there is a possibility to localize the observed crystal defects. Received: 1 August 1997 / Revised: 5 March 1998 / Accepted: 5 March 1998     

Effect of growth conditions on the structure of two-dimensional latex crystals: experiment

The growth kinetics and structure of two-dimensional crystals of fine latex particles on solid substrates have been studied using a variety of microscopic techniques: optical microscopy, surface plasmon resonance microscopy, transmission electron microscopy, scanning electron microscopy and atomic force microscopy. A circular-shaped crystal is grown from a thin layer of a latex suspension by a two-step mechanism: nucleation and crystal growth. Here we report an experimental study of the factors influencing the crystallization process, especially focusing on the water evaporation rate, the liquid meniscus at the crystal boundary, the particle size and concentration, the substrate, etc. Crystals of good quality and structure are grown at a high evaporation rate (low humidity) favoring a convection-dominated influx of particles from the suspension. The particle diffusion plays a role at suppressed evaporation thus causing an increase in the number of crystal defects. The dynamics of the meniscus slope leads to growth instability resulting in a sequence of multilayer rings. A hexagonal lattice prevails in the final crystal whereas a square lattice is observed in the transition regions between two different hexagonal multilayers. These general trends of the crystallization process are the same for different particle diameters (19 nm, 55 nm, 144 nm and 1.696 μm), volume fractions (0.001–0.01) and substrates (bare and metal-coated glass and mica). Received: 8 February 1999 Accepted in revised form: 12 May 1999     

Grain characterization of nanostructured TiN coatings prepared by reactive plasma spraying

Nanostructured TiN coatings were prepared by reactive plasma spraying, and the microstructure and the grain morphology of the coatings were analyzed by XRD, SEM and TEM. The results show that the coatings were mainly composed of TiN and Ti₃O phases (other phases were not detected), and they had the typical sprayed lamellar structures with few pores being visible. In the planar view, nanoscale grains with particle diameters ranging from 60 to 120 nm were observed in the coatings, and both fine equiaxial and columnar grains were found in some zones of the coatings. But in the cross‐sectional view, the TiN coatings were mainly composed of columnar grains, with no qualities of equiaxial grains being found. Thus, it can be concluded that the TiN coatings are composed of columnar grains, and the columnar grains are nanostructural, equiaxial grains in their cross‐section. Copyright © 2007 John Wiley & Sons, Ltd.     

A phenomenological model for the undulating twist grain boundary-C* phase

We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discovered undulating twist grain boundary-C* (UTGB_C*) phase. In the UTGB_C* phase, the smectic C* (SmC*)-like blocks and the grain boundaries separating them undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic blocks as interfaces with an anisotropic interfacial tension. At moderate chiral strengths we find a TGB_A-TGB_C-SmC* sequence. As the chiral strength is increased this goes to the sequence TGB_A-UTGB_C*-SmC*. Such sequences have been observed experimentally.     

Chain Architecture Dependent 3‐Dimensional Supramolecular Assembly of Rod‐Coil Molecules with a Conjugated Hexa‐p‐phenylene Rod

Summary: We have prepared hexa‐p‐phenylene based rod‐coil molecules with identical coil volume fractions, but different poly(propylene oxide) (PPO) coil architectures (linear versus dibranched), and investigated their self‐assembling behavior in the solid state by small angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. Rod‐coil molecules with a linear PPO coil showed a honeycomb‐like lamellar assembly of rod segments with hexagonally arrayed PPO coil perforations. In contrast, the rod‐coil molecules with dibranched PPO coils self‐organized into rod bundles with a body centered tetragonal symmetry surrounded by a PPO coil matrix. These results demonstrate that the steric hindrance at the rod/coil interface arising from coil architectural variation is a dominant parameter governing supramolecular rod assembly in the rod‐coil system.

TEM images and schematic illustrations of the self‐assembled structures of rod‐coil molecules with linear (left) and dibranched (right) PPO coils, respectively.     

Scattering of two-dimensional analogs of disordered lamellae

Optical Fourier transformation is used to study some two-dimensional (2D) analogs of disordered lamellae microstructure. A connected network of thick lines is used as the 2D representation of the disordered lamellar structures which have been observed in surfactant solutions. It is shown that, in these amorphous structures, the position of the scattering peak is still given by the underlying lattice and the observed scattering can be decomposed as a fair approximation, as a product of a local form factor and a structure factor due to the correlations of the underlying lattice.     

纳米掺铁二氧化钛的sol-gel法制备与表征 (Ⅱ)纳米掺铁二氧化钛的微晶特性与晶体生长

本文结合透电镜分析研究了sol-gel方法制备的纯二氧化钛和掺铁二氧化钛干凝胶的晶化过程,计算了在不同的煅烧温度下二氧化钛微晶的晶胸参数,晶粒度,畸变等参数的变化关系,应用非晶物质晶化晶核生长速率议程计算的昌粒生长活化能表明晶粒生长分为两个阶段,临界点大约为相变温度,纯的和掺铁的二氧化钛在两个阶段的晶粒生长活化能分别约为20．8kJ.mol^-1,70.9kJ.mol^-1和26．6kJ.mol^-1,78.8kJ.mol^-1.这个差别可能是由于相主过程首先发生在小晶粒上,导致小晶粒生长较为困难所致。     

纳米掺铁二氧化钛的sol-gel法制备与表征 (Ⅱ)纳米掺铁二氧化钛的微晶特性与晶体生长

本文结合透射电镜分析研究了 sol-gel方法制备的纯二氧化钛和掺铁二氧化钛干凝胶的晶化过程。计算了在不同的煅烧温度下二氧化钛微晶的晶胞参数,晶粒度,畸变等参数的变化关系。应用非晶物质晶化晶核生长速率方程计算的晶粒生长活化能表明晶粒生长分为两个阶段,临界点大约为相变温度。纯的和掺铁的二氧化钛在两个阶段的晶粒生长活化能分别约为 20.8kJ· mol-1,70.9kJ· mol-1和 26.6kJ· mol-1,78.8kJ· mol-1。这个差别可能是由于相变过程首先发生在小晶粒上,导致小晶粒生长较为困难所致。     

Crystallization of silver carboxylates from sodium carboxylate mixtures

Silver carboxylates can be made by the reaction of silver nitrate and the corresponding sodium carboxylates. The length of the alkyl chain has a significant impact on the product behavior. In this study, 18, 20, and 22 carbon chains (stearate, arachidate, and behenate, respectively) have been selected. All three sodium carboxylates are very insoluble in water at room temperature. Solutions are obtained above the Krafft temperature, which precipitates lamellar crystals if cooled at the proper cooling rate. Depending on the chain length, metastable morphologies, such as vesicles and tiny fibers, can be seen consecutively before hexagonal plates form. The carboxylate with the shorter chain length reaches equilibrium more quickly. All three silver carboxylates also take on a lamellar structure. Small-angle X-ray scattering (SAXS) shows that the d spacing of the crystals increases as the chain length increases. Cryo-TEM illustrates that the crystallites are the result of micelle nucleation and micelle aggregation. In addition, the crystallization process in the presence of silver bromide nanocrystals has been investigated. In the initial stage, an epitaxial interface is formed between the silver carboxylate crystallites and the cubic silver bromide grains. Budlike and strandlike structures grow because of it. The consequent strand enclosure restrains the crystal growth, which reduces the size and changes the morphology of the crystals.     

Synthesis of crystalline skutterudite superlattices using the modulated elemental reactant method

A series of samples ((AB)(x)(CD)(y))(z) were prepared containing both short repeat units (AB and CD) and long repeat units ((AB)(x)(CD)(y)), where the short repeat units were designed to have the composition appropriate to form square M(4)Sb(12) skutterudites (M = Fe, Co, or Ir; square = vacancy, La, or Y). X-ray diffraction and reflectivity were used to follow the evolution of the films from amorphous, layered materials to crystalline skutterudite superlattices as a function of annealing temperature and time. In all cases, the short repeat units interdiffused and crystallized the expected skutterudite, while the long repeat period persisted after annealing. The skutterudites crystallize with random crystallographic orientation with respect to the substrate. The observed splitting of the peaks in the high-angle diffraction data from the IrSb(3)/CoSb(3) sample indicates the formation of a novel superlattice structure with each grain having a random crystallographic orientation of the skutterudite lattice with respect to the superlattice direction.