Kinds Of Lamellae Research Articles

Self-assembly Behavior of Symmetrical Linear ABCA Tetrablock Copolymer: A Self-consistent Field Theory Study

ABCA tetrablock copolymers offer new opportunities for design of materials with novel structures. Using real-space self-consistent field theory and simulation, we systematically examined the self-assembly behavior of linear ABCA tetrablock copolymers in a 2D space. The simulation was carried out under conditions of symmetrical compositions and interactions. We focus on the influence of chain length ratio of block A and interactions between block A and other blocks B and C on the self-assembly behavior of the copolymer system. The simulation results show that most of the structures self-assembled by the ABCA tetrablock copolymers are centrosymmetric, such as diblock-like lamella phase, two kinds of lamellae with beads at interface, two kinds of hierarchical lamella phase, hexagonal honeycomb-like phase, lamella phase with mixed BC and hexagonal spheres with mixed BC. Furthermore, we find that a novel noncentrosymmetric Janus spheres can be obtained when the interaction between blocks B and C is strong, whereas a noncentrosymmetric lamella phase was obtained at weak interaction between blocks B and C. Phase diagrams for the ABCA tetrablock copolymers with different interaction strength between blocks B and C are constructed by comparing free energies of candidate ordered structures. In addition, studies on the metastable behavior of the system reveal that enthalpy plays an important role in the metastable behavior of the ABCA tetrablock copolymer system. Our work can provide useful guide for structure control of such kind of tetrablock copolymers in experiments.

A study of ADMET polyethylene with 21‐carbon branches on every 15th compared to every 19th carbon: What a difference four extra backbone methylenes make

ABSTRACTPrecision polyethylenes with 21‐carbon alkyl branches precisely spaced on every either 15th or 19th carbon along the polymer backbone lead to the formation of two kinds of lamellae, yielding different thicknesses during the crystallization process. Thinner lamellae originate from side‐chain crystallization, whereas thicker lamellae are formed by cocrystallization of the branch and the main chain. Side‐chain crystallization (separate from main chain crystallization) is favored when the branch is placed on every 15th carbon. Cocrystallization (side chain with main chain) is favored with the branch on every 19th carbon. Both form stable hexagonal crystal units. A branch spacing separation of just four carbons along the main chain makes a remarkable difference in crystallization behavior. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3090–3096

Baltic Sea Gastrotricha—one new species and one new record of Chaetonotida from Poland

Gastrotricha is a cosmopolitan phylum of aquatic and semi-aquatic invertebrates that comprise about 820 described species, which are divided into two orders: Chaetonotida Remane, 1925 [Rao & Clausen, 1970] and Macrodasyida Remane, 1925 [Rao & Clausen, 1970]. They inhabit natural as well as artificial habitats in diverse marine, freshwater, and semi-aquatic ecosystems (e.g. peatbogs, alder woods, riparian forests). Until now, 29 species of gastrotrichs from the Polish Baltic Sea region (including three freshwater species which were found in estuaries) were known. Sixteen species belong to Chaetonotida and thirteen to Macrodasyida. During this study we found two species, Heterolepidoderma sinus spec. nov., and Aspidiophorus lamellophorus Balsamo, Hummon, Todaro et Tongiorgi, 1997 which is new to the Baltic Sea fauna. H. sinus spec. nov. has distinct cuticular reinforcements in the anterior dilatation of the pharynx. Moreover, it is characterized by two kinds of lamellae: one type is represented by small triangular lamellae which aris from lateral scales, the second type is large and clearly visible and arises from ventral scales. None of the Heterolepidoderma species known so far has two types of lamellae. A. lamellophorus was previously known only from the Mediterranean Sea. The finding of two new gastrotrich species in the Baltic Sea shows that the knowledge of these small invertebrates in the area is still far from complete.

Phase-specific elastic/plastic interface interactions in layered NiAl–Cr(Mo) structures

The depth-dependent, as-grown and deformation-induced strain and dislocations partitioned through the interfaces in a two-phase layered NiAl–Cr(Mo) structure are directly measured at the mesoscale using 3-D X-ray microdiffraction. It is demonstrated that in the as-grown, undeformed state, neighboring submicron Cr solid solution and NiAl eutectic lamellae (doped with ∼3% Mo) form a heterointerface with 180° rotation around a 〈112〉 pole. It is shown that the mechanical response to the indentation of a layered composite with alternating Cr(Mo)–NiAl lamellae is distinct from the response of single-phase materials. In the center of the indent, after the load is released, the NiAl lamellae are under compressive forward stresses (with the same sign as the indentation-induced compression) while Cr solid solution lamellae are under tensile back stresses (with opposite sign from the indentation load). The depth-dependent alternation of compressive/tensile residual strains in the neighboring Cr solid solution and NiAl lamellae is understood in the framework of the Mughrabi’s composite model considering two types of structure elements: harder and softer regions. Under indentation, both kinds of lamellae are assumed to deform compatibly. After the load is released, residual forward stresses are formed in the harder lamellae, and back stresses are formed in the mechanically softer lamellae. Line-broadening analysis of the intensity distribution along the diffraction vector reveals a 15-times increase in dislocation density in the near-surface zone in the center of the indent. Such a large increase is typical for severe deformation.

Theoretical Investigation of Nano-Scale Organization in Blends of Semicrystalline/Semicrystalline Polymers by Small Angle X-ray Scattering

A theoretical model has been derived for the calculation of small-angle X-ray scattering (SAXS) data from stacks of crystalline lamellae in which two or more kinds of lamellae are mixed within the same stack according to different statistical models, ranging from the fully random mode, to alternated one, up to block configurations. The model has been used to study the complex lamellar morphology which develops from melt in some blends obtained by mixing two semicrystalline samples of isotactic polypropylene (iPP) characterized by different degree of stereoreoregularity, similar molecular masses, and melting temperatures of ≈162 °C for the more stereoregular component iPP1 and ≈74 °C for the less stereoregular one iPP2. It is shown that for samples crystallized in conditions far from thermodynamic equilibrium, the observed SAXS intensity profiles may easily show absence of correlation peaks, associated with high amount of diffuse scattering, not easy to unravel. Using our theoretical approach we show that featureless SAXS intensity distribution may correspond to formation of disordered lamellar stacks with lamellae of the two components randomly mixed within the same stacks. The tendency of iPP1 and iPP2 to form mixed lamellar stacks with a random configuration provides clear evidence that the two components are miscible in the melt.

Two new species of Paraprionospio (Polychaeta: Spionidae) from the Grand Caribbean region and comments of the genus status

Material earlier recorded from the Grand Caribbean region as Paraprionospio pinnata (Ehlers, 1901) was reexamined and referred to two new species: Paraprionospio tamaii sp. nov. and P. yokoyamai sp. nov. Paraprionospio tamaii sp. nov. has dorsal crests on segments 20–29, notopodial lamellae oval on chaetigers 1–2, reniform on chaetiger four, neuropodial lamellae abovate on chaetiger three, and branchiae with two kinds of lamellae: (1) bifoliate, rounded lamellae positioned basally, and (2) flabellate lamellae positioned from the middle to distal region of branchiae. Paraprionospio yokoyamai sp. nov. has no dorsal crests, notopodial lamellae triangular and elongate on chaetiger 1–2, subtriangular on chaetiger four, neuropodial lamellae cordate on chaetiger three, and branchiae with three kinds of lamellae: (1) small subtriangular (unipinnate) in the basal region, (2) bifoliate in the middle region and (3) flabellate from the middle to distal region. None specimen revised from the Grand Caribbean region was identified as Paraprionospio pinnata. The geographic distribution of this ‘cosmopolitan’ species should be carefully verified. An identification key to currently recognized species of Paraprionospio is provided.

On the Stability of Lyotropic Lamellar Liquid Crystals and the Thicknesses of Their Lamellae

The stability of lamellar lyotropic liquid crystals was investigated using a thermodynamic formalism. The interfacial tension γ was expressed in terms of the thicknesses δ1 and δ2 of the two kinds of lamellae. For given thicknesses, the system might be stable if γ ≥ 0. In adition, the derivatives ∂f/∂δi of the excess free energy f, due to van der Waals, hydration, steric, electrostatic, and undulation interactions, must be negative for the lamellar structure to be stable. The thermodynamic conditions employed show that the domain of existence of the lamellar phase is restricted to narrow ranges of layer thicknesses and parameters of interaction. The effect of the bending modulus and of the ionic strength on the stability was examined. The thicknesses of the two kinds of lamellae were calculated using equilibrium and mass balance equations. It was shown that the nonlinearity of these equations could be responsible for the observed deviations from the “ideal dilution law”. A comparison between the calculated and experimental results was made.

Molecular packing in self-assembled monolayers of normal alkane on Au(111) surfaces

Self-assembled monolayer of normal hexatriacontane ( n-C 36H 74) is formed at the interface between a solution of n-C 36H 74 in dodecane and a reconstructed Au(111) surface. Two kinds of lamellae have been observed, in which the long axis of n-C 36H 74 molecule is either perpendicular to or tilted 63.5° to the trough of the lamellae. The packing structures in both kinds of the lamellae are different. Theoretical calculations based on molecular mechanics have been performed to elucidate the ways of the molecular packing in the self-assembled monolayers on gold.

Further study on double-melting endotherms of isotactic polypropylene

The origins of the single- and double-melting endotherms of isotactic polypropylene crystallized at different temperatures were studied carefully by differential scanning calorimetry, wide-angle X-ray diffraction, and small-angle X-ray scattering. The experimental data show that spontaneous crystallization occurs when the crystallization temperature is lower than 117 degrees C; thus the lamellae formed are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae leads to double endotherms. On the other hand, when the crystallization temperature is higher than 136 degrees C, two major kinds of lamellae with different thickness are developed during the isothermal process, which also results in the double-melting endotherms. In the intermediate temperature range the lamellae formed are perfect, and there is only a single peak in the distribution of lamellar thickness. This explains the origin of the single-melting endotherm. (C) 2000 John Wiley & Sons, Inc.

On the origin of the thermodynamic stability of lyotropic lamellar liquid crystals

A thermodynamic formalism is developed for lamellar liquid crystals, in which the free energy is written as the sum of two terms: one which treats the two kinds of lamellae (water and oil) as bulk media and a second which (i) accounts for the thinness of the lamellae and (ii) includes the interactions among them. By “thin” we mean thicknesses small compared to the range of interaction forces between the molecules of each of the lamellae. On the basis of this free energy expression, equations are derived which can provide information regarding the maximum dilution of the liquid crystal with water or hydrocarbons. Considering planar layers and only van der Waals type interactions between molecules, we find that there are conditions under which the lamellar liquid crystals display the unlimited swelling observed experimentally. The effect of the undulation disorder of the interfaces between the lamellae is also discussed.

Tarnishing in vivo and in vitro of a low-gold alloy related to its structure.

Studies on tarnishing of a low-gold alloy, both as-cast and solid-solution-annealed, have been carried out in vivo and compared with that of a type III gold alloy. Small polished samples were partly embedded in complete dentures for nine months. Generally, the low-gold alloy in the as-cast state was found to be more prone to tarnishing than were the other two materials, followed by the same alloy, solid-solution-annealed. These results are in agreement with both previous and present in vitro studies using 2% solutions of Na2S. The tarnish attacks followed the pattern of Cu segregations in vitro, and frequently also in vivo. Previous findings in vitro, on the other hand, show that tarnishing occurs predominantly on Ag-rich phases. The link is to be sought in the structural details. In the Cu-rich regions along grain boundaries and in interdendritic positions, eutectic particles with thin lamellae (approximately equal to 0.05 micron) of alternating Ag and Cu enrichments occurred. In these areas, we found a more unstable matrix with increased tendency to splitting into similar, but even thinner, lamellae. Most likely, these lamellae of alternating compositions act as galvanic cells. The cathodic reduction of oxygen probably takes place mainly on the Cu-rich lamellae, which are Au-enriched on the surface due to initial selective dissolution of Cu. With increasing time, attacks up to several micron in depth comprising both kinds of lamellae tend to occur at the grain boundary regions in an in vivo test, probably by a fretting corrosion mechanism.

CRYSTALLIZATION OF POLYETHYLENETEREPHTHALATE UNDER MOLECULAR ORIENTATION

Crystallization of polyethyleneterephthalate (PET) was carried out in a steady state flow using a cone-plate type viscometer. The resultant texture was examined by means of (micro) differential thermal analysis (DTA) and of electron microscopy mainly as a function of shear rate (i.e., rotation speed of the cone). In order to avoid the rearrangement occuring during the DTA measurements, the crystallized films were treated with 70% aqueous solution of monoethyl amine and the extracted crystals were also examined by DTA and electron microscopy. The DTA thermograms revealed that two types of crystals with different melting temperatures were formed during the isothermal crystllization. The relative amount of the crystals with the higher melting temperature increased with the increasing rate of shear. The crystals with the lower melting temperature were susceptible to the amine treatment and apt to rearrange on heating, suggesting their instability. The electron microscopy revealed that in the case of the low shear, lamellae grown mormal to the direction of the shear were piled up along the shear direction just as in the case of polyethylene but in the case of higher shear, the two kinds of lamellae, big and very small, coexisted, the fomer having aligned along the direction of shear. It is postulated that the big lamellae might be grown from the bundle-like nuclei formed by intermolecular crystallization, which would be facilitated by molecular orientation and would show a higher melting temperature than that of the irregular lamellae resulted from the usual chain folding.

Das Lamellare Dünn-Dick-Muster der Chloroplasten von Chlamydomonas, Euglena, Fucus, Vaucheria

As supposed in a previous paper, Heitz (1960), the “lamellar thin-thick pattern” is found in four algae of very different taxonomic position. It is suggested that the presence of thin and thick lamellae as well in the chloroplasts with grana as without grana, is a common, significant characteristic. Perhaps there is a functional difference between the two kinds of lamellae. The thick lamellae in the midst of the two thin lamellae in the Algae (and in the liverwort Anthoceros) are perhaps the “precursors” of the many thick lamellae in the so called grana of the higher plants.