Chevron-like Structure Research Articles

Co-crystal sustained by π-type halogen-bonding inter-actions between 1,4-di-iodo-perchloro-benzene and naphthalene.

The formation and crystal structure of a co-crystal based upon 1,4-di-iodo-perchloro-benzene (C6I2Cl4) as the halogen-bond donor along with naphthalene (nap) as the acceptor is reported. The co-crystal [systematic name: 1,2,4,5-tetra-chloro-3,6-di-iodo-benzene-naphthalene, (C6I2Cl4)·(nap)] generates a chevron-like structure that is held together primarily by π-type halogen bonds (i.e. C-I⋯π contacts) between the components. In addition, C6I2Cl4 also inter-acts with the acceptor via C-Cl⋯π contacts that help stabilize the co-crystal. Within the solid, both aromatic components are found to engage in offset and homogeneous face-to-face π-π stacking inter-actions. Lastly, the halogen-bond donor C6I2Cl4 is found to engage with neighboring donors by both Type I chlorine-chlorine and Type II iodine-chlorine contacts, which generates an extended structure.

Revision of Triplicatella (Orthothecida, Hyolitha) with preserved digestive tracts from the early Cambrian Chengjiang Lagerstätte, South China

ABSTRACT The cap-shaped shells of Triplicatella are known almost exclusively from small shelly fossil assemblages with articulated specimens showing unequivocally that they represent the operculum of a hyolith. Abundant specimens of Triplicatella opimus from the fine-grained shales of the Chengjiang Lagerstätte of South China with soft-part preservation are documented herein. The soft tissues, including the feeding apparatus and complex digestive system, in T. opimus strongly suggest that Triplicatella was a deposit feeder. The digestive tract of T. opimus consists of two limbs, a spiral loop folded into a chevron-like structure and a slightly recurved to straight anal tube, which are preserved as reddish-black traces enriched in iron. The new anatomical information obtained from T. opimus in the Chengjiang Biota suggests an intermediate stage in the development of the characteristic folded gut of orthothecid hyoliths. The new anatomical information reported here shows that Triplicatella is one of the best-preserved early members of the Orthothecida and promotes our understanding of the general anatomy and evolution of the Hyolitha.

An orthothecid hyolith with a digestive tract from the early Cambrian of Bornholm, Denmark

The hyolith assemblage from the early Cambrian of Bornholm, Denmark, shows a higher diversity than contemporary assemblages in Baltoscandia. The most common species in the Green Shales (Læså Formation, Norretorp Member, Cambrian Stage 3), is Hyolithes [=Hyolithus] (Orthotheca) johnstrupi Holm, 1893. A specimen of this species shows a well-preserved and almost complete digestive tract, folded into an approximately 22 mm long chevron-like structure comprised of at least 20 arcuate loops on the ventral side and a flattened, gently sinuous to straight anal tube on the dorsal side. The thin, phosphatic outer shell layer of the conch is crushed under the digestive tract due to compaction while the digestive tract is preserved in three dimensions and appears undisturbed. The shape of the digestive tract is similar to that of the middle Cambrian Guduguwan hardmani (Etheridge) from Australia and the lower Cambrian specimens from Russia described by Meškova & Sysoev. The Danish specimen is probably an adult, lending support to the idea that the orthothecid digestive tract becomes more complex during ontogeny. Hyolithus (Orthotheca) johnstrupi is revised and here referred to Circotheca Sysoev, 1958.

Thick smectic shells

The known ground state of ultrathin smectic films confined to the surface of a sphere is described by four +1/2 defects assembled on a great circle and a director which follows geodesic lines. Using a simple perturbative approach we show that for thick smectic films on a sphere with planar anchoring this solution breaks down, distorting the smectic layers. The instability happens when the bend elastic constant exceeds the anchoring strength times the radius of the inner sphere. Above this threshold, the formation of a periodic chevron-like structure, observed experimentally as well, relieves geometric frustration. We quantify the effect of thickness and curvature of smectic shells and provide insight into the wavelength of the observed texture.

A coarse-grained simulation study of mesophase formation in a series of rod–coil multiblock copolymers

The mesophase behaviour of a rod-coil multiblock copolymer is assessed by means of a new soft-core simulation model, which is suitable for the simulation of combinations of isotropic and anisotropic particles. The simulations demonstrate the presence of isotropic melt, micelles, lamellar, nematic and gyroid phases, with ordered phases able to grow spontaneously from the isotropic melt. The influence of increasing the length of the rigid rod-component of the polymer is studied, with mesophase stability enhanced by increases in rod length. Increased nematic phase stability is demonstrated also as the length of rod component is increased. For longer rods (smaller fraction of coils) the models show evidence of metastable chevron-like structures that initially form on cooling from the polymer melt. These are eventually lost in favour of true lamellar ordering over long annealing runs. The structure of molecules within the phases formed is also assessed. Chains in the lamellar phase are shown to produce both bridging and loop behaviour, with the latter preferred slightly. Simulations at a low occupied volume fraction, corresponding to self-assembly in solution, demonstrate the formation of a structurally ordered nanowire.

Orientation of polyoxymethylene by rolling with side constraints

In this paper, we describe the application of the constrained rolling process to produce highly oriented polyacetal bars with enhanced mechanical properties. In this process, the heated polymer billet is deformed in a channel formed in the circumference of the bottom roll that provides lateral constraint to the material as it deforms. It is a process that has attracted interest due to its capability to produce thick cross-sectional oriented products continuously and at moderate production speeds. Here the focus is on two commercial grades of polyoxymethylene (a) Delrin® 100 and (b) Tarnoform® 300. Tarnoform®, unlike Delrin®, is a copolymer. The compression behaviour of these grades has been investigated in a plane strain channel die to determine the optimum constrained rolling conditions. Samples were then rolled to different reduction ratios close to but below the crystalline melting temperature of the two grades.The modulus and strength increased almost linearly with reduction ratio. Rolled Delrin® exhibited higher modulus and strength than Tarnoform®. Under impact loading, with the initial notch perpendicular to the rolling direction, the fracture process was incomplete for both resins with the specimens exhibiting a hinge type break. Structural investigations of the rolled samples were carried out by wide and small angle X-ray diffraction. The structures produced were very similar to those produced in plane strain compression test. The pole figures from the (100) reflection suggest that the c axes of the POM crystals are oriented along the rolling direction while ab planes showed clustering of orientation of (100) normals in six directions. SAXS patterns from the rolled samples with the X-ray beam parallel to the force direction showed two-point patterns that suggest the transformation of the spherulitic morphology to a fibrillar structure in this direction. However, perpendicular to the rolling direction, four-point patterns were obtained that suggest cooperative kinking of the lamellae during deformation to produce a chevron-like structure. The enhancement in properties as a result of molecular orientation suggests that these materials can have major commercial applications.

Evolution of lamellar orientation and crystalline texture of various polyethylenes and ethylene-based copolymers in plane-strain compression

Deformation-induced orientation behavior of a series of polyethylenes and ethylene copolymers was studied by WAXS and SAXS. Plane-strain compression was employed as a deformation mode. Compression experiments were performed at room temperature and at 80 °C. Activity of crystallographic slip systems supported by interlamellar shear during deformation process resulted in formation of the (100)[001] primary texture component with chain axis aligned along the flow direction, FD, and (100) planes perpendicular to the loading direction, LD. A second texture component was produced on unloading by {310} twinning of primary component. Intensity of that twinning was controlled primarily by the molecular weight of polymer, through modification of the molecular network and its stress response on applied strain. Molecular weight influenced also the orientation behavior on the lamellar level. In all polymers studied the lamellar shear, intensive on the beginning of deformation and exhausting around the true strain of 0.75, induced cooperative kinks of lamellae. In samples of relative low molecular weight further advancement of deformation led to an additional intense fragmentation of lamellar structure and restructurization into new long period in the direction of flow ( e>1.2). In polymers of higher molecular weight such an intense fragmentation did not occur due to topological constraints. Consequently, the lamellar structure of heavily deformed polyethylenes of low molecular weight consisted of relatively small blocks, oriented with their normals approximately along FD, while high molecular weight species, in absence of intense lamellae fragmentation, developed the structure of relatively long lamellar crystals, oriented in a chevron-like structure. It was found that the orientation produced by deformation process is controlled entirely by the strain applied to the material, while the differences induced by modification of the stress due to the change of the deformation temperature are of minor consequence.

Stability of the chevron domain at triple-line reconstructions

The line of emergence of a grain boundary at the surface of a face centered cubic crystal may decompose into a nanoscale triangular prism with a chevronlike structure. This behavior has been found in gold, and we address here the generality of this phenomenon. The stability of the chevron structure is studied and discussed for the case of a special tilt grain boundary in copper, nickel, and aluminum. Atomistic simulations predict a chevron decomposition for Cu and Ni, but not for Al. The stability of Al against line decomposition was confirmed by atomic resolution electron microscopy. The distinct behavior of these crystals is related to their stacking fault energies.

An X-ray Study of Devices Containing a N*???SmC*Liquid Crystal Mixture

The structure of a ferroelectric liquid crystal mixture with an N∗ → SmC∗ transition in skew aligned cells has been studied by X-ray diffraction measurements of the layer normal distribution. The rubbing directions of the cells were set parallel, and skewed at ±9°, ±18° and ±40°. The cell with parallel alignment directions showed two sets of layers with normals twisted in-plane by ∼±20° from the rubbing direction. The layer normal distribution of each set of layers was consistent with a chevron-like structure but with an additional untilted region, probably at the apex. It was also observed that the tilt of the chevron arms was weakly temperature dependent, with their normals tilting out of the plane of the device by an angle of 9° to 12° on cooling. The cells with alignment skewed by ±9° and ±18° also showed chevron-like structures which are probably developed in the middle of the cell with each arms tilted by ∼20° and with normals slightly twisted in-plane by ±4° with respect to the mean rubbing direction. However, there were additional components of the layer-normal distribution, which are believed to be a continuous distribution of layer orientations between the middle of the cell and layers near the substrate with normals twisted further away from the mean rubbing direction. The cell with alignment directions skewed by ±40° showed only a chevron structure with the normals to its two arms twisted with respect to each other by 2° and tilted out of plane by ∼17°.

Synthesis and crystal structure of Cu 2NiO(B 2O 5) and Cu 2MgO(B 2O 5)

A transport reaction synthesis technique has been used to prepare single crystals of two pyroborate compounds having the formulas Cu 2NiO(B 2O 5) and Cu 2MgO(B 2O 5). The two compounds are isostructural and crystallize in the monoclinic space group P2 1/ c. Cu 2NiO(B 2O 5): a=3.2003(10), b=14.775(3), c=9.097(3), β=93.28(4), V=429.4(2) Å 3, Z=4; and Cu 2MgO(B 2O 5): a=3.2401(6), b=14.790(2), c=9.147(2), β=94.88(2), V=436.7(2) Å 3, Z=4. The structures of Cu 2NiO(B 2O 5) and Cu 2MgO(B 2O 5) were, respectively, refined from 804 and 1000 independent reflections to the final residuals R1=0.0366, wR2=0.0911 and R1=0.0231, wR2=0.0644. Both compounds exhibit a chevron-like structure built up of ribbons, made of edge-connected copper and nickel–oxygen polyhedra, running along the (1 0 0) direction. These ribbons are connected from one another via oxygen atoms and the cohesion of the three-dimensional network is ensured by [B 2O 5] entities. Cu in part occupies the position for Ni or Mg, so that the compounds actually are solid solution compounds. Ni or Mg atoms are octahedrally coordinated by oxygen, while the two pure Cu sites show [4] and [4+1] coordination, for Cu(1) and Cu(2), respectively. The ELNES B-K edge spectra for the two compounds support that the borate group present is [B 2O 5].

Structure evolution and phase behavior of polymer blends under the influence of shear

The effect of shear flow on the phase behavior, structure formation and morphology of a near-critical composition blend of poly(styrene- co-acrylonitrile)/poly(methyl methacrylate) (SAN/PMMA) and an off-critical composition blend of SAN/poly(ε-caprolactone) (SAN/PCL) were investigated. Both blends exhibited lower critical solution temperature (LCST) behavior. Rheo-SALS (small angle light scattering) was used to characterize the time-dependent structure evolution during flows with shear rates from 0.01 to 0.05 s −1 and stresses from 8 to 60 kPa. Two types of transient “dark-streak” scattering patterns were observed. One featured a dark streak that diverged with increasing scattering vector, q, which corresponded to a chevron-like structure in the morphology of the blend. The other type of dark streak converged with increasing q, and may be due to a spinodal structure that was stretched in the flow direction. A capillary viscometer was used to achieve shear stresses as high as 400 kPa for the SAN/PCL blends, and microscopy analysis of microtomed extrudates revealed a dispersed phase morphology of cylindrical PCL-rich domains highly oriented in the flow direction. No shear-induced phase transitions were observed for either blend for the range of shear stress studied.

Dynamics of disordered patterns in electroconvection of homeotropically aligned nematic liquid crystals

We report on experiments in nematics with homeotropic alignment where the electrohydrodynamic instability occurs after a bend Freedericksz transition. The threshold of convection, the wave number at onset, and the threshold for the transition between order and disorder in the presence of a stabilizing magnetic field agree with theory. Whereas in the oblique-roll range the behavior of the correlation time of the disordered state exhibits near onset the expected scaling behavior, the disordered state in the normal-roll range at zero magnetic field shows no dynamics below a well-defined value ${\ensuremath{\varepsilon}}_{s}$ of the reduced control parameter, in contrast to theoretical predictions. At ${\ensuremath{\varepsilon}}_{s}$ one has reproducibly a continuous and reversible transition to a dynamic state. We attribute these features to the appearance of chevronlike structures.

Taxol Induces Microtubule-Rough Endoplasmic Reticulum Complexes and Microtubule-Bundles in Cultured Chondroblasts

Taxol induces a vast increase in the number of microtubules (MTs) in functional chondroblasts. The drug also induces a marked change in MT distribution. In control cultures, anti-tubulin stains long, fine, sinuous filaments radiating from a perinuclear center. In taxol-treated cells, anti-tubulin stains stubby, straight, chevron-like structures that assume a striking antipodal distribution. Such MT-bundles are relatively stable: they persist for over 48 h after removal of taxol, and even for 16–24 h in Colcemid. Many of these supernumerary MTs bind to, and align on, the cytoplasmic face of the rough endoplasmic reticulum (RER). In binding, the MTs displace the numerous ribosomes that normally stud the surface of the cisternae of the RER. The bound MTs form a remarkably uniform layer with center-to-center spacings of 40 nm. The attached parallel arrays of MTs achieve lengths of over 10 μm. These bound MTs not only dislodge ribosomes from the RER surface, but they also zip together adjacent ER complexes, forming tiers of two to eight cisternae. Numerous cytoplasmic bundles of hexagonally-ordered MTs are also induced. When closely aligned, the MTs assume a crystalline configuration with a six-fold symmetry, a central MT being surrounded by six equidistant MTs. A single cell can have over 100 MT-bundles and the number of MTs per bundles varies from 2–30. The forces aggregating cytoplasmic MT-bundles probably differ from those that bind MTs to the RER. Taxol also fragments the prominent Golgi complex that characterizes actively secreting chondroblasts. No obvious morphologic relationship has yet been detected between these induced MTs and other organelles such as intermediate-sized filaments, microfilaments, mitochondria, Golgi cisternae, or secretory vesicles.