Pseudohexagonal Structure Research Articles

On the plastic deformation of the amorphous component in semicrystalline polymers

The molecular orientation of the amorphous component of semicrystalline high-density polyethylene (HDPE) induced by plane strain compression was studied by wide-angle X-ray scattering measurements utilizing the pole figure technique and separation of the scattering produced by the crystalline and amorphous components. It was found that the oriented amorphous component produced by large strain plastic deformation consists of domains of extended chain segments, closely packed in a two-dimensional pseudo-hexagonal aggregation, which are separated by less ordered regions. The deformation leads to the formation of a texture of the amorphous component which is common to the whole sample. In this texture, the direction of the chains coincides with the direction of flow, and one of the (100) pseudo-planes of the pseudo-hexagonal structure in every domain is perpendicular to the loading direction. It was suggested that the most important deformation mechanisms in the ordered amorphous component were the glide of the chain segments along their axes and the slip of the pseudo-planes of the ordered chains in the direction perpendicular to the chain axis, with resembling the crystallographic slip processes. Such a specific deformation of the amorphous layers most probably resulted from the strong constraint imposed by the slip deformation in the crystalline component to which the amorphous component is intimately connected.

Characterization of the Structure and Catalytic Behavior of AlF3−x(OH)xwith Aluminum Successively Replaced by Chromium and Magnesium

The calcination of α-AlF3·3H2O forms a β-AlF3phase which is catalytically active for dismutation of C1-hydrocarbons such as dichlorodifluoromethane. The stepwise replacement of aluminum by chromium and magnesium leads to considerable alterations in structure and surface properties of the calcination products. This is accompanied by significant changes in the catalytic activity. The synthesis of the catalysts was carried out by coprecipitation of mixed metal fluoride trihydrates and subsequent calcination procedures. The stepwise replacement with chromium leads to a rebuilding of the lattice from the pseudo-hexagonal tungsten bronze (HTB) structure of β-AlF3into a cubic pyrochlore structure of CrF3−x(OH)x. Hexagonal β-CrF3or mixed crystals β-(Al,Cr)F3were not obtained via this route. The maximum catalytic activity for CCl2F2dismutation was obtained for the 50% chromium sample, which is accompanied by a maximum BET surface area and a maximum number of Lewis acid sites. CrF3−x(OH)xexhibits a dramatic loss of catalytic activity as well as BET surface area. Possible explanations are given by comparing the pseudo-hexagonal tungsten bronze structure with the cubic pyrochlore structure with regard to accessibility of the Lewis acid metal cations. The presence of hydroxyl groups within the pyrochlore lattice enables the formation of hydrogen bridge bonds which is accompanied by a shielding of the metal cations. In the case of the replacement with magnesium the aim was to tune the strength of the Lewis acidity. The catalytic activity of the calcined samples passes through a maximum at the 10% magnesium sample. Due to the decrease of the strength of Lewis acid sites with further increasing Mg content, the catalytic activity is more and more diminished. At 50% Mg and higher the strength of the Lewis acid sites is no longer sufficient to catalyze the dismutation.

Tertiary structure of an amyloid immunoglobulin light chain protein: a proposed model for amyloid fibril formation.

An immunoglobulin light chain protein was isolated from the urine of an individual (BRE) with systemic amyloidosis. Complete amino acid sequence of the variable region of the light chain (VL) protein established it as a kappa I, which when compared with other kappa I amyloid associated proteins had unique residues, including Ile-34, Leu-40, and Tyr-71. To study the tertiary structure, BRE VL was expressed in Escherichia coli by using a PCR product amplified from the patient BRE's bone marrow DNA. The PCR product was ligated into pCZ11, a thermal-inducible replication vector. Recombinant BRE VL was isolated, purified to homogeneity, and crystallized by using ammonium sulfate as the precipitant. Two crystal forms were obtained. In crystal form I the BRE VL kappa domain crystallizes as a dimer with unit cell constants isomorphous to previously published kappa protein structures. Comparison with a nonamyloid VL kappa domain from patient REI, identified significant differences in position of residues in the hypervariable segments plus variations in framework region (FR) segments 40-46 (FR2) and 66-67 (FR3). In addition, positional differences can be seen along the two types of local diads, corresponding to the monomer-monomer and dimer-dimer interfaces. From the packing diagram, a model for the amyloid light chain (AL) fibril is proposed based on a pseudohexagonal spiral structure with a rise of approximately the width of two dimers per 360 degree turn. This spiral structure could be consistent with the dimensions of amyloid fibrils as determined by electron microscopy.

Crystal Structure of Poly(p-Aminohippuric Acid) Produced by Thermal Polymerization.

Synthesis of poly (p-aminohippuric acid) (PPAHA) was carried out using two different polymerization techniques: thin-film melt-polymerization and single-crystal thermal-polycondensation. Crystal structure and morphology of PPAHA were examined by means of the electron microscopy. Lamellar crystals were observed on the thin-film melt-polymerized specimen. The electron diffraction pattern from the lamellar crystal suggested the pseudohexagonal lattice structure. Comparing the result with the packing of chains in the unit cell of poly (p-phenylene terephthalamide) crystal, it was concluded that the unit cell of PPAHA was orthorhombic (a=0.48nm, b=0.84nm and c(chain axis)=1.9nm), the space group of which was reasonably assumed to be P212121, and that the diffraction pattern was superimposition of the reflections from the twins with three different orientations (± 60° and ± 120°).In the case of the thermally polymerized material from the p-aminohippuric acid single crystal, numerous fibrils (100-200nm in diameter and 50-200μm in length) appeared over the surface of the specimen after annealing longer than 1000h, in which the molecular chains were considered to be normal to the basal plane, since the diffraction pattern always corresponded to the projection of the a *-b* plane in the reciprocal lattice of PPAHA crystal.

Fiber Structure Formation in High-speed Melt Spinning of Sheath-Core Type Bicomponent Fibers.

High-speed melt spinning of sheath-core type bicomponent fibers was performed and the change of fiber structure with increasing take-up velocity was investigated. Four kinds of polymers, poly (ethylene terephthalate) (PET), polypropylene (PP), high-density polyethylene (PE) and polystyrene (PS), were selected and three sets of core/sheath combinations, PET/PP, PET/PE and PET/PS, were studied. The structure of each component in high-speed spun bicomponent fibers was analyzed through the birefringence, wide-angle X-ray scattering pattern and DSC thermogram measurements. The development of the molecular orientation and the starting of the orientationinduced crystallization of PET in PET/PP and PET/PE bicomponent fibers occurred at lower take-up velocities as compared to those in the single component spinning. On the other hand, PP in PET/PP fiber showed low orientation pseudo-hexagonal structure at all the take-up velocities investigated and the birefringence of PE in PET/PE fibers obtained at 6km/min was almost negligible, meaning fiber structure formation of PP and PE components was suppressed. On the contrary, in the PET/PS combination, structure formation of PET was suppressed and there was no indication of orientation-induced crystallization even at 7km/min. Oppositely, the molecular orientation of PS component was enhanced. The mechanism of the mutual interaction of two components on structure formation in the high-speed spinning of bicomponent fibers was speculated based on the characteristics of the constituent polymers, and it was concluded that the structure formation of a component having higher temperature-dependent elongational viscosity and higher solidification temperature is expected to be enhanced.

Morphology and Structure of Nylon 68 Single Crystals

The structure and morphology of nylon 68 single crystals were studied by transmission electron microscopy. There are two crystal phases. A monoclinic phase with a = 0.960 ± 0.005 nm, b = 0.826 ± 0.005 nm, and γ = 115 ± 1°, when viewed along the chain axis, c, is the usual form for crystals cooled slowly from the crystallization temperature. This lattice is the same, within experimental error, as that of monoclinic crystals of nylon 46, a polymer which has the same chain structure, but with amide groups more closely spaced. Nylon 68 crystals quenched into nonsolvent from the crystallization temperature are found in a pseudohexagonal phase which has parameters a = b = 0.97 nm and γ = 120°. When monoclinic crystals are heated, they transform, gradually, into the pseudohexagonal phase ; the transformation is complete at 203 °C, and melting takes place at 234 °C. The crystals grow from solution in the pseudohexagonal phase which is stable at high temperatures ; they usually revert to the monoclinic phase on cooling, but the pseudohexagonal phase can be obtained at room temperature by quenching to below the glass transition temperature. Monoclinic nylon 46 single crystals were examined on heating. They transformed to the pseudohexagonal structure at 245°C before melting took place at 295°C.

Twinning as a clue to a CDW-like behavior and to a lack of SME in plutonium metal

The twinning systems of alpha phase Pu metal have been reported by Zocco et al. (Proc. XII Int. Congr. for Electron Micros., 1990, p. 1014). Twinning modes and charge-density wave (CDW) states have both been reported for the CDW materials 2HTaSe 2 and TiNi alloys. The latter alloys also show shape memory effect (SME). The similarities in properties of α-Pu and TiNi alloys have recently been pointed out by Sandenaw (J. Nucl. Mater., 189 (1992) 343). A possible clue to a CDW-like behavior and a lack of SME in α-Pu may be in the twinning systems observed. Microtwinning has been suggested as interfering with flow of itinerant electrons and thus causing a rise in electrical resistivity as reported for the TiNi alloys (Wang et al., J. Appl. Phys., 43 (1972) 97). The latter workers report a one-to-one correspondence between mechanical “memory” effect and electrical resistivity. α-U exists with many twinning modes (Frank, Acta Metall., 1 (1953) 71) but does not show any CDW behavior except at low temperatures. Therefore, twinning alone cannot be the source of a CDW-like behavior in α-Pu metal. Bak ( Rep. Prog. Phys., 45 (1982) 587) explained that the conduction electron density in a distorted phase is spacially modulated to form the CDW which accompanies the periodic lattice distortion (PLD). There appear to be competing periodicities (a PLD) in the complex α-Pu structure because the eight crystallographically different Pu atoms of the unit cell are seen to occur in a different but reproducible pattern in each of three directions on the a-c plane of the distorted pseudo hexagonal structure (Smith et al., ACS Symp. Series, No. 216, Plutonium Chemistry (1983) Fig. 1). It is suggested that the modulation of the conduction electrons, in three competing periodicities (chains) of the α-Pu lattice, should result in three different competing CDW-like states. The interference between three different CDW-like states may lead to the inability to see any one CDW-like state in α-Pu by neutron diffraction. The α-Pu structure is more complex than those of the CDW materials 2HTaSe 2 and TiNi alloys which also show twinning and a hexagonal structure. The twinning planes and twinning (shear) directions of single-crystal TiNi have been described with only the number one and zeros. (Miyazaki and Wayman, Acta Metal., 36 (1988) 181). The twinning planes and twinning directions of α-Pu were described by Zocco et al. with the numbers 1, 2, 3, 5 and 7 (and zeros). The greater complexity of twinning systems in α-Pu metal may explain an inability to see CDW states by TEM. This complexity could explain the lack of SME in Pu metal.

Studies on the crystallization and melting of nylon-6,6: 1. The dependence of the Brill transition on the crystallization temperature

The isothermal crystallization process from the melt in nylon-6,6 was followed in situ by X-ray diffraction. At all the crystallization temperatures studied, the samples crystallized into the pseudohexagonal (hexagonally packed) Brill structure, which transformed into the triclinic structure on subsequent cooling to room temperature from the crystallization temperature. This transition occurs about 40°C lower than the respective crystallization temperature. Also the transformation temperature ( T B) displays a hysteresis effect on heat cycling, with T B being higher on heating than on cooling. These findings affirm the first-order transition character of T B, contrary to some previous suggestions. They also imply gradation in perfection and/or crystal size while in the Brill structure. It follows further that any difference in structure and property, as determined at room temperature, must reside in the Brill structure at the stage of formation and can only be reflected indirectly by the transformed structures observed at room temperature. With reference to spherulites, the latter has important implications for the arrangement of hydrogen bonding within the Brill structure. On the other hand, crystallization directly into the triclinic Bunn structure remains a possibility for crystallization from solution, awaiting confirmation by X-ray diffraction.

Zur lumineszenz von biaktiviertem yttriumorthoborat

Powder materials of yttriumorthoborate coactivated by trivalent bismuth and gadolinium or praseodymium and gadolinium described here have been prepared by solid state reaction. These luminophors crystallize in the vaterite type. Therefore they have a pseudo-hexagonal structure. There are two crystallographically different sites for the cations. In addition to X-ray diffractograms, emission and excitation spectra and spectra of the diffuse reflection of the luminophors have been studied at room temperature along with the influence of temperature and UV radiation on emission of the samples. Bi III and Pr III ions sensitize Gd III luminescence. There is only Gd III luminescence in the emission spectra. The peak of maximum intensity is at 31 888 cm −1. Therefore the luminophors could be used in medicine for therapeutical purposes.

Features of the structure of poly- bis-trifluoroethoxyphosphazene extrudates

The special structural features of the mesophase of bis-trifluoroethoxyphosphazine extrudates are studied by X-ray diffraction analysis, differential scanning calorimetry, and rheological methods with shear and longitudinal flow. It is shown that in the region of 453–493 K the rheological properties and a number of structural characteristics of a polymer melt mesophase are significantly changed, with an accompanying thermal effect. On the basis of the data obtained it is suggested that over this temperature range conformational transformation of the macromolecules of polymer in the mesomorphic state takes place, with formation of a structure which is intermediate between a unidimensional layerized and a two-dimensional pseudo-hexagonal structure.

Mesophase structure of flexible‐chain polymers

AbstractA structure of polyethylene (PE) and poly[bis(2, 2,2‐trifluoroethoxy)phosphazene] (PPh) has been studied by means of the X‐ray diffraction technique, scanning electron microscopy and differential scanning calorimetry. The polymers under investigation are examples of the organic and inorganic mesophase flexible‐chain polymers not containing the mesogenic groups. When heated above the melting point both polymers form the condis crystal mesophase state which is stable in a wide temperature range. The PE mesophase reveals the pseudohexagonal structure, while the structure of mesophase PPh has some peculiarities arising from the sharp increase in the chain stiffness within a certain temperature range. The PPh mesophase structure is characterized by the perfect one‐dimensional long‐range order of the monomolecular layers and by the short‐range order only along the two other directions within the layer. This structure may be due to the difference between the interactions of the inorganic backbones and regularly alternating side trifluoro‐ethoxy groups.

Adsorption on well-defined solid surfaces chloride adsorption on a (110) face of silver

Abstract The adsorption of bromide has been studied on a (110) face of silver by means of mixed solutions with hexafluorophosphate as base electrolyte. From the differential capacity-potential curves, the adsorbed charge of bromide was determined as functions of the electrode charge and the potential. The components of the inner-layer capacity have been determined for the two peaks always observed on the (110) face in the presence of specific adsorption. For the more negative peak potential, the component at constant amount adsorbed confirms the essential contribution of the orientation of water dipoles to the existence of this peak. On the other hand, the component at constant electrode charge indicates a non0linear electrode charge dependence of the standard Gibbs energy of adsorption. For the less negative peak potential, the two components of the inner-layer capacity seem to show that the bromide adsorption is mainly responsible for the appearance of the second peak. Different structures of the layer of adsorbed ions are proposed for different coverages of the surface. A pseudohexagonal structure, equivalent to the c (4×2) structure, corresponds to the adsorbed charge determined at saturation.

Adsorption on well-defined solid surfaces: Bromide adsorption on a (110) face of silver

The adsorption of bromide has been studied on a (110) face of silver by means of mixed solutions with hexafluorophosphate as base electrolyte. From the differential capacity-potential curves, the adsorbed charge of bromide was determined as functions of the electrode charge and the potential. The components of the inner-layer capacity have been determined for the two peaks always observed on the (110) face in the presence of specific adsorption. For the more negative peak potential, the component at constant amount adsorbed confirms the essential contribution of the orientation of water dipoles to the existence of this peak. On the other hand, the component at constant electrode charge indicates a non0linear electrode charge dependence of the standard Gibbs energy of adsorption. For the less negative peak potential, the two components of the inner-layer capacity seem to show that the bromide adsorption is mainly responsible for the appearance of the second peak. Different structures of the layer of adsorbed ions are proposed for different coverages of the surface. A pseudohexagonal structure, equivalent to the c (4×2) structure, corresponds to the adsorbed charge determined at saturation.

Direct atomic imaging of solid surfaces: IV. Dislocations on Au(100)

Direct observation of the metastable gold (100) 1 X 1 surface by high resolution electron microscopy provides evidence for surface Shockley partial dislocations. This result is correlated with the known 5 X 20 reconstruction to a hexagonal overlayer since the dislocations possess a pseudo-hexagonal core structure. This implies a dislocation mechanism for the phase transformation from 1 X 1 to 5 X 20. It is proposed that pipe diffusion along the dislocation cores or mass transport via dislocation glide or climb could explain the rapid atomic migration required during the phase transition from 1 X 1 to hexagonal.

Magnetic resonance study of the crystal symmetry of praseodymium bromate nonahydrate

Electron spin resonance measurements on Nd ions in Pr(BrO3)39H2O (PrBR) have been carried out at 4.2 K to examine the symmetry of the rare-earth sites. The results are not compatible with recent diffraction studies which identified the crystal as hexagonal with D3h rare-earth site symmetry. The spectra show a large number of inequivalent sites with probably only reflection symmetry, repeating every 60° in the plane perpendicular to the long axis of the crystal. These results appear to be consistent with the twinned pseudohexagonal C122v structure also proposed for this compound.

Structure of Ag ON Rh and its effect on the adsorption of D 2 and CO

Ag vapor deposited on Rh(100) is investigated as a possible model for the corresponding bimetallic cluster catalyst. Ag overlayer growth follows the Frank-Van der Merwe mechanism at 300 K and the Stranski-Krastanov mechanism at 640 K. The first Ag overlayer grows epitaxially with the Rh(100) surface and forms two-dimensional islands. The second layer reconstructs forming a pseudohexagonal lattice structure with a 19% greater Ag atom density than the first layer. Neither alloy formation nor dissolution of Ag into the Rh crystal lattice is observed. The presence of Ag decreases the capacity of the Rh(100) surface for both D 2 and CO chemisorption. In both cases, loss of adsorption sites on the Rh surface is attributed to physical site blocking by islands of Ag. Very weakly bound CO adsorption sites are observed which are attributed to CO bonding to Ag atoms in the first Ag overlayer. The Ag/Rh(100) system is found to be very similar to the analogous SiO 2 supported catalyst.

Ferroelectric polymers and applications

Abstract Ferroelectricity in polymers such as polyvinyli-dene fluoride (PVF2) was recognized as possible in the last decade, since they are semicristalline, two at least of the cristalline phases are polar, and polariza tion/field hysteresis loops were observed; nevertheless, no Curie temperature nor associated structural phase transition were reported, and therefore, we have assumed that these transition could occur in the vecinity of the melting temperature. Considering the pseudo hexagonal structure of the polar phase, Kepler and Broadhurst have suggested that the macromolecular chains could cooperatively rotate by sixty degrees discrete jumps (6 sites model) under applied field, which conformation energy calculations has justified; more precisely, polarization switching has been analysed in terms of a king propagation model (Taylor). These models have been experimentally confirmed by Servet, who has observed by X-Rays diffraction and IR absorption analysis these discrete rotations of the macromolecular chains under applied field. Copolymers of PVF2 with polytrifluoroethylene (PTrFE or PVF3) present a true ferroelectric character, since a structural transition from the high temperature non polar phase (PVF3 phases to the low temperature polar phase (βPVF2 phase) occurs at a temperature lower than the melting temperature, which depends upon the relative concentration PVF2/PVF3. Extrapolation of this dependance for pure PVF2 gives for this polymer the melting point as the Curie temperature, as we has already assumed. Applications of these feroelectric polymers begin on a large scale, since they are now commercially available from different manufacturers, usually as thin films (thickness ≤50 to 100μm) and the developpment stage, in larger thicknesses (up to 1 to 2 mm). The main applications are piezoelectric transducers, for underwater acoustics, microphones and loudspeakers and electrical energy storage and production.

The valence of Sm in a monolayer on Al(100)

Submonolayer and monolayer films of Sm deposited on an Al(100) crystal surface have been studied by LEED and X-ray photoemission spectroscopy. Layers deposited at — 100°C or RT and without subsequent annealing were disordered and composed of Sm in both divalent and trivalent forms. Annealed films showed a pseudo hexagonal structure and were predominantly trivalent. We conclude that a perfect monolayer of Sm on Al(100) is composed wholly of trivalent atoms.

Two new copper(II) hydroxoiodides

As evidenced by X-ray diffraction, two new hydroxoiodides of copper have been synthesized and their compositions were deduced from analytical data of their mixtures. One of the compounds, with Cu/I = 4 represented by the formula Cu 4(OH) 7I(2.5H 2O), had a fcc structure with a = 9.43 Å and the other with Cu/I = 2.5, represented by Cu(OH) 1.4(O) 0.2(I) 0.1(I 3 0.1(0.74H 2O) appeared to have a hexagonal (or pseudo-hexagonal) structure with a = 3.78 and c = 6.61 Å. The newly found Cu(II) hydroxoiodides constitute, together with Cu 2(OH) 3I, the only known instances in which the ions Cu(II) and I − are present in compounds.

A high-pressure x-ray study of Nylon 11

An x-ray diffraction study of Nylon 11 was carried out at high pressures and high temperatures. Careful measurements made on both wet and dry samples at atmospheric pressure indicated that the structure of Nylon 11 suggested by Slichter was not quite correct. Some distortion of the planar conformation causes a shortening of the c repeat distance from the value required for a fully extended conformation. The compressibility of the crystal lattice at higher pressures up to 20 kbar was studied and related to the anisotropic bonding present in the structure. A crystal transition at atmospheric pressure previously unreported was observed to occur at 95 °C, the triclinic structure (α phase) changing to a pseudohexagonal structure. The influence of pressure on this transition was studied. It was found that the increase in transition temperature with pressure dTt/dP (∼15 K/kbar) was greater than the melting temperature increase with pressure dTm/dP (∼9 K/kbar). A pressure-temperature phase diagram for Nylon 11 was obtained and a triple point was observed at 310 °C and 14.5 kbar. At pressures above 14.5 kbar the α phase is stable until melting.