Pseudohexagonal Phase Research Articles

Large-scale fabrication of H 2(H 2O)Nb 2O 6 and Nb 2O 5 hollow microspheres

Hollow micro-sized H 2(H 2O)Nb 2O 6 spheres constructed by nanocrystallites have been successfully synthesized via a bubble-template assisted hydrothermal process. In the reaction process, H 2O 2 acts as a bubble generator and plays a key role in the formation of the hollow structure. An in situ bubble-template mechanism has been proposed for the possible formation of the hollow structure. The spherelike assemblies of these H 2(H 2O)Nb 2O 6 nanoparticles have been transformed into their corresponding pseudohexagonal phase Nb 2O 5 through a moderate annealing dehydration process without destroying the hierarchical structure. Optical properties of the as-prepared hollow spheres were investigated. It is exciting that the absorption edge of the hollow Nb 2O 5 microspheres shifts about 18 nm to the violet compared with bulk powders in the UV/vis spectra, indicating its superior optical properties.

Microstructure and viscoelasticity in thermotropic copolyesters: the influence of monomer concentration

This research investigates the influence of monomer composition on the thermal transitions, mi crostructure, and viscoelastic properties of thermotro pic liquid crystalline polymers (LCPs) based on 1, 4-hydroxy-benzoic acid (B) and 2,6-hydroxy-naphthoic acid (N). Compositions B–N of 1:1, 1:3, 3:1, and 2:1 mol% were studied. The solid-to-nematic transition T s→n was greatly influenced by the monomer composition, ranging from 230 to 280 °C. However, the decomposition temperature T dec was independent of composition, all the materials degrading at T dec = 509 °C. Polarized optical microscopy showed a threaded texture in the nematic phase. Fiber X-ray diffraction showed aperiodic meridional maxima for all LCPs, the number of maxima depended on composition. B–N 3:1 and 1:3 mol% favor the orthorhombic crystallographic phase whereas the 1:1 and 2:1 mol% crystallized in a pseudohexagonal phase. Dynamic time sweeps showed that the nematic phases are stable at the temperatures studied. Strain sweeps showed the existence of a linear viscoelastic (LVE) regime. The LVE properties ranged from predominantly viscous (G′ G″) and with a well-defined rubber-like regime for the 1:3 mol% composition, resembling the rheology of flexible polymer melts. The persistence length q appears to be the parameter driving the rheological behavior.

High Pressure Crystallization of HDPE Droplets

Dispersions of high density polyethylene (HDPE) particles in polystyrene (PS) were produced by interfacially driven breakup of nanolayers in multilayered systems that were fabricated by means of layer-multiplying coextrusion. The droplet size distribution was controlled by the HDPE initial individual layer thickness: 14, 40 or 120 nm. These unique systems allowed us to demonstrate unequivocally for the first time that the HDPE pseudohexagonal phase formation required nucleation on foreign surfaces or on crystals formed in the orthorhombic form at atmospheric pressure. The dispersions of HDPE particles in the PS matrix were subjected to annealing at high pressure of 480 MPa at 235 °C, in conditions where the pseudohexagonal phase forms in HDPE. To reach the desired conditions the samples were first heated and then subjected to high pressure or they were first pressurized and then heated. The samples treated according to the first route melted and transformed into the pseudohexagonal phase from the molten state whereas the second route allowed for transformation of orthorhombic crystals to the pseudohexagonal phase without previous melting. The lamella thickness formed under high pressure was estimated on the basis of melting temperature. The HDPE particles, annealed under high pressure without previous melting, exhibited high melting temperature, above 140 °C, corresponding to the lamella thickness above 50 nm, that indicated thickening in the pseudohexagonal phase. The HDPE particles that were first melted and then pressurized behaved differently depending on their sizes: thick lamellae were formed under high pressure only in particles large enough to contain heterogeneities able to nucleate HDPE bulk crystallization. These results point out that nucleation, presumably heterogeneous, is advantageous, or even necessary, for the formation of the HDPE pseudohexagonal phase.

Preparation and spectroscopic properties of Nb 2O 5 nanorods

Nb 2O 5 nanorods have been prepared using water/ethanol media. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–visible absorption and photoluminescence spectroscopy. The as-prepared Nb 2O 5 nanorods appeared to be single pseudohexagonal (TT-Nb 2O 5) phase. From the photoluminescence spectrum, two emission bands at 407 and 496 nm, respectively, were observed. The origin of the luminescence was discussed in detail.

Investigation of phase transitions of a pseudo-hexagonal phase of [Rh(NH3)5Cl]2[Re6S8(CN)6]·3H2O during thermal decomposition

A general motif of the crystal structure of [Rh(NH3)5Cl]2[Re6S8(CN)6]·3H2O is examined, and the cluster anions are found to form a pseudo-hexagonal sublattice. The thermal decomposition of [Rh(NH3)5Cl]2[Re6S8(CN)6]·3H2O is studied, and it is shown that in helium atmosphere thermolysis occurs through the formation of intermediate amorphous phases. The final product obtained at 1200°C is a disordered single-phase solid solution of Re0.75Rh0.25 based on the structure of rhenium. Powder X-ray diffraction data for solid solutions in the system of Rh-Re are surveyed. It is demonstrated that the data for phases prepared by the thermal decomposition of coordination compounds better match the theoretical state diagram than the experimental one. The dependence of atomic volume on the composition of solid solutions of RexRh1−x is derived.

Elaboration of Ta2O5 Thin Films Using Electrostatic Spray Deposition for Microelectronic Applications

The deposition of tantalum oxide thin films on SiOxNy/Si substrates using electrostatic spray deposition was investigated. Different microstructures (dense, reticulated, porous) depending on the process parameters such as the nature of the precursor solution, substrate temperature, nozzle-to-substrate distance, and precursor solution flow rate were obtained. The evolution of film thickness and morphology versus deposition time was discussed. The films deposited at temperatures ranging from 100 to 210 °C were found amorphous. A pure pseudo hexagonal δ-Ta2O5 single phase was identified by X-ray diffraction and Raman spectrocopy after annealing from 650 °C and above.

Investigation on the Brill transition of polyamide 618

The Brill transition of even-even polyamide 618 was investigated using differential scanning calorimetry (DSC), temperature-dependent wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). The X-ray diffraction results indicate that the melt-crystallized sample of polyamide 618 transforms from the triclinic unit cell to the pseudo-hexagonal phase in the range of 120–180°C. In this range, the thermograph of polyamide 618 presents a broad endothermal peak. From the FTIR spectra, it was found that during the transition process of polyamide 618, the intensity of the intra-sheet hydrogen bonds becomes weak. At the same time, the CH2-amide bonds twist, and the all-trans conformation of methylene sequences is disordered by inserting the gauche conformation. The CH2 segments are in a mobile state because of the enhanced stretching and twisting vibrations of the C-CO and C-N bonds.

Polymorphism in Nylon‐11: Characterization using HTWAXS and HTFTIR

AbstractNylon‐11 was crystallized into different crystalline phases by appropriate methods. The α phase was obtained by precipitating nylon‐11 in 1,4‐butanediol. The melt crystallized sample at 175 °C gave pseudohexagonal phase. The γ phase was obtained by casting nylon‐11 in trifluroacetic acid (TFA). The change in the structure was monitored in situ by high temperature wide‐angle X‐ray scattering (HTWAXS) and high temperature Fourier transform infrared spectroscopy (HTFTIR). The studies have shown that nylon‐11 is one of the few nylons that exhibit extensive degree of polymorphism. HTFTIR studies revealed, apart from the conformational differences between various phases of nylon‐11, some information on the formation of γ phase.

Crystalline Phases in Nylon-11: Studies Using HTWAXS and HTFTIR

Nylon-11 was crystallized into different crystalline phases by appropriate methods. The change in the structure during heating was monitored in situ by wide-angle X-ray scattering (WAXS). The α phase obtained by precipitating nylon-11 in 1,4-butanediol started transforming into the pseudohexagonal phase on heating, but the transformation was not fully completed before melting. The melt crystallized sample at 175 °C gave the pseudohexagonal phase, which transformed into the α‘ phase on cooling to room temperature at about 100 °C. The d spacing of the 001 reflection also showed a change at the transition temperature. The melt-quenched sample showed the pseudohexagonal phase, which did not change during heating, but on cooling transformed into α‘ phase. The γ phase was obtained by casting nylon-11 in trifluoroacetic acid (TFA), and it changed into the pseudohexagonal phase at about 110 °C on heating. The d spacing of the 001 reflection depended on the way in which it was obtained. The studies have shown that nylon-11 is one of the few nylons that exhibit an extensive degree of polymorphism. High-temperature Fourier transform infrared spectroscopy (HTFTIR) studies revealed, apart from the conformational differences between various phases of nylon-11, some information on the formation of the γ phase.

The phase diagram of poly(4-hydroxybenzoic acid) and poly(2,6-hydroxynaphthoic acid) and their copolymers from X-ray diffraction and thermal analysis

Homopolymers and copolymers of 4-hydroxybenzoic acid (HBA) and 2,6-hydroxynaphthoic acid (HNA) have been studied with differential scanning calorimetry and temperature-resolved wide angle X-ray diffraction. All polymers have more than one disordering transition between the glass transition (between 400 and 430K) and decomposition (between 710 and 750K). The first transition in PHBA at 616–633K is from orthorhombic rigid crystals to a conformationally disordered pseudo-hexagonal phase (condis phase). The two higher transitions are first, a further disordering process to a hexagonal condis crystal, and then a change to an anisotropic melt (liquid crystal) at about 800K, with increasing decomposition above 750K. In PHNA, orthorhombic crystals change above 600K to an orthorhombic condis crystal structure, which go to an anisotropic melt at 750K, and subsequent decomposition. In addition, using empirical entropy rules that account for the changes during the transitions from the crystal to the disordered mobile phases, an effort is made to understand the disorder and mobility, and to arrive at a non-equilibrium phase diagram of the copolymer system. The existence of a single, but up to 200K wide, glass transition and remaining high crystallinity of the copolyesters, indicate partial solubility of the repeating units in all phases. The new data are compared to and brought into agreement with the large number of prior measurements and often unclear interpretations.

繰り返し温度変化による高強力ナイロン６繊維の構造変化

Nylon 6 fibers are used as tire cord in carcass of automobile tire for rubber reinforcement to keep internal pressure. During run state, the internal part of tire is heated up to 100 °C and cooled by stopping, resulting in the same thermal history of Nylon 6 tire cord. It has been reported that crystal phase of Nylon is transformed to other phase under high temperature condition. In room temperature, two reflection peaks assigned to α-phase were observed by wide angle X-ray diffraction measurement for Nylon 6 fiber. On heating, these two peaks became one peak of pseudohexagonal phase at around 70 °C. This transition referred as the Brill transition took place repeatedly by heating and cooling. Also, tensile strength decreased with increasing temperature by heating and increased by cooling. For higher-order structure measured by small angle X-ray scattering, the long period of Nylon 6 lamella was constant on cyclic temperature changes. On the other hand, it was observed that the slope against fiber direction and the size of lamella were irreversibly changed at around 70 °C. It has been reported in former study that the Brill transition was observed at higher temperature region than 150 °C because samples were Nylon 6 pellets and films. Accordingly, the Brill transition temperature changed much lower in high oriented Nylon 6 fiber.

Simulations of the mobile phase of polyethylene

Results are presented from atomistic molecular dynamics simulations of the mobile pseudo-hexagonal phase of polyethylene, which occurs under conditions of elevated pressure and temperature. Three different types of model are considered, all of which employ periodic boundary conditions. The first model consists of n-alkane sequences (48×–C 24H 48–) that are bonded across the simulation box boundaries to produce chains that are effectively infinite in extent. On heating, at high pressure, this system displays a rotator phase, in which the chains retain an all- trans conformation, and rotate as semi-rigid units. A second model, consisting of finite n-alkanes (48×C 24H 50) displays the same behaviour at low temperatures, but at high temperature and pressure forms a conformationally disordered rotator phase, characterised by a large proportion of gauche defects and a significant lattice expansion. The final model considered contains long n-alkanes (24×C 102H 206) which contain jog defects and each pass twice through the simulation box. This model forms a conformationally disordered rotator phase at high temperature and ambient pressure. The behaviour of the three models, in terms of the variations in chain conformation and rotational and translational dynamics, are compared. The conformationally disordered phases provide useful representations of the experimentally observed mobile phase.

Stereochemistry Driven Distribution of 1,4-Diaminocyclohexane Residues over the Crystalline and Amorphous Phase in Copolyamides 4.14/1,4-DACH.14. A Solid-State NMR and Temperature-Dependent WAXD Study

Following our previous study on the stereoselective cocrystallization of linear and cyclic dicarboxylic acid residues in copolyamides 12.6/12.1,4-cyclohexanedicarboxylic acid (12.6/12.1,4-CHDA, Vanhaecht et al., Macromolecules 2004, 37, 421), we have now investigated a series of copolyamides containing trans or cis isomers of 1,4-diaminocyclohexane (1,4-DACH), viz. 4.14/1,4-DACH.14. Solid-state NMR studies and WAXD experiments demonstrate that the cis isomer is present in the amorphous regions of the copolyamide, whereas the trans isomer is located in both the crystalline and the amorphous phase. Similar to the results for the trans-1,4-CHDA-based copolyamides, trans-1,4-DACH moieties are probably built into the crystalline structure with the cycloaliphatic ring oriented perpendicular to the hydrogen-bonded crystal sheets. Temperature-dependent WAXD patterns show that, unlike in the homopolyamide 4.14, a pseudohexagonal phase is not formed at the Brill temperature in the copolyamide. Instead, crossing rat...

Temperature dependent wide angle X-ray diffraction studies on the crystalline transition in water saturated and dry polyamide 6/66 copolymer

The crystalline transition in water saturated and dry polyamide 6/66 copolymer (ratio 4:1 by mol) was investigated by means of temperature dependent wide angle X-ray diffraction (WAXD). The polyamide 6/66 copolymer (PA6/66) exhibits a poorly developed α-phase at room temperature after being cooled down from the melt. Uptake of water improves the crystalline structure by mobilizing the amorphous phase, thus facilitating chain relaxation in the crystalline phase. Upon heating, the effect of the glass transition on the change of the crystal lattice constants is seen. Further heating leads to a gradual crystalline transition from the α-phase to a pseudohexagonal phase. Different from the behavior in polyamide 6, this pseudohexagonal phase does not further transform to a high temperature α′-phase before melting. The delay of the crystalline transition in the water saturated PA6/66––as compared to the dry material can be understood as a result of the better ordered crystalline structure in it.

Melting and crystallization behavior of poly(tetrafluoroethylene) by temperature modulated calorimetry

Temperature-modulated differential scanning calorimetry (TMDSC) in the quasi-isothermal mode is applied to investigate melting and crystallization of poly(tetrafluoroethylene) (PTFE) obtained from aqueous dispersion, both melt-crystallized and native (as-polymerized). The differences, shown in the past between the melting behavior of melt-crystallized and native PTFE, have been confirmed and further evidenced through this technique. A large reversing heat capacity is present in the melting and crystallization regions. As proposed by many authors, the presence of the reversing heat capacity can be related to surface melting and crystallization. It can occur on the growth and/or fold surfaces and it has been shown that it is larger for those macromolecules having higher chain mobility that allows rearrangements on the crystal surface. In the present case, the large observed reversing heat capacity can be related to the very high sliding ability of PTFE chains in the pseudohexagonal phase, which is much larger than that of most semicrystalline polymers. Due to the crystal–crystal transition at 30 °C, which can be described as a fusion in the longitudinal direction, melting of PTFE can be considered intermediate between the irreversible melting of macromolecules and the completely reversible isotropization of liquid crystalline polymers.

Investigation of Structural Processes During Indentation of Polymers by Synchrotron Radiation Microdiffraction

A Vickers microindentation setup has been integrated into the scanning setup used at the European Synchrotron Radiation Facility (ESRF) microfocus beamline (ID 13). Ex situ wide‐angle x‐ray scattering experiments performed on an isotactic polypropylene (iPP) single fiber show changes in chain orientation and breakup of the monoclinic α‐modification into domains due to plastic deformation in the indented zone. The systematic mapping of the perturbed zone allows spatially dependant structural changes to be determined. Similar experiments for a Vectra single fiber show a partial transformation of the pseudohexagonal phase into the orthorhombic phase. In situ experiments allow following structural changes as a function of time and applied force. During indentation of an iPP fiber, domain doubling during indentation and single domain recovery after the tip has been retracted is observed.

A WAXD and Solid-State NMR Study on Cocrystallization in Partially Cycloaliphatic Polyamide 12.6-Based Copolymers

To study the influence of the stereochemistry on the possibility of cocrystallization of linear and cyclic aliphatic residues in copolyamides, a series of copolyamides 12.6/12.1,4-cyclohexanedicarboxylic acid with variable compositions were synthesized. From solid-state NMR studies it could be deduced that cis-1,4-CHDA is present in the amorphous regions whereas the trans residues are located in both the crystalline and the amorphous phase. WAXD patterns confirm the presence of trans-1,4-CHDA inside the crystals and reveal that the cycloaliphatic ring is most likely oriented in a direction perpendicular to the crystal sheets that contain the hydrogen bonds. As a result, the intersheet distance is increased compared to that of polyamide 12.6. Furthermore, the rings prevent the genesis of a pseudohexagonal phase above the Brill temperature. Exceptionally, crossing rather than merging of the (100) and the combined (010)−(110) WAXD reflections is observed with increasing temperature, indicating that the inter...

High-temperature X-ray diffraction studies on polyamide6/clay nanocomposites upon annealing

The influence of nanodispersed clay on the alpha crystalline structure of polyamide 6 (PA6) was examined in-situ with X-ray diffraction (XRD) between room temperature and melting. In pure PA6 upon annealing the alpha crystalline phase was substituted by an unstable pseudohexagonal phase at 150degreesC, then it transformed into a new stable crystalline structure - high temperature alpha' phase above the transition temperature. However, in PA6/clay nanocomposite (PA6CN), the alpha phase did not present crystalline phase transition on heating. The increase in the annealing temperature only led to continuous intensity variation. The different behaviors were caused by the confined spaces formed by silicate layers, which constrained the mobility of the polymer chains in-between.

Polyethylene Unit Cell and Crystallinity Variations as a Consequence of Different Cross-Linking Processes

Changes of crystallinity and unit cell parameters for a low-density polyethylene as a consequence of cross-linking by dicumyl peroxide and by electron beam irradiations are compared. Polyethylene samples cross-linked by irradiation in the solid state present variations essentially only of the a unit cell parameter while those cross-linked in the melt state present similar variations of both a and b unit cell parameters. A strict analogy with unit cell variations observed for ethene random copolymers with propene and with bulkier comonomers, respectively, is shown. This suggests inclusion of cross-links for solid-state cross-linked samples, which would lead to a gradual transformation from an orthorhombic to a pseudohexagonal phase, and exclusion of cross-links for melt state cross-linked samples, which would generate tensions at the orthorhombic crystallite interphases.

Influence of Crystallization Conditions on the Crystal Order and Dynamic Thermal Behavior of Nylon 1010

Nylon 1010 crystals with different size and perfection were prepared under various crystallization conditions. Temperature-varied X-ray diffraction and transmission electron microscopy were used to investigate the structure and morphology of these crystals. The pseudo-hexagonal phase (also known as γ modification) of nylon 1010 was obtained by heating the quenched sample to a high temperature, where this phase is stable. Cooling of the γ modification sample immediately resulted in a Brill transition into the triclinic form. The WAXD and TEM results showed that the crystal structure at room temperature as well as its thermal behavior on heating is strongly influenced by the crystallization conditions. No Brill transition temperature was observed for large or perfect crystals in both the heating and the cooling process, i. e. for melt-crystallized spherulitic crystals and lamellar single crystals grown from dilute solution. This shows that the Brill transition depends on the size and perfection of the crystal.