Time-of-flight Rietveld neutron structure refinement and quantum chemistry study of Y-α-sialon

A Y-α-sialon was prepared using a specially designed temperature regime supporting formation of α-sialon and the amount of non-crystalline phase was remarkably reduced by a chemical treatment. Chemical composition of the sample was determined by EDX analysis. Crystal structure of Y-α-sialon was refined using high quality time-of-flight neutron powder diffraction data taken with the General Purpose Powder Diffractometer (GPPD) at the IPNS, Argonne National Laboratory. Unrestrained structure refinement was done in the P31c space group. The list of simultaneously refined parameters included profile parameters, atomic coordinates, individual isotropic temperature parameters and the occupancy parameter of Y. The contents of Y in the cell refined to 0.38(1) and calculated Si–N distances in tetrahedra varied from 1.716(3) to 1.773(3) Å, but their distributions in the respective tetrahedra differ as in the parent structure of α-Si₃N₄. Anisotropic temperature parameters refinement, as well as constrained refinements of the occupancy parameters of Si–Al and N–O pairs, were also attempted, but they all ended with unphysical values. Variation of the background was analyzed using RDF formalism. Three peaks centered around 1.7 Å (with a shoulder at 2.2 Å), 2.8 Å and 3.5 Å proved that the composition of amorphous phase was close to that of crystalline. Ab initio cluster calculations performed at the B3LYP/6-31G** level of theory excluded configuration with O atoms bonded to three Si atoms and proved that the most probable is the configuration where an O atom is shared by three Al atoms.     

Determination of the α, β, and γ crystalline phases of poly(vinylidene fluoride) films prepared at different conditions

Samples containing the three crystalline phases of poly(vinylidene fluoride), α, β, and γ, have been obtained under distinct crystallization conditions. Samples containing exclusively unoriented β phase have been obtained by crystallization from dimethylformamide (DMF) solution at 60°C. Oriented β phase has been obtained by uniaxial drawing, at 80°C, of an originally α phase sample. Samples containing exclusively α phase have been obtained by melting and posterior cooling at room temperature. Samples containing both α and γ phases have been obtained by melt crystallization at 164 °C for 16 and 36 h. Presence of the crystalline phases in each sample were confirmed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), wide‐angle X‐ray scattering (WAXD), polarized light optical microscopy (PLOM), and scanning electron microscopy (SEM). Infrared absorption bands identifying unequivocally the presence of β and γ phases in a sample are presented. It is shown that solution crystallization at T < 70°C always results in the β phase, regardless of the solvent used. Melt temperatures of the respective phases have also been determined. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3272–3279, 2006     

Phase transformation from α′- to β′-sialon by liquid infiltration in Y-Si-Al-O-N system

The formation of β′-sialon from α′-sialon during infiltration of a glass matrix has been investigated. Sintered α′-sialon containing large grains of about 5 μm has been resintered within a glass matrix of which the composition is in equilibrium with β′-sialon. During resintering the glass matrix has penetrated into sintered α′-sialon specimen and induced the transformation of α′- into β′-sialon. No trace of α′ is found within β′ grains formed. The nucleation of β′ grains is thus believed to occur not on the preexisting α′ grains but in the penetrated glass matrix. The concentration of Al in β′ grains increases from their centre towards their edge, implying that the glass matrix has continuously penetrated with resintering time. Continuous penetration of glass matrix and thus a concentration gradient in the penetrated glass is also confirmed by a directional growth of 12H polytype grains formed between transformed β′ and bulk glass matrix.     

Phase stability and melting behavior of the α and γ phases of nylon 6

The phase stability and melting behavior of nylon 6 were studied by high‐temperature wide‐angle X‐ray diffraction and differential scanning calorimetry (DSC). The results show that most of the α phase obtained by a solution‐precipitation process [nylon 6 powder (Sol‐Ny6)] was thermodynamically stable and mainly melted at 221°C; the double melting peaks were related to the melt of α crystals with different degrees of perfection. The γ phase formed by liquid nitrogen quenching (sample LN‐Ny6) melted within the range 193–225°C. The amorphous phase converted into the γ phase below 180°C but into the high‐temperature α phase at 180–200°C. Both were stable over 220°C. α‐ and γ*‐crystalline structures were formed by annealing but were not so stable upon heating. Typical double melting peaks were shown on the DSC curve; melt recrystallization happened within the range 100–200°C. The peak at 210°C was mainly due to the melting of the less perfect crystalline structure of the γ phase and a fraction of the α phase; the one at 219°C was due to the high‐temperature α‐ and γ‐phase crystals. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The preparation and characterisation of crystalline α-zirconium phosphate

Crystalline α-zirconium phosphate has been prepared by four methods. The products have been characterised by chemical and thermal analysis, X-ray and infrared spectroscopy, surface area, porosity and particle size determinations and electron microscopy. A method is recommended for the preparation of crystalline α-zirconium phosphate.     

Effects of α‐form or β‐form nuclei on polymorphic crystalline morphology of poly(butylene adipate)

The effects of α‐form and β‐form nuclei on polymorphic morphology of poly(butylene adipate) (PBA) upon recrystallization from the molten state up to various T_max values were examined by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and polarized light microscopy (PLM). In this study, PBA with complex melting and polymorphism behaviour was used as a model for examining different types and extents of residual nuclei. As the PBA initially containing the sole α‐crystal was brought to a molten state of various T_max, the extents of trace α‐form crystal nuclei varied and were dependent on T_max. Furthermore, it did not matter whether, initially, the PBA contained α‐ or β‐form crystals (or both) because only a single type of α‐nuclei could be left upon treatment to the molten liquid state at T_max. Therefore, only the α‐crystal in PBA had ‘memory capacity’ in the molten liquid state while the β‐crystal did not. This was so because the latter had been completely transformed into the solid state prior to being heated into a liquid. PBA crystallized before α‐nuclei could be packed into α‐crystal, regardless of the crystallization temperature (T_c). For recrystallization from molten PBA without any nuclei, the crystalline polymorphism was correspondingly influenced by T_c. Copyright © 2005 Society of Chemical Industry     

α‐ and β‐crystalline forms of isotactic polypropylene investigated by nanoindentation

Under special crystallization conditions from the melt, both α‐ and β‐forms of isotactic polypropylene were produced simultaneously. The α‐ and β‐spherulites of polypropylene were differentiated under optical microscope, allowing the nanoindentation of individual spherulites of each crystallographic form. Elastic modulus and hardness of β‐spherulites were found to be 10 and 15% respectively lower than in α‐spherulites. The higher stiffness of α may be related to the particular interlocked structure with cross‐hatched lamellae, and to a lower molecular mobility in the crystallites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 195–200, 1999     

Acetylcholine Promotes Binding of α‐Conotoxin MII at α3β2 Nicotinic Acetylcholine Receptors

α‐Conotoxin MII (α‐CTxMII) is a 16‐residue peptide with the sequence GCCSNPVCHLEHSNLC, containing Cys2–Cys8 and Cys3–Cys16 disulfide bonds. This peptide, isolated from the venom of the marine cone snail Conus magus, is a potent and selective antagonist of neuronal nicotinic acetylcholine receptors (nAChRs). To evaluate the impact of channel–ligand interactions on ligand‐binding affinity, homology models of the heteropentameric α₃β₂‐nAChR were constructed. The models were created in MODELLER with the aid of experimentally characterized structures of the Torpedo marmorata‐nAChR (Tm‐nAChR, PDB ID: 2BG9) and the Aplysia californica‐acetylcholine binding protein (Ac‐AChBP, PDB ID: 2BR8) as templates for the α₃‐ and β₂‐subunit isoforms derived from rat neuronal nAChR primary amino acid sequences. Molecular docking calculations were performed with AutoDock to evaluate interactions of the heteropentameric nAChR homology models with the ligands acetylcholine (ACh) and α‐CTxMII. The nAChR homology models described here bind ACh with binding energies commensurate with those of previously reported systems, and identify critical interactions that facilitate both ACh and α‐CTxMII ligand binding. The docking calculations revealed an increased binding affinity of the α₃β₂‐nAChR for α‐CTxMII with ACh bound to the receptor, and this was confirmed through two‐electrode voltage clamp experiments on oocytes from Xenopus laevis. These findings provide insights into the inhibition and mechanism of electrostatically driven antagonist properties of the α‐CTxMIIs on nAChRs.     

Radiation-induced copolymerization of α,β,β-trifluoroacrylonitrile with α-olefin

Homopolymerization and copolymerization of α,β,β-trifluoroacrylonitrile (FAN) with γ-olefins were carried out in bulk by γ-ray irradiation at 25°C. FAN gives very small quantities of brown and greasy low molecular weight polymer. Cyano groups in FAN polymer were found to be readily hydrolyzed to acid amide groups in the atmosphere. FAN was found to copolymerize with ethylene, propylene, and isobutylene via a radical mechanism to form equimolar copolymers in a wide range of monomer compositions. The polymerization rate increases linearly with FAN fraction in the monomer mixture. These copolymers are also hydrolyzed in the atmosphere, and the hydrolysis proceeds with more difficulty for the copolymer with higher α-olefin. The reactivity ratios r₁ (FAN) and r₂ (α-olefin) were determined to be 0.01 and 0.12 for the FAN/ethylene copolymerization and 0.01 and 0.07 for the FAN/propylene copolymerization. These results confirm that an alternating copolymerization takes place in the FAN/α-olefin system.     

Rheological properties of the liquid crystalline poly(γ-benzyl α, L-glutamate) gels in benzyl alcohol

We have determined the degree of stiffness of a poly(γ-benzyl α, L-glutamate) (PBLG) chain in benzyl alcohol by measuring the intrinsic viscosities of dilute Solutions with differing molecular weights. Viscoelastic properties in oscillatory shear flow have been studied and the dependence of the loss and storage moduli on temperature, composition, and frequency are reported. We have also studied the transient shear stress relaxation behavior of the PBLG gel at different temperatures and concentrations. A comparison has been made between these gels and a classical poly(dimethyl siloxane) (PDMS) network, as well as, typical glassy polymers. Shear creep and recovery measurements have been made for this system. Some extensional step strain experiments using lubricated squeezing have been investigated. Tensile experiments have been made to determine stress-strain relationship during elongation. Preliminary experiments using the impulse approach to viscoelasticity further indicate the high elastic contribution in the gel.     

Native celluloses on the basis of two crystalline phase (Iα/Iβ) system

A survey by X-ray diffractometry was carried out to confirm the two crystalline phase Iα/Iβ(triclinic/monoclinic) system of native celluloses. We have investigated cellulose samples from 12 different origins and measured the d-spacings with reasonable precision. The samples were then subjected to the hydrothermal annealing that brings the cellulose crystals to the monoclinic type; d-spacings were reevaluated afterward. From the statistical analysis using observed d-spacings, all the cellulose samples were categorized into the two groups that coincide with the two crystalline phase systems: one is the algal-bacterial type rich in triclinic phase and the other is the cotton-ramie type of the monoclinic phase. © 1993 John Wiley & Sons, Inc.     

Photoelectrochemical phenomena at the interface Cu/Cu(II), glycine, α- or β-alanine

The analysis of photopotentials and photocurrents induced by laser beam perturbation of the interface Cu/solution of Cu(II)—glucine, α- or β-alanine complexes, has shown that phase layers containing Cu₂O display the properties of p-type semiconductors, except for the β-alanine system which displays n-type conductivity. In accordance with results of the analysis of thermodynamic and kinetic stability of Cu₂O layers the highest level of photoresponse is observed in the β-alanine system. Inversion of the photocurrent sign has been registered in the latter system at a certain cathodic overvoltage, the value of which depends on both the pH of solution and the laser emission intensity.     

Polymerization of α, α-disubstituted-β-propiolactones and lactams. 9. ABA block copolymers of α-methyl-α-butyl-β-propiolactone and pivalolactone

ABA block copolymers were prepared by the anionic polymerization of α-methyl-α-butyl-β-propiolactone, MBPL (B block), and pivalolactone, PL (A blocks). The MBPL block had a very low decree of crystallinity and a glass temperature of ? 13°C, so phase separation with extensive crystallization of the PL blocks gave thermoplastic elastomers when the MBPL block constituted the principal and continuous phase. The observed crystallinity and melting point of 40–45°C in the MBPL homopolymer have not been previously reported. Measurements were obtained by electron microscopy of the initial size distribution of the PL domains as a function of copolymer composition and degree of polymerization, and on the effect of annealing on this parameter. Tensile strengths and elongations at break were both less than those previously observed for equivalent ABA block copolymers of PL and α-methyl-α-propyl-β-propiolactone.     

Poly(α,α,α′,α′-tetrafluoro-p-xylylene)

Poly(α,α,α′,α′-tetrafluoro-p-xylylene) was prepared by the pyrolysis of cyclo-di-(α,α,α′,α′-tetrafluoro-p-xylylene) and by the pyrolysis of α,α′-bis(alkylsulfonyl)-α,α,α′,α′-tetrafluoro-p-xylene. The pyrolysis of α,α′-dibromo-α,α,α′,α′-tetrafluoro-p-xylylene also gave the polymer, but the method is less satisfactory. Poly(α,α,α′,α′-tetrafluoro-p-xylylene) shows remarkable thermal and oxidative stability at elevated temperatures. Useful mechanical and electrical properties are retained after aging for 3000 hr at 250° in air. After initial change due to crystallization, tensile strength remains near 10,000 psi, elongation above 5%, and dielectric constants and dissipation factors at approximately 2.4 and .001, respectively.     

Reactions of α-Arylhydrazono-β-oxo-β-phenyl-propionitriles

ω-Cyanoacetophenone was coupled with a variety of aromatic diazonium chlorides to give the corresponding arylazo derivatives 1 a – g . Analysis of the IR and UV spectra of these compounds indicated that they exist mainly as the corresponding arylhydrazones. Compounds 1 a – d, f, g reacted with diazomethane to yield the N-methyl-arylhydrazones 2 a – f . 3-Amino-4-arylazo-5-phenylpyrazoles 5 a – g and 4-arylazo-5-imino-1,3-diphenyl-2-pyrazolines 6 a – g were obtained by the interaction of 1 a – g with hydrazine hydrate and with phenylhydrazine, respectively.     

Fracture behaviour of microcrack-free alumina–aluminium titanate ceramics with second phase nanoparticles at alumina grain boundaries

Alumina + 10 vol.% aluminium titanate composites were obtained by colloidal filtration and reaction sintering of alumina and titania. The materials were dense with aluminium titanate grains of average sizes 2.2–2.4 μm located mainly at alumina triple points. The reaction sintering schedule promoted the formation of additional nanometric grains, identified as aluminium titanate using STEM–EDX analysis between the alumina grains. This special microstructure led to a change of the toughening mechanism from the typical crack bridging reported for microcrack-free composites fabricated from alumina and aluminium titanate powders to microcracking.

The identification of microcracking as the main toughening mechanism was done from the analysis of stable fracture tests of SENVB samples in three points bending and fractographic observations. Monophase alumina materials with similar grain sizes were used as reference.

Different fracture toughness parameters were derived from the load–displacement curves: the critical stress intensity factor, K_IC, the critical energy release rate, G_IC, the J-Integral and the work of fracture, γ_WOF, and the R curves were also built. The comparison between the linear elastic fracture parameters and the non-linear ones revealed significant toughening and flaw tolerance.     

Preparation and Nucleophilic Reactions of β-Fluoro-γ-Alkyl Halides

Problems related to the preparation of β-fluoro-γ-alkyl halides and the nucleophilic reactions of these compounds are described. These compounds are found to be less reactive than the corresponding halides without the fluorine atom. Nucleophilic reactions of these halides are S_N2 reactions, and no allylic rearrangements are observed in any case.     

Micro-structural study of Yaozhou celadons (Tang to Yuan Dynasty): probing crystalline and glassy phases

The Yaozhou kiln complex is a representative production center of ancient northern China, famous for the celadon production. In this work, bubbles, glassy matrix and residual crystals of celadon glazes produced from the Tang to Yuan Dynasty were analyzed by using optical microscopy, Raman spectroscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). The results revealed that the Song, Jin and Yuan productions present bigger bubble and higher area ratios of the Si-O bending over stretching modes than the Tang and Wudai productions. This is consistent with firings at higher temperatures during Song, Jin and Yuan Dynasties. It is also in agreement with the historical studies, which revealed the change from wood-firing to coal-firing during Song Dynasty. The observation of calcium phosphate in Yaozhou productions indicated that the glaze ash had been used. No iron-based particle was identified by Raman spectroscopy in the glazes of all periods. The green color is certainly due to iron ion dispersed in the glassy matrix. Our study also confirmed no significant change in glaze raw materials used for Yaozhou productions from Tang to Yuan Dynasty.