Stable Orthorhombic Form Research Articles

In situ analysis of melt-drawing behavior of ultrahigh molecular weight polyethylene films with different molecular weights: roles of entanglements on oriented crystallization.

Ultrahigh molecular weight polyethylene (UHMW-PE) films having different molecular weights (MWs) were melt-drawn at 150 °C. The stress-strain curve for higher-MW film exhibits higher stress on the characteristic plateau region and a subsequent steeper increase of stress due to strain hardening. Structural changes during such melt-drawing were analyzed using in situ wide-angle X-ray diffraction measurements. Hexagonal crystallization occurs at the beginning of the plateau region, independent of the sample MW. Once this hexagonal reflection intensity is saturated, it remains constant even at the later stage of draw. In contrast, orthorhombic reflection intensities gradually increase with increasing draw strain. Both of these oriented crystallizations into plateau hexagonal and increasing orthorhombic forms are accelerated with increasing MW. Correspondingly, the higher amount of extended chain crystals (ECCs) was confirmed from morphological observation for the resultant melt-drawn films of the higher-MW sample. Deep entanglements can effectively transmit the applied stress; thus, the oriented amorphous melts induce rapid hexagonal crystallization with disentangling shallow entanglements, which subsequently transforms into orthorhombic form. Such hexagonal crystallization plays the role of a thermodynamic pathway for growing such ECCs, where the stable orthorhombic form gradually accumulates with increasing draw strain.

Two Crystal Forms of 4-{(2E)-2-[1-(4-Methoxyphenyl)ethylidene] hydrazinyl}-8-(trifluoromethyl)quinoline

The Schiff base, 4-{(2E)-2-[1-(4-methoxyphenyl)ethylidene] hydrazinyl}-8-(trifluoromethyl)quinoline, crystallizes in two polymorphic forms depending on the solvent. One of these forms is monoclinic (1M), space group P21/c with a = 10.2906(10) A, b = 8.9211(7) A, c = 18.4838(15), β = 97.271(8)°, and the other is orthorhombic (1O), space group Pbca, unit-cell parameters: a = 13.6485(12) A, b = 9.0588(9) A, c = 27.400(2) A. The molecules in either crystalline form have similar bond lengths and angles, but one is nearly planar while the other has a significant twist. In monoclinic form the dihedral angle between terminal ring planes is 17.26(8)° while in the orthorhombic one it is 26.11(5)°, and in this latter case the central chain is almost coplanar with the quinoline ring system while in the former these two planes are significantly twisted. The crystal structures of both forms are determined by the interplay of van der Waals forces and weak directional interactions C–H···F, π···π stacking, and—in the case of 1M—short intermolecular C–F···N contact. The crystals of 1M decomposes slowly into the powder while the other form is stable. The powder diffraction pattern of the product of decomposition of 1M is similar to that calculated for 1O. This suggests that the decomposition is a consequence of the phase transition of the less stable monoclinic into more stable orthorhombic form. The less stable monoclinic polymorph of the title compound decomposes into the powder which has the crystal structure of the more-stable orthorhombic polymorph.

Terahertz Spectroscopy of Histidine Enantiomers and Polymorphs

We have measured terahertz and powder x-ray diffraction spectra of D-histidine, L-histidine, and DL-histidine. The as-received D and L material exists in two different polymorphs: D-histidine is in the metastable monoclinic form, while L-histidine is in the stable orthorhombic form. For both the L and D enantiomers, recrystallization of the as-received material results in a mixture of the monoclinic and orthorhombic forms.

A Novel Hybrid Chromatography−Crystallization Process for the Isolation and Purification of a Natural Pharmaceutical Ingredient from a Medicinal Herb

In this work, a hybrid chromatography−crystallization separation process is suggested and is used to isolate and purify the antimalarial drug artemisinin from the Chinese medicinal herb Artemisia annua. Extraction of the aerial parts of A. annua by an organic solvent yielded a complex mixture of natural products. A hybrid separation process was therefore proposed to pursue higher recovery and purity of artemisinin. The crude extract was first separated by flash column chromatography, and the fractions containing artemisinin were then separated with a two-step antisolvent crystallization process to isolate artemisinin of high purity. The antisolvent crystallization process was designed on the basis of the solubility of artemisinin in different solvents and the crystallization behaviour of the two polymorphs of artemisinin. The first step in the crystallization process was used to remove impurities from the solution, and the second step was used to isolate the stable orthorhombic form of artemisinin. The results of the present work demonstrate the great potential of combining the advantages of column chromatography and crystallization to isolate and purify a target compound from plant extracts. The separation efficiency of the process can be improved by the synergistic effect of chromatography and crystallization processes.

Chromatography‐Crystallization Hybrid Process for Artemisinin Purification from Artemisia annua

AbstractThe feasibility of using a chromatography‐crystallization hybrid separation process for isolation and purification of artemisinin from the herbal plant Artemisia annua was studied. After separating the crude extracts of Artemisia annua by flash column chromatography, a two‐step antisolvent crystallization process was used for further isolation and purification. The two‐step antisolvent crystallization process was designed based on the solubility of artemisinin in different solvents and solvent mixtures. The first step crystallization process was to remove impurities from the solution and the second step to isolate artemisinin. The crystallization behavior of the two polymorphs of artemisinin during the antisolvent crystallization process was also studied by performing crystallization in solutions of artemisinin in acetonitrile and acetone, respectively. It was observed that the formation of the two polymorphs of artemisinin in acetone solution follows the Ostwald's rule of stages. Fast feeding of the antisolvent (water) resulted in crystallization of the metastable triclinic form which then transformed to the stable orthorhombic form through a solvent‐mediated transformation mechanism. The stable orthorhombic form crystallized out when water was fed slowly. Crystallization from acetonitrile always yielded the stable orthorhombic form regardless of the feeding rate of the antisolvent. Finally, a chromatography‐crystallization hybrid separation process was proposed and tested. The results of the present work demonstrated the feasibility of combining the advantages of column chromatography and crystallization to improve efficiency of the isolation of artemisinin from Artemisia annua.

New developments in X-ray storage phosphors

We found a significant PSL effect in Eu2+-doped fluorozirconate glasses (ZBLAN) which were additionally doped with Br− or Cl− ions. The PSL is attributed to the characteristic emission of Eu2+ present in nano-crystallites of BaBr2 or BaCl2, which form in the glass upon annealing. The metastable hexagonal form of BaX2(X=Br,Cl) is always formed first before it is converted into the stable orthorhombic form. The particle size increases upon annealing and so does the PSL efficiency of the glass ceramic. However, there is a saturation of the PSL efficiency, which is for Br− doping about 9% and for Cl−-doping about 80% of the Eu-doped BaFBr standard. The particle size was determined by transmission electron microscopy (TEM). The TEM results show a clear tendency for bigger particles for longer annealing at the expense of its number. The particle size for the most efficient phosphor is about 100nm.

The correlation between crystalline behavior of polyethylene segments and hydrogen bonds among carboxyl groups in ethylene–Acrylic Acid copolymers

The fine structure of the methylene rocking bands obtained by high resolution and cryogenic FTIR spectroscopy was utilized to characterize the crystalline behavior of polyethylene (PE) segments in an ethylene–acrylic acid copolymers. The hydrogen bonds among the carboxyl groups play a cross-linking role and prevent the PE segments from adopting the stable orthorhombic form. As a result, a lower content of the orthorhombic form and higher contents of the monoclinic form and amorphous region of PE segments are found in the samples. Quenching treatment of the samples strengthens this effect. On the other hand, the crystallization of the PE segments causes strain on the hydrogen bonds among carboxyl groups. Consequently, the distorted carboxyl dimers are easy to break at elevated temperature.

Two-Dimensional Short-Range Magnetic Order in the Tetragonal Spinel Li2Mn2O4

Metastable, tetragonal Li2Mn2O4, prepared by chemical insertion of Li into the cubic spinel, LiMn2O4, was studied by dc magnetic susceptibility and neutron diffraction. The susceptibility data show a very broad maximum at about 120 K, a sharp maximum at about 50 K, and no evidence for Curie−Weiss behavior even up to 600 K. This is consistent with the dominance of short-range, antiferromagnetic correlations attributed to the geometrical frustration inherent in the Mn3+ sublattice which is a slightly distorted pyrochlore lattice, a three-dimensional array of corner-sharing tetrahedra. The temperature dependence of two diffuse peaks in the neutron diffraction data correlate well with the 50 K anomaly but show, remarkably, an asymmetric Warren line-shape characteristic of two-dimensional correlations with no sign of long-range order down to 1.6 K. The magnetic reflections can be indexed as (2 0) and (1 3) of a √3 × √3 magnetic structure arising from a Kagomé layer. The structure and properties of tetragonal Li2Mn2O4 are compared with the thermodynamically stable orthorhombic form.

Abrupt high/low-transition in flux-grown Mg-cordierite single crystals with hour-glass structure

Mg-cordierite single crystals with a composition Mg 1.98 Al 4.08 Si 4.95 O 18 were grown from a PbO/V 2 O 5 -flux according to the method of Lee & Pentecost (1976). They exhibit an optically visible hour-glass structure with a distinctly higher face-specific Pb concentration up to 0.83 weight % (=0.02 Pb per formula unit) on the (0001) basal faces. Determination of the distortion index by Weissenberg photographs shows the crystals to be hexagonal (Δ=0.0) without any deviation from the space group P6/mcc. Nevertheless, the optic axial angle 2V X varies between 0 o and 20 o in different parts of one single crystal, and features reminiscent of twinning perpendicular to (0001), combined with undulatory extinction can be observed. Heating of the crystals at 1290 o C for 10 hours leads to the disappearence of the hour-glass structure and of the undulatory extinction in the now orthorhombic crystals, while 2V X rises to a homogeneous value of nearly 90 o . During heating at 1290 o C, the flux-grown single crystals remained hexagonal up to durations of 3.5 hours; after 4 hours a rather abrupt change to the stable orthorhombic form (Δ∼0.20) was noted. Thus, at this temperature, the total heating time necessary to transform the metastable hexagonal single crystal into twinned orthorhombic ones is much less, by a factor of 5, than for intergrown polycrystalline aggregates of Mg-cordierite. In fact, the high/low transition itself occurs within less than 30 minutes, whereas polycrystalline aggregates tend to produce intermediate structural states over considerable periods of time

Volumetric study of the α→α′ phase transformation of cerium and its implications regarding the crystal structure of α′-cerium.

The volume change of cerium metal at the α→α′ phase transformation at 51 kbar is found to be (1.5±1) % from macroscopic measurements made in the sliding-anvil apparatus SAD 2. This value supports the proposal by Zachiarasen and Ellinger that the high-pressure forms of cerium above 51 kbar are a stable orthorhombic form with the α-uranium type of structure and a metastable monoclinic body-centered form with a deformed face-centered structure and invalidate all the other crystal structures reported so far for α′-cerium.

The Electronic Structure of Furan-quinones. Polymorphism in Dinaphtho[2, 1–2′, 3′] furan-8, 13-dione

Abstract Three polymorphic modifications, a stable orthorhombic form (α) and two metastable monoclinic forms (β and γ), of Dinaphtho[2,1–2′,3′]furan-8,13-dione were identified by single crystal X-ray diffraction. Heats of transformation, measured calorimetrically, forβ→ α and γ → α are 2.33 cals/gram and 2.07 cals/gram, respectively. The long wavelength absorption spectrum of the α form and the fluorescence spectra of the three polymorphs are reported. The optical properties of the polymorphs indicate an increase in intermolecular interaction on going from the α to β to γ crystal.

On the structure of tungstic oxide in the colloidal state

Tungstic oxide crystals in the colloidal state were seen under the electron microscope. Three different types of particles were found: (1) Very small particles the shapes of which were not resolved, (2) rectangular plates, (3) large hexagonal plates. Electron diffraction from the fine particles gave rings consistent with a cubic structure. The other two types gave diffraction patterns consistent with the orthorhombic structure of tungstic oxide obtained by X-ray analysis. It was interpreted that in the process of growth in the colloidal state, the tungstic oxide crystals occur in a metastable cubic structure which ultimately changes into the stable orthorhombic form.