Crystal Structure and Non-Hydrostatic Stress-Induced Phase Transition of Urotropine Under High Pressure

High-pressure behavior of hexamethylenetetramine (urotropine) was studied in situ using angle-dispersive single-crystal synchrotron X-ray diffraction (XRD) and Fourier-transform infrared absorption (FTIR) spectroscopy. Experiments were conducted in various pressure-transmitting media to study the effect of deviatoric stress on phase transformations. Up to 4 GPa significant damping of molecular librations and atomic thermal motion was observed. A first-order phase transition to a tetragonal structure was observed with an onset at approximately 12.5 GPa and characterized by sluggish kinetics and considerable hysteresis upon decompression. However, it occurs only in non-hydrostatic conditions, induced by deviatoric or uniaxial stress in the sample. This behavior finds analogies in similar cubic crystals built of highly symmetric cage-like molecules and may be considered a common feature of such systems. DFT computations were performed to model urotropine equation of state and pressure dependence of vibrational modes. The first successful Hirshfeld atom refinements carried out for high-pressure diffraction data are reported. The refinements yielded more realistic C−H bond lengths than the independent atom model even though the high-pressure diffraction data are incomplete.     

Thermal behavior of ethylene copolymers with di- and tri-alkenylsilsesquioxane comonomers synthesized by post-metallocene catalysts

Journal of Thermal Analysis and Calorimetry - This paper reported thermal properties of ethylene copolymers with di- and tri-alkenylsilsesquioxanes (POSS) synthesized by bis(phenoxy-imine) Ti, Zr,...     

Pressure-induced pseudorotation in crystalline pyrrolidine

Pressure of 1.14(3) GPa leads to the crystallization of a new β phase of pyrrolidine, where every second molecule pseudorotates to the CH(2)-off envelope conformation, energetically less favored than the NH-off envelope. The β phase is triclinic, space group P ?1, but it is isostructural with monoclinic phase α, where all molecules are in the N-off envelope conformation. The energy associated with the pseudorotation constitutes only about 5% of the total energy change. The energetically unfavorable axial N-H position is retained.     

Equations for two-phase flows: A primer

The main goal of these notes is to give a review of the equations for two phase flow problems with an interface between the two phases in a self-contained way, and, in particular, to properly include surface tension into the interface balance equations.     

Ethylene/POSS copolymerization behavior of postmetallocene catalysts and copolymer characteristics

Copolymerization of ethylene with iso‐butyl substituted monoalkenyl(siloxy)‐ or monoalkenylsilsesquioxane (POSS) comonomers over bis(phenoxy‐imine) and salen‐type titanium and zirconium catalysts was studied. It was found that the polyreaction performance was significantly depended by the kind of the catalyst and by the structure and concentration of POSS in the feed. The POSS comonomer was efficiently incorporated into the polymer chain at up to 0.2 mol %. The differences in the copolymer compositions as the functions of the catalyst kind and the POSS comonomer were observed, including the varied number‐average sequence length of ethylene and unsaturated end groups, as determined by ¹H NMR and FT‐IR. The presence of POSS comonomers affected also the melting and crystallization behavior of the copolymers, as evidenced by DSC, because of influence on the polymer chain arrangement. The POSS units could act as the nucleating agents. Moreover, the crystal and structural parameters of ethylene/POSS copolymers were evaluated on the basis of X‐ray results, and the limited self‐aggregation of POSS incorporated into the polymer chain, the small number and size of POSS aggregates, and the increased crystallinity degree of copolymers were demonstrated. The ethylene/POSS copolymers produced by postmetallocenes offered also high thermal stability and interesting morphological properties. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3918–3934     

Biphasic ethylene polymerisation using 1-n-alkyl-3-methylimidazolium tetrachloroaluminate ionic liquid as a medium of the Cp2TiCl2 titanocene catalyst

A systematic analysis was performed on a series of 1-n-alkyl-3-methylimidazolium tetrachloroaluminates (where alkyl = ethyl, butyl, hexyl, and octyl), applied as a medium of the Cp₂TiCl₂ titanocene catalyst, to evaluate the influence of the physical properties of the ionic liquids on the polymerisation reaction carried out in the biphasic ionic liquid/hexane mode. Two alkylaluminium compounds, AlEtCl₂ and AlEt₂Cl, were used as activators. The influence of the activator/catalyst molar ratio on the performance of the ethylene polymerisation was determined for each ionic liquid studied. The best results were obtained using 1-n-octyl-3-methylimidazolium tetrachloroaluminate. For the titanocene catalyst immobilised in the ionic liquid, AlEtCl₂ turned out to be a better activator than AlEt₂Cl in our studies. The properties of the polyethylene product have also been presented.     

Polar symmetry in new high-pressure phases of chloroform and bromoform

Polar ordering has been induced by pressure in solid chloroform (trichloromethane), CHCl3, and bromoform (tribromomethane), CHBr3, obtained by isochoric and isothermal freezing in a diamond anvil cell. Structures of these new polymorphs have been determined by single-crystal X-ray diffraction, CHCl3 at 0.62 and 0.75 GPa and CHBr3 at 0.20 and 0.35 GPa. Despite different centrosymmetric structures of all low-temperature phases of CHCl3 (space group Pbcn) and CHBr3 (P6(3)/m, P1, and P3), the high-pressure phases are isostructural in space group P6(3). The polar phase of CHBr3 is formed at 295 K, already at the freezing pressure of approximately 0.1 GPa, while CHCl3 transforms from the Pbcn phase into the P6(3) phase between 0.62 and 0.75 GPa. It has been demonstrated that the electrostatic contribution to halogen...halogen and H...halogen interactions in the CHCl3 and CHBr3 molecular crystals is favorable for the polar aggregation and that this effect intensifies with increasing pressure.     

Tests for specific nonparametric relations between two distribution functions with applications

Let (X,Y) be a random vector and let G and H be the marginal distributions of X and Y, respectively. In this paper, we propose two tests, one of Kolmogorov‐Smirnov type and the other of Wilcoxon type, for the null hypothesis Ψ(G) = H against the alternative Ψ(G) < H, where Ψ() is a function such that Ψ(G) is a distribution function. The tests are based on the empirical distribution functions of the observations on X and Y, which are dependent. We obtain their asymptotic null distributions. A suspected relationship between the distribution functions of two dependent outcomes can be specified as a hypothesis to be tested in examples like the load sharing models, record values, and auction bidding models. As an application, we consider in detail the problem of testing the effect of load sharing in two component parallel systems.