Remarks on stabilization of second-order evolution equations by unbounded dynamic feedbacks

In this paper, we consider second-order evolution equations with unbounded dynamic feedbacks. Under a regularity assumption, we show that observability properties for the undamped problem imply decay estimates for the damped problem. We consider both uniform and non-uniform decay properties.     

Synthesis of a thermally stable hybrid acene-thiophene organic semiconductor via a soluble precursor

An acene fused-thiophene hybrid p-semiconductor exhibiting high thermal stability has been synthesized via a soluble precursor bearing sterically interacting trimethylsilyl groups. [structure: see text]     

Synthesis,characterization, and computational study of the trans-IO2F5(2-) anion

The combination of CH(3)CN solutions of [N(CH(3))(4)][F] and a mixture of cis- and trans-[N(CH(3))(4)][IO(2)F(4)] produces the novel trans-IO(2)F(5)(2)(-) anion. Under the given conditions, only the trans-IO(2)F(4)(-) anion acts as a fluoride ion acceptor, thus allowing the separation of isomerically pure, soluble cis-IO(2)F(4)(-) from insoluble trans-IO(2)F(5)(2)(-). The trans-IO(2)F(5)(2)(-) and cis-IO(2)F(4)(-) anions were characterized by infrared and Raman spectroscopy and theoretical calculations at the LDFT and HF levels of theory. The trans-IO(2)F(5)(2)(-) anion has a pentagonal-bipyramidal geometry with the two oxygen atoms occupying the axial positions. It represents the first example of a heptacoordinated main group AO(2)X(5) species and completes the series of pentagonal-bipyramidal iodine fluoride and oxide fluoride species. The geometries of the pentagonal-bipyramidal series IO(2)F(5)(2)(-), IOF(5)(2)(-), IF(5)(2)(-), IOF(6)(-), IF(6)(-), and IF(7) and the corresponding octahedral series IO(2)F(4)(-), IOF(4)(-), IF(4)(-), IOF(5), IF(5), and IF(6)(+) were calculated by identical methods. It is shown how the ionic charge, the oxidation state of the iodine atom, the coordination number, and the replacement of fluorine ligands by either an oxygen ligand or a free valence electron pair influence the stuctures and bonding of these species.     

Ionic conductivity of and Raman spectroscopy investigation in binary oxosalts (1 − x)AgPO3xAg2SO4 glasses

(1 ? x)AgPO₃xAg₂SO₄ homogeneous glasses obtained by quenching of a melt of the two salts are pure ionic Ag⁺ conductors. The RT conductivity is increased from 2.5 × 10^?7 to 4 × 10^?6 (Ω cm)^?1 when the ratio of Ag₂SO₄ is increased from 0 to 0.3. Raman spectroscopy shows that no modifications of the (PO₃)^∞ chain skeleton occur by adding Ag₂SO₄. The low-frequency Raman band lying at about 55 cm^?1 is quantitatively correlated to Ag⁺ oscillations, the hopping distance decreasing from 3.0 to 2.7 Å if a jump process between regular Ag⁺ sites is considered.     

Approach to improving the toughness of TGMDA/DDS epoxy resin by blending with thermoplastic polymer powders

A series of hot-melt processable thermosetting compositions was prepared by blending N,N,N′,N′-tetraglycidyl-4,4′ -diaminodiphenyl-methane/4,4′-diaminodiphenylsulfone (TGMDA/DDS) epoxy resin and thermoplastic polymer powders with average particle size below 30 μm. The basic thermoplastic polymers were either a high T_g amorphous cardo polyimide (T_g=350°C) or commercial semicrystalline PA6 and PA12 polyamides. The resulting heterogeneous mixtures showed viscosity values below 5000 cps suitable for prepregging process. After cure, phase-separated morphologies were maintained with a rather limited interphase miscibility as demonstrated by thermomechanical analysis. Scanning electron microscope examination of fracture surfaces pointed out a strong adhesion between the powder particles and the surrounding polyepoxy network, particularly for the potentially reactive polyamide structures. Moreover, as shown by differential scanning calorimeter analysis, the crystallinity ratio of the PA6 and PA12 powders was lowered due to melting during thermal polymerization. The fracture toughness properties of the powder-containing materials were compared with those of a fully miscible cardo polyimide–TGMDA/DDS blend coming from an homogeneous resin composition. The best improvement in fracture energy was obtained for the powder-modified resins. The most effective composition filled with 16 wt% of powdered polyimide exhibited a fourfold increase in G_IC (388 J/m² versus 100 J/m²) without compromising the epoxy thermomechanical stability (T_g=227°C versus 223°C).     

Synthese,structure cristalline et analyse vibrationnelle de l'hexathiohypodiphosphate de lithium Li4P2S6

The crystalline compound Li₄P₂S₆ is obtained either by devitrification of Li₄P₂S₇ glass at 450°C with sulfur formation or by crystallisation at 450°C of a Li₂S, P and S melt. The structure determination has been solved by X-ray diffraction on a monocrystal. The unit cell is hexagonal $\text{P6}{}_3\text{mcm}$ with a = 6.070(4), c = 6.577(4) Å, V = 209 ų, Z = 1. Intensities were collected at 293°K with Kα (λ = 0.71069 Å) Mo radiation on an automatic Nonius CAD-4 diffractometer. The structure was solved under the assumption of random disorder of P atoms over two sites (occupancy factor of 0.5). Anisotropic least-squares refinement with W = 1 gave R = 0.047 for 90 independent reflections and 9 variables. The structure is built according to an ABAB sequence sulfur packing. Per unit cell, four out of six octahedral sites are occupied by Li ions, and the other two are statistically filled (0.5) by PP pairs. The PP central bond (2.256(13) Å) links two staggered PS₃ groups (PS = 2.032(5) Å) to form the D_3d symmetry P₂S^4?₆ anion. Infrared and Raman spectra show features very similar to those of Na₄P₂S₆, 6H₂O and M^IIPS₃ compounds. A new assignment in terms of symmetry species is proposed for the P₂S₆ internal modes, which is confirmed by a normal coordinate calculation using a valence force field; the stretching force constants f_PP and f_PS are equal to 1.6 and 2.7 mdyne Å^?1, respectively.     

(2‐Thienylmethyl)ammonium ­trichlorostannate(II): a hybrid salt

The structure of the title hybrid salt, (C₅H₈NS)[SnCl₃], is built up from segregated layers of organic cations and Sn polyhedra. [SnCl₃]^? groups are linked together by weak Sn?Cl interactions to form a one‐dimensional polymeric chain of anions.     

Computational schemes for modeling proton transfer in biological systems: Calculations on the hydrogen bonded complex [CH3OH · H · NH3]+

Different schemes are explored for the calculation of the proton transfer process in the hydrogen bonded cation [CH₃OH · H · NH₃]⁺. Results from ab-initio calculations with the STO-3G, 3-21G and 4-31G basis sets, are compared in search for an efficient reliable scheme to study the potential energy curves for the proton transfer. The curve constructed from the lowest energies calculated with the frozen optimized geometries of the two possible pairs of proton donor and acceptor fragments, (i.e., CH₃OH/NH₃ and CH₃OH/NH) is in good agreement with that obtained when all the fragments of the hydrogen bonded complex are completely optimized simultaneously.