Ferrocenylenesilylene Polymers as Coatings for Tapered Optical-Fiber Gas Sensors

In this paper we report the use of ferrocenylenesilylene polymers as coatings for tapered optical-fiber sensors. The principle of operation of this device is based upon environmentally induced changes in the refractive index of the polymer layer which change the power transmitted through the tapered fiber. The results for two sensor arrays fabricated using the ferrocenylenesilylene polymer [( ⁵-C₅H₄)Fe( ⁵-C₅H₄)MePhSi] _m and copolymer {[( ⁵-C₅H₄)Fe( ⁵-C₅H₄)SiPhMe] _n [( ⁵-C₅H₄)Fe( ⁵-C₅H₄)Me₂Si] _m are presented. We also show that the sensitivity of this device is a function of the taper beat length.     

Palladium-Mediated Ring-Opening Reactions of Strained Cyclotetrasilanes

Room-temperature ring-opening polymerization of the strained monomer 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane mediated by palladium complexes was attempted. Examination of the reaction products between the cyclotetrasilane and the metal complex at high concentrations indicated that insertion of one cyclotetrasilane molecule between the ligand–palladium bond was followed by reductive elimination of the tetrasilane. Among the palladium complexes examined were (³-allyl)PdCp [Cp=⁵-cyclopentadienyl], [(³-allyl) PdCl]₂, (³-allyl)PdCl(PPh₃), (³-allyl)PdCl(PPh₃)₂, (³-allyl)Pd(PPh₃)₃ and PdCl₂(PPh₃)₂. Reaction of 1,1,2,3,4-pentamethyl-2,3,4-triphenylcyclotetrasilane with (³-allyl)PdCp provides similar products.     

Synthesis and Redox Properties of a π-Conjugated Cyclobutadienecobalt Polymer Containing Ferrocenyl Groups

The reaction of CpCo(PPh₃)₂, in which Cp= ⁵-cyclopentadienyl, with a -conjugated diacetylene, FcCC–o-C₆H₄–CCFc, in which Fc=ferrocenyl, was found to give a cyclobutadienecobalt mononuclear complex, { ⁴-C₄Fc₂(o-FcC₆H₄)₂}CoCp (1), the crystal structure of which was determined by X-ray crystallography. In contrast, the reaction of CpCo(PPh₃)₂ with FcCC–p-C₆H₄–CCFc affords a cyclobutadienecobalt polymer, [p-C₆H₄( ⁴-C₄Fc₂)CoCp] _n (2). The monocobalt complex 1 shows reversible 1e^– and 3e^– redox waves at E ⁰=0.116 and 0.350 V vs Ag/Ag⁺, and the polymer complex 2 shows two chemically reversible redox waves at E ⁰=0.143 and 0.219 V for the oxidation of the ferrocenyl moieties in the cyclic voltammogram. Crystal data are as follows: (1, C₆₅H₄₉CoFe₄), triclinic, space group P\={1} (No. 2), a=13.547(4), b=16.197(4), c=11.763(4) Å, =106.79(2), =97.93(3), =97.12(3), V=2410(1) ų, Z=2.     

Monolayer dispersion of molybdenum on silica

Laser Raman spectroscopy (LRS) was used to characterize the hydrated and dehydrated states of surface-supported Mo⁶⁺ on Davison 952 silica gel. Silica-supported Mo samples were prepared from MO₂(³-C₃H₅)₄ and (⁵-C₅H₅)₂Mo₂(CO)₄. Metal loadings of 5.1–7.8 wt% Mo were studied. Crystallites of MoO₃ were not observed at these high loadings. An upper limit for dispersing dehydrated, isolated Mo⁶⁺ was found at about 1 Mo atom/nm², which corresponds to the isolated silanol density of silica.     

Correlation between configuration/conformation of zirconocenes on the stereoselectivity of the propylene polymerization reaction

Summary The catalytic performance (activity and polymer properties) of metallocenes with different symmetries in combination with methylaluminoxane (MAO) in the polymerization of propylene has been investigated at different temperatures, under standardized reaction conditions. The zirconocene rac-ethylene (⁵-1-indenyl) zirconium (IV) dichloride, with C₂ symmetry, produces isotatic polypropylene and isopropylidene(⁵-cyclopentadienyl (⁵-9-fluorenyl) zirconium (IV) dichloride, with C _S symmetry, syndiotactic polypropylene. The degree of the tacticity of these polymers increases with decreasing polymerization temperature. Only atactic polypropylene was formed with the unbridged zirconocenes bis(⁵-cyclopentadienyl) zirconium (IV) dichloride and bis(⁵-indenyl zirconium (IV) dichloride at any temperature investigated (10–60°C).     

[η]-M-relationship for polyacrylamide in aqueous 0.1 M Na2SO4 solution

Summary The intrinsic viscosity [] and the molecular weight M_W of 8 pure, unbranched polyacrylamide (PAAm) samples — prepared in our laboratory — were determined and lead to the following formula [] = 0,0194 · M_W ^0,70(cm³/g) Solvent: aqeous 0.1 M Na₂SO₄ solution; T=298K; M_W/M_n-2,5 . In addition a comparison to the []-M-relationships of PAAm in other solvents is given.     

Electrodeposition, composition and structure of Zn—Cr alloys

The effect of polyethylene glycol (PEG 1500) as additive and of deposition conditions on Zn—Cr alloy electrodeposition from an acidic sulfate electrolyte at room temperature, without agitation was investigated. PEG polarizes the overall cathodic reaction and inhibits Zn deposition. Cr codeposition with Zn starts at a cathodic potential of about –1,95 V vs Hg/Hg₂SO₄, which is reached at current density of about 20 A dm^–2 in galvanostatic conditions. Zn—Cr alloy coatings containing up to 28 at % Cr were obtained depending on the plating conditions. SEM observations showed an island-like structure, formed by the local growth of crystals, which covered the surface during further deposition. In the first stages of electrodeposition the powder diffraction spectra contain lines of b.c.c. -(Zn,Cr) phase (a 3.02 Å). After 30 s deposition time weak lines of Zn-based phase (a 2.67 Å, c 4.90 Å) appear, and become clearly visible in coatings deposited for 90 s. The average Cr content in the alloy coatings decreases with advancing deposition. The as-plated surface contains C in organic compounds and Zn(OH)₂. After 50 min sputtering, Zn and a mixture of Cr, Cr₂O₃ and Cr₇C₃ were found. The presence of organic C and O, probably from inclusions of PEG, were also detected.     

Structure and thermal stability of zinc–nickel electrodeposits

ZnNi alloys were electrodeposited from a chloride bath on steel substrates. The effect of nickel bath concentration on chemical composition, structure and microstructure of the deposits is demonstrated. From 0 to 13 nickel, the phases obtained do not correspond to that reported on the thermodynamic phase diagram. It is shown that the substitution of zinc by nickel is responsible for the formation of distorted _d and _d phases corresponding to the supersaturated hexagonal phase of zinc and to the unsaturated cubic phase of Zn–Ni alloy, respectively. Differential scanning calorimetry indicates that the thermal instability of the alloys containing up to 13 wt of nickel, results from the crystallization of the phase from the _d and _d phases at around 200 °C and 250 °C, respectively.     

Synthesis and Properties of PPV–Based (η<Superscript>6</Superscript>–Arene)Cr(CO)3–Containing Polymers Having Alkyldiphenylamine or Triarylamine in the Main Chain

Summary Novel poly(p–phenylenevinylene)–based (⁶–arene)Cr(CO)3–containing polymers including the alkyldiphenylamine or triarylamine units in the main chain were synthesized by the Homer–Wadsworth–Emmons coupling reaction. The structures of the polymers were confirmed by NMR and FT–IR spectra. The polymers were soluble in common organic solvents such as THF, CHCl₃, and CH₂Cl₂. The polymer showed interaction between the electron–withdrawing (⁶–arene)Cr(CO)³ unit and the unshared electron pair on the nitrogen atom along the polymer backbone.     

Crystal structure and morphology of electrodeposited zinc-iron binary alloys

Deposits of zinc-iron alloy have been prepared galvanostatically from a sulphate bath and the crystal structure has been determined by X-ray diffraction and transmission electron microscopy measurements. The electrodeposited zinc-iron alloys have metastable structures and the individual phases coexist over wide composition ranges. The phases are identified as (10073 at % zinc), (8748 at % zinc), ₁(7862 at % zinc) and (620 at % zinc). Thec andc/a in the h.c.p. lattice of the -phase decrease continuously with decrease of zinc concentrations, and the latter changes from 1.86 to 1.60 (a andc are the lattice constants of the -phase in the direction of thea- andc-axes, respectively). The -phase particles exhibit a hexagonal plate-like morphology which is thin in the direction of thec-axis. The morphology of the electrodeposits changes from plate-like to pyramidal shape when fine -phase particles (100 nm) start to form surrounding the -phase platelets, and then to lenticular or granular in the /₁ duplex region. The -phase forms in the low zinc concentration region and changes the electrodeposits to a fine cuboidal morphology.     

Application of the accelerated Tafel plot technique to corrosion kinetics: Role of double layer through simulation

Results on the simulated -t transient response of an actively corroding system under accelerated Tafel plot (ATP) conditions have revealed the influence of input parameters (i, ) and system parameters (C _dl,i _corr andb) and explained the observed dependence of kinetic parameters (arrived at on the basis of the intercept-slope method) on in certain time domains. New improved methods, which eliminate such dependence and give uniform corrosion rate data over all time domains, are described in the paper.Nomenclature ATP accelerated Tafel plot - transfer coefficient - b Tafel slope (V) - C _d double layer capacitance (F cm^–2) - i initial value of the exponentially decaying current (A) - E _c corrosion potential (V) - overpotential (V) - n overpotential corresponding to maximum in -t transient (V) - F Faraday constant (C mol^–1) - i _corr corrosion current density (A cm^–2) - n number of electrons involved in charge transfer step - p intercept of ATP (V) - R gas constant (JK^–1 mol^–1) - R _p polarization resistance ( cm²) - S slope of ATP, i.e. d/dt (V) - S _av average of theS values at > _el (V) - S _meas slope of the linear -t region, i.e. d/dt (Vs^–1) - T temperature (K) - t time (s) - t _m time corresponding ton _m in the -t transient (s) - time constant of the exponentially decaying current pulse (s) - _el electrode time constant given byR _p C _d (s)     

Synthesis of UHMW-PE by a Highly Active Homogeneous System: η5-(C5H5)2Ti(η1-OC(O)C6H5)2/MAO

Summary The complex ⁵-(C₅H₅)₂Ti (¹-OC(O)C₆H₅)₂ was synthesized and activated with MA0 for ethylene polymerization. The resultant polymer is UHMW-PE with a viscosimetric molecular weight of Mv = 6.8 x 10⁶ to 0.7 x 10⁶ g/mol dependent on the temperature of the reaction. The effects of the Al/Ti molar ratio and temperature of the reaction on the catalytic activity and polymer properties were studied.     

Static and dynamic radii of polystyrene in ethylbenzene and tetrahydrofuran

Summary Hydrodynamic radii, R _H ^D and R _H , determined, respectively, from — translational diffusion coefficients and intrinsic viscosities are compared with radii of gyration S_Z for polystyrene of narrow molecular weight distribution in good-solvent systems, ethylbenzene and tetrahydrofuran. The S_Z data indicate THF is of comparable solvating power to ethylbenzene which, in turn, based on literature data is similar to benzene in its affinity for polystyrene. The ratios ₁ = S_z/R _H ^D and ₂=R _H /R _H ^D are much larger for polystyrene in ethylbenzene than in tetrahyaroruran. These results are interpreted to indicate the presence of a large draining effect in the ethylbenzene system.     

Relationship between preferential adsorption coefficient and intrinsic viscosity in ternary systems

Summary The relation proposed between the preferential adsorption coefficient, , and the intrinsic viscosity, ¦¦ ¦¦=¦¦+AK has been applied in this paper to previously published data. This equation is found to be valid in theta condi tions and far away from them. The obtained results are compared to the ones calculated with the Dondos-Benoit equation.     

High resolution transmission electron microscopic (HRTEM) determination of the preferentially exposed faces onγ-Al2O3 andη-Al2O3

High resolution transmission electron microscopy has been used to examine the crystallites in -Al₂O₃ and -Al₂O₃. -Al₂O₃ has been found to be nearly hexagonally shaped crystallites whose identity distances and angles of fringe patterns indicate that (110) planes are preferentially exposed. -Al₂O₃ showed preferentially exposed surfaces of (100), (110) and super imposed (111)/(211). This information is valuable to the catalyst researcher trying to model the surfaces of these two supports.     

Interaction of methane with surfaces of silica,aluminas and HZSM-5 zeolite. A comparative FT-IR study

Infrared investigations on the interaction of methane with silica, aluminas (, and ) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over these oxides and HZSM-5 zeolite. Our findings featured that the infrared inactive ₁ band (2917 cm^–1) of a gaseous methane molecule became active and shifted to lower frequencies (2900 and 2890 cm^–1) when it adsorbed on the surfaces of these adsorbents. Our results also demonstrate that hydroxyl groups played a very important role in methane adsorption over the acidic oxides and the HZSM-5 zeolite. When interaction between the hydroxyl groups and methane took place, the band shift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-OH-Al>Al-OH>Si-OH, which is in accordance with the order of their acidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, -alumina and HZSM-5 zeolite could exchange with CD₄ at temperatures higher than 773K, while those on -alumina could exchange at a temperature as low as 573 K. Another interesting observation was the formation of formate species over Al₂O₃ (both and ) at temperatures higher than 473 K. The formate species would decompose to CO₂, or produce carbonate at much higher temperatures. Formation of formate species was not observed over silica and HZSM-5 under similar conditions, -Al₂O₃ did not adsorb or react with methane in any case.     

Functionally substituted tetramethylcyclopentadienes: Synthesis of 1-(p-hydroxyphenyl)-2,3,4,5-tetramethylcyclopentadiene and its use in the synthesis of an organometallic polymer

Reaction of 2,3,4,5-tetramethylcyclopent-2-enone (1) withp-LiC₆H₄OC-Me₂OMe, followed by treatment with aqueous acid afforded 1-(p-hydroxyphenyl)-2,3,4,5-tetramethylcyclopentadiene (2). This new ligand was then used in the synthesis of the functionally substituted organomolybdenum reagent (⁵-C₅Me₄-p-C₆H₅OH)Mo(CO)₂(NO) (3). Treatment of a preformed 1/1 styrene/maleic anhydride copolymer ( ) with 5 mol% 3 led to chemical incorporation of the organometallic species into the polymer. The final product contained 3 mol% of the organometallic moiety.     

Effects of process conditions on the formation of microporous membranes via solid-liquid thermally induced phase separation

Poly(phenylene sulfide) (PPS) membranes were formed via solid-liquid thermally induced phase separation. The effects of nucleation density () on final membrane structure were investigated. was varied by changing dissolution temperature (T _d, the temperature at which the melt-blend is formed) and polymer concentration in the initial polymer-diluent mixture.     

Investigation of water electrolysis by spectral analysis. I. Influence of the current density

The potential (or current) fluctuations observed under current (or potential) control during gas evolution were analysed by spectral analysis. The power spectral densities (psd) of these fluctuations were measured for hydrogen and oxygen evolution in acid and alkaline solutions at a platinum disk electrode of small diameter. Using a theoretical model, some parameters of the gas evolution were derived from the measured psd of the potential fluctuations, such as the average number of detached bubbles per time unit, the average radius of the detached bubbles and the gas evolution efficiency. The influence of the electrolysis current on these parameters was also investigated. The results of this first attempt at parameter derivation are discussed.Nomenclature b Tafel coefficient (V^–1), Equation 46 - C electrode double layer capacity (F) - e gas evolution efficiency (%) - f frequency (Hz) - f _p frequency of the peak in the psd _v and _i (Hz) - F Faraday constant, 96 487 C mol^–1 - l electrolysis current (A) - J electrolysis current density (mA cm^–2) - k slope of the linear potential increase (V s^–1), see Fig. 1 - n number of electrons involved in the reaction to form one molecule of the dissolved gas - r _b radius of a spherical glass ball (m) - r _e radius of the disk electrode (m) - R _e electrolyte resistance () - R _p polarization resistance () - R _t charge transfer resistance () - u ₁ distribution function of the time intervals between two successive bubble departures (s^–1) - v _g mean volume of gas evolved per unit time (m³ s^–1) - v _t gas equivalent volume produced in molecular form per unit time (m³ s^–1) - V ₀ gas molar volume, 24.5×10^–3 m³ at 298 K - x ₀ time pseudoperiod of bubbles evolution (s) - Z electrode electrochemical impedance () Greek characters _e dimensionless proportional factor (Equation 19) - slope of log /logJ and loge/logJ curves - number of bubbles evolved per unit time (s^–1) - _a activation overpotential (V) - _ci concentration overpotential of reacting ionic species (V) - _cs concentration overpotential of dissolved molecular gas (V) - _ohm ohmic overpotential (V) - _t total overpotential (V) - v parameter characteristic of the gas evolution pseudoperiodicity, Equation 13 (s^–1) - time constant of the double layer capacity change (s) - _v power spectral density (psd) of the potential fluctuations (V² Hz^–1) - _i power spectral density (psd) of the current fluctuations (A² Hz^–1) Special symbols spatial average of the overpotential _j over the electrode surface - time averaged value of - _j fluctuation of around - <> mean value of the total overpotential jump amplitude due to a bubble departure - <I> mean value of the current jump amplitude due to a bubble departure Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.