Mobility study of amorphous polymers by high-resolution NMR at solid-state

Summary Mobility of amorphous polymers such as poly(vinyl chloride) (PVC), atactic polypropylene (PPA) and ethylene-propylene-diene terpolymer (EPDM) were studied by proton spin-lattice relaxation time (T₁ H), proton spin-lattice relaxation time in the rotating frame (T₁ ^H ), and dipolar dephasing pulse sequence. The data are discussed in terms of mobility and by the distribution of configurational sequences.     

C-13 Rotating frame relaxation of solid polymers

Summary ¹³c T₁ of some polymers has been measured at various temperatures. This relaxation is sensitive to other motions than those determining the proton relaxation. The T₁'s of polyethylene and poly(ethylene-vinylacetate) show minima which correspond to second moments of 108 s^–2 and 4·10⁸ s^–2, respectively, in contrary to the theoretical value of 4·10⁹ s^–2. The reason is a strongly restricted motion — e. g. torsional oscillation — which cannot average out completely the dipolar interaction. In poly(methylmethacrylate) and in poly(vinylacetate) a decrease of the restriction with increasing temperature has been observed.Presented at the 22nd Microsymposium, Characterization of Structure and Dynamics of Macromolecular Systems by NMR Methods, Prague, CSSR, July 20–23, 1981     

Mobility study of amorphous polymers by high-resolution NMR at solid-state

Summary Mobility of amorphous polymers such as poly(vinyl chloride) (PVC), atactic polypropylene (PPA) and ethylene-propylene-diene terpolymer (EPDM) were studied by proton spin-lattice relaxation time (T₁ H), proton spin-lattice relaxation time in the rotating frame (T₁H), and dipolar dephasing pulse sequence. The data are discussed in terms of mobility and by the distribution of configurational sequences.     

Convective Regime of Filtration Combustion of Energetic Materials in a Cocurrent Flow of Their Combustion Products

The convective regime of filtration combustion of energetic materials in a cocurrent flow of their combustion products is studied using a model with extremely simplified kinetics and heat transfer, which shows instability of the process. It is shown that the more accurate twotemperature model describes a steadystate regime. In this regime, the gas temperature on the hot boundary of the heating zone is well below the combustion temperature, and the solidphase temperature is well below the temperature proposed in recent studies on this topic. It is pointed out that the twotemperature approach is unjustified and intragranular nonisothermicity must be taken into account for convective regimes. It is shown that the threetemperature model, which takes into account this effect, does not give a stable steadystate solution.     

Dependence of the Time‐Space Structure of the Chemical‐Reaction Zone on the Initial Density of the Explosive

The results of a study of the chemicalreaction zone structure for a number of high explosives (HEs) are discussed. It is found that an increase in the initial density of the HEs leads to a structural change in the chemicalreaction zone involving elimination of the Von Neumann spike at the critical density point. Conversely, as the initial density of the explosive decreases, the ratios of the gasdynamic parameters (in particular, mass velocities) at the Von Neumann point to the same parameters at the Jouguet point increase. It is shown that to explain the indicated regularities, one does not need to invoke the assumption of an increasing contribution of the exothermic decomposition of the HE the shock. The results can be explained within the framework of classical Zeldovich–Von Neumann–Döring theory with an ordinary shock at the detonationwave front and, hence, with a nearly zero contribution to the total energy release in the chemical reaction zone.     

Pore Propagation Directions in P+ Porous Silicon

A comparative study is presented on the pore propagation directions of porous silicon layers (PSL) formed on p⁺-type substrates of different orientations. PSLs were formed on plain (0 0 1) and (1 1 1) silicon wafers as well as on structured (0 0 1) wafers containing facets of various orientations. During anodization, regular pores follow the 0 0 1 direction on the (0 0 1) planes. While on the (1 1 1) planes fewer regular pores develop and seemingly propagate closely to the 1 1 1 direction. These results indicate that the pores propagate perpendicular to the surface i.e. along the field lines when the surface orientation is either (0 0 1) or (1 1 1).When the silicon surface provided (1 1 0) orientation (Chuang, Collins, and Smith, 1989), or its position is in between the (0 0 1) and (1 1 1) planes then the pores do not propagate perpendicular to the surface but along the 0 0 1 direction.All the phenomena exhibited might be explained by presuming that during formation, the pores propagate along the 1 0 0 directions, and that those 1 0 0 directions are preferred which are closely to the field lines. In PSLs formed on (0 0 1) surfaces the field lines and the 0 0 1 crystallographic direction are coincident. However, in the (1 1 1) oriented wafer where three equally probable 1 0 0 directions exist around the field lines, more irregular structure of PSLs will develop.     

Defensive steroids from a carrion beetle (Silpha novaboracensis)

Two novel steroids, 3,7-dihydroxy-14-pregn-4-en-15,20-dione (1) and 3,7, 20-trihydroxy-14-pregn-4-en-15-one (4), have been characterized from the rectal gland ofSilpha novaboracensis (Coleoptera: Silphidae). Judging from the function of comparable pregnanes in another species ofSilpha, the compounds may play an antipredator role.Paper No. 81 of the seriesDefense Mechanisms of Arthropods. Paper No. 80 is Eisner, T., Deyrup, M., Jacobs, R., and Meinwald, J.,J. Chem. Ecol. 12:1407–1415.     

Pore Size Distribution and Chord Length Distribution of Porous VYCOR Glass (PVG)

A porous VYCOR-glass of porosity c 30% was analyzed by use of nitrogen adsorption (NA), mercury intrusion (MI) and small-angle scattering (SAS). The distribution density of the pore diameter was determined from the SAS experiment, based on the stereological information for a fixed order range L = 40 nm.A pore can be described by use of two random variables, which depend on each other: The pore diameter d and the chord length l. In a first step, an assumption free data evaluation method yields the second derivative of the SAS correlation function (r). Then, based on the intimate connection between (r) with random chord lengths, an interpretation of the first two mean peaks was performed. These peaks reflect the chord length distributions of pore and wall. The problem of the allocation of the peaks has been solved based on the information of the NA and MI experiments. The transformation of the distribution densities of the pore diameters V _M(d) (obtained by MI a experiment) and V _N(d) (obtained by a MI experiment) into chord length distribution densities A _M(l) and A _N(l) have allowed the clear interpretation of (r). It was possible to separate the chord distributions of the pores from those of the walls. The first (r) peak reflects the chord length distribution density (l) of the pores (first moment l¯ = 10.6 nm) and the second one that of the walls f(m) (first moment m¯ = 21 nm). It follows c 30%. The average mean chord length is d¯ _lm 15 nm. The second moment of (l) is 108 nm².Finally, from the separated function (l), the diameter distribution density of the pores V _SAS(d) has been obtained. V _SAS(d) was calculated, neither assuming a defined mathematical function type of the distribution nor a certain shape or dimension of the pore. The first and second moments of V _SAS(d) are 7 nm and 74 nm². From comparing the three distribution densities V _SAS(d), V _M(d) and V _N(d) it can be concluded that the assumption of cylindrical pores is fulfilled.While the chord length distribution density of the walls is a highly symmetrical function, which can be approximated by a Gauss term, the pores have an unsymmetrical chord distribution density with the PVG.     

Effect of gas evolution on current distribution and ohmic resistance in a vertical cell under forced convection conditions

The volume fraction of gas bubbles in a vertical cell with a separator was evaluated on the basis of the Bruggemann equation by taking into account the increase in velocity of the rising gas bubbles when fresh solution without gas bubbles is supplied to the bottom of the cell at constant velocity. This enhancement of the velocity results from an increase in the volume of gases evolving at the working electrode. The following three cases for overpotential at the working electrode were considered: no overpotential, overpotential of the linear type and of the Butler-Volmer type. The volume fraction, _h , at the top of the cell was expressed as a function of the dimensionless height of the cell and kinetic parameters. The total cell resistance can be expressed by {(2/5 _h )[1– _h )^–3/2–1+ _hD;]+µ}₁ d ₁/wh, where ₁ is the resistivity of the solution without gas bubbles,d ₁ the interelectrode distance,w the cell width,h the cell height and the parameter involving overpotential and resistance of the separator. It was found that there is an optimum value of the interelectrode distance. The optimum value is about a quarter of the value for the case of constant gas rise velocity, which corresponds to a closed system.Nomenclature b linear overpotential coefficient - C proportionality constant given by Equation 2 - d ₁ interelectrode distance - d ₂ thickness of the separator - F Faraday constant - h height of the cell - i current density - l total current - t ₀ exchange current density - k parameter given byd ₁(z)^1/2 - n number of electrons transferred - p gas pressure - r dimensionless cell resistance defined by Equation 16 - R gas constant - R _t total cell resistance - T temperature - u auxiliary function defined by Equation 37 - v solution velocity in the cell - v ₀ solution velocity at the bottom of the cell - v _h solution velocity at the top of the cell - V voltage at the working electrode - V _eq voltage at the working electrode when no current flows - w width of the electrode - y axis in the vertical direction from the bottom of the cell - z dimensionless variable fory, defined by Equation 8 - z _h dimensionless variable forh, defined by [C(V– V _eq/(₁ d ₁ v ₀]h - anodic transfer coefficient in the Butler-Volmer equation - volume fraction of gas bubbles in the cell - _h volume fraction of gas bubbles at the top of the cell - dimensionless cell voltage, given by Equation 38 - Butler-Volmer overpotential - Butler-Volmer overpotential when current density,I/wh, flows through the electrode, as described in Equation 42 - µ parameter representing either µ_S, µ_S + µ_L or µ_S + µ_BV - µ_BV ratio defined by Equation 41 - µ_L ratio defined byb/(₁ d ₁) - µ_S ratio defined by ₂ d ₂/₁ d ₁ - ₁ resistivity of the solution phase without gas bubbles - ₁(y) resistivity of the solution phase with gas bubbles at levely - ₂ resistivity of the separator - kinetic parameter in the Butler-Volmer equation, given by Equation 39     

Fertilizer requirements for specified yield targets. I. Theoretical derivation of mathematical models for the computation of soil and fertilizer nutrient efficiencies

The conventional deduction procedure of computation of soil () and fertilizer () nutrient efficiencies for the amount of fertilizer required for specified yield targets does not make provision of the amount of soil nutrient derived by crops from the available pool of soil nutrients not accounted for in the amount extracted by a soil test procedure. The derivation of two mathematical models, viz., Tamil Nadu Agricultural University Model I [TNAU Model I] and Model II [TNAU Model II] is reported in this paper which aim at computing the soil () and fertilizer () nutrient efficiencies not accounted for by the conventional method.In the case of TNAU Model I, the relationship between the nutrient uptake (U) and the soil (S) and the fertilizer (F) nutrients was established by assuming a functional relationship of the type U =S +F such that 0 1 and 0 1. In TNAU Model II the same relationship was established as U =S +F + such that 0 1, 0 1 and > 0. The term in the latter model is a measure of the amount of soil nutrient the crop absorbs from a slowly available pool of nutrients not accounted for in the amounts extracted by the soil test procedure employed or applied through fertilizer.The field verification of these models is reported elsewhere.     

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.     

Dielectric properties of anodic oxide layers on tantalum

The dielectric properties of the anodically formed oxide layers on tantalum in contact with electrolyte were analysed by measuring the frequency and temperature dependence of the impedance. It has been found that the frequency dependence of the series capacitance and resistance component of the impedance in the audio frequency range are in accordance with Young's relation. In order to explain such behaviour the electrical resistivity is assumed to vary exponentially with distance through the oxide layer. This variation can be ascribed to the occurrence of the exponential change of oxygen vacancies in the anodic layer during the growth of the oxide layer. The activation energy was obtained from the temperature dependence of the series capacitance. In the paper the unsimplified Young's relations have been proved to be K-K transformable.Nomenclature C _s series capacitance (F) - R _s series resistance () - f frequency of applied signal (Hz) - x integration variable of frequency (Hz) - A area (cm²) - K characteristic length (cm^–1) - d oxide layer thickness (cm) - y distance through oxide (cm) - a _C slope of linear part of 1/C _s against logf plot (Equation 5) - a _R slope of linear part ofR _s against 1/f plot (Equation 3) - relative permittivity of oxide layer - (y) resistivity at distancey ( cm) - (0) resistivity on positiony=0 ( cm) - (d) resistivity on positiony=d ( cm) - T absolute temperature (K) - k Boltzmann constant (eV K^–1) - activation energy (eV) - z complex variable,z=x+iy, - Res residue     

On the NiMoO4 oxidative dehydrogenation of propane to propene: some physical correlations with the catalytic activity

The - and -phases of NiMoO₄ have been investigated with different techniques (X-ray diffraction, electrical conductivity, IR spectroscopy) in order to tentatively rationalise the different catalytic activities observed in the oxidative dehydrogenation of propane to propene. XRD analyses have shown that at 595 ° C, the -phase is already present but a temperature of 700 ° C is required to obtain a full conversion into a pure -phase. Electrical conductivity showed the presence of anionic vacancies. It is proposed that propene is formed by the reaction of propane with surface O^2- anions. The -phase is almost twice more selective in propene formation than the -phase for comparable conversion at identical temperatures. This could derive from different oxygen environments on the active catalytic site.     

The possibility of studying thermal motion correlation by multiple pulse NMR

Summary We present the results of a calculation of the spin-lattice and spin-spin relaxation for dipolar solid when it is irradiated by the WAHUHA and MREV-8 pulse sequences. The spin-spin relaxation rate which is determined by the second-order dipolar interaction term of the average Hamiltonian depends on thermal motion of four atoms. It has been shown that from the temperature dependence of this relaxation rate the degree of correlated atomic motion can be estimated.Presented at the 22nd Microsymposium, Characterization of Structure and Dynamics of Macromolecular Systems by NMR Methods, Prague, CSSR, July 20–23, 1981     

Behavioral and neurosensory responses of the boll weevil,Anthonomus grandis Boh. (Coleoptera: Curculionidae), to fluorinated analogs of aldehyde components of its pheromone

Competitive field tests with -fluorinated analogs of compounds III and IV (III--F and IV--F, respectively) of the boll weevil,Anthonomus grandis Boh., aggregation pheromone showed these compounds, when combined with the other pheromone components [(±)-I and II], to be as attractive as grandlure [(+)-I, II, and III+IV]. Dose-response curves constructed from electroantennograms of male boll weevils to serial stimulus loads of III, IV, III--F, IV--F, and the corresponding acyl fluorinated analogs (III-acyl-F and IV-acyl-F) showed the -fiuorinated analogs to be as active as the pheromone components (threshold=0.1 g), while the acyl fluorinated analogs had a 10-100 x higher threshold (=1-10 g). Single-neuron recordings showed that IV neurons and II neurons (Dickens, 1990) responded to IV--F and III--F, respectively, while IV-acyl-F and III-acyl-F were inactive. Since a previous study showed compounds I, II, and IV to be essential for behavioral responses in the field, it seems likely that the activity of the -fluorinated analogs observed here is due to the stimulation of IV neurons by IV--F as indicated in single neuron recordings.     

Mass transfer to planar and expanded metal electrodes in bubble columns

Mass transfer rates at planar electrodes and electrodes of expanded metal placed in the centre of a bubble column were measured. The gas velocity and the physical properties of the electrolytic solutions were varied and different types of expanded metal were investigated. In some cases increases in the mass transfer coefficient over the planar electrode value of more than 100% were obtained. Dimensionless correlations are presented for the different systems.Nomenclature A mean mesh aperture - D diffusivity - D _c column diameter - g acceleration due to gravity - Ga Galileo number =gL ³/v ² - Gr Grashof number =gL ³/v ² - k mass transfer coefficient - L electrode height - r radial position - R column radius - Re Reynolds number =R _h V _s/ - R _h hydraulic radius = / - Sc Schmidt number = /D - Sh Sherwood number =kL/D - V_s superficial velocity - gas void fraction - _M porosity of expanded metal - kinematic viscosity - density - electrode area per unit volume - electrode area per unit net area     

Hydrogenolysis of alkanes and olefin polymerization on the silica-supported zirconium hydrides: A comparative DFT study

The model reactions of ethylene polymerization and hydrogenolysis of linear alkanes (propane, n-butane, and n-pentane) on the silica-supported zirconium hydrides (Si–O)₃Zr^IVH, (Si–O)₂Zr^IVH₂, and (Si–O)₂Zr^IIIH were studied using the DFT approach. Catalytic processes under study were shown to occur involving different surface hydrides. The ethylene polymerization was found to proceed at comparable rates on the zirconium monohydrides, (Si–O)₃ZrH, and dihydrides, (Si–O)₂ZrH₂. Cleavage of linear alkanes on the monohydrides (Si–O)₃ZrH is thermodynamically unfavorable; however, the dihydrides (Si–O)₂ZrH₂ can act as catalysts of the process under mild conditions. Hydrides of the trivalent zirconium, (Si–O)₂Zr^IIIH, can also contribute to the hydrogenolysis reaction. A feature of all the systems studied is low regioselectivity of the corresponding processes.     

Self-regulating waves for order n exothermic reactions in condensed media

An estimate is made of the accuracy in determining the wave speed for an exothermic reaction of order 1n3 in a condensed phase by means of the semi-infinite reaction zone and certain other analytical methods. The boundaries have been revised for the existence of steady-state nondegenerate and stable wave states in the activation parameter plane. For n2, a solution has been obtained on the existence and uniqueness of a stationary reaction wave. The semi-infinite zone method and numerical solution of the nonstationary equations for concentration and temperature (1n3) have been used to determine how the characteristic width of the reaction wave varies with the propagation speed as the initial reagent concentration varies.Leningrad Chemical Machinery Research Institute, 193167, St. Petersburg. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 6, pp. 61–68, November–December, 1994.     

Cooperative inclusion of sodium 1-pyrenesulfonate by γ-cyclodextrin

Summary The interaction of -cyclodextrin(-CD) with sodium 1-pyrenesulfonate(PS) was studied spectrophotometrically. -CD was found to cause much larger decrease in the absorption maxima of PS than -CD. The fluorescence spectra of PS in the presence of -CD showed excimer emission, while those of PS with -CD showed only monomer emission, indicating that -CD forms 12 (-CDPS) complexes in which two PS molecules are included in the -CD cavity in a face-to-face fashion. The binding isotherm showed a sigmoidal curve. The association constants were estimated by computer simulation of the binding curve. The 12 (CDPS) complex was found to be much more stable (K=10⁶ M^–1) than the 11 complex (K=1 M^–1). At high concentration of -CD another -CD cooperates in binding two PS molecules, resulting in the formation of a 22 complex.     

Current distribution in a two-dimensional narrow gap cell composed of a gas evolving electrode with an open part

On the basis of the observation of gas bubbles evolved by electrolysis, a two-dimensional vertical model cell composed of electrodes with open parts for releasing gas bubbles to the back side is proposed. The model cell consists of two layers. One layer forms a bubble curtain with a maximum volume fraction of gas bubbles in the vicinity of the working electrode with open parts. The other. being located out of the bubble layer, is a convection layer with a small volume fraction distributed in the vertical direction under forced convection conditions. The cell resistance and the current distribution were computed by the finite element method when resistivity in the back side varied in the vertical direction along the cell. The following three cases for overpotential were considered: no overpotential, overpotential of the linear type and overpotential of the Butler-Volmer type. It was found that the cell resistance was determined not only by the interelectrode gap but also by the percentage of open area and in some cases by the superficial surface area. The cell resistance varied only slightly with the distribution of the bubble layer in the back side.Nomenclature b linear overpotential coefficient given byb=/i - C proportionality constant given by Equation 15 - d ₁ distance between front side of working electrode and separator - d ₂ thickness of separator - F Faraday constant - I total current per half pitch - i current density at working electrode - i ₀ exchange current density - L length of a real electrolysis cell - n number of electrons transferred in electrode reaction - O _p percentage of open area given by Equation 1 - p pitch, i.e. twice the length of the unit cell, defined by 2(BC) in Fig. 4 - q thickness of bubble curtain, defined by (AM) in Fig. 4 - R gas constant - r _t total cell resistance - r unit-cell resistance defined by (V – V _eq)/I - r _rs residue ofr from sum ofr ₀ andr - r ₀ ohmic resistance of solution when0 _p=0 - r resistance due to overpotential when0 _p=0 - s electrode surface ratio or superficial surface area given by Equation 2 for the present model - T absolute temperature - t thickness of working electrode defined by EF in Fig. 4 - V cell voltage - V _eq open circuit potential difference between working and counter electrodes - solution velocity in cell - ₀ solution velocity at bottom of cell - w width of working electrode, defined by 2(DE) in Fig. 4 - x abscissa located on cell model - y ordinate located on cell model - anodic transfer coefficient - linear overpotential kinetic parameter defined byb/[_bc(p/2)] - d infinitesimally small length on the boundary - volume fraction of gas bubbles in cell - dimensionless cell voltage defined bynF(V – V _eq)/RT - overpotential at working electrode - Butler-Volmer overpotential kinetic parameter defined by [nFi _0bc(p/2)]/RT - coordinate perpendicular to boundary of model cell - ₁ resistivity of bubble-free solution - ₂ resistivity of separator - _bc resistivity of bubble curtain - potential in cell