Variation of glass transition temperature with direction in unidirectional glass fiber-reinforced composites

Summary A theoretical expression of the value of the glass-transition temperatureT _g at any direction of orientation, defined by the angle subtended between this direction and the average axis of fibers; was introduced. Dilatometric, as well as dynamic tests were executed for a type of unidirectional composite in order to compare the respective values ofT _g derived from these tests. The experimental results were also compared with theoretical values derived from the theoretical expression. A satisfactory agreement between, theory and experiment was established.With 5 figures and 1 table     

Hydrocarbon long-range interaction coefficients from point charge model formulae

Two-body interaction coefficientsC6 andd4 and the three-body coefficient 3 are calculated for interactions between like species for a variety of hydrocarbons. Three different point charge models are employed and results are similar when Frost model Lewis set wavefunctions are used and in agreement with other literature values where available. We also consider simple Slater-Kirkwood type formulae for the coefficients and it turns out that very similar results to theab initio model values are found ford4. The formulae can also be used in conjunction with molecular fragment wavefunctions.     

Non-additivity in water-ion-water interactions

The three-body system Li⁺(H₂O)₂ was analyzed to study that non-additive part of the interaction potential which can be obtained by the Hartree-Fock approximation.For long and intermediate distances the three-body correction was found to be well represented by the induction energy, where bond dipoles are induced on each water molecule by point charges located on the (unpolarizable) lithium ion and on the other molecule respectively: for shorter distances this approximation was corrected by means of an exponential repulsive term. Such a potential model for non-additive interactions was extended to the more general situation Li⁺(H₂O)_n, and Monte-Carlo calculations were carried out on clusters containing up to six water molecules; comparison with other simulation results and with available data showed a significantly improved agreement with experiment. Tentative values for H are presented for n =7, 8,..., 20, where experimental data are not available.     

Formal Mathematical Analysis of the Existence of the Common Intersection Point in Relation to Determining the Parameters Describing Ion Adsorption at the Oxide/Electrolyte Interface: Comparison of the Triple and Four-Layer Models

For most oxide/electrolyte systems potentiometric titration curves measured for different ionic strengths have a Common Intersection Point (CIP) which corresponds to the Point of Zero Charge (PZC). However, there are systems where a CIP exists but the surface charge at this point does not equal zero (PZC CIP). In this paper theoretical analysis of the systems in which the PZC and CIP do not coincide is presented. It is based on the well-known 2-pK surface charging approach and Triple Layer Model (TLM) as well as the Four Layer Model (FLM) of the electric double layer. The appropriate mathematical criterion for CIP existence was applied with detailed derivations, both for TLM and FLM. Having determined in this manner the parameter values, one can draw proper conclusions about the features of oxide/electrolyte adsorption systems, in which PZC and CIP do not coincide. The values of adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both of the experimental titration isotherms, and to the individual isotherms of electrolyte cation adsorption measured using radiometric methods.     

A theoretical study of 1-amino-3-butene and 3-butene-1-thiol

Conformational analysis of 1-amino-3-butene and 3-butene-1-thiol was carried out using the 4-21G basis set. The conformers obtained were subjected to 6-31G^* single-point analysis for the calculation of energies, charge distributions, and dipole moments. The geometries and stabilities obtained are in good agreement with available experimental data. The results are interpreted in terms of intramolecular hydrogen bonding and anomeric interactions: Some of the most stable conformers of both molecules have intramolecular hydrogen bonds between the hydrogens of the amino or thiol groups and the electrons of the double bond. The 4-21G geometries were refined to obtain rotational constants closer to the experimental values.     

Comparison between different ways to determine diffusion coefficient and by solving Fick’s equation for spherical coordinates

The following document covers the determination of the diffusion coefficient of two powder materials: LiFePO₄ and LiMn₂O₄ by using potentiostatic intermittent titration technique (PITT) and impedance spectroscopy methodology and compares relevant results with the following relation: , which is obtained by solving Fick’s spherical coordinate equation (where I ₀ is the initial step current in the PITT experiment, R is the particle radius, Q is the charge that intercalated during the step, and α is the percentage of the theoretical intercalated charge). This procedure allowed the verification of the validity of the spherical model for the powder materials, the accuracy of the expression proposed for the diffusion coefficient determination, and the correctness of the measures that had been taken.     

Interaction of monovalent and divalent counterions with some carboxylic polysaccharides

Eleven samples of carboxylic polysaccharides were studied. The activity coefficients γ have been measured for monovalent (Na⁺) and divalent (Ca²⁺) counterions. There is no specific influence of the structure of the chain on γ values. Agreement with theoretical values confirms the rigidity of the chain; for low charge density, the theoretical treatment seems to be incorrect. Selectivity is discussed in term of selectivity coefficient K and free energy of exchange ΔG; ΔG is linearly related to the charge density but K which characterizes the competition of the two counterions is sensitive to the nature of the chain. The carboxymethylamyloses present a larger selectivity whose origin is not discussed here. The last point treated is the intrinsic constant of dissociation of polyacids. The pK₀ values are practically independent of the nature of the polyelectrolyte and of the charge density; the values are close to the pK₀ of monomeric unit and are not affected by the position of ? COOH in the anhydroglucose ring.     

Charge carrier photogeneration on some substituted polyacetylenes

The photogeneration of charge carriers was studied with the following polymers: poly-[N-(2-propinyl)-phenothiazine] (PPPT) and copolymers of N-(2-propinyl)-carbazole with N-(2-propinyl)-phenothiazine (PCz+PPT) and N-(2-propinyl)-carbazole with phenylacetylene (PCz+PA). In the case of PCz+PA, the experimentally found dependence of the photogeneration efficiency on the strength of an externally applied electric field could be well fitted with the curve calculated on the basis of Onsager's model of geminate recombination. In the cases of PPPT and PCz+PPT, on the other hand, the experimental values deviated strongly from the theoretical curve at field strengths between 10⁶ and 10⁷ V/m.Equal values for the separation distancer ₀ and the primary charge carrier yield ₀ were found for all polymers:r ₀=2.0 nm and ₀=0.20 at _inc=254 nm;r ₀=2.5 nm and ₀=0.15 at _inc=355 nm.With PPT and PCz+PPT a strong dependence of the electric resistance on the humidity content of surrounding air was observed.     

MM3(96) CONFORMATIONAL ANALYSIS OF d-GLUCARAMIDE AND X-RAY CRYSTAL STRUCTURES OF THREE d-GLUCARIC ACID DERIVATIVES—MODELS FOR SYNTHETIC POLY(ALKYLENE d-GLUCARAMIDES)

An exhaustive conformational analysis of d-glucaramide was carried out using MM3(96) [MM3(96). Molecular Mechanics Software used with permission from N.L. Allinger; University of Georgia]. Nine torsion angles were each driven in increments of 120°, generating 19,683 starting conformations. Each conformation was then fully energy-minimized using MM3's block diagonal/full matrix optimization option at dielectric constants of both 3.5 and 6.5. Conformer populations were calculated based on the modeling results and calculated theoretical average ¹H vicinal coupling constants were compared to experimental values obtained in D ₂O solution. Crystal structures of three acyclic d-glucaric acid derivatives (N, N′-dimethyl- d-glucaramide, dipotassium d-glucarate, and sodium potassium d-glucarate) are reported. These structures and that of previously reported monopotassium glucarate correspond closely with model conformations that were within one kcal/mol of the global minimum.     

Rational Design of Poly(2,7‐Carbazole) Derivatives for Photovoltaic Applications

A reliable model that can be used to estimate the electronic properties (i.e., the HOMO, LUMO, and band gap energies) of conjugated polymers would be a great tool for applications in organic electronics such as light‐emitting diodes, field‐effect transistors, and photovoltaic cells. Recently, poly(2,7‐carbazole) derivatives have shown promising results when used as an active donor layer in bulk heterojunction photovoltaic cells with power conversion efficiency exceeding 7%. By using a simple correlation between density functional theory (DFT) theoretical calculations performed on six model compounds (using the repeating unit) and experimental data from the six corresponding polymers, an accurate estimation of the HOMO energy level, the LUMO energy levels, and the band gap of several poly(2,7‐carbazole) derivatives was obtained. According to the theoretical data obtained for more than one hundred repeating units, fourteen new copolymers that can be used as p‐type materials in bulk heterojunction solar cells were selected and synthesized. Experimental data obtained from these materials were then used to refine the correlation between DFT and experimental data of poly(2,7‐carbazole) derivatives.

     

Potentiometric Titrations of Rutile Suspensions to 250°C

A stirred hydrogen electrode concentration cell was used to conduct potentiometric titrations of rutile suspensions from 25 to 250°C in NaCl and tetramethylammonium chloride media (0.03 to 1.1m). Hydrothermal pretreatment of the rutile improved titration reproducibility, decreased titration hysteresis, and facilitated determination of the point of zero net proton charge (pHznpc). These pHznpc values are 5.4, 5.1, 4.7, 4.4, 4.3 (±0.2 pH units), and 4.2 (±0.3 pH units) at 25, 50, 100, 150, 200, and 250°C, respectively. The difference between these pHznpc values and pK_w(the neutral pH of water) is rather constant between 25 and 250°C (−1.45 ± 0.2). This constancy is useful for predictive purposes and, more fundamentally, may reflect similarities between the hydration behavior of surface hydroxyl groups and water. A three-layer, 1pKa surface complexation model with three adjustable parameters (two capacitance values and one counterion binding constant) adequately described all titration data. The most apparent trend in these data for pH values greater than the pHznpc was the increase in proton release (negative surface charge) with increasing temperature. This reflects more efficient screening by Na⁺relative to Cl⁻. Replacing Na⁺with the larger tetramethylammonium cation for some conditions resulted in decreased proton release due to the less efficient screening of negative surface charge by this larger cation.     

Structural,mechanical, electronic and thermal properties of KZnF3 and AgZnF3 Perovskites: FP-(L)APW+lo calculations

This study presents a theoretical prediction of the structural, mechanical, electronic and thermal properties of the zinc-based Perovskites (AgZnF₃ and KZnF₃) within the framework of Density Functional Theory (DFT) using All-electron self consistent Full Potential Augmented Plane Waves plus local orbital FP-(L)APW + lo method. To make our work comparable and reliable, several functional were used for the exchange-correlation potential. Also, this study intends to provide a basis and an improvement for updating either the values already predicted by other previous work (by using obsolete functional) or to predict them for the first time. GGA-PBE and GGA-PBEsol were used to predict the structural properties of AgZnF₃ and KZnF₃ Perovskites such as lattice parameter, bulk modulus and its pressure derivative and the cohesive energy. For these properties, the found values are in very good agreement; also those found by GGA-PBEsol are closer to other available previous and experimental results. The electronic properties of these materials are investigated and compared to provide a consolidated prediction by using the modified Becke Johnson potential TB-mBJ with other functional; the values found by this potential are closer to the available proven results and show that these materials exhibit an indirect gap from R to Γ point. The charge densities plot for [110] direction and QTAIM (Quantum Theory of Atoms in Molecules) theory indicate that ionic character is predominate for (K, Ag, Zn)F bonds. Finally, the effect of temperature and pressure on the unit cell volume, the heat capacity C_V and entropy were studied using the quasi-harmonic Debye model.     

Effect of molecular size on the threshold energy producing a51Cr hot atom

The lowest retention values of⁵¹Cr hot atom in various forms of the -diketonetype complexes, Cr(bzac)₃, Cr(dpm)₃ and Cr(dbm)₃, as well as Cr(acac)₃ as a reference, were obtained in the experimental condition in which they are regarded as the primary retentions. From these retention values, the threshold energies for producing a hot atom, the appearance energies, were estimated using the calculated relationship between the primary retention and appearance energy, which was obtained by the recoil energy spectra computed by the method recently developed by us. The appearance energy increased steeply with increasing molecular weight of the complex. A theoretical interpretation for this result was attempted by setting a model in which a hot atom is produced in the center of the complex and receives the stopping action by the surrounding ligand molecule. The agreement between the observed and calculated values was good.     

Optical isotope shifts and changes in nuclear charge radii of stable Ti isotopes

Isotope shifts and hyperfine structures in three optical transitions of TiI have been investigated by using laser induced resonance fluorescence in a collimated atomic beam. From the isotope shifts data, changes of the mean square charge radii of the stable titanium isotopes have been determined for the first time. Using a combined analysis with muonic atom data on root mean square nuclear charge radii, improved model independent r ^21/2 values for the odd-even Ti isotopes have been obtained.     

Orbital alignment dependence of electron transfer cross sections

This paper reports experimental and theoretical results for the influence of target excitation and orbital alignment on the charge exchange process for the system H⁺-Na(3s, 3p). The experimental velocity range covers for the first time the region around and beyond the matching velocity of 0.47 a.u. of the Na(3p) state, i.e. the velocity characteristic of the orbital motion of the valence electron. The cross section parameters are found to depend sensitively on collision velocity, with a qualitatively different behaviour below and above the matching velocity. The results for the orbital alignment dependence support the intuitive picture that, when going beyond the matching velocity, electron transfer becomes increasingly favoured when the orbital velocity of the active electron has a component parallel to the collision velocity. Agreement with earlier experimental and theoretical results at lower velocities is good and allows conclusions about the quality of theoretical approximations in the various velocity regimes.     

Ionic association in aqueous solutions of strong electrolytes

A more accurate calculation of relaxation effects obtained with the standard Debye-Hückel-Onsager model has been presented recently and is here applied to several aqueous 1:1 electrolytes. The variation of the standard deviation between calculated and observed equivalent conductivities withK _A leads to an ill-defined minimum; but, where data over a wide concentration range are available, the minimum corresponds to values of the contact distancea which approximate to estimates from ionic dimensions. It is therefore proposed that, although preciseK _A values from conductance cannot be determined, the most probable values are those associated with realistic estimates ofa. When data cover a limited concentration range, minimum standard deviations are often indeterminate or vary greatly for duplicate runs. It is shown that reasonable values ofK _A can be obtained from such data if comparison is made at estimated values ofa.Notation The symbols not defined in the text are the following b e ²/kTa for 1:1 electrolytes - e electronic charge - k Boltzmann gas constant - T absolute temperature - dielectric constant of solvent - –(3/2y)(_e0/₀)     

A Thermodynamic Analysis of Ion Adsorption in the Metal Oxide/Electrolyte Systems in which PZC and CIP do not Coincide

A theoretical-numerical analysis of two adsorption systems composed of the same kind of oxide- TiO₂, and of two different electrolytes, NaCl and CsCl is presented. For one kind of the electrolyte (NaCl), PZC and CIP coincide, whereas they are different for the other (CsCl) electrolyte. The analysis is carried out by applying the popular TLM model, and by drawing formal-mathematical consequences of CIP existence in both kinds of adsorption systems. The values of the adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both experimental titration isotherms, and to the individual isotherms of cation adsorption measured using radiometric methods. That theoretical-numerical analysis suggests, that the inequality PZC (pK ^int _a1 + pK ^int _a2) may be a general feature of the oxide/electrolyte systems including the systems in which PZC and CIP coincide.     

Nonlinear Impedance of Liquid In–Ga Electrode in Aqueous Solutions of a Symmetrical Surface-Inactive Electrolyte

Experimental data related to the potential dependence of nonlinear characteristics of the electrical double layer at a liquid In–Ga electrode in aqueous solutions of a symmetrical surface-inactive electrolyte are obtained for the first time. It is shown that, as opposed to polycrystalline Cd and Pb electrodes, on a liquid (atomically smooth) In–Ga electrode, as on Hg, there is a clear intersection of the potential dependences of a nonlinear signal for different concentrations of a 1–1-valence surface-inactive electrolyte at one point. The intersection point exactly corresponds to the potential of zero charge of an electrode undistorted by specific adsorption of ions. It is established that, when estimating hydrophilicity of metals by a nonlinear impedance method, most information is provided by the region of average negative charges, rather than by the region near zero charge. It is shown that, as opposed to a linear impedance method, the nonlinear impedance method makes it possible to determine, directly from experiment, quantities that directly characterize the metal–solvent chemisorption interaction in a pure form; at the same time, these quantities are criteria of lyophilic nature of metals. Quantities that characterize the metal–solvent chemisorption interaction, obtained by the linear and nonlinear impedance methods are in good agreement, which confirms the validity of the approach we proposed earlier for separating the difference between reciprocal capacitances of the inner part of the electrical double layer on Hg and metal M, ^Hg _M C ^-1 _i= 1/C ^Hg _i– 1/C ^M _iinto physical (^Hg _M C ^–1)_physand chemical constituents. This coincidence also confirms correctness of numerical values obtained earlier for quantities (^Hg _M C ^–1)_phys.     

Polyolefin Macroradical Properties: A Semiempirical Quantum Chemistry Study

Summary: Semiempirical quantum chemistry simulations have been used to estimate the properties of mid‐chain alkyl radicals of ethylene, propylene and styrene oligomers depending on the chain length. The values of spin density and charge on the radical site proved to be almost unchanged for oligomer radicals having from 2 to 11 repeated units. Bond strength parameters of bonds neighboring the radical site demonstrate stable values starting from pentamers. The reliable inference is that the electron structure of polyethylene, poly(propylene) and polystyrene (PS) macroradicals may be simulated by means of relatively short oligomers. The obtained data indicate clearly that the polystyrene tertiary alkyl radical has noticeably decreased values of both spin and charge onto the radical site as well as very decreased strength of β‐bonds. For that reason the PS tert‐alkyl radical is estimated as that having decreased activity and increased susceptibility to chain scission. The most probable cause is delocalization of the free electron onto the neighboring aromatic ring.

Spin density distribution in tertiary radical of polystyrene.