Length scale effects in epoxy: The dependence of elastic moduli measurements on spherical indenter tip radius

Significant increases in the measured elastic moduli with decreasing indentation depth have been previously found in various polymers by indentation tests with a Berkovich tip at micro-to nanometer length scales. These increases in the determined elastic moduli were related to second order displacement gradients which increase with decreasing depth when a conical tip is applied. When a spherical tip is applied, such depth dependence should not be present as the second order displacement gradients remain essentially unchanged with indentation depth. However, these gradients should be proportional to the radius of the spherical tip. To examine the notion of second order displacement gradient dependence in measurements of elastic moduli, indentation experiments are conducted on epoxy with spherical tips of different nominal radii. Accounting for tip imperfections, an increase in the determined elastic moduli is found with decreasing tip radius, which corroborates the notion of second order displacement gradient dependence.     

The dependence of the electronic conductivity of carbon molecular sieve electrodes on their charging states

The dependence of the electronic conductivity of activated carbon electrodes on their potential in electrolyte solutions was examined. Kapton polymer films underwent carbonization (1000 degrees C), followed by a mild oxidation process (CO(2) at 900 degrees C) for various periods of time, to obtain carbons of different pore structures. A specially designed cell was assembled in order to measure the conductivity of carbon electrodes at different potentials in solutions. When the carbon electrodes possessed molecular sieving properties, a remarkable dependence of their conductivity on their charging state was observed. Aqueous electrolyte solutions containing ions of different sizes were used in order to demonstrate this phenomenon. As the average pore size of the activated carbons was larger, their molecular sieving ability was lower, and the dependence of their conductivity on their charging state regained its classical form. This behavior is discussed herein.     

Kinetic Analysis of the Divergence of Reaction Pathways in the Chiral Lewis Base Promoted Aldol Additions of Trichlorosilyl Enol Ethers: A Rapid‐Injection NMR Study

The aldol addition reaction of trichlorosilyl enol ethers and aldehydes with and without chiral Lewis base catalysts has been kinetically analyzed. The uncatalyzed reactions display classic first‐order dependence on each component. The reactions catalyzed by bulky chiral phosphoramide 5 were examined by ReactIR and exhibited first‐order dependence on the catalyst. To examine the kinetic behavior of the reaction catalyzed by phosphoramide 4 , a Rapid‐Injection (RI) NMR apparatus was constructed and employed to allow rapid in‐situ mixing and monitoring of the reaction course. The aldol addition catalyzed by 4 displayed second‐order dependence on phosphoramide, thus providing direct evidence that two catalyzed pathways (with complimentary stereochemical consequences) exist that depend on phosphoramide structure and concentration. Arrhenius activation parameters for all three reactions showed striking characteristics of negligible enthalpies and extremely high entropies of activation. Comparison of these values was precluded by the existence of complex preequilibria in the catalyzed process.     

Landau theory-based estimates for viscosity coefficients of uniaxial and biaxial nematic liquid crystals

Using Landau theory, it is shown that eight phenomenological parameters are needed to describe and distinguish the twelve viscosity coefficients of a biaxial nematic phase, or the five viscosity coefficients of a uniaxial nematic phase. The dependence of the coefficients on the macroscopic uniaxial and biaxial order parameters is established. Since these order parameters are determined by the anisotropies of the dielectric constant, we show that it should be possible to determine values for all eight of the phenomenological parameters of the theory from measurements of the temperature dependence of the five viscosities of a uniaxial phase.     

Kinetics and mechanism of base catalysed ethyl cyanoformate addition to aldehydes

The mechanism by which tertiary amines catalyse the formation of cyanohydrin carbonates from aldehydes and alkyl cyanoformates is investigated by means of a kinetic study. The reaction rate shows a second order dependence on amine concentration unless the amine is sterically hindered, when the rate has a first order dependence on amine concentration. The catalytic activity of the amine correlated with its pK_aH. On the basis of these results, a mechanism is proposed in which the amine acts as a base to activate a water molecule, which reacts with the ethyl cyanoformate generating cyanide in situ.     

Kinetics and mechanism of palladium(II) chloride catalysed oxidation of unsaturated acids by vanadium(V)

Summary The kinetics of the palladium(II) catalysed oxidation of acrylic, methacrylic and crotonic acid by vanadium(V), in acid medium at constant ionic strength exhibit zeroth order dependence on vanadium(V) and first order dependence on palladium(II) and the unsaturated acid. Complex formation between the palladium(II) species and the unsaturated acid, with possible exchange of chloride ion and hydrogen ion in two successive steps, was invoked. The reaction rate is determined by a rearrangement leading to elimination of chloride ion. A plausible mechanism is proposed.     

Kinetics and mechanistic aspects on electron‐transfer process for permanganate oxidation of poly(ethylene glycol) in aqueous acidic solutions in the presence and absence of Ru(III) catalyst

The kinetics and mechanism of oxidation of poly(ethylene glycol) (PEG) by the permanganate ion as a multiequivalent oxidant in aqueous perchlorate solutions at an ionic strength of 2.0 mol dm⁻³ has been investigated spectrophotometrically. The reaction kinetics was found to be of complex in nature. The pseudo–first‐order plots showed curves of inverted S‐shape, consisting of two distinct stages throughout the entire course of reaction. The first stage was relatively slow, followed by a fast reaction rate at longer time periods. The first‐order dependence in [MnO₄⁻], fractional first‐order dependence in [H⁺], and fractional first‐order kinetics in the PEG concentration for the first stage have been revealed in the absence of the Ru(III) catalyst. The influence of the Ru(III) catalyst on the oxidation kinetics has been examined. The oxidation was found to be catalyzed by the added Ru(III) catalyst. The First‐order dependence on the catalyst and zero order with respect to the oxidant concentrations have been observed. The kinetic parameters have been evaluated, and a tentative reaction mechanism consistent with the kinetic results is suggested and discussed.     

Kinetics and mechanism of the oxidation of glycine by N-bromosuccinimide

A systematic kinetic study on the oxidation of glycine by N-bromosuccinimide (NBS) in presence of mercuric acetate in acetic acid—water media has been made. Near first order dependence inNBS and glycine and near inverse first order dependence in hydrogen ion concentrations have been observed. A negligible ionic strength effect and a positive dielectric effect have been observed. Various rate parameters have been computed and hydrocyanic acid identified as the end product. On the basis of the kinetic data, a mechanism of the reaction has been proposed.With 3 Figures     

Kinetics of oxidation of pantothenic acid by chloramine‐T in perchloric acid and in alkaline medium catalyzed by OsO4: A mechanistic approach

Kinetics of oxidation of pantothenic acid (PA) by sodium N‐chloro‐p‐toluenesulfonamide or chloramine‐T (CAT) in the presence of HClO₄ and NaOH (catalyzed by OsO₄) has been investigated at 313 K. The stoichiometry and oxidation products are same in both media; however, their kinetic patterns were found to be different. In acid medium, the rate shows first‐order dependence on [CAT]_o, fractional‐order dependence on [PA]_o, and inverse fractional‐order on [H⁺]. In alkaline medium, the rate shows first‐order dependence each on [CAT]_o and [PA]_o and fractional‐order dependence on each of [OH^?] and [OsO₄]. Effects of added p‐toluenesulfonamide and halide ions, varying ionic strength, and dielectric constant of medium as well as solvent isotope on the rate of reaction have been investigated. Activation parameters were evaluated, and the reaction constants involved in the mechanisms have been computed. The proposed mechanisms and the derived rate laws are consistent with the observed kinetics. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 201–210, 2005     

SAXS instrumental broadening and the order dependence of peak width

The width at half-height of SAXS discrete diffraction peaks has been used to compute the number of polyethylene lamellae in a stack. The dependence of this width on diffraction order reflects the nature and magnitude of lattice fluctuations. Within the literature there have been conflicting reports on this order dependence and hence on the nature of the lattice fluctuations. Previous studies have neglected the effects of instrumental broadening. These present studies show that instrumental effects can account for some 40% of the observed first-order peak width and drastically change the ratio of peak widths as a function of order. These studies, carried out on isothermally grown polyethylene single-crystal mats, also demonstrate the importance of the functions chosen to represent the various broadening factors. A mat was made up consisting of randomly stacked lamella with two distinct fold periods. The scattering from this mixed mat could not be described by either of the prevailing theories.     

Kinetics and mechanism of esterification of epoxy resin in presence of triphenylphosphine

The kinetics of esterification of bisphenol‐A based epoxy resin with acrylic acid in presence of triphenylphosphine has been studied. The reaction exhibits a first‐order rate dependence with respect to [Epoxy] and [Catalyst]. A first‐order dependence of rate with respect to [Acid] has been observed during a particular kinetic run. However, a retarding effect of [Acid] on the rate has been observed by increasing the initial concentration of acid. A suitable mechanism consistent with the kinetic data is proposed and discussed. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 280–285, 2004     

Oxidation of 3‐(3,4‐dihydroxy phenyl)‐l‐alanine (levodopa) and 3‐(3,4‐dihydroxy phenyl)‐2‐methyl‐l‐alanine (methyl dopa) by manganese(III) in pyrophosphate media: Kinetic and mechanistic study

Manganese(III) (Mn(III)) has been stabilized in weakly acidic solution by means of pyrophosphate and the nature of the complex was elucidated spectrophotometrically. Stoichiometry of Mn(III)‐oxidation of levodopa and methyl dopa in pyrophosphate medium was established in the pH range 2.5–4.0 by iodometric and spectrophotometric methods. The reaction shows a distinct variation in kinetic order with respect to [Mn(III)], a first‐order dependence in the pH range 1.9–2.6, decreasing to fractional order above pH 3. Other common features include first‐order dependence on [dopa], positive fractional order dependence on [H⁺], and inverse first‐order dependence on [Mn(III)] in the pH range studied. The effects of varying ionic strength and solvent composition were studied. Added ions such as SO₄^2? and ClO₄^? alter the reaction rate, probably due to the change in the formal redox potential of Mn(III)–Mn(II) couple because of the changes in coordination environment of the oxidizing species. Evidence for the transient existence of the free radical intermediate is given. Cyclic voltametric sensing of levodopa and methyl dopa has ruled out the formation of dopaquinones as oxidation products in the pH range studied. Activation parameters have been evaluated using the Arrhenius and Erying plots. Mechanisms consistent with the kinetic data have been proposed and discussed. These studies are expected to throw some light on dopa metabolism. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 449–457, 2001     

Kinetic study of ruthenium(VI) -- catalyzed oxidation of 2-methoxy- ethanol by aqueous alkaline hexacyanoferrate(III)

The kinetics of ruthenium(VI) catalyzed oxidation of 2-methoxyethanol by hexacyanoferrate(III) ion in an aqueous alkaline medium at constant ionic strength shows zero order dependence on hexacyanoferrate(III) and first order dependence on Ru(VI). Dependence of substrate concentration shows a Michaelis – Menten type behaviour. The rate increases with the decrease in alkali concentration. A reaction mechanism involves the formation of an intermediate complex between the substrate and ruthenium(VI). This complex decomposes slowly, producing ruthenium(IV), which is reoxidized by hexacyanoferrate(III) in subsequent steps. The theoretical rate law obtained is in complete agreement with the experimental observations.     

Investigation of the viscoelastic properties of 4-propoxy-biphenyl-4-carbonitrile

ABSTRACT

We report on the temperature-dependent measurements of dielectric permittivity, birefringence, elastic constants and rotational viscosity for 4-propoxy-biphenyl-4-carbonitrile in the nematic region. The temperature dependence of the three elastic constants was determined from studies of the Freédericksz transition. The thermal dependence of elastic constants shows features similar to the literature (bend > splay > twist). Elastic constants are proportional to the square of the order parameter. Temperature-dependent dielectric characterisation was carried out at a frequency of 10 kHz. The compound shows positive dielectric anisotropy in the nematic phase. The rotational viscosity is found to be relatively low. Temperature dependence of order parameter is estimated using Haller’s method. The figure of merit was also calculated as a function of temperature.     

Radiation-induced graft polymerization of acrylic acid onto fluorinated polymers. I. Kinetic study on the grafting onto poly(tetrafluoroethylene-ethylene) copolymer

Direct radiation-induced grafting of aqueous acrylic acid (AAc) onto poly(tetrafluoroethylene-ethylene) (ET) film has been studied. The effect of grafting conditions such as monomer concentration, exposure dose, dose rate, and film thickness on the grafting yield was investigated. The dependence of the grafting rate on monomer concentration was found to be 1.2 order. The dependence of the grafting rate on dose rate was found to be 0.6 order regardless of the film thickness. The relationship between the grafting rate and film thickness gave a negative first-order dependence. The results suggest that the grafting process is mainly controlled by monomer diffusion, and it was concluded that this grafting system proceeded by the front mechanism. The swelling behavior increases linearly with degree of grafting. The electrical conductivity and mechanical properties for the trunk and grafted polymer were investigated at different irradiation doses in air and under vacuum irradiations. © 1992 John Wiley & Sons, Inc.     

Nuclear spin dynamics using time-dependent projection operators: application to the saturation of dipolar order in slowly rotating samples

An extension of the projection operators method is presented by considering explicit time-dependent projection operators. The usefulness of the present formalism is demonstrated by an investigation of nonadiabatic corrections to the evolution of a many-body system under a slow motion. A theoretical and experimental study of the saturation of nuclear spins dipolar order induced by a slow sample rotation is presented. Theoretically, the master equation of the dipolar order beyond the limit of an adiabatic evolution is established. It is shown how the time dependence of the projection operators is related to saturation of the dipolar order. A formal expression of the saturation rate is derived and its dependence upon the angle between rotation axis and external magnetic field is derived. Comparison with experimental data obtained on polycrystalline adamantane validates our theoretical approach.     

The effect of pretransitional phenomena on the characteristics of induced ferroelectric liquid-crystalline systems

The effect of different types of phase transition on the temperature dependence of the helical pitch and the spontaneous polarization is investigated for mixtures of smectic C liquid crystals and a chiral dopant, based on salicylidenanylene derivatives. The observed phenomena are explained by assuming that the temperature dependence of the pitch is connected with the growth of the disclination loops near the second order phase transition, and that the subsurface electric field can unwind the helix in chiral substances with strong piezoelectric effects which occur for large polarization values.     

Gamma-ray yield dependence on bulk density and moisture content of a sample of a PGNAA setup: A Monte Carlo study

Monte Carlo calculations were carried out to study the dependence of γ-ray yield on the bulk density and moisture content of a sample in a thermal-neutron capture-based prompt gamma neutron activation analysis (PGNAA) setup. The results of the study showed a strong dependence of the γ-ray yield upon the sample bulk density. An order of magnitude increase in yield of 1.94 and 6.42 MeV prompt γ-rays from calcium in a Portland cement sample was observed for a corresponding order of magnitude increase in the sample bulk density. On the contrary the γ-ray yield has a weak dependence on sample moisture content and an increase of only 20% in yield of 1.94 and 6.42 MeV prompt γ-rays from calcium in the Portland cement sample was observed for an order of magnitude increase in the moisture content of the Portland cement sample. A similar effect of moisture content has been observed on the yield of 1.167 MeV prompt γ-rays from chlorine contaminants in Portland cement samples. For an order of magnitude increase in the moisture content of the sample, a 7 to 12% increase in the yield of the 1.167 MeV chlorine γ-ray was observed for the Portland cement samples containing 1 to 5 wt.% chlorine contaminants. This study has shown that effects of sample moisture content on prompt γ-ray yield from constituents of a Portland cement sample are insignificant in a thermal-neutrons capture-based PGNAA setup.     

The effect of pretransitional phenomena on the characteristics of induced ferroelectric liquid-crystalline systems

The effect of different types of phase transition on the temperature dependence of the helical pitch and the spontaneous polarization is investigated for mixtures of smectic C liquid crystals and a chiral dopant, based on salicylidenanylene derivatives. The observed phenomena are explained by assuming that the temperature dependence of the pitch is connected with the growth of the disclination loops near the second order phase transition, and that the subsurface electric field can unwind the helix in chiral substances with strong piezoelectric effects which occur for large polarization values.