Power law of nucleation rate of folded-chain single crystals of polyethylene

The number-average molecular weight (M _n) dependence of the primary nucleation rate (I) of polyethylene (PE) folded-chain single crystals was studied in the ordered phase. We observed that the M _n dependence of I is mainly controlled by the diffusion process of polymer chains within the interface between a nucleus and the melt and/or within the nucleus. The results show that I decreases with increasing M _n and follows a power law I∝M _n ^−2.3 for the ordered phase. It is named the power law of the nucleation rate. In a previous article we showed that for the disordered phase I∝M _n ⁻¹. In this article, we concluded that I decreases with increasing M _n and follows a universal power law, I∝M _n ^−H for both ordered and disordered phases. The power H depends on the degree of order of the crystalline phase, which is related to the morphology. Received: 13 September 2000 Accepted: 15 November 2000     

Dynamics of the surface layer in cyanobiphenyl–aerosil nanocomposites with a high silica density

Composites were prepared from an aerosil and 4-n-alkyl-4′cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of ～7?g aerosil in 1?cm³ of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10^?2–10⁹?Hz) in a large temperature range (220–370?K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel–Fulcher–Tammann law, which is found to be typical for glass-forming liquids. The quasi two-dimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd–even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface).     

ELECTRICAL CONDUCTIVITY OF IODINE DOPED POLY(PHTHALOCYANINATOSILOXANE)/PPTA BLEND FIBERS

Poly(phthalocyaninatosiloxane), [Si(Pc)O]n, with the number average degree of polymerization of about 130 was prepared by heating its monomer Si(Pc) (OH)_2, in solid state at 420℃for 42 hrs at 10~(-3) torr dynamic vacuum. The [Si(Pc)O]n powder was iodine doped with I_2-bensene solution for 48 hrs. Pure iodine doped poly(phthalocyaninatosiloxune), {[Si(Pc)O]I_y}n, fibers and {[Si(Pc)O]I_y}n/poly(p-pbenylene terephthalamide) blend fibers were wet-spun with dry nltrogen-sealed Teflon lined device. D.C. electrical conductivity of the fibers was measured by the four-probe method with an automated charge transport measurement system from 80K to room temperature. It was found that the dependence of conductivity, σ, on temperature, T, could fit a group of thermal fluctuation-induced tunnelling (TFIT) equations, and that the dependence of conductivity on volume fraction, φ, of the iodine doped {[Si(Pc)O]I_y}n could fit a group of modified percolation equations. A thrce-dimensioual composite plot of σ-1 / T-φshows that these two groups of equations match each other quite well. It has been pointed out that for the blend fibers their composition is the most important factor for both mechanical and electrical properties.     

The mechanism in the elimination kinetics of 2-chloropropionic acid in the gas phase

The elimination kinetics of 2-chloropropionic acid have been studied over the temperature range of 320–370.2°C and pressure range of 79–218.5 torr. The reaction in seasoned vessel and in the presence of the free radical suppressor cyclohexene, is homogeneous, unimolecular, and obeys a first-order rate law. The dehydrochlorination products are acetaldehyde and carbon monoxide. The rate coefficient is expressed by the following Arrhenius equation: log k₁(s^?1) = (12.53 ± 0.43) – (186.9 ± 5.1) kJ mol^?1 (2.303RT)^?1. The hydrogen atom of the carboxylic COOH appears to assist readily the leaving chloride ion in the transition state, suggesting an intimate ion pair mechanism operating in this reaction.     

Synthesis and magnetic properties of LaSr2CoMnO7

A new oxide phase LaSr₂CoMnO_7.00±0.05, a member of the Raddlesden-Popper homologous series A _{n + 1}B _n O_{3n + 1} (n = 2), was synthesized, and its structure and magnetic properties characterized. The unit cell of this phase was indexed in terms of space group I4/mmm with the parameters a = 0.3831(2) nm, c = 2.0070(1) nm. The magnetic susceptibility as a function of temperature obeys the Curie-Weiss law in the range 250–350 K. At low temperatures, magnetic anomalies are observed due to the competition between antiferromagnetic and ferromagnetic ordering and spin-glass transition. The charge state of transition metals was ascertained proceeding from magnetic measurements and XANES spectra.     

Thiotropolone as a Chromogenic Reagent for the Microdetermination of Copper(II)

Thiotropolone forms a brown water-insoluble complex with copper(II), extracttable into chloroform and other non-polar solvents. The 1:2 complex shows maximum absorption at 450 nm and obeys Beer's law upto 4.55 ppm. The sensitivity of the reaction is 0.0046 μg Cu(II)/cm² for log I₀/I=0.001 with molar absorptivity of 1.36×10⁴. The usefulness of the procedure has been enhanced to the determination of copper in various alloys and synthetic mixtures.     

Rate constant of the recombination of free electrons and holes in AgCl at 295 K

The transient kinetics of the loss of electrons generated by light pulses in powdered AgCl has been studied by the microwave photoconductivity method (36 GHz) at 295 K. At high light intensities,I ₀ > 10¹⁴ photon cm^–2 per pulse, the kinetics obeys the second-order law. The rate constant of the recombination of free electrons and holes is equal to 2·10^–12 cm³ s^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2234–2236, September, 1996.     

Investigation of transitions and relaxation processes in polystyrene by using positron annihilation

The glass transition and relaxation processes in polystyrene resins with the number average molecular weight ranging from 7.0·10² to 9.8·10⁴ were studied with the positron annihilation technique. The pick-off annihilation lifetime of ortho-positronium (₃) and its intensity (I ₃) were measured in the temperature range from 20 to 430 K. The glass transition temperature (T _g) was determined as an onset temperature coefficient of ₃.T _g shows the molecular weight dependence in these samples. BelowT _g, local motions were detected by measurements ofI ₃. The local motions could be observed above 100 K in this experiment.I ₃ show the minimum at around 250 K and it does not show molecular weight dependence.     

Rate constants for the decarboxylation of the t-butoxicarbonyl radical and concurring radical terminations directly obtained by kinetic ESR

Rate constants of various simultaneous reactions of t-butoxicarbonyl and t-butyl radicals generated by photolysis of t-butylpivalate in n-heptane are directly determined by kinetic electron spin resonance. The temperature dependence of the decarboxylation reaction t-BuO?O → t-Bu^. + CO₂ obeys log )K/S^?1( = 13.8?49.0/θ where θ = 2.303 RT/kJ ^. mol^?1. The self- and cross-termination of the radicals are diffusion limited.     

Spectrophotometric determination of thallium as thallium tetriodide

Iodide ions react with thallic ions at pH 2–8 to form a complex iodide, which is suitable for the spectrophotometric determination of thallium. The reaction has a sensitivity of 0.05 μg Tl per cm² for log $\text{I}{}_0\text{I}$ = 0.001 and obeys Beer's law up to 40 p.p.m. Optimum conditions for the reaction have been established. The standard deviation is 0.6%. The effects of temperature and pH, the ratio of thallium to reagent, stability of the complex, its conformity to Beer's law, and the rate of color formation were studied. The effect of many diverse ions was examined.     

A Theoretical Investigation into the Luminescent Properties of d8‐Transition‐Metal Complexes with Tetradentate Schiff Base Ligands

A theoretical investigation on the luminescence efficiency of a series of d⁸ transition‐metal Schiff base complexes was undertaken. The aim was to understand the different photophysics of [M‐salen]ⁿ complexes (salen=N,N′‐bis(salicylidene)ethylenediamine; M=Pt, Pd (n=0); Au (n=+1)) in acetonitrile solutions at room temperature: [Pt‐salen] is phosphorescent and [Au‐salen]⁺ is fluorescent, but [Pd‐salen] is nonemissive. Based on the calculation results, it was proposed that incorporation of electron‐withdrawing groups at the 4‐position of the Schiff base ligand should widen the ³MLCT–³MC gap (MLCT=metal‐to‐ligand charge transfer and MC=metal centered, that is, the dd excited state); thus permitting phosphorescence of the corresponding Pd^II Schiff base complex. Although it is experimentally proven that [Pd‐salph‐4E] (salph=N,N′‐bis(salicylidene)‐1,2‐phenylenediamine; 4E means an electron‐withdrawing substituent at the 4‐position of the salicylidene) displays triplet emission, its quantum yield is low at room temperature. The corresponding Pt^II Schiff base complex, [Pt‐salph‐4E], is also much less emissive than the unsubstituted analogue, [Pt‐salph]. Thus, a detailed theoretical analysis of how the substituent and central metal affected the photophysics of [M‐salph‐X] (X is a substituent on the salph ligand, M=Pt or Pd) was performed. Temperature effects were also investigated. The simple energy gap law underestimated the nonradiative decay rates and was insufficient to account for the temperature dependence of the nonradiative decay rates of the complexes studied herein. On the other hand, the present analysis demonstrates that inclusions of low‐frequency modes and the associated frequency shifts are decisive in providing better quantitative estimates of the nonradiative decay rates and the experimentally observed temperature effects. Moreover, spin–orbit coupling, which is often considered only in the context of radiative decay rate, has a significant role in determining the nonradiative rate as well.     

Synthesis and magnetic properties of the complex oxide La<Subscript>1.5</Subscript>Sr<Subscript>1.5</Subscript>CuMnO<Subscript>6.67</Subscript>

A new oxide phase, La_1.5Sr_1.5CuMnO_6.67, related to the Ruddlesden-Popper homologous series A _{n + 1}B _n O_{3n + 1} (n = 2) was synthesized. The magnetic susceptibility of the obtained compound obeys the Curie-Weiss law above 114 K. At 14 K, a magnetic anomaly related to the transition to the spin-glass state was detected.     

Effect of alkyl substituents structures and added ions on the phase transition of polymers and gels prepared from methyl 2‐alkylamidoacrylates

The effects of structure alteration of alkyl groups and addition of ions to solutions of new thermosensitive polymer series of poly(methyl 2‐alkylamidoacrylate)s were investigated. Potential advantages of the thermosensitive polymers are their simplicity of functionalization and transition temperature control that result from their unique α,α‐disubstituited structures. Poly(methyl 2‐propionamidoacrylate) (PMPA) and poly(methyl 2‐isobutyracrylate) (PMIBA) were thermosensitive polymers, and poly(methyl 2‐acetamidoacrylate) and poly(methyl 2‐n‐butyramidoacrylate) were completely water‐soluble and ‐insoluble, respectively. The PMIBA solution showed endotherm during the phase transition, while endotherm was not detected for PMPA. The difference between the two polymers resulted from the size of the hydrophobic groups. MPA gel was prepared by copolymerization with N,N′‐methylenebis(acrylamide) and temperature‐induced volume change of the gel was continuous. The salting‐out effect of inorganic ions on PMPA solution and MPA gel followed the Hofmeister series. The inorganic (I^? and SCN^?) and organic ions (ⁿPr₄N⁺ and ⁿBu₄N⁺), which showed the salting‐in effect, were indicated to directly interact with PMPA chains. These ions widened the temperature range of the phase transition of the PMPA solutions. This reduced cooperativity of the phase transition was caused by size decrease of cooperative domains, which resulted from the interaction of the ions with the polymer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4942–4952, 2005     

The kinetics and mechanisms of methyl 2-bromopropionate and 2-bromopropionic acid pyrolyses under maximum inhibition

The kinetics of the gas phase pyrolyses of methyl 2-bromopropionate and 2-bromopropionic acid were studied in a seasoned, static reaction vessel and under maximum inhibition of the free radical suppressor toluene. The working temperature and pressure range was 310–430°C and 26.5–201.5 torr, respectively. The reactions proved to be homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficients are expressible by the following equations: for methyl 2-bromopropionate, log k₁(s^?1) = (13.10 ± 0.34) ? (211.4 ± 4.4)kJ mol^?1(2.303RT)^?1; for 2-bromopropionic acid, log k₁(s^?1) = (12.41 ± 0.29) ? (180.3 ± 3.4)kJ mol^?1(2.303RT)^?1. The bromoacid yields acetaldehyde, CO and HBr. Because of this result, the mechanism is believed to proceed via a polar five-membered cyclic transition state.     

Cooperative Effect of CH···O Bonds in Models for Biological Systems

In order to calculate CH···O interactions appearing in biologically important systems, a preliminary study is performed for smaller models. MO bond indices I_XHY are calculated for dimers involving HCN, acetic acid, acetamide, glyoxal and formamide, as well as a ribose33-U35 pair appearing in tRNA^Phe and 3MU-U, also related to RNA. The distance dependence obeys the same pattern for I_OHO, I_NHO and I_CHO. It is shown that the negative I_XHY sign is related to the lengthening of the XH distance; the fluctuation of net charges for the involved atoms is related to the bond index. All results point at a definite cooperative effect.     

Effect of the reference chain dimension on the viscoelastic and stress–strain behavior of poly(n-butyl methacrylate) networks

The linear viscoelastic and stress-strain behavior of poly(n-butyl methacrylate) networks at a content of crosslinking agent (ethyleneglycol dimethacrylate) of c? 0–1 × 10^?4 mole/cm³ was investigated in the main transition and rubberlike region in the temperature interval from 20 to 150°C. The dependence of the unperturbed chain dimensions on temperature was determined from thermoelastic measurements in the rubberlike region; this dependence was unaffected by the content of crosslinking agent. Application of time–temperature superposition to the linear viscoelastic behavior did not give a continuous superimposed curve in the proximity of the rubberlike region; superposition within the whole time region required introducing the change of the unperturbed chain dimensions with temperature. This correction was sufficient for a sample with a higher content of the crosslinking agent. However, for loose networks (c< 0.1 × 10^?4 mole/cm³) it was insufficient, because of another relaxation mechanism in the region of high temperatures. It was found that the intensity and temperature dependence of this relaxation mechanism, which is probably due to a change of the number of entanglements with temperature, are connected with the magnitude and the temperature dependence of the C₂ constant of the Mooney-Rivlin equation.     

Dependence of the rate of formation of nitrate ions in water on the intensity and frequency of ultrasound waves

The dependence of the rate of the sonochemical formation of nitrate ions in water saturated with air on the intensity (within 0–2.5 W/cm²) and frequency (20–1600 kHz) of ultrasound waves was studied. The acoustic power was measured by the comparative calorimetric method. The reaction was conducted in the kinetic region, where the reaction rate was independent of the rate of mass transfer processes, such as the degasification and stirring of the solution and the depletion of the reactants, being determined only by the formation of radicals in cavitation bubbles. It was demonstrated that, within the indicated range of ultrasound frequencies, the dependence of the reaction rate on the ultrasound intensity behaved as follows: at intensities below 0.1–0.2 W/cm², the w(I) dependence is nonlinear and can be roughly approximated by a quadratic function; at I > 0.2 W/cm², w(I) becomes linear. This behavior can be explained in the following way: at I < 0.2 W/cm², with increasing I, both the fraction of acoustic power absorbed by the cavitation cloud and the reaction rate in the cavitation cloud increase; as I increases still further, nearly the entire acoustic power is absorbed by the cavitation cloud, and the w(I) dependence becomes linear. To make it possible to compare the sonochemical effects of ultrasound waves at different frequencies, a criterion K was introduced, which was defined as the slope of the w(I) plot within its linear portion (at I > 0.2 W/cm²). The K(f) dependences passes through a maximum at a frequency of f ∼ 100 kHz; at frequencies of f > 500 kHz, K ∼ f ⁻¹.     

Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic solution

The kinetics of oxidation of ferrocyanide by N-bromosuccinimide (NBS) has been studied spectrophotometrically in aqueous acidic medium over temperature range 20–35 °C, pH = 2.8–4.3, and ionic strength = 0.10–0.50 mol dm⁻³ over a range of [Fe²⁺] and [NBS]. The reaction exhibited first order dependence on both reactants and increased with increasing pH, [NBS], and [Fe²⁺]. The rate of oxidation obeys the rate law: d[Fe³⁺]/dt = [Fe(CN)₆]^4–[HNBS⁺]/(k ₂ + k ₃/[H⁺]). An outer-sphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Addition of both succinimide and mercuric acetate to the reaction mixture has no effect on the reaction rate under the experimental conditions. Mercuric acetate was added to the reaction mixture to act as scavenger for any bromide formed to ensure that the oxidation is entirely due to NBS oxidation.     

Mechanism of photogeneration of free carriers in quasi-one-dimensional photosemiconductors: Application tc polydiacetylene

The dependence of the quantum yield of free charge carriers on the electric field strength in quasi-one-dimensional photosemiconductors is rationalized. The occurrence of the trapping and nongeminate recombination in the bulk of the material is shown to be responsible of nonzero yield at zero external field. Current i vs. excitation intensity g plot obeys the law i∼g^2/3 for polydiacetylene single crystal as expected from theory presented.     

Diantimony Complexes [CpR2Mo2(CO)4(μ,η2-Sb2)] (CpR=C5H5, C5H4tBu) as Unexpected Ligands Stabilizing Silver(I)n (n=1–4) Monomers,Dimers and Chains

Synthesis and reactivity of transition metal compounds bearing “naked” pnictogen atoms is an active research area with remarkable bonding patterns observed in the formed compounds. Within this field, intense investigations on the coordination behavior of complexes possessing P_n and As_n (2≤n≤5) moieties have been conducted. However, studies on heavier analogues have been ignored so far due to arduous challenges related to low yields and moderate air stability. Herein, we present the first in-depth study addressing the reactivity of organometallic complexes containing Sb-donor atoms with several Ag^I salts. These reactions afforded twelve unprecedented aggregates as monomers, dimers as well as three- and four-membered chains of Ag^I ions claimed in the literature to be inaccessible. Interatomic distances as well as computational evidence obtained with help of several different methods suggest the presence of Ag⋅⋅⋅Ag interactions in all complexes containing more than one Ag^I ion.