Comparison of thermal curing behavior of liquid and solid urea–formaldehyde resins with different formaldehyde/urea mole ratios

This study was undertaken to compare thermal cure kinetics of urea–formaldehyde (UF) resins, in both liquid and solid forms as a function of formaldehyde/urea (F/U) mole ratio, using multi-heating rate methods of differential scanning calorimetry. The requirement of peak temperature (T _p), heat of reaction (ΔH) and activation energy (E) for the cure of four F/U mole ratio UF resins (1.6, 1.4, 1.2 and 1.0) was investigated. Both types of UF resins showed a single T _p, which ranged from 75 to 118 °C for liquid resins, and from 240 to 275 °C for solid resins. As the F/U mole ratio decreased, T _p values increased for both liquid and solid resins. ΔH values of solid resins were much greater than those of liquid resins, indicating a greater energy requirement for the cure of solid resins. The ΔH value of liquid UF resins increased with decreasing in F/U mole ratio whereas it was opposite for solid resins, with much variation. The activation energy (E _a) values calculated by Kissinger method were greater for solid UF resins than for liquid resins. The activation energy (E _α ) values calculated by isoconversional method which showed that UF resins in liquid or solid state at F/U mole ratio of 1.6 followed a multi-step reaction in their cure kinetics. These results demonstrated that thermal curing behavior of solid UF resin differed greatly from that of liquid resins, because of a greater branched network structure in the former.     

Mechanism of electron excitation energy degradation in solutions

The inductive-resonant mechanism of electronic energy degradation is proposed and proved for rare earth ions, transition metal ions and simple molecules (NO^?₂) in solutions. The interaction of two oscillators is considered, that is the vibronic interaction corresponding to the radiation spectrum of an excited ion or molecular and the vibrational spectrum corresponding to the excitation of high frequency vibrations of the solvent. The calculation of the energy degradation rare constant (k_degr.) by Förster's formula is shown to give k_degr. values of the same order as the experiment. Such a treatment can quantitatively explain all experimental regularities of the degradation process, for instance, the dependence of k_degr. on the distance from the electronic excitation centre to the nearest high frequency vibration gravity centre. It is shown that the suggested mechanism corretly explains the deuteration effect, the dependence of k_degr. on ΔE (energy gap) and the variation of this dependence for differenct classes of compounds. The possibility of proving the validity of the suggested model for the case of complex organic molecules is discussed.     

Picosecond laser studies on the effect of structure and environment on intersystem crossing in aromatic carbenes

A large solvent polarity effect on the rate of singlet to triplet intersystem crossing (k_ST) has been observed in the carbenes, diphenylcarbene (DPC) and dicycloheptadienylidene (DCHD). It is found that both k_ST and the energy splitting (ΔE_ST) separtaing the singlet and triplet states decrease as the solvent polarity increases for the aromatic carbenes. This “inverse” gap effect, i.e. the time for intersystem crossing decreases with increasing energy gap, is explained by an off-resonance intersystem crossing from the singlet to a sparse triplet vibronic manifold characteristic of a small energy gap. The trend in ΔE_ST, which is proposed to be responsible for the variation in k_ST for DPC, DCHD and structurally related aromatic carbenes, is suggested to arise from the variation in the bond angle of the central methylene carbon atom.     

Supplement on applicability of the Kissinger equation in thermal analysis

The relative errors (e%) in the determination of the activation energy from the slope of the Kissinger straight line ln(β/βT _p²) vs. 1/T _p (β is the heating rate) are in-depth discussion. Our work shows that the relative errors is a function containing the factors of x _p and Δx _p, not only x _p (x _p = E/RT _p, E is the activation energy, T _p is the temperature corresponding to maximum process rate, R is the gas constant). The relative error between E _k and E _p will be smaller with the increase of the value of x and/or with the decrease of the value of Δx. For a set of different heating rates in thermal analysis experiments, the low and close heating rates are proposed from the kinetic theory.     

Halogenation of ketones with N-halosuccinimides under solvent-free reaction conditions

Several aryl substituted ketones, cyclic ketones, 1,3-diketones and a β-ketoamide were halogenated with N-halosuccinimides under solvent-free reaction conditions (SFRC) at various temperatures (20-80 °C), whereas less enolized ketones required the presence of an acid catalyst (p-toluenesulfonic acid, PTSA). Bromination of substituted acetophenones obeys first order kinetics v=k_Br[ketone] and the following correlation with the keto-enol equilibrium constant: log k_Br=0.3pK_E+C₁, less enolized substrates being more reactive; the moderate positive charge developed in the rate determining step was confirmed by the Hammett correlation (ρ=−0.5). On the other hand, in cyclic ketones an opposite relation was observed: log k_Br=−0.6pK_E+C₂, indicating higher reactivity of substrates with higher enolization constant (K_E). The important role of the nature of the solvent (MeCN, MeOH) in preorganization of the ketone-NBS-PTSA mixture prior to SFRC bromination was found.     

Prediction of ortho substituent effect in alkaline hydrolysis of phenyl esters of substituted benzoic acids in aqueous acetonitrile

The second-order rate constants k for the alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C₆H₄CO₂C₆H₅, in aqueous 50.9% acetonitrile have been measured spectrophotometrically at 25°C. The log k values for meta and para derivatives correlated well with the Hammett σ_m,p substituent constants. The log k values for ortho-substituted phenyl benzoates showed good correlations with the Charton equation, containing the inductive, σ_I, resonance, σ^○ _R, and steric, E _s ^B, and Charton υ substituent constants. For ortho derivatives the predicted (log k _X)_calc values were calculated with equation (log k _ortho)_calc = (log k ^H _AN)_exp + 0.059 + 2.19σ_I + 0.304σ^○ _R + 2.79E _s ^B ? 0.0164ΔEσ_I — 0.0854ΔEσ^○ _R, where DE is the solvent electrophilicity, ΔE = E _AN — E _H20 = ?5.84 for aqueous 50.9% acetonitrile. The predicted (log k _X)_calc values for phenyl ortho-, meta- and para-substituted benzoates in aqueous 50.9% acetonitrile at 25°C precisely coincided with the experimental log k values determined in the present work. The substituent effects from the benzoyl moiety and aryl moiety were compared by correlating the log k values for the alkaline hydrolysis of phenyl esters of substituted benzoic acids, X-C₆H₄CO₂C₆H₅, in various media with the corresponding log k values for substituted phenyl benzoates, C₆H₅CO₂C₆H₄-X.     

STUDIES ON THE KINETICS OF PROPYLENE POLYMERIZATION WITH THE COMPLEXTYPE TiCl_3 CATALYST

The active center concentration C_p, the rate constant k_p, and the activation energy of chain propagation E_p in the polymerization of propylene with complex-type TiCl_3-(C_2H_5)_2AlCl catalyst system were studied. The Mn was corrected by (?) value determined by GPC. The values thus obtained for C_p, k_p, and E_p at 50℃were 3.01 mol/mol Ti, 6.27 1/mol·sec, and 5.10 Kcal/mol respectively.The kinetic parameters were compared with those obtained from conventional TiCl_3·AlCl_2 catalyst, showing that the higher activity of the complex-type catalyst over the conventional catalyst is not only due to the higher C_p of the former, but to a greater extent due to the increase of the k_p value.     

Synthesis of poly(p-phenylene)s having alternating sugar and alkyl substituents by Suzuki coupling polymerization and evaluation of their main-chain conformations

This paper reports synthesis of poly(p-phenylene)s (PPPs) having alternating sugar and alkyl substituents by Suzuki coupling polymerization of a 1,4-dibromobenzene monomer having peracetylated glucose residues with a 1,4-benzene bis(boronic acid) having alkyl chains using Pd(PPh₃)₄ in a mixed solvent of THF and NaHCO₃ aq. at reflux temperature. The polymerization proceeded with the progress of frequent deacetylation, and thus, the crude product was acetylated, followed by the isolation procedures, giving the PPP having alternating peracetylated glucose and alkyl substituents. The structure of the isolated product was confirmed by the ¹H and ¹³C NMR measurements to be the desired PPP derivative. The M_n values were estimated by GPC analysis with DMF as the eluent to be 5400-12,700. The deacetylation of the polymer completely took place using sodium methoxide in methanol/THF. The conformation of the main-chain was evaluated by the CD spectrum in comparison with that of PPP only with the glucose substituents, indicating that the present PPP derivative had the flexible nature of the main-chain by introduction of the alkyl-substituted units between glucose-substituted units. The Suzuki coupling of a 1,4-dibromobenzne monomer having disaccharide substituents with the benzene bis(boronic acid) monomer was also performed under the similar conditions. The product was precipitated from the reaction mixture, which was simply isolated by filtration. The isolated polymer was purified further by reprecipitation into diethyl ether and its structure was a PPP having free disaccharide or monosaccharide residues. This indicated occurrence of complete deacetylation as well as partial degradation of the glycosidic linkages in the disaccharides during the polymerization. The main-chain of the obtained polymer had also the flexible nature. The fluorescence spectra of the obtained PPP derivatives in this study were also measured.     

Kinetics of non-isothermal crystallization in Cu50Zr43Al7 and (Cu50Zr43Al7)95Be5 metallic glasses

The crystallization kinetics of Cu₅₀Zr₄₃Al₇ and (Cu₅₀Zr₄₃Al₇)₉₅Be₅ metallic glasses was studied using differential scanning calorimetry (DSC) at four different heating rates under non-isothermal condition. The glass transition temperature T _g, the onset temperature of crystallization T _x, and the peak temperature of crystallization T _p of the two metallic glasses were determined from DSC curves. The values of various kinetic parameters such as the activation energy of glass transition E _g, activation energy of crystallization E _p, Avrami exponent n and dimensionality of growth m were evaluated from the dependence of T _g and T _p on the heating rate. The values of E _g and E _p, calculated from many different models, are found to be in good agreement with each other. The average values of the Avrami exponent n are (2.8 ± 0.4) for Cu₅₀Zr₄₃Al₇ metallic glass and (4.2 ± 0.3) for (Cu₅₀Zr₄₃Al₇)₉₅Be₅ metallic glass, which are consistent with the mechanism of two-dimensional growth and three-dimensional growth, respectively. Finally, the parameter H _r, S, and crystallization enthalpy ΔH _c are introduced to estimate the glass-forming ability and thermal stability of metallic glasses. The result shows that the addition of Be improves the glass-forming ability and thermal stability of Cu₅₀Zr₄₃Al₇ metallic glass.     

A study of the electrical conduction and devitrification behaviour of the amorphous chalcogenide Te81Ge15As4

The thermal spectrum of thedc conductivity of Te₈₁Ge₁₅As₄ during a consecutive heatingcooling cycle within the temperature rangeT _g > T> T _m indicated that the heating and cooling parts of the cycle do not coincide and phase transformation were ohserved The activation energies of conduction (δE) for the initial amorphous, the liquid and the final crystallized phases are 0.35, 0.30 and 0.16 eV, respectively. The growth of conductive regions during isothermal annealing of the amorphous solid phase was studied and correlated to the corresponding structural changes recorded by using microphotography and X-ray diffraction. The structure of the investigated three-component composition can be represented as a solid solution based on tellurium character. A value of 2.4 eV was obtained for the activation energy of crystallization from non-isothermal transition data.     

Kinetic description of bimolecular reactions with low energy cross sections in dilute gases

The rate ν of bimolecular chemical reaction A+A=B+C is analyzed for simple models of reactive cross sections. Collisions of particles colliding with energy E larger than a relatively low characteristic energy E _Lare either non-reactive (reversed Prigogine-Xhrouet model = rPX) or the ability to react is decreasing for E>E _L(reversed line-of-centres model = rLC). After solution of the Boltzmann equation analytical expressions for the distribution function f and the rate coefficient k have been derived. It is shown that the Arrhenius activation energy E _Ais small and even negative for sufficiently small E _L. The non-equilibrium corrections to ν are small.     

Intramolecular energy band dispersion of n-C36H74 observed by angle-resolved photoemission with synchrotron radiation

The energy band dispersion E = E(k) is determined for the valence bands in a long-chain alkane n-C₃₆ H₇₄ by angle-resolved photoemission from an oriented polycrystalline sample using synchrotron radiation. Comparison with theoretical calculations for a single chain shows good qualitative agreement for the position, width, and dispersion of the valence band. This is the first observation of energy band dispersion in an organic molecular solid.     

Copolymerization of styrene and butadiene in emulsion. II. Relative rates of crosslinking and propagation

For the styrene–butadiene emulsion copolymerization (71 parts butadiene:29 parts styrene) the ratio of the rate coefficients for crosslinking, k_x, and propagation, k_p, have been determined at 5, 15, and 25°C by using an adaption of the method of Morton and co-workers. These ratios yield a value of 4.85 kcal/mole for the difference in activation energy between crosslinking and propagation, E_x ? E_p. Since the relative frequency of crosslinking and propagation depends upon the copolymer composition, and hence upon the free monomer ratio and the temperature, the range of application of these data is more limited than in a simple homopolymerization.     

Etude cinetique de la polymerisation cationique du cyclopentadiene amorcee par l'hexachloroantimoniate de triphenylmethyle

A study of cyclopentadiene polymerization, initiated by φ₃C⁺SbCl₆^? in methylene chloride solution, has been carried out at temperatures between ?70 and +20° using a dilatometric method. An overall external second order with respect to monomer has been found. At very low temperature (?70°), the concentration of active centres remains low and roughly constant, in agreement with a quasi-stationary state assumption. Between ?50 and + 10°, experimental determination of (k_p. M^*), obtained from variation of v_p and [M] with time, shows that the concentration of centres goes through a maximum, sharper and more rapidly reached as the temperature is raised. Initiation is slower than propagation and active centres are rapidly destroyed when termination becomes faster than initiation. This explains the partial conversions and the observed maximum for concentration of active centres. Propagation and unimolecular termination rate constants have been determined at each temperature: activation energies are E_p = ?8 ± 0·5 kcal mole^?1 and E_p = ?0·3 ± 0·1 kcal mole^?1. These negative values can be explained by an exothermic process of solvation of active centres, leading to more reactive propagating species.     

Kinetic regularities of catalytic ethylene polymerization on single- and multi-site cobalt and vanadium bis(imino)pyridine complexes

The number of active centers C _p in the homogeneous complexes LCoCl₂ and LVCl₃ (L = 2,6-(2,6-R₂C₆H₃N=CMe)₂C₅H₃N; R = Me, Et, ^t Bu) and the propagation rate constants k _p have been determined by the radioactive ¹⁴CO quenching of ethylene polymerization on these complexes in the presence of the methylaluminoxane (MAO) activator. For the systems studied, a significant portion of the initial complex (up to 70%) transforms into polymerization-active centers. The catalysts based on the cobalt complexes are single-site, and the constant k _p in these systems is independent of the volume of substituent R in the ligand, being (2.4?3.5) × 10³ L mol^?1 s^?1 at 35°C. The much larger molecular weight of the polymer formed on the complex with the tert-butyl substituent in the aryl rings of the ligand compared to the product formed on the complex with the methyl substituent is due to the substantial (～11-fold) decrease in the rate constant of chain transfer to the monomer. At the early stages of the reaction (before 5 min), the vanadium complexes contain active centers of one type only, for which k _p = 2.6 × 10³ L mol^?1 s^?1 at 35°C. An increase in the polymerization time to 20 min results in the appearance, in the vanadium systems, of new, substantially less reactive centers on which high-molecular-weight polyethylene forms. The number of active centers C _p in the 2,5-tBu₂LCoCl₂ and 2,6-Et₂LVCl₃ systems with the MAO activator increases as the polymerization temperature is raised from 25 to 60°C. The activation energies of the chain propagation reaction (E _p) have been calculated. The value of E _p for complex 2,5-tBu₂LCoCl₂ is 4.5 kcal/mol. It is assumed that the so-called “dormant” centers form in ethylene polymerization on the 2,6-Et₂LVCl₃ complex, and their proportion increases with a decrease in the polymerization temperature. Probably, the anomalously high value E _p = 14.2 kcal/mol for the vanadium system is explained by the formation of these “dormant” centers.     

Energy distribution of charge carriers in solids in highly non-equilibrium states considering cascade transitions and inelastic scattering

A study is made of the cascade process, which describes the energy loss and multiplication of highly non-equilibrium secondary electrons and holes in crystalline platinum irradiated by low-energy electrons. The pair-creation scattering rates are evaluated in the framework of statistical model that takes into account the electron band structure of platinum. Kinetic equations for the excited electron and hole energy distributions are solved numerically in the isotropic scattering approximation for some primary (excitation) energies E_p that do not exceed the plasma energy E_F+ℏω_pl.     

Picosecond study of an electron transfer in the first excited state of dodci

Electron transfer from the first excited singlet state of a polymethine cyanine dye. DODCI, to various electron acceptors (p-benzoquinone,p-dinitrobenzene and methylviologen) was investigated using picosecond fluorescence and absorption spectroscopy. The electron transfer to methylviologen was confirmed by conventional nanosecond laser spectroscopy. Its efficiency, as expressed by the ratio k₅/(k₄ + k₅) = 0.07. can be explained by coulombic repulsion in the initial radical pair. On the other hand, although fluorescence quenching by p-BQ and p-DNB is very efficient, no electron transfer was observed.     

Crystallization kinetics of calcium carbonate at a stoichiometric ratio of components

The formal kinetics of calcium carbonate crystallization in aqueous solutions is studied at a stoichiometric ratio of Ca²⁺ and CO₃^2- ions. The kinetics of the process was monitored by convenient and reliable methods (complexometric analysis for calcium in an aqueous solution and energy dispersive and microscopic measurement of solid particle sizes). The effect the temperature and degree of supersaturation have on the periods of induction and mass crystallization and the equilibrium concentration of calcium ions in solution is estimated at continuously controlled pH and solution ionic strength. The kinetic parameters (n, k, τ_1/2, E_a) of calcium carbonate crystallization are calculated. It is shown that calcium carbonate with a calcite structure formed at a stoichiometric ratio of reagents, and changes in the temperature (25–45°C) and the solution’s degree of supersaturation (2–6) within the considered range had no effect on the characteristics of the solid phase.     

Study of kinetics of thermal degradation and to determine the activation energies of polyamides based on s-triazine

A series of ten polyamides was prepared by the high-temperature polycondensation of 4,6-bis(N-(4-(benzoylchloride)amino))-2-(N-phenyl- piperazin-1-yl)-1,3,5-triazine with different aromatic and aliphatic diamines. The synthesized polyamides were analyzed by physico-chemical properties such as solubility, density, viscosity etc. The structure of prepared polyamides was evaluated by ¹H NMR and FTIR spectrum. Thermogravimetric analysis used to study the kinetics of thermal degradation of some synthesized polyamides. Broido, Horowitz & Metzger, Coats Redfern and Chan et al. models were applied to respective thermograms to determine the activation energy (E_a). Activation energy data shows that the polyamides obtained from aromatic diamine has greater stability than the polyamides obtained from aliphatic amine.