Kinetic and structural aspects of gelation in polyacrylonitrile-propylene carbonate system

Structural rearrangements in solutions of high-molecular-mass polyacrylonitrile in propylene carbonate upon phase separation were studied and quantitatively characterized by means of turbidity spectroscopy and oscillation rheometry with the use of previous data obtained via the light transmission technique. The kinetic features of separation of the new phase and the particle size growth were established. It was shown that there is a time lag between the onset of physical-gel formation in the system and the onset of phase separation. The average particle size that corresponds to the transition of the system into gel and determines the gel morphology at the subsequent further stages was found for different temperatures.     

Kinetic modeling of promotion and inhibition of temperature on photocatalytic degradation of benzene vapor

This study investigated the effects of temperature, humidity, and benzene concentration on the photocatalytic oxidation of benzene vapor over titanium dioxide. An annular packed-bed photocatalytic reactor was employed to determine the intrinsic oxidation rates for the photocatalysis of benzene. Degussa P-25 TiO₂ was used as the photocatalyst and a 15 W near-UV lamp (350 nm) was used as the light source. The experiments were conducted at influent benzene concentrations of 250–450 ppmv, water vapor concentrations of 13,500–27,500 ppmv, and reaction temperatures ranging from 100 to 200 °C. Benzene oxidation rates increased with temperature below 160–180 °C, but decreased with temperature above 160–180 °C. Raising the reaction temperature increased the chemical reaction rates but reduced the reactant adsorption rate on TiO₂ surfaces. The overall reaction rate increased with temperature, indicating that the reduction of reactant adsorption rate did not affect the overall reaction, and thus the chemical reaction was the rate-limiting step. As the chemical reaction rate gradually exceeds the reactant adsorption rate with temperature, the rate-limiting step was shifted from the chemical reaction to the reactant adsorption. Additionally, the competitive adsorption between benzene and water for the active sites on TiO₂ resulted in the promotion and inhibition of reaction rate by humidity. This study developed a modified bimolecular Langmuir–Hinshelwood kinetic model to simulate the temperature and humidity related promotion and inhibition of the photocatalysis of benzene. The correlation developed here was used as a basis for determining the apparent activation energy of 0.76 kcal/mol and adsorption enthalpies of benzene and water of −20.1 and −13.7 kcal/mol.     

Kinetic modeling of aqueous phenol degradation by UV/H2O2 process

A dynamic kinetic model for the oxidation of phenol in water by an UV/H₂O₂ process is developed. The model is based on the elementary chemical and photochemical reactions, initiated by the photolysis of hydrogen peroxide into hydroxyl radicals. The model is validated by using experimental data obtained from the open literature for an actual UV/H₂O₂ process. Using those data and the developed kinetic model, kinetic rate constants for phenol intermediates, catechol and hydroquinone, are estimated. Moreover, the optimum initial hydrogen peroxide concentration is estimated by means of the validated model. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 40: 34–43, 2008     

Kinetic studies on the aminolysis of someO-aryl oximes in benzene

Pyrrolidinolysis and piperidinolysis ofO-aryl oximes show third order dependence on amine with no uncatalytic route. The effect of temperature on separate routes and overall rate is reported and explained.     

Dynamic thickening investigation of the gelation process of PAM/PEI system at high temperature and high pressure

The gelation process of organically polymer gel was investigated by dynamic thickening measurements. Rheological measurements were used to evaluate the viscosity of the gel. During the gelation process, high temperature resulted in higher rate of crosslinking. Rigid and stable gel was formed in neutral and alkaline media, and the higher of the pH value, the faster of the gelation process. However, gel could not be formed in acid condition. Moreover, the rate of crosslinking increased with the increase of concentration of polymer and crosslinker. The addition of NH₄Cl elongated the gelation time significantly, but played a negative role in the gel strength, while a rigid gel was formed in the presence of Sodium acetate or trisodium citrate dehydrate. This paper summarizes the results and discusses how various parameters affect the gelation process of the gel.     

Kinetic analysis of the process of thermal degradation of poly-N-vinylcarbazole and its derivatives

Thermal studies of poly-N-vinylcarbazole and its derivatives containing ?Cl, ?Br, ?I and ?CH₃ as functional groups were carried out by means of thermogravimetry. The kinetics of the processes of thermal degradation of these polymers was examined. Kinetic parameters, i.e. apparent activation energy, reaction order and pre-exponential factor, were evaluated by using an integral method.     

Kinetic behaviour of commercial catalysts for methane reforming in ethanol steam reforming process

Ethanol steam reforming has been studied in a fluidized bed （in order to ensure bed isothermicity） on commercial catalysts for methane reforming. The results allow analyzing the effect of temperature （in 300-700℃ range）, and both metal and support nature on the reaction indices （ethanol conversion, yields and selectivities to H2 and byproducts （CO2, CO, CH4 and C2H4O））. Special attention has been paid to catalysts＇ stability by comparing the evolution of the reaction indices with time on stream at 500°C （minimum CO formation） and 700℃ （minimum deactivation by coke deposition）. Although they provide a slightly lower H2 yield, the results evidence a good behaviour of Ni based catalysts, indicating that they are an interesting alternative of more expensive Rh based ones.     

Kinetic gelation predictions of species aggregation in tetrafunctional monomer polymerizations

During the polymerization of tetrafunctional monomers, a heterogeneous distribution of species exists throughout the reaction mixture and dramatically complicates the network structural evolution. This work attempts to quantify this heterogeneity using kinetic gelation simulation predictions of a defining parameter called the heterogeneity index. The heterogeneity index directly measures how heterogeneous the polymerization is with respect to interactions of like species or pairs of species. Examples of the species are polymer segments, monomeric double bonds, pendant double bonds, initiator molecules, radicals, and free volume. By implementing this index, it is clear that the heterogeneity in the polymer network dramatically changes as a function of conversion and polymerization conditions. In particular, microgel formation and monomer pooling were characterized and quantified with the heterogeneity index. In addition to characterizing the structural heterogeneity, the influence of the heterogeneity on the initiator efficiency and trapping of free radicals was also studied and qualitatively compared to experimentally observed behavior. ©1995 John Wiley & Sons, Inc.     

The thermal degradation behaviour of polydimethylsiloxane/montmorillonite nanocomposites

A series of novel polydimethylsiloxane/montmorillonite (PDMS/MMT) nanocomposites was prepared. The thermal degradation behaviour of these nanocomposites was studied by means of Thermal Volatilization Analysis (TVA) and Thermogravimetric Analysis (TGA). The major degradation products were identified as cyclic oligomeric siloxanes from D₃ to D_7, and higher oligomeric siloxane residues. Other minor degradation products include methane, bis-pentamethylcyclotrisiloxane, propene, propanal, benzene and dimethylsilanone. The results demonstrate that the nanoclay significantly alters the degradation behaviour of the PDMS network, modifying the profile of the thermal degradation and reducing the overall rate of volatiles evolution. The results also indicate that the nanoclay promotes the formation of dimethylsilanone and benzene by inducing low levels of radical chain scission.     

The adsorption of benzene and n-hexane on aluminum oxide

Summary Benzene is physically adsorbed on Al₂O₃ at 20–100° in greater amount than is n-hexane. In the range 100–150°, chemisorption of both hydrocarbons begins and decreases with increase in temperature.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 900–902, May, 1965 Original article submitted July 22, 1964     

Kinetics of thermo-oxidative degradation of polypropylene/aluminum trihydroxide/organo Fe-montmorillonite nanocomposites

In the article, the thermal oxidative degradation kinetics of pure polypropylene/aluminum trihydroxide (PP/ATH) and PP/ATH/organo Fe-montmorillonite (Fe-OMT) nanocomposites were investigated using Kissinger, Friedman and Flynn–Wall–Ozawa methods. The results showed that thermal oxidative degradation of PP/ATH/Fe-OMT nanocomposites to PP/ATH were complex reaction: the whole process of thermal oxidative degradation were composed with the decomposition of ATH, the cracking and charring of the backbone chains of PP, and the oxidative degradation of char, which the curses of energy mutative with the process of thermal oxidative degradation. The control steps were different in each degradation stage. The activation energy was high in the original degradation stage. It was due to the molecular structure and may closely relate with onset temperature. In the intermediate process, the activation energy was low. In the last stage of the degradation, the activation energy was graveled because the carbon may be oxidized. In the whole process of thermal oxidative degradation, the activation energy of PP/ATH/Fe-OMT nanocomposite was higher than that of PP/ATH.     

Polymerization of terminal alkynes by a new Ziegler–Natta catalyst system: Scandium naphthenate/trialkylaluminum

The scandium naphthenate/trialkylaluminum system was investigated as a catalyst in the polymerization of substituted alkynes. Phenylacetylene was converted to a moderately crystalline, high cis polymer. The alkyl-substituted acetylenes produced rubbery or solid materials of relatively high molecular weight. Infrared (IR), ultraviolet (UV), ¹H-NMR, and ¹³C-NMR studies of the polyalkylacetylenes revealed high cis conformation, but apparently twisted from planarity to decrease molecular orbital overlap in the polyconjugated chain.     

Growth process of polymers near the gelation threshold

We will review experimental results obtained recently on the determination of the laws governing the growth process of polymer clusters as the gel point is approached. The exponent γ which characterizes the increase of the mean weight-average molecular weight M_w as the gel point is approached and the exponent τ which characterizes the mass distribution were measured on different chemical systems. They were found to be independent of the chemical system (within experimental error) and very close to exponent values calculated by computer simulations following the percolation model. Therefore, the sol-gel transition is a critical phenomenon of connectivity belonging to the same class of universality as percolation.     

Magnetic field effect on photocatalytic degradation of benzene over Pt/TiO2

The magnetic field effect on heterogeneous photocatalytic degradation of benzene over Pt/TiO2 has been observed for the first time. The coupling effect between magnetic field and photo field influences the conversion of benzene and production of CO2 in different modes.     

Kinetic study on degradation of gaseous acetone over thin-film TiO2 photocatalyst in a continuous flow system

The effects of mixing condition by baffles, inlet concentration and face velocity on the photocatalytic degradation (PCD) of gaseous acetone over thin-film TiO₂ have been examined in a continuous flow system. The PCD rate increased upon installing baffles at a low inlet concentration and a high face velocity.     

Kinetic behaviour of non-isothermal lime hydration

The rate of the solid—liquid reaction between lime and water depends on the physicochemical properties of lime and on the solid-to-liquid ratio. The hydration is usually carried out under non-isothermal conditions and at high solid-to-liquid ratios, leading to a first reaction step in which the solid breaks down with a fast increase in temperature. The second step is much slower. In this work a mathematical model based on the structural properties of the solid was used to describe the second step of the hydration leading to the interfacial reaction and diffusion parameters and activation energies for both parameters.The kinetic model was able to simulate the non-isothermal results of the adiabatic system in the range of variables studied.     

Viscoelastic behaviour during gelation of various molar-mass polyvinylchloride solutions and polymerization temperatures

The gel formation of various 10% molar-mass polyvinyl-chloride (PVC) sample solutions and polymerization temperatures in di-(2-ethylhexyl)phthalate (DOP) was studied by measuring dynamic moduli as functions of gelation (or ageing) time and frequency. The propensity to form gels increases with increasing molar mass and decreasing polymerization temperature. Extraction of a commercial PVC with acetone separates it into a relatively highly crystalline and a nearly non-crystalline polyvinyl chloride.