Light-induced crosslinking polymerization of a novel N-substituted bis-maleimide monomer

The photoinduced polymerization of a N-alkyl substituted bis-maleimide (MI) has been studied by real-time infrared spectroscopy. The polymerization of the neat monomer proceeds rapidly and extensively because of both its great absorbance in the UV-range, and the presence on the alkyl chain of easily abstractable hydrogen atoms, which are needed to produce the initiating radicals. Such photoinitiator-free resin was found to be less sensitive to oxygen inhibition than typical UV-curable acrylate resins, the peroxyl radicals formed being capable to propagate the chain reaction. This N-substituted bis-maleimide was also successfully used as monomeric photoinitiator to induce the crosslinking polymerization of acrylate monomers through the free radicals formed upon its photolysis.     

Methylene blue as a retarder of free radical polymerization: 1. Polymerization of acrylonitrile,methyl methacrylate and styrene

Methylene blue (MB), a redox dye, in monomeric form significantly retards the free radical polymerization of styrene (ST) and acrylonitrile (AN) in N,N-dimethyl formamide solvent. The effect is small in methyl methacrylate (MMA) polymerization. Partly reduced MB is found to be a stronger retarder than MB. The rate constants for the oxidation of the polymer radicals by MB at 60°C follows the order ST>AN>MMA. The results cannot be explained when only taking into consideration the reactivity and polarity parameters of the radicals and the substrate, as is done in the Q-e scheme. The low rate constant with the PMMA radical is attributed to steric effects. The current work has shown that single molecules of MB are effective chain terminators and this is in sharp contrast with the conclusion reached by Chen who postulated that only the aggregates of MB are the chain terminators in the polymerization of acrylamide in water.     

Controlling melt reactions during preparing long chain branched polypropylene using copper N,N-dimethyldithiocarbamate

Effect of copper N,N-dimethyldithiocarbamate (CDD) on melt reactions during preparing long chain branched polypropylenes (LCB-PP) via free radical grafting was studied. The structure and rheological properties of the modified PPs were characterized. The results showed that CDD could efficiently control two side reactions, i.e. degradation of PP backbone and homopolymerization of multifunctional monomer (trimethylol propane triacrylate (TMPTA)) in the presence of peroxide. Meanwhile the addition of CDD also increased the efficiency of forming LCB structure. The reaction between CDD and active free radicals (carbon centered and alkoxy species) led to forming in situ dithiocarbamate radicals, which cannot attack PP backbone and are weaker initiator for TMPTA. The resultant dithiocarbamate radicals could react with the PP macroradicals and the acrylic radicals reversibly, which prolong the life time of PP macroradical and increase the reaction probability between macroradicals. The obtained LCB-PP showed high melt strength.     

Modeling of nitrogen/diborane/methane/hydrogen plasma for nanocrystalline diamond growth: Comparison with experimental data

Gas phase thermodynamic equilibrium calculations involving N₂/B₂H₆/CH₄/H₂ mixtures were performed to investigate the chemical interactions leading to boron incorporation and microstructure variations in microwave plasma assisted chemical vapor deposition of diamond films. Molecular fractions of several BH_x (x = 1–3) species were calculated to study the incorporation mechanism of boron atom into diamond structure. A strong influence of the BH in causing the boron incorporation level in diamond lattice is confirmed by the correlation of its modeled equilibrium composition in the gas phase with boron content as determined experimentally. Nitrogen addition leads to nanocrystallinity and a reduction in boron incorporation due to a decrease in BH as additional B/N/H radicals are formed in the gas phase. We also obtained a good degree of agreement between the theoretically predicted CH₃/CN gas phase ratio and the experimental surface roughness trends as measured for all samples.     

Viscous flow effects in the periodic pressure cycling of gas-phase catalytic reactions

The previous study of Hamer and Cormack[1], on the increased effectiveness of gas phase catalytic reactions under conditions of oscillating external pressure, is extended to include viscous flow effects. For this purpose, the full, time dependent equations of viscous flow, diffusion and chemical reaction for a first order, isothermal reaction in a porous catalyst are solved numerically.The inclusion of viscous effects results in a new dimensionless parameter, N. It is shown that in general the catalyst effectiveness factor is smaller for smaller N. It is concluded, however, that even though viscous effects might be very important for extremely low permeability catalysts, viscous effects can be neglected for operating conditions such that N > 1. For one example cited this corresponds to a permeability of 0.2 millidarcy.     

Photochemically masked benzophenone: Photoinitiated free radical polymerization by using benzodioxinone

A new photoinitiating system for free radical polymerization of methyl methacrylate (MMA) is reported. This system consists of benzodioxinone and hydrogen donors such as triethylamine (TEA), N,N-dimethyl ethanol amine (DMEA) and tetrahydrofuran (THF). A feasible mechanism involves photoinduced formation of benzophenone from benzodioxinone and subsequent hydrogen abstraction of photoexcited benzophenone from hydrogen donors to yield radicals capable of initiating polymerization of MMA.     

Pro- and Antioxidative Effect of α-Tocopherol on Edible Oils, Triglycerides and Fatty Acids

Using advanced electron paramagnetic resonance techniques (EPR), oxidation of crude vegetable oils and their components (fatty acids and triglycerides) by radicals generated from hydrogen peroxide was investigated. The correlation rotational times were determined allowing us to characterize radicals formed during edible oils oxidation. Additionally ¹H- and ¹⁴N-hyperfine coupling constants differentiate the fatty acids dependently on their unsaturation. The acids with a higher number of unsaturated bonds exhibit higher A_N values of PBN/·lipid adduct. The waste oil with high free fatty acids content underwent the oxidation reaction more efficiently, however due to saturation and the high content of the fatty acids the carbon-centered radicals formed (upon hydrogen peroxide radicals) and their PBN (N-tert-butyl-α-phenylnitrone) adducts were less stable. The antioxidant effect was dependent on the amount of α-tocopherol added. In small amounts of up to 0.35 mg/1 g of fatty acid or triglyceride, it inhibited the creation of PBN/·lipid adducts while with higher amounts it intensified adduct formation. The α-tocopherol (AT) addition influence was also studied as spin scavenging dependence and indicated that any addition of the antioxidant in the investigated samples led to free radical scavenging and the effect increased with the increase in AT content.     

Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures

Gas hydrates from CO₂/N₂ and CO₂/H₂ gas mixtures were formed in a semi-batch stirred vessel at constant pressure and temperature of 273.7 K. These mixtures are of interest to CO₂ separation and recovery from flue gas and fuel gas, respectively. During hydrate formation the gas uptake was determined and the composition changes in the gas phase were obtained by gas chromatography. The rate of hydrate growth from CO₂/H₂ mixtures was found to be the fastest. In both mixtures CO₂ was found to be preferentially incorporated into the hydrate phase. The observed fractionation effect is desirable and provides the basis for CO₂ capture from flue gas or fuel gas mixtures. The separation from fuel gas is also a source of H₂. The impact of tetrahydrofuran (THF) on hydrate formation from the CO₂/N₂ mixture was also observed. THF is known to substantially reduce the equilibrium formation conditions enabling hydrate formation at much lower pressures. THF was found to reduce the induction time and the rate of hydrate growth.     

Relationship of coal extraction with free radicals and coal macerals

Extraction and de-ashing of coals with N-methyl pyrrolidone (NMP) under mild conditions has been reported recently. In this work, we have studied about a dozen American coals, along with some of their residues and extracts after treatment with NMP using in situ ESR (Electron Spin Resonance) spectroscopy of free radicals. At room temperature, N-₈ (number of free radials/g) decreases whereas the percentage extraction increases with atomic H/C of coals. Also N₈ for extract (residue) is lower (higher) than that for coals, ESR spectra of the extract exhibits only the single broad component and the weighted average of N₈ for the extract and the residue nearly equals that of the parent coal. These results have led to the hypothesis that the treatment with NMP is an extraction process in which exinites and to a lesser degree vitrinites are favored, whereas the inertinites are nearly rejected by NMP.     

Absolute Concentration Measurements of Chemically-Important Flame Radicals

A fundamental understanding of flame chemistry is a necessary part of designing and optimizing combustors to meet environmental regulations while maintaining system efficiency. We have performed quantitative, absolute concentration measurements for several free radicals to test various aspects of the predictive capabilities of the natural gas combustion chemistry mechanism, GRI-Mech 2.11, the chemical mechanism for methane combustion. The CH, NO, OH, and HCO radicals were studied in a series of low-pressure methane/O₂/N₂ flames using the technique of laser-induced fluorescence. The results show that GRI-Mech 2.11 predicts well the hydrocarbon oxidation chemistry in lean and near stoichiometric flames, but appears to possess some inadequacies for rich flames. Of special importance is the finding that the rate coefficient for the important CH + N₂ reaction in the current version of GRI-Mech appears to be too low. Also, reburn of NO is correctly predicted in a lean flame but not a rich one.     

Photomodulated voltammetry investigations on the benzyl radical

Benzyl radicals were generated photochemically directly in an electrochemical cell, using diphenylacetone as a precursor. The half-wave potentials and the plateau current of benzyl radical reduction and respectively oxidation were determined in acetonitrile, propylene carbonate, N,N-dimethylformamide and dimethylsulfoxide. While half-wave potentials and transfer coefficients were independent of chopper frequency and phase setting, the plateau current was markedly influenced by these parameters. The lifetime of the radical was obtained from the dependence of the phase shift of the current on the angular frequency. The influence of precursor concentration, light intensity and nature of the solvent on the radical lifetime was investigated.     

The pressure beneath a growing rising bubble

On the basis of potential theory, the equations of motion and the associated pressure field are derived for an idealized growing spherical gas bubble rising in an inviscid liquid under the influence of gravity from a horizontal plate-orifice and from a free standing nozzle. The dimensionless pressure (p_N?p₀)/πga at the gas source N (where p₀ is the undisturbed ambient pressure, a the instantaneous bubble radius and π the liquid density) is found to rise to a maximum when the dimensionless bubble position h/a = 1·55 for bubbles formed at a plate orifice and (125/48)^{$\text{1}\text{3}$} ? 1·38 for bubbles formed at a free standing nozzle. These positions of maximum pressure correspond well to experimentally observed positions at which the gas supply to bubbles grown at constant flow rate in water is cut off by the collapse of the neck linking bubble and gas source. The volumes of bubbles at this instant are predicted theoretically and compare well with experimentally determined values.     

E.s.r. study of free radicals in poly(vinyl carbazole)

Poly(N-vinylcarbazole) was γ-irradiated at 77 K. The analysis of e.s.r. spectra showed that cation radicals centred at N were produced in the system. Hyperfine splitting constants are: a^N=0.57 mT and a^H_β=0.68 mT. The dependence of the rate constant of free radical decay on pressure (1–800 MPa) was studied at various temperatures (90–170°C). Activation volumes were determined for two characteristic pressure and temperature ranges. The results of e.s.r. measurements are compared with those obtained from dynamic-mechanical measurements.     

Study of elementary reactions of free radicals formed on oxide and platinum containing catalysts

The formation of alkylperoxy radicals on catalysts CuCr₂O₄/ -Al₂O₃, Pt /--Al₂O₃ and on alumina upon the interaction of amines and alcohols with oxygen has been observed applying the matrix isolation ESR technique. Surface-generated gas phase radicals are shown to initiate a radical chain reaction. The efficiency of the yield of radicals depends on the catalyst mass and its arrangement in a reactor: the desorption of radicals into the gas phase occurs primarily from the outer geometrical surface of catalyst granules.     

Free radicals in coal and coal conversions. 6. Effects of liquefaction process variables on the in-situ observation of free radicals

To determine the effects and relative importance of process variables in coal liquefaction, a uniquely designed and fabricated high-pressure/high-temperature electron spin resonance (e.s.r.) apparatus is used to monitor the in-situ formation and behaviour of free radicals, which are generally assumed to be the key factor. It is concluded that the temperature is the most significant single process variable that affects free radical formation; for Powhatan No. 5 coal there is a 9-fold increase in going from 400 to 460 °C. At 460 °C the other process variables tested can affect significantly the free radicals significantly, but at 400 °C these variables have essentially no effect on free radicals formation. The next most significant effect is due to the combination of solvent nature and residence time. Tetralin and the SRC-II heavy distillate quench the free radicals from Powhatan No. 5 to the same extent with one significant difference. In tetralin the maximum concentration is observed shortly after the slurry achieves its highest temperature, whereas in the SRC-II heavy distillate experiments the concentration is still increasing, at 460 °C, even after 1 h of reaction. The heating time, pressures and types of gas used affect the free radical concentration to a much smaller extent. The conversions obtained in the in-situ e.s.r. experiments using SRC-II heavy distillate as the solvent are somewhat lower than those obtained with tetralin as the solvent. The corresponding oil yields with tetralin are considerably higher than with SRC-II heavy distillate.     

Stability of Y-TZP during hydrothermal treatment: neutron experiments and stability considerations

The degradation of yttria-stabilised zirconia in humid atmospheres at temperatures around 250 °C is influenced by a number of microstructural parameters. The corrosive attack starts from the surface and is associated with the tetragonal to monoclinic transformation. In order to investigate structural changes chemically homogeneous single phase t-powders had been hydrothermally treated in a D₂O-atmosphere and subsequently investigated by neutron diffraction. Slight structural changes were observed after water treatment: a general contraction of both a and c parameters, hence, the “free” structural z(oxygen) parameter as well as the c/a-ratio are closer to the values of cubic zirconia. In this paper the structural changes due to the penetration of water radicals during hydrolysis are discussed. They give rise to stresses which then have an impact on the energy balance between the tetragonal and the monoclinic phase (martensitic transformations). Since the powder state can be assumed to be mostly free from macro-straining, the changes of the lattice constants were used to deduct a shape change tensor. Hence assuming linear elastic behaviour (isotropic elastic modulus of 200 GPa for both a surface grain and its environment) maximum stresses of 328 MPa for the a-axis and 488 MPa for the c-axis were found. Thus, water radicals basically lead to further tensile stresses in the surface which increase the energy difference between the t and m phase, i.e. a further destabilisation.     

Reduction of (NO x + N2O) on different catalysts in their joint presence in the nitrous gas

To select a catalyst for purification of the tail gas from (NO _x + N₂O) present in the nitrous gas in the production of nitric acid, we (1) studied the composition of the tail gas in UKL-7 units, as well as in the combined schemes 1/3.5 at and AK-72 and AK-72M on large-scale selective (SCP) and nonselective (NSCP) catalytic purification plants, and (2) carried out pilot tests of different catalysts supported on γ-Al₂O₃, calcium aluminate, and CaO using industrial gas. Tests were carried out in a reactor 1 l in volume at flow rates of 12000 h^?1 and, for the ruthenium catalyst, at 25500 h^?1. NSCP was shown to decrease the N₂O content in the tail gas to 0.002 vol %. In SCP processes, NO _x is completely reduced but N₂O is formed. Over the Zr catalyst, NO _x can be reduced without the production of N₂O. Over the ruthenium catalyst, approximately 62% N₂O is decomposed at 410–540°C.     

Mechanism of the Reduction by Ammonia of Nitrates Stored onto a Pt–Ba/Al2O3 LNT Catalyst

The mechanism involved in the formation of N₂ and of N₂O during the reduction of nitrates stored onto a Pt–Ba/Al₂O₃ LNT catalyst is investigated using labeled NO and unlabeled ammonia, in the presence and in the absence of NO in the gas phase. The reduction of the stored NO _x species (labeled nitrates) with NH₃ leads to the selective formation of N₂. Based on the isotopic distribution, it appears that N₂ formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NO and NH₃ at metal Pt sites. When the reduction of the stored nitrates is carried out in the presence of NO in the gas phase, NO is preferentially reduced. This implies that the rate determining step of the reduction of nitrates by ammonia is likely associated with the release of stored NO _x . Negligible amounts of nitrous oxide have been observed during the NH₃-TPSR with adsorbed nitrates, whereas relevant quantities of N₂O have been detected at low temperatures (below 180 °C) in the runs performed in the presence of NO in the gas phase. The data converge to indicate that N₂O formation involves the presence of gaseous NO and this suggests that the formation of nitrous oxide occurs either through the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NH _x species.     

Reduction of NO in the exhaust gas by reaction with N radicals

N atoms can serve as a reducing agent for NO in the exhaust gases from fossil fuel power plants. In order to investigate the reduction of NO, synthetic exhaust gas was plasma treated in a dielectric barrier discharge (DBD) (direct treatment) as well as treated by adding nitrogen atoms generated in a pure nitrogen DBD (remote treatment). A DBD with a coaxial electrode geometry was used. Oxygen could be added to the synthetic exhaust. A complete NO reduction was achieved in dry, oxygen free exhaust gas with direct treatment, but oxygen and humidity in the exhaust gas promotes formation of N_xO_y and reduces the efficiency of NO reduction. A 90% NO reduction was achieved with remote treatment. Remote treatment is insensitive to oxygen in the exhaust, but requires large amounts of nitrogen and energy. Fourier transform infrared spectroscopy and ultraviolet absorption spectroscopy were employed in order to detect NO, NO₂, and N₂O in the treated exhaust gas stream.     

Hydrogen separation and purification in membranes of miscible polymer blends with interpenetration networks

This study demonstrates the successful implications of blending technique cum chemical modification for the fabrication of high performance polymeric membranes for gas separation applications. The effect of variation in composition on miscibility and microstructure, gas permeability and selectivity of blend membranes is investigated. It is found that augmentation in PBI composition results in enhancement in gas separation performance of membranes which is attributed mainly to the effect of diffusivity selectivity. Analysis of the microstructure of membranes confirms the variations in chain packing density, d-spacing and segmental mobility of polymer chains as a result of blending. Separation performance of membranes is further ameliorated through chemical modification of blend constituents. Modification of PBI phase with p-xylene dichloride brings about slight improvements in selectivity performance, especially for H₂/CO₂ and H₂/N₂. In contrast, the selectivity of membranes is improved significantly after cross-linking of Matrimid phase with p-xylene diamine. The results indicate that higher tendency of Matrimid toward cross-linking reaction contributes more in controlling the transport properties of membranes through diffusion coefficient by increase in chain packing density and diminishing the excess free volumes. Results obtained in this study reveal the promising features of developed membranes for gas separation applications with great potential for hydrogen separation and purification on industrial scale.