Polymerization of organic compounds in an electrodeless glow discharge. XI. Pressure in glow discharge

The pressure of a steady-state flow of monomer, p₀, changes to a new steady-state flow pressure, p_g, in glow discharge. The value of p_g is dependent on the flow rate of monomer, the pumping-out rate of the vacuum system for the product gas (which is hydrogen in many cases of plasma polymerization of hydrocarbons), and the characteristic hydrogen yield of a monomer associated with plasma polymerization. The relationships between these factors were established and examined for plasma polymerizations of acetylene, ethylene, and acrylonitrile.     

Polymerization of organic compounds in an electrodeless glow discharge. VIII. Dependence of plasma polymerization of acrylonitrile on glow characteristic

The effects of experimental conditions (i.e., flow rate, pressure, discharge wattage, and glow characteristics) on the plasma polymerization of acrylonitrile were investigated. It was found that the glow characteristic is highly dependent on both flow rate and discharge wattage and that the plasma polymerization depends strongly on the glow characteristic. However, when experimental conditions are selected to maintain a fully developed glow in the tail flame portion of rf discharge, plasma polymerization is independent of discharge wattage and pressure. The polymer deposition rate is linearly proportional to the monomer flow rate. The deviations from this ideal situation are generally attributable to incomplete glow or partial glow under conditions which caused the deviation. The “character” of the glow largely determines the chemistry of the system. Consequently, the properties of polymers formed under different glow characteristics are also different.     

Polymerization of organic compounds in an electrodeless glow discharge. V. Amines and nitriles

Polymerizations of amines and nitriles in a glow discharge generated by inductive coupling of 13.5-MHz radio frequency are investigated by employing the measurement of closed-system pressure change. It is found that nitrogen in those compounds are incorporated into polymer in a nearly stoichiometric manner and that the polymerization of those compounds are very much similar to that of hydrocarbons. Hydrogen detachments, opening of double bond and/or cyclic structure, opening of triple bond including C?N, and opening of aromatic and heteroaromatic structures contribute to polymerization of amines and nitriles.     

Polymerization of organic compounds in an electrodeless glow discharge. IX. Flow-rate dependence of properties of plasma polymers of acetylene and acrylonitrile

Properties (free-radical concentration, gas permeabilities, internal stress, and contact angle of water) of plasma polymers of acetylene and of acrylonitrile were investigated as a function of flow rate of monomer in an electrodeless glow discharge. It was found that the monomer flow rate has a strong influence on free-radical concentration, gas permeabilities, and internal stress but little influence on the contact angle of water. The discharge power has little effect on properties when the full glow is maintained in the reactor. Gas permeabilities decrease with increasing concentration of free radicals in plasma polymers.     

Polymerization of organic compounds in an electrodeless glow discharge. VI. Acetylene with unusual comonomers

Polymerizations of acetylene with nonpolymerizing gases and vapors such as H₂O, N₂, and CO in a plasma generated by inductive coupling of 13.5 MHz radio frequency are investigated. It is found that acetylene copolymerizes with those comonomers and that properties of copolymers are distinctly different from that of plasma polymer of acetylene. The copolymerization with H₂O has a significant effect in reducing the trapped free radicals in the plasma polymer (to a nonexistent level). Infrared and electron spin resonance (ESR) spectroscopies and elemental analysis of polymers are used to investigate the incorporation of H₂O, N₂, and CO into the plasma polymers.     

Polymerization of organic compounds in an electrodeless glow discharge. IV. Hydrocarbons in a closed system

The polymerization of hydrocarbons was investigated by measuring the hydrogen yield during the glow discharge polymerization in a closed system. It was found that the pressure change in the glow discharge polymerization of hydrocarbons was mainly due to the production of hydrogen and to the loss of vapor phase monomer by polymerization. The opening of triple or double bonds and cyclic structures plays an important role in the polymerization of hydrocarbons; however, these are not exclusive mechanisms. The major polymerization mechanism for saturated normal hydrocarbons seems to be by the formation of free radicals due to hydrogen abstraction and the recombination of these primary radicals. The polymerization due to this mechanism also seems to occur concurrently during the polymerization of hydrocarbons with multiple bond and/or cyclic structures. Aromatic hydrocarbons polymerize with very low hydrogen production, indicating that the utilization of an aromatic double bond is the major mechanism of polymerization.     

Polymerization of styrene in an electrodeless glow discharge

The polymerization (polymer deposition) rate of styrene in an electrodeless glow discharge from styrene vapor and a mixture of styrene vapor and gas (H₂, He, A, and N₂) was investigated. The rate of polymerization, R, was found to be independent of the discharge power. The rate of polymerization of the pure monomer was found to be proportional to the square of monomer pressure p_M. The addition of gas increased the rate of polymerization depending upon the partial pressure of the gas, p_x, and R can be generally expressed by R = a[p_M]²{1 + b[p_x]}. The value of b is dependent of the type of gas and follows the order of N₂, > A > He > H₂. The distribution of polymer deposition was found to be nearly independent of the partial pressure of the gas and of the discharge power with N₂ and H₂ as plasma gas; however, with He and A, the distribution is highly dependent on the partial pressure of the gas and on the discharge power. The study strongly suggests that polymerization occurs in the vapor phase and that the growing polymer radicals deposit on the surface of the discharge vessel, yielding highly crosslinked polymer deposition.     

Polymerization in an electrodeless glow discharge. III. Organic compounds without olefinic doublebond

The rates of polymer deposition from various organic compounds which do not contain an olefinic doublebond in an electrodeless glow discharge were studied. The polymerization rates of these unconventional monomers are by and large similar to those of olefinic monomers reported in the previous study (part II). The rate of polymer deposition R₀ from pure monomer flow can be characterized, according to the analysis used in part II, by R_o = apM² and R₀ = kF_w, where pM is the vapor pressure of the monomer, F_w is the weight basis flow rate of the monomer. Type A monomers which predominantly polymerize and type B monomers which decompose in a glow discharge were also found with these unconventional monomers. The effects of structural factors on the values of a amd k and on the classification of types A and B were examined. These structures and groups—aromatic, heteroaromatic, nitrogen-containing (e.g., >NH,? NH₂,? CN), Si-containing, and olefinic doublebond—favor the polymerization. These structures and groups—oxygen-containing chlorine, aliphatic hydrocarbon chains, and cyclic hydrocarbon chains—favor the decomposition of the monomer in a glow discharge. It is postulated that the polymerization of organic compounds proceed by the recombination of excited species (probably free radicals) created by glow discharge and reexcitation followed by further recombinations in the vapor phase and at the interface.     

Polymerization in an electrodeless glow discharge. II. Olefinic monomers

The rates of polymer deposition from various olefinic monomers in an electrodeless glow discharge were studied. The previously found empirical relationship (with styrene in part I) between the rate of polymer deposition R, the monomer pressure p_M, and gas pressure p_x in a steady-state flow system (i.e., R = a(p_M)² [1 + b(p_x)], R being nearly independent of the discharge power) was also found with all monomers investigated. (The effect of gas was examined with nitrogen in this study.) However, it was found that the polymer deposition is controlled by the monomer flow rate and R_o (in pure monomer flow) is proportional to the flow rate of monomer F_w (based on the weight); i.e., R_o = kF_w, where k is a characteristic rate constant of the polymerization. Olefinic monomers can be generally classified into two major groups, i.e., type A monomers which predominantly polymerize, and type B monomers which decompose in a glow discharge. Type B monomers have smaller values of a and k compared to type A monomers. The values of a and k for type A monomers both increase with increasing molecular weight of the monomer. The values of k for all monomers investigated are within roughly an order of magnitude, indicating that the reactivity levels of monomers are very similar in a glow discharge polymerization.     

Effect of electrodeless glow discharge on polymers

The effects of gas plasma generated by electrodeless (inductive coupling) glow discharge on polymers were investigated as functions of gas pressure, discharge power, exposure time, and type of plasma gas. A remarkable similarity between the plasma susceptibilities of low molecular weight organic compounds and polymers was observed; i.e., polymers which have ether, carbonyl, ester, or carboxylic acid attached to a nonaromatic structure are very susceptible to plasma. The weight loss was proportional to the exposure time and exposed area. The discharge power and type of gas were found to have a great influence on both the rate of weight loss and the morphology of the exposed surface. The predominant effect of plasma on polymers was found to be degradation (manifested by weight loss). The crosslinking effect was found to be marginal with many polymers; however, significant crosslinking was observed with double bond-containing polymers. The crosslinking was examined by swelling the treated films. With copolymers of styrene–butadiene, 4-vinylpyridine–butadiene, methacrylic acid-butadiene, and acrylic acid–butadiene, the crosslinking was greatly dependent on the discharge power, the butadiene content of the copolymers, and the exposure time. Both degradation and crosslinking by gas plasma were generally limited to the exposed surface; however, the propagation of crosslinking in the direction of thickness was observed with copolymers of styrene–butadiene. The plasma of organic vapor also cause degradation of plasma-susceptible polymers, particularly at high wattage, although the deposition of polymer occurs simultaneously.     

Polymerization of organic compounds in an electrodeless glow discharge. VII. A fluid mechanic aspect of plasma and its effect on the polymer deposition rate

The deposition rate of polymer from plasma of an organic compound is influenced by the fluid mechanical aspect of plasma. Using cylindrical reactors which have constriction of various size, and utilizing the tail-flame portion of inductively coupled r.f. (13.5 MHz) glow discharge, it is demonstrated that the polymer deposition rate is proportional to the ratio of surface area/volume of the reactor tube, which has been often neglected in studies of polymer deposition rate.     

Wettability of glow discharge polymers

Treatment of good adhering glow discharge polymerized propylene (GDPP) coatings with reactive gas plasma from oxygen, nitrogen, or water (produced in a tubular reactor operation at 27.1 MHz) results in surfaces characterized by more hydrophilic interactions. Zisman's plots indicate a change in surface energy after such treatment. Transmission electron microscopy depicts that the pronounced improvement in wettability of GDPP polymer after oxygen plasma treatment in part results from the high surface development in the polymer. However, the effect is not permanent. The wettability of plasma-treated polymer diminishes with time and reaches a limiting value in 2–3 months indicating structural rearrangements at the polymer surface. Argon, carbon monoxide, or bromotrichloromethane plasma does not change the polymer surface wettability significantly. In contrast to the O₂ plasma-treated GDPP polymer, a glow discharged polymer synthesized from ?-caprolactam maintains its inherent hydrophilicily over an extended period of time.     

Adhesion of glow discharge polymers

Various reaction parameters in a propylene glow discharge polymerization were investigated with the objective of synthesizing films having good adhesion to metal and glass substrate. Monomer flow rate (at a constant pumping efficiency) was found to exert a significant effect on the quality of polymer adhesion to substrates. Good adhering polymeric films were obtained only at high flow rates where the deposition rate decreases with an increase in monomer flow rate. This observation applied also to ethylene, propane, allyl bromide, and ?-caprolactam. Transmission electron microscopy of the freshly deposited polymer films indicated a defect-free structure of the polymer surface obtained at high flow rates. The polymer surface obtained at low flow rates had a bead structure superimposed by uniformly placed circular defects.     

Adhesion of glow discharge polymers to metals and polymers

Adhesion of glow discharge polymers to metals and polymers in an adhesive joint was measured by lap-shear test and immersion in hot water of 70°C °C for an extended time. A glow discharge polymer was deposited onto polymers [polyethylene and poly(tetrafluoroethylene)] and metals (aluminum and stainless steel) prior to when the polymer and metal were joined. It is found that the lap-shear strength is enhanced by coating the surfaces of these substrates with plasma film produced from methane, ethylene, and acetylene, and that deterioration of the adhesive bonding part, when immersed in hot water of 70°C, is strongly dependent on the gas used as well as operational conditions where a polymer film is formed. The adhesion of a polymer produced from methane on the polymer and metal is strong enough to apply for durable, adhesive joints.     

Enhanced thermionic emission of mayenite electride composites in an Ar glow discharge plasma

Mayenite electride has attracted increasing research interests because of its unique electronic properties. The thermionic emission behavior of the mayenite electride is relatively unknown. Previous studies revealed that mayenite electride exhibited a bare work function ranged from 2.1–2.6 eV when the thermionic emission was tested in vacuum, and enhanced emission currents could be achieved by applying a super-high external electric field. In this paper, the thermionic emission behavior and the corresponding effective work function of two types of mayenite electride based composites, mayenite electride-titanium and mayenite electride-carbon, were investigated in an Ar glow discharge plasma at elevated temperatures (400–1000 K) without applying a high external electric field, which is critical for the application in electric propulsion and other aerospace apparatus, yet never had been done before. During the testing, the thermal equilibrium process and plasma sheath expansion were observed. The effective work function of the two mayenite electride composites were determined as a function of temperature. The Rasor-Warner model was applied to determine the bare work function and adsorption-site density of the mayenite electride based composites. Results suggested that the adsorption of Ar ions led to the enhanced thermionic emission (~30 A/m² at 985 K) and low effective work functions (0.9–2.2 eV) of mayenite electride based composites, without the need of applying high electric fields. Our findings will pave the ways for the application of mayenite electride and its composites as the thrust cathode materials for electric propulsion where plasma is present.     

溶液中辉光放电等离子体化学反应研究

分析了溶液中辉光放电等离子体过程中水分子在等离子体层、等离子体-溶液界面和主体溶液中的反应历程;介绍了水溶液体系中辉光放电等离子体电解引发的等离子体合成反应和高级氧化反应;对有机溶剂体系中的辉光放电等离子体电解反应进行了介绍.实验表明甲醇溶液辉光放电等离子体电解主要产物是氢气,还有少量一氧化碳、甲烷、乙烷、丙烷、1,3,5-三(口恶)烷和水等,气相产物中氢气摩尔分数在86%以上.根据甲醇溶液中GDE过程中得到的产物分析,提出了甲醇分解的可能反应历程.     

Preliminary experiment of surface hardening of polymers by glow discharge polymerization

Polymers including polyethylene, polycarbonate, and polytetrafluoroethylene were modified by glow discharge polymerization to enhance surface hardness. The surface hardness of the polymer substrates could be improved by glow discharge polymerizations of silicon-containing compounds. The mixture of tetramethylsilane (TMS) and oxygen was more effective than tetramethoxysilane to improve the surface hardness. The surface hardness improved by the glow discharge polymerization strongly depended on the nature of the polymer substrates to be modified. The adhesion between polymer films prepared from the TMS/O₂ mixture by glow discharge polymerization and the polymer substrates was good.     

Characterization of diamond fluorinated by glow discharge plasma treatment

The surface fluorination of diamond by treatment in glow discharge plasmas of CF₄ for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition (HFCVD). Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF₄ for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy (SEM). Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy (EPRS), revealing that untreated diamond possesses an appreciable density of free radicals (6×10¹² g⁻¹) which initially falls with treatment time in the CF₄ plasma but increases for long treatment times.     

Degradation of indole in aqueous solution using contact glow discharge plasma

Exhaustive indole oxidation in aqueous solution was studied using contact glow discharge plasma. The results indicated that the rate of indole degradation increases with the decrease in the solution conductivity. The degradation rate can be enhanced under the following situations. First, the increase in temperature. Second, introduce active carbon and hydrogen peroxide to the solution. Third, the degradation process is performed in alkaline or acidic media rather than the neutral media. Fourth, add Fe²⁺ to solution to undergo Fenton’s reaction. However, n-butanol was found decelerate the degradation of indole. Some major intermediates produced during the degradation were detected by using both HPLC and GC-MS.     

Effect of glow discharge plasma on rhodium-based catalyst for oxygenates synthesis

Rhodium-based catalysts were prepared by impregnation, treated with glow discharge plasma, characterized by X-ray diffraction, X-ray photoelectron spectroscopy, H₂ temperature-programmed reduction, H₂ temperature-programmed desorption and CO temperature-programmed desorption, and investigated for oxygenate synthesis from CO hydrogenation. Based on the characterization results, plasma treatment endowed the samples with smaller particle size, higher dispersion of active components, and an enrichment of active components on the surface as well. As a result, the reducibility and adsorption properties were modified. In catalytic tests, the catalytic activity for CO hydrogenation over the samples treated by plasma was improved remarkably: the conversion of CO and the yield of oxygenates increased at most by a factor of 78.62% and 51.96%, respectively, while the selectivity of ethanol and methanol in the oxygenates was enhanced as well. Translated from Natural Gas Chemical Industry, 2006, 31(1): 21–24 [译自: 天然气化工]