Formation Of Contact Glow Discharge Electrolysis Plasma And Its Application In Polymerization

Contact glow discharge electrolysis (CGDE) is a novel electrochemical process in which plasma is sustained by dc glow discharge between an electrode and the surface of electrolyte. The conventional electrolysis is developed into CGDE when the applied voltage is sufficiently high in aqueous media. There are a lot of energetic species produced by CGDE, such as H2O2,·OH,·H, e-aq, HO2·and so on, and these energetic species can diffuse into the solution around the anode. Therefore, CGDE can provide rich active intermediates for chemistry reaction in aqueous solution. This paper consists of seven parts, studying in detail the formation of plasma by CGDE, the polymerization induced by CGDE and the application of CGDE to the treatment of wastewater.(1) The characteristics and method to produce cold plasma were described in brief. The formation of plasma by CGDE depends on temperature, stirring, electrode materials, electrode dimensions and so on. The CGDE plasma has some significant effects on wastewater treatment, chemical synthesis and initiation polymerization. In addition, there was a mini-review concerning the polymerization method and technology. The plasma-initiated polymerization and its application were emphasized. It is indicated that the CGDE plasma can initiate suspension polymerization, emulsion polymerization and aqueous solution polymerization.(2) Current-voltage characteristics of glow discharge electrolysis process were analyzed, and the different physical phenomena and chemical effects were observed at the different discharge stages. The effects of solution conductivity and electrode polarity on the current-voltage curves were examined. The concentrations of·OH and·H were detected by high performance liquid chromatography (HPLC) and the H2O2 was determined by the redox titration. The formation rate constants of three energetic species were calculated based on the experimental data under different applied voltage, solution conductivity and pH. The yield of hydrogen peroxide obtained from experiment was higher than that expected on the basis of Faraday's law. Moreover, taking 4-nitrophenol as an example of contaminants and tert-butanol as a scavenger of hydroxyl radical, the role of energetic species in degrading organic compounds was examined in detail.(3) An effective and green technique for polymerization to produce polymethylmethacrylate (PMMA) was developed, in which the chemical initiators were replaced by the CGDE plasma. The highest molecular weight (Mn) and the lowest polydispersity index (PDI) of the resulting polymers were 1.12×10~6 and 1.21, respectively. The effects of various important parameters on the conversion, Mn and PDI, such as the discharge voltage, discharge time, the content of methylmethacrylate (MMA), the amount of suspension stabilizer (polyvinyl alcohol), polymerization temperature and post polymerization time were examined in detail. Mn and PDI of polymer can be controlled with the control of discharge parameters and polymerization conditions. PMMA was characterized by gel permeation chromatographer (GPC), Fourier transform infrared spectroscopy (FT-IR), cold field emission scanning electron microscopy (FESEM), nuclear magnetic resonance (1H NMR) and thermogravimetric analysis (TGA). Results indicated that PMMA prepared by the CGDE plasma and PMMA by 2, 2′-azobisisobutyronitrile (AIBN) as an initiator had similar chemical structure, tacticity and thermal stability. However, the morphology of these two polymers was different, that is, the diameter of polymer particle by AIBN was 0.5～1 m while one by the CGDE plasma was 0.2～0.5 m. The polymerization of MMA by the CGDE plasma obeys the first order kinetic law; however, there is an obvious induction period during polymerization process.(4) Poly(methylmethacrylate-co-styrene) was prepared using suspension polymerization initiated by the CGDE plasma. The effects of various important parameters on the conversion, Mn and PDI, such as the discharge voltage, discharge time, the content of monomer, polymerization temperature, post polymerization time and oxygen were investigated. Mn and PDI of the resulting polymers were 2.26×10~5 and 1.78, respectively. The copolymer obtained was characterized by GPC, FT-IR, 1H NMR, FESEM and TGA. The kinetic and mechanism of suspension polymerization induced by the CGDE plasma was discussed. The results indicated that the polymerization obeys the first order kinetic law and radical mechanism.(5) The emulsion polymerization of MMA initiated by CGDE was studied. The effects of various important parameters on the conversion, Mn and PDI, such as the discharge voltage, discharge time, the content of monomer, the amount of emulsor, polymerization temperature and post polymerization time were investigated. A polymer with higher Mn and lower PDI was obtained. The polymer obtained was characterized by GPC, FT-IR, 1H NMR, FESEM and TG-DTA. The results indicated that PMMA has, in most cases, the syndiotactic structure with high thermal stability, having a diameter of 0.5~1 m, approximately.(6) A convenient and effective technique for polymerization to produce the poly(acrylic acid-co-acrylamide)/montmorillonite superabsorbent composite in aqueous solution was developed. The superabsorbent with water absorbency about 1024 g/g for distilled water as well as 56 g/g for 0.9% NaCl solution was obtained. The effects of various important parameters on the water absorbency of superabsorbent composite, such as the discharge voltage, discharge time, the ratio of acrylic acid to acrylamide, neutralization of acrylic acid, the amount of crosslinking agent and montmorillonite added were examined in detail. The superabsorbent composite was characterized by FT-IR, SEM, TEM and TGA. Results indicated that montmorillonite was effectively compounded with polymer, moreover, the water absorbency, water retention and thermal stability of the superabsorbent composite prepared by CGDE was higher than that of the superabsorbent composite by initiator under the same polymerization conditions.(7) The degradation of catechol by the CGDE plasma was studied. Ultraviolet (UV) absorption spectra and HPLC were used to monitor the degradation process and to identify the major intermediates. The results illustrated that the pH of solution, discharge voltage, initial concentration and stirring speed played an important role in the degradation of catechol, especially, Fe2 + ion having a remarkable catalytic effect. A kinetic model of catechol degradation was proposed on the basis of experimental data. The rate constant and the concentration of catechol calculated by the kinetic model were in good agreement with experimental data. The studies on degradation mechanism demonstrated that mass transfer became the rate-determining step of the degradation of organics by CGDE. The whole degradation process was analyzed from the point of view of thermodynamics, and the energy efficiency of the degradation induced by CGDE must be improved.