Ar Atmospheric Pressure Plasma Research Articles

A novel combination of non-thermal plasma process and a heterogenous nanophotocatalyst-sorbent comprising zinc sulfide, graphene oxide and polyaniline as an efficient strategy for treatment of organic-contaminated water

A dielectric barrier discharge (DBD) reactor was designed in the present study to generate non-thermal atmospheric pressure Ar plasma. A heterogeneous p-n-p visible-driven photocatalyst comprising ZnS, Graphene oxide, and Polyaniline (ZGP) was also synthesized via chemical polymerization. A novel combination of photocatalyst and plasma treatment was then developed for the synergetic removal of organic pollutants from aqueous solutions. The addition of the p-n-p photocatalyst (5 mg) to the dye solution (malachite green, 50 mL, 20 mg/L) under plasma radiation resulted in a 97 % removal rate in 12 min. Scavenging studies confirmed a decline in the effectiveness of the reactive species in the following order: OH• > e־ > O2־• > h+. The changes in surface chemistry of the recovered photocatalyst were probed by XPS, and possible involved reactions were suggested. The synergetic factor of 1.53 in this system was a remarkable achievement comparable with the records of air or oxygen plasma reactors. The absence of hazardous by-products was confirmed by GC–MS and AAS techniques. The treated aqueous solution was utilized to evaluate its effect on lentil seed growth and trout blood factors, which led to promising results with several advantages but no negative effects.

Optimized Gingiva Cell Behavior on Dental Zirconia as a Result of Atmospheric Argon Plasma Activation.

Several physico-chemical modifications have been developed to improve cell contact with prosthetic oral implant surfaces. The activation with non-thermal plasmas was one option. Previous studies found that gingiva fibroblasts on laser-microstructured ceramics were hindered in their migration into cavities. However, after argon (Ar) plasma activation, the cells concentrated in and around the niches. The change in surface properties of zirconia and, subsequently, the effect on cell behavior is unclear. In this study, polished zirconia discs were activated by atmospheric pressure Ar plasma using the kINPen®09 jet for 1 min. Surfaces were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and water contact angle. In vitro studies with human gingival fibroblasts (HGF-1) focused on spreading, actin cytoskeleton organization, and calcium ion signaling within 24 h. After Ar plasma activation, surfaces were more hydrophilic. XPS revealed decreased carbon and increased oxygen, zirconia, and yttrium content after Ar plasma. The Ar plasma activation boosted the spreading (2 h), and HGF-1 cells formed strong actin filaments with pronounced lamellipodia. Interestingly, the cells' calcium ion signaling was also promoted. Therefore, argon plasma activation of zirconia seems to be a valuable tool to bioactivate the surface for optimal surface occupation by cells and active cell signaling.

Characteristics of atmospheric pressure Ar-plasma around a spherical particle: Numerical study

The characteristics of atmospheric pressure Ar-plasma such as the spatial distributions of number densities, fluxes, and temperatures of electrons and heavy plasma particles, as well as the spatial distribution of electric potential around an individual spherical particle, were studied numerically on the basis of the hydrodynamic (diffusion) equations for plasma components. The governing equations were formulated for plasma that is not in thermal and ionization equilibrium. The boundary conditions near the particle were set on the edge of the space charge layer (sheath) adjacent to the particle surface. The nonlinear problem was solved taking into account the temperature dependencies of transport and kinetic coefficients. The heat flux introduced by the plasma into the particle was calculated and compared with the results of the simple heat conduction model. The range 10 − 5 – 10 − 4 m of particle radius and the range 6–18 kK of unperturbed plasma temperature were considered.

Effect of small amount of CF4 addition on He and Ar atmospheric pressure plasma jet and influence of relative permittivity of downstream material on plasma–surface interaction

CF4 is an important source of fluorine groups in atmospheric pressure plasma jet (APPJ). In order to obtain reactive fluorine species under atmospheric pressure, noble gas (Ar or He) and CF4 are usually mixed and used as the working gas of APPJ. In this paper, the differences in the discharge dynamics on He/CF4 APPJ and Ar/CF4 APPJ are investigated experimentally. Meanwhile, combined with simulation, the effects of downstream targets with different relative permittivity on the radial propagation range of the plasma plume and the distribution of F-containing reactive species are studied. It is discovered that the addition of a small amount of CF4 (20 sccm) will increase the intensity of He/CF4 APPJ due to the contribution of Penning ionization of metastable He with CF4. Differently, the addition of CF4 will continuously lead to a significant decrease in the intensity of Ar/CF4 APPJ. The radial propagation range of He/CF4 APPJ on the target surface decreases with the increase in the relative permittivity of the downstream target. The smaller relative permittivity inhibits the axial propagation speed of APPJ, but it increases the radial propagation range of reactive species. The larger relative permittivity promotes the production of F-containing reactive species and their flux intensity on the target surface.

On the green aurora emission of Ar atmospheric pressure plasma

The Ar atmospheric pressure plasma was found to be an excellent laboratorial source for green aurora emission. However, the characteristic and production mechanism of the green aurora emission of the Ar atmospheric pressure plasma are still not clear. In this work, an Ar plasma in a long glass tube which emits intense green aurora light is investigated. With the long glass tube, it can be concluded that the green aurora emission in the Ar plasma is not owing to the mixture of Ar plasma plume with the surrounding air. It is also found that the green aurora emission often appeared beyond the active electrode when the active electrode is placed at the downstream of the gas flow. The green emission disappears when the traces amount of O2 or N2 (about 0.05%–0.07%) is added to Ar. This is because the O2 molecules deactivate the upper state O(1S), which results in the decrease of the green emission. On the other hand, when N2 is added, Ar metastable atoms are quenched by N2, which results in the decrease of O atoms and eventually leads to the decrease of the green emission intensity. The intensity of the green aurora emission increases when the driving voltage frequency increases from 1 to 10 kHz. More importantly, it is found that the green aurora emission is not affected when a grounded stainless steel needle is in contact with the plasma plume. Thus, the green emission is not driven electrically. All these findings are helpful for the understanding of the physics and its applications of atmospheric pressure plasma jet in space physics, laser physics and other application areas.

Vacuum UV (VUV) Photo-Oxidation of Polyethersulfone (PES)

International need for water quality is placing a high demand on separation technology to develop advanced oxidative processes for polyethersulfone (PES) membranes to help improve water purification. Therefore, VUV photo-oxidation with a low pressure Ar plasma was studied to improve the hydrophilicity of PES by flowing oxygen over the surface during treatment. X-ray photoelectron spectroscopy (XPS) detected a decrease in the C at% (4.4 ± 1.7 at%), increase in O at% (3.7 ± 1.0 at%), and a constant S at% (5.4 ± 0.2 at%). Curve fitting of the XPS spectra showed a decrease in sp2 C-C aromatic group bonding, and an increase in C-O, C-S, O=C-OH, sulphonate (-SO3) and sulphate (-SO4) functional groups with treatment time. The water contact angle decreased from 71.9° for untreated PES down to a saturation level of 41.9° with treatment. Since scanning electron microscopy (SEM) showed no major changes in surface roughness, the increase in hydrophilicity was mainly due to oxidation of the surface. Washing the VUV photo-oxidized PES samples with water or ethanol increased the water contact angle saturation level up to 66° indicating the formation of a weak boundary layer.

Plasma Enhanced Fluorine-Free Superhydrophobic Polyester (PET) Fabric with Ultra-Robust Antibacterial and Antibacterial Adhesion Properties

Superhydrophobic antibacterial fabric possesses properties of antibacterial and antibacterial adhesion and shows huge demand in the field of medical textiles. However, current technologies are unable to fully address this. Hence, a simple method is highly desirable. Herein, the pristine polyester (PET) fabric is immersed into the solution containing ZnO nanoparticle and polydimethylsiloxane (PDMS), and the fiber surfaces are uniformly covered by a ZnO-PDMS layer after being treated by low pressure Ar plasma. The weight gain rate of the treated fabric is 3.5%, which is basically unchanged, and the air permeability, moisture permeability, and tensile properties of the fabric are basically not affected. It is found that the water contact angle (WCA) of the fabric is over 162.7°and sliding angle (SA) is less than 10°. The stable binding of PDMS and PET fibers induces a robust superhydrophobicity; even after 300 washing cycles and 600 friction cycles, it still remains superhydrophobic. The antibacterial rates of Escherichia coli and Staphylococcus aureus before washing were 99.89% and 99.85%, respectively, and after 100 cycles of washing, the antibacterial rates decreased to 99.36% and 99.17%, respectively. Therefore, it shows a good development prospect in the application of protecting clothing or textiles that require good antibacterial properties (such as bed sheets, duvet covers, etc.).

Interactions between atmospheric pressure plasma jet and deionized water surface

This study describes atmospheric pressure Ar plasma jet interactions with and without deionized (DI) water. The plasma jet was operated under an applied voltage in the range 1.8–2.5 kV with a 50 kHz sinusoidal wave. The dissipated power without DI water measured at the grounded copper tape increased along with the applied voltage. However, the power with DI water saturated above 2.3 kV. The hydroxyl radicals and plasma electrons, as well as the emission intensities of atomic oxygen and argon increased rapidly via plasma interaction with DI water beyond an applied voltage of 2.3 kV, whereas the electron temperatures decreased significantly. The experimental results show the possibility of identifying the region of interaction between plasma and DI water surface using these electrical and optical characteristics, as well as the plasma parameters of electron temperature and density.

Characterization of a microsecond pulsed non-equilibrium atmospheric pressure Ar plasma using laser scattering and optical emission spectroscopy

A non-equilibrium atmospheric pressure argon (Ar) plasma excited by microsecond pulse is studied experimentally by laser scattering and optical emission spectroscopy (OES), and theoretically by collisional-radiative (CR) model. More specifically, the electron temperature and electron density of plasma are obtained directly by the laser Thomson scattering, the gas temperature is measured by laser Raman scattering, the optical emissions of excited Ar states of plasma are measured by OES. The laser scattering results show that the electron temperature is about 1 eV which is similar to that excited by 60 Hz AC power, but the gas temperature is as low as 300 K compared to about 700 K excited by 60 Hz AC power. It is shown that the microsecond pulsed power supply, rather than nanosecond ones, is short enough to reduce the gas temperature of atmospheric pressure plasma to near room temperature. The electron temperature and electron density are also obtained by CR model based on OES, and find that the intensities of the optical emission intensity lines of 727.41, 811.73, 841.08, 842.83, 852.44 and 912.86 nm of Ar can be used to characterize the behavior of electron density and electron temperature, it is very useful to quickly estimate the activity of the atmospheric pressure Ar plasma in many applications.

Evaluation of biomarkers for haemocomopatibility of polymer biomaterials

Tubes are indispensable in our daily life, mechanical engineering and biomedical fields. However, the practical applications of tubes are sometimes limited by their poor wettability. Reported herein is hydrophilization of the tube inner wall by transferred atmospheric pressure plasma (TAPP). An Ar atmospheric pressure plasma jet (APPJ) is used to induce He TAPP inside polytetrafluoroethylene (PTFE) tube to perform inner wall surface modification. Optical emission spectrum (OES) is used to investigate the distribution of active species, which are known as enablers for surface modification, along the TAPP. Tubes’ surface properties demonstrate that after TAPP treatment, the wettability of the tube inner wall is well improved due to the decrease of surface roughness, the removal of surface fluorine and introduction of oxygen. Notably, a deep surface modification can significantly retard the aging of the obtained hydrophilicity. The results presented here clearly demonstrate the great potential of TAPP for surface modification of the inner wall of tube or other hollow bodies, and thus a uniform, effective and long-lasting surface modification of tube with any length can be easily realized by moving the tube along its axis.

Free radical generation by non-equilibrium atmospheric pressure plasma in alcohol–water mixtures: an EPR-spin trapping study

Free radical species in aqueous solution—various alcohol–water reaction mixtures—by exposure to non-equilibrium cold atmospheric pressure Ar plasma (CAP), were monitored using electron paramagnetic resonance spin-trapping techniques with 3, 5-dibromo-4-nitrosobenzene sulfonate as a water soluble nitroso spin trap. The major radical species were formed by H-abstraction from alcohol molecules due to ·OH radicals. In the ethanol–water mixture ·CH2CH2OH produced by H abstraction from CH3 group of the ethanol and ·CH3 radicals were detected. The latter was due to the decomposition of unstable CH3·CHOH to form the ·CH3 radicals and the stable formaldehyde by C–C bond fission. These intermediates are similar to those observed by reaction with ·OH radicals generation in the H2O2–UV photolysis of the reaction mixtures. The evidence of ·CH3 radical formation in the pyrolytic decomposition of the reaction mixtures by exposure to ultrasound or in methane irradiated with microwave plasma have been reported previously. However, the pyrolytic ·CH3 radicals were not found in both plasma and H2O2–UV photolysis condition. These results suggests that free radicals produced by Ar-CAP are most likely due to the reaction between abundant ·OH radicals and alcohol molecules.

Building 3D porous, elastic and hydrophilic current collectors: Prolonging the cycling life of Si based anode for lithium ion batteries

Huge volume variation plagued silicon for large-scale application in high energy density lithium ion batteries serving as anodic materials, making active layers cracked or delaminated from current collectors after several cycles. To address this issue, a free-standing 3D porous carbon based current collector with recoverable mechanical robustness is dedicatedly designed and fabricated. Atmospheric pressure Ar plasma jet irradiation is applied to transit its surface wettability from hydrophobic to hydrophilic, making it compatible with the aqueous slurry casting process. Embedding Si nanoparticles without any architecture modification into such a state-of-the-art current collector delivers an excellent reversible capacity (1138mAhg−1), prolonged cycling performance (>400cycles) at the current density of 0.5Ag−1 and low capacity fading rate (<0.09% per cycle). This also provides considerable opportunities as a current collector of choice for the energy storage units in the ever-growing flexible and wearable electronic devices.

Spatial distributions of O, N, NO, OH and vacuum ultraviolet light along gas flow direction in an AC-excited atmospheric pressure Ar plasma jet generated in open air

Ground-state O (3Pj) and N (4Sº) atoms, nitric oxide (NO) and hydroxyl (OH) radicals, and vacuum ultraviolet (VUV) radiation emitted from an AC-excited Ar atmospheric pressure plasma jet (APPJ) operated in open air were measured using VUV absorption spectroscopy, laser induced fluorescence spectroscopy and optical emission spectroscopy, respectively. The O (3Pj) density decreased with increasing distance from the plasma head, and the decrease rate in the edge region of the plasma jet around 8 mm distance was especially high. On the other hand, the N (4Sº) atom density was almost constant up to approximately 10 mm and increased with the distance over 12 mm. The relative NO density increased with the distance up to 10 mm and then became saturated, although the OH density was significantly reduced from the plasma jet exit of the head, and could be observed in the plasma jet only up to the distance of 5 mm. Emissions due to N, H and O atoms were observed in the VUV region and the emission intensity for N atoms (wavelength: 174.3 nm) decreased exponentially with increasing the distance. The results obtained with the Ar APPJ operated in open air conditions indicate the generation of many different reactive oxygen and nitrogen species by gas-phase reactions, and the composition of the reactive species is strongly dependent on the distance from the plasma head. These results are very useful for clarifying the kinetics of reactive species in APPJs and their application in various technological fields.

Gold nano-particle formation from crystalline AuCN: Comparison of thermal, plasma- and ion-beam activated decomposition

In this work, in addition to the conventional thermal process, two non-conventional ways, the plasma and ion beam activations are described for preparing gold nanoparticles from microcrystalline AuCN precursor. The phase formation at plasma and ion beam treatments was compared with that at thermal treatments and the products and transformations were characterized by thermogravimetry-mass-spectrometry (TG-MS), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). TG-MS measurements in Ar atmosphere revealed that AuCN decomposition starts at 400°C and completes at ≈700°C with evolution of gaseous (CN)2. XPS and TEM show that in heat treatment at 450°C for 1h in Ar, loss of nitrogen and carbon occurs and small, 5–30nm gold particles forms. Heating at 450°C for 10h in sealed ampoule, much larger, 60–200nm size and well faceted Au particles develop together with a fibrous (CN)n polymer phase, and the Au crystallites are covered by a 3–5nm thick polymer shell. Low pressure Ar plasma treatment at 300eV energy results in 4–20nm size Au particles and removes most of the nitrogen and part of carbon. During Ar+ ion bombardment with 2500eV energy, 5–30nm size Au crystallites form already in 10min, with preferential loss of nitrogen and with increased amount of carbon residue. The results suggest that plasma and ion beam activation, acting similarly to thermal treatment, may be used to prepare Au nanoparticles from AuCN on selected surface areas either by depositing AuCN precursors on selected regions or by focusing the applied ionized radiation. Thus they may offer alternative ways for preparing tailor-made catalysts, electronic devices and sensors for different applications.

Study of the Characteristics of Cold Ar Atmospheric Pressure Plasma Jet Generated With Inverted Tapered Tube

The atmospheric pressure plasma jet (APPJ) has become one of the hot research issues in plasma science and technology. In this paper, a cold Ar APPJ is generated using a dielectric barrier discharge configuration device equipped with an inverted tapered tube (ITT). Its characteristics are investigated by optical and electrical diagnostics, including discharge images, voltage–current waveforms, and optical emission spectra (OESs). The parameters, such as discharge power, gas temperature, and electronic excitation temperature (EET), are calculated and analyzed. All the above characteristics and parameters are compared with those of Ar APPJ generated with a conventional straight tube (CST). Furthermore, by numerical simulation, the flow field distribution of the ITT and CST is obtained and compared. The results show that the use of the ITT can raise Ar flow velocity, improve the distribution of Ar mole fraction, and Ar flow field. Moreover, the discharge power for the ITT is lower than that for the CST, so are the gas temperature and EET. The difference between theOESs is less obvious. Finally, the plasma plume develops along the inner surface of the CST with bits of apparent filamentary discharge, but along the central axis of the ITT and manifested as a stable and homogeneous discharge, which might be more promising for material processing and biomedical application in some cases.

Simultaneous and long-lasting hydrophilization of inner and outer wall surfaces of polytetrafluoroethylene tubes by transferring atmospheric pressure plasmas

Plasma hydrophilization is a general method to increase the surface free energy of materials. However, only a few works about plasma modification focus on the hydrophilization of tube inner and outer walls. In this paper, we realize simultaneous and long-lasting plasma hydrophilization on the inner and outer walls of polytetrafluoroethylene (PTFE) tubes by atmospheric pressure plasmas (APPs). Specifically, an Ar atmospheric pressure plasma jet (APPJ) is used to modify the PTFE tube’s outer wall and meanwhile to induce transferred He APP inside the PTFE tube to modify its inner wall surface. The optical emission spectrum (OES) shows that the plasmas contain many chemically active species, which are known as enablers for various applications. Water contact angle (WCA) measurements, x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) are used to characterize the plasma hydrophilization. Results demonstrate that the wettability of the tube walls are well improved due to the replacement of the surface fluorine by oxygen and the change of surface roughness. The obtained hydrophilicity decreases slowly during more than 180 d aging, indicating a long-lasting hydrophilization. The results presented here clearly demonstrate the great potential of transferring APPs for surface modification of the tube’s inner and outer walls simultaneously.

Surface modification of tube inner wall by transferred atmospheric pressure plasma

Tubes are indispensable in our daily life, mechanical engineering and biomedical fields. However, the practical applications of tubes are sometimes limited by their poor wettability. Reported herein is hydrophilization of the tube inner wall by transferred atmospheric pressure plasma (TAPP). An Ar atmospheric pressure plasma jet (APPJ) is used to induce He TAPP inside polytetrafluoroethylene (PTFE) tube to perform inner wall surface modification. Optical emission spectrum (OES) is used to investigate the distribution of active species, which are known as enablers for surface modification, along the TAPP. Tubes’ surface properties demonstrate that after TAPP treatment, the wettability of the tube inner wall is well improved due to the decrease of surface roughness, the removal of surface fluorine and introduction of oxygen. Notably, a deep surface modification can significantly retard the aging of the obtained hydrophilicity. The results presented here clearly demonstrate the great potential of TAPP for surface modification of the inner wall of tube or other hollow bodies, and thus a uniform, effective and long-lasting surface modification of tube with any length can be easily realized by moving the tube along its axis.

(Invited) Plasma Synthesis and Structural Control for Carbon Nanowalls for Novel Nano-Bio Devices

Carbon nanowalls (CNWs) are one of carbon nanomaterials. They consist of graphene sheets standing vertically on substrates, so that they have self-aligned three-dimensional structures. Significant recent attention has been focused on the functionalities of the CNWs because of their unique morphologies and excellent electrical properties. For example, since they have large surface-to-volume ratios and very high aspect ratios, they are expected as catalyst supporting materials in fuel cells, field emitters, and various kinds of templates. In addition, high-density graphene edges on the top-region of CNWs promise expression of novel functionalities and applications. In this study, we have investigated controlled synthesis techniques of carbon nanowalls, which have suitable morphologies and electrical properties for each device. Density ratios of hydrogen (H) and fluorocarbon (CFx (x=1-3)) radicals are essential to determine the density and height of CNWs, because the H radicals enhance desorption of fluorine (F) atoms from the adsorbed CFx. Ar+ ion irradiation is also important for the nucleation and vertical-growth of graphene sheets. According to observation results of their initial growth using ellipsometry and scanning electron microscopy, not only nucleation but also vertical-growth of graphene sheets could occur only under Ar+ ion irradiation with suitable energy and flux for each process. For example, when Ar+ion energy is too much, no film deposition occers due to etching. On the other hand, since too low energy ion bombardment can not induce nucleation of nanographen, only continuous amorphous carbon film grows on the substrate surface. Furthermore, when ion flux is too high, continuous amorphous carbon films grow too, since deposition rates of carbon related radicals much higher than growth rate of vertical graphene sheets. By using a supercritical fluid, high-density metal nanoparticles or metal oxide ones can be uniformly supported on the whole surface area of CNWs. For example, TiO2nanoparticles-supported CNWs efficiently desolve methylene blue molecules, which are typical test reagent for photocatalytic performance under UV irradiation. Pt nanoparticles-supported CNWs also show fine catalyst performance at cyclic voltammetry (CV) measurements. In addition, they also show very high durability at high potential cycle tests, which demonstrate start and stop operations of fuel cells. Furthermore, the CNWs also show multimodal and unique characteristic as cell culturing template. Preliminarily, the wide-range control of surface wettability of CNWs from superhydrophilic to superhydrophobic can be realized by the post-growth plasma treatments. Then, the dependence of the HeLa cell culturing rate and morphological change on the surface wettability of CNWs scaffolds was systematically evaluated. The cell culturing rates were significantly dependent on the CNW densities, although the surface wettability of the CNWs was not significantly dependent. Morphological changes of the cells were not significantly dependent on the density of CNWs. On the other hand, using the chemically surface-modified CNWs as electrodes, the CV characteristics were measured in PBS (PBS, pH 7) containing bovine serum albumin (BSA). Using the as-grown CNW electrode without Ar atmospheric pressure plasma treatment, weak oxidation and reduction peaks were observed in anode peak potential at 0.2 V and cathodic peak potential at -0.3 V, respectively. On the other hand, in the case of typical surface-oxidized CNW electrode, a broad oxidation peak was recorded with an anode peak potential of 0.2 V. A high peak reduction current was also observed in the cathodic peak potential at -0.75 V. In the case of surface-oxidized CNW electrode of low height, the CV profile exhibited small peak currents compared with the case using typical CNW-oxide electrode, due to the decrease of immobilized BSA on the reduced surface area. These results indicate that the high potentials of CNWs as a novel platform for nano-bio applications.

Interruption in Propagation of an Ar Atmospheric Pressure Plasma Jet Surrounded With Distilled Water

Surrounded with distilled water, a classic argon atmospheric pressure discharge device is employed to study the propagation of the plasma. By examining the high-speed photographs, a phenomenon of interruption is found at different flow rates. At 3 L/min, the interruption appears only at the downward stage of the applied voltage after the plasma jet has run into the ambient air. As the flow rate reaches up to 15 L/min, however, the interruption can be obtained at a rather low applied voltage and exists in both the upward and downward stages. It is believed that the active species flowing from the upstream to the downstream region may be the main cause.

Polymerization of Maleic Anhydride Induced by Non-Equilibrium Atmospheric Pressure Argon Plasma

The non-equilibrium atmospheric pressure Ar plasma was applied for the polymerization of maleic anhydride (MA). The deposited films were analyzed by using Fourier transform infrared spectroscopy (FT-IR) proving the monomer was successfully polymerized with retaining the functional groups. The intensity of optical emission spectroscopy (OES) of the plasma jet was found to become weaker when the monomer was introduced into the jet. This was interpreted as the result of the energy transfer from the metastable Ar to the monomer. It was proposed that the excited MA changed into π-π* transition state to produce dimer biradicals which initiate the polymerization.