Surface & Coatings Technology 205 (2011) 5312–5317

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Surface & Coatings Technology

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ESR study of atmospheric pressure plasma jet irradiated aramid fibers

Jie Sun a,b, Lan Yao a,b, Shiyuan Sun a,b,c, Yiping Qiu a,b,⁎a Key Laboratory of Textile Science and Technology, Ministry of Education, Chinab Department of Technical Textiles, College of Textiles, Donghua University, Shanghai 201620, Chinac College of Textiles and Clothing Engineering, Dezhou University, Shandong 253023, China

⁎ Corresponding author at: Department of TechnicaDonghua University, Shanghai 201620, China. Tel.: +867792627.

E-mail address: ypqiu@dhu.edu.cn (Y. Qiu).

0257-8972/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.surfcoat.2011.05.045

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 January 2011Accepted in revised form 24 May 2011Available online 7 June 2011

Keywords:Atmospheric pressure plasma jetAramid fiberMoistureFree radicalsElectron spin resonance

Electron spin resonance (ESR) spectroscopy is used to investigate the free radicals trapped in aramid (Kevlar 49)fibers by an atmospheric pressure plasma jet treatment. The effects of various plasma operational conditions andfiber moisture content on free radical products are evaluated. It demonstrates that both free radical formationreaction and coupling reaction occur and compete with each other on the fiber surface during the plasmatreatment and thefinal free radical outcomes are determinedby the competition results, depending on treatmentconditions. Fiber moisture content not only has a tremendous influence on the concentration of the trapped freeradicals but also affect their evolution. Radicals trapped in moistened fibers show poorer stability than thosetrapped in dry fibers during storage in nitrogen. However, when the plasma treated samples are stored in a dryenvironment allowing oxygen access to the fiber surface, free radicals trapped in dry fibers decay rapidly whilethose in moistened fibers increase within the chosen storage period.

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1. Introduction

The commercial aramid fiber, poly (p-phenylene terephthalamide),or PPTA, has receivedmuchattention due to its high tensile strength andmodulus, good thermal resistance, and low specific density [1–3].Whenit is used as a reinforcing fiber in composites, fiber surface modificationmay be necessary to improve the fiber/matrix interfacial shear strength[4,5]. As an effective and environmentally favorable technique, plasmatreatments provide an ideal means for modifying surface properties ofpolymeric fibers [6–8]. Of which, the recently emerged atmosphericpressure plasma techniqueprevails over lowpressure plasmas due to itsadvantages such as no need for a vacuum system, online processcapabilities, high efficiency, and scalability to a larger area [9,10].

However, a potential problem for atmospheric pressure plasmatreatment is that water molecules absorbed into a hygroscopicsubstrate may interact with the plasma. It has been reported thatmoisture sorption, especially by hydrophilic fibers, leads to acontinuous change in the structure and hence properties of the fiberdue to swelling [11,12]. Therefore, the moisture in the air and thewater molecules absorbed into the surface layer of the substrate couldpotentially affect the results of atmospheric pressure plasmatreatment [13,14]. Much work has been done in this regard in ourprevious studies [15,16]. For aramid fibers, our studies show that theplasma etching effect is facilitated by increased moisture regain of the

fibers, and more polar groups are introduced onto the fiber surface,leading to a more significant improvement in the interfacial bondingstrength between the fiber and epoxy. It follows that the moisturecontent of the substrate is a unique and important parameter inatmospheric pressure plasma treatment just as such parameters asoutput power, gas flow rate, and duration.

Plasma surface modification is accomplished by active speciesgenerated during the gas discharge through interacting physically orchemically with the substrate surface molecules [17]. Most of theinteractions involve formation of free radicals on the substrate surfaceand it is these free radicals that play vital roles in initiating plasmainduced reactions due to the reactive nature of their unpairedelectrons [18]. In addition, plasma induced free radicals are alsoneeded to initiate grafting polymerizations for surface modification ofpolymers. Therefore, study of such radicals (radical quantity,structure, reactivity and stability) is of outmost importance forunderstanding of the nature of the plasma treatment and fordiscussion on comparable basis about the experimental data obtainedunder various plasma treatment conditions. Electron spin resonance(ESR) is one of the most reliable methods for studying and identifyingfree radicals [19]. ESR measurement has been used in the study ofthermal degradation or stress induced free radicals in aramid fibers[20,21]. However, little has been reported about free radicalsgenerated by plasma treatments, especially the atmospheric pressureplasmas although abundant reports are available on the plasmassurface modification of aramid fibers. Measurement of such radicalscan be used as chemical diagnostic tool for evaluation of plasmatreatment effectiveness in experimental studies and practical appli-cations of plasma treatments [22].

5313J. Sun et al. / Surface & Coatings Technology 205 (2011) 5312–5317

In this study, plasma induced free radicals trapped in aramid fiberand their behaviors were studied using ESR measurement. The effectsof various plasma-operational parameters such as output power,duration, gas flow rate and jet-to-substrate distance (JTSD) wereinvestigated. Furthermore, the influence of fiber moisture content onthe free radical outcome and evolution was also studied under variousstorage conditions.

2. Experimental

2.1. Material

The aramid fibers used in this study were Kevlar 49 supplied byDuPont Company. The average diameter of a single fiber was about12 μm, and the tensile modulus was 124 GPa. Before the plasmatreatment, the fibers were washed with acetone for 10 min and thenwith distilled water for another 10 min, and dried in a vacuum oven atroom temperature for 24 h to remove the surface finishes andcontamination.

In order to prepare samples with different moisture regain (MR)defined as the mass of the absorbed moisture divided by the dry massof the substrate, the fibers were divided into three groups. One groupwas kept in a 5% relative humidity (RH) dessicator for plasmatreatment. Another group was balanced in 20 °C and 65% RH for atleast 24 h before plasma treatment. Still another group wassuspended above water in a sealed container with 85% RH for 24 h.The MR for the three groups was calculated as 0.5%, 3.5%, and 4.8%respectively. Fibers with 3.5% MR were generally used for plasmatreatment unless otherwise specified.

2.2. Plasma treatment

The plasma treatment was carried out using an APPJ, modelAtomfloTM-R manufactured by Surfx Technologies, USA. Employingcapacitively coupled electrode design, the source used helium as thecarrier gas and produced a stable discharge at atmospheric pressurewhen a 13.65 MHz radio frequency power was applied. More detailedillustration about the apparatus has been given in our previouspublication [13]. The plasma applicator had a diameter of 25 mm andwas mounted perpendicular to the sample platform. Table 1 showsthe processing parameters, when one parameter changed, the otherparameters were kept constant.

2.3. ESR measurement

Electron spin resonance spectroscopy was carried out on a BrukerEMX-8/2.7 X-band ESR spectrometer. The frequency ν of the appliedelectromagnetic (microwave) radiation was fixed at 9.8 GHz (withinthe X band) and themagneticfield Bwas varied in order to generate theESR spectra. The sample was contained in a quartz tube (i.d.=5 mm)sealed with a plastic cab and mounted perpendicular to the ESRmagnetic field. For comparison, each sample was prepared with anequal mass. The spectra were recorded at room temperature in themagneticfield ranging from3430 G to3530 Gandwere given as thefirstderivative of the signal intensity as a function of themagnetic field. Theresonance value for an isotropic singlet signal was taken as the

Table 1Processing parameters of atmospheric pressure plasma jet treatment.

Parameter Possible values

Output power (W) 40, 60, 80Helium flow rate (LPM) 20, 30, 40Oxygen content (%) 1Treatment time (s) 8, 16, 32, 48Jet-to-substrate distance (mm) 1.5, 3, 6

resonance position B0, the value of a magnetic field where the signalarose, or g value, which depended on the chemical environment of theunpaired electron concerned [23,24]. The g value can be determined inan ESR experiment by measuring the field and the frequency at whichresonance occurs. In principle, quantitative measurements of absolutefree radical concentration (spinnumber/cm2) are basedon comparisonsof the areas under the resonance absorption (ESR signals) curvesobtained from the tested samples with those from freshly preparedstandards of a known concentration. In that case, the double integralmethod may be adopted to calculate the areas under the ESR signal.However it is technologically difficult to obtain a standard sample andmake quantitative measurements of the absolute free radical concen-tration of our samples. Therefor this study compares the free radicalintensity generated on the same kind ofmaterial under different plasmatreatment conditions or post-treatment storage conditions. We noticedthat these samples give similarly shaped spectra that have the same linewidth. The height of a derivative signal, i.e. the vertical distance betweenthe maximum and the minimum, was used as a measurement of itsintensity [24,25]. The change in the signal height is proportional to thechange in the radical content.

3. Results and discussion

3.1. Radical species and fiber moisture regain dependence of radicalconcentration

Fig. 1 presents the ESR spectra of the aramid fibers with differentMR exposed to the same APPJ treatment condition (32 s, 60 W, He/O2:20/0.2 LPM, JTSD: 3 mm). The spectra (except for samples used forradical decay test) weremeasured 3 h after the plasma treatment. Theparamagnetic centers showed a single, symmetrical ESR absorption ofg=2.0045 and the line width ΔΒpp=18 G. The resonance spectra areanalogous with the spectra of PPTA degraded by photo irradiation,heating or stress [26,27]. They are generally regarded as to be ascribedto delocalized carbon-centered free radical structure resulting fromsecondary hemolytic bond scission and crosslinking reactions and theformation of conjugated structure [21,26]. The absence of anyhyperfine splitting implied that the observed radicals were the resultsof extensive bond-breaking and bond-forming reactions of the fibersurface molecules [21]. No significant differences in the spectra lineshape were observed under different conditions, while, the ampli-tudes of the spectra increased with the decrease of fiber MR. Asevidenced in Fig. 1, the free radical concentration in the relatively drysample (0.5%MR)was almost twice as much as that of the wet sample(4.8%MR). The results indicated that the existence of water moleculesin the fiber has a significant impact on the plasma-induced free radicalyield.

Fig. 1. ESR spectra of aramid fibers with different moisture regain irradiated by APPJ(60 W, 32 s, He/O2: 20/0.2 LPM, JTSD: 3 mm).

5314 J. Sun et al. / Surface & Coatings Technology 205 (2011) 5312–5317

This phenomenon may be attributed to the change of moleculararrangement on or near the substrate surface resulted from theinvasion of the water molecules. It is known that hydrogen bondscontribute to the intermolecular force in polyamides, normally withenergy of 14.6–21.7 KJ/mol [28]. When an aramid fiber is exposed to ahumid environment, water diffuses into its amorphous regions, andhydrogen bonds between the molecular chains are disrupted in favorof forming hydrogen bond between the amide group and the watermolecules [16]. According to Nissan's theory [29], the dissociationmodes of the hydrogen bonds depend much on the amount ofabsorbed moisture, and when the water absorption reaches a certainlevel, the breaking down of hydrogen bonds occurs cooperatively in acluster. Therefore, a high amount of moisture absorption can result ina more relaxed molecular structure and a decrease of fibercrystallinity, facilitating the plasma etching reactions [16,30]. It ispossible that the activated low molecular weight compoundsproduced by intense plasma etching and the absorbed watermolecules in fiber could combine with the free radicals immediatelyafter they were formed [18]. In addition, increasedmolecular mobilitydue to the looser macromolecular structure at higher moisture regaincould make free radical recombination easier [31], which may alsoaccount for the low free radical intensity.

3.2. Plasma treatment condition dependence of radical concentration

Fig. 2 shows the influence of plasma treatment RF power on freeradical generation for the aramid fiber plasma treated for 32 s. Freeradical concentration increased slightly first as the RF powerincreased from 40 to 60 W, and then dropped substantially at 80 W.This relation reflected that, when the power went beyond a certainlevel, further increasing the powermight not further increase or couldeven decrease the free radical concentration. Similar phenomenonwas also observed in earlier reports [18,19]. During the process ofplasma–substrate interactions, generation and decay reactions of thefree radicals both occurred and competed with each other [32]. Theincrease in output power is related to the increase in the plasmadensity [33], namely, higher power brought about an increase in thenumber and the activity of the active species in the plasma jet. Thisfeature induced more intense surface molecular fragmentation andenhanced the plasma efficiency in generating instantaneous freeradicals. It may be surmised that when the instantaneous concentra-tion of free radicals generated on the fiber surface exceeded a giventhreshold, the coupling reaction could become the dominantpathway, resulting in a reduction of final detectable free radicalconcentration [19]. As a result, at higher output power, the totalenergy required to generate the critical concentration of free radicals

Fig. 2. Progressive changes of the ESR spectra for APPJ irradiated aramid fibers as afunction of RF frequency (32 s, He/O2: 20/0.2 LPM, JTSD: 3 mm).

was reached in a shorter time [19]. A further exposure to the plasmadischarge made the free radical recombination reaction increasinglymore competitive, leading to a reduction in final free radicalconcentration.

Fig. 3 showed the progressive change in ESR spectra of the aramidfiber treated by APPJ at 80 W in terms of plasma duration, providingan evidence of the above explanation. For the sample with 8 s ofplasma treatment duration at 80 W, the free radical concentration iscomparable to that of sample treated for 32 s at 60 W, while a declinein free radical concentration could be observed with prolongedplasma treatment. As stated above, intra- or inter-chain crosslinkingthrough radical recombination may account for the phenomenon.

For the samples treated by APPJ at 80 w for 32 s (JTSD: 3 mm), theprogressive change in ESR spectra related to the gas flow rate is givenin Fig. 4. It appeared that the radical concentration increasedappreciably as the gas flow rate switched from 20 to 30 LPM, andthen tended to level off at a higher flow rate. The results indicated thatthe gas flow rate had a significant effect on the free radical generation,which may be explained as follows. A high flow rate quicklytransported the active species to the substrate surface [17], resultingin an enhanced plasma emission intensity and consequently a higherefficiency in creating free radicals. However, coupling reaction ispromoted by the rising in instantaneous free spin concentration dueto the close vicinity of unstable radicals. Therefore, a further increasein the gas flow rate did not make much more radical yields.

Fig. 5 exhibits the influence of JTSD on the free radical outcomes forKevlar fibers treated at 80 W for 32 s. No obvious change in freeradical concentration could be observed as the JTSD increased from1.5 to 3.0 mm. At atmospheric pressure, the mean free path for gasmolecules are extremely short (about 6×10−5 mm), which thusprovides great opportunity for active species in plasma and air gasmolecules to collide with one another. As the distance increases to acertain level, the velocity and activity of active species greatlydecrease when they reach the substrate surface due to the highcollision probability [34]. Besides, in atmospheric pressure plasma,atomic oxygen and nitrogen as well as meta-stable molecules, whocontribute to chain scission reaction, will be formed in a very shortdistance from the nozzle [35]. Therefore, with the increase in JTSD, theplasma efficiency in generating free radicals could be hindered.However unexpectedly, it seems that the free radical concentrationfor 6 mm distance was higher than that of the two smaller distances.In this regard, we may explain it from the free radical recombinationmechanism as discussed above. The higher detectable amounts of freeradicals generated in the sample treated at 6 mm indicated that, freeradical formation reaction was more competitive than radicalcoupling reaction at that distance.

Fig. 3. Progressive changes of the ESR spectra for APPJ irradiated aramid fibers as afunction of plasma duration (80 W, He/O2: 20/0.2 LPM, JTSD: 3 mm).

Fig. 4. Progressive changes of the ESR spectra for APPJ irradiated aramid fibers as afunction of Helium flow rate (32 s, 80 W, JTSD: 3 mm).

a

b

Fig. 6. Time evolution of the ESR spectra for APPJ irradiated dry (0.5% MR) aramid fibers(60 W, He/O2: 20/0.2 LPM, JTSD: 3 mm) upon storing at different conditions: (a) undernitrogen atmosphere; (b) in a desiccator allowing oxygen access to the samples.

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3.3. Radical decay

In order to gain an insight into the evolution behavior of living freeradicals trapped in the aramid fiber (MR 0.5%) in contact with oxygen,the plasma irradiated (60 W, He/O2:20/0.2 LPM, JTSD: 3 mm) fiberswere stored under two different conditions. One group was sealedinside a quartz tube filled with inert nitrogen gas, and the other groupwas contained in an open quartz tube and stored in a dessicator with5% RH. In the latter circumstance, air can flow through the openingallowing the trapped living free radical to react insufficiently withoxygen. Fig. 6 shows the ESR spectra changewith the storage time. Fig.6(a) and (b) presents the ESR spectra measured from the fibers storedin nitrogen for 3–40 h, and those contacted insufficiently with oxygenfor the same periods respectively.

After 40 h of storage, while the shape of the ESR spectra remainedunchanged for both samples, the intensity decreased in varyingdegrees depending on the storage conditions. For the samples storedin conditions allowing insufficient contact with oxygen, the freeradical intensity reduced 50% in 15 h, and then decreased graduallywith prolonged storage. However, under nitrogen atmosphereprotection, only a small reduction in the signal intensity occurredafter the first 15 h storage and 80% of the initially trapped free radicalsstill remained in the fiber even after 40 h storage. This result indicatedthat in nitrogen, the free radicals trapped in the aramid fibers wererelatively stable and the recombination reaction was not prone tooccur at room temperature, consistent with the characteristic ofdelocalized carbon-centered radical [21]. When oxygen had access tothe plasma treated aramid fiber, small oxygen molecule could

Fig. 5. Progressive changes of the ESR spectra for APPJ irradiated aramid fibers as afunction of JTSD (80 W, He/O2: 20/0.2 LPM, JTSD: 3 mm).

penetrate the open, hydrogen-bonded structure of the aramids andscavenge radicals [27], thus accounting for the obvious drop in freeradical concentration.

For comparison, the lifetime of radicals trapped in the wet aramidfiber (4.8% MR) through plasma treatment at 60 w for 32 s (He/O2:20/0.2 LPM, JTSD: 3 mm) were also examined by storage in thesame two different conditions as described above for storing plasmatreated dry samples. During storage in nitrogen atmosphere, theradical concentration decreased gradually with the storage time. Asshown in Fig. 7(a), after 40 h of storage, about 27% of the initialradicals decayed. The relatively higher decay rate over that of the drysamples could be attributed to the remaining water molecules in thefiber bulk, which could quench the radical as scavenger [36]. Duringstoring in a dessicator contacting insufficiently with oxygen, surpris-ingly, the radical concentration took on an upward tendency with thestorage time. It seems in contradiction to the decay law for freeradicals that reported previously [37,38]. Therefore, there must besome newly formed free radicals emerged during the storage in thiscondition. Among other circumstances, three factors existed makingthe reaction conditions different from others: 1) the remained watermolecules in the fiber; 2) dry storage environment (RH 0.5%); and 3)oxygen access to the fiber. According to literature [39,40], OH radicalcan still be generated or converted from the other radicals or ionsafterglow of plasma region with the presence of water molecules. Theinitiating reactions involving the excitation of H2Omolecule are givenin reactions (1)–(4):

e þ H2O→e þ •H þ •OH ð1Þ

a

b

Fig. 7. Time evolution of the ESR spectra for APPJ irradiated moistened (4.8% MR)aramid fibers (60 W, He/O2: 20/0.2 LPM, JTSD: 3 mm) upon storing at differentconditions: (a) under nitrogen atmosphere; (b) in a desiccator allowing oxygen accessto the samples.

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Oð1DÞ þ H2O→2•OH ð2Þ

2H2O→H2O2 þ H2 ð3Þ

2H2O→H3Oþ þ e þ •OH ð4Þ

Therefore, it is possible that the fiber surface adsorbed water alsotook part in the interaction with plasma under the bombardment ofthe plasma active species, and brought about the formation of activespecies such as OH radical, H radical, H2, H2O2, H3O+etc. when thetreated samples were placed into the dessicator while allowingoxygen access to the fiber, the possible propagation reactions are [41]:

H• þ O2→HO2• ð5Þ

HO2 þ O2 þ •H→1O2 þ H2O2 ð6Þ

O2 þ •OH→1O2 þ OH

� ð7Þ

H• þ H2O2→H2O þ •OH ð8Þ

•OH þ H2O2→H2O þ HO2 ð9Þ

As a result, more reactive oxygen species (ROS) especially OHradical are formed. The OH radical is highly reactive and play a crucialrole for their strong oxidation in many physiochemical processes[42,43]. M. Dizdaroglu etc.[44] pointed that the OH radical is able toadd to double bonds of DNA base to form OH-adduct radicals and alsocan abstracts an H atom from the melthyl group of thymine to formallylic radicals. Research in literature [45] demonstrated that, in theabsence of ionizing radiation, OH radical can abstract a hydrogen atomfrom the protein polypepitide backbone to form a carbon-centeredradical. Therefore, it is reasonable to speculate that it is the reactionbetween the OH radial and fiber matrix that leads to the generation ofnew free radicals. In addition, water-absorption by silica gel in thedessicator may stimulate such reactions as (8), (9) that produce H2O,which to some extent may also stimulate the whole reaction cycle forfree radical generation. Further experiments are needed to clarify theexact mechanism for this phenomenon.

4. Conclusions

The ESR spectra of aramid fibers treated with atmosphericpressure plasma were single and symmetric lines with 18 G linewidth located close to the free electron g value. The signal intensitydecreases obviously with the increase of fiber moisture regain. Anychange in plasma operation parameters such as output power,duration, JTSD, and gas flow rate could bring about a variation inthe free radical yield. During the plasma treatment, both free radicalformation reaction and coupling reaction occurred and competedwith each other, which determined the final free radical outcomes.Fiber moisture regain also had a great impact on the evolution ofplasma generated free radicals. For plasma treated dry fibers, the freeradical showed excellent stability under nitrogen atmosphereprotection, while the radical intensity decreased rapidly when thefibers contacted insufficiently with oxygen in a dry environment. Forthe plasma treated moistened fibers, free radical decay rate innitrogen atmosphere was higher than that of the dry samples, andmost surprisingly, an increase in free radical concentration within thetime period chosen here was observed during storage in the same dryconditions with access to oxygen.

Acknowledgments

This work was supported by the National High TechnologyResearch and Development Program of China (No. 2007AA03Z101),the State Key Program of National Natural Science of China(No.51035003), Natural Science Foundation for the Youth (No.50803010 and 60904056), and the Program of Introducing Talentsof Discipline to Universities (No. B07024), and the FundamentalResearch Funds for the Central Universities. We also want to thankProfs. Chen and Lin in the Institute of Nutritional Science at ChineseAcademy of Sciences for their help in the ESR measurements.

References

[1] H.H. Yang, Kevlar Aramid Fiber, JohnWiley & Sons, Chichester, U.K., 1993.[2] D.A. Mooney, J.M.D. MacElroy, Langmuir 23 (2007) (1811) 11804.[3] W.R. Hall III, W.F. Knoff, J. Eng. Fibers Fabr. 3 (2008) 15.[4] J. Kalantar, L.T. Drzal, J. Mater. Sci. 25 (1990) 4194.[5] Y.J. Hwang, Y. Qui, C. Zhang, B. Jarrard, R. Stedeford, J. Tsai, Y.C. Park, M. McCord, J.

Adhes. Sci. Technol. 17 (2003) 7.[6] T. Schuman, B. Adolfsson, M. Wikstrom, Prog. Org. Coat. 54 (2005) 88.[7] Morent, N. De Geyter, J. Verschuren, K.D. Clerck, P. Kiekens, C. Leys, Surf. Coat.

Technol. 202 (2008) 427.

5317J. Sun et al. / Surface & Coatings Technology 205 (2011) 5312–5317

[8] Hocker, Pure Appl. Chem. 74 (2002) 423.[9] Y. Qiu, Y.J. Hwang, C. Zhang, B.L. Bures, M. Mccord, J. Adhes. Sci. Technol. 16 (2002)

449.[10] T.C. Montie, K. Kelly-Wintenberg, J.R. Roth, IEEE Trans. Plasma Sci. 28 (2000) 41.[11] J.F. Fuzek, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 140.[12] M. Fukuda, H. Kawai, J. Polym. Sci. B Polym. Phys. 35 (1997) 1423.[13] L. Liu, Q. Jiang, T. Zhu, J. Appl. Polym. Sci. 102 (2006) 242.[14] Y. Liu, H. Xu, L. Ge, L. Han, H. Yu, Y. Qiu, J. Adhes. Sci. Technol. 21 (2007) 663.[15] Y. Ren, C.X. Wang, Y.P. Qiu, Appl. Surf. Sci. 253 (2007) 9283.[16] L. Zhu, W.H. Teng, H.L. Xu, Y. Liu, Q. Jiang, C.X. Wang, Surf Coat. Tech. 202 (2008)

1966.[17] M.H. Jung, H.S. Choi, Thin Soild Film 515 (2006) 2295.[18] H.S. Sabharwal, F. Denes, L. Nielsen, R.A. Young, J. Agric. Food Chem. 41 (1993) 2202.[19] L. Andreozzi, V. Castelvetro, G. Cianluca, L. Corsi, M. Faetti, E. Fatarella, F. Zulli, J.

Colloid Interface Sci. 289 (2005) 455.[20] I.M. Brown, T.C. Sandreczki, Polymer 25 (1984) 759.[21] J.R. Brown, K.C. Hodgeman, Polymer 23 (1982) 365.[22] M. Kuzuya, K. Ito, S. Kondo, Y. Yamauchi, Thin Solid Film 345 (1999) 85.[23] R.V. Parish, NMR, NQR, EPR, and Mössbauer spectroscopy in inorganic chemistry,

Ellis Horwood Limited, Chichester, 1990.[24] M.J. Larsen, Y. Ma, H. Qian, H. Toftlund, P.B. Lund, E.M. Skou, Solid State Ionics 181

(2010) 201.[25] M. Trihi, J.L. Duroux, M.J. Hyvernaud, M. Bernard, Appl. Radiat. Isot. 47 (1996) 156.[26] D.J. Carlsson, L.H. Gan, D.M. Wiles, J. Polym. Sci. A Polym. Chem. 16 (1978) 2353.

[27] D.J. Carlsson, L.H. Gan, D.M. Wiles, J. Polym. Sci. A Polym. Chem. 16 (1978) 2365.[28] S. Wu, Polymer Interface and Adhesion, Marcel Dekker, Inc., New York, 1982.[29] A.H. Nissan, Macromolecules 10 (1977) 660.[30] Z.Q. Gao, S.J. Peng, J. Sun, L. Yao, Y.P. Qiu, Curr. Appl. Phys. 10 (2010) 230.[31] J.R. Chen, J. Appl. Polym. Sci. 62 (1996) 1325.[32] N. Inagaki, Plasma Surface Modification and Plasma Polymerization, Technomic

Pub. Co., Inc., Lancaster, Pennsylvania, 1996.[33] Z. Falkenstein, J. Appl. Phys. 83 (1998) 5095.[34] W. Rakowski, J. Soc. Dyers Colour. 113 (1997) 50.[35] G.J.H. Brussaard, K.G.Y. Letourneur, M. Schaepkens, M.C.M. van de Sanden, D.C.

Schram, J. Vac. Sci. Technol. B 21 (2003) 61.[36] T. Memetea, V. Stannett, Polymer (20) (1979) 469.[37] Y. Zhao, M. Wang, Z. Tang, G. Wu, Radiat. Phys. Chem. 79 (2010) 29.[38] M.J. Larsen, Y. Ma, H. Toftlund, P.B. Lund, E.M. Skou, Solid State Ionics 181 (2010)

201.[39] Y. Guo, X. Liao, D. Ye, J. Environ. Sci. 20 (2008) 1429.[40] R. Ono, T. Oda, J. Electrostat. 55 (2002) 333.[41] W.T. Shin, N. Sung, Environ. Eng. Res. 10 (2005) 144.[42] F. Liu, W. Wanga, W. Zheng, Y. Wang, Eur. Phys. J. D. 42 (2007) 435.[43] M. Valko, C.J. Rhodes, J. Moncol, M. Izakovic, M. Mazur, Chem. Biol. Interact. 160

(2006) 1.[44] M. Dizdaroglu, P. Jaruga, M. Birincioglu, H. Rodriguez, Free Radic. Biol. Med. 32

(2002) 1102.[45] E.R. Stadtman, Science 257 (1992) 1220.