repetitively pulsed atmospheric pressure discharge treatment of rough polymer surfaces...

IOP PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGY

Plasma Sources Sci. Technol. 17 (2008) 035024 (11pp) doi:10.1088/0963-0252/17/3/035024

Repetitively pulsed atmospheric pressuredischarge treatment of rough polymersurfaces: I. Humid air dischargesAnanth N Bhoj1,3 and Mark J Kushner2,4

1 Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana, IL 61801, USA2 Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA

E-mail: [email protected] and [email protected]

Received 8 March 2008, in final form 29 May 2008Published 31 July 2008Online at stacks.iop.org/PSST/17/035024

AbstractPlasmas generated at atmospheric pressure are used to functionalize the surfaces of polymersby creating new surface-resident chemical groups. The polymers used in textiles andbiomedical applications often have non-planar surfaces whose functionalization requirespenetration of plasma generated species into sometimes complex surface features. In thisregard, the atmospheric pressure plasma treatment of a rough polypropylene surface wascomputationally investigated using a two-dimensional plasma hydrodynamics modelintegrated with a surface kinetics model. Repetitively pulsed discharges produced in adielectric barrier–corona configuration in humid air were considered to affix O. Macroscopicnon-uniformities in treatment result from the spatial variations in radical densities whichdepend on the polarity of the discharge. Microscopic non-uniformities arise due to the higherreactivity of plasma produced species, such as OH radicals, which are consumed before theycan diffuse deeper into surface features. The consequences of applied voltage magnitude andpolarity, and the relative humidity on discharge dynamics and radical generation leading tosurface functionalization, are discussed.

1. Introduction

Plasma functionalization of polymers entails the modificationof the topmost surface layers with chemically different atomsor functional groups to modify surface properties. Forexample, atmospheric pressure plasmas in air [1–3] arecommonly used to functionalize the surfaces of hydrocarbonpolymeric materials by affixing O-containing groups. Giventhe hydrocarbon backbone (represented by RH) some ofthese groups are alcohols (R–OH), peroxy (R–OO•), carbonyl(R–C=O) and acid (O=R–OH) [4, 5]. Higher O/C ratios inthe chemical composition of the topmost layers increase thesurface energy and so improve properties such as adhesion,wettability or reactivity.

There is interest in using plasmas to modify thesurface properties of natural and synthetic textiles as being

3 Present address: Novellus Systems, Inc., 3011 N. 1st St, San Jose,CA 95134, USA.4 Author to whom any correspondence should be addressed.

more environmentally friendly than wet processing [6, 7].Plasma treatment of textile fibers improves shrink resistance,hydrophilicity and color fastness [8,9]. Textile fabrics usuallyconsist of fibers in a warp and weft arrangement, creatingsurface features of a few to tens of micrometers [10]. Since theentire fabric surface is not completely exposed to the discharge,the ability of plasma produced reactive species to penetrate intosurface features is important to the quality and uniformity oftreatment [11–13].

Biomedical surfaces are also often non-planar and rough,and functionalized by plasmas to enhance (or prevent) desiredinteractions with living cells [14]. For example, plasmatreated implants and grafts have been used for many years.Polymer substrates are also modified using plasmas for cellmicro-patterning where selectively activated regions of thesurface are affixed with functional groups which promotecell adhesion [15]. Tissue engineering routinely involves thefunctionalization of topologically rough scaffold-like surfacesto promote cell adhesion and proliferation [16, 17].

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Plasma Sources Sci. Technol. 17 (2008) 035024 A N Bhoj and M J Kushner

Common to both of these classes of applications is thedesire that the plasma process uniformly treats the surfaceindependently of the local topography which requires thatreactive plasma produced species penetrate into this roughness.Gaining a deeper understanding of the interaction betweengas phase transport processes and surface reaction pathwaysleading to functionalization will enable optimization of therelative abundance of functional groups on the surface andeven the creation of tailored functional group gradients [18].

In this paper, and in part II [19], we discuss the resultsfrom a multiscale computational investigation of the plasmatreatment of rough and porous polypropylene (PP) surfaces ina repetitively pulsed atmospheric pressure corona dischargeusing an integrated two-dimensional plasma hydrodynamics–surface kinetics model. Discharges in air can efficientlyfunctionalize PP surfaces with O-containing groups due tothe abundant generation of reactive oxygen containing gasphase species such as O, OH and O3, and the fact that O2can affix to radical sites. Although charged particles do notplay a large role in directly functionalizing surfaces, we foundthat the charged particle dynamics indirectly influence theuniformity and the magnitude of treatment. For example, thepolarity of the voltage pulse has an effect on the extent ofplasma penetration into rough features. With positive coronadischarges, the electron density near small features (a few tomany micrometers) is low due to the formation of a cathodefall-like region, thereby reducing the amount of electron impactdissociation in the immediate vicinity of the feature. Radicalstake longer to penetrate into features by diffusion whereasrecombination of positive ions increases the supply radicalsat the surface.

We found that variations in the relative humidity affectthe microscopic uniformity of surface functionalization. Inlow humidity discharges, gas phase OH radicals are generatedclose to the surface as products of H abstraction by O atoms,in densities comparable to that produced by electron impactof H2O in higher humidity discharges. At higher appliedvoltages, the energy deposition per pulse increases, producingmore O atoms. This results in a higher overall degree offunctionalization but at the cost of increased macroscopic non-uniformity.

The model used in the investigation is described insection 2 and the results from these simulations are discussedin section 3. Our concluding remarks are in section 4.

2. Description of the model and reactionmechanisms

The two-dimensional modeling platform, non-PDPSIM, hasclosely coupled modules addressing plasma dynamics and gasphase kinetics. These modules are the same as describedin [20]. Briefly, the plasma dynamics module is used to solvePoisson’s equation for the electric potential simultaneouslywith the multi-fluid charged and neutral species conservationequations on an unstructured mesh. Poisson’s equation and theconservation equations for charged species are simultaneouslysolved. The conservation equations for electron temperature,neutral species densities and flow properties are sequentially

integrated in a time-splicing manner following the updatesof Poisson’s equation and charged particle densities. Theseupdates are then followed by integration of the continuityequations for surface-resident species. A radiation transportmodule is used to track photons emitted from designatedexcited species. These photons photoionize the gas and interactwith the polymer surface.

The model is capable of simulating the repetitively pulseddischarges (a few to tens of kilohertz). In these discharges,the duration of the pulse is typically short (10s ns at most)compared with the time between pulsing (usually �100s µs).During the discharge pulse, all of the equations addressingelectrostatics, and charged and neutral particle transport aresolved. After the termination of the pulse and charged particledensities have diminished to negligible values, the interpulseperiod (IP) begins. During the IP, only the conservationequations for neutral transport, and gas and surface reactionkinetics are integrated. After the IP, the discharge is reinitiated.This procedure is continued until quasi-steady state conditionsare achieved. At this point, and with prior knowledge thatfurther evolution of the surface coverages will not significantlyaffect the plasma properties, the time-varying fluxes of allplasma species at all locations on the surfaces are recordedduring an additional discharge pulse and IP. The recordedfluxes are then interpolated as a function of time whileexecuting the surface kinetics module (SKM) for additionalpulses and IPs.

The SKM is the same as that described in [20]. TheSKM consists of a modified surface site-balance algorithmimplemented at the nodes of specified materials that borderthe plasma. Briefly, the initial conditions for the SKM are thesurface coverages of species found on the virgin polymer.The SKM accepts fluxes of gas phase species and photons,and implements a user-specified surface reaction mechanismconsisting primarily of gas phase species reacting on thesurface sites. The mechanism also allows for photochemistryand reactions between surface sites. Outputs from the SKMinclude the fractional coverage of surface-resident species andreturning fractions for gas phase reactants and products.

The reaction mechanism used for humid air gas phasechemistry is essentially the same as that described in [21]. Gasspecies include N2, N2(A 3�), N∗∗2 , N

+2 , N

+, N+4 , N, N(2D),

O2, O2(a 1�), O2(b 1�), O+2, O−2 , O

−, O(1D), O(1S), O+,O, O3, H2O, H2O+, H2, H, OH and HO2. N∗∗2 nominallydenotes N2(b 1�, b′ 1�) but is used as a lumped stateincluding transitions higher than N2(A 3�). In order topropagate the positive corona streamer, photoionization wasincluded with N∗∗2 being the emitting species with a lifetimeof 5 ns, producing photoionization of O2 with a cross sectionof 10−19 cm−2. Electron impact reactions with N2, O2 andH2O including vibrational excitation, electronic excitation,attachment and ionization are included in the calculation ofthe electron energy distribution for generating rate coefficientseven though the vibrational states are not explicitly includedin the model.

The most important gas phase reactions of relevance tosurface treatment consist of neutral chemistry, initiated by

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electron impact reactions of O2 and H2O producing O andOH radicals,

e + O2 → O + O + e, (1)e + H2O → H + OH + e. (2)

At atmospheric pressure, three-body reactions dominatethe radical kinetics, where O reacts with O2 to form O3 and Hreacts with O2 to form HO2,

O + O2 + M → O3 + M (k = 6.9 × 10−34T −1.25g cm6 s−1),(3)

H + O2 + M → HO2 + M (k = 6.1 × 10−32T −1.6g cm6 s−1),(4)

where k is the rate coefficient at room temperature unless thereis specific temperature dependence noted. There are few majorgas phase consumption pathways for O3 at room temperatureand so it tends to accumulate from pulse to pulse. O3 doesreact at slow rates with H to form OH, with O to form O2 andwith OH to form HO2,

H + O3 → OH + O2 (k = 1.4 × 10−10e−470/Tg cm3 s−1),(5)

O + O3 → O2 + O2 (k = 8 × 10−12e−2060/Tg cm3 s−1),(6)

OH + O3 → HO2 + O2 (k = 1.9 × 10−12e−1000/Tg cm3 s−1).(7)

O3 does undergo other dissociative processes, forexample, reacting with O2 to form O2 and O. This process, aswell as others, are included in the reaction mechanism howeverthey are important only at higher gas temperatures than ofinterest here.

HO2 reacts slowly with O, H and OH forming radicalsor stable products. OH and O react mutually at slow rates toform H,

HO2 + O → OH + O2 (k = 2.9 × 10−11e−200/Tg cm3 s−1),(8)

HO2 + OH → H2O + O2 (k = 8 × 10−11 cm3 s−1), (9)HO2 + H → H2O + O (k = 2.4 × 10−12 cm3 s−1), (10)O + OH → H + O2 (k = 2.3 × 10−11e110/Tg cm3 s−1).

(11)

The surface reaction mechanism is essentially the sameas that discussed in [20]. Surface reactions are initiated byH abstraction from the hydrocarbon backbone (RH) by gasphase radicals, principally O and OH, creating active surfacealkyl sites (R•). The probability of a reaction may vary with thelocal H bonding at the site of the C atom due to bond polarityeffects, and that variation is accounted for in the mechanism.As a result, secondary and primary C atom sites were lessreactive than tertiary and secondary sites by a factor of 5–10,respectively. The initiating steps are

RH + O → R• + OH p = 10−3, (12)

RH + OH → R• + H2O p = 0.1, (13)

where p is the probability of reaction for the tertiary sites. R•

sites rapidly react with O and O3 to form alkoxy (R–O•) groups,

R• + O → R–O• p = 0.1, (14)R• + O3 → R–O• + O2 p = 1.0. (15)

In contrast, the reaction of R• sites with O2 to form peroxy(R–OO•) groups has a lower probability, but due to the largerdensities of O2 are fairly rapid,

R• + O2 → R–OO• p = 10−3. (16)Reactions between surface sites also occur (with the rate

coefficients specified in units of cm2 s−1). For example,abstraction of H atoms from surrounding sites by R–O• groupsresults in alcohol (R–OH) groups with a rate coefficient of10−16–10−15 cm2 s−1,

R–O• + RH → R–OH + R•, (17)which leaves an alkyl radical to sustain further reactions.Alternatively the C–C bond at the R–O• site cleaves, leading tochain scission and the formation of carbonyl (HR=O) groups,and the creation of a new R• radical site at rates of 103 s−1,

R–O• → R=O + R•. (18)The reaction of carbonyl groups with O and OH createscarbonyl radicals (•R=O), which, upon further reaction withOH, results in an acid group (HO–R=O),

HR=O + O → •R=O + OH, (19)HR=O + OH → •R=O + H2O, (20)

•R=O + OH → HO − R=O. (21)Continued chain scission similar to that occurring with alkoxygroups leads to the formation of CO2, a terminal step in thereaction chain,

•R = O + O → RH + CO2. (22)Other termination steps include the recombination of alkylradicals on the backbone with H and OH radicals,

R• + H → RH p = 0.2, (23)R• + OH → R–OH p = 0.2. (24)

The contribution of ions to the surface reactions inatmospheric pressure discharges is less clear. The highcollisionality of atmospheric pressure discharges results inions having near thermal energy close to the surface. It isfeasible for ions arriving at the surface to contribute to surfacechemical reactions in a manner similar to their neutral radicalcounterparts. For instance, thermal O+ and O+2 can reacton the surface in a manner similar to O and O2 since theyare typically neutralized prior to striking the surface. Non-reactive ions may still break chemical bonds due to the potentialenergy they bring or due to secondary electron emission.There are no specific data available in the literature for

3


the secondary electron emission coefficient (γ ) on dielectricsurfaces due to ion bombardment at atmospheric pressurethough estimates give γ = 0.01 [22]. In the context ofsurface chemical reactions, ion bombardment accompanied bysecondary electron emission most likely leads to the formationof alkyl sites (R•). A sensitivity analysis was performed byvarying γ from 10−3–1.0. It was determined that for thetransient discharges investigated in this work, the contributionsof ion-initiated reactions to surface functionalization are notsignificant due to the short pulse durations.

Photons emitted from the plasma are thought to play amore important role in surface kinetics [23–25]. UV photonsemitted with energies greater than 4–5 eV can cleave C–Cbonds forming alkyl sites. Skurat, et al [26] proposed that UVphotochemistry on hydrocarbon polymers proceeds via threemajor pathways, the dominant one being the elimination of H2forming adjacent alkyl radical sites (–·RR·–). These sites havea higher probability of undergoing scission to form R• sitesthan recombining [23],

RH + hν → •RR• + H2, (25)•RR• → R=R, (26)

•RR• → R• + R•. (27)The other two pathways result in the formation of R•

radicals by breaking either the C–H bond releasing atomichydrogen or the C–C bond scission in the PP backbone,

RH + hν → R• + H, (28)

RH + hν → R• + R•. (29)Skurat et al [26] estimated the relative rates of equation (29) areabout 4–5 times faster than those of equation (28). In this work,the total quantum yield of photons leading to the formation ofalkyl (R•) sites was taken to be 0.2 [27].

Surface radicals (R•) on adjacent backbones can react witheach other to crosslink [28–31]. The crosslinking reactionsincluded in the mechanism include

R• + R• → R × R p = 10−15 cm2 s−1, (30)

2(•RR•) → R × R p = 10−15 cm2 s−1, (31)where × indicates a crosslinked site.

3. Functionalization of rough surfaces in humid airdischarges

The humid air, atmospheric pressure discharge functionaliza-tion of PP with rough surface features of a few micrometerswas computationally investigated. The device considered isa corona–dielectric barrier similar to industrial web-treatmentdevices [32], and is shown schematically in figure 1 as imple-mented in the model. The geometry is symmetric across thecenterline. The upper metal electrode is housed in a dielectricwith an exposed tip and is typically powered at a few to 10 kHz.The gap between the tip of the upper electrode and the lowergrounded metal electrode is 2 mm. The rough PP surface is

Figure 1. Schematic of the corona treatment device. (a) Typicalweb-roller arrangement for polymer treatment. (b) Modelrepresentation of the discharge device with magnification of therough polymer surface having micrometer-sized strand-like features.

on the lower grounded electrode and so it effectively operatesas a dielectric-barrier discharge. Roughness on the polymersurface is resolved having strand-like features of a few mi-crometers to resemble textiles or scaffold-like surfaces for celladhesion. Note that the fine surface structure is included onlywithin 600 µm of the centerline to reduce the computationalburden. In the following discussion, macroscopic uniformityrefers to scale lengths of the discharge, up to a few millimeters.Microscopic uniformity refers to scale lengths of the surfacefeatures, many to tens of micrometers. These uniformities arebeing examined for a stationary surface (that is, a non-movingweb) without forced gas flow and so reflect the fundamentalprocesses. As discussed in [20], macroscopic non-uniformitiesmay be averaged out by motion of the web and forced gas flow.

The unstructured mesh is locally refined close to thepowered electrode and near the surface features with aresolution of about 1 µm so that reactor- and surface-scaleprocesses can be simultaneously resolved. Regions distantfrom the discharge zone have node spacings of 100 µm. Thetotal number of nodes is 21 296 with 9643 of these being in theplasma. Results from simulations with a smaller number ofsurface features and increased resolution did not significantly

4


Figure 2. Plasma properties after the first breakdown pulse in anegative humid air discharge. (a) Densities of electrons, positiveions and oxygen atoms. (b) Densities of electrons, O+2 , negativesurface charge and O atoms in the vicinity of the surface. Thecontour labels are fractions of the maximum density noted in eachfigure.

differ. The base case conditions are atmospheric pressureand N2/O2/H2O = 79/20/1 (relative humidity of 30%). Thedischarge operates at 10 kHz by biasing the powered electrodewith unipolar 15 kV pulses of either negative or positivepolarity.

The densities of electrons, all positive ions and O atoms atthe time the plasma closes the gap after a propagation time of1.6 ns are shown in figure 2 for a −15 kV pulse. The avalanche,terminated by charging of the PP, produces a plasma densityof 1013 cm−3 in the center of the gap and nearly 1014 cm−3 atthe tip of the electrode. The major radicals for PP processingproduced by electron impact dissociation are O and OH withdensities of a few times 1013 and 1012 cm−3 in the vicinity ofthe PP surface.

Densities of electrons, O+2, O, OH and surface charges atthe end of the discharge pulse near the surface are also shownin figure 2. The electrons and ions are able to penetrate intothe surface features only to a limited extent. The size of the

Figure 3. Plasma properties midway during the first interpulseperiod (duration 100 µs) of the negative discharge. (a) Density of O,OH and O3 in bulk plasma. (b) Density of O, OH, O3 and HO2 inthe vicinity of the surface. The contour labels are fractions of themaximum density noted in each figure.

features are commensurate with the Debye length (λD ≈ 1–10 µm). However, at times during and after the pulse, theion density inside the features is low enough that it is notmandatory that charged particle densities be quasi-neutral. Themore mobile electrons in the leading edge of the avalanchenegatively charge the surface of the features with local volumedensities of up to 100 µC cm−3. This surface charging, whichoccurs dominantly at sites with large view angles to the plasma,prevents electrons from deeply penetrating into the features.What penetration of electrons there is continues to produceO atoms by electron impact dissociation, though at the end ofthe discharge pulse, radicals are largely absent from deep insidethe features. Positive ions are accelerated into the features bythe negative surface charge and fill the features with densitiesof 1011 cm−3.

The densities of O, O3 and OH at 50 µs, halfway throughthe IP, are shown in figure 3(a). At this time, the peak densityof O atoms in the discharge region has been reduced by more

5


than an order of magnitude largely by the formation of O3,producing an ozone density of nearly 1015 cm−3. Since themain pathway for gas phase production of OH is electronimpact of H2O, the density of OH steadily decreases duringthe IP in the bulk.

The densities of O, OH, O3 and HO2 near the surfacefeatures near the end of the IP of 100 µs are shown infigure 3(b). At this late time, diffusion has enabled thedensities of species that slowly react with the surface to beessentially uniform over the scale of the roughness. Neutralradicals continue to penetrate into surface features by diffusion.O atoms, further depleted by the formation of O3 and reactionswith the surface, have densities of 5 × 1011 cm−3 inside thefeatures. O3 and HO2, species which tend to accumulate due totheir low reactivity, have densities of 8×1014 and 9×1013 cm−3inside the features. Earlier in the IP, the surface is a net sourceof OH as a result of H abstraction by O atoms. By the end of theIP, these sources decrease with the depletion of the O atoms.As a result, the surface becomes a net sink for OH given itsown reactivity with the surface, and the density of OH insidefeatures decreases.

The densities of electrons, positive ions and O atomsduring breakdown for a positive voltage pulse are shownin figure 4(a). As is characteristic of positive coronastreamers compared with negative streamers, the plasmacolumn is more confined because the positive corona is morecritically dependent on space charge and photoionization forpropagation. However, the charged particle densities in thebulk plasma are commensurate between the positive and thenegative discharges. With electrons drifting upwards, whenthe avalanche strikes the surface charging is positive and thedischarge spreads out forming a sheath like, cathode fall regionat the surface. The source of electrons in this region isdominantly by secondary emission by ions from the surface.The bulk of the ions are driven into the features by electricfield drift producing densities of 1011–1012 cm−3 and surfacecharge of up to a positive 1 µC cm−3. Since few electrons enterinto the features (


Figure 5. Plasma properties at the end of the first interpulse period(duration 100 µs) of the positive discharge. (a) Densities of O, OHand O3 in bulk plasma. (b) Densities of O, OH, O3 and HO2 in thevicinity of the surface. The contour labels are fractions of themaximum density noted in each figure.

positive discharge provides for more uniform fluxes on themacroscopic scale.

The densities surface groups on the PP in the negativedischarge during a single IP period [alkyl, (R•)] and afterseveral IPs [alkoxy (R–O•) and peroxy (R–OO•)] are shownin figure 7. Alkyl radicals are rapidly generated during andimmediately after a discharge pulse by abstraction of H atomsfrom the PP backbone by O and OH created within a fewmean-free paths of the surface. The alkyl coverage reachesa maximum fractional value of 2 × 10−5 (2 × 1010 cm−2)a few microseconds into the IP. For the remainder of theIP, alkyl sites are consumed by O, O3 and O2 from the gasphase producing alkoxy (R–O•) and peroxy (R–OO•) groups.This reduces the density of alkyl sites to


(a)

(b)

(c)

Figure 7. Coverage of functional groups with position along therough surface in repetitively pulsed negative discharges at differenttimes. (a) alkyl radicals (R•), (b) alkoxy (R–O•) and (c) peroxy(R–OO•).

The analogous time evolutions of alkyl (R•), alkoxy(R–O•) and peroxy (R–OO•) coverages in the positivedischarge are shown in figure 8. As with the negative discharge,the alkyl sites are regenerated during and immediately aftereach pulse, but to densities in excess of 2×1011 cm−2 due to thelarger fluxes of OH. These sites are consumed by passivationdominantly by O3 and O2 in the interpulse period. In contrast tothe negative discharge, there is greater uniformity on the scaleof a few millimeters. This results from the spreading of thepositive discharge on the PP, which reduces spatial gradientsin the fluxes of O and O3. However, slower radical penetration

(a)

(b)

(c)

Figure 8. Coverage of functional groups with position along therough surface in repetitively pulsed positive discharges at differenttimes. (a) alkyl radicals (R•), (b) alkoxy (R–O•) and (c) peroxy(R–OO•).

into features results in increased microscopic non-uniformityof alkoxy sites.

The coverage of alcohol (R–OH) and carbonyl (R–C=O)groups after 1 s of treatment is shown in figure 9 for positive andnegative discharges. There are qualitative differences in bothsmall-scale and large-scale coverages. Although the negativecorona has higher coverages of carbonyl groups on axis, themore local production of O and OH in the positive coronadischarges at larger radii increases the coverage of all groupsat larger radii. Microscopic variations in both carbonyl andalcohol groups are larger in the positive discharge, similar tothat of the alkoxy coverage. The larger fluxes of more reactive

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Figure 9. Coverage of selected groups along the rough surface after1 s treatment in repetitively pulsed negative and positive discharges(a) carbonyl (R–C=O) and (b) alcohol (R–OH).

OH provides for higher and more uniform peak coverage butdifferentiates more between sites having larger and smallerview angles to the plasma. As such, the choice of polarity ofthe discharge does give some degree of control of compositionand uniformity. Positive corona discharges tend to have moremacroscopic uniformity due to the spreading of the dischargeacross the surface. At the same time, positive dischargeshave less microscale uniformity due to the limited ability forelectrons to penetrate deeper into the microstructure wherethey may produce radicals.

Through parametrization of the model and sensitivitystudies of reaction probabilities, it was found that the mostlikely role of low energy ions—abstracting H atoms from PPto create alkyl radical sites—was not significant in atmosphericpressure corona discharges. The reaction probability for an ionM+ reacting with a PP site to abstract an H atom,

M+ + RH → R• + M–H, (32)was varied between 10−3 and 1.0. The resulting change insurface functionalization over a treatment time of 1 s wasnegligible. This was to be expected since the ions have amuch shorter lifetime compared with neutral radicals and theirdensities do not accumulate pulse to pulse. As a result, theirtime-averaged fluxes to the surface are generally orders ofmagnitude smaller than O, OH and O3. Although low energyions might also produce chain scission and crosslinking, the

Figure 10. Photon properties and photon initiated products for anegative discharge. (a) Photon fluxes at the end of the dischargepulse and (b) density of crosslinked sites after 1 s of treatment. Thesharp microscopic variations arise due to line of sight photontransport being shadowed by surface features.

low fluxes of ions (and lack of accumulation on a pulse-to-pulsebasis) reduce the importance of these processes.

Crosslinking is a terminal process for consumptionof surface radical sites by creating non-reactive sites.Crosslinking can be a significant process when the flux ofreactive gas species is low, such as in rare gas discharges. Indischarges where the flux of passivating species is large, thelikelihood for radical sites to react with and be passivated bygas phase species prior to crosslinking is large. UV photonsfrom the plasma contribute to crosslinking by adding anotherpathway for chain scission reactions.

For example, the UV photon flux along the surface at theend of breakdown for a −15 kV discharge pulse is shown infigure 10(a). These fluxes generally track the spatial gradientsin plasma density. The line of sight transport of photons resultsin shadowed surface features receiving a lower photon flux thansites that are directly exposed to the discharge. The density ofcrosslinked sites resulting from the UV illumination after 1 sof processing is shown in figure 10(b). Crosslinking competeswith functionalizing and passivating gas phase reactions. Atthose sites where fluxes of reactive gas phase species islarge, surface radicals are more likely to be passivated thancrosslinked. For the negative discharge, these fluxes arehighest on axis and so the density of crosslinked sites is loweron axis. For the same reasons, the densities of crosslinked sites

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Figure 11. Surface properties after 1 s treatment in repetitivelypulsed discharges. (a) Surface coverage of peroxy (R–OO•) for−12.5, −15 and −17.5 kV. (b) Surface coverage of alcohol (R–OH)groups while varying the relative humidity.

are larger inside features on axis than outside the features. Thisresults from the fluxes of passivating species being lower insidethe features but there being sufficient penetration of the plasmainto features that produces photons that photon produced chainscission reactions occur. Off axis, the penetration of plasmainto features is less and the shadowing of photon fluxes from thehigh plasma density regions is more severe. In those locations,there is more photon-induced crosslinking at the top of featuresthan inside features.

The consequences of the magnitude of the negative appliedvoltage on the coverage of peroxy groups (R–OO•) after 1 sof treatment are shown in figure 11(a) With an increase inapplied voltage, the densities of radicals generated in thedischarge increase and so surface coverages increase. Themacroscopic uniformity decreases with increasing voltage dueto a narrowing of the discharge (more rapid gap closure)producing larger gradients in radical production. These higherfluxes on axis begin to saturate the surface sites at about 0.7coverage. Microscopic variations in coverage do not changesignificantly as the local spatial dynamics of reactive speciesfluxes are not sensitive functions voltages.

Relative humidity or the fraction of H2O, [f (H2O)] inair ultimately determines the densities of OH generated in thedischarge. Larger f (H2O) generally produce larger fluxes ofOH. These radicals contribute both to more rapid H abstractionand to the slower OH addition to alkyl (R•) sites producingalcohol (R–OH) groups. The effect of f (H2O) on the coverage

of alcohol groups is shown in figure 11(b). At low f (H2O),tens of ppm, the large fluxes of O atoms that abstract H atomslocally produce OH densities that are comparable to the plasmaproduced flux of OH in discharges having high f (H2O) of afew per cent. As a result, increasing f (H2O) has a limitedeffect on functionalization and production of alcohol groupson the macroscopic scale. Since plasma produced OH fluxescome from outside of the features and OH radicals have largerreaction probabilities with the surface, there is little penetrationof plasma produced OH into features. The OH fluxes insidefeatures dominantly result from H abstraction by O atoms. Asa result, the alcohol coverage outside features is more sensitiveto f (H2O) than inside features.

4. Concluding remarks

The functionalization of rough PP surfaces with featuresof a few micrometers using repetitively pulsed atmosphericpressure discharges in humid air was computationallyinvestigated. Electrons and ions produced during the pulsepenetrate into the rough surface features to a limited extent.The penetration of charged species depends on the dischargepolarity. In negative discharges, there is limited penetration ofelectrons into surface features which locally produce reactivespecies by electron impact. In positive discharges a sheath-like region near the surface prevents electrons from penetratinginto the surface features and so there is less local productionof radicals. Longer lived radicals are able to penetrate into allsurface features over timescales of hundreds of microseconds.The more reactive radicals such as OH are depleted nearexposed surface features which reduces fluxes penetratinginside the features. In atmospheric pressure discharges,the contributions of ions to surface chemistry are limited.Although the contribution of UV photons is small, somecrosslinking does occur at locations where reactive fluxes ofpassivating radicals are low.

The dynamics of the discharge and radical generationhave significant effects on the degree and uniformity offunctionalization. The macroscopic distribution of reactivespecies in the positive discharge is more uniform due tospreading of the discharge along the surface, and this leadsto higher macroscopic uniformity of treatment. However poorplasma penetration into features produces more microscopicnon-uniformities than in the negative discharge. Increasinghumidity had a limited effect on alcohol coverage due to theformation of OH at the surface by H abstraction by O atoms.

Acknowledgments

This work was supported by the National Science Foundation(CTS-0520368). The authors thank Dr Mark Strobel for hisadvice and guidance.

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1. Introduction2. Description of the model and reaction mechanisms3. Functionalization of rough surfaces in humid air discharges4. Concluding remarks Acknowledgments References

repetitively pulsed atmospheric pressure discharge treatment of rough polymer surfaces...

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