mjk@iastate.edu http://uigelz.ece.iastate.edu PLASMA SURFACE INTERACTIONS FOR ATMOSPHERIC PRESSURE FUNCTIONALIZATION OF POLYMERS Mark J. Kushner Iowa State University Ames, IA 50011 USA mjk@iastate.edu http://uigelz.ece.iastate.edu March 2007 EUJapan_0307_01

Optical and Discharge Physics ACKNOWLEDGEMENTS Group Members Ananth N. Bhoj Ramesh Arakoni Natalie Babeava Funding Agencies 3M Corporation Semiconductor Research Corporation National Science Foundation Air Force Office of Scientific Research Iowa State University Optical and Discharge Physics EUJapan_0307_02

Optical and Discharge Physics AGENDA Introduction to plasma functionalization of polymers Description of nonPDPSIM Corona treatment of Polymers: Pulsing, Flowing, Moving Optimizing Uniformity Concluding Remarks Iowa State University Optical and Discharge Physics EUJapan_0307_03

Optical and Discharge Physics FUNCTIONALIZATION OF POLYMER SURFACES  Functionalization occurs by chemical interaction of plasma produced species - ions, radicals and photons with the surface. (a) (b) (c) Example: H abstraction by O atom enables affixing O atoms as a peroxy site. Increase surface energy  increase wettability.  Process treats the top few layers. Wettability on PE film with 3 zones of treatment. Courtesy: http://www.polymer-surface.com Iowa State University Optical and Discharge Physics EUJapan_0307_04

Filamentary Plasma 10s – 200 mm SURFACE MODIFICATION OF POLYMERS Pulsed atmospheric filamentary discharges (coronas) routinely treat commodity polymers like poly-propylene (PP) and polyethylene (PE). Filamentary Plasma 10s – 200 mm Iowa State University Optical and Discharge Physics EUJapan_0307_05

COMMERCIAL CORONA PLASMA EQUIPMENT Optical and Discharge Physics Sherman Treaters Tantec, Inc. Iowa State University Optical and Discharge Physics EUJapan_0307_06

Optical and Discharge Physics MODELING OF FUNCTIONALIZATION OF POLYMERS Goals: Investigate fundamental physics of plasma-surface interactions for functionalization for industrially relevant conditions. Develop scaling laws to be able to customize surface functionalization. Methodology: Integrated multi-scale modeling of corona discharge. Test system: Functionalization of polypropylene in oxygen containing plasmas. Iowa State University Optical and Discharge Physics EUJapan_0307_07

Optical and Discharge Physics MODELING PLATFORM nonPDPSIM nonPDPSIM is organized around modules addressing different physical processes having different timescales. Iowa State University Optical and Discharge Physics EUJapan_0307_08

CHARGED PARTICLE TRANSPORT Optical and Discharge Physics Poisson’s equation for the electric potential F = electric potential Nj = density qj = charge s = surface charge Transport equations for conservation of the charged species j Gj = Flux Sj = sources due to collisions, photons Surface charge balance Iowa State University Optical and Discharge Physics EUJapan_0307_09

CHARGED PARTICLE TRANSPORT Optical and Discharge Physics Electron fluxes given by Sharfetter-Gummel form  = mobility, D = diffusion coefficient, E = Electric field Automatically selects upwind-or-downwind Ion fluxes given by SG or by accounting for full momentum Pj = partial pressure  ij = collision frequency v = velocity Finite volume discretization to insure 100% conservative as solution of Poisson’s equation requires 10-6 to 10-7 resolution. Iowa State University Optical and Discharge Physics EUJapan_0307_10

Optical and Discharge Physics BULK-BEAM ELECTRON ENERGY TRANSPORT Bulk Electrons: Continuum electron energy equation. Integrate implicitly using method of Successive-Over-Relaxation. Transport coefficients obtained by solving (and periodically updating) spatially homogeneous Boltzmann’s equation for the electron energy distribution. Secondary electrons emitted in high E-fields: Monte Carlo simulation. Iowa State University Optical and Discharge Physics EUJapan_0307_11

Optical and Discharge Physics RADIATION TRANSPORT Radiation transport is important due to: Ionization and chemical reactivity by absorption in gas phase Photo-electrons produced from surfaces Photochemistry induced on surfaces. Excited states Nj(r’) emit photon. Absorption along path r’’ by Nk with obscurations. Ni ionized by photon at r Capture in Greens function Iowa State University Optical and Discharge Physics EUJapan_0307_12

Optical and Discharge Physics FLUID MODULE : NEUTRAL PARTICLE TRANSPORT Fluid averaged values of mass density, mass momentum and thermal energy density obtained in using unsteady algorithms. Continuity: Momentum: Energy:  Individual neutral species diffuse within the bulk flow. Iowa State University Optical and Discharge Physics EUJapan_0307_13

Optical and Discharge Physics SURFACE KINETICS MODULE The surface kinetics module accounts for plasma surface interactions. Surface site balance model executed along the surface-plasma boundary. Fractional coverage of surface species Derive sticking coefficients and reaction probabilities to feed back to plasma and neutral transport modules. Iowa State University Optical and Discharge Physics EUJapan_0307_14

LARGE DYNAMIC RANGE: TIME SLICING Optical and Discharge Physics Vastly different timescales addressed using a variant of time-slicing which leverages time different timescales required to come into adiabatic equilibrium. Iowa State University Optical and Discharge Physics EUJapan_0307_15

Optical and Discharge Physics EXAMPLE OF MESH: POLYMER TISSUE SCAFFOLDING Iowa State University Optical and Discharge Physics EUJapan_0307_16

Optical and Discharge Physics ADAPTIVE NONEQUILIBRIUM ELECTRON TRANSPORT As the plasma evolves, regions will become “non-equilibrium” where electron transport is poorly described by fluid equations. Large gradients in electric field. Time rate of change of electric field is large compared to collision frequency. To properly address transport, a kinetic approach is used for the electron energy distribution for position and time, f(r,,t). These regions are addressed using an adaptive mesh technique with “sensors” that detect where are “non-equilibrium” conditions. Use a kinetic Monte Carlo technique in those regions. Iowa State University Optical and Discharge Physics EUJapan_0307_17

Optical and Discharge Physics ADAPTIVE EMCS PROCESS Sensor identifies non-equilibrium region in unstructured mesh. Dynamics determine scale to be resolved. Rectilinear mesh is superimposed over non-equilibrium region. Monte Carlo particles are launched from edges of overlayed mesh to obtain f(r,,t). Iowa State University Optical and Discharge Physics EUJapan_0307_18

Optical and Discharge Physics BREAKDOWN IN HID LAMP: THE IONIZATION FRONT  Te  [Sources]  [e]  MCS 10 eV 1021 cm-3s-1 1013 cm-3  Ionization front with steep gradients in [e] and electron impact sources moves across the gap.  The EMCS sensor identifies the region of the ionization front and maps an adaptive MCS mesh onto it. Animation Slide-GIF MIN MAX  Ar, 30 Torr, 2000V, 100 ns Iowa State University Optical and Discharge Physics EUJapan_0307_19

Optical and Discharge Physics BREAKDOWN IN HID LAMP: THE IONIZATION FRONT  Te  [Sources]  [e]  MCS 10 eV 1021 cm-3s-1 1013 cm-3  Ionization front with steep gradients in [e] and electron impact sources moves across the gap.  The EMCS sensor identifies the region of the ionization front and maps an adaptive MCS mesh onto it. Animation Slide-AVI MIN MAX  Ar, 30 Torr, 2000V, 100 ns Iowa State University Optical and Discharge Physics EUJapan_0307_19A

PRIMER ON SURFACE CHEMISTRY Optical and Discharge Physics  Polypropylene structure  Functional groups are when treated in O2 containing plasmas: Alkyl Alkoxy Carbonyl Alcohol Peroxy Acid R R-O R=O R-OH R-OO O=R-OH Iowa State University Optical and Discharge Physics EUJapan_0307_20

PRIMER ON SURFACE CHEMISTRY Optical and Discharge Physics Ratio of O, OH, O2 and O3 fluxes determine surface composition. Magnitude of fluxes and residence time determines importance of surface-surface reactions. Iowa State University Optical and Discharge Physics EUJapan_0307_21

Optical and Discharge Physics FORCED GAS FLOW AND WEB MOVEMENT Gas Flow Polymer surfaces are continuously treated at web speeds of a few m/s with residence times in plasma of up to a few ms. Non-air gas mixtures are often “forced flowed” through gap to customize radicals to surface. Iowa State University Optical and Discharge Physics EUJapan_0307_22

Optical and Discharge Physics FORCED GAS FLOW AND WEB MOVEMENT 10 cm Web Motion 2 mm Translate surface mesh points to account for web motion. - 5 kV, 1 atm, He/O2/H2O=89/10/1 Inter-electrode gap: 2 mm Gas flow: 0 – 30 slpm (many m/s) Web speed: 0-8 m/s Iowa State University Optical and Discharge Physics EUJapan_0307_23

Optical and Discharge Physics DYNAMICS OF THE FIRST PULSE: Te, SOURCES Te peaks at the ionization front initiated near the electrode and propagates toward the PP surface. Electron sources by electron impact ionization track the maximum in Te.  Te 0-9 eV  Electron Source 5x1020-5x1023 cm-3s-1 Animation Slide-GIF  - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0–2 ns, no flow Iowa State University Optical and Discharge Physics MIN MAX log scale EUJapan_0307_24

REPETITIVELY PULSED DISCHARGES – [e] Optical and Discharge Physics Electron avalanche from the powered electrode. The pulse duration a few ns Terminated by charging of dielectric. Peak [e] of a few 1014 cm-3.  1014 cm-3 0.01 100 log scale  He/O2/H2O=89/10/1, -5 kV, 10 kHz, 1 atm Animation Slide-GIF Iowa State University Optical and Discharge Physics EUJapan_0307_25

Optical and Discharge Physics POST-PULSE REACTANT DENSITIES Important radicals are those containing O atoms. Post pulse radical densities: [OH] 1014 cm-3 [O] 1015 cm-3  He/O2/H2O=89/10/1, -5 kV, 10 kHz, 1 atm Iowa State University Optical and Discharge Physics EUJapan_0307_26

Optical and Discharge Physics POST-PULSE REACTANT DENSITIES Pulse to pulse variation in radical densities is nominal. Some small decrease in densities by polymer due to gas heating and.  He/O2/H2O=89/10/1, -5 kV, 10 kHz, 1 atm Iowa State University Optical and Discharge Physics EUJapan_0307_27

Optical and Discharge Physics HOW NONEQUILIBRIUM CAN A 1-ATM STREAMER BE? Even at 1 atm, gradients in electric fields can be large enough to require kinetic schemes. Differences are not large, but can make factors of two differences in plasma densities. Iowa State University Optical and Discharge Physics EUJapan_0307_28

REPETITIVELY PULSED DISCHARGES - GAS HEATING Gas temperature rises by 5-10 K in the discharge zone, and is convected away after the pulse. Corresponding change in gas density. Higher rep-rates leave heated gas in discharge zone. TG 300-305 K  r (2.8-2.9 10-4 g-cm-3)  He/O2/H2O=89/10/1, -15 kV, 1 kHz, 10 slpm, 1 atm. 0.0 1 Iowa State University Optical and Discharge Physics EUJapan_0307_29 Animation Slide-GIF

[O] – WITHOUT AND WITH FORCED GAS FLOW Optical and Discharge Physics  Without forced flow  [O] 1015 cm-3  With forced flow Flow O atoms are generated by electron impact with every pulse. Rapid reactions to form ozone (O + O2 + M  O3 + M) deplete the O atoms within 10s s of discharge pulse. Little change in O atom distribution with and without gas flow. He/O2/H2O=89/10/1 10 kHz, 0 or 30 slpm, 1 atm. Animation Slide-GIF Iowa State University Optical and Discharge Physics 0.001 1 log scale EUJapan_0307_30

[O] – WITHOUT AND WITH FORCED GAS FLOW Optical and Discharge Physics  Without forced flow  [O] 1015 cm-3  With forced flow Flow O atoms are generated by electron impact with every pulse. Rapid reactions to form ozone (O + O2 + M  O3 + M) deplete the O atoms within 10s s of discharge pulse. Little change in O atom distribution with and without gas flow. He/O2/H2O=89/10/1 10 kHz, 0 or 30 slpm, 1 atm. Animation Slide-AVI Iowa State University Optical and Discharge Physics 0.001 1 log scale EUJapan_0307_30A

[O3] – WITHOUT AND WITH FORCED GAS FLOW Optical and Discharge Physics  Without forced flow  [O3] 3 x 1014 cm-3  With forced flow Flow O3 is relatively unreactive and accumulates from pulse to pulse. Without forced flow, diffusion distributes O3 up- and downstream. With forced flow, a plume of O3 extends downstream. He/O2/H2O=89/10/1 10 kHz, 0 or 30 slpm, 1 atm. Animation Slide-GIF Iowa State University Optical and Discharge Physics 0.001 1 log scale EUJapan_0307_31

[O3] – WITHOUT AND WITH FORCED GAS FLOW Optical and Discharge Physics  Without forced flow  [O3] 3 x 1014 cm-3  With forced flow Flow O3 is relatively unreactive and accumulates from pulse to pulse. Without forced flow, diffusion distributes O3 up- and downstream. With forced flow, a plume of O3 extends downstream. He/O2/H2O=89/10/1 10 kHz, 0 or 30 slpm, 1 atm. Animation Slide-AVI Iowa State University Optical and Discharge Physics 0.001 1 log scale EUJapan_0307_31A

[OH] – WITHOUT AND WITH FORCED GAS FLOW Optical and Discharge Physics  Without forced flow  [OH] 1014 cm-3  With forced flow Flow OH has an intermediate reactivity between O and O3. Some modest accumulation occurs with a small plume downstream with forced flow. He/O2/H2O=89/10/1 10 kHz, 0 or 30 slpm, 1 atm. Animation Slide-GIF Iowa State University Optical and Discharge Physics 0.001 1 log scale EUJapan_0307_32

[OH] – WITHOUT AND WITH FORCED GAS FLOW Optical and Discharge Physics  Without forced flow  [OH] 1014 cm-3  With forced flow Flow OH has an intermediate reactivity between O and O3. Some modest accumulation occurs with a small plume downstream with forced flow. He/O2/H2O=89/10/1 10 kHz, 0 or 30 slpm, 1 atm. Animation Slide-AVI Iowa State University Optical and Discharge Physics 0.001 1 log scale EUJapan_0307_32A

ALKYL COVERAGE (NO FLOW, NO MOTION) Optical and Discharge Physics Alkyl sites (R) are rapidly produced by H abstraction by O and OH. RH + O  R + OH Alkyl sites are slowly passivated by O2 to form peroxy sites (R-OO) R + O2  R-OO As O3 accumulates, alkyl sites are further depleted to form alkoxy sites R + O3  R-O + O2 Animation Slide-GIF  - 5 kV, 1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s Iowa State University Optical and Discharge Physics EUJapan_0307_33

ALKYL COVERAGE (NO FLOW, NO MOTION) Optical and Discharge Physics Alkyl sites (R) are rapidly produced by H abstraction by O and OH. RH + O  R + OH Alkyl sites are slowly passivated by O2 to form peroxy sites (R-OO) R + O2  R-OO As O3 accumulates, alkyl sites are further depleted to form alkoxy sites R + O3  R-O + O2 Animation Slide-AVI  - 5 kV, 1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s Iowa State University Optical and Discharge Physics EUJapan_0307_33A

PEROXY COVERAGE (NO FLOW, NO MOTION) Optical and Discharge Physics Peroxy sites are produced following each pulse by O2 reactions with alkyl sites R + O2  R-OO Peroxy sites are relatively unreactive and only slowly are depleted by H abstraction R-OO• + RH  R-OOH + R• As a result, peroxy sites accumulate pulse to pulse. Animation Slide-GIF  - 5 kV, 1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s Iowa State University Optical and Discharge Physics EUJapan_0307_34

PEROXY COVERAGE (NO FLOW, NO MOTION) Optical and Discharge Physics Peroxy sites are produced following each pulse by O2 reactions with alkyl sites R + O2  R-OO Peroxy sites are relatively unreactive and only slowly are depleted by H abstraction R-OO• + RH  R-OOH + R• As a result, peroxy sites accumulate pulse to pulse. Animation Slide-AVI  - 5 kV, 1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s Iowa State University Optical and Discharge Physics EUJapan_0307_34A

Optical and Discharge Physics ALKYL, PEROXY COVERAGE WITH FLOW Alkyl (R) sites are still rapidly produced and passivated with each pulse. The plume of OH radicals downstream produce a tail of R sites. Peroxy sites accumulate downstream.  1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 30 slpm Flow Animation Slide-GIF Iowa State University Optical and Discharge Physics EUJapan_0307_35

Optical and Discharge Physics ALKYL, PEROXY COVERAGE WITH FLOW Alkyl (R) sites are still rapidly produced and passivated with each pulse. The plume of OH radicals downstream produce a tail of R sites. Peroxy sites accumulate downstream.  1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 30 slpm Flow Animation Slide-AVI Iowa State University Optical and Discharge Physics EUJapan_0307_35A

SURFACE COVERAGES WITH FLOW Optical and Discharge Physics With stationary web, repetitive treatment of same sites produce large densities peroxy groups. Other groups are “etched away” by continual flux of O and OH radicals.  1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 30 slpm Flow Iowa State University Optical and Discharge Physics EUJapan_0307_36

Optical and Discharge Physics ALKYL, PEROXY COVERAGE WITH WEB MOTION The rate of alkyl (R) passivation is fast compared to web motion. Little R moves downstream. Peroxy (R-OO) sites being long lived, move with the web downstream. Web speed 4 m/s (no flow)  1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 4 m/s Animation Slide-GIF Web Iowa State University Optical and Discharge Physics EUJapan_0307_37

Optical and Discharge Physics ALKYL, PEROXY COVERAGE WITH WEB MOTION The rate of alkyl (R) passivation is fast compared to web motion. Little R moves downstream. Peroxy (R-OO) sites being long lived, move with the web downstream. Web speed 4 m/s (no flow)  1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 4 m/s Animation Slide-AVI Web Iowa State University Optical and Discharge Physics EUJapan_0307_37A

SURFACE COVERAGES WITH WEB MOTION Optical and Discharge Physics With moving web and flow motion out of high radical fluxes allow surface-surface reactions to change functionality. Ratio of Peroxy/Carbonyl Flow-no motion: 6 Motion-no flow: 1.7 Web  1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 30 slpm Iowa State University Optical and Discharge Physics EUJapan_0307_38

SURFACE COVERAGES WITH FLOW AND WEB MOTION With flow and motion in same direction, sites move “under plume” of radicals. Effectively larger fluence increases radical processes. Ratio of Peroxy/Carbonyl Flow-no motion: 6 Motion-no flow: 1.7 Motion-flow 2.8 Choice of flow and motion provides control over functional groups. Flow Web  1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 30 slpm Iowa State University Optical and Discharge Physics EUJapan_0307_39

TUNING PEROXY COVERAGE Optical and Discharge Physics vs WEB SPEED, FLOW Higher web speeds reduce residence time in plasma zone and so reduce fluence of radicals. Peroxy (R-OO) coverage decreases. With sites being long lived move with the web downstream. PRF adjusted for same J/cm2.  No flow  1 atm, He/O2/H2O=89/10/1, 0.022 s  100 cm/s Web Iowa State University Optical and Discharge Physics Flow EUJapan_0307_40

PEROXY, ALKOXY COVERAGE Optical and Discharge Physics vs O2 FRACTION O2 fraction allows additional control over functionality. Large O2 More O Even more O3. RH + O  R + OH (Alkyl) R + O2  R-OO (Peroxy) R + O3  R-O + O2 (Alkoxy) Higher f(O2) favors Alkoxy. Ratio of Peroxy/Alkoxy f(O2) = 1%: 7 f(O2) = 30%: 4.4 Iowa State University Optical and Discharge Physics EUJapan_0307_41

Optical and Discharge Physics FUNCTIONALIZATION OF POLYMER TISSUE SCAFFOLDING  E. Sachlos, et al. Corona functionalization of rough polypropylene resembling tissue scaffold. 1 atm, He/O2/H2O, 10 kHz Optimize uniformity in small structures. Iowa State University Optical and Discharge Physics EUJapan_0307_42

Optical and Discharge Physics OPTIMIZE CHEMISTRY, UNIFORMITY WITH GAS MIXTURE Balance of peroxy (R-OO), alkoxy (R-O) and alcohol (R-OH) groups can be controlled by composition of fluxes. Example: He/O2/H2O e + O2  O + O + e e + H2O  H + OH + e O + O2 + M  O3 + M Large f(O2), small f(H2O): Small OH fluxes, large O3 fluxes Small f(O2), large f(H2O): Large OH fluxes, small O3 fluxes Impact on polypropylene surface chemistry RH + O  R + OH (slow rate) RH + OH  R + H2O (fast rate) R + O2  R-OO (slow rate but a lot of O2) R + O3  R-O + O2 (fast rate) R + OH  R-OH (fast rate) Iowa State University Optical and Discharge Physics EUJapan_0307_43

Optical and Discharge Physics CONTROLLING FLUX OF OZONE TO SURFACE Pulsed corona discharge, 10 kHz He/O2/H2O = 99-X /X/1 After short discharge pulse, flux of O atoms is large. At end of interpulse period, flux of O atoms is negligible as most O has been converted to O3. Flux of O3 increases by nearly 100 with increasing f(O2). Non-uniform O3 fluxes results from reaction limited transport into microstructure. Iowa State University Optical and Discharge Physics EUJapan_0307_44

Optical and Discharge Physics CONTROLLING FLUX OF OZONE TO SURFACE O2 fluxes at any finite mole fraction; peroxy PP-OO formation dominates. Large O2 produces large O3 fluxes which favors alkoxy PP-O. Small O2 increases OH fluxes by H2O dissociation and so alcohol PP-OH fractions increase. Small scale uniformity is dominated by reactivity of O3 and in ability to penetrate deep into crevices. Low O3 but moderate OH optimizes uniformity. He/O2/H2O = 99-X /X/1 Iowa State University Optical and Discharge Physics EUJapan_0307_45

Optical and Discharge Physics CONCLUDING REMARKS Functionalization of low value materials for high value applications has great promise. Even at atmospheric pressure, fluxes of radicals can be tailored to provide desired functionality. Modeling of industrial processes requires some care to details: flow, motion of web, repetition rates, photon-surface-interactions. Iowa State University Optical and Discharge Physics EUJapan_0307_45