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MULTISCALE SIMULATION OF FUNCTIONALIZATION OF SURFACES USING ATMOSPHERIC PRESSURE DISCHARGES *

MULTISCALE SIMULATION OF FUNCTIONALIZATION OF SURFACES USING ATMOSPHERIC PRESSURE DISCHARGES * Ananth N. Bhoj a) and Mark J. Kushner b) a) Department of Chemical and Biomolecular Engineering University of Illinois, Urbana, IL 61801 bhoj@uiuc.edu

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MULTISCALE SIMULATION OF FUNCTIONALIZATION OF SURFACES USING ATMOSPHERIC PRESSURE DISCHARGES *

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  1. MULTISCALE SIMULATION OF FUNCTIONALIZATION OF SURFACES USING ATMOSPHERIC PRESSURE DISCHARGES* Ananth N. Bhoja) and Mark J. Kushnerb) a)Department of Chemical and Biomolecular Engineering University of Illinois, Urbana, IL 61801 bhoj@uiuc.edu b)Department of Electrical and Computer Engineering Iowa State University, Ames, IA 50011 mjk@iastate.edu http://uigelz.ece.iastate.edu 32nd IEEE International Conference on Plasma Science, 18th – 23rd June 2005, Monterey, CA. *Work supported by the NSF.

  2. OUTLINE  Functionalization of Surfaces using Plasmas  Modeling Platform  Pulsed Coronas in Humid Air, He/O2/H2O mixtures  Uniformity of surface treatment O2 fraction Discharge Polarity  Surface microstructure  Concluding remarks Iowa State University Optical and Discharge Physics ICOPS05_agenda

  3. FUNCTIONALIZATION OF SURFACES USING PLASMAS  Functionalization of surfaces occurs by their interaction with plasma species - ions, radicals, and photons. For hydrocarbon polymer:  Surface groups created in an O2 containing plasma include  Acids –C(OH)=O  Carbonyl -C=O  Alkoxy –C-O Peroxy –C-O-O  Alcohols –C-OH Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_01

  4. SURFACE MODIFICATION OF POLYMERS  Pulsed atmospheric discharges (coronas) are widely used in industry to treat commodity polymers like polypropylene (PP). Tantec, Inc.  Functionalization increases surface energy and enhances wettability and adhesion. Akishev, et al, Plasmas Polym., 7, 261 (2002). PE Film Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_02

  5. Untreated PP Plasma Treated PP TREATMENT OF NON-PLANAR SURFACES • M. Strobel, 3M  Roughness of surface to be treated may range from 100s nm to few µm. • SEM of polyester fabric (Borcia, et al, Plasma Sources Sci. Technol, 12, 235 (2003).  Penetration of plasma species into surface features determines the extent and uniformity of functionalization. Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_03

  6. SCHEMATIC OF THE 2-D MODELING PLATFORM • Unstructured mesh • Plasma hydro-dynamics (multi-fluid, Poisson’s equation) • Radiation transport • Integrated plasma-surface kinetics model Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_04

  7. DBD TREATMENT OF PP SURFACE WITH MICROSTRUCTURE • Corona treating a polymer on the grounded electrode acts as a DBD. • Gas mixtures  He/O2/H2O  Humid air - N2/O2/H2O • How does uniformity of treatment vary over microscopic and macroscopic scales when repetitively pulsed?  PRF – 10 kHz Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_05

  8. PLASMA PROPERTIES IN He/O2/H2O MIXTURES Potential (V) Te (eV) [Positive Ions]cm- 3 [O] cm- 3 - 5000 – 0 0 – 7.5 1011– 10141011– 1014 - 5 kV, 760 Torr He/O2/H2O=98/1/1, 0 – 2.5 ns ANIMATION SLIDE Iowa State University Optical and Discharge Physics MINMAX ANANTH_ICOPS05_06

  9. PLASMA PENETRATION INTO SURFACE FEATURES [e] cm- 3 1010– 1013 t = 2.5 ns t = 5 ns [O] cm- 3 1010– 1013 t = 5 ns t = 2.5 ns [Positive ions] cm- 3 1011– 1013 t = 2.5 ns t = 5 ns - 5 kV, 760 Torr, He/O2/H2O=98/1/1 Iowa State University Optical and Discharge Physics MINMAX ANANTH_ICOPS05_07

  10. EFFECT OF O2 FRACTION ON [O] DENSITY He/O2/H2O = 98 / 1 / 1 He/O2/H2O = 89 / 10 / 1 t = 5 ns t = 3 ns  At higher O2 fraction, the density of reactive [O] radicals at the surface increases during the pulse. He/O2/H2O = 69 / 30 / 1 [O] cm- 3 2x1011 2x1014 t = 3 ns (log scale) - 5 kV, 760 Torr Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_08

  11. SURFACE REACTION MECHANISM FOR PP  Gas phase radicals abstract H from the surface and produce radical sites on the PP surface.  Further reactions lead to formation of functional groups like peroxy, alkoxy, alcohols and other groups1. 1R. Dorai and M.J.Kushner, J. Phys. D 36, 666 (2003). Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_09

  12. DENSITY OF SURFACE RADICALS • ALKYL (PP ) are formed by O + PP  PP + OH • Nonuniformities result from both fluxes and shadowing. • ALKOXY (PP-O) O3 + PP PP-O O + PP PP-O take longer to form but eventually dominate. Iowa State University Optical and Discharge Physics - 5 kV, 760 Torr, 10 kHz, He/O2/H2O=98/1/1 ANANTH_ICOPS05_10

  13. DENSITY OF -OH FUNCTIONAL GROUPS • Non-uniformities occur on the macroscopic and microscopic (< 1 µm) length scales; and are site dependent - 5 kV, 760 Torr, He/O2/H2O=98/1/1, 10 kHz Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_11

  14. TEMPORAL VARIATION OF FLUXES WITH O2 FRACTION IP START IP END • Larger fraction of reactive [O] is lost in the interpulse period at higher O2 fraction by three-body recombination and ozone formation. • Optimum composition is at O2 = 10% with significant [O] flux maintained through the long interpulse period. • At position along surface: 0.05 cm, He/O2/H2O=99-x/x/1 • IP - interpulse period (duration – 100 ms at 10 kHz) Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_13

  15. SPATIAL VARIATION OF FLUXES WITH O2 FRACTION [O2]=10% [O2]=75% • As O2 fraction increases, there is greater non-uniformity in fluxes of [O] and O3 at the surface. • At end of IP, He/O2/H2O=99-x/x/1 • IP - interpulse period (duration – 100 ms at 10 kHz) Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_13

  16. EFFECT OF O2 FRACTION ON TREATMENT • Macroscopic non-uniformities in –OH and –OO ultimately arise from significant spatial variation of O, O3 and OH fluxes. • Microscopic scale non-uniformities arise due to transport limitations of reactive [O] at higher O2 fractions. - 5 kV, 760 Torr, He/O2/H2O=99-x/x/1, 10 kHz, 1 s Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_14

  17. HUMID AIR - EFFECT OF VOLTAGE POLARITY - 15 kV, 2.5 ns +15 kV, 10 ns  [O]  [e]  [e] 1011 - 1014 - 15 kV, 2.5 ns 1010 - 1013 +15 kV, 10 ns • Positive discharge spreads more uniformly on surface. • Surface recombination of O+ enhances [O] at surface. 1011 - 1014 5x1011 - 5x1014 760 Torr, N2/O2/H2O=79/20/1 cm- 3 Iowa State University Optical and Discharge Physics MAX MIN log scale ANANTH_ICOPS05_15

  18. EFFECT OF POLARITY ON TREATMENT • Pulsed positive polarity operation gives higher uniformity of functionalization due to more uniform formation of [O] and O3 near the surface. 760 Torr, N2/O2/H2O=79/20/1, 10 kHz, 0.15 s Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_16

  19. IONS AND PHOTONS IN SURFACE CHEMISTRY • UV photons from the plasma can cleave surface bonds. • Ions striking the surface transfer energy and can activate sites. • Postulate creation of intermediate active sites. • Active sites may crosslink in addition to standard reaction pathways. Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_17

  20. NON-UNIFORMITY DUE TO ‘SHADOWING’ [CROSSLINKS] • Line-of-sight photon transport fails to reach deep inside surface features. • Crosslinking is relatively higher on surface features exposed to the avalanche where ion and photon fluxes are higher. - 5 kV, 760 Torr, He/O2/H2O=98/1/1, 10 kHz, 0.2 s Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_18

  21. CONCLUDING REMARKS  Multiscale modeling using unstructured meshes was applied to study DBD corona treatment of PP surface with microstructure.  Non-uniformity in functionalization occurs on the macroscopic (few mm) and microscopic ( < 1 µm) scales.  In He/O2/H2O mixtures, the O2 fraction can be used to optimize uniformity of treatment.  In humid air, pulsed positive polarity discharges produce more uniform functionalization.  ‘Shadowing’ by surface microstructure results in non-uniform crosslinking with a mechanism using photon and ion fluxes. Iowa State University Optical and Discharge Physics ANANTH_ICOPS05_19

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