1. 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 b) Department of Electrical and Computer Engineering Iowa State University, Ames, IA 50011 mjk@iastate.edu http://uigelz.ece.iastate.edu 32 nd IEEE International Conference on Plasma Science, 18 th – 23 rd June 2005, Monterey, CA. *Work supported by the NSF.

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

3. Iowa State University Optical and Discharge Physics FUNCTIONALIZATION OF SURFACES USING PLASMAS ANANTH_ICOPS05_01  Functionalization of surfaces occurs by their interaction with plasma species - ions, radicals, and photons. For hydrocarbon polymer:  Surface groups created in an O 2 containing plasma include  Acids –C(OH)=O  Carbonyl -C=O  Alkoxy –C-O  Peroxy –C-O-O  Alcohols –C-OH

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

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

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

7. Iowa State University Optical and Discharge Physics DBD TREATMENT OF PP SURFACE WITH MICROSTRUCTURE ANANTH_ICOPS05_05  Corona treating a polymer on the grounded electrode acts as a DBD.  Gas mixtures  He/O 2 /H 2 O  Humid air - N 2 /O 2 /H 2 O  How does uniformity of treatment vary over microscopic and macroscopic scales when repetitively pulsed?  PRF – 10 kHz

8. MIN MAX Iowa State University Optical and Discharge Physics PLASMA PROPERTIES IN He/O 2 /H 2 O MIXTURES  - 5 kV, 760 Torr He/O 2 /H 2 O=98/1/1, 0 – 2.5 ns ANANTH_ICOPS05_06 Potential (V)T e (eV) [Positive Ions] cm - 3 [O] cm - 3 - 5000 – 0 0 – 7.5 10 11 – 10 14 10 11 – 10 14 ANIMATION SLIDE

9. Iowa State University Optical and Discharge Physics PLASMA PENETRATION INTO SURFACE FEATURES ANANTH_ICOPS05_07  - 5 kV, 760 Torr, He/O 2 /H 2 O=98/1/1 [e] cm - 3 t = 2.5 nst = 5 ns t = 2.5 ns t = 5 ns [O] cm - 3 t = 2.5 ns t = 5 ns [Positive ions] cm - 3 MIN MAX 10 10 – 10 13 10 11 – 10 13 10 10 – 10 13

10. Iowa State University Optical and Discharge Physics  At higher O 2 fraction, the density of reactive [O] radicals at the surface increases during the pulse. ANANTH_ICOPS05_08  - 5 kV, 760 Torr (log scale) [O] cm - 3 EFFECT OF O 2 FRACTION ON [O] DENSITY He/O 2 /H 2 O = 98 / 1 / 1 He/O 2 /H 2 O = 69 / 30 / 1 t = 5 ns 2x10 11 t = 3 ns 2x10 14 He/O 2 /H 2 O = 89 / 10 / 1 t = 3 ns

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

12. Iowa State University Optical and Discharge Physics DENSITY OF SURFACE RADICALS ANANTH_ICOPS05_10  - 5 kV, 760 Torr, 10 kHz, He/O 2 /H 2 O=98/1/1  ALKYL (PP  ) are formed by O + PP  PP  + OH  Nonuniformities result from both fluxes and shadowing.  ALKOXY (PP-O  ) O 3 + PP   PP-O  O + PP   PP-O  take longer to form but eventually dominate.

13. Iowa State University Optical and Discharge Physics DENSITY OF -OH FUNCTIONAL GROUPS ANANTH_ICOPS05_11  - 5 kV, 760 Torr, He/O 2 /H 2 O=98/1/1, 10 kHz  Non-uniformities occur on the macroscopic and microscopic (< 1 µm) length scales; and are site dependent

14. Iowa State University Optical and Discharge Physics TEMPORAL VARIATION OF FLUXES WITH O 2 FRACTION ANANTH_ICOPS05_13  Larger fraction of reactive [O] is lost in the interpulse period at higher O 2 fraction by three-body recombination and ozone formation.  Optimum composition is at O 2 = 10% with significant [O] flux maintained through the long interpulse period.  At position along surface: 0.05 cm, He/O 2 /H 2 O=99-x/x/1  IP - interpulse period (duration – 100  s at 10 kHz) IP END IP START

15. Iowa State University Optical and Discharge Physics SPATIAL VARIATION OF FLUXES WITH O 2 FRACTION ANANTH_ICOPS05_13  As O 2 fraction increases, there is greater non-uniformity in fluxes of [O] and O 3 at the surface.  At end of IP, He/O 2 /H 2 O=99-x/x/1  IP - interpulse period (duration – 100  s at 10 kHz) [O 2 ]=75% [O 2 ]=10%

16. Iowa State University Optical and Discharge Physics EFFECT OF O 2 FRACTION ON TREATMENT ANANTH_ICOPS05_14  Macroscopic non-uniformities in –OH and –OO ultimately arise from significant spatial variation of O, O 3 and OH fluxes.  Microscopic scale non-uniformities arise due to transport limitations of reactive [O] at higher O 2 fractions.  - 5 kV, 760 Torr, He/O 2 /H 2 O=99-x/x/1, 10 kHz, 1 s

17. Iowa State University Optical and Discharge Physics HUMID AIR - EFFECT OF VOLTAGE POLARITY ANANTH_ICOPS05_15  760 Torr, N 2 /O 2 /H 2 O=79/20/1 cm - 3 - 15 kV, 2.5 ns MAX  Positive discharge spreads more uniformly on surface.  Surface recombination of O + enhances [O] at surface. 10 10 - 10 13 +15 kV, 10 ns - 15 kV, 2.5 ns +15 kV, 10 ns 10 11 - 10 14  [O]  [e] MIN 10 11 - 10 14 5x10 11 - 5x10 14 log scale  [e]

18. Iowa State University Optical and Discharge Physics EFFECT OF POLARITY ON TREATMENT  Pulsed positive polarity operation gives higher uniformity of functionalization due to more uniform formation of [O] and O 3 near the surface. ANANTH_ICOPS05_16  760 Torr, N 2 /O 2 /H 2 O=79/20/1, 10 kHz, 0.15 s

19. ANANTH_ICOPS05_17 Iowa State University Optical and Discharge Physics 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.

20. Iowa State University Optical and Discharge Physics NON-UNIFORMITY DUE TO ‘SHADOWING’ ANANTH_ICOPS05_18  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/O 2 /H 2 O=98/1/1, 10 kHz, 0.2 s [CROSSLINKS]

21. Iowa State University Optical and Discharge Physics CONCLUDING REMARKS ANANTH_ICOPS05_19  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/O 2 /H 2 O mixtures, the O 2 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.