NEW OPPORTUNITIES IN PLASMA-SURFACE INTERACTIONS FOR FUNCTIONALIZATION OF SURFACES* Ananth Bhoj, Natalie Babaeva, Rajesh Dorai and Mark J. Kushner Iowa State University 104 Marston Hall Ames, IA May 2005 *Work supported by National Science Foundation, 3M Inc. DAMOP_0505_01

Iowa State University Optical and Discharge Physics AGENDA  Plasmas for modification of surfaces  Functionalization of polymers  Challenges for adapting commodity processes for high value materials.  Opportunities for AMO  Concluding Remarks DAMOP_0505_02

Iowa State University Optical and Discharge Physics PLASMAS FOR MODIFICATION OF SURFACES  Plasmas are ideal for producing reactive species (radicals, ions) for modifying surface properties to achieve desired mechanical or chemical functionality.  Plasma processing that adds or remove molecules from surfaces to achieve this functionality span orders of magnitude in conditions:.  Etching for micro- electronics fabrication (<100’s mTorr)…. Peter Ventzek…prior talk.  Functionalization of polymers (atmospheric pressure) DAMOP_0505_03

 Low pressure  Low throughput  High precision  Grow expensive materials  High tech  High pressure  High throughput  Low precision  Modify cheap materials  Commodity Web Treatment of Films $0.05/m 2 $1000/cm 2 Microelectronics EXTREMES IN CONDITIONS, VALUES, APPLICATIONS Iowa State University Optical and Discharge Physics DAMOP_0505_04

 Can commodity processes be used to fabricate high value materials? Iowa State University Optical and Discharge Physics CREATING HIGH VALUE: COMMODITY PROCESSES $0.05/m 2 $1000/cm 2 ? DAMOP_0505_05  Where will, ultimately, biocompatible polymeric films fit on this scale? Artificial skin for $0.05/cm 2 or $1000/cm 2 ?  What are the opportunities for AMO physics to build the knowledge base to meet this challenge?

LOW COST, COMMODITY FUNCTIONALIZATION OF POLYMERS DAMOP_0505_06

Iowa State University Optical and Discharge Physics SURFACE ENERGY AND FUNCTIONALITY OF POLYMERS  Most polymers, having low surface energy, are hydrophobic.  For good adhesion and wettability, the surface energy of the polymer should exceed of the overlayer by  2-10 mN m -1. DAMOP_0505_07

Iowa State University Optical and Discharge Physics PLASMA SURFACE MODIFICATION OF POLYMERS  To improve wetting and adhesion of polymers atmospheric plasmas are used to generate gas-phase radicals to functionalize their surfaces. Untreated PP Plasma Treated PP  M. Strobel, 3M  Polypropylene (PP) He/O 2 /N 2 Plasma  Massines et al. J. Phys. D 31, 3411 (1998). DAMOP_0505_08 Hydrophilic Hydrophobic

Iowa State University Optical and Discharge Physics POLYMER TREATMENT APPARATUS DAMOP_0505_09  Filamentary Plasma 10s – 200  m

Iowa State University Optical and Discharge Physics COMMERCIAL CORONA PLASMA EQUIPMENT  Tantec, Inc.  Sherman Treaters DAMOP_0505_10

Iowa State University Optical and Discharge Physics REACTION MECHANISM FOR HUMID-AIR PLASMA  Initiating radicals are O, N, OH, H  Gas phase products include O 3, N 2 O, N 2 O 5, HNO 2, HNO 3. DAMOP_0505_11

Iowa State University Optical and Discharge Physics REACTION PATHWAY DAMOP_0505_12

Iowa State University Optical and Discharge Physics FUNCTIONALIZATION OF THE PP SURFACE  Untreated PP is hydrophobic.  Increases in surface energy by plasma treatment are attributed to the functionalization of the surface with hydrophilic groups.  Carbonyl (-C=O)  Alcohols (C-OH)  Peroxy (-C-O-O)  Acids ((OH)C=O) DAMOP_0505_13 Boyd, Macromol., 30, 5429 (1997).  Polypropylene, Air corona  The degree of functionalization depends as gas mix, energy deposition and relative humidity (RH).

Iowa State University Optical and Discharge Physics POLYPROPYLENE (PP) POLYMER STRUCTURE  The surface energy of polypropylene [C 2 H 3 (CH 3 )] n is increased by hydrogen abstraction (ions, radicals photons) followed by passivation by O atoms, in this case forming peroxy groups. DAMOP_0505_13A

Iowa State University Optical and Discharge Physics SITE SPECIFIC REACTIVITY  Three types of carbon atoms in a PP chain:  Primary – bonded to 1 C atom  Secondary – bonded to 2 C atoms  Tertiary – bonded to 3 C atoms  The reactivity of an H-atom depends on the type of C bonding. Reactivity scales as: H TERTIARY > H SECONDARY > H PRIMARY DAMOP_0505_14

Iowa State University Optical and Discharge Physics PP SURFACE REACTION MECHANISM: INITIATION  The surface reaction mechanism has initiation, propagation and termination reactions.  INITIATION: O and OH abstract H from PP to produce alkyl radicals; and gas phase OH and H 2 O. DAMOP_0505_15

Iowa State University Optical and Discharge Physics PP SURFACE REACTION MECHANISM : PROPAGATION  PROPAGATION: Abundant O 2 reacts with alkyl groups to produce “stable” peroxy radicals. O 3 and O react to form unstable alkoxy radicals. DAMOP_0505_16

Iowa State University Optical and Discharge Physics PP SURFACE REACTIONS : PROPAGATION / AGING  PROPAGATION / AGING: Peroxy radicals abstract H from the PP chain, resulting in hydroperoxide, processes which take seconds to 10s minutes. DAMOP_0505_17

Iowa State University Optical and Discharge Physics PP SURFACE REACTION MECHANISM : TERMINATION  TERMINATION: Alkoxy radicals react with the PP backbone to produce alcohols and carbonyls. Further reactions with O eventually erodes the film. DAMOP_0505_18

Iowa State University Optical and Discharge Physics GLOBAL_KIN AND SURFACE KINETICS  Reaction mechanisms in pulsed atmospheric air plasma treatment of polymers have been investigated with global kinetics and surface models.  GLOBAL_KIN  2-Zone homogeneous plasma chemistry (bulk plasma, boundary layer)  Plug flow  Multilayer surface site balance model  Circuit module  Boltzmann derived f(  ) DAMOP_0505_19

Iowa State University Optical and Discharge Physics BASE CASE: n e, T e  Ionization is dominantly of N 2 and O 2,  e + N 2  N e + e,  e + O 2  O e + e.  After a few ns current pulse, electrons decay by attachment (primarily to O 2 ).  Dynamics of charging of the dielectrics produce later pulses with effectively larger voltages; residual preionization and metastables also persist.  N 2 /O 2 /H 2 O = 79/20/1, 300 K  15 kV, 9.6 kHz, 0.8 J-cm -2  Web speed = 250 cm/s (460 pulses) DAMOP_0505_20

Iowa State University Optical and Discharge Physics GAS-PHASE RADICALS: O, OH  Electron impact dissociation of O 2 and H 2 O produces O and OH. O is consumed primarily to form O 3 ; OH is consumed by both bulk and surface processes.  After 100s of pulses, radicals attain a periodic steady state. DAMOP_0505_21 OO  OH NN

Iowa State University Optical and Discharge Physics PP SURFACE GROUPS vs ENERGY DEPOSITION  Surface concentrations of alcohols, peroxy radicals are near steady state with a few J-cm -2.  Alcohol densities decrease at higher J-cm -2 energy due to decomposition by O and OH to regenerate alkoxy radicals.  Air, 300 K, 1 atm, 30% RH  Ref: L-A. Ohare et al., Surf. Interface Anal. 33, 335 (2002). DAMOP_0505_22

 Increasing RH produces OH which react with PP to form alkyl radicals, which are rapidly converted to peroxy radicals by O 2. PP-H + OH (g)  PP  + H 2 O (g) PP  + O 2(g)  PP-O 2   Alcohol and carbonyl densities decrease due to increased consumption by OH to form alkoxy radicals and acids. Iowa State University Optical and Discharge Physics HUMIDITY: PP FUNCTIONALIZATION BY OH PP-OH+ OH (g)  PP-O  + H 2 O (g), PP=O  + OH (g)  (OH)PP=O DAMOP_0505_23

Iowa State University Optical and Discharge Physics COMMODITY TO HIGH VALUE  Control of O to O 3 ratio using He/O 2 mixtures can be used to customize surface functionalization.  1 atm, He/O 2, 15 kV, 3 mm, 9.6 kHz, 920 pulses.  As the material value increases (cents to dollars /cm 2 ?) higher process refinement is justified to customize functionalization. DAMOP_0505_24

Iowa State University Optical and Discharge Physics COMMODITY TO HIGH VALUE  1 atm, He/O 2 / H 2 O, 15 kV, 3 mm, 9.6 kHz, 920 pulses.  Additional “tuning” of functionalization can be achieved with sub-mTorr control of water content.  Small water addition “tuning” of functionalization can be achieved with sub- mTorr control of water content.  H and OH reduce O 3 while promoting acid formation. DAMOP_0505_25

THE CHALLENGE: COMMODITY PROCESSING FOR HIGH VALUE MATERIALS DAMOP_0505_26

Iowa State University Optical and Discharge Physics DAMOP_0505_27 THE ROLE OF PLASMAS IN BIOSCIENCE  Plasmas, to date, have played important but limited roles in bioscience.  Plasma sterilization  Plasma source ion implantation for hardening hip and knee replacements.  Modification of surfaces for biocompatibility (in vitro and in vivo)  Artificial skin  The potential for commodity use of plasmas for biocompatibility is untapped.  Low pressure rf H 2 O 2 plasma (

Iowa State University Optical and Discharge Physics “HIGH VALUE” PROCESSING - CELL MICROPATTERNING DAMOP_0505_28  Low pressure “microelectronics-like” plasmas are used to pattern selective substrate regions with functional groups for cell adhesion.  These processes have costs commensurate with microlectronics: high value, high cost. 1 Andreas Ohl, Summer School, Germany (2004).  PEO - polyethyleneoxide  pdAA – plasma deposited acrylic acid

Iowa State University Optical and Discharge Physics DAMOP_0505_29 ATMOSPHERIC PRESSURE PLASMAS: THE CHALLENGE  Controlling functional groups on polymers through fundamental understanding of plasma-solid interactions will enable engineering large area biocompatible surfaces.  10,000 square miles of polymer sheets are treated annually with atmospheric pressure plasmas to achieve specific functionality. Cost: < $0.05 /m 2  Low pressure plasma processing technologies produce biocompatible polymers having similar functionalities. Cost: up to $100’s /cm 2 ($1000’s/cm 2 for artificial skin)  Can commodity, atmospheric pressure processing technology be leveraged to produce high value biocompatible films at low cost? The impact on health care would be immeasurable. $0.05/m 2 $1000/cm 2 ?

Iowa State University Optical and Discharge Physics  The surface modification of polymers (such as PP) by atmospheric pressure corona DBDs is a geometrically complex but cheap process.  The plasma is filamentary non-uniformly producing reactants  The surface is at best rough and at worst a mesh of strands.  Can these surfaces be functionalized to meet high value standards? POLYMER PROCESSING BY CORONA DBDs DAMOP_0505_30

Iowa State University Optical and Discharge Physics  Continuity (sources from electron and heavy particle collisions, surface chemistry, photo-ionization, secondary emission), fluxes by modified Sharfetter-Gummel with advective flow field.  Poisson’s Equation for Electric Potential:  Electron energy equation:  Photoionization, electric field and secondary emission: DESCRIPTION OF nonPDPSIM: CHARGED PARTICLE, SOURCES DAMOP_0505_31

Iowa State University Optical and Discharge Physics CAN COMMODITY PROCESSES PRODUCE HIGH VALUE MATERIALS  Demonstration: corona-rod, 2 mm gap, 15 kV pulse, N 2 /O 2 /H 2 O =79.5 / 19.5 / 1, 1 atm  Tantec, Inc. DAMOP_0505_32

E/N, T e, SOURCES, ELECTRON DENSITY  Pulse is initiated with electron emission from tip of cathode.  Development of plasma streamer deforms potential producing large electric field. Pulse is terminated with dielectric charging.  N 2 /O 2 /H 2 O =79.5 / 19.5 / 1, 1 atm, -15 kV, 0-15 ns  E/N MINMAX Animation Slide DAMOP_0505_33 TeTe  Net Ionization TeTe  [e]

Iowa State University Optical and Discharge Physics POST PULSE RADICAL DENSITIES  Radical and ion densities at end of pulse are as high as 10s ppm. Temperature rise is nominal due to short pulse duration. OO O2(1)O2(1) MINMAX  N 2 /O 2 /H 2 O =79.5 / 19.5 / 1, 1 atm, 15 kV, 0-15 ns  N 2 (A) DAMOP_0505_34  H, OH

Iowa State University Optical and Discharge Physics  Electrons penetrate surface features on the polymer to a limited extent due to surface charging. SURFACE INTERACTIONS: ELECTRON DENSITY  -15 kV, 760 Torr, N 2 /O 2 /H 2 O=79.5/19.5/1 MIN (log scale) MAX 1.35 ns1.40 ns 2x x x x ns 2x x ns 1x x ns [e] cm  m DAMOP_0505_35

Iowa State University Optical and Discharge Physics  Radicals striking the surface penetrate into the features by diffusion.  Unlike charged species, with time, the density of radicals such as [O], increases inside these features. SURFACE INTERACTIONS: [O] DENSITY  -15 kV, 760 Torr, N 2 /O 2 /H 2 O=79.5/19.5/1 1x x x x ns 5x x ns 7.0 ns MIN (log scale) MAX [O] cm ns1.4 ns 10  m DAMOP_0505_36

Iowa State University Optical and Discharge Physics FUNCTIONAL GROUP DENSITIES ON POLYPROPYLENE  1 atm, N 2 /O 2 /H 2 O=79.5/19.5/1, 1.5 ms, 10 kHz. DAMOP_0505_37

Iowa State University Optical and Discharge Physics FUNCTIONALIZATION OF SCAFFOLDING DAMOP_0505_38  Functionalization of scaffolding-like surfaces for cell adhesion.  Can uniformity be maintained over micro-and macroscopic lengths.  Use 1 atm, He/O 2 /H 2 O mixtures to optimize.

Iowa State University Optical and Discharge Physics FUNCTIONALIZING PP SCAFFOLDING: HIGH O 2 (He/O 2 /H 2 O = 69/30/1) DAMOP_0505_39  High O 2 produces O 3 and rapid alkoxy formation.  Reactivity of O 3 limits transport and produces long- and short-scale nonuniformities.  1 atm, He/O 2 /H 2 O = 69/30/1

Iowa State University Optical and Discharge Physics FUNCTIONALIZING PP SCAFFOLDING: LOW O 2 (He/O 2 /H 2 O = 89/10/1) DAMOP_0505_40  Lower O 2 produces less O 3 and limits alkoxy formation.  Overall uniformity becomes reaction limited, producing smoother functionalization.  1 atm, He/O 2 /H 2 O = 89/10/1

Iowa State University Optical and Discharge Physics REMINDER: LOCAL STRUCTURE MATTERS DAMOP_0505_41  The reactivity of =C-H to gas phase species depends and with other surface species on their local bonding and orientation on surface.  1 atm, N 2 /O 2 /H 2 O = 79.5/19.5/1  Experimental evidence suggest reactivity scales as: H TERTIARY > H SECONDARY > H PRIMARY

Iowa State University Optical and Discharge Physics COVERAGE OF PEROXY [=C-O-O  ] BY BONDING AT 10 ms DAMOP_0505_42  Primary and secondary sites with large view angles are rapidly functionalized to peroxy.  Alkyl tertiary sites lag and are susceptible to OH, O 3 passivation  1 atm, N 2 /O 2 /H 2 O = 79.5/19.5/1

Iowa State University Optical and Discharge Physics COVERAGE OF PEROXY [=C-O-O  ] BY BONDING AT 140 ms DAMOP_0505_43  Long term production of O 3 and reactions between surface species favor secondary and tertiary sites.  Uniformity improves (mostly).  1 atm, N 2 /O 2 /H 2 O = 79.5/19.5/1

Iowa State University Optical and Discharge Physics PROCESSING COMPLEX SHAPES DAMOP_0505_44  Functionalization of parts with complex shapes with dimensions larger than reaction length of radicals requires plasma to penetrate into structure.  Demonstration case: grooved disk with 30  m slots.

Iowa State University Optical and Discharge Physics PROCESSING COMPLEX SHAPES: PLASMA PENETRATION DAMOP_0505_45  Plasma penetrates through grooves but shadow some surfaces.  Charging of surface steers plasma  Electron density (max = cm -3 ) Animation Slide-GIF  1 atm, N 2 /O 2 /H 2 O = 79.5/19.5/1, 2 ns MINMAX

Iowa State University Optical and Discharge Physics PROCESSING COMPLEX SHAPES: O ATOM DENSITY  Plasma penetrates through grooves but shadow some surfaces.  Charging of surface steers plasma  O atom density (max = cm -3 ) DAMOP_0505_46  1 atm, N 2 /O 2 /H 2 O = 79.5/19.5/1, 2 ns MINMAX Animation Slide-GIF

Iowa State University Optical and Discharge Physics COMMENTS: PHOTONS AND CHARGING  Unlike neutral radicals that eventually diffuse into nooks-and- crannies, shadowing (photons) and local electric fields (surface charging) produce highly non-uniform profiles.  What affect does UV illumination and charging have on reactivity? DAMOP_0505_47  1 atm, N 2 /O 2 /H 2 O = 79.5/19.5/1, 2 ns MINMAX

Iowa State University Optical and Discharge Physics THE CHALLENGE  Can established AMO theory and measurement techniques developed for gas phase species be extended to produce reaction probabilities on the surfaces of solid polymers?  Can scaling laws be developed for going from molecules to surfaces?  For example, how different are….. DAMOP_0505_48

Iowa State University Optical and Discharge Physics OPPORTUNITIES AND CONCLUDING REMARKS  The interaction of plasma produced species with polymer surfaces is an exceedingly rich field of study.  The are very (very [very]) few fundamental studies capable of producing reaction probabilities of even simple systems such as O atoms on polypropylene or polyethylene.  Probabilities for reactions between surface species are only now becoming quantified. (Session C1: “Interaction of Slow Electrons with Biomolecules”)  Photon and charging effects on rates……unknown.  Improving our fundamental understanding and predictive capability (and leveraging commodity techniques) will revolutionize fields such as health products. DAMOP_0505_49