PLASMA SURFACE INTERACTIONS FOR ATMOSPHERIC PRESSURE FUNCTIONALIZATION OF POLYMERS Mark J. Kushner Iowa State University Ames, IA 50011 USA [email protected]@iastate.edu

PLASMA SURFACE INTERACTIONS FOR ATMOSPHERIC PRESSURE

FUNCTIONALIZATION OF POLYMERS

Mark J. KushnerIowa State UniversityAmes, IA 50011 USA

[email protected] http://uigelz.ece.iastate.edu

March 2007

EUJapan_0307_01

mailto:[email protected]

http://uigelz.ece.iastate.edu/

Iowa State University

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

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AGENDA

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Introduction to plasma functionalization of polymers

Description of nonPDPSIM

Corona treatment of Polymers: Pulsing, Flowing, Moving

Optimizing Uniformity

Concluding Remarks

FUNCTIONALIZATION OF POLYMER SURFACES

Functionalization occurs by chemical interaction of plasma produced species - ions, radicals and photons with the surface.

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

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Iowa State UniversityOptical and Discharge Physics

(a)(b)

(c)

Pulsed atmospheric filamentary discharges (coronas) routinely treat commodity polymers like poly-propylene (PP) and polyethylene (PE).


SURFACE MODIFICATION OF POLYMERS

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Filamentary Plasma 10s – 200 m



COMMERCIAL CORONA PLASMA EQUIPMENT

Tantec, Inc.

Sherman Treaters

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MODELING OF FUNCTIONALIZATION OF POLYMERS

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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.



MODELING PLATFORM nonPDPSIM

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nonPDPSIM is organized around modules addressing different physical processes having different timescales.


CHARGED PARTICLE TRANSPORT

Poisson’s equation for the electric potential

= electric potential Nj = density qj = charge = surface charge

Transport equations for conservation of the charged species j

j = Flux Sj = sources due to collisions, photons

Surface charge balance

)( j

jj Nq

jjj S

t

N

materialj

j Sqt

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CHARGED PARTICLE TRANSPORT

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.

iijijj

j

jjjjj

j

j vvNM

ENqvP

Mt

1

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i1/2 D

ni1 ni exp(x)1 exp(x)

qq

i 1 ix

D

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.

EBeeeeeee STTkTTLTStkTn

2

5/

2

3

Iowa State UniversityOptical and Discharge PhysicsEUJapan_0307_11

c

v t

vfvfEq



RADIATION TRANSPORT

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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

rdrrGrNrNdt

rdN

rr

rdrrNexp

rrG

ijj

jii

kijkk

j

ijij

3

2

,

4,



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.

)pumps,inlets()v(t

i

iii ENqvvNkTt

v

i i

iiiipp EjHRvPTcvTt

Tc

SV

T

iTiii SS

N

ttNNDvtNttN

FLUID MODULE : NEUTRAL PARTICLE TRANSPORT

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Optical and Discharge PhysicsEUJapan_0307_14

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.

Vastly different timescales addressed using a variant of time-slicing which leverages time different timescales required to come into adiabatic equilibrium.


LARGE DYNAMIC RANGE: TIME SLICING

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EXAMPLE OF MESH: POLYMER TISSUE SCAFFOLDING

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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.



ADAPTIVE EMCS PROCESS

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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).

[Sources]

BREAKDOWN IN HID LAMP: THE IONIZATION FRONT


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.

MIN MAX

[e] Te

10 eV MCS

Ar, 30 Torr, 2000V, 100 ns

Animation Slide-GIF

1013 cm-3 1021 cm-3s-1

BREAKDOWN IN HID LAMP: THE IONIZATION FRONT

Iowa State UniversityOptical and Discharge PhysicsEUJapan_0307_19A

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.

MIN MAX

[e] 1013 cm-3

[Sources] 1021 cm-3s-1

Te 10 eV

MCS

Ar, 30 Torr, 2000V, 100 ns

Animation Slide-AVI



PRIMER ON SURFACE CHEMISTRY

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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



PRIMER ON SURFACE CHEMISTRY

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Ratio of O, OH, O2 and O3 fluxes determine surface composition.

Magnitude of fluxes and residence time determines importance of surface-surface reactions.



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.

FORCED GAS FLOW AND WEB MOVEMENT

Gas Flow

FORCED GAS FLOW AND WEB MOVEMENT

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2 mm

10 cm

- 5 kV, 1 atm, He/O2/H2O=89/10/1 Inter-electrode gap: 2 mm

Web Motion

Gas flow: 0 – 30 slpm (many m/s) Web speed: 0-8 m/s


Translate surface mesh points to account for web motion.



DYNAMICS OF THE FIRST PULSE: Te, SOURCES

- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0–2 ns, no flow

Animation Slide-GIF

MIN MAX log scale

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Te 0-9 eV

Electron Source 5x1020-5x1023 cm-3s-1

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.

REPETITIVELY PULSED DISCHARGES – [e]


0.01 100 log scale

1014 cm-3

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.

He/O2/H2O=89/10/1, -5 kV, 10 kHz, 1 atm Animation Slide-GIF

POST-PULSE REACTANTDENSITIES


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

POST-PULSE REACTANTDENSITIES


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

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.

0.0 1

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

(2.8-2.9 10-4 g-cm-3)

He/O2/H2O=89/10/1, -15 kV, 1 kHz, 10 slpm, 1 atm.

Animation Slide-GIF



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.

0.001 1 log scale

Without forced flow [O] 1015 cm-3

Animation Slide-GIF

[O] – WITHOUT AND WITH FORCED GAS FLOW

With forced flow Flow


Optical and Discharge PhysicsEUJapan_0307_30A

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.


0.001 1 log scale

Without forced flow [O] 1015 cm-3

Animation Slide-AVI

[O] – WITHOUT AND WITH FORCED GAS 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.


0.001 1 log scale

Without forced flow [O3] 3 x 1014 cm-3

Animation Slide-GIF

[O3] – WITHOUT AND WITH FORCED GAS 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.


0.001 1 log scale

Without forced flow [O3] 3 x 1014 cm-3

Animation Slide-AVI

[O3] – WITHOUT AND WITH FORCED GAS FLOW




OH has an intermediate reactivity between O and O3.

Some modest accumulation occurs with a small plume downstream with forced flow.


0.001 1 log scale

Without forced flow [OH] 1014 cm-3

Animation Slide-GIF

[OH] – WITHOUT AND WITH FORCED GAS FLOW




OH has an intermediate reactivity between O and O3.

Some modest accumulation occurs with a small plume downstream with forced flow.


0.001 1 log scale

Without forced flow [OH] 1014 cm-3

Animation Slide-AVI

[OH] – WITHOUT AND WITH FORCED GAS FLOW


- 5 kV, 1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s Iowa State UniversityOptical and Discharge Physics

ALKYL COVERAGE (NO FLOW, NO MOTION)

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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


ALKYL COVERAGE (NO FLOW, NO MOTION)

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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


PEROXY COVERAGE (NO FLOW, NO MOTION)

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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


PEROXY COVERAGE (NO FLOW, NO MOTION)

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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

1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 30 slpm


ALKYL, PEROXY COVERAGEWITH FLOW

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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.

Animation Slide-GIFFlow



ALKYL, PEROXY COVERAGEWITH FLOW

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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.

Animation Slide-AVIFlow



SURFACE COVERAGES WITH FLOW

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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.

Flow

1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 4 m/s


ALKYL, PEROXY COVERAGEWITH WEB MOTION

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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)

Animation Slide-GIF

Web

1 atm, He/O2/H2O=89/10/1, 10 kHz, 0.022 s, 4 m/s


ALKYL, PEROXY COVERAGEWITH WEB MOTION

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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)

Animation Slide-AVI

Web



SURFACE COVERAGES WITH WEB MOTION

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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



SURFACE COVERAGES WITH FLOW AND WEB MOTION

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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.

WebFlow

1 atm, He/O2/H2O=89/10/1, 0.022 s


TUNING PEROXY COVERAGEvs WEB SPEED, FLOW

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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.

Web

No flow

100 cm/s

Flow



PEROXY, ALKOXY COVERAGEvs O2 FRACTION

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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



FUNCTIONALIZATION OF POLYMER TISSUE SCAFFOLDING

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Corona functionalization of rough polypropylene resembling tissue scaffold.

1 atm, He/O2/H2O, 10 kHz

Optimize uniformity in small structures.

E. Sachlos, et al.



OPTIMIZE CHEMISTRY, UNIFORMITY WITH GAS MIXTURE

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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)



CONTROLLING FLUX OF OZONE TO SURFACE

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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.



CONTROLLING FLUX OF OZONE TO SURFACE

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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



CONCLUDING REMARKS

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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.

Documents

PLASMA SURFACE INTERACTIONS FOR ATMOSPHERIC PRESSURE FUNCTIONALIZATION OF POLYMERS Mark J. Kushner Iowa State University Ames, IA 50011 USA [email protected]@iastate.edu