LLNL-PRES-725837

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC

Highly Crosslinked Polymers for Transparent Ablators

21st Target Fabrication Meeting 2017LDRD 15-ERD-020

Xavier Lepró

Salmaan Baxamusa, Paul Ehrmann, Joe

Menapace, Johann Lotscher, Swanee

Shin, Richard Meissner, Ted LaurenceMarch 14, 2017

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To achieve ignition, symmetry is the key!….not only physically, but chemically too!

Chemically vapor deposited (CVD) polymer makes an ideal ablator capsule: amorphous, low atomic number, and can be easily doped with high-Z atoms

~ 200 m

“Ablator capsule”

• Imploding capsule compresses hydrogen-filled interior, leading to Deuterium+Tritium fusion.

• Capsule must be topographically and chemically symmetric (round, smooth, no bumps, homogenous chemical composition).

2 mm 0

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The changes of properties of plasma

polymers with time are related to the

concentration of trapped free radicals.

J. Macromol. Sci. A, 10, 383 (1976)

Plasma CVD polymers are not equivalent to their liquid-phase counterparts!

Plasma CVD polymers do not have a well-defined chemical structure

Advantages of plasma CVD polymers:• Single-step synthesis and casting

• Conformal over complex geometries

• Solvent-free (highly pure)

• Very smooth (nm RMS)

• Highly crosslinked However…

Polym. Degrad. Stab. 122 (2015), 1333

I=13 mW/cm2

C● + O2 COOH

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Our Approach: Produce well-defined polymers that can replace amorphous Hydrogenated Carbon

Non-plasma energy sources can be used for gas-phase polymerization!

initiated Chemical Vapor Deposition

iCVD

Macromolecules 39, 2006, 3688-94

By using a thermally liable initiator, monomer molecules remain

unchanged until polymerization

Similar to Traditional polymerization

Photolytic

Pyrolytic

PROPERTIESSTRUCTURE

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Ablators have to endure stringent processing conditions

Spherical MandrelConformal coating

(Low stress film)

Pyrolysis

300°C

Hydrocarbon

polymer capsule

ablator

Polish/cleanLaser drill

AssemblyInspection

20 K

Hydrogen solid

monocrystal layerImplosion!

Thermally stable

Mechanical stiff

1.

2.

3.

Chemical inert

Photostable4.Transparent5.Cryostable6.Amorphous CH

Symmetric

No oxygen

Smooth

7.

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Ablator manufacturing requires highly resilient, cross-linked polymers

iCVD enables the synthesis andconformal coatings of polymeric filmsof highly crosslinked polymers.

n

CH3

PDVB

Poly(Divinylbenzene)

Divinylbenzene(p-DVB)

• Highly crosslinked

• Optically transparent

• No oxygen content

• Similar to polystyrene

• Stability via aromaticity

• No crystallizable groups

Lack of plasma-induced defects make iCVDPDVB transparent and photo-stable

iCVD - PDVB Plasma Polymer

Infusible and insoluble

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Lack of plasma-induced defects make iCVD PDVB transparent and photostable

UV-Vis spectrumTransparent to visible light

PhotochemistryNo visible light photochemistry

Baxamusa et al, Chem. Vap. Dep. 2015

200 250 300 350 400 450 500 550 6000

20

40

60

80

100

Wavelength (nm)

Tra

ns

mis

sio

n %

GDP

PDVB

Plasma polymer

iCVD PDVB

No photon absorption

No photo-oxidation=

0 5x105

1x106

2x106

0

100

200

300

400

3 h

Plasma polymer 1 mW

Plasma polymer 10 mW

85 h

PDVB anneal 280°C 5 mW

PDVB as-deposited 5 mW

[O

H]/

[CH

]1

00

0

Dose (mJ/cm2)

70 h

= 405 nm

iCVD PDVB

(dry air)

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200 400 600 800 1000

0.0

0.2

0.4

0.6

0.8

1.0

1-T

wavelength, (nm)

Fresnel Reflection

iCVD PDVB free-standing films are mechanically robust

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

2

4

6

8

10200°C annealed

Lo

ad

on

sa

mp

le (

mN

)

Displacement into surface (m)

as-deposited PDVB

Young’s Modulus invariant after 200°C anneal

5.7±0.2GPa

Stiffer than plasma polymer!

Stable density changes only ~1%

~25 µm thick films

0 1 2 3 40

1

2

3

4

25°C

50°C

100°C

125°C

150°C

175°C

200°C

225°C

250°C

280°C

PD

VB

mass (

mg

)

Volume (mm3)

200°C

=1.06 g/cm3

EGDP ~3.5GPavs.

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PDVB is thermally stable with no glass transition

ThermogravimetryStable at 300oC

0 50 100 150 200 250 300 350 40090

91

92

93

94

95

96

97

98

99

100

Temperature (oC)

Ma

ss

(%

)

<1.5% volatilesConfirmed DVB via mass spec

Mass Spectrometry

CH2

CH2

CH2

CH2

CH2

CH3CH3

CH2 Reversible thermal expansion

50 100 150 200 250

1020

1040

1060

1080

1100

thic

kn

es

s (

nm

)

Temperature (°C)

heating

coolin

g

in-situ ellipsometryRevealed no crystallinity or

glass transition

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iCVD-PDVB is strong, transparent, thermally resistant… but can it be used to make capsules?

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Film stress on coatingscan produce crippling,substrate warping ordelamination

Is film stress low enough?

It is important to measure film stress!

Undesirable for smooth & symmetrical ablators

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Measuring film stressLow stress coatings are a key requirement to make ablators

Deposition of a thin film generatesstress on a substrate which bendsaccordingly

Substrate changes radius of curvature, R

Measured by Interferometry

𝜎𝑓 =𝐹𝑓

𝑑𝑓𝑤=

Ys𝑑𝑠2

6𝑅 1 − 𝜈𝑠 𝑑𝑓

Stoney equation

Generated stress is related solely to substrate properties and film thickness

2 in PDVB coated wafers

f : Film stress

Ys : Substrate Young’s Modulus

ds : Substrate thickness

s : Substrate Poison’s ratio

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Thick PDVB films show near-zero intrinsic stressArbitrary film thickness are possible!

0 5 10 15 20 25

0

10

20

30

40

50

60

280°C

200°C

100°C

as-deposited

Str

ess

(M

Pa)

thickness (m)

50°C

PDVB intrinsic film stress

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Film stress is related to unreacted monomer… and can be removed by evaporation

Annealing removes unreacted monomer

0 50 100 150 200 250 300

-10

0

10

20

Str

es

s c

ha

ng

e,

(M

Pa

)

Temperature, T (°C)

1st annealing

Tb DVB=192°C at 1 atm

0 50 100 150 200 250 300

-40

-30

-20

-10

02

nd annealing

Str

es

s c

ha

ng

e,

(M

Pa

)

Temperature, T (°C)

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We have successfully incorporated Si dopantPolymer design allows doping

Current monomer:

+ Si dopant

Approach:

Si-doped PDVB

Si-PDVB

~1 µm thick film

1600 1400 1200 1000 800 600

IR Inte

nsity

wavenumber (cm-1)

Bands associated

with Silicon

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We can coat flat surfaces, but what about spheres?In the works: Devising substrate-shaker alternatives

Coated Uncoated

Glass spheres

Results so far are promising:

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Summary

• iCVD allows polymer design.

• Highly crosslinked polymers such as PDVB can be conformationally synthesized.

• PDVB polymers have shown to have high transparency, induce low film stress while being mechanically strong.

• PDVB is more chemically stable than GDP and does not photo-oxidize as easy.

• High-Z elements can be used for polymer doping: Si-doped PDVB.

SEM

Film Cross-section

25.2 m

Si wafer

PDVB

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Current technology: CVD plasma polymerizationProduces plastic ablators of amorphous C-H materials (GDP)

Given that plasma breaks organic molecules at different locations , plasma “polymers” do not have a defined chemical composition, are prone to aging and react with humidity and light.

Gas feed

Helical resonator

Vacuum chamber

Plasma glow discharge

Glow discharge

Organic gas

Plasma “polymer”

or Glow Discharge Polymer

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Fragmentation in Plasma Polymers leads to chemical instability

Plasma CVD polymer

Goal

J. Pol. Sci. A., 32, 1399 (1994)

▪

▪

▪

▪

▪

BUT…

▪

▪

▪

• Oxygen uptake in polymers hinders ignition.• Not optically clear• Limits control over material properties.

GDP