Divinylbenzene (DVB) Shells

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Divinylbenzene (DVB) Shells

• High Average Power Laser Program Workshop
• UCLA
• Los Angeles, CA
• June 2-3, 2004
• Jon Streit
• Diana Schroen

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Review
4 mm Diameter Foam Shell

• 300 micron DVB Foam Wall
• CH Polymer
• 1-3 Micron Cell Size
• 20 - 120 mg/cc
• 1-5 micron Carbon Overcoat

• Shell formed through microencapsulation
• Overcoat applied with interfacial polycondensation

• Status at last review
• Nonconcentricity reduced. Further improvement
  needed.
• Characterization of wet shells routine.
• Low dry yields of overcoated shells. Shrinkage
  of PVP noted.
• Overcoat surface smoothness increased. Further
  improvement necessary.

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Shell Production Status
Formation / Gelation Understanding of gelation and NC increased. Time, PAA concentration, agitation all factors.
Characterization Characterization of wet shells routine. Need to quantify characterization accuracy has arisen.
Overcoating Problems with PVP overcoat persist. Alternative overcoat chemistry being explored.
Supercritical Drying Drying problems suspected to be caused primarily by overcoat shrinkage.
Scale-Up Beginning cooperative effort with GA.



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Full vs. Partial Fill Flask
Characterization of all shell batches has been
completed. Superiority of the partially full
flask has been confirmed.
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0.05 PAA vs. 0.1 PAA
0.01 PAA generally results in lower
nonconcentricity.
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Longer Gelation Time and NC

• Analysis of the collected data also indicated
  that a longer gelation time reduces
  nonconcentricity. Some of the shells with the
  lowest nonconcentricity used Benzyl Ether as the
  solvent (longer time until shape is set).
• A small carefully controlled study was
  performed to confirm these trends.

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PAA and Gelation Time Results
Batch PAA Gelation Time NC
64A 0.05 Standard 15
64B 0.05 Longer 10
64C 0.1 Standard 10
66B 0.1 Longer 5
66C 0.15 Standard 30
67C 0.15 Longer 33
Absorbance data is now collected using a
spectrophotometer to determine if the degree of
polymerization obtained during the
pre-polymerization step affects NC.
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Alternative Gelation Method

• An alternative DVB gelation method was
  developed at GA for LLE (Don Czechowicz, Abbas
  Nikroo, Reny Paguio, Masa Takagi).
• This method uses a dual initiator (low and high
  temperature) system.
• The lower decomposition temperature of the
  first initiator allows the shape of the shell to
  be set at less than 35 C, thus simplifying
  density matching and temperature control while
  increasing gelation time.
• The temperature is then increased to allow to
  finish the reaction with the second initiator.
• Capsules made by this method can be rapidly
  exchanged to IPA without shells breakage, but no
  direct comparison of shell strength has been made.

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PVP Limitations / Problems

• The PVP overcoat tends to delaminate from the
  shell surface during the exchange and drying
  processes.
• The PVP overcoat tends to shrink during the
  drying process resulting in foam densification
  and small shell diameter.
• Tris was added as an additional cross-linker to
  try to reduce this trend. Shells produced with
  tris do not shrink as much, but tend to have
  rougher surface finish. Shells with tris adhere
  to the shell only marginally better.
• PVP polymer forms at the oil/water interface
  and grows away from the interior of the shell.
  This may or may not be significant depending on
  the location of the oil/water interface and the
  foam shell boundary.

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PVP Overcoat Tests

• Due to poor results in overcoat survival and
  surface finish using 4-chlorotoluene as the oil
  for the overcoat reaction, diethyl phthalate and
  dibutyl phthalate were substituted for the oil
  phase.
• Organic solvent, time, small molecule
  cross-linker, and solvent exchange were all
  varied to try to improve the overcoat.

Water Reactant Solvent Reaction Time Exchange
PVP DEP 5, 10, 30 min Flush and dry with organic solvent or exchange to IPA
PVP Tris DEP 5, 10, 30 min Flush and dry with organic solvent or exchange to IPA
PVP DBP 5, 10, 30 min Flush and dry with organic solvent or exchange to IPA
PVP Tris DBP 5, 10, 30 min Flush and dry with organic solvent or exchange to IPA
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PVP Results
Interferometer surface roughness measurement of
overcoat. PVP reacted for 30 minutes with DEP as
solvent, RMS 34 nm

• Dry yields were low for all PVP overcoat
  trials. The best surface finish achieved was for
  the reaction conditions above.

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PVP Alternatives

• To combat the shrinkage and adherence problems
  encountered with PVP, other polymers formed
  through interfacial polymerization are being
  investigated.
• PVP historically been chosen for its optical
  transparency. With an opaque foam this is no
  longer important.
• Initial runs of the application of a
  poly(ethylene terephthalate) type overcoat has
  been applied to DVB shells.
• This type of coating has been shown to grow
  towards the oil side of the interface (into the
  shell).
• The degree of crossliking can be roughly
  controlled as both a difunctional and a
  trifunctional monomer can be used.

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Initial PET results
Interferometer surface roughness measurement of
overcoat. PET type polymer reacted for 10
minutes with DBP as solvent, RMS 107 nm

• Initial PET type polymer results were promising.

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Future Work

• Continue to study nonconcentricity including GA
  two initiator process.
• Continue to study the PVP and alternative
  overcoating processes and study the effects of
  chemistry and reaction conditions on overcoat
  surface roughness.
• Continue collaboration and site visits with GA.