JES Divinylbenzene (DVB) Shells High Average Power Laser Program Workshop UCLA Los Angeles, CA June 2-3, 2004 Jon Streit Diana Schroen

JES Review 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. 300 micron DVB Foam Wall – CH Polymer – ~1-3 Micron Cell Size – mg/cc 1-5 micron Carbon Overcoat Shell formed through microencapsulation Overcoat applied with interfacial polycondensation 4 mm Diameter Foam Shell

JES 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-UpBeginning cooperative effort with GA.

JES Full vs. Partial Fill Flask Characterization of all shell batches has been completed. Superiority of the partially full flask has been confirmed.

JES % PAA vs. 0.1 % PAA 0.01 % PAA generally results in lower nonconcentricity.

JES 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.

JES PAA and Gelation Time Results BatchPAA % Gelation Time NC% 64A0.05Standard15 64B0.05Longer10 64C0.1Standard10 66B0.1Longer5 66C0.15Standard30 67C0.15Longer33 Absorbance data is now collected using a spectrophotometer to determine if the degree of polymerization obtained during the “pre- polymerization step” affects NC.

JES 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.

JES 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.

JES 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 PVPDEP 5, 10, 30 min Flush and dry with organic solvent or exchange to IPA PVP + TrisDEP PVPDBP PVP + TrisDBP

JES 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.

JES 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.

JES 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.

JES 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.