Divinylbenzene (DVB) Shells Foam Buckle Pressure & Uniformity

1. Divinylbenzene (DVB) Shells Foam Buckle Pressure & Uniformity High Average Power Laser Program Workshop University of Wisconsin Madison, WI September 24-25, 2003 Jon Streit Diana Schroen

2. Review • 4 mm Diameter Foam Shell • 300 micron DVB Foam Wall • CH Polymer • ~1-3 Micron Cell Size • 20 - 120 mg/cc • 1 micron Carbon Overcoat • Inner Water Phase • Organic Phase • Stripping Phase • Status at last review: • Fabricated approximately 4 mm diameter shells with 300 µm walls at 100 mg/cc and 50 mg/cc. • Overcoated shells with a poly(vinyl phenol) overcoat. • Developed in-house characterization method. • All overcoated shell cracked during drying process. • No characterization of overcoat.

3. Formation / Gelation Characterization Overcoating Supercritical Drying Shell Production Flow • Shell production work areas: • Problem: Shell Nonconcentricity • Possible Solution: Design / Obtain new apparatus for gelation with improved temperature and agitation control. • Problem: Time Consuming • Possible Solution: Automate • Problem: Time consuming, Poor initial surface finish • Possible Solution: Automate, Optimize chemistry, reaction conditions • Problem: Low yield for dried, overcoated shells • Possible Solution: Continue drying / overcoating optimization

4. Fail Shells Flow Through Tube Pass Optical Cell Shells Sorted We are Beginning to Work with GA to Develop a Flow Through System To Shell Characterization Sort Droplet Generator Heated Coil IPA Rinse Benzyl Salicylate • Would agitation be adequate? Microscopes (2 Views)

5. Agitation & Temp During Gelation • Shell characterization data relating non-concentricity and density matching has been scattered. • Current generator set-up has poor temperature control and non-uniform agitation. • To remedy this, a new generator set-up and two new agitation methods have been devised. New set-up will control stripping temp and receiving bottle temp until transfer to new agitator. Current flask angle and heat source contact area.

6. Gelation Equipment A hybridization oven has been ordered to gel shells using a carousel motion. The angle the tube is held in the carousel can also be adjusted to provide a sloshing motion for periodic deformation of the shell. A bottle rotator that is submersible in a heated bath is being designed for superior heat transfer and uniform agitation.

7. Benzyl Ether as Dibutyl Phthalate Replacement • Benzyl ether was tested as a dibutyl phthalate substitute to allow gelation and characterization in the same solvent. • Same density and similar solubility parameters as DBP. • Index of refraction is higher (1.56 vs. 1.49) Shell Gelled using DBP in Water Shell Gelled using BE in Water Shell Gelled using BE in BE

8. Cell Size 100 mg/cc DVB Polymerized using Dibutyl Phthalate as the Solvent. 100 mg/cc DVB Polymerized Using Benzyl Ether as a Solvent. Cell Size is Larger Using Benzyl Ether

9. Fiber Size 100 mg/cc DVB TPX Foam Fiber Size is difficult to Quantify for 100 mg/cc DVB. The structure resembles that of the “string of pearls” structure of an aerogel.

10. Overcoating • Overcoating experiments have begun. • Surface roughness is greater then seen in previous work. • We are varying reaction time, pH, reactant concentration, PVP molecular weight, crosslinker, and crosslinker concentration to try to reduce the surface roughness. • SEM photos of initial overcoat have been taken.

11. Overcoat SEM Images 5000x, 20K PVP, 5 min 25000x, 20K PVP, 5 min At 5 minutes, the overcoat does not look complete and appears rough.

12. Overcoat SEM Images 5000x, 20K PVP, 15 min 25000x, 20K PVP, 15 min At 15 min, the overcoat shows some improvement, but still rough.

13. Supercritical Drying • Yields from drying shells are still low – at best 30%. • Initially we thought that slowing the rate of CO2 bleed off would increase yields – but this seems to have little effect. Dried Uncoated Shell • Overcoating polymerization and chemistry factors that affect overcoating strength and adhesion to the capsule are now being studied to increase dry yield. Dried Overcoated Shell

14. Buckle Pressure Hole in Foam Overcoat Failure Crack in Foam • Shells do not dimple as expected. Failure modes are through gross foam failure, overcoat delamination, or cracks in the foam. • Permeation is rapid for the initial overcoats – shells do not yield if pressure is increased at a moderate rate.

15. Future Work • Study nonconcentricity results for new droplet generator / agitator set-up with relation to density matching. • Continue to study and streamline the overcoating process and study the effects of overcoating chemistry and reaction conditions on overcoat surface roughness. • Explore methods to eliminate shell rupture problem during supercritical drying including overcoating chemistry and reaction condition effects. • Continue generating buckle pressure data.