Generation of Microcellular Materials via Polymerization in Carbon Dioxide

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Abstract - Beckman - 9870925 Aerogels are crosslinked materials which exhibit both very low bulk density (less than 100 kg/m3) and very small average pore size (less than 1 nanometer). They have been proposed for use as a catalyst supports (specific surface area can top 1000 m2/g), highly efficient insulation, and as membranes. While they exhibit fascinating physical and chemical properties, aerogels are relatively time-consuming to produce and the silica-based versions (the most prevalent type) are quite brittle. In summary, aerogels are of interest because the small pore size and high pore density gives very high specific surface area, while the low bulk density allows for efficient use of materials. It would be interesting and useful to explore more efficient means by which aerogels can be produced, and to expand the range of physical properties available to them. A number of researchers have explored generation of microcellular organic polymers via phase separation (induced by temperature, non-solvent addition, crosslinking, etc.) in both liquids and supercritical fluids, yet production of materials with both small pores and low foam density (reduction of over 90% compared to the bulk polymer) has not been achieved. Given this background, the PI's plan to generate low density, microcellular materials via a non-linear polymerization in a dilute carbon dioxide (CO2) solution where premature precipitation of the polymer is prevented via the use of the highly CO2 -soluble reactive precursors. To overvcome the low rate of reaction inherent to dilute reactant concentration, they plan to generate precursors where the reactive end groups tend to aggregate in carbon dioxide, thus generating local concentrations significantly higher than the nominal, mean field composition. They plan to synthesize and characterize CO2-philic reactive oligomers with CO2-phobic end groups, confirm that the model reactive oligomers are miscible with carbon dioxide, generate porous materials via reaction of model oligomers in CO2 followed by solvent removal above 31C, and explore changes in cellular microstructure as the molecular weight between crosslinks, and the identity of the CO2-philic and CO2-phobic groups is varied. Preliminary work with crosslinked polyurethanes in carbon dioxide has shown that they can indeed design a crosslinked polymer system that exhibits a single phase (swollen network without without an excess CO2 phase) in the presence of high pressure CO2. Further, using reactive polymer systems in liquids, they have shown that use of precursors with solvent-philic repeat units and solvent-phobic reactive end groups enhances the rate of reaction in dilute solution through aggregation of end groups.