Generation of Microcellular Materials via Polymerization in Carbon Dioxide

摘要- Beckman - 9870925 气凝胶是表现出非常低的堆积密度（小于100 kg/m3）和非常小的平均孔径（小于1纳米）的交联材料。 它们已被提议用作催化剂载体（比表面积可高达1000 m2/g）、高效绝缘体和膜。 虽然它们具有迷人的物理和化学特性，但气凝胶的生产相对耗时，并且基于二氧化硅的版本（最普遍的类型）非常脆。 总之，气凝胶是令人感兴趣的，因为小的孔径和高的孔密度提供非常高的比表面积，而低的堆积密度允许有效地使用材料。 这将是有趣的和有用的，探索更有效的方法，通过它可以生产气凝胶，并扩大其物理性质的范围。 许多研究人员已经探索了通过相分离（由温度、非溶剂添加、交联等诱导）来产生微孔有机聚合物。在液体和超临界流体中，还没有实现同时具有小孔和低泡沫密度（与本体聚合物相比减少超过90%）的材料的生产。 鉴于这一背景，PI计划通过在稀二氧化碳（CO2）溶液中的非线性聚合来产生低密度微孔材料，其中通过使用高度可溶于CO2的反应性前体来防止聚合物的过早沉淀。 为了克服稀释反应物浓度所固有的低反应速率，他们计划生成反应性端基倾向于聚集在二氧化碳中的前体，从而生成显著高于标称平均场组成的局部浓度。 他们计划合成和表征具有疏CO2端基的亲CO2反应性低聚物，确认模型反应性低聚物与二氧化碳混溶，通过模型低聚物在CO2中的反应生成多孔材料，然后在31 ℃以上去除溶剂，并探索细胞微观结构的变化，因为交联之间的分子量，以及亲CO2和疏CO2基团的身份是不同的。 在二氧化碳中使用交联聚氨酯的初步工作已经表明，他们确实可以设计在高压CO2存在下表现出单相（溶胀网络而没有过量CO2相）的交联聚合物体系。 此外，使用液体中的反应性聚合物体系，他们已经表明，使用具有亲溶剂重复单元和疏溶剂反应性端基的前体通过端基的聚集提高了稀溶液中的反应速率。

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.