Generation of Microcellular Materials via Polymerization in Carbon Dioxide

摘要-Beckman-9870925气凝胶是一种具有极低堆积密度(小于100 kg/m~3)和极小平均孔径(小于1纳米)的交联型材料。它们已被提议用作催化剂载体(比表面积可达1000平方米/克)、高效绝缘和膜。虽然气凝胶表现出令人着迷的物理和化学特性，但气凝胶的生产相对耗时，基于二氧化硅的版本(最流行的类型)相当易碎。总而言之，气凝胶之所以令人感兴趣，是因为它的小孔尺寸和高孔道密度提供了非常高的比表面积，而低体积密度允许有效地利用材料。探索生产气凝胶的更有效的方法，并扩大气凝胶可用的物理性质的范围，将是有趣和有用的。许多研究人员探索了通过相分离(由温度、非溶剂添加、交联剂等诱导)生成微孔有机聚合物的方法。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(与主体聚合物相比减少90%以上)的材料的生产。在此背景下，PI计划通过在稀释二氧化碳(CO2)溶液中进行非线性聚合来生成低密度微孔材料，其中通过使用高度溶解于CO2的活性前体来防止聚合物过早沉淀。为了克服稀释反应物浓度固有的低反应速度，他们计划产生前体，在这些前体中，活性端基往往聚集在二氧化碳中，从而产生明显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二氧化碳疏水端基的亲二氧化碳反应性低聚物，确认模型反应性低聚物与二氧化碳相容，通过模型低聚物在二氧化碳中的反应生成多孔材料，然后在31℃以上去除溶剂，并探索随着交联剂之间分子量的变化，细胞微观结构的变化，以及亲二氧化碳和憎二氧化碳基团的相同之处。对二氧化碳中的交联型聚氨酯的初步研究表明，他们确实可以设计出一种在高压二氧化碳存在下呈现单一相(膨胀的网络而没有多余的二氧化碳相)的交联型聚合物体系。此外，在液体中使用反应性聚合物体系，他们已经表明，使用具有亲溶剂重复单元和疏溶剂活性端基的前体通过端基的聚集提高了稀溶液中的反应速度。

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.