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This work evaluates the effects of solvents and a block copolymer surfactant on pore structures in polyimide aerogels synthesized via sol-gel reaction process. Specifically, cross-linked polyimide gel networks are synthesized in single or mixed solvents from a combination of dimethylformamide, N-methylpyrrolidone, and dimethylacetamide and supercritically dried to obtain aerogels. The bulk density, pore size, and mechanical properties of aerogels are determined. The results show that gel times are strongly dependent on the electron acceptance ability of the solvent system and concentration of the surfactant. At longer gel times, the polyimide strands coarsen and the pores in aerogel shift from predominantly mesoporous to macroporous state with corresponding reduction in compressive modulus. The block copolymer surfactant also slows down gelation and coarsens the polyimide strands but only weakly affects the compressive modulus of the aerogels.

This work focuses on the fabrication of polyimide aerogel microparticles of diameter 200−1000 μm from a surfactant-free, two-phase, silicone oil/dimethylformamide (DMF) oil-in-oil (O/O) system using a simple microfluidic device. The polyimide sol prepared in DMF is turned into droplets suspended in silicone oil in the microfluidic device. The droplets are guided to a heated silicone oil bath to accelerate sol−gel transition and imidization reactions, thereby yielding spherical, discrete gel microparticles that do not undergo coalescence. The discrete gel microparticles are isolated and supercritically dried to obtain aerogel microparticles. The microparticle size distribution shows dependence on dispersed and continuous phase flowrates in the microfluidic channels. The microparticle surface morphology shows dependence on the silicone oil bath temperature.

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The water-in-oil emulsion-templating method is used in this work for fabrication of open cell aerogel foams from syndiotactic polystyrene (sPS). A surfactant-stabilized emulsion is prepared at 60-100 °C by dispersing water in a solution of sPS in toluene. sPS gel, formed upon cooling of the emulsion to room temperature, locks the water droplets inside the gel. The gel is solvent exchanged in ethanol and then dried under supercritical condition of carbon dioxide to yield the aerogel foams. The aerogel foams show a significant fraction of macropores with a diameter of a few tens of micrometers, defined as macrovoids that originated from the emulsified water droplets. In conjunction, customary macropores of diameter 50-200 nm are derived from sPS gels. The macrovoids add additional openness to the aerogel structures. This paper evaluates the structural characteristics of the macrovoids, such as diameter distribution, macrovoid interconnect density, and skin layer density, in conjunction with the final aerogel foam properties.

A fused filament fabrication process is used for conversion of polyimide aerogels into bicontinuous gyroid structures to obtain enhanced elasticity. A sacrificial hollow mold in an inverse gyroid shape is first fabricated from high impact polystyrene and is filled with the polyimide sol. After the sol−gel transition, the mold is dissolved in a solvent to yield the gel structure of a gyroid shape. The gel is supercritically dried to recover a free-standing aerogel structure with ultrahigh porosity (98.9%) and low bulk density (0.0146 g/cm 3 ). The bicontinuous gyroid structure of the aerogel offers significant elasticity and high elongation at break compared to an otherwise brittle monolithic aerogel of polyimide. The ability to create intricate aerogel structures opens up a large potential in applications such as sensor housing and acoustic barriers of arbitrary shape and size and load-bearing porous thermally insulating structures that allow breathing.

This work focuses on the fabrication and organic dye adsorption capacity of pill-shaped polyimide aerogel microparticles of diameter approximately 200 μm and length approximately 1000 μm. First, spherical polyimide sol droplets suspended in silicone oil are generated in a microfluidic droplet generator. Second, these droplets are guided into a contraction flow channel to deform them into pill-shaped droplets. Third, the pill-shaped droplets undergo sol–gel transition in a heated section of the flow channel and turn into particles. Finally, the pill-shaped aerogel microparticles are recovered via solvent exchange and supercritical drying and characterized. The data indicate that the flow rates of polyimide sol and silicone oil strongly influence the length of the microparticles while surfactants, although not necessary to obtain pill-shaped particles, exert strong influence on the aspect ratio of the particles. The aspect ratio of particles and the internal morphology are discussed using hydrodynamic and interfacial forces. The ionic dye adsorption capacity of these pill-shaped particles is determined and compared with that of spherical gel microparticles.