This study evaluates the potential of polymer aerogel monoliths in removing airborne nanoparticles. Macroporous monolithic aerogels of -form syndiotactic polystyrene (sPS) are synthesized for this purpose via thermoreversible gelation of a solution of sPS in a good solvent followed by supercritical drying. The air permeability and airborne nanoparticle removal efficiency are determined as function of the bulk density of aerogels. The data reveal a power-law dependence of particle removal efficiency and air permeability on bulk density. The data also reveal that efficiency greater than 99.95% can be achieved if the bulk density is kept at 0.042 g/cm 3 or higher. These materials show air permeability of the order of 10-10 m 2. The gradient density aerogels produced via sequential injections of sPS solutions show improvements in filtration efficiency attributed to additional skin layers. This idea is exploited in designing gradient density aerogel filters that offer higher efficiency and higher air permeability at a much lower bulk density than the single density monoliths of the same thickness.

This study evaluates the role of small mesopore volume fraction in aerogel monoliths in achieving high efficiency airborne nanoparticle filtration. The diffusional flow in mesopores (diameter 2-50 nm) is more effective in nanoparticle capture than the viscous flow regime encountered in macropores (diameter > 50 nm). Substantial macropore fraction, however, is essential for achieving high air permeability. This idea is tested by evaluating the filtration efficiency of 25-150 nm diameter airborne sodium chloride nanoparticles and the permeability of air through hybrid aerogel monoliths with about 2-4 % by volume mesopore content. The hybrid aerogels are prepared by growing discontinuous silica aerogel particles in the macropores of-form syndiotactic polystyrene (sPS). The results show that air permeability is not much affected by the silica content although the particle capture efficiency is increased to greater than 99.95 % due to the contribution of diffusional deposition aided by the mesopores. The aerogels develop cracks and reduce filtration efficiency at high silica content due to brittle nature of silica particle networks.

The role of electrostatic force on separation of airborne nanoparticles is evaluated in this work by considering a hybrid monolithic aerogel of syndiotactic polystyrene (sPS) and polyvinylidene fluoride (PVDF). The sPS part accounts for open pore structures in the monolith, while the PVDF chains contribute spontaneous polarity for particle capture by the electrostatic force. The hybrid aerogels are fabricated by thermoreversible gelation of sPS from a solution with PVDF in tetrahydrofuran followed by supercritical drying of the gel. sPS is present as the δ-form clathrate crystalline phase and PVDF as α- and γ-form crystalline phases in the hybrid. The presence of PVDF induces significant static charges on the surfaces of hybrid aerogels. The filtration efficiency is determined by passing airborne NaCl nanoparticles with diameter in the range 25-150 nm through the filter media. The experimental data reveal that air permeability of the hybrid system (∼10 m) is close to that of sPS monoliths. The hybrid materials show filtration efficiency ≥99.999% in comparison to 98.889% observed for a sPS monolith with the same solid content.

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