Polymer aerogels for efficient removal of airborne nanoparticles

High Modulus, Strut-like poly(ether ether ketone) Aerogels Produced from a Benign Solvent

Poly(ether ether ketone) (PEEK) was found to form gels in the benign solvent 1,3-diphenylacetone (DPA). Gelation of PEEK in DPA was found to form an interconnected, strut-like morphology composed of polymer axialites. To our knowledge, this is the first report of a strut-like morphology for PEEK aerogels. PEEK/DPA gels were prepared by first dissolving PEEK in DPA at 320 °C. Upon cooling to 50 °C, PEEK crystallizes and forms a gel in DPA. The PEEK/DPA phase diagram indicated that phase separation occurs by solid-liquid phase separation, implying that DPA is a good solvent for PEEK. The Flory-Huggins interaction parameter, calculated as χ12 = 0.093 for the PEEK/DPA system, confirmed that DPA is a good solvent for PEEK. PEEK aerogels were prepared by solvent exchanging DPA to water then freeze-drying. PEEK aerogels were found to have densities between 0.09 and 0.25 g/cm3, porosities between 80 and 93%, and surface areas between 200 and 225 m2/g, depending on the initial gel concentration. Using nitrogen adsorption analyses, PEEK aerogels were found to be mesoporous adsorbents, with mesopore sizes of about 8 nm, which formed between stacks of platelike crystalline lamellae. Scanning electron microscopy and X-ray scattering were utilized to elucidate the hierarchical structure of the PEEK aerogels. Morphological analysis found that the PEEK/DPA gels were composed of a highly nucleated network of PEEK axialites (i.e., aggregates of stacked crystalline lamellae). The highly connected axialite network imparted robust mechanical properties on PEEK aerogels, which were found to densify less upon freeze-drying than globular PEEK aerogel counterparts gelled from dichloroacetic acid (DCA) or 4-chlorphenol (4CP). PEEK aerogels formed from DPA were also found to have a modulus-density scaling that was far more efficient in supporting loads than the poorly connected aerogels formed from PEEK/DCA or PEEK/4CP solutions. The strut-like morphology in these new PEEK aerogels also significantly improved the modulus to a degree that is comparable to high-performance crosslinked aerogels based on polyimide and polyurea of comparable densities.

Effects of NO2 aging on bismuth nanoparticles and bismuth-loaded silica xerogels for iodine capture

The capture of long-lived radioactive iodine (129I) from oxidizing off-gasses produced from reprocessing used nuclear fuel is paramount to human health and environmental safety. Bismuth has been investigated as a viable iodine getter but the phase stability of bismuth-based sorbents in an oxidizing environment have not yet been researched. In the current work, bismuth nanoparticle-based sorbents, as free particles (Bi-NPs) and embedded within silica xerogel monoliths made with a porogen (TEO-5), were exposed to I2(g) before and after aging in 1 v/v% NO2 at 150 °C. For unaged sorbents, BiI3 was the dominant phase after iodine capture with 8-30 mass% BiOI present due to native Bi2O3 on the surface of the unaged nanoparticles. After 3 h of aging, 82 mass% of the Bi-NPs was converted to Bi2O3 with only a small amount of iodine captured as BiOI (18 mass%). After aging TEO-5 for 3 h, iodine was captured as both BiI3 (26 %) and BiOI (74 %) and no Bi2O3 was detected.". Additionally, bismuth lining the micrometer-scale pores in the TEO-5 led to enhanced iodine capture. In a subsequent exposure of the sorbents to NO2 (secondary aging), all BiI3 converted to BiOI. Thus, direct capture of iodine as BiOI is desired (over BiI3) to minimize loss of iodine after capture.

Meso- and Macroporous Polymer Gels for Efficient Adsorption of Block Copolymer Surfactants

An understanding of surfactant adsorption at solid-liquid interfaces is important for solving many technological problems. This work evaluates surfactant adsorption abilities of high surface area (200-600 m²/g), high porosity (>90%), hierarchically structured open pore polymer gels. Specifically, the interactions of a nonionic block copolymer surfactant, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with three polymer gels, namely, syndiotactic polystyrene (sPS), polyimide (PI), and polyurea (PUA) offering different surface energy values, are evaluated at surfactant concentrations below and well above the critical micelle concentration (CMC). Two distinct surfactant adsorption behaviors are identified from the surface tension and nuclear magnetic resonance data. At concentrations below CMC, the surfactant molecules adsorb as a monolayer on polymer strands, inferred from the Langmuir-type adsorption isotherm, with the adsorbed amount increasing with the specific surface area of the polymer gel. The study reports for the first time that the gels show a strong surfactant adsorption above CMC, with the effective surfactant concentration in the gel reaching several folds of the CMC values. The effective surfactant concentration in the gel is analyzed using surfactant micelle size, polymer surface energy, and pore size of the gel. The findings of this study may have strong implications in liquid-liquid separation problems and in the removal of small dye molecules, heavy metal ions, and living organisms from aqueous streams.

Surfactant-free oil-in-oil emulsion-templating of polyimide aerogel foams

A surfactant-free oil-in-oil emulsion-templating method is presented for fabrication of monolithic polyimide aerogel foams using monomer systems that produce fast sol–gel transition. An aerogel foam is a high porosity (∼90%) material with coexisting meso- and macropores inherent to aerogels with externally introduced micrometer size open cells (macrovoids) that are reminiscent of foams. The macrovoids are introduced in polyimide sol using surfactant-free emulsion-templating of droplets of an immiscible liquid that are stabilized against coalescence by fast sol–gel transition. Three immiscible liquids – cyclohexane, n-heptane, and silicone oil – are considered in this work for surfactant-free emulsion-templating. The aerogel foam monoliths, recovered by supercritical drying, exhibit smaller size macrovoids when n-heptane and cyclohexane are used as emulsion-templating liquid, while the overall porosity and the bulk density show weak dependence on the emulsion-templating liquid.

Zirconia aerogels for thermal management: Review of synthesis, processing, and properties information architecture

Zirconia aerogels are porous nanomaterials with high specific surface areas and low thermal conductivities that are suitable for a wide range of functions. The applications of zirconia aerogels include numerous uses in thermal management systems that are specifically beneficial in aeronautics and aerospace systems. This review seeks to detail the synthesis, processing, and characterization of these unique materials. However, the many distinctive synthesis pathways and processing conditions of zirconia aerogels can make the optimization of these materials difficult, potentially inhibiting further development. Independent variables in the synthesis process alone include zirconium precursor, rare earth stabilizer, solvent system, gelation agent, and surfactant templating agent. If only two distinct options were available for each synthetic variable, there would be up to 32 different synthetic pathways; if there were three options for each variable, 243 different synthetic pathways would be possible. Apart from the gel synthesis, processing conditions, including drying method, drying temperature, drying solvent, and sintering temperature, as well as various techniques used to characterize aerogels, need to be considered. To mitigate the sheer volume of synthetic parameters, this review uses an architected information structure to contemplate approximately 600 aerogel materials, along with the synthesis and processing conditions that make each material unique. By utilizing this information structure, containing over 10,000 relationships amongst 3,800 nodes, the connection between specific properties of zirconia aerogels and the pathways used to produce them can be more easily visualized, leading to a more effective understanding of the many variables that are used in the synthesis and processing of these materials. This review seeks to utilize data science in a way that can elucidate structure-property relationships in colloidal chemistry, providing a more efficient way to evaluate the synthesis and processing of materials with high experimental dimensionality.

Investigation of Microstructures and Air Permeability of Aerogel-Coated Textile Fabric Materials

This study focuses on fabrication of aerogel-coated macroporous polyester fabrics for the purposes of filtration of nanometric airborne particles and potential application in facemasks. Syndiotactic polystyrene (sPS) and polyimide (PI) gels that provide respectively majority macropores (diameter > 50 nm) and mesopores (diameter 2 to 50 nm) are coated onto woven polyester fabrics via a dip coating process. The resultant materials are supercritically dried to obtain aerogelcoated fabrics. The results show that sPS is more suitable for the dip coating process. However, evaporation of the solvent during handling of gel-coated fabrics leads to closure of the surface pores that are later recovered via solvent annealing. The resultant aerogel-coated fabrics offer high air permeability (∼10–10 m2) and high filtration efficiency (> 99.95%) of airborne sodium chloride test particles of size 25 to 150 nm.

Nanoporous Crystalline Composite Aerogels with Reduced Graphene Oxide

High-porosity monolithic composite aerogels of syndiotactic polystyrene (sPS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) containing reduced graphene oxide (r-GO) were prepared and characterized. The composite aerogels obtained by supercritical carbon dioxide (scCO₂) extraction of sPS/r-GO and PPO/r-GO gels were characterized by a fibrillar morphology, which ensured good handling properties. The polymer nanoporous crystalline phases obtained within the aerogels led to high surface areas with values up to 440 m² g⁻¹. The role of r-GO in aerogels was studied in terms of catalytic activity by exploring the oxidation capacity of composite PPO and sPS aerogels toward benzyl alcohol in diluted aqueous solutions. The results showed that, unlike sPS/r-GO aerogels, PPO/r-GO aerogels were capable of absorbing benzyl alcohol from the diluted solutions, and that oxidation of c.a. 50% of the sorbed benzyl alcohol molecules into benzoic acid occurred.

Polyimide aerogels with novel bimodal micro and nano porous structure assembly for airborne nano filtering applications

Aerogels have presented a very high potential to be utilized as airborne nanoparticles' filtration media due to their nanoscale pore size and extremely high porosity. The filtering performance of aerogels, such as air permeability and filtration efficiency, is highly related to the configuration of aerogels' nanostructure assembly. However, as aerogel morphology is formed with respect to the intermolecular forces during the gelation stage, tailoring the aerogel nanostructure assembly is still a challenge. In this work, a novel strategy for tailoring polyimide aerogel nanostructure assembly is proposed by controlled disturbing of the intermolecular forces. From the results, the nanostructure assembly of the 4,4′-oxydianiline (ODA)–biphenyl-tetracarboxylic acid dianhydride (BPDA) polyimide aerogel is tailored to a uniform bimodal micro and nano porous structure. This was achieved by introducing the proper fraction of thermoplastic polyurethane (TPU) chains to the polyimide chains in the solution state and through a controlled process. The fabricated polyimide/TPU aerogels with bimodal morphology presented enhanced filtration performance, with 30% improved air permeability and reduced cell size of 3.51 nm over the conventional ODA–BPDA polyimide aerogels. Moreover, the fabricated bimodal aerogels present the reduced shrinkage, density, and effective thermal conductivity of 6.3% and 0.063 g cm⁻³, 28.7 mW m⁻¹ K⁻¹, respectively. Furthermore, the bimodal polyimide/TPU aerogels show the higher porosity of 96.5 vol% along with increased mechanical flexibility over the conventional polyimide aerogel with comparable backbone chemistry.

Aerogels have presented a very high potential to be utilized as airborne nanoparticles' filtration media due to their nanoscale pore size and extremely high porosity.

Surfactant-Free Process for the Fabrication of Polyimide Aerogel Microparticles

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.

Solvent Effects on Tuning Pore Structures in Polyimide Aerogels

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.

Open Cell Aerogel Foams via Emulsion Templating

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.

Tuning Porous Networks in Polyimide Aerogels for Airborne Nanoparticle Filtration

The suitability of monolithic polyimide aerogels as filter media for removal of airborne nanoparticles was investigated in this work by considering two solvents, N-methylpyrrolidone (NMP) and dimethylformamide (DMF) for tuning of meso- and macropore content. Polyimide gels were synthesized from the chemical reactions between solutions of pyromellitic dianhydride, 2,2'-dimethylbenzidine, and 1, 3, 5-triaminophenoxylbenzene. The gels were dried via supercritical drying in CO₂ to obtain the aerogels. The porosity of polyimide aerogels was varied by changing the initial concentration of the solids in the solutions in the range of 2.5-10 wt %. The resulting aerogels show high porosity (91-98%), high specific surface area (473-953 m²/g), low bulk density (0.025-0.12 g/cm³), and solvent dependent macro- and mesopore content. The monoliths with bulk density of >0.05 g/cm³ produced high values of nanoparticle filtration efficiency (>99.95%) with air permeability of the order of 10^-10 m². A strong proportional relationship was observed between the macropore content and air permeability and between the mesopore content and high filtration efficiency. Specimens prepared in DMF and NMP offered the same level of filtration efficiency, but the former provided a factor of 2 higher air permeability due to much greater proportion of macropores.

Thermoreversible Gelation of Poly(ether ether ketone)

Solutions of poly(ether ether ketone) in dichloroacetic acid have been shown to form monolithic, thermoreversible gels at temperatures ranging from 10 to 140 °C. A phase diagram was constructed over broad concentration and temperature ranges, and the phase boundary suggests an upper critical solution temperature (UCST) behavior. Furthermore, poly(ether ether ketone) (PEEK) gels were solvent-exchanged with water to form hydrogels and subsequently lyophilized to form PEEK aerogels. The PEEK aerogels of density 0.2 g/mL were found to be highly porous and composed of uniform 200 nm morphological features. The crystal structure of the PEEK hydrogels and aerogels was found to be identical to that of melt-crystallized PEEK. The mechanical properties of the PEEK aerogels (in compression) were found to be superior to conventional silicate aerogels of comparable density. This report is the first example of a monolithic, thermoreversible gel of PEEK and the first demonstration of PEEK hydrogels and aerogels.