Quantitative Evaluation of the Hierarchical Porosity in Polyimide Aerogels and Corresponding Solvated Gels.

Aerogels are promising materials for many aerospace applications, including high-performance antennae and flexible insulation, because of their inherent low density and high surface areas. Polymer aerogels, especially polyimide aerogels, provide excellent mechanical properties beyond traditional silica aerogels while maintaining the required thermal stability. Polyimide aerogel surface area, porosity, and pore volume are important properties; however, these measurements are traditionally conducted on the aerogel after removal of the solvent. Because of this, the impact of synthetic control and solvent presence on the nanoscale to mesoscale structure of polyimide aerogels in functional applications is unclear. In this report, we use small-angle neutron scattering to determine the dry and solvated skeletal strut size and composition of polyimide aerogels to deduce the impact of solvation on the structure of complex aerogel struts. Our results show that the aerogel contains a hierarchical assembly of pores, with pores present both within and between the supporting struts. This translates to a material with solvent in the larger pores, as well as absorbed in the supporting polyimide skeleton. The amount of solvent uptake in the struts varies with the solvent and polyimide properties. The insight from these results provides pathways to determine the correlations between aerogel nano- and mesoscale structural characteristics, fabrication processes, and their performance in functional applications such as polymeric battery separators. These results also broaden the characterization tools of polymeric aerogels that differentiate between dry and solvated nano- and mesoscale structures that exist in common operating conditions.

Flexible Polyimide Aerogels with Dodecane Links in the Backbone Structure.

Polyimide aerogels using 1,12-dodecyldiamine (DADD), 3,3'-dimethylbenzidine (DMBZ), and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and cross linked using 1,3,5-triaminophenoxybenzene (TAB) were synthesized. Substitution of the aromatic diamine, DMBZ, with varying amounts of the aliphatic diamine, DADD, increases the flexibility in the backbone structure of the prepared aerogel. These aerogels are also lightweight, low density, have a low dielectric constant, and high modulus. Their overall properties (density, shrinkage, porosity, dielectric constant, water uptake, and modulus) and potential use as a conformal substrate for lightweight, high-performance antennas are discussed.

Toward Improved Optical Transparency of Polyimide Aerogels.

Highly translucent polyimide aerogels were prepared by combining equimolar amounts of pyromellitic dianhydride, 4,4'-hexafluoroisopropylidene di(phthalic anhydride) (6FDA), and 2,2'-dimethylbenzidine and cross-linking with 1,3,5-benzenetricarbonyl trichloride. A multivariable statistical design of experiments was used to perform a comparison study between three variables used to fabricate the aerogels: formulated repeat unit (n) of polyimide oligomers, 6FDA fraction of total dianhydride (0-50 mol %), and total polymer concentration in solution (7-10 wt %). Polymers with 25 mol % 6FDA in the backbone structure were found to produce polyimide aerogels with high optical transmission and low haze. These aerogels also possessed higher surface areas and very narrow nanoscale pore size distribution. Because of the decreased thermal conductivity with increasing amount of 6FDA in the backbone, these aerogels may find use where the combination of high optical transparency and thermal impedance is desired, such as insulated window panes. To this end, future efforts will focus on reducing the yellow color of the polyimide aerogels.

Hierarchical Morphology of Poly(ether ether ketone) Aerogels.

The phase diagram for the thermoreversible gelation of poly(ether ether ketone) (PEEK) in 4-chlorophenol (4CP) was constructed over broad temperature and concentration ranges, revealing that PEEK is capable of dissolving and forming gels in both 4CP and dichloroacetic acid (DCA) up to a concentration of 25 wt %. Highly porous aerogels of PEEK were prepared through simple solvent exchange followed by one of two drying methods of solvent removal from the wet gel: freeze-drying or supercritical CO2 fluid extraction (SC-drying). The field-emission scanning electron microscopy analysis showed that gelation of PEEK in 4CP, followed by SC-drying, produced aerogels with well-defined lamellar aggregates as compared to less ordered aggregates formed from DCA. Mechanical properties (in compression) were shown to improve with increasing density, resulting in equivalent compressive moduli at comparable density, regardless of the preparation method (gelation solvent selection, concentration variation, or drying method). Nitrogen adsorption-desorption isotherms indicate that PEEK aerogels are comprised of mesopores (2-50 nm diameter pores) formed from stacked crystalline lamella. PEEK aerogels prepared using SC-drying exhibit higher Brunauer-Emmett-Teller surface areas than freeze-dried aerogels of comparable density. The ultra-small-angle X-ray scattering/small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering analysis revealed a hierarchical morphology of the PEEK aerogels with structural features from PEEK crystallites to agglomerates of stacked lamella that spanned a wide range of length scales. SANS contrast-matching confirmed that the morphological origin of the principle scattering feature in PEEK aerogels is stacked crystalline lamella. Nitrogen sorption measurements of porosity and the specific surface area of the PEEK aerogels were correlated with the SAXS analysis to reveal a remarkably high surface area attributed to the platelet-like, lamellar morphology. Contact angle and contact angle hysteresis (CAH) revealed that low-density PEEK aerogels (ρ < 0.15 g/cm3) have water contact angles above the superhydrophobicity cutoff angle (>150°) and a very low CAH near 1°.

Increased Flexibility in Polyimide Aerogels Using Aliphatic Spacers in the Polymer Backbone.

Polyimide aerogels are mechanically strong porous solids with high surface area, low density, and dielectric constants close to 1, making them ideal materials for use as substrates for lightweight antennas. Increasing the flexibility of the polyimide aerogels extends the usefulness for conformal antennas for use on small aircraft such as unmanned air vehicles or personal air mobility vehicles. To this end, polyimide aerogels made with aromatic amines with 4-10 methylene units as flexible spacers between aromatic rings in the backbone have been fabricated. Substituting 25-75 mol % of fully aromatic 2,2'-dimethylbenzidine with these flexible diamines increases the flexibility of polyimide aerogels, making them bendable at thicknesses up to 2-3 mm. The density, dielectric constants, thermal and moisture stability, and mechanical properties of these aerogels were assessed to understand the effect of the amount and length of the methylene spacers on these properties.

Triboelectric Nanogenerators Made of Porous Polyamide Nanofiber Mats and Polyimide Aerogel Film: Output Optimization and Performance in Circuits.

Triboelectric nanogenerators (TENGs) have been attracting a tremendous amount of attention since their discovery in 2012. Finding new means to enhance energy output is an ongoing pursuit. Herein, we introduce a new type of high-performance TENG composed of highly porous polyamide (PA) nanofiber mats and polyimide aerogel films. We have demonstrated that the thickness of the porous triboelectric materials, which is attained by stacking multiple layers of triboelectric materials, has a profound effect on the triboelectric output performance of TENGs. The triboelectric output increased when PA increased from one layer to six layers. However, it decreased when PA was further increased to 12 layers. With an optimum material thickness, a TENG with only a 2 cm2 effective device size achieved a high output voltage of 115 V and a current of 9.5 µA under a small compressive pressure (30 kPa). A peak power density of 1.84 W/m2 was achieved on a 4.7 MΩ external load. The TENG was able to light 60 light-emitting diodes easily and quickly charge capacitors with different capacitance to 6 V, indicating an outstanding energy harvesting ability. In addition, the performance of multiple TENGs connected in different ways, as well as the performance of TENGs in resistive/inductive/capacitive circuits, were investigated. These findings provide new insight into the working principles of TENGs in complex circuits.

Nanostructure-Dependent Marcus-Type Correlation of the Shape Recovery Rate and the Young's Modulus in Shape Memory Polymer Aerogels.

Thermodynamic-kinetic relationships are not uncommon, but rigorous correlations are rare. On the basis of the parabolic free-energy profiles of elastic deformation, a generalized Marcus-type thermodynamic-kinetic relationship was identified between the shape recovery rate, Rt( N), and the elastic modulus, E, in poly(isocyanurate-urethane) shape memory aerogels. The latter were prepared with mixtures of diethylene, triethylene, and tetraethylene glycol and an aliphatic triisocyanate. Synthetic conditions were selected using a statistical design of experiments method. Microstructures obtained in each formulation could be put into two groups, one consisting of micron-size particles connected with large necks and a second one classified as bicontinuous. The two types of microstructures could be explained consistently by spinodal decomposition involving early versus late phase separation relative to the gel point. Irrespective of microstructure, all samples showed a shape memory effect with shape fixity and shape recovery ratios close to 100%. Larger variations (0.35-0.71) in the overall figure of merit, the fill factor, were traced to a variability in the shape recovery rates, Rt( N), which in turn were related to the microstructure. Materials with bicontinuous microstructures were stiffer and showed slower recovery rates. Thereby, using the elastic modulus, E, as a proxy for microstructure, the correlation of Rt( N) with E was traced to a relationship between the activation barrier for shape recovery, Δ A#, and the specific energy of deformation, (reorganization energy, λ), which in turn is proportional to the elastic modulus. Data were fitted well ( R2 = 0.92) by the derived equations. The inverse correlation between Rt( N) and the elastic modulus, E, provides a means for qualitative predictability of the shape recovery rates, the fill factors, and the overall quality of the shape memory effect.

Polyimide Aerogels Using Triisocyanate as Cross-linker.

A family of polyimide (PI)-based aerogels is produced using Desmodur N3300A, an inexpensive triisocyanate, as the cross-linker. The aerogels are prepared by cross-linking amine end-capped polyimide oligomers with the triisocyanate. The polyimide oligomers are formulated using 2,2'-dimethylbenzidine, 4,4'-oxydianiline, or mixtures of both diamines, combined with 3,3',4,4'-biphenyltetracarboxylic dianhydride, and are chemically imidized at room temperature. Depending on the backbone chemistry, chain length, and polymer concentration, density of the aerogels ranged from 0.06 to 0.14 g/cm3 and Brunauer-Emmett-Teller surface areas ranged from 350 to 600 m2/g. Compressive moduli of these aerogels were as high as 225 MPa, which are comparable to, or higher than, those previously reported prepared with similar backbone structures but with other cross-linkers. Because of their lower cost and commercial availability as cross-linker, the aerogels may have further potential as insulation for building and construction, clothing, sporting goods, and automotive applications, although lower-temperature stability may limit their use in some aerospace applications.

Effect of Bulky Substituents in the Polymer Backbone on the Properties of Polyimide Aerogels.

With unique advantages over inorganic aerogels including higher strengths and compressive moduli, greater toughness, and the ability to be fabricated as a flexible thin film, polymer aerogels have the potential to supplant inorganic aerogels in numerous applications. Among polymer aerogels, polyimide aerogels possess a high degree of high thermal stability as well as outstanding mechanical properties. However, while the onset of thermal decomposition for these materials is typically very high (greater than 500 °C), the polyimide aerogels undergo dramatic thermally induced shrinkage at temperatures well below their glass transition (Tg) or decomposition temperature, which limits their use. In this study, we show that shrinkage is reduced when a bulky moiety is incorporated in the polymer backbone. Twenty different formulations of polyimide aerogels were synthesized from 3,3,'4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 4,4'-oxidianiline (ODA) or a combination of ODA and 9,9'-bis(4-aminophenyl)fluorene (BAPF) and cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC) in a statistically designed study. The polymer concentration, n-value, and molar concentration of ODA and BAPF were varied to demonstrate the effect of these variables on certain properties. Samples containing BAPF showed a reduction in shrinkage by as much as 50% after aging at elevated temperatures for 500 h compared to those made with ODA alone.

Highly Porous, Rigid-Rod Polyamide Aerogels with Superior Mechanical Properties and Unusually High Thermal Conductivity.

We report here the fabrication of polyamide aerogels composed of poly-p-phenylene-terephthalamide, the same backbone chemistry as DuPont's Kevlar. The all-para-substituted polymers gel without the use of cross-linker and maintain their shape during processing-an improvement over the meta-substituted cross-linked polyamide aerogels reported previously. Solutions containing calcium chloride (CaCl2) and para-phenylenediamine (pPDA) in N-methylpyrrolidinone (NMP) at low temperature are reacted with terephthaloyl chloride (TPC). Polymerization proceeds over the course of 5 min resulting in gelation. Removal of the reaction solvent via solvent exchange followed by extraction with supercritical carbon dioxide provides aerogels with densities ranging from 0.1 to 0.3 g/cm3, depending on the concentration of calcium chloride, the formulated number of repeat units, n, and the concentration of polymer in the reaction mixture. These variables were assessed in a statistical experimental study to understand their effects on the properties of the aerogels. Aerogels made using at least 30 wt % CaCl2 had the best strength when compared to aerogels of similar density. Furthermore, aerogels made using 30 wt % CaCl2 exhibited the lowest shrinkage when aged at elevated temperatures. Notably, whereas most aerogel materials are highly insulating (thermal conductivities of 10-30 mW/m K), the polyamide aerogels produced here exhibit remarkably high thermal conductivities (50-80 mW/(m K)) at the same densities as other inorganic and polymer aerogels. These high thermal conductivities are attributed to efficient phonon transport by the rigid-rod polymer backbone. In conjunction with their low cost, ease of fabrication with respect to other polymer aerogels, low densities, and high mass-normalized strength and stiffness properties, these aerogels are uniquely valuable for applications such as lightweighting in consumer electronics, automobiles, and aerospace where weight reduction is desirable but trapping of heat may be undesirable-applications where other polymer aerogels have to date otherwise been unsuitable-creating new opportunities for commercialization of aerogels.

Moisture-Resistant Polyimide Aerogels Containing Propylene Oxide Links in the Backbone.

Polyimide aerogels made using anhydride-capped oligomers from 4,4'-oxydianiline (ODA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) cross-linked with 1,3,5-tri(aminophenoxy)benzene (TAB) have been reported with very good mechanical properties but poor resistance to moisture. Replacing 50 mol % of the ODA with poly(propylene glycol)bis(2-aminopropyl ether) (PPG) with an average molecular weight of 230 g/mol in the oligomer backbone gives aerogels with water contact angles of 80°. The aerogels also absorb very little moisture on soaking in water. The aerogels also shrink less with increasing PPG concentration and therefore have significantly lower density and higher porosity than those made without PPG. Mechanical properties of the aerogels increased with increasing density, regardless of the polymer backbone. Brunauer-Emmett-Teller (BET) surface area of the aerogels studied ranged from 300 to 400 m2/g, depending mainly on PPG concentration. The high moisture resistance makes them promising materials for substrates for lightweight antennas as well as insulation for a variety of applications.

Polyimide aerogels with amide cross-links: a low cost alternative for mechanically strong polymer aerogels.

Polyimide aerogels combine high porosity, low thermal conductivity, flexibility, and low density with excellent mechanical properties. However, previously used cross-linkers, such as 1,3,5-triaminophenoxybenzene (TAB), 2,4,6-tris(4-aminophenyl)pyridine (TAPP), or octa(aminophenoxy)silsesquioxane (OAPS), either are not commercially available or are prohibitively expensive. Finding more cost efficient cross-linkers that are commercially available to synthesize these aerogels is crucial for making large scale manufacturing attractive. Herein, we describe an approach to making polyimide aerogels starting with amine capped oligomers that are cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC). BTC is a lower cost, commercially available alternative to TAB, TAPP, or OAPS. Aerogels made in this way have the same or higher modulus and higher surface area compared to those previously reported with either TAB or OAPS cross-links at the same density. While the cross-link structure is an amide, the thermal stability is not compromised most likely because the cross-link is only a small part of the composition of the aerogel. Onset of decomposition depends primarily on the backbone chemistry with 4,4'-oxidianiline (ODA) being more thermally stable than 2,2'-dimethylbenzidine (DMBZ), similar to those previously reported with other cross-links.

Dielectric and other properties of polyimide aerogels containing fluorinated blocks.

The dielectric and other properties of a series of low-density polyimide block copolymer aerogels have been characterized. Two different anhydride-capped polyimide oligomers were synthesized: one from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 4,4'-oxidianiline (ODA) and the other from biphenyl-3,3',4,4'-tetracarboxylic dianhydride and ODA. The oligomers were combined with 1,3,5-triaminophenoxybenzene to form a block copolymer networked structure that gelled in under 1 h. The polyimide gels were supercritically dried to give aerogels with relative dielectric constants as low as 1.08. Increasing the amount of 6FDA blocks by up to 50% of the total dianhydride decreased the density of the aerogels, presumably by increasing the free volume and also by decreasing the amount of shrinkage seen upon processing, resulting in a concomitant decrease in the dielectric properties. In this study, we have also altered the density independent of fluorine substitution by changing the polymer concentration in the gelation reactions and showed that the change in dielectric due to density is the same with and without fluorine substitution. The aerogels with the lowest dielectric properties and lowest densities still had compressive moduli of 4-8 MPa (40 times higher than silica aerogels at the same density), making them suitable as low dielectric substrates for lightweight antennas for aeronautic and space applications.

Low dielectric polyimide aerogels as substrates for lightweight patch antennas.

The dielectric properties and loss tangents of low-density polyimide aerogels have been characterized at various frequencies. Relative dielectric constants as low as 1.16 were measured for polyimide aerogels made from 2,2'-dimethylbenzidine (DMBZ) and biphenyl 3,3',4,4'-tetracarbozylic dianhydride (BPDA) cross-linked with 1,3,5-triaminophenoxybenzene (TAB). This formulation was used as the substrate to fabricate and test prototype microstrip patch antennas and benchmark against state of practice commercial antenna substrates. The polyimide aerogel antennas exhibited broader bandwidth, higher gain, and lower mass than the antennas made using commercial substrates. These are very encouraging results, which support the potential advantages of the polyimide aerogel-based antennas for aerospace applications.

Tailoring properties of cross-linked polyimide aerogels for better moisture resistance, flexibility, and strength.

Combinations of rigid and flexible aromatic diamines were used to tailor the properties of octa(aminophenyl)-silsesquioxane (OAPS) cross-linked polyimide aerogels. 2,2'-Dimethylbenzidine (DMBZ) or p-phenylenediamine (PPDA) was used in combination with the more-flexible diamine, 4,4'-oxydianiline (ODA). The amount of rigid diamine was varied from 0% to 100% of the total diamines in the backbone. The resulting aerogels vary in density, shrinkage, porosity, surface area, mechanical and thermal properties (depending on the type of diamine and the proportions of rigid diamine to flexible diamine used). Replacing ODA with PPDA increases shrinkage that occurs during gelation and processing, while increasing the DMBZ fraction decreases shrinkage. Replacing ODA with 50 mol% of DMBZ maintains the flexibility of thin films, while the moisture resistance of the aerogels is greatly improved.

Mechanically strong, flexible polyimide aerogels cross-linked with aromatic triamine.

Polyimide gels are produced by cross-linking anhydride capped polyamic acid oligomers with aromatic triamine in solution and chemically imidizing. The gels are then supercritically dried to form nanoporous polyimide aerogels with densities as low as 0.14 g/cm(3) and surface areas as high as 512 m(2)/g. To understand the effect of the polyimide backbone on properties, aerogels from several combinations of diamine and dianhydride, and formulated oligomer chain length are examined. Formulations made from 2,2'-dimethylbenzidine as the diamine shrink the least but have among the highest compressive modulus. Formulations made using 4,4'-oxydianiline or 2,2'dimethylbenzidine can be fabricated into continuous thin films using a roll to roll casting process. The films are flexible enough to be rolled or folded back on themselves and recover completely without cracking or flaking, and have tensile strengths of 4-9 MPa. Finally, the highest onset of decomposition (above 600 °C) of the polyimide aerogels was obtained using p-phenylene diamine as the backbone diamine with either dianhydride studied. All of the aerogels are suitable candidates for high-temperature insulation with glass transition temperatures ranging from 270-340 °C and onsets of decomposition from 460-610 °C.

Tailoring mechanical properties of aerogels for aerospace applications.

Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.

Polyimide aerogels cross-linked through amine functionalized polyoligomeric silsesquioxane.

We report the first synthesis of polyimide aerogels cross-linked through a polyhedral oligomeric silsesquioxane, octa(aminophenyl)silsesquioxane (OAPS). Gels formed from polyamic acid solutions of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), bisaniline-p-xylidene (BAX) and OAPS were chemically imidized and dried using supercritical CO(2) extraction to give aerogels having density around 0.1 g/cm(3). The aerogels are greater than 90 % porous, have high surface areas (230 to 280 m(2)/g) and low thermal conductivity (14 mW/m-K at room temperature). Notably, the polyimide aerogels cross-linked with OAPS have higher modulus than polymer reinforced silica aerogels of similar density and can be fabricated as both monoliths and thin films. Thin films of the aerogel are flexible and foldable making them an ideal insulation for space suits, and inflatable structures for habitats or decelerators for planetary re-entry, as well as more down to earth applications.