The 3D Printing of Polyimide Aerogels

Polyimide aerogels are a nanoporous material made by extracting the liquid portion of a wet gel and replacing it with air, while still maintaining the porous solid internal architecture. This results in a material with extremely low densities, low thermal conductivities, high internal surfaces areas, and low dielectric constants. Currently, complex aerogel forms are not possible, since they require molds in order to be shaped. To overcome this challenge, we are attempting to use various additive manufacturing techniques to produce complex architectures. Three different approaches are being studied to accomplish this. One is to print using a viscous wet gel and have the structure solidify at the end of the print, another is to use a mixing tip, where a less viscous gel and the final chemical that causes gelation will be mixed as it is printing, allowing it to gel layer by layer, and the third is to use a UV curable aerogel with a UV light following behind to solidify the gel as it prints. To have these approaches be successful, the printing parameters and the aerogel's chemical formulation need to be optimized. The more harmonious these two factors can be made, the more defined and complex the resulting structure can be. Preliminary results show that 3D printing polyimide aerogels are a viable option, but further optimization must occur for reliable printing

Use of Nanofibers to Strengthen Hydrogels of Silica, Other Oxides, and Aerogels

Research has shown that including up to 5 percent w/w carbon nanofibers in a silica backbone of polymer crosslinked aerogels improves its strength, tripling compressive modulus and increasing tensile stress-at-break five-fold with no increase in density or decrease in porosity. In addition, the initial silica hydrogels, which are produced as a first step in manufacturing the aerogels, can be quite fragile and difficult to handle before cross-linking. The addition of the carbon nanofiber also improves the strength of the initial hydrogels before cross-linking, improving the manufacturing process. This can also be extended to other oxide aerogels, such as alumina or aluminosilicates, and other nanofiber types, such as silicon carbide

Di-Isocyanate Crosslinked Aerogels with 1, 6-Bis (Trimethoxysilyl) Hexane Incorporated in Silica Backbone

Silica aerogels are desirable materials for many applications that take advantage of their light weight and low thermal conductivity. Addition of a conformal polymer coating which bonds with the amine decorated surface of the silica network improves the strength of the aerogels by as much as 200 times. Even with vast improvement in strength they still tend to undergo brittle failure due to the rigid silica backbone. We hope to increase the flexibility and elastic recovery of the silica based aerogel by altering the silica back-bone by incorporation of more flexible hexane links. To this end, we investigated the use of 1,6-bis(trimethoxysilyl)hexane (BTMSH), a polysilsesquioxane precursor3, as an additional co-reactant to prepare silica gels which were subsequently cross-linked with di-isocyanate. Previously, this approach of adding flexibility by BTMSH incorporation was demonstrated with styrene cross-linked aerogels. In our study, we varied silane concentration, mol % of silicon from BTMSH and di-isocyanate concentration by weight percent to attempt to optimize both the flexibility and the strength of the aerogels

Carbon Nanofiber Incorporated Silica Based Aerogels with Di-Isocyanate Cross-Linking

Lightweight materials with excellent thermal insulating properties are highly sought after for a variety of aerospace and aeronautic applications. (1) Silica based aerogels with their high surface area and low relative densities are ideal for applications in extreme environments such as insulators for the Mars Rover battery. (2) However, the fragile nature of aerogel monoliths prevents their widespread use in more down to earth applications. We have shown that the fragile aerogel network can be cross-linked with a di-isocyanate via amine decorated surfaces to form a conformal coating. (3) This coating reinforces the neck regions between secondary silica particles and significantly strengthens the aerogels with only a small effect on density or porosity. Scheme 1 depicts the cross-linking reaction with the di-isocyanate and exhibits the stages that result in polymer cross-linked aerogel monoliths

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 150°) and a very low CAH near 1°

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\u27-dimethylbenzidine as the diamine shrink the least but have among the highest compressive modulus. Formulations made using 4,4\u27-oxydianiline or 2,2\u27dimethylbenzidine 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

Meta-Aerogels: Auxetic Shape-Memory Polyurethane Aerogels

Shape-memory poly(isocyanurate-urethane) (PIR-PUR) aerogels are low-density monolithic nanoporous solids that remember and return to their permanent shape through a heating actuation step. Herein, through structural design at the macro scale, the shape-memory response is augmented with an auxetic effect manifested by a negative Poisson\u27s ratio of approximately -0.8 at 15% compressive strain. Thus, auxetic shape-memory PIR-PUR monoliths experience volume contraction upon compression at a temperature above the glass transition temperature of the base polymer (Tg ≈ 30 °C), and they can be stowed indefinitely in that temporary shape by cooling below Tg. By heating back above Tg, the compressed/shrunk form expands back to their original shape/size. This technology is relevant to a broad range of industries spanning the commercial, aeronautical, and aerospace sectors. The materials are referred to as meta-aerogels, and their potential applications include minimally invasive medical devices, soft robotics, and situations where volume is at a premium, as for example for storage of deployable space structures and planetary habitats during transport to the point of service