Properties of gradient polyimide aerogels prepared through layer‐by‐layer assembly

Abstract

A series of density gradient polyimide (PI) aerogels were prepared using layer‐by‐layer assembly and radial freezing method. Briefly, a layer of poly(amic acid) (PAA) ammonium salt aqueous solution was first radially frozen. Then, the second layer of PAA ammonium salt (PAS) aqueous solution of different concentration was added on the top of the first PAS layer and radially frozen. A multilayer gradient PAS solid sample could therefore be fabricated by repeating this similar procedure. The density gradient PI aerogels were obtained after freezing drying and final thermal imidization treatment. Each layer of gradient PI aerogels had anisotropic pore structure, which consisted of tube‐like pores along the radial direction and toward the center axis of the cylindrical samples. The compressive strength of five‐layer gradient PI aerogel was higher than that of three‐layer gradient and single‐layer PI aerogels with the same density. The gradient PI aerogels exhibited anisotropic heat transfer behavior in the direction of density gradient, and heat transfer from the higher density side to the lower density side was faster.

Scale‐up fabrication and advanced properties of recycled polyethylene terephthalate aerogels from plastic waste

“…Second, toxic chemicals are often required in the gelation step which poses several hazards. Additionally, the use of carbon dioxide (CO 2 ), a common solvent used in solvent exchange, means that it is not an environmentally friendly production process 24–26 . Although aerogels dried through supercritical drying are often used as the benchmark, limitations in its fabrication makes it difficult and expensive to produce, especially on an industrial scale.…”

Ultrasound‐assisted freeze‐drying strategy to enhance nanoparticle dispersion in aerogels: A case study on polyimide/silica nanocomposite aerogel

“…However, disadvantages such as brittle and nano‐dust release limit the further application of nano‐silica aerogel 4 . Polyimide aerogel is a highly promising material for thermal protection due to its excellent mechanical properties, low thermal conductivity and processability 5,6 . To overcome the drawbacks of nano‐silica aerogels mentioned above, the preparation of nano‐silica/polyimide composite aerogels has been intensively investigated 5 .…”

“…4 Polyimide aerogel is a highly promising material for thermal protection due to its excellent mechanical properties, low thermal conductivity and processability. 5,6 To overcome the drawbacks of nano-silica aerogels mentioned above, the preparation of nano-silica/ polyimide composite aerogels has been intensively investigated. 5 The use of a PI sheet layer to support nano-silica particles has been found to effectively address the issue of fragility associated with nano-silica aerogels, while simultaneously maintaining their thermal insulation and flame retardant properties.…”