Anisotropic polyimide aerogels fabricated by directional freezing

High ion barrier hydrogel with excellent toughness achieved by directional structures

Owing to their nontoxicity, environmental friendliness, and high biocompatibility, physically cross-linked hydrogels have become popular research materials; however, their high water content and high free volume, along with the weak bonding interactions inherent to ordinary physically cross-linked hydrogels, limit their application in fields such as flexible devices, packaging materials, and substance transport regulation. Here, a structural barrier approach based on directional freezing-assisted salting out was proposed, and the directional structure significantly enhanced the barrier performance of the hydrogel. When the direction of substance diffusion was perpendicular to the pore channel structure of the directional freezing-PVA hydrogel (DFPVA), the Cl- transmission rate was 57.2% for the uniform freezing-PVA hydrogel (UFPVA). By adjusting the concentration of the salting-out solution and the salting-out time, the crystallinity and crystal domain size of the hydrogel could be further changed, optimizing and regulating the barrier performance of the hydrogel, with the best Cl- unit permeability being 36.02 mg mm per cm2 per day. Additionally, DFPVA had excellent mechanical properties (stress of 6.47 ± 1.04 MPa, strain of 625.85 ± 61.58%, toughness of 25.77 ± 3.72 MPa). Due to the barrier and mechanical properties of the direct structure, DFPVA is suitable as a drug carrier for slow drug release in vitro.

Nanocellulose-based aerogels for water purification: A review

Water purification using thin membranes at high pressures through adsorption and size exclusion is the widely used mechanism due to its simplicity and enhanced efficiency compared to other traditional water purification methods. Aerogels have the potential to replace conventional thin membranes considering their unmatched adsorption/absorption capacity and higher water flux due to their unique highly porous (99 %) 3D structure, ultra-low density (~1.1 to 500 mg/cm³), and very high surface area. The availability of a large number of functional groups, surface tunability, hydrophilicity, tensile strength and flexibility of nanocellulose (NC) makes it a potential candidate for aerogel preparation. This review discusses the preparation and employment of NC-based aerogels in the removal of dyes, metal ions and oils/organic solvents. It also offers recent updates on the effect of various parameters that enhance its adsorption/absorption performance. The future perspectives of NC aerogels and their performance with the emerging materials chitosan and graphene oxide are also compared.

Recent advances in tailoring and improving the properties of polyimide aerogels and their application

With the rapid advancements in technology and growing aerospace applications, there is a need for effective low-weight and thermally insulating materials. Aerogels are known for their ultra-lightweight and they are highly porous materials with nanopores in a range of 2 to 50 nm with very low thermal conductivity values. However, due to hygroscopic nature and brittleness, aerogels are not used commercially and in daily life. To enhance the mechanical and hydrophobic properties, reinforcement materials such as styrene, cyanoacrylates, epoxy along with hydroxyl, amines, vinyl groups are added to the surface. The addition of organic materials resulted in lower service temperatures which reduce its potential applications. Polyimides (PI) are commonly used in engine applications due to their suitable stability at high temperatures along with excellent mechanical properties. Previous research on polyimide aerogels reported high flexibility or even foldability. However, those works' strategy was mainly limited to altering the backbone chemistry of polyimide aerogels by changing either the monomer's compositions or the chemical crosslinker. This work aims to summarize, categorize, and highlight the recent techniques for improving and tailoring properties of polyimide aerogels followed by the recent advancements in their applications.

Electromagnetic Interference Shielding Performance of Anisotropic Polyimide/Graphene Composite Aerogels

Anisotropic polyimide (PI)/graphene composite aerogels were fabricated by unidirectional freezing. A poly(amic acid) (PAA) ammonium salt/graphene dispersion was first synthesized by mixing together PAA, H₂O, triethylamine (TEA), and graphene and then was successively subjected to one-way freezing, freeze-drying, and thermal imidization. The one-way growth of ice crystals endowed the composite aerogels with highly arranged tubular pores. The PI/graphene composite aerogels possessed anisotropic conductivity, electromagnetic interference (EMI) shielding, heat transfer, and compression performance. Moreover, the composite aerogels with low density (0.076 g·cm^-3) exhibited high EMI shielding effectiveness (SE) of 26.1-28.8 dB, and its specific EMI SE value achieved 1373-1518 dB·cm²·g^-1 when the graphene content was 13 wt %. The main electromagnetic interference shielding mechanism of these composite aerogels was microwave absorption. The composite aerogels had excellent thermal stability, and their 5% weight loss temperature was higher than 546 °C in nitrogen. This research provided an easy and environmentally friendly approach to prepare lightweight and anisotropic PI-based composite aerogels.