The ultra-low temperature environment in the aerospace field poses a major challenge to the performance of elastic materials in spacecraft and related equipment. At present, most traditional inherently elastic materials usually lose their inherent elasticity in ultra-low temperature environments. The existing solutions are mainly elastic aerogels based on carbon and ceramic structures; these newly emerged carbon and ceramic aerogels promote the development of elastic materials for ultra-low temperature environments, but their complex manufacturing processes and high costs limit their further development. Applications.

In this context, the latest report shows a polymer aerogel composed of low-cost chitosan and melamine-formaldehyde resin, which is superelastic at liquid nitrogen temperature (77K). Among polymer materials, polyimide (PI) has significant resistance to extreme conditions. However, the thermal imidization after freeze-drying in the above strategy inevitably leads to a shrinkage deformation of up to 40%, which greatly impairs the compressibility of the elastic PI aerogel. In addition, due to the incomplete salinization of polyamic acid (PAA), the decomposition of PAAS in water cannot be completely avoided, resulting in the low molecular weight of the elastic PI aerogel, which affects its resilience performance. The recent appearance of electrospun nanofiber PI aerogels provides an effective way to avoid the massive shrinkage and decomposition of PAAS in water, but the combination of electrospinning processes complicates the entire manufacturing process and increases costs.

In view of this, the team of Professor Ye Mingxin and Professor Shen Jianfeng of Fudan University developed a mass-produced and low-cost oriented dimethyl sulfoxide crystal-assisted cryogelation and freeze-drying strategy, and realized the covalent cross-linked polyimide (PI) ) Superelasticity (99% elastic compression strain) of aerogel at ultra-low temperature (4 K). After severe thermal shock (ΔT=569 K), the elastic loss is almost zero, and it can withstand more than 5000 compression cycles. Anti-fatigue performance.

Figure 1. Schematic diagram of constructing a superelastic PI aerogel. Design and synthesize PI aerogels with covalent cross-links, radially distributed microstructures and different shapes.

Figure 2. DMSO solvent covalently cross-linked PI aerogel preparation process.

[Mechanism of Highly Elastic PI Aerogel] 

The author uses the good solubility of DMSO to realize the chemical imidization process and covalent cross-linking structure to make PI aerogel, which synergistically reduces the volume shrinkage of the resulting aerogel. The PI aerogel prepared by DMSO chemical imidization shows a volume shrinkage of less than 7.3%, which is far better than the thermal imidization of 19.5-25.3%. The covalent cross-linked structure can usually endow PI aerogels with better heat resistance and mechanical properties, and can inhibit structural damage caused by thermal stress shock during high-temperature thermal annealing.

Figure 3. The structure and morphology of PI aerogels.

[Mechanical properties of high elastic PI aerogel]

The PI aerogel also exhibits amazing super-elasticity at deep low temperatures of 4 K. Even after being subjected to a thermal shock between 4 K and 573 K, the PI aerogel still maintains a compressibility and compressibility of up to 99% strain. Perfect recoverability, no obvious structural damage was observed.

Figure 4. Mechanical properties of PI aerogels.

Figure 5. Mechanical properties of PI aerogels in various environments.