Background:
In recent years, there has been a lot of attention in the spotlight for smart temperature-controlled textiles with personal thermal management, which provide advanced thermal comfort to the wearer at a wide range of ambient temperatures, thereby minimizing energy consumption. Especially in some occasions, such as aerospace and fire scenes, lightweight, ultra-thin fabrics are required to ensure the wearer's productivity and safety. Emerging aerogel fibers Inheriting the three-dimensional (3D) porous structure of aerogel and the flexibility of fibers, the emerging aerogel fiber has the characteristics of light weight, high porosity and multi-functional integration, showing great potential in the field of intelligent temperature control textiles. The highly porous structure imparts low thermal conductivity (23-50 mW m-1 k-1) to the aerogel fibers, which greatly inhibits heat loss. In addition, its unique microstructure can also provide favorable conditions for the integration of smart materials such as phase change materials. As a result, aerogel fibers and their composite fabrics show great potential for personal thermal management.
Research Results
Here, the Donghua University research team reports a rapid, scalable, cross-linked polyimide (CPI) aerogel fiber fabrication strategy for wet spinning and cost-effective atmospheric pressure drying via a UV-enhanced dynamic gelation strategy. A spinning solution based on photosensitive polyimide (PPI) with trifluoromethyl was designed and synthesized. Ultraviolet light-enhanced dynamic gelation can induce rapid crosslinking of PPI spinning solution within 10 s under ultraviolet light, achieving rapid sol-gel transition, thus forming a strongly crosslinked gel backbone, which can effectively prevent the structure collapse of CPI aerogel fibers during atmospheric pressure drying. Using their method, high-strength aerogel fibers up to hundreds of meters in length can be prepared quickly and continuously in 7 h, which is much faster than the previously reported method, which typically takes 49-94 h. The thickness of the ultra-thin aerogel fiber is 07 mm, which is about 1 8 of down, has almost the same thermal insulation properties as down. In addition, in a proof-of-concept study, CPI aerogel fabrics can be combined with shape memory materials to make smart heat-adapted fabrics for personal thermal management. Their research provides a scalable production method for the development of aerogel fibers and textiles to meet advanced application scenarios. The related research work was published in the top journal Nature Communications with the title of "Fast and Scalable Production of Crosslinked Polyimide Aerogel Fibers for Ultrathin Thermoregulating Clothes". Congratulations!
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Figure 1Preparation and morphology of CPI aerogel fibers Cross-linked polyimide (CPI) aerogel fibers are prepared by wet spinning, solvent exchange, and atmospheric pressure drying using a UV-enhanced dynamic gelation strategy. PPI has excellent solubility and stability in common solvents, which ensures the formation of a spinning solution and improves processability. Subsequently, PPI and photoinitiator (IrgaCure2100) were dissolved in NMP to prepare a spinning solution that showed high zero-shear viscosity and shear-thinning behavior, favoring the spinning process. The spinning solution is extruded to form filaments, which can be quickly converted into gel fibers under ultraviolet irradiation. Subsequently, solvent exchange and atmospheric pressure drying are followed to obtain CPI aerogel fibers. In a UV-enhanced dynamic gelation strategy, randomly distributed PPI chains are rapidly cross-linked under UV irradiation, forming a highly crosslinked, high-strength gel network. In addition, the rapid gelling process and atmospheric drying strategies provide great flexibility for rapid, continuous, and scalable manufacturing of CPI aerogel fibers. The prepared CPI aerogel fibers have a perfect fiber morphology and are approximately 300 m in diameter. The corresponding high-magnification scanning electron microscope images show highly porous nanostructures formed by a network of three-dimensional interconnected nanofibers, demonstrating typical aerogel morphology. Figure 1e shows a roll of prepared CPI aerogel fibers up to several hundred meters in length, which can be woven into a large CPI aerogel fabric with a length of more than 1 m by a semi-automatic braiding machine. Therefore, their strategy is expected to be an alternative to high-throughput, scalable aerogel fiber fabric preparation methods. 
Figure 2The sol-gel transition process of PPI is realized by the UV-enhanced dynamic gelation strategyIn response to UV irradiation, the double bonds of PPI are induced to polymerize with free radicals to form a strongly cross-linked polyimide gel. FTIR spectroscopy showed that the vinyl characteristic peak (C = C extension) of PPI-100 at 1628 cm-1 gradually disappeared with the extension of UV irradiation time. Based on the change in the absorption peak area of the double bond, the conversion rate of the double bond was as high as 93 under 30 s of ultraviolet irradiation2%。
Figure 3The gelation and skeletal strength of CPI gel fibers further visualizes the sol-gel transition process by squeezing the PI and PPI spinning solutions into the NMP solvent under UV radiation. After extrusion, PI diffuses rapidly in the solvent and is unable to maintain the fiber shape, whereas PPI-100 is able to form stable cross-linked gel fibers in the solvent, suggesting that UV-induced sol-gel transitions are effective. The changes in storage modulus and loss modulus of Pi and PPI solutions before and after UV irradiation are shown in Figure 3b. After UV irradiation, the storage modulus and loss modulus of the PPI-100 solution increased by an order of magnitude. In addition, the storage modulus of PPI-100 (about 11,700 Pa) after UV irradiation is much higher than the loss modulus (about 5020 Pa), and it appears as a gel. However, the modulus of the PI solution did not change much after UV irradiation, and the storage modulus was less than the loss modulus, indicating that it was in a sol state. More importantly, the CPI gel fibers prepared by UV-enhanced dynamic gelation strategy had high tensile strength and elastic modulus, and the strength increased with the increase of hema grafting rate. This is because PPI with a high grafting rate can provide more cross-linking sites for gel formation, resulting in a higher cross-linking density for the resulting gel fibers. Therefore, due to the stability and high strength of the gel backbone, the gel fibers can be further collected into rolls without damaging the shape, which can effectively inhibit shrinkage during solvent exchange and promote the construction of three-dimensional porous structures after drying. To further demonstrate the advantages of UV-enhanced dynamic gelation strategies in the construction of aerogel fibers, they compared gelation kinetics with previously reported gelation strategies such as condensation reactions, chemical cross-linking, and ionic cross-linking. They recorded the variation of gel modulus over time obtained by several gelation strategies. The UV-enhanced gelation strategy induces a rapid sol-gel transition in the precursor solution while imparting a higher modulus (approximately 11,700 Pa) to the gel. In contrast, the gel modulus produced by condensation reactions, chemical cross-linking, and ionic cross-linking is less than 10 Pa. This weak gel backbone inevitably requires supercritical or freeze-drying to remove solvents, thus preventing the three-dimensional backbone from collapsing. In addition, these gelation strategies exhibit slow gelation kinetics due to the low diffusion velocity of crosslinkers or gelling agents (e.g., Ca2+, urea) during spinning. Notably, the high-strength gel backbone prepared by the UV-enhanced dynamic gel strategy has fast gel kinetics that prevents the structural collapse of aerogel fibers during atmospheric pressure drying. In addition, CPI-100, which contains a large number of methyl trifluoromethyl groups, has a hydrophobic surface and low capillary pressure, which further avoids structural collapse during atmospheric drying. Therefore, with their strategy, it takes only about 7 hours to prepare aerogel fibers by wet spinning, solvent exchange, and atmospheric drying. 
Figure 4CPI Fiber Morphology and physical properties of fabricsConclusions and prospects
In conclusion, cross-linked polyimide aerogel fibers were easily prepared by wet spinning by UV-enhanced dynamic gelation strategy. Photosensitive polyimides can be crosslinked and gelated within 10 seconds upon UV irradiation, enabling an immediate sol-to-gel transition and maintaining fiber shape after extrusion. CPI's strongly cross-linked gel backbone has a high storage modulus of about 11,700 Pa, so it can effectively prevent the structural collapse of aerogel fibers during solvent exchange and atmospheric drying. The resulting CPI-100 aerogel fibers have a high specific modulus (390.).9 kn m-1 kg-1), which can be woven into aerogel fabrics. Ultra-thin aerogel fabric (0.)7 mm thick) is impressive, showing a temperature difference of 200 on a 108 hot table, which is equivalent to 54 mm thick down jacket. In addition, in the proof-of-concept study, a smart heat-adapted textile was designed by combining CPI aerogel fibers with phase change materials, which realized intelligent thermal regulation in high-temperature environments. This work opens up vast possibilities for the fabrication of high-performance and multifunctional aerogel fibers in a scalable and cost-effective manner, demonstrating great potential for personal thermal management and beyond.