Aerogels are lightweight materials with extensive microscale pores, which could be used in thermal insulation, energy devices, aerospace structures, as well as emerging technologies of flexible electronics. However, traditional aerogels based on ceramics tend to be brittle, which limits their performance in load-bearing structures. Due to restrictions posed by their building blocks, recently developed classes of polymeric aerogels can only achieve high mechanical strength by sacrificing their structural porosity or lightweight characteristics.
A research team led by Dr Lizhi Xu and Dr Yuan Lin from the Department of Mechanical Engineering of the Faculty of Engineering of the University of Hong Kong (HKU), has developed a new type of polymer aerogel materials with vast applicational values for diverse functional devices.
In this study, a new type of aerogels was successfully created using a self-assembled nanofiber network involving aramids, or Kevlar, a polymer material used in bullet-proof vests and helmets. Instead of using millimetre-scale Kevlar fibres, the research team used a solution-processing method to disperse the aramids into nanoscale fibrils. The interactions between the nanofibers and polyvinyl alcohol, another soft and “gluey” polymer, generated a 3D fibrillar network with high nodal connectivity and strong bonding between the nanofibers. “It’s like a microscopic 3D truss network, and we managed to weld the trusses firmly together, resulting in a very strong and tough material that can withstand extensive mechanical loads, outperforming other aerogel materials,” said Dr Xu.
The team has also used theoretical simulations to explain the outstanding mechanical performance of the developed aerogels. “We constructed a variety of 3D network models in computer, which captured the essential characteristics of nanofibrillar aerogels,” said Dr Lin, who led the theoretical simulations of the research. “The nodal mechanics of fibrillar networks are essential to their overall mechanical behaviours. Our simulations revealed that the nodal connectivity and the bonding strength between the fibres influenced the mechanical strength of the network by many orders of magnitudes even with the same solid content,” said Dr Lin.
“The results are very exciting. We not only developed a new type of polymer aerogels with excellent mechanical properties but also provided insights for the design of various nanofibrous materials,” said Dr Xu, adding, “the simple fabrication processes for these aerogels also allow them to be used in various functional devices, such as wearable electronics, thermal stealth, filtration membranes, and other systems,”
The research was published in Nature Communications, in an article entitled “Ultrastrong and multifunctional aerogels with hyperconnective network of composite polymeric nanofibers”.
Link of the paper: https://www.nature.com/articles/s41467-022-31957-2
港大機械工程成功研發具備優異力學性能的新型氣凝膠材料
氣凝膠是一種具備微觀多孔結構的輕質材料，其在熱防護、能源存儲、航空航太設備、柔性電子等領域有著重要的潛在應用價值。然而，傳統陶瓷的氣凝膠材料的力學脆性，限制了承受載荷變形情況下的使用。新近研發的高分子氣凝膠因為受限於可用的材料成分，只能在犧牲輕質和多孔性的前提下實現較高的力學強度。
由香港大學（港大）機械工程系助理教授徐立之博士及副教授林原博士領導的研究團隊，利用一種常用於製造防彈衣和頭盔的合成纖維，研發出一種新型超強韌的氣凝膠材料，其力學性能明顯優於其他同類的氣凝膠。
該新型氣凝膠材料製備方法簡易，具潛力開發應用於可穿戴器件、熱學隱身、濾膜結構以及其他多種新型功能器件中。研究結果已在《自然-通訊》刊登，文章題爲 ”Ultrastrong and multifunctional aerogels with hyperconnective network of composite polymeric nanofibers”。
研究團隊利用自行組裝的芳綸納米纖維網路，構建出強韌多孔的新型高分子氣凝膠。對位元芳綸材料具有優異的力學性能，常用於防彈衣、頭盔等高強結構中。與常見的宏觀尺寸芳綸纖維不同，研究團隊利用溶液加工方法製備芳綸納米纖維，再利用納米纖維和聚乙烯醇之間的相互作用，構建具備高連接度和高節點強度的三維纖維網路。
「這就像是微觀尺度上的桁架網路，我們想辦法把這些桁架牢固地焊接到了一起，從而產生一種非常強韌的材料，它的力學性能明顯優於其他類似的氣凝膠。」徐立之博士解釋說。
林原博士負責領導相關的理論模擬工作，闡明了氣凝膠的強韌化機理，他説：「我們在電腦中構建了一系列的三維纖維網路，再現了納米纖維氣凝膠的重要結構特徵。纖維網路節點的力學行為對於宏觀力學特性非常重要。我們的模擬研究表明，即使在相同的固含量下，不同的節點連接度和節點結合能造成宏觀機械強度上幾個數量級的差別。」
「這些研究結果非常令人振奮。我們不僅開發出了一種力學強韌的新型氣凝膠材料，同時也探明了納米纖維網路普適的強韌化機理，這對於其他類似材料的研發也具有重要的指導意義。」徐博士補充說。
Figure 1. a – Schematics of the assembly process of our composite nanofiber aerogels. 複合納米纖維氣凝膠的製造過程示意圖。
Figure 1. b - Scanning electron microscope image showing the micro-structure of the developed aerogel materials. 掃描電子顯微鏡所顯示氣凝膠的微觀結構。
Figure 1. c - The aerogel possesses desirable features such as lightweight, semi-transparent and high load bearing capability. 開發的氣凝膠具有輕質、半透明和高承載能力等理想特性。
Figure 1. d - Both high toughness and tensile modulus are achieved by our composite nanofiber aerogels, when compared to other polymeric aerogels. 與其他聚合物氣凝膠相比，我們的複合納米纖維氣凝膠具有高韌性和高拉伸模量。
Dr Lizhi Xu (Right) and Dr Yuan Lin (Left). 徐立之博士(右)和林原博士(左)。
Dr Lizhi Xu (Right) and Dr Yuan Lin (Left). 徐立之博士(右)和林原博士(左)。
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