It is conceivable that distinct chemical crosslinking between fibrils may affect mechanical properties of the network 23. Although most of them can form 3D microfibrillar network, their achievable mechanical strengths vary drastically by a few orders of magnitude (e.g., from 10 −2 MPa to 10 1 MPa) even with similar levels of solid content. Indeed, a range of biomacromolecules including cellulose 11, 12, 13, 14 and chitin 15, 16, 17, or synthetic polymers such as polyimide (PI) 18, 19, 20 and polyurethane (PU) 21, 22, were explored for the creation of nanofoams or those referred to as aerogels. Nevertheless, achieving a high mechanical strength with currently available chemistries remains difficult due to the weak interactions among fibrils 10. Recently, aerogels or foams from self-assembled polymeric nanofibers have drawn wide attention due to their structural similarity to biological tissues 9. However, it is difficult to design microscale topology or fabricate 3D structures with this method. Electrospinning techniques were adopted for creating flexible polymeric membranes involving nanofibers 7, 8. Aerogels prepared by sol-gel processes exhibit high stiffness originating from the ceramic constituents, but their intrinsic brittleness may limit their application under macroscopic deformation 4, 5, 6. For example, metamaterials created from stereolithographic patterning allow rational design for desired mechanical properties 1, 2, 3, but the required structural ordering creates challenges for large-scale production. Although extensive efforts have been devoted to the engineering of lightweight materials with 3D microfibrillar network, approaches to high mechanical strength and scalable fabrication are still limited. As exemplified by the microstructures of cartilage, trabeculated bones, and plant tissues, these three-dimensional (3D) microfibrillar networks afford a combination of physical strength, lightweight, mass permeability, and surface functionality. Nature exploited such design for the building of a variety of load-bearing biological tissues.
Porous network assembled from fibrillar elements represents an efficient structural design for materials.
The mechanistic insights and manufacturability provided by these robust microfibrillar aerogels may create further opportunities for materials design and technological innovation. Furthermore, their simple processing techniques allow fabrication into various functional devices, such as wearable electronics, thermal stealth, and filtration membranes. Indeed, the polymeric aerogels achieved both high specific tensile modulus of ~625.3 MPa cm 3 g −1 and fracture energy of ~4700 J m −2, which are advantageous for diverse structural applications. As revealed by theoretical simulations of 3D networks, these features at fibrillar joints may lead to an enhancement of macroscopic mechanical properties by orders of magnitude even with a constant level of solid content. The interactions between the nanoscale constituents lead to assembled networks with high nodal connectivity and strong crosslinking between fibrils. Here, we report ultrastrong polymeric aerogels involving self-assembled 3D networks of aramid nanofiber composites.
Despite extensive efforts, achieving high mechanical properties for synthetic 3D microfibrillar networks remains challenging. Three-dimensional (3D) microfibrillar network represents an important structural design for various natural tissues and synthetic aerogels.