Metamaterials with negative magnetic permeability, negative permittivity, negative thermal conductivity, et al. having great importance in various application fields including electromagnetism, acoustics, mechanics, thermotics. Among which, auxetic architectures with a negative Poisson's ratio describe the degree of transverse deformation of a given material when it is axially stretched or compressed. Such designable auxetic architectures possess intriguing mechanical properties including superior shear resistance, indentation resistance, fracture toughness and damping performance, promising a diverse range of applications, particularly under stringent conditions such as in aerospace, defence and smart systems as well as biomedical devices. Nevertheless, designing and manufacturing auxetic architectures with mutli-scale structures is still challenging. By considering the advantages of aerogels in the design and processing of auxetic architectures, it is envisaged that auxetic architectures will be endowed with a more hierarchically porous structure and a maximised range of physical properties. Consequently, our group （http://www.aerogel-online.com/）prepared Kevlar nanofiber dispersion ink with suitable rheological properties in DMSO/KOH solution, then 3D fibrous Kevlar aerogel architectures with porosity at multiple scales have been designed and fabricated through a new additive manufacturing strategy, i.e., integration of direct ink writing and freeze-casting with non-toxic solvent-based inks followed by special drying techniques. Difffferent from previous water-based freeze-casting or direct micro extrusion, this approach exhibits an advanced capacity of manufacturing auxetic architectures from a diverse range of polymer aerogels or nanomaterial aerogels with a higher resolution, faster printing rate and lower energy cost. In addition, 3D polymeric aerogel based auxetic architectures smoothly welded between the adjacent struts, showing the mechanical integrity of the resulted architecture. The highly porous 3D nanofifibrous Kevlar aerogel architectures achieve excellent mechanical properties with an ultralow density (down to 11.9 mg∙cm-3) and large specifific surface area (up to 350 m2∙g-1 ).
Figure 1. Schematic illustration of the 3D printing process of a Kevlar aerogel auxetic architecture with a negative Poisson's ratio.
The Poisson's ratio is tunable in a wide range, spanning from -0.8 to 0.4, by adjusting the spatial arrangement of the designed pattern struts. In the case of the 3DKAAA with ʋ=−0.8, the strong adhesion between the struts and junctions, randomly-entangled nanofibre network within the struts and multi-stage fracture mechanism made such ultra-lightweight auxetic aerogel architectures tougher with more energy absorption (80.1 J∙g-1) compared with the structure with a positive Poisson's ratio (e.g., 25.9 J∙g-1 for ʋ =0.4). Finally, these fibrous Kevlar aerogel architectures have been further functionalized into hydrophobic, luminescent and thermoresponsive architectures by using fluorocarbon resin, functional dyes and organic phase-change materials respectively. This work opens an avenue for the design and fabrication multifunctional auxetic architectures and auxetic-composites for potential diverse applications in many emerging fields.
Figure 2. Tensile mechanical properties of the 3D printed Kevlar aerogel auxetic architectures.
Figure 3. The multi-functionalization of the 3D-KAAA.
This work was published on Journal of Materials Chemistry A (Journal of Materials Chemistry A. 2020, DOI: 10.1039/d0ta02590a). The first author is Qingqing Cheng, a doctoral candidate in University of Science and Technology of China. The corresponding authors are Prof. Xuetong Zhang and Prof. Wenhui Song (University College London），other authors including Dr. Yang Liu (Imperial College London)、Prof. Jing Lyu and Prof. Qiang Lu (Soochow University).
Reproduced from Polymer Science and Technology WeChat official account：https://mp.weixin.qq.com/s/DlotXY_sDDm5Xm0GZFqvSg