Controllable and Predictable Viscoelastic Behavior of 3D Boron‐Doped Multiwalled Carbon Nanotube Sponges

Abstract

3D carbon nanotube (CNT)-based macrostructures are the subject of extensive attention because the outstanding properties of 1D and 2D nanostructures have not been fully translated into key engineering applications. Generation of 3D CNT architectures with covalent junctions could endow the new materials with extraordinary mechanical properties. In this study, detailed experimental characterization and statistical comparison are carried out on 3D boron-doped multiwalled CNT (CB x MWNT) sponges with covalent junctions and undoped multiwalled CNT (undoped-MWNT) sponges without junctions. By investigating the plastic, elastic, viscoelastic, and dynamic viscoelastic properties of both sponges, as well as the dependency of these mechanical properties on material morphology, the CB x MWNT sponge is found to be a more predictable and stable material than the undoped-MWNT sponge. Statistical comparison proves that the excellent properties of the CB x MWNT are attributed to its "elbow-like" junctions inside the 3D networks, which prevent permanent buckling and bundling of the CNTs under extreme loading. Thus, by optimizing the covalent junctions in 3D CNT sponges, their functional behavior can be controlled and regulated. These fi ndings may promote applications of 3D CNT sponges in various fi elds, including biomedical or high-precision devices in which lightweight, controllable, and reliable mechanical properties are always desirable.

Hyperelasticity of three-dimensional carbon nanotube sponge controlled by the stiffness of covalent junctions

“…This result also validates our initial hypothesis that a stretch along the CNT axis rarely happens in a compressed sponge due to its incomparable stiffness compared to that of junctions. Moreover, the simulation output gives an insight into our previous observation that the 3-D CNT sponge with covalent junctions shows a comparatively smaller modulus than its pristine counterpart, undoped-MWCNT sponges [8], while the presence of elbow-like junctions endows the sponge with controllable and remarkable mechanical flexibility. Considering that the junction stiffness could be regulated by the dopant For both CBxMWCNT and N-MWCNT sponges, the predicted junction stiffness varies with the sponge density.…”

“…Moreover, we found that the MWCNT sponges with covalent junctions have drastically different mechanical behaviors with the pristine (undoped) sponges, including the plastic, elastic, viscoelastic, and dynamic properties. CBxMWCNT sponge is demonstrated as a more predictable and stable material than the undoped-MWCNT sponge [8]. This difference implies that covalent junctions may play a critical role in shaping the mechanical behaviors of 3D CNT sponges.…”

“…This difference implies that covalent junctions may play a critical role in shaping the mechanical behaviors of 3D CNT sponges. Since the CNTs are modeled as multi-segment chains, the present constitutive model may not be suitable for the undoped CNT sponges [8]. It is also important to note that the time-dependent viscoelastic behaviors and plastic properties of the sponges are not incorporated in this model.…”

“…Different from uniformly aligned CNT bundles or CNT films, 3-D sponges composed of randomly oriented CNTs have been fabricated to achieve isotropic material properties [4e6]. The 3D CNT sponges with hierarchical structures were observed to exhibit excellent mechanical properties such as controllable viscoelasticity and high electrical conductivity as a result of increased surface area [7,8]. Their multifunctional characteristics have been exploited for a number of applications, such as hydrogen storage, tissue engineering and environmental-friendly reusable sorbent [4,9,10].…”

“…However, actual roles of these parameters and their mechanical contributions remain largely elusive. The stressestrain curves of CNT sponges were mostly explained using linear elastic or viscoelastic models [5,8]. No proper constitutive model is available to correlate the mechanical behaviors of CNT sponges with the CNT morphology or junction properties.…”