Abstract
Nanoscale building blocks of many materials exhibit extraordinary mechanical properties due to their defect-free molecular structure. Translation of these high mechanical properties to macroscopic materials represents a difficult materials engineering challenge due to the necessity to organize these building blocks into multiscale patterns and mitigate defects emerging at larger scales. Cellulose nanofibrils (CNFs), the most abundant structural element in living systems, has impressively high strength and stiffness, but natural or artificial cellulose composites are 3-15 times weaker than the CNFs. Here, we report the flow-assisted organization of CNFs into macroscale fibers with nearly perfect unidirectional alignment. Efficient stress transfer from macroscale to individual CNF due to cross-linking and high degree of order enables their Young's modulus to reach up to 86 GPa and a tensile strength of 1.57 GPa, exceeding the mechanical properties of known natural or synthetic biopolymeric materials. The specific strength of our CNF fibers engineered at multiscale also exceeds that of metals, alloys, and glass fibers, enhancing the potential of sustainable lightweight high-performance materials with multiscale self-organization.
Highlights
The quest for more eco-friendly and energy-efficient technologies accentuates the need to develop lightweight structural materials with exceptional mechanical performance from renewable resources.[1]
Macroscale fibers from nanoscale Cellulose nanofibrils (CNFs) are fabricated by hydrodynamic alignment of the fibrils from a surface-chargecontrolled sol.[22]
In the core flow, charged CNF fibrils are free to rotate due to electrostatic repulsions and Brownian motion (Figure 1a, position 1), only restrained by fibril−fibril interactions
Summary
The quest for more eco-friendly and energy-efficient technologies accentuates the need to develop lightweight structural materials with exceptional mechanical performance from renewable resources.[1]. An overarching challenge in structural materials fabrication is to translate the extraordinary mechanical properties of nanoscale building blocks (e.g., tensile strength and Young’s modulus) to the macroscale bulk materials.[4] This problem arises from the fundamentally nonideal stress transfer from the macro- to molecular scale that prevents efficient utilization of the high mechanical performance of nanoscale building blocks. Flow-assisted assembly is a promising method for fabricating large, well-ordered edifices of nanoscale objects.[10−13] the colloidal behavior of CNF in liquids is known to be more complicated than that of isotropic nanomaterials, monodispersed nanorods, or carbon nanotubes due to broad distribution of length, process-induced deformations, facile gelation into a disorganized glassy state, and complexity of CNF−CNF interactions in different orientations.[9,14] Hydrodynamic stresses from extensional flows are known to effectively break dense colloidal aggregates and to produce dispersions with steady-state ordering of materials, in contrast to shear flows.[15,16] Inspired by the architecture of the S2 layers, we here make use of insights into the behavior of nanofibrils under flow and organize them into dense macroscale fibers with in situ-controlled organization that resolve the problems of multiscale stress transfer discussed above.[11,17−21]
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