Flow-assisted assembly is a promising method for fabricating large, well-ordered edifices of nanoscale objects. (10−13) However, 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-controlled organization that resolve the problems of multiscale stress transfer discussed above. (11,17−21)

The architecture of wood, especially the outer cell wall layer (S2 layer) that possesses the highest strength and stiffness among all layers, provides leads for structural design of uniaxial high-performance materials. (5) The S2 layer is made of semicrystalline CNFs that are aligned and embedded in a matrix of hemicellulose and lignin to form macrofibers. Being cross-linked to hemicellulose and ligninan abundance of carboxyl groups, (6) the crystalline regions of CNF contain the backbone of cellulose molecules, which makes them stiff (Young’s modulus of 130–150 GPa) and strong (∼1.0–3.0 GPa). (5,7,8) Unfortunately, macroscopic materials from these structural components have mechanical properties that are 3–15 times short of the theoretical and experimentally determined values characteristic of the individual fibrils due to difficulties in the assembly of CNFs into macroscale dense structures, promoting efficient stress transfer between them and inhibiting the occurrence of stochastic defects. (3,9)

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) Nature has long developed abilities to tightly control the structural features of its high-performance finite size building blocks with well-ordered arrangements at nano- and molecular level. (2,3) Lately, scientists have been seeking ideas of mimicking natural materials’ architecture based on engineering design principles, typically called “bioinspired assembly”. An overarching challenge in structural materials fabrication is to translate the extraordinary mechanical properties of nanoscale building blocks (, 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. Poor adhesion and building block misalignment creates large amount of nanoscale defects that limits the materials performance at scales most common to human technologies. (2,3)

Results and Discussion ARTICLE SECTIONS Jump To

–) groups that allows the supramolecular interactions between CNFs to self-organize the fibrils into a well-packed state with maximized CNF–CNF contacts (position 4) (see Macroscale fibers from nanoscale CNFs are fabricated by hydrodynamic alignment of the fibrils from a surface-charge-controlled sol. (22) In this process, it is vital to align the fibrils in the suspension before “locking” the nanostructure into metastable colloidal glass. This was accomplished using well-established fundamentals of extensional flow fields (13,23) provided by a double flow-focusing channel ( Figure 1 a). In the core flow, charged CNF fibrils are free to rotate due to electrostatic repulsions and Brownian motion ( Figure 1 a, position 1), only restrained by fibril–fibril interactions. Note that the electrostatic repulsion caused by the dissociated COOH groups on the surface of CNFs is much higher than the attractive van der Waals forces at neutral to slightly alkaline pH. The first sheath flow of deionized (DI) water supports electrostatic repulsion and prevents transition into the glass state in contact with channel walls; it also aligns the fibrils toward the flow direction (position 2). (13,17) Before the alignment is diminished by the Brownian diffusion, the second flow of low pH acid enhances the fibril alignment (position 3), while reducing the electrostatic repulsion between the fibrils due to protonation of carboxyl (COO) groups that allows the supramolecular interactions between CNFs to self-organize the fibrils into a well-packed state with maximized CNF–CNF contacts (position 4) (see Figure S1 for further details on the experimental setup). The continuous threads obtained from the flow-induced assembly were subsequently held at their ends and air-dried. Characterization of the assembled structure with scanning electron microscopy (SEM) ( Figure 1 b) sampled in the longitudinal direction showed uniformly sized CNF fibers with dense and near perfectly aligned fibrils without obvious packing defects or voids ( Figure S2 ). Micrographs of fiber cross sections sampled in the transversal direction confirm the dense fibrillar packing and reveal a well-defined layered structure ( Figure 1 c).

Figure 1 Figure 1. Assembly of nanostructured CNF fibers. (a) Schematic of double flow-focusing channel used for CNF assembly. The CNF suspension is injected in the core flow (light brown color), DI water (blue color) in the first sheath, and acid at low pH (light green color) in the second sheath flows. Arrows show the flow direction. Hydrodynamic and electrostatic interactions at different positions along the channel are illustrated schematically on the right. Position 1, poor fibril alignment due to Brownian diffusion and electrostatic repulsion (illustrated with the dashed arrows) caused by dissociated carboxyl (−COOH) groups on the fibrils surface. Position 2 hydrodynamically induced alignment (illustrated by solid, green arrow) occurs during acceleration/extension. Position 3 further increase in alignment during acceleration/extension, and in position 4, following the acid addition, Brownian diffusion is minimized due to the transition of CNF suspension to an immobilized volume filling arrested state due to protonation of the COO– groups. For illustration, the relative size of the fibrils has been magnified around 300 times. The use of acid for transforming the free-flowing CNF suspension to a fibrous colloidal glass ensures that the electrostatic repulsions are replaced by van der Waals forces in the protonated carboxyl groups. This complete removal of electrostatic repulsion is not possible with simple electrolytes.(22) (b) SEM image of the fiber surface, where the dense fibrillar network with well-preserved anisotropic arrangement can be seen. (c) SEM image of the cross section of the fiber, showing the aligned nanofibrils. Scale bars in (b) and (c) are 3 μm and insets are 400 nm.

In situ monitoring of CNF assembly in solution was essential for successful adaptation of multiple conditions. Polarization microscopy shows the alignment of fibrils (in situ alignment can be quantified using synchrotron-based microfocus small-angle X-ray scattering (μSAXS) (–1) are calculated ( monitoring of CNF assembly in solution was essential for successful adaptation of multiple conditions. Polarization microscopy shows the alignment of fibrils ( Figure 2 a), with gradually increasing birefringence from randomized suspensions to aligned fibrils. (13) Concurrently,alignment can be quantified using synchrotron-based microfocus small-angle X-ray scattering (μSAXS) ( Figure 2 b) (see Methods for details). Local order parameters of CNF suspensions with different lengths of the constituting fibrils (CNF-550 of 590 nm and CNF-1360 of 391 nm, the suffix represent the surface charges in μequiv g) are calculated ( Figure 2 c and Figures S3 and S4 ). An order parameter of 1 represents a fully aligned state of the fibrils (in the direction of fiber preparation), and 0 corresponds to an isotropic fibril distribution.

Figure 2 Figure 2. In situ study of the alignment, dealignment and rotary diffusion of fibrils. (a) Microscopy image of the channel used for μSAXS measurements, placed between two cross-polarized filters rotated 45° from the vertical axis (white arrows). White color corresponds to the birefringence signal obtained for the CNF-550 suspension. Numbers represent the positions where in situ measurements were carried out. Scale bar is 1 mm. (b) μSAXS scattering diffractograms at different positions along the channel for CNF-550 suspension (top row) and CNF-1360 suspension (bottom row). Curved lines represent the beam stopper. (c) Local order parameters calculated from the μSAXS scattering diffractograms as a function of downstream position normalized with the channel width (h). (d) Birefringence signal obtained in a single flow-focusing channel for the CNF-550 suspension. Scale bar is 1 mm. The black squares along the center line represent the positions where the D r values are estimated. (e) Birefringence signal obtained at z/h = 3 as a function of time when the flow is viciously stopped. The inset shows the initial decay of the birefringence signal for the CNF-1360 suspension. The dashed line indicates the fit of the initial decay to measure the D r . Negative and positive time represents the condition before and after stopping the flow, respectively. (f) D r as a function of downstream position along the center line. The vertical dashed line shows the position z/h = 3 from where the birefringence decays are plotted in (e). This position also corresponds to the filled black square at z/h = 3 in (d). The colors in (e) and (f) correspond to different suspensions: CNF-550 (red) and CNF-1360 (blue).

z/h = 1) in the CNF suspension. At the beginning of the focusing step (z/h = 2), the order parameter decreases due to deceleration of the core flow followed by a sudden increase after the focusing due to acceleration (3 ≤ z/h < 4). Subsequently, the order parameter decreases slightly (4 ≤ z/h < 7) before increasing again during acceleration in the contraction (7 ≤ z/h < 10). The decrease in order parameter after the focusing and contraction steps (4 ≤ z/h < 7; 10 ≤ z/h < 15) indicates nanofibril relaxation toward isotropy, primarily due to Brownian diffusion.D r ) of CNF-1360, which is twice that of to CNF-550 ( Initially, the shear from channel walls gave rise to some ordering (= 1) in the CNF suspension. At the beginning of the focusing step (= 2), the order parameter decreases due to deceleration of the core flow followed by a sudden increase after the focusing due to acceleration (3 ≤< 4). Subsequently, the order parameter decreases slightly (4 ≤< 7) before increasing again during acceleration in the contraction (7 ≤< 10). The decrease in order parameter after the focusing and contraction steps (4 ≤< 7; 10 ≤< 15) indicates nanofibril relaxation toward isotropy, primarily due to Brownian diffusion. (13) Substantial differences between the alignment and disorganization behavior of CNF-550 and CNF-1360 suspensions are observed: the shorter the fibrils (CNF-1360), the faster the process of alignment and dirorganization ( Figure 2 c). This effect is due to the diffusivity based on length distributions of the nanofibrils as given by the rotary diffusion coefficient () of CNF-1360, which is twice that of to CNF-550 ( Figure 2 d–f) (see Methods for details).

Establishing a relationship between fibril characteristics (length, surface charge) and mechanical properties, particularly when fabrication technique involves fibrils under flow induced stretching, is vital to provide the foundation for future rational design of materials with targeted performance maxima. For a comprehensive understanding, we fabricated another set of fibers (CNF-820 with mean fibrils length of 683 nm) and compared the mechanical properties of CNF-550, CNF-820, and CNF-1360. The stress–strain curves ( Figure 3 a) show an initial linear region (pseudoelastic), followed by significant deviation from linearity (plastic region). The “knee” in the curve represents the elastic–plastic transition and is attributed to yielding mechanisms related to sliding of the fibrils. (9) CNF-550 and CNF-820 show similar stress–strain behavior with a modulus and strength of ∼70 GPa and ∼1200 MPa, respectively ( Figure 3 a–d). This could be due to the similar mean length of the constituent fibrils and indicates that strength and stiffness of the prepared fibers are relatively independent of fibril surface charge. (24) This is further supported by a significant decrease in strength (630 MPa) by reducing fibril length to 391 nm (CNF-1360). Moreover, the lower modulus of CNF-1360 (45 GPa) is due to the relatively low fibril orientation, as verified by wide-angle X-ray scattering (WAXS) ( Figure 3 e). The orientation index for CNF-550 fibers is 0.92 (order parameter = 0.70), whereas CNF-1360 fibers show 0.83 (order parameter = 0.53). In general, the stiffest materials tend to be the strongest if provided with defect-free structure and strong interfaces to ensure adequate load transfer and cohesion within the material. (2,3) The strength and stiffness of our fibers vividly demonstrate the importance of connectivity and bonding at the fibril–fibril interfaces facilitating multiscale stress transfer.

Figure 3 Figure 3. Tensile mechanical properties and nanostructure characterization of the prepared fibers. (a) Stress–strain curves for nanostructured fibers made from fibrils of different lengths indicating their influence on tensile mechanics of CNF fibers prepared from double flow-focusing channel measured at 50% relative humidity (RH). Effect of (b) physical (RH) and (c) chemical (cross-linking with BTCA) approaches for tuning the tensile mechanical properties of fibers (prepared from CNF-550 suspension). Plots comparing (d) Young’s modulus and ultimate strength of different fibers prepared in this work. LHC and CL stand for low humidity condition and cross-linking, respectively. Error bars correspond to the standard deviation obtained from 10 samples for each case. (e) Azimuthal integration of the (200) scattering plane of the diffractograms for CNF-550 and CNF-1360 samples. Diffractogram corresponding to a CNF-550 fiber is shown in the inset.

m value of 28.9, indicating very few defects inside the fibers. As one might partially expect from previous studies of bioinspired nanocomposites, strengthening of interfibrillar interactions and removal of water molecules from the fibril surface resulted in marked improvement of fiber mechanics with respect to other CNF composites but also led to embrittlement and low failure strain. At high humidity, water acts as a plasticizer and allows extended elastic deformations, thus reducing the stiffness and increasing the strain. To further extend the property range of these materials, we evaluated different approaches based on physical (varying the ambient condition) and chemical (covalent cross-linking of the fibrils) strategies. With CNF-550 fibers conditioned at 14% RH for a period of 40 h before testing, CNF-LHC exhibited a modulus and strength values as high as 82 ± 4 GPa and 1320 ± 85 MPa, respectively ( Figure 3 b). Similar strength (1320 ± 56.5 MPa) is calculated from the Weibull analysis, (25,26) with avalue of 28.9, indicating very few defects inside the fibers. As one might partially expect from previous studies of bioinspired nanocomposites, strengthening of interfibrillar interactions and removal of water molecules from the fibril surface resulted in marked improvement of fiber mechanics with respect to other CNF composites but also led to embrittlement and low failure strain. At high humidity, water acts as a plasticizer and allows extended elastic deformations, thus reducing the stiffness and increasing the strain. (27) To reduce the humidity effect, the CNF-550 fibers were cross-linked by 1,2,3,4-butane tetracarboxylic acid (BTCA) that was used to neutralize the suspensions during the flow-based assembly of the fibers. BTCA creates covalent bridges between CNF fibrils, (28) replicating to some degree the cross-links between cellulose and lignin/hemicellulose. (6) The average strength of cross-linked fiber (CNF-CL) tested at 50% RH increased to 1430 MPa (highest measured value of 1570 MPa) with negligible change in modulus ( Figure 3 c,d). Chemical cross-linking introduces covalent bonds between the fibrils, which improves the connectivity and stress transfer. (29)

Additionally, the high strain to failure obtained for the un-cross-linked CNF fibers (∼6%) is rather uncommon for highly oriented structures. Structural changes and interfibrillar molecular interactions were further investigated by cyclic loading–unloading tests in the post-yield regime ( Figure S5 ). The post-yield modulus may decrease due to the formation of cracks or increase due to reorientation of the fibrils (as for the case of random-in-plane CNF network) along the test direction. (30) Interestingly, the Young’s modulus in the present case remains unchanged upon unloading at post-yield strain values (see Methods for details). This suggests a lack of structural changes, and the mechanisms of plastic deformation must involve reformable secondary bonds as in the case of stick–slip. (31) This was further verified by molecular dynamics (MD) simulations ( Figure S6 ), where the number of hydrogen bonds (per unit area) remains constant during the relative sliding of the fibrils. (32) Upon cross-linking the CNF fibers, the plastic deformation is substantially reduced, and the stress–strain curve becomes relatively linear as the secondary interactions are replaced by covalent bonds ( Figure 3 c). Although interfibrillar interactions are dominated by the relatively weak hydrogen bonds and van der Waals forces, the highly aligned state of the fibrils amplifies their effects due to collective synergy of molecular interlocking, leading to stiffening and effective energy dissipating mechanisms (stick–slip and molecular zip-up). (31,33)

It is worth highlighting that even the properties of the “weaker” fiber (CNF-1360) have previously been unachievable for CNF fibers fabricated with other approaches. (34−36) Hence, there is a strong and profound benefit of exploiting extensional flow fields for alignment and assembly of nanofibrils (or elongated particles, in general), giving a fresh insight for the proper selection in future high-performance fibers by getting closer to the theoretical limit. Further, the orientation index values for highly aligned CNF-550 reported in this work are only slightly higher than those reported for CNF-based macrofibers fabricated with other approaches. (9,27,37,38) However, the up to 6 times higher strength of our fibers ( Table S1 ) indicates that even in fibers with aligned nanoscale building blocks, interfaces and interactions play a key role in controlling the mechanical properties.

The increased strength and stiffness values of the CNF fibers made by the double flow-focusing method make it feasible to use them for numerous load-bearing applications. (39) The materials data chart ( Figure 4 ) demonstrates that CNF fibers have strength and stiffness that markedly exceed all natural and commercial bio-based materials. This includes natural wood pulp fibers with high orientation of nanoscale crystalline planes, free from damage and natural imperfections (see Methods for details), and wet-spun high aspect ratio nanocelluloses that recently attracted considerable interest. (24,34,35) The latter methods as well as other fiber-drawing techniques used in the past for carbon nanotubes are shear dominated, (40) where a large nozzle diameter is used and thick fibers (25–200 μm) are formed. Consequently, a fibrillar network of lower density and more random orientation of the fibrils are formed, which compromises load transfer between nanoscale building blocks in the macroscopic material. Fibers fabricated through assembly of CNF with these approaches never managed to reach stiffness and strength beyond 35 GPa and 600 MPa, respectively. (9,13)

Figure 4 Figure 4. Tensile mechanical properties of bio-based and selected synthetic fiber materials. Overview of specific ultimate strength versus specific Young’s modulus for a range of bio-based materials, steel, and E-glass from the non-bio-based resources. The region of fibers fabricated in the present work is shown in dark gray. Details on the mechanical properties’ data drawn for different materials are included in the Supporting Information (Table S2).