New tricks help find the strength in fibers

Nanoscale fibers boast impressive mechanical properties often exceeding those of their bulk companions. However, larger-scale materials created from those nanofibers do not always match up to predictions. Now two pieces of research indicate promising strategies for translating the exceptional attributes of nanoscale fibers like carbon nanotubes and cellulose nanofibrils into macroscale materials.

Carbon nanotubes (CNTs) are touted as one of the strongest known materials, but usable fibers made from nanotubes do not achieve the same the impressive physical prowess. The reason is simple: the presence of defects, impurities, random orientations, and different length nanotubes add up to a fiber with compromised strength. Now, however, researchers report that a simple stretching and relaxing process can release initial non-uniform strains in CNT bundles and enable the fabrication of much stronger fibers [Bai et al., Nature Nanotechnology (2018), https://doi.org/10.1038/ s41565-018-0141-z].

“CNTs [have] inherent tensile strength higher than 100 GPa but almost all reported CNT fibers are fabricated using agglomerated CNTs or vertically aligned CNT arrays with components shorter than hundreds of microns and containing plenty of structural defects and impurities, rendering their tensile strengths in the range of 0.5–8.8 GPa,” explains Rufan Zhang of Tsinghua University.

Along with colleagues at Stanford University, the team used a simple approach to produce centimeter-long bundles of ultralong, defect-free CNTs with a tensile strength of over 80 GPa. The key to the strength of the bundles is the way in which the CNTs are produced.

The researchers use gas-flow-directed chemical vapor deposition to synthesize ultralong nanotubes, which have at least one perfectly structured wall. A gas flow focusing strategy gradually assembles the as-grown CNTs via van dear Waals forces into ultralong bundles. Next, however, the researchers undertake a careful process of tightening and relaxing the fiber bundles, which releases the internal strains as the component nanotubes shrink and slip over each other. After repeated cycles of stretching and relaxing, the nanotubes are more uniformly aligned in the bundles and the internal strains are more similar.

The simple process appears to boost the tensile strength of nanotube bundles from as little as 47 GPa to as much as 80 GPa. The researchers believe that their approach could provide a way of synthesizing superstrong fibers, although the issue of producing high quality, ultralong CNTs remains.

“The researchers have made a nice step in terms of achieving bundles of SWCNTs of very high quality that, through a method similar to engineering methods used with bridge cables (bundles composed of many individual wires that all bear load), could exhibit high intrinsic as well as engineering strength,” comments Rodney Ruoff of Ulsan National Institute of Science and Technology (UNIST) in Korea. “It is important to note that these are bundles, not fibers, and that a significant challenge remains in achieving very long fibers composed of CNTs that would also exhibit exceptional strengths.”

Similar issues afflict cellulose nanofibrils, which are the most abundant structural component in living systems like trees and plants. Cellulose nanofibrils have high strength and stiffness but attempts to produce artificial analogues have, to date, produced composite materials up to 15 times weaker.

“One of the biggest challenges in fabricating engineering materials that make use of the often-exceptional properties of nanoscale building blocks is the retention of these properties [at the macroscale],” says L. Daniel Söderberg of KTH Royal Institute of Technology in Sweden.

Together with colleagues at RISE Bioeconomy, DESY in Germany, Stanford University and the University of Michigan in the USA, Söderberg has fabricated an engineering material using nanocellulose that does retain these exceptional mechanical properties [Mittal et al., ACS Nano (2018), https://doi.org/10.1021/ acsnano.8b01084]. The team created continuous fibers (or filaments) from very slender fibrils of nanocellulose, derived from conventional paper pulp fibers.

The key to success is the alignment of the nanocellulose fibrils in the fibers. The researchers first dispersed nanocellulose fibrils in water and used a micro-fluidic concept called flow focusing to process the dispersion into fibers. By excluding Brownian diffusion, which would allow the fibrils to rotate, the process aligns the fibrils along the length of the fibers. The aligned structure is then locked into a gel network by lowering the pH. A continuous fiber can be extracted from the gel, with no restriction on length.

“[Our] continuous, well-defined fibers (or filaments), made from 100% bio-based components (with no fossil-based additives), have a mechanical performance on the same level as glass and Kevlar fibers and perform better than the attributed strength and stiffness of spider dragline silk, widely thought of as the strongest bio-based material,” points out Söderberg.

The process allows the excellent strength and stiffness of nanocellulose fibrils to be translated into engineering-scale fibers. Although the team is only making small amounts of fiber at the moment, they are working with the Swedish research institute RISE Bioeconomy to scale-up the process to produce fibers continuously at high speeds.

“Using these fibers, it will be possible to fabricate 100% bio-based lightweight composites for structurally demanding applications such as automotive products,” says Söderberg. “And because cellulose is compatible with biological tissue, we envisage that materials with our fibers as key components of scaffolds and load-bearing applications in medicine.”

Söderberg believes that applications in medicine could come within the next five years, with lightweight, load-bearing construction applications taking slightly longer to realize.

Markus J. Buehler, McAfee Professor of Engineering at Massachusetts Institute of Technology, agrees that many of the researchers’ ideas could be translated to engineered materials.

“The study reports impressive results that showcase the translation of a biological design paradigm into engineered materials, addressing one of the most challenging problems today," he comments. "The unique architecture is achieved by a clever engineering of the processing of the material, similar to what we see in many other biomaterials such as silk, where an interplay of fluid mechanics, chemistry, and the design of the constituting building blocks leads to the final high-performance material, and ultimate nano-level geometry control that is critical for the outcome.”

Buehler believes that the work offers important insights into the design of hierarchical materials that translate nanoscopic properties to the macroscale.

This article was originally published in Nano Today 21 (2018) 2-4.