A collaboration led by researchers at the University of Bayreuth in Germany has discovered and optimized a polymer fiber with especially high strength and high toughness. The team—which also includes researchers from the Jülich Centre for Neutron Science, Martin Luther University of Halle-Wittenberg, Fraunhofer Institute for Microstructure of Materials and Systems, and RWTH Aachen University in Germany, as well as from Jiangxi Normal University in China and ETH Zürich in Switzerland—reports in the journal Science that the fiber is composed of thousands of highly aligned nanoscale fibers and exhibits mechanical properties similar to the most robust spider silks. The discovery also suggests guiding principles for fabricating strong, tough fibers from other polymers.

Xiaojian Liao, a PhD student supervised by Andreas Greiner and Seema Agarwal at the University of Bayreuth, made the discovery while working with the polymer polyacrylonitrile (PAN). The researchers were trying to create high performance PAN fibers using electrospinning. The fibers had a yarn-like structure with a cross section of 3,000 nanoscale fibers, or nanofibrils. In an attempt to stabilize the nanofibrils for chemical processing, the researchers added the cross-linker poly(ethylene glycol) bisazide (PEG-BA) to a PAN solution before electrospinning. The resulting fiber was stretched and annealed under tension to promote linking between PAN macromolecules. During a routine check of the fiber’s mechanical properties, Liao noticed an unexpected and dramatic increase in strength and toughness. “From that moment on, we were off-routine,” says Agarwal.

Materials with simultaneous high strength (resistance to deformation) and high toughness (resistance to fracture) are in demand for many applications, but the two properties do not often coexist in materials, including fibers. Small-diameter nanofibers with these properties have been made by electrospinning, but they are too fragile for most real-world applications. The fiber created by Greiner’s team demonstrated mechanical properties unprecedented in robust synthetic fibers.

To isolate the source of these properties, the team fabricated PAN fibers without PEG-BA and subjected them to different stretching and annealing conditions. Samples were characterized by stretch ratio (stretched length divided by as-spun length), nanofibril diameter, and nanofibril alignment factor (ranging from 0 for isotopically oriented nanofibrils to 100% for perfectly aligned fibrils). As-spun samples featured a stretch ratio of 1, nanofibril diameters around 1.17 μm, and an alignment factor of about 46.0%.

Three-dimensional x-ray images revealed that nanofibrils in a PAN fiber untwisted and aligned during stretching. Alignment increased with increasing stretch ratio, reaching 99.6% when the fiber was stretched at 160°C to a stretch ratio of 9. The nanofibrils decreased in diameter under stretching to about 0.37 μm. Annealing the fibers under tension had no discernable impact on alignment or diameter.

Next, the researchers fabricated PAN fibers with different concentrations of PEG-BA and subjected them to stretching and annealing under different conditions. Adding PEG-BA had no impact on the nanofibril diameters before or after stretching, but had significant impact on the mechanical properties of the fiber. Tensile tests revealed that the strength of PEG-BA-containing yarns significantly increased with stretching, while toughness significantly increased with annealing.

By experimentally optimizing the concentration of PEG-BA (4% by weight), stretching conditions (stretch ratio of 8 at 160°C), and annealing conditions (4 hours at 130°C), the researchers created fibers with an ultimate tensile strength of 1236 ± 40 MPa and toughness of 137 ± 21 J/g. For comparison, untreated yarns with no PEG-BA had a strength of 72 ± 3.0 MPa and toughness of 76 ± 10 J/g. The optimized fiber had a Young’s modulus equal to that of dragline spider silk, which forms the radial spokes of spider webs and the lifelines by which spiders hang down from the ceiling.  

Polarized Raman spectroscopy revealed that during stretching, PAN macromolecules in the nanofibrils became increasingly aligned with the fiber’s main (long) axis. Wide-angle x-ray imaging showed that crystallinity rose significantly during stretching, from about 59% to 92%. According to the researchers, the dramatic increase in fiber strength likely resulted from the combination of increased crystallinity and increased alignment of the nanofibrils due to stretching.

The dramatic increase in toughness was limited to concentrations of PEG-BA at or below 4% by weight. Experiments with higher concentrations of PEG-BA showed reduced strength and toughness and signs of molecular interactions between the cross-linker and bulk PAN. At concentrations of 4% by weight or less, no molecular weight changes were discernable. “The most plausible explanation for these findings is that the reaction occurs only on the surface of the fibrils,” writes the team.

The researchers propose that as nanofibrils crystallize and their diameters shrink under stretching, the free volume in the polymer is reduced. As a result, PEG-BA macromolecules diffuse to the surface of the fibrils. Then, under annealing, the macromolecules interact to form bridges between nanofibrils that reinforce weaker fibers and increase the yarn’s resistance to fracture. This model suggests three key conditions for extending this work to other polymers: inherently strong fibers, an aligned multifibrillar architecture, and a mechanism for interconnecting fibrils.

“There is a growing consensus that lateral interactions between the crystalline nanoscale structural units in high performance polymer fibers, such as para-aramids and ultrahigh molecular weight polyethylene fiber, are critical to their overall mechanical behavior,” says Yuris Dzenis, a researcher at the University of Nebraska-Lincoln with expertise in advanced nanomaterials and nanomanufacturing techniques, including electrospinning. “The work by Greiner’s group shows that deliberate control of such interactions by introducing linker molecules can lead to increased fiber toughness.”

“While many aspects of the process and the mechanical behavior of the resulting nanostructured yarns are yet to be studied and clarified, the work opens up new opportunities in the field of nano-engineered high-performance polymers that can be achieved at a potentially low cost,” Dzenis says.

The research team is interested in extending this process to other polymers and exploring dozens of other avenues of related research. Among them are better characterization of the material, probing potential applications, and improving processing techniques. “For this we need more hands and machines for yarn preparation,” Greiner says.

Read the abstract in Science.