High-performance fibers are key to many structural and lightweight applications as they are the main reinforcing components of continuous fiber-reinforced composites and fabrics. These fibers are drawn from synthetic polymers such as poly(p-phenylene terephthalamide) (PpPTA) or ultrahigh-molecular-weight polyethylene (UHMWPE). During the drawing process, highly oriented and crystalline nanofibrils of 10–50 nm width form and assemble into larger bundles of 100–500 nm width, thereby creating a hierarchical microstructure. Although it is known that hierarchy generally improves the properties of materials, its role in the failure of synthetic fibers has not been studied. In particular, measuring properties at the intermediate submicrometric scale is challenging. To better understand the role of this organization in the mechanical performance of fibers, the research groups of Yuris Dzenis at the University of Nebraska–Lincoln and Kenneth Strawhecker at the US Army Research Laboratory have taken up the challenge and studied the mechanical interactions between the bundles. Their results were published in ACS Applied Materials and Interfaces (doi:10.1021/acsami.9b23459).

To measure the properties at the submicrometric scale, they developed a special protocol in which a T-shaped notch was first cut using focused ion beam milling. Then, a scanning tunneling microscope probe was inserted into the notch to peel away a thin layer of the fiber that delaminated at the bundle interface. The uncovered polymeric surface was then probed by nanoindentation to measure the interfacial separation energy absorption between bundles. Taylor Stockdale, the first author of the article, explains that “by performing repeated indents at the same location, we could subtract the absorbed energy due to elastic recovery and gain better insight into the energy required to separate two bundles.”

Electron micrograph showing the T-shaped notch made by focused ion beam and cartoon showing the peeling process using the scanning tunneling microscope (STM) probe. Credit: ACS Publications.

Applying this method to two polymers with different crystallinity and chain flexibility, UHMWPE and PpPTA, it was found that both fibers had similar microstructures and tensile properties, and they showed intermediate scale fib-rillation between bundles under tensile failure. However, the interactions between bundles were stronger for PpPTA as compared to UHMPWE, presumably due to more interconnected crystals.

Another interesting finding was that the energy absorbed at the interfaces between the bundles was more than 10 times higher than the energy absorbed at the nanofibril level, of ∼13–27 J.m^–2 and ∼0.3–0.5 J.m^–2, respectively. Although structures fail at their weakest point, the fibrillation happened at the bundles level in their experiments. The researchers point out that to understand the real fracture of the fibers in tension mode, direct in situ characterization of nanofibrils and bundles would be required.

This study contributes to our understanding and quantification of the interaction mechanisms in high-performance fibers. This research could lead to further enhancement of fiber properties by developing a drawing process that would result in an optimum microstructure. Among the many avenues the researchers plan to pursue, “further in situ multiscale testing and extracting individual nanofibrils and nanofibril bundles to perform tensile tests would be of great value,” says Dzenis. “The results can lead to new fundamental scaling models of the discovered unique fractal fracture behavior of hierarchical high-performance fibers.”

Flavia Libonati, an associate professor at the Università di Genova, Italy, and affiliated with the Laboratory for Atomistic and Molecular Mechanics at the Massachusetts Institute of Technology and who did not participate in this study, says that “the fracture mechanisms resemble the failure of fibers present in natural and biological materials and, in particular, the role of the interfaces in the load transfer and the importance of hierarchy on the amplification of the mechanical performance with respect to the building blocks. A deeper understanding of such mechanisms and of the processing–structure–property relationships, via multiscale modeling and experiments, can pave the way toward the design of better advanced materials.”