Lateral bridging features are visible at multiple length scales in the interior of peeled high-performance fiber. Nanoscale fibrils span between neighboring fibril bundles throughout the exposed internal surface and near the crack tip; a fibril bundle is seen spanning between the two crack faces (top). These lateral bridging features may provide key insights to better understanding of complex hierarchical fiber failure behavior. (Image Credit: Joel Brehm, Taylor Stockdale, and Yuris Dzenis; University of Nebraska-Lincoln).High-performance sporting goods, protective armor, and lightweight aerospace components are widely available now thanks to advanced polymer fibers. But despite their widespread application, the reasons for their exceptional mechanical performance is not clear and there have been few major breakthroughs in fiber chemistry, processing or performance in recent years. Now researchers led by Yuris Dzenis of the University of Nebraska and Kenneth Strawhecker of the U.S. Army Research Laboratory in Maryland have uncovered how lateral interactions at different scales determine the performance of these useful polymers [Stockdale et al., ACS Applied Materials & Interfaces 12 (2020) 22256–22267, https://dx.doi.org/10.1021/acsami.9b23459].
“Advanced fibers produced a revolution in structural materials and applications; however, their mechanical behavior through failure is not yet fully understood. We wanted to understand hierarchical failure mechanisms better in these highly anisotropic, high-performance ballistic fibers,” explains Dzenis.
The researchers examined the nano- and microscale structure in examples of the two most common types of high-performance ballistic fibers, rigid-rod para-aramid poly(p-phenylene terephthalamide (or PPTA) and long- chain ultrahigh molecular weight polyethylene (UHMWPE). Both these fibers have a highly hierarchical structure, formed during the drawing process. Microfibrils 10−50?nm in diameter, which the researchers note would be better termed ‘nanofibrils’, form bundles 100−500?nm in width, that together make up micron-sized fibers. While interfacial interactions between nanofibrils have recently been analyzed, no studies have yet been performed at the intermediate scale, point out the researchers. The first hurdle to overcome was sample preparation.
“In previous work, we accessed the fiber interior using reciprocal focused ion beam (FIB)-notches that produce mirror-image fracture surfaces in the two fiber halves, which could then be analyzed by atomic force microscopy,” explains first author Taylor A. Stockdale. “However, this approach is not suitable for the higher loads needed to probe at the intermediate scale. How, then, do you access the fiber interior without contacting the newly exposed surface, while leaving the fiber intact? That was the initial problem to solve.”
Stockdale developed a new sample preparation approach, which uses FIB to cut T-shaped notches in fiber samples that, when peeled, produce long lengths of minimally disturbed internal surfaces. The researchers used nanoindentation to determine the energy required to separate nanofibrils and microscale fibril bundles in each type of high-performance fiber. Nanoindentation analysis at this intermediate lengthscale allowed the researchers to pinpoint and compare lateral interactions at different scales, enabling a clearer picture of advanced fiber mechanical behavior to emerge.
“Interfacial separation energies measured at this scale for the first time for para-aramid and UHMWPE fibers correlate with and provide an explanation of different fibrillation in these fibers during failure,” says Dzenis.
While one might expect more rigid molecules such as PPTA to show longer interfacial fracture lengths, the opposite is observed. The researchers’ nanoindentation studies found stronger separation resistance in PPTA fibers than in the more flexible, long-chain UHMWPE fibers. The performance of these fibers must, therefore, be governed by other mechanisms of lateral interaction. In the hierarchical structure of these materials, nanofibrils are connected together laterally by tie-molecules, nanofibrils connect together fibril bundles, and the final fiber itself is held together by fibril bundles. Separation energies at the nanoscale are low, but at the intermediate scale the researchers recorded higher separation energies. At the macroscale, separation energies are higher still. The results, say the researchers, indicate that there are three distinct scales of lateral interaction in high-performance fibers, determined by the size and frequency of or distance between the tie-molecules, nanofibrils, and fibril bundles. The approach offers a means of predicting when and how high-performance fibers will fail and could provide the basis for entirely new models of fiber fracture mechanics.
“This work opens up new opportunities for further optimization of fiber processing, structure, and properties, which could lead to new, qualitative advances,” say Dzenis and Strawhecker.
David C. Martin, Karl W. and Renate Böer Professor of Materials Science and Engineering at The University of Delaware, believes the results are interesting.
“Understanding the lateral mechanisms of deformation and failure in high strength fibers like these remains an ongoing challenge and is still not well understood,” he says. “While novel synthesis methods used to be fairly common in this field, there hasn’t been as much novel chemistry/molecular design recently, in my opinion. I’m looking forward to [seeing] what [the researchers] will find on a system where the chemistry or structure can be manipulated in a systematic way. Hopefully this [approach] will reveal additional details about how to continue to refine the molecules that could be used in such systems.”
This article was originally published in Nano Today 34 (2020) 100973.