The new superstrong

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Posted on July 12, 2013
Mechanical and industrial engineering assistant professor Marilyn Minus has developed a superstrong fiber that rivals the best in the industry. Credit: Mary Knox Merrill

Mechanical and industrial engineering assistant professor Marilyn Minus has developed a superstrong fiber that rivals the best in the industry. Credit: Mary Knox Merrill

In today’s market for high per­for­mance fibers, used for appli­ca­tions such as bul­let­proof vests, man­u­fac­turers have only four options: Kevlar, Spectra, Dyneema, and Zylon. Made from poly­mers such as poly­eth­ylene, these were the strongest syn­thetic fibers in the world—until recently.

Mar­ilyn Minus, an assis­tant pro­fessor of engi­neering at North­eastern, has devel­oped a type of fiber that is stronger than the first three com­mer­cial prod­ucts men­tioned above, and—even in its first generation—closely approaches the strength of the fourth (Zylon).

Adding small amounts of carbon nanotubes—straight, cylin­drical par­ti­cles made entirely of carbon—to polymer fibers increases their strength mar­gin­ally. But as a grad­uate stu­dent at the Georgia Insti­tute of Tech­nology five years ago, Minus fig­ured that with a little more con­trol, she might be able to turn those modest improve­ments into dra­matic ones. She has spent the last four years at North­eastern proving her hunch.

The fibers cre­ated by Minus’ team are shown in red. Tuning the crys­tal­liza­tion process makes them stronger than any other mate­rial on the market except for Zylon. Image cour­tesy of Mar­ilyn Minus.

In a paper recently released in the journal Macro­mol­e­c­ular Mate­rials and Engi­neering, Minus pre­sented a tun­able process for cre­ating super-​​strong fibers that rival the industry’s very best. As with pre­vious work, Minus’ method inte­grates carbon nan­otubes into the polymer fiber, but rather than serving as simply an added ingre­dient, the nan­otubes now also per­form an orga­ni­za­tional role.

From carbon black powder to metallic par­ti­cles, a variety of mate­rials can guide the for­ma­tion of spe­cific crystal types in a process called nucle­ation. But before carbon nan­otubes, Minus said, “we’ve never had a nucle­ating mate­rial so sim­ilar to poly­mers.”
This sim­i­larity allows the nan­otubes to act likes skates along which the long polymer chains can slide, per­fectly aligning them­selves with one another.

After using tuning the crys­tal­liza­tion process, elec­tron micro­scope imaging shows that the nan­otubes inside the fiber are coated in polymer. Image cour­tesy of Mar­ilyn Minus.

But it’s the crys­tal­liza­tion process that drives the remark­able prop­er­ties recently reported. In their research, Minus and her col­leagues showed that they could easily turn these prop­er­ties on or off. By changing nothing but the pat­tern of heating and cooling the mate­rial, they were able to increase the strength and tough­ness of fibers made with the very same ingre­di­ents.
In the cur­rent research, Minus and her col­leagues worked out the recipe and process for one par­tic­ular polymer: polyvinyl alcohol. “But we can do this with other poly­mers and we are doing it,” she said.

Simply com­bining the nan­otubes and polymer does not induce the polymer to uni­formly coat the nan­otube. Image cour­tesy of Mar­ilyn Minus.

With funding from a new grant from the Defense Advanced Research Projects Agency, Minus will now work out the method for a polymer called poly­acry­loni­trle, or PAN. This is the dom­i­nant mate­rial used to form carbon fibers, which are of par­tic­ular interest in light­weight com­posite mate­rials such as those used in the Boeing 787 air­liner. With the more orga­nized struc­ture afforded by Minus’ method, this mate­rial could see a vast increase in its already great performance.

Source: Northeastern University