Nylon is obviously a choice material for electronic textiles — since there's an established nylon-based textile industry. But the substance also has a piezoelectric crystalline phase via which one tap gives it a build-up of charge ideal for pressure sensing and harvesting energy from ambient motion.
And according to a recent study published in the journal Advanced Functional Materials, scientists have finally found a way to produce the tricky crystal phase with relative ease.
Nylon finally gains official piezoelectric textile status
Sadly, forming nylon into fibers without sacrificing the key crystal structure — needed for the piezoelectric response — isn't a simple project. "This has been a challenge for almost half a century," explained Kamal Asadi, a researcher at Germany's Max-Plank Institute for Polymer Research and professor at the University of Bath in the U.K.
The piezoelectric phase of nylon is appealing not only for electronic textiles, but also for various types of electronic devices, especially where demand exists for something less brittle than the conventional piezoelectric ceramics.
However, the feasible way to produce nylon in decades past while retaining the crystalline phase with a strong piezoelectric response was to melt it, cool rapidly, and finally stretch it to ensure it sets into what's called a smectic δ' phase, reports Phys.org.
How nylon displays piezoelectric behavior
Piezoelectric behavior happens via the amide moieties on the repeating units of a nylon polymer chain, in addition to their interaction with other amide moieties in adjacent chains. When amides are free to align their dipoles in an active electric field, people can exploit the piezoelectric effect of the material in a reaction initially observed back in the 1980s.
However, typically these amides form strong hydrogen bonds with amides on adjacent polymer chains, which locks them into position and stops them from reorienting and aligning, which is critical.
The scientists of the study needed to find a way to induce the phase that leaves inherent amides free to reorient without limiting their morphologies — so they can obtain them without relying on the melt, cool, and stretch out approach of the past.
Researchers try to find new path to piezoelectric nylon
Most research groups worldwide had abandoned their efforts to create piezoelectric films or fibers since the 1990s, but the entrance into Asadi's research group of a "brilliant student who was a textile engineer" — named Saleem Anwar — gave Asadi the impetus to take one more look at the challenge.
The research team considered the crucial factors needed to produce nylon in a phase with strong piezoelectric properties. The typical melt, cool, and stretch method relies on rapidly cooling the nylon — which is why Asadi, Anwar, and their collaborators examined ways to achieve the same result via dissolving the nylon in a solvent, before rapidly extracting the solvent.
Unfortunately, the solvents tend to dissolve the nylon via attacking the hydrogen bonds between the amides — creating hydrogen bonds in their place, which makes extracting the solvent practically impossible, reports Phys.org.
Student has nylon 'eureka' moment with TFA solvent
The big day came when Anwar told Asadi he'd seen something weird while cleaning up with acetone following an experiment — wherein the team had tried to produce nylon films via the use of trifluoroacetic acid (TFA) as the solvent. The nylon solution spills had shifted to a transparent state.
Wondering if this sudden transparency signaled a reaction in progress, the team made a TFA and acetone solution, and then tried to process nylon. The next week, "Saleem came back with his 'eureka' moment — 'I have it!'" exclaimed Asadi, for Anwar.
Anwar had stumbled into the hydrogen bond between acetone and TFA — which is one of the strongest hydrogen bonds known to modern science. So when the researchers placed the solution on a substrate inside a high vacuum to evaporate the solvent, it was "literally like the acetone takes the hand of the TFA molecules and carries them out of the nylon, yielding the piezoelectric crystalline phase," said Asadi.