Nylon seems to be the material of choice for piezoelectric textiles, because the textile industry based on nylon has been very mature, and nylon also has a convenient piezoelectric crystal phase. If you tap it, you will get a perfect charge accumulation. Pressure sensing and harvesting energy from environmental movement.
However, it is not simple to form nylon into a fiber while giving it a piezoelectric crystal structure. Kamal Asadi, a researcher at the Max Planck Institute for Polymer Research in Germany and a professor at the University of Bath in the UK, explained: "This is almost a challenge for half a century." In a recent "Advanced Functional Materials" report, he and His collaborators described how they solved the problem.
The piezoelectric phase of nylon is not only attractive to electronic textiles, but also attractive to various electronic devices, especially for traditional piezoelectric ceramics with high brittleness. However, for decades, the only way to produce crystalline nylon with a strong piezoelectric response is to melt it, cool it quickly, and then stretch it to condense into a smooth δ'phase. The resulting structure is usually tens of microns thick, which is too thick for electronic equipment or electronic textile applications.
The existence of "piezoelectric behavior" stems from the amide groups on the repeating units in the nylon polymer chain and the interaction between them and the amide groups on the adjacent chains. When these amides freely align their dipoles with the electric field, it is possible to take advantage of the piezoelectric effect in the material, which was first observed as early as the 1980s. However, what happens in most crystalline phases of nylon is that these amides form strong hydrogen bonds with amides on other polymer chains, locking their positions and preventing them from reorienting and aligning. Therefore, the challenge we face is to find a way to produce a phase that allows the amide to reorient freely without restricting the morphology it produces during melting, cooling, and stretching.
When most research groups in the world had given up efforts to produce piezoelectric films or fibers in the 1990s, Asadi’s group came to an "outstanding student of textile engineer" Saleem Anwar, which prompted Asadi to pay attention to this issue. . The researchers first considered the basic factors for producing nylon in a phase with strong piezoelectric properties. The method of melting, cooling and stretching depends on rapidly cooling nylon, so Asadi and Anwar and their collaborators studied how to obtain the same effect by dissolving nylon in a solvent and then quickly extracting the solvent. However, solvents tend to dissolve nylon by attacking the hydrogen bonds between amides and form hydrogen bonds in their positions, so it is almost impossible to get rid of the solvent.
One day, when Anwar used acetone to clean up after an experiment, he told Asadi a strange observation that he had tried to use trifluoroacetic acid (TFA) as a solvent to produce nylon film and made a breakthrough. The overflowing nylon solution became transparent. The team suspected that the sudden transparency must indicate a reaction, so they used trifluoroacetic acid and acetone to make a solution and tried to use it to process nylon. Sure enough, the following week, the researchers obtained the desired results.
Anwar accidentally discovered the hydrogen bond between acetone and TFA, which is one of the strongest hydrogen bonds known in the scientific community. Therefore, when the researchers spread the solution on a high-vacuum substrate to evaporate the solvent, as Asadi said: "It's almost like acetone is holding the TFA molecules by the hand, bringing them out of nylon, creating a piezoelectric crystal phase."
The researcher was the first to produce a nylon film with a strong piezoelectric response. But this does not completely solve the problem of producing fibers, because the production method is still incompatible with high vacuum. So they studied other ways to control the solvent extraction rate. They focused on the production of fibers by electrospinning. In the electrospinning process, the electric field attracts the polymer solution into the fibers with a diameter as small as tens of nanometers in width. The high surface area ratio of the fiber produces high solvent extraction. rate. Then, the key is to balance this with the viscosity of the polymer solution and electrospinning conditions so that other factors do not prevent the formation of fibers in the precious δ'stage.
Researchers have found that fibers about 200 nanometers wide have a "sweet spot" among competing factors. The measurement results of the potential generated under a periodic mechanical shock with a frequency of 8 Hz show that the 200-nanometer δ'phase fiber generates 6 V, while the narrower fiber generates less than 0.6 V because of these widths. Factors related to narrowness cause the phase formed by the fiber to have no piezoelectric response.
In fact, in a wide fiber around 1000nm, the fiber is too thick to extract the solvent efficiently and quickly. Nylon forms a γ crystalline phase with only a weak piezoelectric response. In thicker fibers, the piezoelectric response of the γ phase is poor, which is compensated to some extent by the larger fiber volume, which results in a 4V potential. However, the 200nm δ'phase fiber still has the advantage of a more sensitive response.
