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Solution-processable ferroelectric nylons


Scientists at the Max Planck Institute for Polymer Research (MPI-P) led by Dr. Kamal Asadi in cooperation with colleagues of Johannes Gutenberg University of Mainz (JGU) have solved a four-decade challenge of producing very thin nylon films that can be used, for instance, in electronic memory components.

The thin nylon films are several 100 times thinner than human hair and could thus be attractive for applications in bendable electronic devices or for electronics in clothing. The research results have now been published in Science Advances.

As the microelectronic industry is now shifting toward wearable electronic gadgets and electronic (e-)textiles, the comprising electronic materials such as ferroelectrics should be integrated into our clothes. Nylons, a family of synthetic polymers, were first introduced in the 1920s for women's stockings and are nowadays among the most widely used synthetic fibers in textiles. They consist of a long chain of repeated molecular units, so-called polymers, where each repeat unit contains a specific arrangement of hydrogen, oxygen, and nitrogen with carbon atoms.

Besides the use in textiles, it was discovered that some nylons also exhibit so-called ferroelectric properties. This means that positive and negative electric charges can be separated and their state can be maintained. The thereby resulting dipole density is inversely proportional to the length of the aliphatic chain connecting the amide groups along the polymer. Such a polymer with a very short "spacer" is called Nylon-5. This material was synthesized in collaboration with Professor Holger Frey of JGU and his Ph.D. student Philipp von Tiedemann.

The ferroelectric materials are used in sensors, actuators, memories, and energy harvesting devices. The advantage in using polymers is that they can be liquified using adequate solvents and therefore processed from solution at low cost to form flexible thin-films suitable for electronic devices such as capacitors, transistors, and diodes. This makes ferroelectric polymers a viable choice for integration with e-textiles. Although nylon polymers have found significant commercial applications in fabrics and fibers over the years , their application in electronic devices was hindered because it was impossible to create high-quality thin films of ferroelectric nylons by solution processing.

Scientists at the MPI-P in collaboration with researchers from Johannes Gutenberg University Mainz and Lodz University of Technology have now solved this forty-year-old problem. They developed a method to fabricate ferroelectric nylon thin-film capacitors by dissolving nylon in a mixture of trifluoroacetic acid and acetone, and solidifying it again in vacuum. They were able to realize thin nylon films that are typically only a few 100 nanometers thick, several 100 times thinner than human hair.

"Using this method, we have produced extremely smooth thin films. This is very important because it prevents, for example, electrical breakdown of capacitors and the destruction of electronic circuits. At the same time, the smoothness allows for having transparent thin films and eventually transparent electronic devices", said Dr. Kamal Asadi, group leader at the MPI-P.

By using their newly developed method, Asadi's group was able to produce hig- performance nylon capacitors. The scientists subjected the prototypes of the capacitors to extended stress cycles and demonstrated robustness of ferroelectric nylons under millions of operation cycles. The thin nylon films could become an important component for use in flexible electronics in the future and find applications in bendable electronic devices or for electronics in clothing. These new findings pave the way towards multi-functional fabrics that serve as cloth for covering our body and at the same time can generate electricity from our body movement.

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Source: University of Mainz