Solution-processed transparent ferroelectric nylon thin films

Abstract

Ferroelectricity, a bistable ordering of electrical dipoles in a material, is widely used in sensors, actuators, nonlinear optics, and data storage. Traditional ferroelectrics are ceramic based. Ferroelectric polymers are inexpensive lead-free materials that offer unique features such as the freedom of design enabled by chemistry, the facile solution-based low-temperature processing, and mechanical flexibility. Among engineering polymers, odd nylons are ferroelectric. Since the discovery of ferroelectricity in polymers, nearly half a century ago, a solution-processed ferroelectric nylon thin film has not been demonstrated because of the strong tendency of nylon chains to form hydrogen bonds. We show the solution processing of transparent ferroelectric thin film capacitors of odd nylons. The demonstration of ferroelectricity, as well as the way to obtain thin films, makes odd nylons attractive for applications in flexible devices, soft robotics, biomedical devices, and electronic textiles.

Synthesis and Solution Processing of Nylon‐5 Ferroelectric Thin Films: The Renaissance of Odd‐Nylons?

“…The E c of the different molecular weights shows a decreasing trend with increasing molecular weight, starting from 338 ± 18 MV m −1 for lowest (0.7 kg mol −1 ) to 263 MV m −1 for the highest (3.9 kg mol −1 ) molecular weight. The reason for the high E c for solution‐processed odd‐nylons is a higher degree of hydrogen bonding order, as explained in our previous work 4. We have also measured the dielectric constant for all nylon‐5 samples.…”

“…Further, we determine the exact polymeric structure employing end‐group analysis by NMR spectroscopy and matrix assisted laser desorption ionization time of flight mass spectroscopy (MALDI‐ToF MS). Using the samples prepared, we employed a solution quenching technique to obtain extremely smooth and transparent nylon‐5 thin films that are crystallized in the ferroelectric crystalline phase 4. The resulting films were studied with respect to their ferroelectric properties in detail.…”

“…Thin films were prepared by spin‐coating and subsequent solution quenching of the wet films in vacuum. Details of this process are given elsewhere 4. The process yields optically transparent thin films with a thickness in the range of 350 ± 10 nm.…”

Polymer field-effect transistor memory based on a ferroelectric nylon gate insulator

“…Recently, we have demonstrated ferroelectricity in solution processed thin films of odd-nylons using a solvent mixture of TFA : acetone [60 : 40 mole percent (mol%)]. 21 The ferroelectric phase is realized by placing the wet spin-coated film in a vacuum chamber to quickly extract the solvent and provide fast crystallization conditions. The resulting vacuum-quenched thin films show small crystallites and the absence of spherulitic superstructures.…”

“…The solvent-quenching process using a vacuum, to some degree, imitates the supercooling process from a melt, which is known to yield the pseudohexagonal non-spherulitic d 0 -phase. 21 However, thin films produced by vacuum quenching have a roughness of around 5 nm, which is not suited for applications in devices such as FeFETs. 22 We hypothesize that the roughness is due to the extended solvent-quenching time, since the vacuum is not strong enough to quickly extract solvent molecules.…”