Unified Understanding of Ferroelectricity in n‑Nylons: Is the Polar Crystalline Structure a Prerequisite?

Over the past decades, it has been commonly considered that ferroelectricity is closely related to the polar crystalline structure of odd-numbered nylons, and even-numbered nylons should not exhibit ferroelectricity due to their nonpolar crystalline structures. In this work, we ask a fundamental question: Are odd-numbered nylons with polar crystalline structures prerequisites for ferroelectricity? Here, ferroelectric properties are reported for mesomorphic even-numbered nylons (nylons-12 and -6) quickly quenched from the melt, using electric displacement–electric field (D–E) hysteresis loop measurements. From X-ray diffraction and infrared studies, the structure of the mesophases in the quenched samples was considered to contain multiple twists in the chain conformation, resulting in enlarged interchain distance and dangling/weak hydrogen bonds. Upon high field electric poling, the mesophase structure enables dipolar switching of the dangling/weak hydrogen bonds, forming electric-field-induced ferroelectric domains with twisted chain conformations in the crystal. The domain sizes in even-numbered nylons should be smaller than those in odd-numbered nylons, and thus D–E hysteresis loops should be slimmer. This study shows that odd-numbered nylons and polar crystalline structures are not prerequisites for ferroelectricity in nylons. Instead, mesophases with enlarged interchain spacing and disordered hydrogen bonds are the key to ferroelectricity. The knowledge obtained from this study will help us design new nylons and nylon copolymers with defective crystalline structures for enhanced ferroelectric properties.