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

Abstract

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.

Mesophase Structure‐Enabled Electrostrictive Property in Nylon‐12‐Based Poly(ether‐block‐amide) Copolymers

“…After subtraction of the AC electronic conduction (the blue horizontal loop), the D‐E loop became typical ferroelectric shaped. This could be ascribed to the increased permittivity for the amorphous nylon‐12 phase when the temperature was above its T g around 45 °C . The maximum D ( D max ) at 200 MV m −1 for QS P6333 was observed at ≈47 mC m −2 with the apparent ε r being ≈35 (Figure A).…”

“…The broader NH band suggested that hydrogen‐bonding in the nylon‐12 crystals became poorer, confirming the significant structural changes in mesomorphic nylon‐12 crystals for both PEBAX samples after stretching. On the basis of previous reports, we anticipated that these poorly hydrogen‐bonded crystals after stretching should be better to facilitate ferroelectric switching under electric poling.…”

“…The low‐angle (020) reflection indicated the smectic‐like structure from hydrogen‐bonded amide groups. The high‐angle mixed (001/200) reflections indicated the interchain distance . Assuming a monoclinic structure for the mesophase, the b ‐axes for the Q P6333 and Q P7033 were calculated to be 3.15 and 3.21 nm, respectively, shorter than the b ‐axis of 3.27 nm of the γ phase.…”

“…According to previous studies, ferroelectricity in nylon-12 can be achieved from the mesomorphic crystalline structure quenched from the melt. [43,48] Similarly, to achieve the mesophase structure, molten PEBAX samples were quenched from ≈30 °C above T m into liquid nitrogen. DSC was used to study the thermal properties of PEBAX samples (Figure 1).…”

“…The high-angle mixed (001/200) reflections indicated the interchain distance. [48] Assuming a monoclinic structure for the mesophase, the b-axes for the Q P6333 and Q P7033 were calculated to be 3.15 and 3.21 nm, respectively, shorter than the b-axis of 3.27 nm of the γ phase. The longer b-axis for the Q P7033 than that for the Q P6333 suggested that the Q P6333 sample contained more twists along the chain than the Q P7033.…”