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Unified Understanding of Ferroelectricity in n‑Nylons: Is the Polar Crystalline Structure a Prerequisite?
journal contribution
posted on 2016-04-12, 17:33 authored by Zhongbo Zhang, Morton H. Litt, Lei ZhuOver 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.