Understanding the Paraelectric Double Hysteresis Loop Behavior in Mesomorphic Even-Numbered Nylons at High Temperatures

Novel ferroelectric properties, such as slim double and single hysteresis loop (DHL and SHL) behaviors, are attractive for high energy density and low loss dielectric applications. In this study, temperature-dependent ferroelectric behavior was studied for mesomorphic even-numbered nylons (i.e., nylon-12 and nylon-6) using electric displacement electric field (D-E) loop measurements. Upon raising the temperature from room temperature to 100 degrees C, the D-E loops became increasingly narrower, finally leading to slim DHLs with significantly enhanced apparent dielectric constants (i.e., similar to 30 and similar to 60) and small remanent polarizations (i.e., 3.5 and 8.2 mC/m(2)) for quenched and stretched nylon-12 and nylon-6, respectively. Combining wide-angle X-ray diffraction and infrared studies, changes in the mesophases and orientation of hydrogen-bonded amide groups after electric poling were used to unravel the structure ferroelectric property relationship for the even-numbered nylons. At 100 degrees C, the quenched and stretched nylon-12 and nylon-6 films exhibited a paraelectric mesophase with twisted chain conformation and disordered hydrogen bonds. Upon high field poling (>100 MV/m), transient nanodomains could be generated with additional twists in the main chain. The observed DHL behavior was attributed to the electric-field induced reversible transitions between the paraelectric (less twisted chains) and ferroelectric (more twisted chains) states in the mesomorphic crystals of even-numbered nylons. The knowledge gained from this study can inspire potential applications of n-nylons for electric energy storage, e.g., high temperature and high energy density multilayer polymer film capacitors.