Nature of Ferroelectric Behavior in Main-Chain Dipolar Glass Nylons: Cooperative Segmental Motion Induced by High Poling Electric Field

Main-chain dipolar glass polymers such as aromatic nylons are promising for high energy density and low loss dielectric applications because of the limited, noncooperative oscillation of highly dipolar amide groups. However, quenched aromatic nylons have been reported to exhibit significant ferroelectric switching upon high field poling. It is desirable to suppress ferroelectric switching for electric energy storage application, and this requires a fundamental understanding of the nature of ferroelectric behavior in dipolar glass nylons. In this work, a nearly 100% amorphous aromatic nylon, Selar, was used to investigate the origin of ferroelectricity in glassy nylons. Using Fourier transform infrared spectroscopy, it was found that hydrogen-bonding strength played an important role in the ferroelectric switching of amide dipoles in the loosely packed glassy matrix. When hydrogen bonding was weak such as in the quenched film, significant ferroelectric switching took place. In contrast, quenched and annealed films did not exhibit any ferroelectric switching. High-voltage broadband dielectric spectroscopy was used to study molecular and segmental motions in Selar. It was observed that the primary contribution to ferroelectric switching came from cooperative segmental motions (i.e., a combination of various sub-<i>T</i>_g relaxations, where <i>T</i>_g is the glass transition temperature) in the main-chain dipolar glass nylon. This understanding will help us design new aromatic nylons with suppressed segmental motions for high energy density and low loss dielectric applications

Achieving Relaxor Ferroelectric-like Behavior in Nylon Random Copolymers and Terpolymers

High dielectric constant polymers, exhibiting relaxor ferroelectric (RFE) behaviors (i.e., slim single and double hysteresis loops, SHLs and DHLs), are attractive for high energy density and low loss dielectric applications. Utilizing the principle of nanosized ferroelectric domains (nanodomains), this study has designed and developed novel RFE-like polyamides (PAs) based on 11-aminoundecanoic acid, 12-aminododecanoic acid, and <i>N</i>-methyl-11-aminoundecanoic acid (NM11) as an alternative to the high-cost and difficult-to-synthesize poly­(vinylidene fluoride) (PVDF)-based RFE polymers. In the first attempt, quenched and stretched (QS) PA­(11-<i>co</i>-12) copolymers exhibited enhanced ferroelectricity as compared with either nylon-11 or nylon-12. Although relatively narrow hysteresis loops could be achieved at 75 °C, the hydrogen-bonding interaction was not weak enough to induce nanodomains in the nylon copolymers. To further reduce the hydrogen-bonding interaction and achieve nanodomains, a PA­(11-<i>co</i>-12-<i>co</i>-NM11) 30/60/10 (molar ratio) terpolymer (terPA-NCH₃) was synthesized. The NCH₃ groups were expected to participate in the isomorphic crystals, blocking the formation of hydrogen bonds and inducing chain twists in the mesophase. Indeed, the RFE-like behavior with slim SHLs and high dielectric constant (60–70) was successfully achieved for the QS terPA-NCH₃ at high temperatures (>75 °C). Pathways to achieve RFE-like behavior for nylon-based polymers are discussed and compared with those for PVDF-based polymers. The knowledge obtained from this study can inspire potential applications for nylon polymers in advanced electrical and power applications

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 (<i>D</i>–<i>E</i>) loop measurements. Upon raising the temperature from room temperature to 100 °C, the <i>D</i>–<i>E</i> loops became increasingly narrower, finally leading to slim DHLs with significantly enhanced apparent dielectric constants (i.e., ∼30 and ∼60) and small remanent polarizations (i.e., 3.5 and 8.2 mC/m²) 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 °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 <i>n</i>-nylons for electric energy storage, e.g., high temperature and high energy density multilayer polymer film capacitors

High Dielectric Constant Sulfonyl-Containing Dipolar Glass Polymers with Enhanced Orientational Polarization

Dipolar glass (DG) polymers, which utilize sub-<i>T</i>_g orientational polarization (<i>T</i>_g is the glass transition temperature) to enhance dielectric constants, are promising candidates for use in advanced electronic and power applications because conduction of space charges (electrons and impurity ions) is suppressed in the glassy state, and thus, the dielectric loss is low. In this study, we studied the effects of dipole density and dipole arrangement in sulfonyl-containing side-chain DG polymers on their dielectric performance in terms of dielectric constant, energy density, and dielectric loss. Monosulfonyl (i.e., CH₃SO₂-) and disulfonyl [i.e., CH₃SO₂(CH₂)₃SO₂-] groups were quantitatively grafted to polyepichlorohydrin (monosubstitution) and poly­(3,3-bis­(chloromethyl)­oxatane) (bis-substitution), respectively, in order to vary the dipole density and dipole arrangement in the side chains. As a result of orientation polarization from highly polar sulfonyl (4.5 D) groups, these DG polymers exhibited high apparent dielectric constants (7–11.5) in the glassy state with reasonably low dissipation factors (tan δ ∼ 0.003–0.02). It was found that disulfonylated DG polymers exhibited a higher dielectric constant than monosulfonylated DG polymers because of their higher dipole densities. Meanwhile, bis-substituted DG polymers showed a higher dielectric constant than monosubstituted DG polymers. Upon high-field electric poling, reversible transitions between the low-field DG state and the high-field ferroelectric state induced double hysteresis loops, and disulfonylated DG polymers had more significant ferroelectric switching than monosulfonylated DG polymers due to stronger dipolar interactions among the disulfonyl groups. On the basis of the experimental results, monosulfonylated DG polymers, whether mono- or bis-substituted, should be more appropriate for electric energy storage applications

Achieving High Dielectric Constant and Low Loss Property in a Dipolar Glass Polymer Containing Strongly Dipolar and Small-Sized Sulfone Groups

In this report, a dipolar glass polymer, poly­(2-(methyl­sulfonyl)­ethyl methacrylate) (PMSEMA), was synthesized by free radical polymerization of the corresponding methacrylate monomer. Due to the large dipole moment (4.25 D) and small size of the side-chain sulfone groups, PMSEMA exhibited a strong γ transition at a temperature as low as −110 °C at 1 Hz, about 220 °C below its glass transition temperature around 109 °C. Because of this strong γ dipole relaxation, the glassy PMSEMA sample exhibited a high dielectric constant of 11.4 and a low dissipation factor (tan δ) of 0.02 at 25 °C and 1 Hz. From an electric displacement-electric field (D-E) loop study, PMSEMA demonstrated a high discharge energy density of 4.54 J/cm³ at 283 MV/m, nearly 3 times that of an analogue polymer, poly­(methyl methacrylate) (PMMA). However, the hysteresis loss was only 1/3–1/2 of that for PMMA. This study suggests that dipolar glass polymers with large dipole moments and small-sized dipolar side groups are promising candidates for high energy density and low loss dielectric applications