During the past half century, there has been tremendous growth in exploring the chemistry, physics and material science of poly(vinylidene fluoride) (PVDF)-based ferroelectric polymers. With high breakdown strength, easy processing, and outstanding insulating features, these all-organic electrically active materials are attractive for many advanced electrical (e.g., pyroelectric and piezoelectric) applications. Concurrently, the research about ferroelectricity in polymers also began for energy storage applications through controlling crystalline structure and tuning the corresponding ferroelectric properties, e.g., novel relaxor ferroelectricity with slim double- or single- hysteresis-loop (DHL and SHL) behavior in PVDF copolymers and terpolymers. On the other hand, research activities also focused on searching for other classes of ferroelectric polymers, such as ferroelectric polyamides, cyanopolymers, polyureas, and polyuerathane. However, not much progress has been achieved as considerable as what has been achieved for PVDF-based polymers. In this thesis, we focus on the novel ferroelectric behaviors in polyamide-based polymers, trying to deeply understand the origin of ferroelectricity in aliphatic and aromatic nylons, and generalize the relaxor ferroelectric behavior into this hydrogen-bonding system.
It has been commonly considered that only odd-numbered nylons, which prefer polar crystalline structure, are able to show ferroelectric hysteresis loops. In contrast, even-numbered nylons, such as nylon-12 and nylon-6 should not exhibit any ferroelectricity due to their nonpolar crystalline structure. However, in our study, ferroelectric properties are also reported for mesomorphic even-numbered nylons. The structure of the mesophases in quenched samples was considered to contain multiple twists in the chain conformation and dangling/weak hydrogen bonds, which enabled dipolar switching, forming electric-field-induced ferroelectric domains. This study shows that different from ferroelectric PVDF-based polymers, the polar crystalline structure is not the prerequisite for ferroelectricity in nylons. Instead, mesophases with enlarged interchain spacing and disordered hydrogen bonds are the key to ferroelectricity.
With the understanding of ferroelectricity in n-nylons, further weakened hydrogen bonding and more twisted chain conformation (e.g., the all-trans conformation of nylon-12 under poling) are expected to facilitate dipolar relaxation and prevent the large ferroelectric domain from growing up. The simplest way to weaken hydrogen bonds is to raise the temperature. Upon increasing temperature to 100 °C, the D-E loops became increasingly narrower, finally leading to slim DHLs for nylon-6 and nylon-12. 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 crystal of even-numbered nylons.
Furthermore, the SHL relaxor ferroelectric property was successfully achieved in a polyamide terpolymer, PA(11-co-12-co-NM11) based on 11-aminoundecanoic acid (AUA), 12-aminododecanoic acid (ADA), and N-methyl-11-aminoundecanoic acid (NM11), utilizing the principle of nanosized ferroelectric domains (or nanodomains). The hydrogen-bonding interaction was intentionally weakened by chemically introducing defects in the crystalline phase of nylon terpolymers. More importantly, the N-CH3 groups were expected to participate in the isomorphic crystals, blocking the formation of hydrogen bonds and inducing chain twisted in the mesophase.
Finally, the ferroelectric switching behavior in the amorphous phase of a nearly 100% amorphous nylon, Selar, was investigated. Similarly, ferroelectricity in the glassy phase was also highly dependent upon hydrogen-bonding interaction. When the hydrogen-bonding was weak such as in the quenched film, significant ferroelectric switching took place. Otherwise, the quenched and annealed films did not exhibit any ferroelectric switching. High-voltage broadband dielectric spectroscopy was used to study chain segmental motion in Selar. The result indicated that it was the cooperative short-range segmental motions in the main-chain dipolar glass polymer that primarily contributed to the observed ferroelectricity.