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Yu-Feng ZhuZhongbo ZhangMorton H. LittLei Zhu. Macromolecules ... Zhongbo Zhang , David H. Wang , Morton H. Litt , Loon-Seng Tan , Lei Zhu. Angewandte...
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Understanding the Paraelectric Double Hysteresis Loop Behavior in Mesomorphic Even-Numbered Nylons at High Temperatures Zhongbo Zhang, Morton H. Litt, and Lei Zhu* Department of Macromolecular Science and Engineering and Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States S Supporting Information *

ABSTRACT: 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 °C, the D−E 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-fieldinduced 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 nnylons for electric energy storage, e.g., high temperature and high energy density multilayer polymer film capacitors.

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narrow SHLs and DHLs can be achieved.1,3 However, ferroelectric polymers differ from ferroelectric ceramics due to their unique long-chain nature with flexible conformations. It is desirable to thoroughly understand the narrow SHL and DHL behaviors in novel ferroelectric polymers and use this understanding to design new polymers with tailored properties. Currently, narrow SHL and DHL ferroelectric behaviors have only been reported for poly(vinylidene fluoride) (PVDF)based relaxor ferroelectric copolymers via crystal isomorphism (or defect-modified crystals).1,3,20 Unlike relaxor ferroelectric ceramics, nanodomains cannot be achieved by mixing/blending different polymer chains in the crystal. Instead, they need to be achieved via fine-tuning of the polymer chain conformation and thus interchain interactions via repeat-unit isomorphism. Following the discovery of relaxor ferroelectric behavior in electron beam (e-beam)-irradiated P(VDF-TrFE) (TrFE is trifluoroethylene),21,22 a bulky termonomer was terpolymerized into the P(VDF-TrFE) chains to induce twisted chain conformations, which effectively reduces the strong dipolecoupling interactions among the zigzag chains in the crystal.23,24 Here, the termonomers were critical to the

INTRODUCTION Novel ferroelectric behaviors, including slim double and single hysteresis loops (DHLs and SHLs), have emerged for ferroelectric polymers.1 Because of high polarization and low dielectric loss, these novel ferroelectric polymers are attractive for potential applications such as electric energy storage,2−5 electrostrictive/dielectric elastomer actuation,6,7 and electrocaloric cooling.8−10 These novel behaviors have already been achieved for ferroelectric ceramics by generating nanoscale ferroelectric domains (or nanodomains). For example, the polar domain size is only 2−3 nm in Pb(Mg1/3Nb2/3)O3 (PMN), as revealed by high-resolution transmission electron microscopy (TEM).11,12 As a result, the cooperative coupling among the nanodomains is significantly weakened, and slim SHLs with high dielectric constants (a few thousand, i.e., the relaxor ferroelectric behavior13,14) resulted. Similar to ferroelectric ceramics, it is the size and spatial organization of ferroelectric domains that govern the macroscopic ferroelectric properties of polymers. Fundamentally, when the domain size is large, the entire domain must be switched to reverse the macroscopic polarization. As a result, the normal ferroelectric behavior with a rectangular-shaped hysteresis loop is obtained. This property is useful for nonvolatile memory15−17 and piezoelectric transducers in sensors and actuators.18,19 When the polymers contain nanodomains, novel relaxor ferroelectric behavior with © XXXX American Chemical Society

Received: June 1, 2017 Revised: July 11, 2017

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Macromolecules formation of nanodomains via the crystal-pinning effect.25−27 When the pinning effect is weak, such as in the P(VDF-TrFECFE) (CFE is 1,1-chlorofluoroethylene) terpolymer, the twisted conformation is metastable and can be switched to the zigzag conformation by the high poling electric field.1,3 As a result, DHLs are obtained. When the pinning effect is strong, such as in the P(VDF-TrFE-CTFE) (CTFE is chlorotrifluoroethylene), the twisted conformation becomes stable and can survive the high-field electric poling.28 The strong pinning effect also exists in the e-beam-irradiated P(VDFTrFE), where the chemical pinning or cross-linking can fix the twisted chain conformation in the crystal.22,29 Consequently, slim SHLs are achieved with an apparent dielectric constant (εr) as high as ∼70, which is similar to that of water at room temperature (RT).30,31 Given the above understanding of novel ferroelectric behaviors in P(VDF-TrFE)-based copolymers and terpolymers, it was desirable to apply the concept of crystal isomorphisminduced nanodomains in PVDF crystals to other ferroelectric polymers to achieve novel ferroelectric properties. In addition, the synthesis of P(VDF-TrFE)-based terpolymers requires special facilities (explosion-proof autoclaves) and infrastructures (i.e., on-site production of TrFE), which leads to high prices for the terpolymers, inhibiting the near-term commercialization. Therefore, it is highly desirable to search for alternative and cheaper ferroelectric polymers, which also exhibit narrow SHL and DHL ferroelectric behaviors. Polyamides, known as nylons, represent another important class of ferroelectric polymers.32 Compared to PVDF, which has a repeat-unit dipole moment of 2.1 D,33,34 nylons contain amide dipoles that have a higher rigid dipole moment of 3.7 D.35 In addition, nylons are advantageous for their low cost, easy synthesis, and good potential to tune the molecular structures and properties. Ferroelectricity in odd-numbered nylons such as nylon-11 has already been widely reported.32,36−42 Similar to PVDF-based ferroelectric polymers, the ferroelectricity in odd-numbered nylons was considered to be a result of 180°-switching of amide groups in the chains within the ferroelectric domains.39 No ferroelectricity was observed for the polar α phase of nylon-11 because the strong hydrogen-bonding interaction in the tightly packed hydrogenbonded sheets prevented dipole or domain rotation.43,44 Ferroelectric behavior was only obtained in the quenched smectic-like δ′ phase, where interchain spacing is enlarged and the hydrogen-bonding interaction is weakened by the twisted chain conformation and thus disordered hydrogen bonds.45 It has been commonly accepted that nylon ferroelectricity is closely related to the polar crystalline structure of oddnumbered nylons, and even-numbered nylons should not exhibit ferroelectric behavior due to their nonpolar crystalline structures. However, our recent study reported poling electricfield-induced ferroelectric behavior for mesomorphic evennumbered nylons (e.g., nylon-12 and nylon-6).46 More importantly, the ferroelectricity is generated within the crystalline mesophases with twisted chains, rather than from the amorphous phase as reported in the past.47−51 This viewpoint unifies the origin of ferroelectricity in nylons, whether odd- or even-numbered. Basically, the pseudohexagonal mesomorphic structures in the quenched and stretched nylon films are highly defective with twisted chain conformations and disordered/dangling hydrogen bonds. The weakened hydrogen-bonding interaction in the mesomorphic crystals enables easier polarization by the high poling field,

inducing ferroelectric domain formation. As such, a polar crystalline structure should not be a prerequisite for the ferroelectricity observed in n-nylons. However, there is a difference in the ferroelectric behavior between odd- and evennumbered nylons. Because of the stable polar zigzag conformation, odd-numbered nylons tend to favor larger domains when polarized and thus exhibit broad hysteresis loops. On the contrary, even-numbered nylons favor the stable antiparallel zigzag conformation, and the field-induced ferroelectric domains with parallel dipoles are not stable after removal of the poling field. As a result, slimmer hysteresis loops than those of the odd-numbered nylons are observed. Although the normal ferroelectric behavior has been achieved for n-nylons, whether odd- or even-numbered,46 the question still remains: How can one generalize the principle of nanodomains and achieve novel DHL and SHL ferroelectric behaviors for ferroelectric nylons? Fundamentally, the fieldinduced ferroelectric domain size is closely related to the hydrogen-bonding interaction; the weaker the hydrogen bonding, the smaller the ferroelectric domain size. A simple and effective way to weaken the hydrogen-bonding interaction in n-nylons is to raise the temperature. At high temperatures, nylons tend to adopt the high-temperature pseudohexagonal structure with disordered hydrogen bonds, which significantly reduce the hydrogen-bonding interaction. In this work, the temperature-dependent ferroelectric behavior of even-numbered nylons was studied. Indeed, the ferroelectric loops narrowed as the temperature rose. It is intriguing to observe narrow DHLs for mesomorphic nylon-12 and nylon-6 films at 100 °C. This could be attributed to the reversible transformation between the paraelectric state at low fields and the ferroelectric state at high fields. Structure analyses by wideangle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy were employed to unravel the structure− ferroelectric property relationship for even-numbered nylons. In contrast, nylon-11 does not exhibit any DHL behavior at elevated temperatures because of its large ferroelectric domains. Finally, the nature of field-induced ferroelectric behavior in nnylons is discussed and compared to that of PVDF and its random copolymers.

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EXPERIMENTAL SECTION

Materials. Nylon-12 and nylon-6 pellets were purchased from Sigma-Aldrich (St. Louis, MO). Nylon-11 resin (Rilsan Besno TL, melt volume-flow rate 1 cm3/10 min by ISO 1133, Arkema, King of Prussia, PA) was purchased from PolyOne (Avon Lake, OH). All nylon resins were used as received without further purification, except for thorough drying in a vacuum oven at 70 °C for 3 days before meltpressing. Film Fabrication and Processing. Quenched and stretched (QS) nylon films were prepared by hot-pressing using aluminum foil sandwiches. Namely, nylon samples were melted at a temperature about 30−40 °C above their melting temperatures (Tm), i.e., 210 °C for nylon-11 and nylon-12 and 260 °C for nylon-6 for a short period of time (ca. 2 min) to avoid potential oxidative cross-linking of nylons.52 After hot-pressing, the sandwiched samples were immediately quenched into an isopropanol/dry ice bath (about −78 °C). The hot-pressed and quenched films were then uniaxially stretched at room temperature to an extension ratio of ca. 300%, using a home-built stretching apparatus. Quenched, stretched, and annealed samples were obtained by annealing fresh QS samples at 100 °C for 5 min (denoted as QSA@100 °C). Electrically poled QS samples (QSP) were also prepared. Basically, two different types of QSP samples were prepared, i.e., QS samples bipolarly polarized under 200 MV/m (10 Hz) for 25 or 30 cycles at RT (QSP@RT) and at 100 °C (QSP@100 °C), B

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Figure 1. Continuous bipolar D−E loops for fresh QS nylon-11 films at (A) RT, (B) 50 °C, (C) 75 °C, and (D) 100 °C. At each temperature, a total of 5 × 5 continuous D−E loops were performed in order to achieve steady-state ferroelectric behavior. The first three runs are shown in (A), and only the first runs are shown in (B−D). The time interval between each run was ca. 30 s. The loops in (C, D) were obtained by subtracting ac electronic and ionic conductions from the raw data. The detailed procedure is shown in Figure S2. (E) RT continuous bipolar D−E loops for the QSP@100 °C nylon-11 film. (F) RT continuous bipolar D−E loops for the QSA@100 °C nylon-11 film. The poling frequency is 10 Hz with a sinusoidal waveform.

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respectively. For nylon-11, a QS sample bipolarly polarized under 200 MV/m (10 Hz) for 250 cycles at RT (QSP@RT-250) was also prepared. Instrumentation and Characterization. Fourier transform infrared (FTIR) characterization was carried out using a Nicolet IS50R FTIR spectrometer in the transmission mode. The scan resolution was 2 cm−1 with 64 scans. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q100 DSC at a scanning rate of 10 °C/min. For nylon-11 and nylon-12, the scanning range was 0−220 °C. For nylon-6, the scanning range was 0−260 °C. Around 1.5 mg of samples was used in DSC scans to avoid any thermal lag. Two-dimensional (2D) wide-angle X-ray diffraction (WAXD) experiments were performed using a Rigaku MacroMax 002+ equipped with a Confocal Max-Flux optic and a microfocus X-ray tube source operating at 45 kV and 0.88 mA. The X-ray wavelength was 0.1542 nm (Cu Kα). The WAXD patterns were collected using a Fujifilm image plate scanned by a Fujifilm FLA-7000 scanner at a resolution of 50 μm/pixel. One-dimensional (1D) WAXD curves were obtained by integrating the corresponding 2D WAXD patterns radially using the Polar software (version 2.7.4) developed by Stonybrook Technology and Applied Research, Inc. Electric displacement−electric field (D−E) hysteresis loop measurements were performed using a Premiere II ferroelectric tester (Radiant Technologies, Inc., Albuquerque, NM), in combination with a Trek 10/10B-HS high-voltage amplifier (0−10 kV ac, Lockport, NY). The applied voltage had a bipolar sinusoidal or bipolar triangle waveform in the frequency range 5−500 Hz. Silver (Ag) electrodes (2.5 mm2 diameter and ca. 50 nm thick) were evaporated onto both sides of the film samples using an EvoVac deposition system (Angstrom Engineering, Inc., Kitchener, ON, Canada). The metallized film samples were immersed in silicone oil (Fisher 460-M3001) during testing to avoid corona discharge. The temperature was controlled by an IKA RCT temperature controller (Wilmington, NC). A home-built sample fixture with high-voltage cables was used for connecting the electrodes on both sides of the film to the interface of the Radiant ferroelectric tester.

RESULTS AND DISCUSSION Temperature-Dependent Ferroelectric Behavior in Nylon-11. Before studying the high-temperature ferroelectric behavior for even-numbered nylons, the temperature-dependent ferroelectric behavior for nylon-11 (an odd-numbered nylon) was first studied as a comparison. Note that although ferroelectricity in mesomorphic nylon-11 has been documented in the literature,32 its high-temperature ferroelectric behavior has not been thoroughly studied. Figures 1A−D show continuous bipolar D−E hysteresis loops for the QS nylon-11 film at different temperatures. The maximum poling field was 200 MV/m, and the poling frequency was 10 Hz with a sinusoidal waveform. A fresh sample was used at each temperature to avoid any remanent polarization (Pr) from the prior run. In this way, the starting point for the first set of continuous loops was always located at the origin of the plot. Otherwise, the starting point would locate below the origin with a negative Pr. Also, continuous loop tests were used to minimize the effect of the Pr from the previous loop.46,53 At RT (Figure 1A), at least 10 (i.e., 2 × 5) poling cycles were needed to develop nearly steady-state ferroelectric loops for the QS nylon-11 film. On the basis of our previous study,46 there should be hardly any ferroelectric domains in the nonpoled QS nylon-11 film, and quenching from the melt followed by uniaxial stretching generated a mesomorphic crystalline structure. As a result, the first D−E loop appeared to be relatively slim. Upon more electric poling, large ferroelectric domains gradually developed in the crystals because the poorly bonded amide groups in the mesophase were polarizable. After the first and second sets of five continuous loops, Pr values of −23.4 and −32.3 mC/m2 were observed, indicating the formation of polarized large ferroelectric domains induced by the high poling field. After 15 (i.e., 3 × 5) poling cycles, the D− E loops almost saturated with an obvious rectangular shape. To avoid redundancy, the fourth and fifth runs were not shown in C

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Figure 2. 1D WAXD profiles for (A, D) the QS and QSP@RT nylon-11 films at RT, (B, E) the QS and QSP@100 °C nylon-11 films at 100 °C, and (C, F) the QSA@100 °C and QSP@100 °C nylon-11 films cooled back to RT along (A, B, C) the equatorial and (D, E, F) the meridianal directions in the corresponding 2D WAXD patterns (see Figure S3). For each experiment, a new sample was used. (A) and (D) also include the 1D WAXD profiles for the QSP@RT-250 nylon-11 film. Positions of (100)α, (010)α, and (001)α reflections for the α phase nylon-11 are indicated by short bars in the plots. Solid and dashed curves in (A−C) were obtained along the flat-on and edge-on directions for the film samples, respectively.

fixed length. It had been reported before that the distance between hydrogen-bonding sheets [i.e., the (010) planes] could vary considerably, depending on the crystallization and thermal treatment conditions.54,55 For the polarized QS sample, the (010) reflection was located at a much lower q value than that for the (010)α reflection, indicating a larger d010 spacing. The WAXD result in Figure 2A indicates that electric poling pushed the crystal structure toward the stable α phase (i.e., α-like), and the amide hydrogen bonds were aligned parallel to the field direction for the QSP films. Upon further poling for 250 cycles, the (100) and (010) reflections in the edge-on and flat-on profiles became sharper with more separation, suggesting further transition toward the α phase and better orientation of hydrogen bonds. The (001) reflection for the QSP@RT and QSP@RT-250 samples was located at a slightly higher q value than that of the QS sample (Figure 1D). Supposedly, the chains should have adopted more trans conformations after electric poling, leading to a larger d001. However, this was not the case. Instead, the d001 slightly decreased. The shorter d010 in the QSP@RT sample could be due to chain tilting in the crystalline

Figure 1A. The coercive field (EC) reached ∼94 MV/m. Upon further poling at 200 MV/m and RT for 250 (i.e., 50 × 5) cycles, ferroelectric domains could grow even larger, as shown in Figure S1 of the Supporting Information, and the EC could reach 120 MV/m with the Pr being 48 mC/m2. To a large extent, this was facilitated by the intrinsic polar crystalline structure with the zigzag conformation for odd-numbered nylons.32,46 The orientation of ferroelectric domains after extended poling cycles at RT was confirmed by WAXD, as shown in Figures 2A,D. Before electric poling, the QS nylon-11 film exhibited broad and nearly overlapping (100/010) reflections in both flat-on and edge-on WAXD profiles, indicating a pseudohexagonal mesophase with nearly no preferred crystal orientation in the film. However, after 25 poling cycles for the QSP@RT film, a noticeable orientation was observed with the (100) reflection in the edge-on profile and (010) reflection in the flat-on profile. The assignments were based on the fact that the (100) reflection was similar to the (100)α reflection because the hydrogen bonds along the a-axis should have a more or less D

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Macromolecules structure, which was similar to the organization in the triclinic α phase [note that the (001)α located at a higher q value in Figure 2D]. Upon increasing the temperature above the glass transition temperature (Tg ∼ 45 °C), steady-state ferroelectric loops could be achieved within the first five continuous loops for fresh QS nylon-11 films. Obviously, high temperature weakened the intermolecular interaction by disrupting the hydrogen bonds and thus enhanced the dipole and domain switchability. The D−E loops gradually became slimmer, which was reflected by the decreased EC with increasing temperature (see Figure 1B−D). Note that ac electronic and ionic conductivities became so high above 75 °C that they had to be subtracted at 75 and 100 °C to obtain the ferroelectric loops for the QS film (the detailed procedure of subtraction is given in Figure S2). The high electronic and ionic conduction in the QS nylon-11 film prevented further D−E loop study beyond 100 °C because the sample readily broke down electrically. Polarization at high temperatures could also induce the α-like phase with certain crystal orientation in the QSP nylon-11 films (see Figure 2B,D). Compared to the QS sample, which had no crystal orientation at 100 °C, the QSP@100 °C exhibited slightly separated (100) and (010) reflection peaks in the edgeon and flat-on WAXD profiles (Figure 2B), indicating again the transition toward the α phase with certain orientation of the hydrogen-bonded sheets parallel to the electric field direction. The electric poling-induced orientation of hydrogen-bonded −NH− groups was further confirmed by the FTIR study (see Figure S4). After normalizing the symmetric CH2 stretching peaks at 2852 cm−1,39 the polarized nylon-11 films (i.e., QSP@ RT and QSP@100 °C) exhibited a weaker intensity of the hydrogen-bonded N−H stretching at 3295 cm−1 than the QS and QSA@100 °C films. This indicated that the N−H bonds were oriented along the electric field direction (i.e., normal to the films). After cooling back to RT, the (100) and (010) reflections in the flat-on and edge-on profiles further separated for the QSP@ 100 °C film, whereas they did not separate for the QSA@100 °C film. This indicated better α-like phase and more preferred crystal orientation induced by electric poling for the QSP@100 °C film. It was expected that high-temperature electric poling could greatly influence the subsequent ferroelectric properties for the QS nylon-11 film. As shown in Figure 1E, the QSP@ 100 °C film exhibited a much broader hysteresis loop with high EC and Pr values after cooling back to RT. On the contrary, the ferroelectric switchability was largely suppressed for the nonpoled QSA@100 °C film at least up to 200 MV/m (Figure 1F). We speculate that hydrogen-bonding was strengthened by annealing at 100 °C for the QSA@100 °C film, as indicated by the sharper (100/010) reflection in both flat-on and edge-on WAXD profiles (Figure 2C) compared to those of the QS film (Figure 2A). It was interesting to note that the hysteresis loop for the QSP@100 °C film became asymmetric along the E-axis, i.e., −EC = −124 MV/m and +EC = 148 MV/m (Figure 1E). This again could be attributed to the formation of stable (or “permanent”) ferroelectric domains oriented in the negative field direction after 25 cycles of electric poling at 100 °C and cooling to RT. These nonpolarizable domains, which oriented in the negative field direction, generated a negative internal field and decreased the applied external electric field in the sample. As a result, a higher positive poling field and a smaller negative poling field were required to switch the polarizable domains. Compared with the repeated RT poling (see Figure S1), we

conclude that high-temperature poling could have both effects of electric poling (to grow larger domains) and thermal annealing simultaneously, leading to asymmetric and broad hysteresis loops for the QS nylon-11 film. Note that the thermal annealing effect is to enhance the chain mobility for better crystal packing with stronger hydrogen-bonding interaction after cooling back to RT. From the above study, temperature-dependent ferroelectric behavior for nylon-11 is summarized in Scheme 1. Because the Scheme 1. Summary of Temperature-Dependent Ferroelectric Behavior for Nylon-11a

a

HB stands for hydrogen bonding. QS, QSA, and QSP refer to quenched and stretched, quenched, stretched, and annealed, and quenched, stretched, and polarized nylon films, respectively.

ferroelectricity in QS nylon-11 is induced by electric poling in the mesomorphic δ′ phase with poor hydrogen bonding and the stable polar α phase is nonferroelectric,32 the ferroelectric behavior in the QS nylon-11 should be metastable in nature, and there is no genuine ferroelectric phase associated with a well-defined Curie transition temperature (TC). This is rather different from the ferroelectricity in PVDF and its random copolymers such as P(VDF-TrFE), which exhibit a genuine ferroelectric phase with reversible TC (this will be discussed later). Upon thermal annealing above 100 °C, hydrogen bonding is strengthened in the QS sample. Although the crystalline structure still looks like the mesophase (see Figure 2C), the QSA sample becomes somewhat nonpolarizable. When polarized at RT, large ferroelectric domains form, which are facilitated by the polar zigzag chain conformation (top panel of Scheme 1). After removal of the poling electric field, the large domains are long-lived, resulting in a large Pr. These large ferroelectric domains can persist most likely until the Tm. Upon increasing the temperature, hydrogen bonding gradually weakens due to disordering of hydrogen bonds, which leads to smaller field-induced ferroelectric domains. When polarized at 100 °C, slimmer hysteresis loops are observed. Upon removal of the poling field, these ferroelectric domains still lead to a relatively large Pr. After cooling to RT, these ferroelectric domains grow even larger because of both electric poling and thermal annealing effects. As a result, some domains became too large to be polarizable anymore. In this sense, hightemperature polarization of the QS nylon-11 film should be a better way than RT polarization to produce piezoelectric samples with permanently oriented domains. Temperature-Dependent Ferroelectric and Paraelectric Behaviors in the QS Nylon-12 Films. Following the E

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Figure 3. Continuous bipolar D−E loops for fresh QS nylon-12 films at (A) RT, (B) 50 °C, (C) 75 °C, and (D) 100 °C. At each temperature, a total of 5 × 5 continuous D−E loops were performed in order to achieve steady-state ferroelectric behavior. The first three runs are shown in (A), and only the first runs are shown in (B−D). The time interval between each run was ca. 30 s. The loops in (C, D) were obtained by subtracting ac electronic conductivity. The detailed procedure is given in Figure S6A,B. (E) RT continuous bipolar D−E loops for the QSP@100 °C nylon-12 film. (F) RT continuous bipolar D−E loops for the QSA@100 °C nylon-12 film. The poling frequency is 10 Hz with a sinusoidal waveform.

phase such as those observed in PVDF and its random copolymers. Therefore, there is no well-defined/reversible TC for the QS nylon-12. The change of the mesophase structure in the QSP@RT sample was studied by WAXD. Figure 4A shows both flat-on and edge-on 1D WAXD profiles for the equatorial reflections of the QS and QSP@RT nylon-12 films, which were obtained from the corresponding 2D patterns (see Figure S7). From previous reports,45,46 it was claimed that the QS nylon-12 had a mesomorphic phase, which was highly defective and disordered with poor hydrogen bonds in the direction perpendicular to the polymer chains. Because the mesophase is reminiscent of the stable γ phase,56,57 we assumed a pseudohexagonal structure with the chains along the b-axis of the unit cell. The high-angle reflections were thus indexed as the mixed (001) and (200) reflections. For the QS nylon-12 film, both flat-on and edge-on profiles exhibited an overlapped broad (001/200) reflection from the mesophase, indicating no preferred (001) and (200) orientation in the film. The QSP@RT film exhibited nearly the same overlapped (001/200) reflections as the QS film but had slightly narrower peaks. This result showed that poling at RT did not substantially change the pseudohexagonal packing in the QS nylon-12 crystals. However, a small change was noticed for the (020) reflection of the QSP@RT film when compared to the QS film (Figure 2D). After RT poling at 200 MV/m for 25 cycles, the (020) reflection moved to a slightly higher q value, corresponding to a slightly decreased d020 spacing for the smectic-like hydrogen-bonded structure. Note that without any electric poling, nylon-12 tended to favor antiparallel amide bonds. Upon high-field poling, the more twisted chain conformation should induce internal stresses for the chains in the ferroelectric domains. Therefore, after removal of the poling field, the ferroelectric domains in nylon-12 depolarized via restoring the original chain conformations to release these internal stresses. This explains why there was only a slight decrease of the d020 for the QSP@RT nylon-12 film. Ideally, the in situ WAXD experiment should be performed in the presence of the high poling field in order to observe a more dramatic change of the (020) reflection for the QSP@RT film. However,

above study, temperature-dependent dielectric behavior in the QS nylon-12 film was investigated using bipolar D−E loop tests (Figure 3). Upon continuously poling at room temperature for 15 cycles (i.e., 3 × 5 cycles), the electrical behavior showed a transition from the propeller-shaped DHLs (see the first loop) to broad SHLs (see the last five continuous loops) with gradual increases in the maximum D (Dmax) and Pr. This transition was attributed to the continued growth of ferroelectric domains in the mesomorphic crystals under repeated poling cycles at 200 MV/m.46 Different from the QS nylon-11 film (Figure 1A), the steady-state SHLs appeared to be much slimmer. The time interval between two consecutive runs was ca. 30 s. After the first set of five continues loops and 30 s time interval, the Pr dropped from about 24 to 1.67 mC/m2. After the second set of five bipolar loops and 30 s time interval, the Pr slightly increased to 2.14 mC/m2. From the relatively slim D−E loops and small Pr values, we consider that the large ferroelectric domains, which were generated by high-field electric poling, were relatively short-lived (lifetime ∼10−20 s). This could be attributed to the field-induced, unfavorable, twisted chain conformation needed for the parallel arrangement of amide dipoles in even-numbered nylon crystals. As a result, large ferroelectric domains shrank in size due to hydrogen-bond randomization within 30 s after electric poling, leading to small Pr values. However, ferroelectric domains did not entirely vanish, and this could be seen from the following experimental evidence. First, the third and later poling runs at 200 MV/m could quickly reach the steady-state SHLs without the graduate development of ferroelectric domains, such as that observed during the first run (see Figure 3A). It was likely that some remaining small-sized domains helped the subsequent poling of the QSP@RT sample. Second, as shown in Figure S5B, the QSP@RT film exhibited broad transient loops at 500 Hz poling frequency and 150 MV/m poling field. Heating to >40−50 °C (i.e., around the Tg) made these broad transient loops disappear (compare Figure S5D,E with Figure S5B in the Supporting Information). From these results, we consider that the fieldinduced ferroelectric domains for the QS nylon-12 film are also metastable in nature, and they are not the genuine ferroelectric F

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Figure 4. 1D WAXD profiles for (A, D) the QS and QSP@RT (poled for 25 cycles) nylon-12 films at RT, (B, E) the QS and QSP@100 °C nylon12 films at 100 °C, and (C, F) the QS and QSP@100 °C nylon-12 films cooled back to RT along (A, B, C) the equatorial and (D, E, F) the meridianal directions in the corresponding 2D WAXD patterns (see Figure S7). Positions of (001)γ, (200)γ, and (020)γ reflections for γ phase nylon12 are indicated by short bars in the plots. Solid and dashed curves in (A−C) were obtained along the flat-on and edge-on directions for the film samples, respectively.

Figure 5. Bipolar D−E loops for the QS nylon-12 film at 100 °C under different poling frequencies with a triangular wave function: (A) 5, (B) 10, (C) 100, and (D) 500 Hz. For (A) and (B), the ac electronic conduction is subtracted (see Figure S6C,D).

data (see Figure S6A,B). Because of the weakened hydrogen bonding (by hydrogen bond randomization) at elevated temperatures, only five continuous loops were enough for the steady-state ferroelectric behavior. At 50 °C, relatively slim hysteresis loops were observed (Figure 3B). This again could be explained by the weakened hydrogen bonding and reduced ferroelectric domain size for easier ferroelectric reversibility.

the WAXD data collection time was too long (at least 30 min), and the sample always broke down electrically. In the future, synchrotron radiation should be used to perform this in situ WAXD study. High-temperature D−E loops for the QS nylon-12 film are shown in Figures 3B−D. For the ferroelectric loops at 75 and 100 °C, ac electronic conduction was subtracted from the raw G

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Macromolecules When the temperature reached 75 °C, relatively slim DHLs were seen. At 100 °C, well-defined DHL behavior was obtained, which was reminiscent of the novel relaxor ferroelectric behavior reported for the P(VDF-TrFE-CFE) terpolymers.1 The apparent dielectric constant (εr), which was calculated from the slope in the linear part of the hysteresis loops within ±30 MV/m, significantly decreased from 56 at 50 °C to 28 at 100 °C. Judging from the relatively low value of εr = 28 [note that the εr for relaxor ferroelectric P(VDF-TrFE)based terpolymers was reported to be as high as 50−703], the QS nylon-12 film should be paraelectric under low electric fields. Upon poling to high electric fields (>100 MV/m), some ferroelectric domains with more trans conformation started to grow. However, these ferroelectric domains were not stable, and they quickly transformed back to the paraelectric state as the electric field decreased. The reversible paraelectric-toferroelectric transition caused the DHL behavior observed at 100 °C. To understand the transient nature of the field-induced ferroelectric domains in Figure 3D, the QS nylon-12 film was polarized at 100 °C using different frequencies. DHLs with varying broadness were observed (Figure 5). Upon increasing the poling frequency, the DHLs became broader. This could be attributed to the finite lifetime of the field-induced ferroelectric domains at 100 °C. Judging from the loop shape changes in Figure 5, the lifetime of these induced ferroelectric domains should be around 10 ms. When the poling frequency was lower than 100 Hz, the induced ferroelectric domains would disappear before the poling field reached zero. As a result, a narrow DHL was obtained (see Figure 5A,B). When the poling frequency was higher than 100 Hz, there was not enough time for the induced domains to disappear upon decreasing the poling field. Consequently, a broad DHL would be expected (see Figure 5C,D). If the QS nylon-12 is used for hightemperature electric energy storage,3,5 a low poling frequency will be more beneficial in order to achieve low dielectric loss. To understand the DHL behavior, the crystalline structure of the QSP@100 °C film was studied by WAXD at 100 °C, as shown in Figure 4B,E. The same crystalline structure as that of the QS film was seen for the QSP@100 °C film. This was understandable because after poling at 100 °C, the fieldinduced ferroelectric domains, if any, should quickly depolarize via domain randomization and shrinking in size at elevated temperatures. After cooling back to RT, the (001) and (200) reflections were observed at different positions in the edge-on and the flat-on profiles (Figure 4C). Note that their positions were similar to those of (001)γ and (200)γ reflections of the γ phase, respectively. However, the (020) reflection was located at a higher q value than that of the (020)γ reflection (see Figure 4F), indicating a shorter spacing along the chain direction. Therefore, the crystalline structure for the QSP@100 °C film was not the γ phase but still contained more twisted bonds in the main chain. It was likely that electric poling at high temperatures could promote the thermal annealing effect and eventually strengthened certain hydrogen bonds in the field-induced ferroelectric domains. The remanent ferroelectric domains induced by high temperature poling were testified by the D−E loops shown in Figure S8. Before electric poling, the QS nylon12 film exhibited almost linear D−E loops at 100 °C (Figure S8A). This was different from the D−E loops in Figure 3A because of the high poling frequency of 500 Hz and a lower poling field of 170 MV/m. After 20 (4 × 5) cycles of electric

poling (170 MV/m at 10 Hz), DHLs gradually developed for the QS film at 100 °C (Figure S8B). After ca. 30 s, the QSP@ 100 °C film was again polarized at 500 Hz and 100 °C, and broad DHLs are observed in Figure S8C. This result indicated that electric poling at 100 °C was able to induce remanent ferroelectric domains in the sample. These remanent domains could survive after cooling back to RT and caused significantly broadened hysteresis loops for the QSP@100 °C film (see Figure 3E) than those for the QSA@100 °C film (see Figure 3F). For the QSA@100 °C nylon-12 film, it was possible that hydrogen bonding was so much strengthened by thermal annealing that the ferroelectric switching became difficult, although the crystalline structure still resembled the mesophase of the QS film (compare Figure 4C,F with Figure 4A,D). Because the field-induced ferroelectric domains quickly decreased in size and largely randomized, no obvious crystal orientation was observed in FTIR for the QS, QSP@RT, QSA@100 °C, and QSP@100 °C nylon-12 films (see Figure S9). On the basis of the above results, temperature-dependent ferroelectric behavior for nylon-12 is summarized in Scheme 2. Scheme 2. Summary of Temperature-Dependent Ferroelectric Behavior for Nylon-12a

a

HB stands for hydrogen bonding. QS, QSA, and QSP refer to quenched and stretched, quenched, stretched, and annealed, and quenched, stretched, and polarized nylon films, respectively.

After thermal annealing at 100 °C (or above) followed by cooling back to RT, the QS nylon-12 film becomes hardly polarizable due to strengthened hydrogen bonding, although the crystalline structure is still similar to the mesophase of the QS film. Upon electric poling at RT, large ferroelectric domains are generated by rotation and more twists of the polymer chains in the mesophase. The highly twisted bonds in the nylon-12 crystal creates internal stresses, and thus the induced ferroelectric domains are not stable. After removal of the poling electric field, the induced large domains shrink in size and largely randomize to yield small Pr values. However, ferroelectric domains do not completely disappear. Instead, they only vanish after being heated to above 50 °C. When the paraelectric QS film is polarized at high temperatures (e.g., 100 °C), nanodomains can form at high enough poling fields. However, they are transient in nature with a short lifetime (∼10 ms). Upon decreasing the poling field, the sample returns to the paraelectric state. As a result, a narrow DHL is obtained. After removal of poling electric field and cooling back to RT, certain small ferroelectric domains are retained possibly with H

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Figure 6. Continuous bipolar D−E loops for fresh QS nylon-6 films at (A) RT, (B) 50 °C, (C) 75 °C, and (D) 100 °C. At each temperature, a total of 6 × 5 continuous D−E loops were performed in order to achieve the steady-state ferroelectric behavior. Only the first runs are shown in (B−D). The time interval between each run was ca. 30 s. The loops in (C, D) were obtained by subtracting ac electronic conductivity. The detailed procedure is given in Figure S11A,B. (E) RT continuous bipolar D−E loops for the QSP@100 °C nylon-6 film. (F) RT continuous bipolar D−E loops for the QSA@100 °C nylon-6 film. The poling frequency is 10 Hz with a sinusoidal waveform.

this crystal orientation was overlooked. However, it was concluded that the mesophase in the QS nylon-6 film was more reminiscent of the γ form, rather than the α form (different crystalline forms and the Brill transition for nylon-6 should refer to refs 56 and 58). Given the positions of the (001)γ (i.e., the hydrogen-bond direction) and (200)γ (i.e., the direction normal to the hydrogen-bonding sheets) reflections of the γ form (see Figure 7A), we could index the peak reflections in the flat-on and edge-on profiles for the mesophase as (001) and (200), respectively. The observation of the (001) reflection in the flat-on profile indicated that hydrogen bonds were parallel to the film as a result of uniaxial stretching. Similar infilm orientation of hydrogen bonds was also reported for stretched nylon-6 films.59 The reason for reduced polarizability for the QS nylon-6 film could be ascribed to the smaller d200 of 0.412 nm, as compared to the d200 of 0.426 nm in the QS nylon-12 film (Figure 4A), which limits the switching of amide dipoles in the QS nylon-6 film. This leads to a slower transition from the DHLs (see the first five continuous loops) to the ferroelectric SHLs for the QS nylon-6 film. After poling for 30 (6 × 5) continuous cycles, the QSP@RT film exhibited slightly separated (001) and (200) reflections in the flat-on and edgeon WAXD profiles (see Figure 7A). However, the (001) reflection appeared in the edge-on profile, suggesting that highfield electric poling could align the hydrogen bonds in the mesophase along the field direction (i.e., normal to the film). In addition, the (020) reflection for the smectic-like structure along the chain axes was weak, indicating a poor alignment and thus weak hydrogen bonding of the amide bonds in the QS nylon-6 film. After poling, however, the (020) reflection became much stronger. It is possible that electric poling helped the alignment of amide bonds, leading to a long-range ordering of the smectic-like structure along the chain axes. In addition, the d020 of the QSP@RT film became slightly larger (0.788 nm)

broken hydrogen bonds. Consequently, the cooled QSP@100 °C film can be easily polarized to exhibit certain ferroelectric loops. Temperature-Dependent Ferroelectric and Paraelectric Behavior in Nylon-6 Films. To generalize the DHL behavior shown by nylon-12 at elevated temperatures, we continued to study nylon-6 using continuous bipolar D−E loops. Six runs of RT continuous D−E loops for the QS nylon6 film are shown in Figure 6A. Because of the double dipole density, nylon-6 exhibited higher Dmax and Pr than nylon-12. However, the QS nylon-6 film showed reduced polarizability when compared with the QS nylon-12 film polarized at the same poling field. For example, with 10 (2 × 5) continuous poling cycles at 200 MV/m and 10 Hz at RT, the QS nylon-12 film reached steady-state ferroelectric loops. For the QS nylon6 film, at least 30 (6 × 5) poling cycles were necessary to reach the steady-state ferroelectric loop. The Pr gradually increased with continued poling runs; for the first, second, and third runs (ca. 30 s between consecutive runs), the Pr slightly increased from −1.38 mC/m2 to −6.10 and −8.17 mC/m2, respectively. These values were higher than those of the QS nylon-12 film polarized at 200 MV/m and RT, indicating gradual buildup of aligned ferroelectric domains due to stronger hydrogenbonding interaction and thus a slower relaxation rate. To understand the suppressed polarizability of the QS nylon6 film, crystalline structures of the QS and QSP@RT nylon-6 were studied by WAXD. 1D WAXD profiles along the meridianal and equatorial directions are shown in Figures 7A and 7D, respectively. For the QS nylon-6 film, the reflection peak in the edge-on profile (dashed line) was located at a higher q value than that in the flat-on profile (solid line), indicating certain orientation of the mesomorphic (i.e., the socalled β phase55) crystals in the uniaxially stretched film. In our previous study,46 only the flat-on WAXD was studied, and thus I

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Figure 7. 1D WAXD profiles for (A, D) the QS and QSP@RT (poled for 25 cycles) nylon-6 films at RT, (B, E) the QS and QSP@100 °C at 100 °C, and (C, F) cooled back to RT, respectively, along (A, B, C) the equatorial and (D, E, F) the meridianal directions in the corresponding 2D WAXD patterns (see Figure S12). Positions of α phase [(200)α, (002)α, and (020)α] and γ phase [(001)γ, (200)γ, and (020)γ] reflections for nylon-6 are indicated by blue and red short bars in the plots, respectively. Solid and dashed curves in (A−C) were obtained along the flat-on and edge-on directions for the film samples, respectively.

enhanced. Only the first five continuous poling cycles were enough to reach nearly steady-state ferroelectric loops. At 50 °C, the QS nylon-6 film exhibited relatively slim SHLs with EC = 44 MV/m and Pr = 34 mC/m2. At 75 °C, relatively broad DHLs were obtained with EC and Pr being 28 MV/m and 17 mC/m2, respectively. Finally, when temperature reached 100 °C, relatively slim DHLs were seen with the apparent εr being 60 (Figure 6D). Dmax reached ca. 93 mC/m2, nearly double that for the QS nylon-12. Again, these DHLs could be attributed to the reversible paraelectric-to-ferroelectric transition in the mesomorphic nylon-6 crystals. The higher εr and broader loop for the QS nylon-6 film than those for the QS nylon-12 film were attributed to the higher density of amide bonds and thus stronger hydrogen-bonding interaction. It is expected that the DHLs would become increasingly narrower as temperature further increased. However, the breakdown strength substantially decreased due to enhanced electronic conduction in the sample.60 This prevented us from determining ferroelectric properties at even higher temperatures for nylon-6.

than that (0.766 nm) of the QS film. It is possible that the highly disordered chain conformation in the QS nylon-6 mesophase was altered after electric poling and became more extended (or planar zigzag). Similar to the QSP@RT nylon-12 film, we speculate that the field-induced ferroelectric domains in the QSP@RT nylon-6 film should also be metastable. This was confirmed by the heating experiments in Figure S10. Compared with the nonpoled QS film (Figure S10A), the QSP@RT film exhibited much broadened hysteresis loops when polarized at 500 Hz and RT (Figure S10B). After annealing the QSP@RT film at 40 °C for 5 min, the polarizability decreased (Figure S10C). When increasing the annealing temperature to 60 °C, the QSP@RT film eventually became much less polarizable (Figure S10E). This experimental result demonstrated that the field-induced ferroelectric domains in the QSP@RT nylon-6 film were metastable. High-temperature ferroelectric behavior for the QS nylon-6 film was studied using D−E loop tests (Figure 6B−D). At elevated temperatures, dipole and domain mobilities were J

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Figure 8. Bipolar D−E loops for the QS nylon-6 film at 100 °C under different poling frequencies with a triangular wave function: (A) 5, (B) 10, (C) 100, and (D) 500 Hz. The ac electronic conductivity is subtracted for (A) and (B) (see Figure S11C,D).

To understand the DHL behavior for the QS nylon-6 film at 100 °C, the sample was polarized at 100 °C using different poling frequencies (Figure 8). Similar to the case for the QS nylon-12 at 100 °C, relatively slim DHLs were observed when the poling frequency was below 10 Hz, and relatively broad DHLs were seen when the poling frequency was above 100 Hz. Therefore, the lifetime of the field-induced nanodomains was again considered to be around 10 ms. When the poling frequency was slow, the field-induced nanodomains disappeared before the field decreased to zero. As a result, narrow DHLs were obtained (Figure 8A,B). The equatorial and meridianal 1D WAXD profiles for the QS and the QSP@100 °C nylon-6 films at 100 °C are shown in Figure 7B,E. For the fresh QS sample, the (001) and (200) reflections in the flat-on and the edge-on profiles became more overlapped at 100 °C. The QSP@100 °C film showed overlapped (001/200) reflections in both edge-on and flat-on profiles. This was similar to the Brill transition55,58 and was considered to be caused by the back-and-forth electric poling of the twisted nylon-6 chains. The (020) reflection became much more intense for the QSP@100 °C film than that for the QSP nylon-6 films (Figure 7E) because electric poling at high temperatures helped align the amide bonds in a smectic-like structure. After cooling to RT, the QSP@100 °C nylon-6 film showed separated and relatively sharp (001) and (200) reflections in the edge-on and the flat-on profiles, respectively (Figure 7C). The observation of the (200) reflection in the flat-on WAXD profile suggested that the hydrogen-bonded sheets were aligned more or less parallel to the electric field direction. On the contrary, the QSA@100 °C film showed a broad (200) reflection in the edge-on profile, and thus some hydrogenbonded sheets remained parallel to the film (Figure 7C). For the QSP@100 °C film at RT (Figure 7F), the intensity of the (020) reflection was much higher than that of the QSA@100 °C film, indicating the improved smectic-like structure after electric poling at high temperatures. Although the improvement of crystalline structure in the cooled QSP@100 °C film was not that dramatic (e.g., compared to that in the cooled QSA@100 °C film), the ferroelectric behavior could be significantly influenced. Similar to the QSP@100 °C nylon-12 film, the QSP@100 °C nylon-6 film exhibited much better polarizability (Figure 6E) than the nonpoled QSA@100 °C film after cooling to RT (Figure 6F). This result indicated that electric poling at high temperatures created certain ferroelectric domains, which did not completely disappear after cooling to RT for the QSP@ 100 °C nylon-6 film. The orientation of hydrogen-bonded amide groups in various nylon-6 films was also studied by FTIR (see Figure S13). After normalizing the symmetric CH2 stretching band at 2852 cm−1, both QSP@RT and QSP@100 °C films exhibited decreased N−H stretching band intensity at 3295 cm−1. This was consistent with the fact that the amide

bonds in the remanent ferroelectric domains were aligned parallel to the film normal direction by the poling electric field, as observed in the WAXD profiles in Figure 7C. Note that this was somewhat different from the case for nylon-12 (see Figure S9) and could be attributed to the much stronger hydrogenbonding interaction in the ferroelectric domains of nylon-6. Discussion of the Ferroelectric Behavior in Mesomorphic n-Nylons. It is desirable to understand the fundamental physics of ferroelectric behavior observed in mesomorphic n-nylons. Using PVDF and its random copolymers as an example,1,61−63 the following viewpoints are presented for the ferroelectric behavior in long-chain polymers. Fundamentally, intermolecular interactions (dipolar and hydrogen-bonding) and interchain distance in the crystals are important for the observed ferroelectricity in polymers. The intermolecular interaction is intimately dependent upon the dipole moment of the repeat unit and the chain conformation. For the neat PVDF, the rigid dipole moment of the VDF repeat unit is about 2.1 D in the β phase with an all-trans conformation (note that the dipole moment increases to 3.0 D due to the reactive interaction between the rigid molecular dipoles and the electronic polarization64,65). The intermolecular interaction in the β PVDF crystal is weak enough to allow the ferroelectric switching under a high enough poling field (i.e., above the EC ∼ 80 MV/m). However, the intermolecular interaction is still very strong, and large ferroelectric domains easily form, resulting in significant hysteresis. One way to decrease the intermolecular interaction is to copolymerize with monomers that have smaller dipole moments, such as TrFE (dipole moment of 1.05 D) or tetrafluoroethylene (TFE, dipole moment = 0). Indeed, both P(VDF-TrFE) and P(VDF-TFE) random copolymers exhibit enhanced ferroelectricity with a decreased EC [e.g., EC ∼ 50 MV/m for the copolymer with a 50/50 (mol/mol) composition] for the low-temperature ferroelectric phase having an all-trans conformation. However, the intermolecular interaction is still fairly strong, and a relatively large hysteresis is observed. To further decrease the intermolecular interaction, twisted chain conformations must be adopted to avoid direct alignment and interaction of molecular dipoles. Meanwhile, twisted chain conformations increase the interchain distance and ease the dipolar switching of the polymer chains in the crystal. To induce the twisted conformations, large termonomers such as CFE and CTFE are terpolymerized with VDF and TrFE (note that TrFE is required to enable repeat-unit crystal isomorphism; otherwise, the large termonomers are largely excluded from the crystalline region). Because of the reduced intermolecular interaction between twisted chains, relaxor ferroelectric behavior with slim hysteresis loops can be achieved for the P(VDF-TrFE)-based terpolymers. Depending on the stability of the twisted chain conformation, either DHLs or SHLs can be achieved. If the twisted chain conformation can be polarized by the high field K

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hysteresis loop is obtained. In addition to the dipolar interactions among the dangling amide bonds, hydrogen bonding will further strengthen the intermolecular interaction. For example, by simply annealing the QS nylons films at 100 °C for 5 min, the QSA@100 °C nylon films become largely nonpolarizable at 200 MV/m (see Figures 1F, 3F, and 6F), despite the fact that little structural difference from the original mesomorphic phase in the QS films could be detected by WAXD and FTIR. Apparently, 5 min thermal annealing at 100 °C must strengthened the hydrogen bonding, which prevents ferroelectric switching in the QSA@100 °C samples. Third, the twisted chain conformations in mesomorphic nnylons are not stable. This is similar to the cases for the δ and γ PVDF crystals. Upon electric poling, the twisted chains of nylon-11 tend to transform into a more trans conformation, generating nonpolarizable domains. This is exactly observed for the nonpolarizable component in the QS nylon-11 films repeatedly polarized at RT (Figure S1) or 100 °C (Figure 1E). For nylon-12 and nylon-6, highly twisted chains in the fieldinduced ferroelectric domains are not stable. They will transform into less twisted conformations upon removal of the electric field. As a result, DHLs are observed at high temperatures (Figures 5 and 8). Similar to the δ and γ PVDF crystals, there should be no reversible TC for n-nylons (note that the TC for nylon-11 must be above the Tm). As shown in the DSC curves for the QSP@RT nylon-6, -11, and -12 samples (Figure S14), no Curie transition peaks could be observed during the first heating process. Finally, to achieve relaxor ferroelectric behavior for n-nylons, one has to make the twisted conformation more stable. Namely, the twisted chains can easily adopt some polar arrangement in the crystal and not able to transform into the trans conformation. This could be achieved by terpolymerization of nylon-11 and nylon-12 monomers and N-methylated nylon-11 or nylon-12 monomers. We speculate that the bulky N-methyl groups will prevent hydrogen bonding as well as inducing twisted conformations in the main chains. Experiments are currently on the way, and results will be reported in the future.

into a more trans conformation to form transient ferroelectric domains, DHLs will result. This is the case for the P(VDFTrFE-CFE) 59.2/33.6/7.2 (molar ratio) terpolymer.1 If the twisted chain conformation is stable upon electric poling, slim SHLs will result; this is the case for the P(VDF-TrFE-CTFE) 62.2/30.2/7.6 terpolymer.28 However, CTFE is not a prerequisite for the slim SHL behavior. Recently, a P(VDFTrFE-CTFE) 78.1/16.5/5.4 terpolymer was reported to exhibit the DHL behavior,66 probably because the different composition may have favored the metastable twisted conformation. Because of the reduced intermolecular interactions, both P(VDF-TrFE) copolymers (the TrFE content >18 mol %67) and P(VDF-TrFE-X) terpolymers can exhibit thermodynamically reversible Curie transition under ambient pressure. From the above discussion, the ferroelectric behavior of other phases in PVDF and its copolymers can be also understood. First, above the TC, P(VDF-TrFE) exhibits the high-temperature paraelectric phase, where the chains in the crystal adopt highly mobile, mixed TG and TTTG conformations.61 Because of the stable twisted conformation at high temperatures, slim SHLs are observed for the paraelectric phase of P(VDF-TrFE).68 This is different from the DHLs observed for paraelectric nylon-12 and nylon-6 in this study. Second, from the crystal structure point of view, the α phase PVDF should be nonferroelectric. However, because of its twisted chain conformation with reduced intermolecular interaction (ca. 1.2 D/repeat unit) and enlarged interchain spacing, the nonpolar α PVDF can be polarized into the polar δ phase when the poling field is above 100 MV/m.61,69 Third, the δ and γ phases supposedly should exhibit slim hysteresis loops, given their twisted conformation and enlarged interchain spacing. However, no slim loops are observed. This is primarily attributed to the unstable twisted conformation for the δ and γ phases. Upon electric poling above ca. 150 MV/m, the twisted conformation in the δ or γ phase will easily transform into the β phase with the all-trans conformation.61 As a result, no slim hysteresis loops are observed. On the basis of the above discussion, the ferroelectric behavior of n-nylons can be understood as follows. First, the intermolecular interaction in nylon crystals with the all-trans conformation (e.g., the α phase) is too strong to allow any ferroelectric switching due to the large dipole moment of the amide bonds. Note that the rigid dipole moment of a nonhydrogen-bonded amide is 3.7 D.35 When hydrogen bonded, this dipole moment is enhanced and further increases the intermolecular interaction, depending on the hydrogen-bond distance and how many amide groups are bonded together. Second, a viable way to achieve ferroelectric switching for nylon crystals is to have the chains in the crystal adopt twisted conformations to weaken the intermolecular interaction. For the γ phase, the nylon chain contains one twist per repeat unit in the amide N−C(O) bond. Because of this twisted conformation, the γ phase nylon-12 should be more polarizable. Other than the γ phase, the mesophases in quenched n-nylons contain more than one twists in the chain conformation [note that unlike PVDF-based polymers, there are no definite vibrational (i.e., infrared or Raman) signatures for twisted conformations in nylon-based polymers due to the simple aliphatic main chain]. As a result of reduced intermolecular interaction from twisted chains with broken or weakened hydrogen-bonding, ferroelectric switching is achieved for nnylons.46 However, the intermolecular interaction is still strong, and only the normal ferroelectric behavior with a broad

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CONCLUSIONS In summary, the temperature-dependent ferroelectric and paraelectric behavior for mesomorphic nylon-11, -12, and -6 was investigated by revealing the crystalline structure−ferroelectric property relationships. In contrast to the odd-numbered nylon-11, which still exhibited broad ferroelectric SHLs at high temperatures, even-numbered nylons such as nylon-12 and nylon-6 showed novel DHLs because of weakened hydrogenbonding interactions and unfavorable twisted chain conformations in the field-induced ferroelectric domains. At low electric fields, the even-numbered nylons were paraelectric. High electric field poling induced nanodomains, and the reversible paraelectric-to-ferroelectric transition led to the DHL behavior. On the basis of a frequency-dependent D−E loop study, the poling field-induced nanodomains in even-numbered nylons at 100 °C should have a finite lifetime around 10 ms. For poling frequencies below 100 Hz, narrow DHLs were observed, whereas above 100 Hz broad DHLs were seen. Because of a higher dipole density and thus a stronger hydrogen-bonding interaction, the mesomorphic nylon-6 film exhibited a higher Dmax with broader DHLs. These narrow DHLs may show potential for electric energy storage, e.g., to be used in multilayer capacitor films.5 L

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01137. D−E loops for the QSP@RT-250 nylon-11 film, subtraction of ac electronic and/or ionic conduction from raw D−E loops for the QS nylon-11, -12, and -6 films, 2D WAXD patterns for various nylon-11, -12, and -6 films, FTIR spectra for various nylon-11, -12, and -6 films, D−E loop studies of thermal stability of fieldinduced ferroelectric domains for the QSP@RT nylon-12 and -6 films, and DSC thermograms for the QSP@RT nylon-11, -12, and -6 films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(L.Z.) E-mail [email protected]; Tel +1 216-368-5861. ORCID

Zhongbo Zhang: 0000-0001-5294-444X Lei Zhu: 0000-0001-6570-9123 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Science Foundation (DMR-1402733). Z.Z. acknowledges financial support from China Scholarship Council.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01137 Macromolecules XXXX, XXX, XXX−XXX