Unified Understanding of Ferroelectricity in n-Nylons: Is the Polar

Apr 12, 2016 - Over the past decades, it has been commonly considered that ferroelectricity is closely related to the polar crystalline structure of o...
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Unified Understanding of Ferroelectricity in n‑Nylons: Is the Polar Crystalline Structure a Prerequisite? 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: Over the past decades, it has been commonly considered that ferroelectricity is closely related to the polar crystalline structure of odd-numbered nylons, and evennumbered 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 evennumbered 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.

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poly(vinylidene fluoride) (PVDF), nylons, and poly(ethylene terephthalate) (PET).1 The only solid-state paraelectric polar polymers discovered so far are isomorphic crystals of PVDFbased random copolymers [with trifluoroethylene (TrFE) or tetrafluoroethylene (TFE) comonomers]15,16 and terpolymers [with TrFE and 1,1-chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) comonomers] above their Curie temperatures (TC).17 However, accordingly to the Langevin or Onsager theory,18 the dielectric constant of paraelectric dielectrics sensitively depends on temperature (i.e., decreases with increasing temperature). Ferroelectric domains with parallel dipoles (i.e., spontaneous polarization) can form in polar crystalline and liquid crystalline structures of polymers without application of an external electric field.19 The size and spatial organization of these ferroelectric domains are critical for the macroscopic ferroelectric property of polymers. When the domain size is large, the entire domain must be switched in order to switch an individual dipole in the middle of the domain. As a result, normal ferroelectric behavior with a rectangular shaped hysteresis loop is obtained. This normal ferroelectric behavior

INTRODUCTION Recently, a prospective roadmap linking between linear dielectric and nonlinear ferroelectric polymers was proposed by implementing dipolar groups in a polymer matrix.1 With progressively enhanced dipole−dipole or domain−domain interactions, large polarization and thus high dielectric constant with low dielectric loss can be realized for dielectric polymers, which are promising for potential applications such as electric energy storage,1−3 electromechanical actuation,4−6 and electrocaloric cooling.7−9 The first candidate class is dipolar glass polymers, where dipolar groups are confined in the glassy matrix. Because of this confinement, the interaction among neighboring dipoles is limited, and individual dipoles respond to the external electric field more or less independently. In this sense, they are reminiscent of spin glasses in magnetic materials.10,11 Consequently, the dielectric constant can be increased when highly dipolar groups are introduced.12 In general, the enhancement in dielectric constant is more significant for side-chain12,13 rather than main-chain14 polymers because side chains are less sterically hindered than the main chain. The second candidate class is paraelectric polar polymers, where dipolar groups strongly interact with each other to further enhance the dielectric constant. The most common paraelectric behavior is found in polymer melts, such as molten © XXXX American Chemical Society

Received: December 20, 2015 Revised: April 4, 2016

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Macromolecules is suitable for applications such as nonvolatile memory20−22 and piezoelectric transducers in sensors and actuators.19 When the domain size is in the nanometer scale (i.e., nanodomains), novel relaxor ferroelectric (RFE) behavior with narrow single and double hysteresis loops (SHL and DHL) can be obtained.1,17 Effective pinning in repeat-unit isomorphic crystals is important for the generation of nanodomains. For example, DHL can be achieved in P(VDF-TrFE-CFE) terpolymers with weak physical pinning by the large CFE comonomers,17,23,24 while SHL can be achieved either in electron-beam-irradiated P(VDF-TrFE) crystals with chemical pinning (i.e., cross-linking)25−27 or in P(VDF-TrFE-CTFE) terpolymers with strong physical pinning by the even larger CTFE comonomers.28,29 Given the above understanding of the novel ferroelectric behavior in P(VDF-TrFE)-based copolymers and terpolymers, one can apply the concept of nanodomains formed by crystal pinning in isomorphic polymer crystals to other ferroelectric polymer systems. In addition, the synthesis of P(VDF-TrFE)based terpolymers requires special facilities and infrastructures, and their high prices inhibit near-term commercialization. Therefore, it is highly desirable to search for viable but cheaper ferroelectric polymers, which can also exhibit the narrow SHL and DHL ferroelectric behavior. Among other ferroelectric polymers, aliphatic polyamides or nylons have advantages of lower cost, easier synthesis, and better potential to tune the crystalline structure. For ferroelectric nylons, the amide bonds (dipole moment of 3.7 D) are strongly hydrogen-bonded (Hbonded) and form two-dimensional (2D) sheets in their stable crystalline modifications. Note that these stable nylon modifications refer to the crystal structures below the Brill transition temperature,30,31 above which a pseudohexagonal phase with disordered H-bonding is present in the sample. Since the crystalline structures of odd-numbered nylons are polar in nature (i.e., parallel H-bonds), they were predicted and found to exhibit ferroelectricity.32 On the other hand, evennumbered nylons possess nonpolar crystalline structures (antiparallel H-bonds), and hence were expected to be nonferroelectric. In other words, polar crystalline structures have been considered as the prerequisite for ferroelectricity in nylons.32 However, no ferroelectricity could be observed for the polar α phase of nylon-11.33,34 It is considered that the tight crystalline packing and strongly bonded H-bonding sheets in the α phase prevented dipole reversal before electrical breakdown of the sample. Instead, melt-quenched and stretched odd-numbered nylons with a defective crystalline structure (e.g., the δ′ phase for nylon-1135) have been reported to show ferroelectric properties.36−42 Even the oriented γ or γ′ phase (obtained by solvent treatment of the δ′ phase) nylon-11 could exhibit ferroelectric switching.32,43 The ferroelectric switching in the δ′ phase of nylon-11 was evidenced by Fourier transform infrared (FTIR) and wide-angle X-ray diffraction (WAXD) studies on oriented films.39 Neither the quenched δ′ phase nor the γ/γ′ phase of nylon-11 films has well-organized polar and long-range H-bonded structures as that of the α phase. Instead, both phases have a considerable fraction of gauche bonds with short-range and disordered H-bonds. In addition, the interchain distances in both phases are larger than the d-spacing of the H-bonded sheets in the α phase due to the twisted chain conformations, which could facilitate dipole and domain switching in nylon-11 crystals. The above discussion, however, does not clarify the relationship between ferroelectricity in nylons and their

crystalline structures. In other words, is the polar crystalline structure a prerequisite for the ferroelectricity observed in nylon samples? To answer this question, we quickly quenched n-nylon samples (nylons-12, -6, and -11) and confirmed the mesophase formation using WAXD and FTIR studies. All the quenched and stretched n-nylons showed significant D−E hysteresis loops upon electric poling, whether n was even or odd. In contrast, when the quenched samples were annealed at 150 °C for 20 h, the mesophases gradually transformed toward the more stable γ and/or α crystalline forms, and the samples no longer showed ferroelectric hysteresis loops. From these results, we conclude that high-field electric poling induces the formation of ferroelectric domains and thus ferroelectric reversal in the mesophases of all fast quenched n-nylons.

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EXPERIMENTAL SECTION

Materials. Nylon-6 and -12 pellets were purchased from SigmaAldrich (St. Louis, MO). Nylon-11 resin (Arkema Rilsan Besno TL) was purchased from PolyOne (Avon Lake, OH). All nylons were used as received without further purification, except for thorough drying in a vacuum oven at 70 °C for 3 days before hot-pressing. Film Fabrication and Processing. Four types of film samples were fabricated via different combinations of hot-pressing, quenching, stretching, and annealing. (1) Quenched (Q) sample. Using nonsticking aluminum foils, nylon samples were hot-pressed at a temperature about 30−40 °C above their melting temperatures (Tms), i.e., 210 °C for nylon-11 and nylon-12, and 260 °C for nylon-6. After melting at the indicated temperature for 5 min, samples were immediately quenched into an isopropanol/dry ice bath around −78 °C. (2) Quenched and stretched (QS) sample. The hot-pressed and quenched films were uniaxially stretched to an extension ratio of ca. 300% at a speed of 12.7 mm/min, using a home-built stretching apparatus. (3) Quenched, stretched, and annealed (QSA) sample. The QS samples were annealed under tension at 150 °C for 20 h in a vacuum oven. (4) Quenched, annealed, and stretched (QAS) sample. The Q samples were annealed at 150 °C for 20 h, followed by uniaxial stretching (an extension ratio of ca. 300%) at room temperature. Instrumentation and Characterization. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q100 DSC at a scanning rate of 10 °C/min. The Q samples were prepared by quickly quenching the molten samples in tightly sealed hermetic aluminum DSC pans into liquid nitrogen. The quenched and annealed (QA) samples were prepared by annealing the quenched samples at 150 °C for 20 h. The directly annealed (A) samples were prepared by cooling the molten samples to 150 °C for isothermal crystallization for 20 h. Around 2 mg of samples was used for DSC study. Fourier transform infrared (FTIR) was carried out using a Thermo Nicolet Nexus 870 FTIR spectrometer in the transmission mode. The scan resolution was 2 cm−1 with total of 32 scans. Two-dimensional (2D) wide-angle X-ray diffraction (WAXD) experiments were carried out at the synchrotron beamline X27C, National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). The wavelength (λ) of the incident X-ray was 0.1371 nm. A MAR charge-coupled device (CCD) was used as the area detector, and the typical data acquisition time was 30 s. The dspacing was calibrated by silver behenate with the first-order reflection at a scattering vector [q = (4π sin θ)/λ, where θ is the half scattering angle] of 1.076 nm−1. Additional WAXD experiments were performed using a Rigaku WAXD instrument (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 developed by Stonybrook Technology and Applied Research, Inc. B

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Macromolecules Scheme 1. Summary of Different Crystal Modifications in n-Nylons: α Form, γ Form, and the Quenched Form (i.e., Mesophase)a

a

Note: nylon-6 is used here to represent different crystalline modifications of even-numbered nylons.

The D−E hysteresis loop measurements were carried out using a Premiere II ferroelectric tester (Radiant Technologies, Inc., Albuquerque, NM) in combination with a Trek 10/10B-HS highvoltage amplifier (0−10 kV AC, Lockport, NY). The applied voltage had a bipolar sinusoidal waveform in a frequency range of 0.1−1000 Hz. Silver (Ag) electrodes (ca. 2.5 mm2 diameter) with a thickness around 50 nm 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 a silicone oil (Fisher 460-M3001) bath to avoid corona discharge in air. The temperature was controlled by an IKA RCT temperature controller (Wilmington, NC). A low temperature of 0 °C was achieved by cooling the silicone oil bath using a dry ice bath. A home-built sample fixture was used for connecting the electrodes on both sides of the film to the interface of the Radiant ferroelectric tester using highvoltage cables.

most stable phase for nylon-11 and is a common form for nylon-6 as crystallized from the melt. However, it is not a common crystalline form for nylon-12. In the α form of nylon11, all the H-bonded amide dipoles point to the same direction and form a polar crystalline structure, and thus it should be ferroelectric. However, the H-bonding sheets are so tightly packed that no dipole rotation is possible before the electrical breakdown of the sample. As a result, the α form nylon-11 is actually nonferroelectric.33,34 The γ form is another common form for n-nylons, especially for long (n ≥ 12) aliphatic polyamides.31 For nylon-6, the γ form is almost as stable as the α phase. Hence, both modifications may form when crystallized from the melt. Differently, the γ form is the stable phase for nylon-12 during melt crystallization.45,46 From the side view of the γ form in Scheme 1, antiparallel H-bonds are formed between parallel chains in the crystal. To complete all H-bonds, however, a twist must occur at each amide bond [i.e., the C(O)−NH bond].45,46 This conformation makes the unit-cell dimension along chain axis slightly shorter (b = 3.22 nm) than that (3.27 nm) of the planar zigzag conformation. From the top view in Scheme 1, a pseudohexagonal structure is seen with staggered and antiparallel H-bonds. Although the larger interchain distance in the γ form may favor the rotation of amide dipoles under electric poling, dipole switching is still inhibited due to the nonpolar crystalline structure with antiparallel and strong H-bonds. The quenched form of nylons can be prepared by quenching the molten thin films in a dry ice/isopropanol bath, and it is often referred to as the metastable mesophase. Several features have been reported that differentiate it from other stable crystalline phases as well as the amorphous phase. The mesophase is still a crystalline phase but is highly defective and disordered in the direction perpendicular to the polymer chains (see the top view in the right panel of Scheme 1). First, the chain conformation may contain multiple twists per repeat unit (although it is uncertain how many twists and where they

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RESULTS AND DISCUSSION Polymorphism and Mesophase Structure in EvenNumbered n-Nylons. According to previous studies,36−42 ferroelectricity can be achieved in odd-numbered n-nylons only in quenched samples, which exhibit a mesomorphic phase (or mesophase) with more or less random H-bonds.35 After annealing at high temperatures, the samples would transform into thermally stable crystalline structures with more organized H-bonding sheets. As a result, the nylon samples will not exhibit ferroelectricity anymore.37 It is apparent that the crystalline structure of n-nylons plays an important role in their ferroelectricity. Hence, it is necessary to review different crystalline structures, as well as the quenched crystalline form in n-nylons. Scheme 1 shows two stable crystalline forms (i.e., α and γ forms), together with the quenched mesophase, for the nnylons used in this study. For simplicity, nylon-6 is used for the schematic illustration of different crystalline structures. In the α modification, even-numbered n-nylons chains are packed into an antiparallel arrangement with fully extended chain (or planar zigzag) conformation.31,44 This is clearly seen from the side view of the crystalline structure in Scheme 1. The α form is the C

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Figure 1. First heating DSC thermograms for quenched (Q), quenched and annealed (QA), and annealed (A) n-nylon samples: (A) nylon-6, (B) nylon-12, and (C) nylon-11. The heating rate is 10 °C/min.

Figure 2. 2D WAXD patterns and the corresponding 1D profiles for fast quenched thin film (ca. 10 μm) samples: (A) nylon-12 and (B) nylon-6. Reflections for the α and γ nylon-12 and nylon-6 are indicated in the plots.

Figure 3. FTIR spectra for the Q and QA nylon thin films: (A) nylon-12 and (B) nylon-6.

which an immediate cold crystallization peak was observed. The cold crystallization peak was more well-defined for nylon-6 (at 66 °C) than those for nylons-12 and -11 (at 62 and 59 °C, respectively), suggesting that the quenched amorphous phase in nylon-6 had a stronger tendency to cold crystallize than those in nylons-12 and -11. Cold crystallization was not seen for quenched and annealed (QA) and annealed (A) samples, which only showed Tgs between 25 and 75 °C (see Figure 1). After subtracting the heat of crystallization from the cold crystallization peaks, the crystallinities for Q nylon-6, -12, and -11 samples were calculated to be 22, 22, and 19 wt %, respectively (the heats of fusion for perfect nylon-6, -12, and -11 are 230, 224, and 225 J/g, respectively47). These crystallinity values were lower than those (ca. 34−43 wt %, see Figure 1) for the QA and A samples. On the basis of an ultrafast chip-DSC study,48,49 nylons could be quenched into 100% amorphous glasses when the cooling rate was above

are). Second, the chain packing is relatively poor (i.e., pseudohexagonal), and H-bonding among the amide groups is poorly organized. As a result, the interchain distance is enlarged as compared to those of the stable α or γ phase. However, the crystalline structure is more ordered along the chain direction, i.e., smectic-like, with the amide groups aligning into layers (see the side view in the right panel of Scheme 1).35 This mesophase structure is somewhat similar to the high temperature pseudohexagonal phase above the Brill transition temperature.30,31 We speculate that these features can facilitate dipole rotation upon the application of a poling field for both even- or odd-numbered nylons. To obtain the mesophase for n-nylons, the samples were quickly quenched from 30 °C above their Tms into a dry ice/ isopropanol bath at ca. −78 °C. These Q samples were subsequently subjected to the DSC study; the results are shown in Figure 1. For all samples, the Tg was clearly identified, above D

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Figure 4. 2D WAXD patterns for (A) quenched and stretched (QS), (B) quenched, annealed, and stretched (QAS), and (C) quenched, stretched, and annealed (QSA) nylon-12 films at (left) 25 °C, (middle) 150 °C, and (right) cooled back to 25 °C. (D) d001 and d020 changes as a function of temperature for the QS, QAS, and QSA nylon-12 films.

Figure 5. Continuous bipolar D−E loops for (A−D) QS, (E−H) QAS, and (I−L) QSA nylon-12 films. The poling frequency varies from left to right: (A, B, E, F, I, J) 10 Hz, (C, G, K) 100 Hz, and (D, H, L) 1000 Hz with a sinusoidal wave function. All tests were performed at room temperature.

200−500 K/s. These DSC results showed that completely amorphous samples could not be obtained due to a slower cooling rate for the quenching method used in this study. To confirm the mesophase structure for the Q nylon samples, WAXD was used and results for the quenched nylons12 and -6 are shown in Figure 2. Two typical reflections were observed. Because the mesophase is reminiscent of the γ phase, we assume a monoclinic (or pseudohexagonal) structure with the chains along the b-axis of the unit cell. The low-angle (020) reflection is from the smectic-like structure of H-bonded amide groups, and the high-angle broad reflection is mixed (001) and (200). Note that the (020) reflection for the Q nylon-12 was better defined than that for the Q nylon-6. However, the tip of the (001/200) reflection for the Q nylon-12 was not as sharp as that for the Q nylon-6. These observations suggest that the H-

bonded smectic-like structure in the Q nylon-12 was more ordered than that in the Q nylon-6, whereas the lateral chain packing for the Q nylon-12 was poorer than that in the Q nylon-6. FTIR can provide further understanding of chain conformation in the quenched mesophase. Figure 3A shows FTIR spectra for both Q and QA nylon-12 films. Typical absorption bands for the γ form were observed at 1192, 946, and 627 cm−1.50 These γ form bands were still observed for the Q sample; however, the absorption intensities were much weaker than those for the QA sample. For the QA nylon-6 sample, typical α absorption bands were observed at 930, 960, 1024, 1203, 1416, and 1478 cm−1 and γ absorption bands at 973, 1170, 1234, and 1439 cm−1, indicating that both forms coexisted in the crystals.51 Again, the Q nylon-6 film showed E

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Figure 6. Continuous (30) bipolar D−E loops for (A) QS, (B) QAS, and (C) QSA nylon-12 films at room temperature. (D) Continuous 5 bipolar D−E loops for the QS nylon-12 film in (A) after sitting at room temperature for 18 h. The poling frequency and poling electric field are fixed at 10 Hz and 185 MV/m, respectively.

under different poling fields and frequencies are shown in Figures 5A−D. When the poling field was below 100 MV/m at 10 Hz, only linear D−E loops were observed (Figures 5A,B) because no dipole flipping was possible at such low electric fields. As the poling field increased to 120−185 MV/m, DHLs were observed (Figure 5A). Finally, when the poling field increased to 265 MV/m, the QS nylon-12 film exhibited a typical normal ferroelectric loop with a remanent polarization of 22 mC/m2, a maximum D (Dmax) of ca. 60 mC/m2, and a coercive field (EC) of ca. 50 MV/m. Note that the Dmax was close to those reported for nylon-11,37,38 since the dipole density of nylon-12 is almost the same as that of nylon-11. This ferroelectric behavior from a linear loop to a DHL, and then to a broad SHL, with increasing the poling field for the QS nylon12 film was similar to that reported for stretched PVDF and P(VDF-HFP) (HFP is hexafluoropropylene) films.53 This could be explained by the competition between the depolarization field in nanosized ferroelectric domains and the polarization field from the applied field.53,54 Namely, when the depolarization field was stronger than the polarization field at relatively low poling electric fields, a two-step polarization process would take place to result in the DHL behavior. When the polarization field was stronger than the depolarization field at high poling electric fields, only one-step polarization would happen, leading to the normal ferroelectric behavior. Results in Figures 5A,B strongly suggest the existence of normal ferroelectric behavior for the mesomorphic QS nylon-12, an even-numbered nylon which was presumably considered as nonferroelectric. We will show evidence later to differentiate this ferroelectric behavior from the nonlinear D−E loop caused by the amorphous dipoles. After annealing the QS film at 150 °C for 20 h, the QSA nylon-12 film transformed toward the stable γ phase, where Hbonding sheets were gradually established and the interchain distance became smaller (see the d001 after cooling back to 25

all α and γ absorption bands, but with much weaker intensities. These results indicated that the mesophases in Q nylon-12 and -6 films had crystalline structures similar to those in the QA samples, i.e., γ for QA nylon-12 and mixed α/γ for QA nylon-6; however, the chain conformations must be defective with more twists in the polymer backbones. Ferroelectricity in Stretched Nylon-12 Films. To study the ferroelectric behavior of mesomorphic nylon-12, three samples were prepared; (i) QS, (ii) QAS, and (iii) QSA nylon12 films. Preparation details are given in the Experimental Section. The crystalline structures of these three samples were studied by 2D WAXD. The temperature-dependent patterns are shown in Figures 4A−C; the left, middle, and right panels correspond to 25 °C, 150 °C, and cooled back to 25 °C, respectively. From these WAXD patterns, 1D WAXD profiles were obtained (see Figures S1−S3 in the Supporting Information). The corresponding d-spacing changes of d001 and d020 as a function of temperature are shown in Figure 4D. First, the QS sample exhibited the largest d001 and the smallest d020 at 25 °C, whereas the QSA sample showed the smallest d001 and the largest d020. The d001 and d020 were in the middle for the QAS sample. This result implies that the QS sample should be the mesophase, and the QSA sample is close to the γ phase. Upon increasing the temperature, the d001 and d020 remained relatively constant for the mesomorphic QS sample. However, the d001 increased and the d020 decreased for the QSA and QAS samples, suggesting that H-bonding gradually weakened, causing a defective crystal structure at elevated temperatures. With the above understanding of structural differences among QS, QSA, and QAS nylon-12 films, their ferroelectric behavior was studied by bipolar D−E loop tests. Continuous loop tests were used to minimize the effect of remanent polarization from a previous loop.52 Without the interference of remanent polarization, dc conduction in the sample could be easily identified.52 The D−E loops for the QS nylon-12 film F

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Figure 7. Continuous bipolar D−E loops for the QS nylon-12 film at (A) 0, (B) 22, and (C) 50 °C. The poling frequency is 10 Hz with a sinusoidal waveform.

°C in Figure 4D). Consequently, the ferroelectric response substantially diminished even at 265 MV/m, as shown in Figures 5I,J. When the poling electric field was below 185 MV/ m, no obvious ferroelectric loop could be observed (Figure 5I). Even when the poling electric field was increased to 265 MV/ m, only slightly opened S-shaped loops were observed. Note that the QSA sample was not the pure γ phase because its d020 was only 1.48 nm, still smaller than that for the γ phase d020 of 1.61 nm. It is possible that upon applying a higher poling field to the QSA film, the ferroelectric hysteresis loop would appear for the QSA film. Indeed, when the QSA sample was polarized at 400 MV/m for more than two cycles, typical ferroelectric hysteresis loop with increased EC (95 MV/m) and remanent displacement (Drem = 36 mC/m2) values was clearly observed (see Figure S4). This result clearly shows that the ferroelectric behavior of nylon-12 was closely related to the crystalline packing in the sample. Namely, as long as there were enough defects in the crystals and the interchain distance was large enough, a high enough electric field could induce ferroelectric hysteresis. After uniaxial stretching of the annealed sample, the QAS nylon-12 film exhibited weak DHLs at 10 Hz when the poling electric field was below 185 MV/m (Figure 5E). However, as the poling field increased to 265 MV/m, typical ferroelectric SHLs were observed (Figure 5F), which were comparable to those for the QS nylon-12 film (Figure 5B). Note that coldstretching of the annealed film below the Tg could slightly increase the d001 and decrease the d020 as compared to the QSA film (Figure 4D), indicating that certain defects were introduced into the crystalline structure by uniaxial stretching. When the poling electric field was increased to above 200 MV/ m, ferroelectric dipoles and domains became switchable in this defective crystalline structure. Increasing the poling frequency from 10 to 1000 Hz for the QS, QAS, and QSA nylon-12 films at a poling electric field of 185 MV/m (Figures 5C/D, 5G/H, and 5K/L, respectively) should decrease the ferroelectric switching of polar crystalline dipoles/domains because polar crystalline dipoles/domains could not follow the electric field at high enough frequencies. Again, from the QS to the QAS and then to the QSA sample, the ferroelectric switching progressively diminished because of gradually improved H-bonding sheets by high temperature annealing. Not only were the mesophase and poling electric field important, the number of poling cycles (or the total poling time) was also important. After 30 continuous poling cycles at 185 MV/m and 10 Hz, both QS and QAS nylon-12 films exhibited a gradual transition from DHLs to SHLs with

gradually increased Dmax and Drem (Figures 6A,B) until a plateau value was reached. The DHL to SHL transition could be attributed to the continued growth of ferroelectric domains under repeated poling cycles at 185 MV/m. However, both Dmax and Drem values were smaller for the QAS film than the QS film. On the contrary, the QSA film exhibited relatively stable D−E loops with minimum ferroelectric switching (Figure 6C) since the sample was close to the stable γ phase with more developed H-bonding sheets. It is known that nonlinearity in D−E hysteresis loops may have different origins, including ferroelectric switching of domains in polar crystals, dipolar switching in the amorphous phase around Tg, polarization of mobile impurity ions, electronic conduction, and electrochemical reactions.1,55 In order to claim ferroelectricity for the D−E hysteresis loops of nylon-12 films, it is necessary to exclude other origins. First, pure electronic conduction will shift the upper part of the loop upward, making the D−E loop asymmetric with respect to the x-axis.52 This is not observed in Figure 5, indicating negligible electronic conduction in nylon-12 at room temperature. Second, polarization of mobile impurity ions will both broaden the D−E loop and increase the apparent slope.52 The migrational loss of impurity ions in polymers can be easily detected by broadband dielectric spectroscopy (BDS); both the real (εr′) and imaginary (εr″) relative permittivities increase at low frequencies.56 The frequency- and temperature-scan BDS results for the QS nylon-12 film are shown in Figure S5. From these results, the mobility of impurity ions is negligible at room temperature and 10 Hz. This is understandable because room temperature is about 23 °C below the Tg of nylon-12. Therefore, the contribution of impurity ions to the hysteresis loop at room temperature can be neglected. Third, electrochemical reactions do exist for dried nylons, especially above 90 °C.57 It is proposed that the protons in the amide groups can be reduced electrochemically into H2 under high enough fields. However, at room temperature this effect can be ignored. Finally, it is reported that dipolar switching of amorphous dipoles can cause significant dielectric nonlinearity for polymers.58−61 However, this mostly happens near the Tg (i.e., ca. ±15 °C) of the amorphous polymers. At temperatures about 25 °C below2 or above55 the Tg, this nonlinearity from dipolar switching of the amorphous dipoles becomes much less. For example, nonlinearity in D−E loops was significant for a poly(vinylidene chloride-co-acrylonitrile) [P(VDC-co-AN)] random copolymer around its Tg of 50 °C but became much weaker at 25 °C.2 In order to tell whether the D−E hysteresis loops in our nylon-12 films originated from nonlinear dipolar switching of amorphous amide dipoles, the temperature G

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Scheme 2. Schematic Representation of Ferroelectricity Induced by the Poling Electric Field in the Mesophase of Nylon-12a

a The top panel shows switching of small ferroelectric domains under low fields or short times. The bottom panel shows the growth and switching of large ferroelectric domains under high poling field or long times.

Figure 8. 1D WAXD profiles for the QS and QAS nylon-6 films along (A) the meridian and (B) the equator directions. Corresponding 2D WAXD patterns for the QS and QAS films are shown as insets. The α reflections are indicated as blue bars: (020)α at 7.29 nm−1, (200)α at 14.23 nm−1, and (002)α 16.98 nm−1. The γ reflections are indicated as red bars: (020)γ at 7.44 nm−1, (001)γ at 15.33 nm−1, and (200)γ at 15.72 nm−1.

temperature for 18 h. Then, it was subjected to five continuous bipolar poling at 185 MV/m (Figure 6D). If the electric-fieldinduced domains were short-lived (e.g., within minutes), the sample would have returned to the original state and then exhibited the same first five continuous loops as in Figure 6A. However, this was not the case in reality. Rather than showing DHLs in the beginning, broad SHLs with the similar maximum D values were directly observed, suggesting that the electricfield-induced ferroelectric domains were long-lived. From the above D−E loop study, poling electric field and poling time (or cycles) are important for the ferroelectricity in mesomorphic nylon-12 films. The ferroelectricity could be rationalized by the schematic models in Scheme 2. Because of the defective mesophase structure, a number of dangling or weakly H-bonded crystalline dipoles are able to switch upon the application of a high enough poling electric field. At the same time, there could be some strongly H-bonded amide groups, which are unable to rotate with the poling electric field. These strongly H-bonded amide groups serve as physical pinning points in the mesomorphic crystals.17 Because of the randomness of H-bonds in the mesophase, ferroelectric domains must be small. When the poling field is low and/or the poling time is short (i.e., a few cycles), the electric field is unable to invert the

dependence for D−E loops of the QS nylon-12 film was studied. Continuous bipolar D−E loops (six loops at 10 Hz) at 0, 22, and 50 °C are shown in Figures 7A−C, respectively. At 0 °C (about 50 °C below the Tg), typical ferroelectric SHLs were observed at a poling electric field of 265 MV/m (Figure 7A). The EC was 74 MV/m, and the Drem was 34.8 mC/m2. When the temperature increased to 22 °C, the EC and Drem decreased to 44 MV/m and 29 mC/m2, respectively, resulting in slimmer SHLs (Figure 7B). Finally, when the temperature increased to 50 °C, even slimmer SHLs were observed (Figure 7C). Note that the breakdown strength decreased at 50 °C due to enhanced electronic conduction in the sample (this is noticed by the slight upshift for the upper half loops).57 These results could be explained by the weakened H-bonding as temperature increased, which in turn reduced the ferroelectric domain size and increased the ferroelectric reversibility. The enhanced ferroelectric hysteresis as the temperature decreased below the Tg of nylon-12 films indicates that the origin of hysteresis must be ferroelectric switching of ferroelectric domains in the mesophase of nylon-12 films. On the other hand, these ferroelectric domains induced by high electric field were fairly long-lived. For example, after 30 cycles of electric poling at 185 MV/m (Figure 6A), the QS nylon-12 film was left at room H

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Figure 9. Continuous bipolar D−E loops for (A−D) the QS and (E−H) QAS nylon-6 films. The poling frequency varies from left to right: (A, B, E, F) 10 Hz, (C, G) 100 Hz, and (D, H) 1000 Hz with a sinusoidal wave function. All the tests were performed at room temperature.

Figure 10. 1D WAXD profiles for the QS and QSA nylon-11 films along (A) the meridian and (B) the equator directions. Corresponding 2D WAXD patterns for the QS and QSA films are shown as insets. The α reflections are indicated as blue bars: (001)α at 5.62 nm−1, (100)α at 14.50 nm−1, and (010)α at 16.96 nm−1. The γ reflections are indicated as red bars: (020)γ at 4.27 nm−1, (200)γ at 15.08 nm−1, and (001)γ at 15.85 nm−1.

nylon-6 films are shown as insets in Figure 8. The corresponding 1D WAXD profiles along the meridian and equator directions are shown in Figures 8A and 8B, respectively. Broad (001) reflections were observed for both the QS and QAS samples, and the peaks located at a lower q value than those for the γ phases (Figure 8B), indicating a larger interchain spacing. The d020 for the QS nylon-6 film was 0.771 nm, even shorter than the d020 (0.844 nm) for the γ phase (Figure 8A). After annealing at 150 °C for 20 h followed by uniaxial stretching (at a 300% extension ratio), the d020 for the QAS film increased to 0.782 nm. From these results, we concluded that the mesophase was achieved for the QS nylon-6 film. Ferroelectric behavior for the above nylon-6 films was studied by continuous bipolar D−E loops, and results are shown in Figure 9. Because of a higher dipole density than nylon-12, nylon-6 was expected to exhibit more pronounced ferroelectric behavior (i.e., higher maximum and remanent polarizations) than nylon-12. However, this was not the case in reality. When the poling electric field was 185 MV/m at 10 Hz, relatively narrow DHLs with a lower Dmax of 31 mC/m2 was observed for the QS nylon-6 film (Figure 9A), lower than that (42 mC/m2) for the QS nylon-12 (see Figure 5A). When the poling field increased to 265 MV/m at 10 Hz, the Dmax increased to 62 mC/m2 (Figure 9B), similar to that (ca. 60

strong H-bonds, and thus the H-bonded amide groups still maintain their pinning characteristic (see the top right panel of Scheme 2). The switching of nanosized ferroelectric domains results in the DHL behavior, i.e., the nanoconfinement effect as we reported before.17 When the poling field is high and/or the poling time is long (i.e., more cycles), the electric field is able to invert certain strong H-bonds and re-form new parallel Hbonds. As a result, ferroelectric domains can grow into a larger size (see the bottom right panel of Scheme 2). Ferroelectric switching of these large ferroelectric domains results in broad SHLs, i.e., the normal ferroelectric behavior. This explains the DHL to SHL transition in the mesomorphic nylon-12 film as the poling field and/or poling time increases (Figure 6). Therefore, we concluded that the ferroelectric behavior in the mesophase of nonpolar nylon-12 originated from the high electric-field-induced ferroelectric domain formation in defective nylon crystals. Ferroelectricity in Stretched Nylon-6 Films. To generalize the finding of electric-field-induced ferroelectricity for mesomorphic nylon-12, we prepared three nylon-6 films: the QS, QAS, and QSA films. However, the QSA nylon-6 film was too brittle to test, possibly due to a higher crystallinity, and thus it exhibited fairly low electric breakdown strength. This prevented us from obtaining high-field electric properties for the QSA nylon-6 film. 2D WAXD patterns for the QS and QAS I

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Figure 11. Continuous bipolar D−E loops for (A−D) the QS and (E−H) QSA nylon-11 films. The poling frequency varies from left to right: (A, B, E, F) 10 Hz, (C, G) 100 Hz, and (D, H) 1000 Hz with a sinusoidal wave function. All the tests were performed at room temperature.

mC/m2) of the QS nylon-12 (see Figure 5B). When the poling frequency was increased to 100 and 1000 Hz (Figures 9 C,D), nearly linear D−E loops were obtained. This result suggested that the mesophase in the QS nylon-6 film was not as defective as that for the QS nylon-12, probably due to the much shorter alkylene units between neighboring amide bonds. As a result, the fraction of polarizable amide dipoles was lower in the mesomorphic nylon-6 than in the mesomorphic nylon-12. For the QAS nylon-6 film, only linear D−E loops were obtained at all frequencies when the poling electric field was 185 MV/m (Figures 9E,G,H). However, when the poling field increased to 265 MV/m at 10 Hz, certain ferroelectric switching was observed for the QAS nylon-6 film. This was dramatically different from the ferroelectric behavior of the QAS nylon-12 film (Figures 5E−H). Similar to mesomorphic nylon-12 films, the ferroelectricity in mesomorphic nylon-6 should also originate from the crystalline mesophase rather than from the amorphous phase as reported previously.62−65 Ferroelectricity in Stretched Nylon-11 Films. Ferroelectricity in mesomorphic nylon-11 has been well-documented in the literature.32 However, given the ferroelectricity for evennumbered n-nylons, it would be interesting to compare them with nylon-11. Two stretched nylon films were prepared: QS and QSA films. Their 2D WAXD patterns are shown as the insets in Figure 10. 1D WAXD profiles along the meridian and equator directions are shown in Figures 10A and 10B, respectively. The (200/001) reflection was broad and located at a lower q value than that of the (200)γ and (001)γ reflections (Figure 10B). Meanwhile, the (020) reflection was located at a higher q value than the (020)γ reflection (Figure 10A). Both larger interchain distance and shorter layer thickness along the chain axes suggest that the QS nylon-11 film had a mesophase, consistent with previous reports.35 After annealing at 150 °C for 20 h, the (200/001) reflection became two reflections, reminiscent of the (100)α and (010)α reflections of the α phase (Figure 10B). The (100) reflection was located at a q value similar to that of the (100)α, whereas the (010) reflection was located at a q value much lower than the (010)α, indicating a larger d-spacing. Note that the (100)α direction was the Hbonding direction and the (010)α direction was the direction perpendicular to the H-bonding sheets. The d010,α was more sensitive to the annealing temperature whereas the d100,α was

not. In Figure 10A, the (001) reflection for the QSA sample remained at the same position as that for the (020) reflection of the QS sample. This position had a slightly smaller q value than that of the (001)α reflection. Therefore, the crystal structure in the QSA nylon-11 sample was neither the most stable α phase nor the mesophase, but somewhere in between with an enlarged inter-H-bonding sheet distance. Continuous bipolar D−E hysteresis loops for the QS nylon11 film tested at room temperature are shown in Figures 11A− D. When the poling field was up to 185 MV/m, limited ferroelectric switching of nylon-11 dipoles were observed at 10 Hz. At poling frequencies of 100 and 1000 Hz, ferroelectric switching increasingly diminished. This observation is somewhat different from those reported in the literature,41 and the difference is mainly attributed to different poling times, i.e., 100 ms for this study and >500 s for previous reports. As the poling field increased to 265 MV/m, typical ferroelectric SHLs were observed (Figure 11B) because the electric field was high enough to reorient stable H-bonds and to form large ferroelectric domains (see Scheme 2). For the QSA nylon-11 film, no ferroelectric switching could be observed at a poling field of 185 MV/m (Figures 11E,G,H). When the poling field increased to 265 MV/m, some limited ferroelectric switching was observed (Figure 11F). It was clear that after annealing at 150 °C for 20 h the mesomorphic δ′ structure gradually transformed toward the α phase. As a result, ferroelectric switching was inhibited due to the tighter packing of the Hbonding sheets. Comparison of Ferroelectricity for Mesomorphic Even- and Odd-Numbered Nylon Films. From the above results, mesomorphic crystals with disordered H-bonds and enlarged interchain spacing are critical for the generation of electric-field-induced ferroelectricity in both even- and oddnumbered nylons. However, even-numbered nylons do show apparent differences from odd-numbered nylons. First, evennumbered nylons favor nonpolar crystalline structures and oddnumbered nylons favor polar crystalline structures. The domain size induced by the electric field should be smaller for evennumbered nylons than for the odd-numbered nylons. As a result, the ferroelectric loops are slimmer (i.e., lower EC and Drem) for even-numbered nylons than for odd-numbered nylons (e.g., comparing Figures 5B/9B with 11B). Second, evenJ

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numbered nylons should contain certain internal stresses due to more twisted bonds after electric poling to form ferroelectric domains, whereas odd-numbered nylons do not. Upon removal of the poling field, ferroelectric domains in even-numbered nylons tend to depolarize by domain randomization to release internal stresses. Note that the ferroelectric domains do not disappear, as we have shown above that they are relatively longlived. As a result, the remanent polarization for even-numbered nylons quickly decreases, whereas that for odd-numbered nylons remains high after removal of the poling electric field. This is exactly observed in our experiments, as shown in Figure S8. In this sense, ferroelectric even-numbered nylons, unlike the odd-numbered nylons, should not be suitable for piezoelectric applications. Paraelectric Nature for the Amorphous Phases in nNylons. During the review process, a question regarding ferroelectricity in the amorphous phases of n-nylons was raised because it has been reported in the literature and become commonly accepted.32,62−65 To determine whether the amorphous phases in n-nylons are ferroelectric or paraelectric, we carried out detailed D−E loop studies on hot-pressed and isothermally crystallized nylons-12, -6, and -11 films in their most stable crystalline forms (i.e., γ phase for nylon-12 and α phases for nylons-6 and -11); see section V in the Supporting Information. Because of the tight molecular packing in these stable crystalline forms, no ferroelectricity is expected for the crystals. Therefore, any dipolar polarization should originate only from the amorphous phases. From Figures S10−S12 and the corresponding discussion in section V of the Supporting Information, the amorphous phases in nylons-12, -6, and -11 show paraelectric, rather than ferroelectric, responses. Because of migrational loss from impurity ions and the Brownian motion of amorphous dipoles near the Tg, certain loop-opening (i.e., dielectric nonlinearity) is observed, especially at low poling frequencies and high electric fields. This minor dielectric nonlinearity from the paraelectric amorphous phases can superpose upon the major ferroelectric hysteresis from the mesomorphic crystalline phases.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02739. Temperature-dependent 1D WAXD profiles for QS, QAS, and QSA nylon-12 films, continuous bipolar D−E loops for the QSA nylon-12 film with the maximum poling field of 400 MV/m, frequency- and temperaturescan BDS results for QS nylons-12, -6, and -11, comparison of remanent polarization for nylons-11 and -12, and paraelectricity in the amorphous phase of nnylons (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (L.Z.). *E-mail [email protected] (M.H.L.). 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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CONCLUSIONS In summary, the crystalline structure−ferroelectric property relationship was investigated for nylons-12, -6, and -11. It was observed that even-numbered nylons, such as nylons-12 and -6, were also able to show ferroelectric properties, although they have nonpolar crystalline structures (i.e., α and γ forms). Like nylon-11, the key for electric-field-induced ferroelectricity was the highly defective mesophase obtained by fast quenching, rather than the amorphous phase. After quenching the molten nylon films in a dry ice/isopropanol bath, mesophases with disordered H-bonding and twisted chain conformations were obtained. The enlarged interchain distance in the mesophase enabled easier dipole (i.e., dangling or weakly H-bonded amide groups) switching, leading to ferroelectricity. However, only small and metastable ferroelectric domains were obtained for even-numbered nylons because of the preferred nonpolar crystalline organization. This is different from nylon-11, where the polar crystalline structure favors large ferroelectric domains. Therefore, the hysteresis loops of nylons-12 and -6 appeared to be slimmer than those for nylon-11. This understanding of ferroelectricity in the mesophases of n-nylons will help us design new defect-modified nylons, such as nylon random copolymers, to achieve novel ferroelectric properties with narrow SHLs or DHLs.1 K

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DOI: 10.1021/acs.macromol.5b02739 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02739 Macromolecules XXXX, XXX, XXX−XXX