Ferroelectricity, a bistable ordering of electrical dipoles in a material, is widely used in sensors, actuators, nonlinear optics, and data storage. Traditional ferroelectrics are ceramic based. Ferroelectric polymers are inexpensive lead-free materials that offer unique features such as the freedom of design enabled by chemistry, the facile solution-based low-temperature processing, and mechanical flexibility. Among engineering polymers, odd nylons are ferroelectric. Since the discovery of ferroelectricity in polymers, nearly half a century ago, a solution-processed ferroelectric nylon thin film has not been demonstrated because of the strong tendency of nylon chains to form hydrogen bonds. We show the solution processing of transparent ferroelectric thin film capacitors of odd nylons. The demonstration of ferroelectricity, as well as the way to obtain thin films, makes odd nylons attractive for applications in flexible devices, soft robotics, biomedical devices, and electronic textiles.

By revisiting the solution processing of nylons, we have tailored a solvent system with low vapor pressure, strong acidity, and strong hydrogen-bonding character. The solvent can dissolve all aliphatic nylons at room temperature through the formation of hydrogen bonds with the amide groups. The solvent mixture effectively dissolves all nylons, ranging from low odd–numbered nylons (in our case, nylon-5) to commercially available nylon-11. We introduce solution quenching to freeze odd nylons in the poorly organized hydrogen-bonded chain arrangement. Solution-quenched (SQ) thin films have been realized by spin coating of the odd nylon solution for a brief moment, followed by placement of the wet films in vacuum to deplete the solvent. The resulting SQ odd nylon thin films show a ferroelectric behavior. We show that thin films are crystallized in the ferroelectric δ′ phase. We demonstrate the fabrication of extremely smooth and optically transparent films with high quality. The ferroelectric properties of nylon-11 thin films are comparable to those of the thick films. We have compared the performance of nylon-11 ferroelectric capacitor with those of the conventional ferroelectric polymers such as polyvinylidene difluoride (PVDF) and its random copolymer with trifluoroethylene [P(VDF-TrFE)]. Nylon-11 ferroelectric capacitors show superior performance under continuous stress cycles. The demonstration of solution-processed ferroelectric nylon thin films in combination with the simple synthesis and tunability of the chemical structure would potentially unlock the door to an unlimited number of hydrogen-bonded ferroelectric polymers.

The initial enthusiasm about ferroelectric odd nylons has come to a halt because of the inability to solution process ferroelectric thin films, as required for many of the envisioned microelectronic applications. Solution processing of nylon-11 thin films (and generally of all nylons) is a major challenge ( 10 ). Because of the strong hydrogen bond interaction between the amide groups, nylons show insolubility in almost all common solvents. Nylons are soluble in (i) strong acids such as absolute H 2 SO 4 , which protonates the carbonyl bond and breaks the hydrogen bonding between the polymer chains and in (ii) highly polar solvents ( 11 , 12 ), where hydrogen bonding is weakened because of the strong interaction between the amide groups and the polar solvent molecules. To realize solution-processed ferroelectric odd nylon thin films, two challenges should be addressed. First, both solvent types I and II are highly hygroscopic. During film formation processes such as spin coating, water, which is a nonsolvent for nylons, may condense into the drying film, causing vapor-induced phase separation (VIPS), yielding a highly rough film with a cloudy appearance resulting in electrical shorts in capacitors ( 13 , 14 ). Hence, VIPS should be effectively hindered during the process to yield smooth, pinhole-free thin films. Second, the ferroelectric δ′ phase has not been yet demonstrated using a solution processing technique. Therefore, the next challenge is to enforce the crystallization of the odd nylon thin films into the ferroelectric δ′ phase upon solution processing. Processing the thin films from high boiling point solvents, e.g., m-cresol, typically leads to the α phase, whereas using low boiling point solvents, such as trifluoroacetic acid (TFA), typically yields the γ phase ( 8 ). Both phases are not ferroelectric and therefore undesired. The realization of smooth, optically transparent, pinhole-free solution-processed odd nylon thin films that are crystallized in the ferroelectric phase will be a major breakthrough in the field of ferroelectric polymers.

Nearly three decades ago, it was found that the odd nylons show ferroelectric properties ( 1 , 2 ). Nylons are made of repeating aliphatic units connected by amide functional groups. The letter n in n-nylon refers to the number of carbon atoms in a repeat unit. The amide groups have a large dipole moment of 3.7 D, and at the same time, they have a strong tendency to form hydrogen-bonded sheets ( 3 ). The crystalline structure, as well as ferroelectric properties, of nylons can be fine-tuned by varying the chemical structure, blending, and copolymerization ( 4 – 6 ). Nylon-11, a chemical structure shown in the inset of Fig. 1A , is the most studied ferroelectric member of the family. Nylon-11 shows five crystalline phases: α, α′, γ, δ, and δ′ ( 7 , 8 ). Ferroelectricity is only demonstrated for the δ′ phase, a poorly organized hydrogen-bonded chain structure that enables the dipole (re)orientation and switching upon the application of an electric field ( 1 , 9 ). The ferroelectric nylon-11 capacitors are fabricated by the uniaxial stretching of melt-quenched thick films, which yields free-standing foils with thickness of typically tens of micrometers.

RESULTS AND DISCUSSION

Ferroelectricity in SQ nylon thin films The ferroelectric hysteresis loop of SQ nylon-11 thin films is shown in Fig. 1A. Thin films have been prepared by spin coating from 4 weight % solution of nylon-11 in TFA:acetone [60:40 mole percent (mol %)] solvent mixture, followed by immediate solvent quenching by placing the wet films in vacuum. For the field below 200 mega volts (MV)/m, the dielectric displacement, D, response is linear with respect to the applied electric field, E. Increasing the field beyond 200 MV/m opens hysteresis in the D-E loop. The loops are saturated when the field reaches 420 MV/m. The maximum remanent polarization, P r , amounts to 4.7 μC/cm2. As a benchmark measurement, we also determine P r and coercive field, E c , of 4.5 μC/cm2 and 110 MV/m, respectively, for the melt-quenched-stretched (MQS) nylon-11 thick film as shown in fig. S1. The P r values for SQ thin films agree well with those of MQS thick films and with those reported in the literature (1, 3, 9, 15). The E c for SQ nylon-11 thin films amounts to 200 MV/m, which is higher than the benchmark value of MQS films (110 MV/m). We shall clarify this difference by the subtle difference in microstructure in the next section. The ferroelectric loop of the SQ thin film of nylon-5 is given in Fig. 1B. Nylon-5 shows P r = 12.5 μC/cm2. Higher P r values for nylon-5 are expected because the P r for odd nylons increases linearly with the dipole density, i.e., 1.34 × 10−2 D/Å3 as compared to 2.94 × 10−2 D/Å3 for nylon-11 and nylon-5, respectively (4). Nylon-5 also exhibits higher E c (300 MV/m). Notably, the D-E measurements have been performed repeatedly at fields exceeding 450 MV/m, and the capacitors have survived the test. Therefore, the breakdown field strength is well above 450 MV/m. Because of the larger number of hydrogen bonds per unit length, an increase in the E c of nylon-5 is expected. Figure 1 is an unambiguous demonstration of solution-processed ferroelectric nylon thin films.

Solvent mixture of TFA:acetone Nylon thin films processed from TFA-only solution (under exactly the same film formation conditions) do not show any ferroelectric behavior, in sharp contrast to TFA:acetone. Therefore, we set out to study the TFA:acetone solvent mixture. The boiling point of TFA, as shown in Fig. 2A, substantially increases upon increasing acetone fraction. Conversely, the vapor pressure of the mixture decreases. The mixing of TFA with acetone is an exothermic process. At an intermediate composition of 40 mol % of acetone, the boiling point of the mixture is highest (i.e., 113°C) and the vapor pressure is lowest (i.e., 1.6 kPa) among all compositions. Upon further increase in the acetone mole fraction, the boiling point drops and the vapor pressure increases. TFA:acetone solvent mixture therefore shows negative deviation from Raoult’s law. Fig. 2 Dissolution mechanism of nylons in the TFA:acetone mixture. (A) Boiling point (B. P.) and (B) vapor pressure (V. P.) of the solvent mixture of TFA:acetone as a function of acetone mole fraction (X acetone ). The inset shows the schematic of the interactions between TFA and acetone molecules for 50:50 mol % TFA:acetone. (C) 1H shift of the NMR (850.3 MHz at 298 K) spectra for different solvent mixtures of TFA:acetone-d 6 as a function of acetone mole fraction. (D) 1H shift of the NMR (850.3 MHz at 298 K) spectra of nylon-11 solution for different solvent mixtures of TFA:acetone-d 6 as a function of acetone mole fraction. The solubility region for nylons is marked in green. The insolubility region due to the shielding of the proton is marked in red. The inset shows the schematic of the shielding of TFA by acetone molecules for 75:25 mol % TFA:acetone. For a deeper understanding, we have performed solution nuclear magnetic resonance (NMR). We use acetone-d 6 to monitor the proton shift of TFA. The solvent composition is changed from 100 mol % TFA to 100 mol % acetone-d 6 . The mixture of TFA and acetone show additional carbonyl signals compared to the pure acetone-d 6 (fig. S2A). The additional signals are due to the exchange of hydrogen and deuterium, which can be easily explained with a keto-enol tautomerism (fig. S2B). The intermediate complex of TFA-H-acetone suggests that the acidic proton of TFA is shared with acetone molecules. Higher acidity of the solvent mixture is shown by the shift in proton spectra (fig. S3A). The positions of the 1H chemical shifts are shown in Fig. 2C. TFA shows a proton peak at 12.47 parts per million (ppm). Upon the addition of acetone, the proton peak shifts linearly to a higher value of 14.30 ppm for 50:50 mol % TFA:acetone. The slope of the proton shift amounts to 3.7 ppm per mole fraction of acetone. For acetone between 60 and 90 mol %, the slope of the proton shift is much reduced to 0.6 ppm per mole fraction of acetone. We ascribe the proton shift to the formation of hydrogen bonds between TFA and acetone. For the individual components, the intermolecular hydrogen bonding is not present. In the mixture, however, strong hydrogen bonding is formed between carbonyl oxygen of acetone and H+ of TFA. As a result, the tendency of the mixture molecules to go to vapor phase is reduced and the boiling point is increased. Moreover, the addition of acetone to the TFA leads to weaker binding of H+ to TFA and eventually the deshielding of H+. For 50:50 mol % TFA:acetone, the interaction schematics is shown in the inset of Fig. 2B, where the proton of TFA is shared between TFA and acetone. The hydrogen bonds between TFA and acetone have enthalpy of more than −10 kcal/mol and are among the strongest hydrogen bonds (16). Acetone is not a solvent for nylons. In the next step, we perform the solubility test for nylon-11 in TFA:acetone mixtures. We find that a TFA:acetone mixture up to 50 mol % acetone dissolves nylons very well, whereas a 40:60 mol % TFA:acetone mixture does not dissolve nylons. The solubility region is marked in green in Fig. 2. The reason of insolubility of the nylon-11 at acetone concentration beyond 50 mol % is that the H+ is covered with a sheath of acetone molecules. A schematic of 25:75 mol % TFA:acetone is shown in the inset of Fig. 2D. Because of the shielding of the H+ by the acetone molecules, the proton cannot attack the hydrogen bonding between the bulky nylon chains, rendering nylons insoluble. We note that the same results are obtained for nylon-5. To investigate the effect of the solvent mixture interaction with nylons, we monitor the acid proton shift for the nylon-11 solution in TFA:acetone. TFA shows a proton peak at 12.47 ppm (Fig. 2C). Nylon in TFA shifts the proton peak to 12.58 ppm (Fig. 2D). The shift indicates that the proton is shared between amide groups of nylon-11 and TFA. The addition of 10 mol % acetone, however, led to a stronger shift to 13.10 ppm, which increased linearly to 14.53 ppm at 50 mol %. The 50:50 mol % TFA:acetone is therefore the best solvent mixture to dissolve nylon-11. We have used the 60:40 mol % composition for solution processing of the films because of its highest boiling point and lowest vapor pressure. We note that diffusion order NMR (DOSY) (fig. S4) has shown no degradation of nylon-11 because of the increased acidity of TFA:acetone solvent mixture.

Microstructure study of the solution-processed thin films We first study the surface topography of a conventionally spin-coated nylon-11 thin film, i.e., without SQ, from TFA:acetone solvent mixture. Because of the high boiling point of the solvent mixture and the strong interactions between TFA:acetone and nylon, the solvent evaporates very slowly from the film during the spin coating. Hence, spin coating times of nearly 4 min are required to obtain seemingly dry films with typical thickness of 500 nm. Respective atomic force microscopy (AFM) height image is shown in Fig. 3A. The topography shows the formation of a co-continuous coarse microstructure due to VIPS with a root mean square (RMS) roughness that amounts to 48 nm. These films do not show any ferroelectric behavior. Fig. 3 Transparent solution-processed nylon-11 thin films. Tapping mode AFM height image of the (A) conventionally spin-coated and (B) SQ thin films. (C) Ultraviolet-visible absorption as a function of wavelength on a double logarithmic scale of the conventionally spin-coated thin film and the SQ thin films. The dashed lines are the calculated absorbance using Eq. 1. The inset shows optical quality of the SQ thin films; the images of the logo of the Max Planck Institute for Polymer Research are taken through the SQ thin film (left) and the conventionally spin-coated films (right). Photo credit: Saleem Anwar, Max Planck Institute for Polymer Research. (D) Evolution of the roughness of the conventionally spin-coated and the SQ thin film upon variation in thicknesses. The RMS roughness is measured by AFM height topography, while calculated roughness is determined using optical absorption measurement. The calculated roughness agrees well with the experimental roughness obtained by AFM. The dashed lines are guide to the eye. The AFM height topography of the SQ thin film (Fig. 3B) shows the formation of an extremely smooth surface. The application of vacuum quickly depletes the wet film from the solvent. VIPS is effectively hindered, and a very fine microstructure is obtained with an RMS roughness that amounts to only 4 nm. To corroborate on the optical quality of the SQ thin film, we have measured the absorbance of the thin films as a function of wavelength, as shown in Fig. 3C. The SQ thin film shows interference fringes because of extreme film smoothness. For conventionally spin-coated films, the absorbance increases by two orders of magnitude without any interference patterns. The absorbance is fitted using the equation (17) α sc = 1 d ( 2 π ( n f − n a ) σ RMS λ ) 2 (1)where d is the film thickness, n f is the refractive index of nylon-11, which is 1.52 (18), n a is the refractive index of ambient, σ RMS is the roughness of the film, and λ is the wavelength of light. The calculated σ RMS from Eq. 1 for different film thicknesses is in good agreement with the experimental values of those obtained by AFM height topography (Fig. 3D). We note that the roughness for the SQ thin film remains well below 5 nm for different thicknesses. Furthermore, we have measured transparency and haze, as shown in fig. S5. The SQ thin film shows haze of only 0.3%, which is an order of magnitude lower than that of the conventionally spin-coated films.

Crystalline structure We have determined the crystallinity from differential scanning calorimetry (DSC) (fig. S6). The SQ thin film and the benchmark MQS thick film show crystallinities of 25 and 26%, respectively. The SQ thin film shows crystallinity comparable to that of the MQS thick film. To gain insight into the crystalline structure of the nylon-11 SQ thin film, we have performed wide-angle x-ray diffraction (WAXD) and compared the results with WAXD diffractograms of the MQS thick film. The MQS film of nylon-11 has a polar δ′ phase, which is characterized by a metastable mesophase with randomly oriented hydrogen bonds along the backbone and between the adjacent chains (15, 19). The MQS film (Fig. 4A) shows a WAXD peak at 4.79 nm−1, which corresponds to the low-angle (001) reflection of the δ′ phase, and is assigned to the smectic-like arrangement of the amide groups along the polymer chain with a d-spacing of 1.311 nm (3, 15, 20). The peak at 15.08 nm−1 is broad. To resolve the peak, we have performed WAXD along the parallel and perpendicular to the stretch direction (see fig. S8). The observed scattering angles for the (100) and (010) show only a small shift, meaning that (100) and (010) peaks are merged, and the peak value correspond to a d-spacing of 0.417 nm. The (100) and (010) peaks are assigned to the interchain distance along the hydrogen bonds and the intersheet distance between the hydrogen-bonded sheets, respectively (15, 20). We note that both reflections have almost identical position indicating low intermolecular order with hydrogen bonds that are randomly oriented along the backbone and between adjacent chains (3, 19). Fig. 4 Ferroelectric order in the SQ thin film of nylon-11. (A) WAXD patterns for the SQ thin film and the benchmark MQS film. The δ′ phase of nylon-11 has (001) peak at low q values and a broader peak at higher q values due to the superposition of (100)/(010) reflections. The solid lines show the deconvolution of the (100) and (010) reflections. The reported literature values for the δ′ phase are shown by black bars. (B) Room-temperature Fourier transform infrared (FTIR) spectra of the amide I and amide II bands for the SQ thin film and MQS film of nylon-11. (C) 1H magic angle spinning (MAS) and (D) 13C cross-polarization/MAS (CP/MAS) solid-state NMR spectra of the SQ thin film and MQS film. a.u., arbitrary units. Nylon-11 SQ thin films are readily crystalized in the ferroelectric δ′ phase. The WAXD pattern of the SQ film shows reflections similar to that of the δ′ phase MQS film. The (100)/(010) peak, however, shows broadening and a marginal shift in the position toward a higher q value of 15.30 nm−1 and a lower d-spacing of 0.411 nm. We have compared in table S1 the position of (100)/(010) and (001) peaks for all different crystalline forms of nylon-11. The observed d-spacing for the (100)/(010) peak of the SQ thin film can be assigned to γ, γ′, or δ′ phase. However, the low-angle (001) reflection of the SQ film corresponds to a d-spacing of 1.311 nm and matches exactly with the one for the MQS film and the reported literature values for nylon-11 δ′ phase. The slight shift of the (100)/(010) peak for the SQ thin film is due to better intramolecular order, with hydrogen-bonded amide groups forming two-dimensional sheets (3). The ferroelectric switching in nylon-11 stems from the alignment of the intersheet hydrogen bonds. Lower intersheet spacing of the hydrogen bonds in the SQ thin film compared to the MQS film results in stronger intersheet dipolar interactions, and therefore, a larger E c is required to switch the orientation of the dipoles in the SQ thin film. We note that a similar shift in coercive field to higher values has been reported for acid-treated nylon-11 MQS thick films (21, 22). On the basis of the WAXD, the nylon-11 SQ thin film is crystallized in the δ′ phase but with better order of the hydrogen-bonded amide dipoles. To further corroborate on hydrogen bonding in SQ thin film, we have performed Fourier transform infrared (FTIR) spectroscopy and solid-state NMR on both MQS film and SQ thin film. Full infrared (IR) scan for both films is shown in fig. S7. We focus on the hydrogen-bonded peaks, i.e., amide I and amide II, where amide I is attributed to the stretching of the C═O double bonds while amide II is due to the in-plane bending mode of N─H and the stretching mode of the central amide ─N─CO─ bond (20, 23, 24). Amide I and amide II peaks for MQS thick films occur at 1636 and 1544 cm−1, respectively. The peaks for the SQ thin film, however, occur at slightly different wavenumbers (1634 and 1547 cm−1 for amide I and amide II, respectively). The position of the amide bands is sensitive to the details of the nylon chain packing and the interactions between the amide groups. It has been shown that, upon decreasing disorder, amide I and amide II bands shift to, respectively, lower and higher wavenumbers (25). The observed shifts of the amide I and amide II bands for the SQ thin film in comparison to the MQS film indicate the formation of more ordered hydrogen bonding in the SQ thin film, in agreement with WAXD data. Final confirmation of better order in thin film comes from the solid-state NMR spectroscopy on the MQS film and the SQ thin film. 1H magic angle spinning (MAS) NMR spectra shown in Fig. 4C indicate an additional signal intensity of weakly hydrogen-bonded 1H sites observed around 6 to 7 ppm in the SQ thin film, which is significantly weaker in the MQS film, in agreement with WAXD and FTIR spectroscopy. However, we observed more pronounced differences between the MQS film and the SQ thin film in the 13C cross-polarization/MAS (CP-MAS) spectra shown in Fig. 4D. The 13C CP/MAS NMR signals of carbonyl sites observed at 173.5 ppm and the two CH 2 units next to the hydrogen-bonding amide groups observed at 40.4 and 36.8 ppm match perfectly for the MQS film and the SQ thin film. For the aliphatic chain between the amide groups, however, we observed significant differences between the two films. The NMR signal of the aliphatic chains splits into two signals: the signal of the CH 2 segments in trans conformation at 33.0 ppm and the signal of the segments in gauche conformation at 30.4 ppm. In the MQS film, the CH 2 segments are predominantly in gauche conformation with only a minor trans contribution. In contrast, the trans-gauche distribution in the SQ thin film has a comparable population of both conformations. Taking into account that both films have a similar crystallinity and that CH 2 chains in noncrystalline regions will adopt preferably gauche conformations for entropic reasons, we conclude that aliphatic CH 2 chains in crystalline MQS nylon-11 adopt a crankshaft-like chain structure between neighboring amide units while the prominent chain structure in the SQ nylon-11 thin film is significantly closer to the all-trans zigzag. The SQ thin film shows a strong conformational difference in comparison with the MQS film. The peak related to the trans conformation is much stronger, in perfect agreement with both WAXD and FTIR spectroscopy.