Transparent ferroelectric nylon thin films treated in solution | EXCLUSIVE OFFER !

Abstract

Ferroelectricity, a bistable control of electric dipoles in a material, is widely used in sensors, actuators, nonlinear optics and data storage. Traditional ferroelectrics are based on ceramics. Ferroelectric polymers are inexpensive, lead-free materials that offer unique features such as design freedom made possible by chemistry, easy solution-based low temperature treatment and mechanical flexibility. Of the technical polymers, the odd nylons are ferroelectric. Since the discovery of ferroelectricity in polymers, nearly half a century ago, no thin film of ferroelectric solution-treated nylon has been demonstrated, due to the strong trend of nylon chains. to form hydrogen bonds. We show the solution treatment of transparent thin film ferroelectric capacitors of odd nylons. The demonstration of ferroelectricity, as well as the way to obtain thin films, make odd nylons attractive for applications in soft devices, soft robotics, biomedical devices and electronic textiles.

INTRODUCTION

Nearly three decades ago, it was found that odd nylons have ferroelectric properties (1, 2). Nylons consist of repeating aliphatic units linked by amide functional groups. The letter not in not-Nylon means the number of carbon atoms in a repeating 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 the ferroelectric properties of the nylons can be adjusted by varying the chemical structure, mixing and copolymerization (46). Nylon-11, a chemical structure shown in the box of Figure 1A, is the most studied ferroelectric member of the family. Nylon-11 has five crystalline phases: α, α, γ, Ύ and Ύ. (seven, 8). Ferroelectricity is only demonstrated for the Ύ 'phase, a poorly structured structure of the hydrogen-linked chain that allows (re) orientation and switching of the dipole when an electric field is applied (1, 9). Ferroelectric capacitors made of nylon-11 are made by uniaxial stretching of thick films quenched in the molten state, giving free-standing sheets typically of several tens of microns in thickness.

Fig. 1 Ferroelectric nylon thin films treated in solution.

reE hysteresis loop (top) and switching current (bottom) of (A) nylon 11 and (B) nylon-5, respectively. The insets show the respective chemical structures.

The initial craze for ferroelectric odd nylons has stopped due to the impossibility of treating ferroelectric thin films in solution, as required by many microelectronic applications envisioned. The solution treatment of thin layers of nylon-11 (and generally of all nylons) is a major challenge (ten). Due to the strong hydrogen bonding interaction between amide groups, nylons exhibit insolubility in almost all common solvents. Nylons are soluble in (i) strong acids such as absolute H2SO4, which protonates the carbonyl bond and breaks the hydrogen bond between the polymer chains and in (ii) highly polar solvents (11, 12), where the hydrogen bond is weakened because of the strong interaction between the amide groups and the polar solvent molecules. To make ferroelectric thin layers of solution-treated uneven nylon, two challenges must be addressed. First, both types of solvents I and II are highly hygroscopic. During film forming processes such as spin coating, water, which is not a solvent for nylons, can condense in the drying film, causing vapor-induced phase separation (VIPS), producing a very rough film with a hazy appearance causing shorts in the capacitors (13, 14). Therefore, VIPS should be effectively prevented during the process to produce smooth, thin films without pinholes. Secondly, the ferroelectric ÎŽ 'phase has not yet been demonstrated by a solution treatment technique. Therefore, the next challenge is to force the crystallization of the odd nylon thin films into the ferroelectric ÎŽ phase during solution processing. The treatment of thin films from high boiling point solvents, for example m-cresol, generally leads to phase a, while the use of low boiling point solvents, such as trifluoroacetic acid (TFA), usually gives phase y (8). Both phases are not ferroelectric and therefore undesirable. The production of odd nylon thin films treated with a smooth, optically transparent and crystalline solution in the ferroelectric phase will constitute a major advance in the field of ferroelectric polymers.

By revisiting the solution treatment of nylons, we adapted a solvent system with low vapor pressure, strong acidity and strong hydrogen bond character. The solvent can dissolve all the aliphatic nylons at room temperature by forming 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 the quenching of the solution to freeze odd nylons in the poorly organized arrangement of hydrogen-bonded chains. Solution quenched thin films (SQ) were made by spin coating the strange nylon solution for a short time, followed by placing the wet films under vacuum to remove the solvent. The resulting thin SQ nylon layers have a ferroelectric behavior. We show that thin films are crystallized in the ÎŽ-ferroelectric phase. We demonstrate the manufacture of extremely smooth and optically transparent high quality films. The ferroelectric properties of nylon-11 thin films are comparable to those of thick films. We compared the performance of the ferroelectric nylon 11 capacitor with conventional ferroelectric polymers such as polyvinylidene fluoride (PVDF) and its random copolymer with trifluoroethylene (P (VDF-TrFE)). Ferroelectric Nylon-11 capacitors exhibit superior performance under continuous stress cycles. The demonstration of solution-processed ferroelectric nylon thin films, combined with the simple synthesis and tunability of the chemical structure, would potentially open the door to an unlimited number of ferroelectric hydrogen-bonded polymers.

RESULTS AND DISCUSSION

Ferroelectricity in nylon SQ thin films

The ferroelectric hysteresis loop of nylon-11 SQ thin films is shown in Figure 1A. Thin layers were prepared by centrifugation from a 4% by weight solution of nylon 11 in TFA: acetone (60:40 mol%) with a solvent mixture, followed by quenching. immediate solvent by placing the wet films under vacuum. For the field below 200 mega volts (MV) / m, the dielectric displacement, re, the response is linear with respect to the applied electric field, E. The increase of the field beyond 200 MV / m opens the hysteresis in the reE loop. The loops are saturated when the field reaches 420 MV / m. The maximum remanent polarization, Pr, equivalent to 4.7 ÎŒC / cm2. As a reference measure, we also determine Pr and coercive field, Evs, 4.5 ÎŒC / cm2 and 110 MV / m, respectively, for the thick film of nylon-11 stretched in the molten-quenched state (MQS), as shown in fig. S1. the Pr The values ​​of the SQ thin films correspond well to those of the MQS thick layers and to those reported in the literature (1, 3, 9, 15). the Evs for SQ thin layers of nylon-11 reach 200 MV / m, which is higher than the reference value of MQS films (110 MV / m). We will clarify this difference by the subtle difference of microstructure in the next section.

The ferroelectric loop of the nylon-5 thin film SQ is given in FIG. 1B. Nylon-5 watch Pr = 12.5 ÎŒC / cm2. Higher Pr values ​​for nylon-5 are expected because the Pr for odd nylons, increases linearly with dipolar density, ie 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 presents higher Evs (300 MV / m). In particular, the reE measurements were made repeatedly at fields above 450 MV / m, and the capacitors survived the test. Therefore, the intensity of the breakdown field is much higher than 450 MV / m. Due to the higher number of hydrogen bonds per unit length, an increase in Evs Nylon-5 is provided. Figure 1 is an unambiguous demonstration of thin films of ferroelectric nylon treated in solution.

TFA solvent mixture: acetone

Thin nylon films treated from a TFA solution only (in exactly the same film-forming conditions) exhibit no ferroelectric behavior, which contrasts sharply with TFA: acetone. Therefore, we started to study the solvent mixture TFA: acetone.

As shown in Figure 2A, the boiling point of TFA increases substantially as the acetone fraction increases. Conversely, the vapor pressure of the mixture decreases. The mixture of TFA with acetone is an exothermic process. For an intermediate composition of 40 mole% acetone, the boiling point of the mixture is the highest (ie 113 ° C) and the vapor pressure is the highest. low (ie 1.6 kPa) among all compositions. As the mole fraction of acetone increases again, the boiling point decreases and the vapor pressure increases. TFA: the mixture of acetonic solvents thus has a negative deviation from Raoult's law.

Fig. 2 Mechanism of dissolution of nylons in the mixture TFA: acetone.

(A) Boiling point (B.P.) and (B) vapor pressure (V.P.) of the solvent mixture of TFA: acetone as a function of the mole fraction of acetone (Xacetone). The inset shows the pattern of interactions between TFA and acetone molecules for a TFA / acetone ratio at 50:50 mol%. (VS) 1Displacement H of the NMR spectrum (850.3 MHz to 298 K) for different solvent mixtures of TFA: acetonere6 depending on the mole fraction of acetone. (re) 1Displacement H of the NMR spectrum (850.3 MHz to 298 K) of a solution of nylon-11 for different solvent mixtures of TFA: acetonere6 depending on the mole fraction of acetone. The solubility region of the nylons is marked in green. The region of insolubility due to proton shielding is marked in red. The inset shows the pattern of TFA protection by acetone molecules for 75:25 mol% TFA: acetone.

For further understanding, we performed nuclear magnetic resonance solution (NMR). We use acetonere6 monitor the proton transfer of TFA. The solvent composition was changed from 100 mol% TFA to 100 mol% acetone.re6. The mixture of TFA and acetone has additional carbonyl signals compared to the pure acetone mixture.re6 (Fig S2A). The additional signals are due to the exchange of hydrogen and deuterium, which is easily explained by a keto-enol tautomerism (Figure S2B). The intermediate complex of TFA-H-acetone suggests that the acid proton of TFA is shared with the acetone molecules. The higher acidity of the solvent mixture is illustrated by the shift of the proton spectra (Fig. S3A). The positions of 1The chemical shifts H are illustrated in FIG. 2C. TFA shows a proton peak at 12.47 parts per million (ppm). When adding acetone, the peak of the proton goes linearly to a higher value of 14.30 ppm for 50:50 mol% TFA: acetone. The slope of the proton displacement is 3.7 ppm per mole fraction of acetone. For acetone between 60 and 90 mol%, the slope of the proton transfer is greatly reduced to 0.6 ppm per mole fraction of acetone. We attribute the proton shift to the formation of hydrogen bonds between TFA and acetone. For individual components, intermolecular hydrogen bonding is not present. However, in the mixture, a strong hydrogen bond is formed between the carbonyl oxygen of acetone and H+ of TFA. As a result, the tendency of the mixing molecules to pass into the vapor phase is reduced and the boiling point is increased. In addition, the addition of acetone to the TFA results in a weaker binding of H+ to TFA and finally the disenchantment of H+. For 50:50 mol% TFA: acetone, the interaction patterns are shown in the box of Figure 2B, where the TFA proton is partitioned between TFA and acetone. The hydrogen bonds between TFA and acetone have an enthalpy above -10 kcal / mol and are among the strongest hydrogen bonds (16).

Acetone is not a solvent for nylons. In the next step, we carry out the solubility test of nylon 11 in TFA: acetone mixtures. We find that a TFA: acetone mixture up to 50 mol% acetone dissolves the nylons very well, while a 40:60 mol% mixture of TFA: acetone does not dissolve the nylons. The solubility region is marked in green in Figure 2. The reason for the insolubility of nylon-11 at an acetone concentration greater than 50 mol% is that the H+ is covered with a sheath of acetone molecules. A scheme of 25: 75 mol% TFA: acetone is presented in the box of Figure 2D. Because of the H shielding+ by the acetone molecules, the proton can not attack the hydrogen bond between the large nylon chains, making the nylons insoluble. We note that the same results are obtained for nylon-5.

To investigate the effect of the interaction of the solvent mixture with nylons, we monitor the proton acid shift for the nylon-11 solution in TFA: acetone. TFA shows a proton peak at 12.47 ppm (Fig 2C). Nylon in the TFA displaces the proton peak at 12.58 ppm (Figure 2D). The offset indicates that the proton is shared between the amide groups of nylon-11 and TFA. The addition of 10 mol% of acetone, however, resulted in a larger shift at 13.10 ppm, which increased linearly to 14.53 ppm to 50 mol%. 50:50 mol% TFA / acetone is therefore the best solvent mixture for dissolving nylon-11. We used the 60:40 mol% composition for the solution treatment of the films because of its highest boiling point and lowest vapor pressure. We note that the diffusion order NMR (DOSY) (Fig. S4) showed no degradation of nylon-11 due to the increased acidity of the TFA: acetone solvent mixture.

Microstructural study of thin films treated in solution

We first study the surface topography of a conventional spin-coated nylon-11 thin film, i.e. without SQ, from the TFA: acetone solvent mixture. Due to 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 spin coating. Therefore, centrifugation deposition times of nearly 4 minutes are required to obtain apparently dry films with a typical thickness of 500 nm. The height image of the respective Atomic Force Microscopy (AFM) is shown in Figure 3A. The topography shows the formation of a co-continuous coarse microstructure due to the VIPS with a 48-nm RMS. These films show no ferroelectric behavior.

Fig. 3 Clear nylon-11 thin films treated in solution.

AFM height image in tapping mode (A) classically drained and (B) SQ thin films. (VS) Ultraviolet visible absorption versus wavelength on a logarithmic double scale of conventional centrifugally coated thin film and SQ thin films. The dashed lines represent the absorbance calculated using Eq. 1. The inset indicates the optical quality of thin films SQ; The logo images of the Max Planck Institute for Polymer Research are taken through the SQ thin film (left) and conventional spin-coated films (right). Photo credit: Saleem Anwar, Max Planck Institute for Polymer Research. (re) Evolution of the roughness of the conventionally centrifugally coated thin film and the thin film SQ during the variation of thicknesses. The RMS roughness is measured by the AFM height topography, while the calculated roughness is determined by a measurement of optical absorption. The calculated roughness corresponds well to the experimental roughness obtained by AFM. The dotted lines guide the eye.

The AFM height topography of the thin film SQ (Fig. 3B) shows the formation of an extremely smooth surface. The application of the vacuum quickly exhausts the wet film of the solvent. VIPS is effectively hampered and a very fine microstructure is obtained with an RMS roughness of only 4 nm.

To corroborate the optical quality of the SQ thin film, we measured the absorbance of thin films as a function of wavelength, as shown in Figure 3C. The SQ thin film has interference fringes due to the extreme fineness of the film. For conventional spin-coated films, the absorbance increases by two orders of magnitude without any pattern of interference. The absorbance is adjusted using the equation (17)

αCaroline from the south=1re(2π(notFnota)σRMSλ)2

(1)or re is the thickness of the film, notF is the index of refraction of nylon-11, which is equal to 1.52 (18) nota is the refractive index of the ambient, σRMS is the roughness of the film and λ is the wavelength of the light.

The calculated σRMS Equation 1 for different film thicknesses is in good agreement with the experimental values ​​of those obtained by the AFM height topography (Fig. 3D). We note that the roughness of the thin film SQ remains well below 5 nm for different thicknesses. In addition, we measured transparency and haze, as shown in fig. S5. The SQ thin film has a haze of only 0.3%, which is an order of magnitude less than conventional spin coated films.

Crystal structure

We have determined the crystallinity by differential scanning calorimetry (DSC) (Figure S6). The thin film SQ and the thick film MQS of reference have crystallinities of 25 and 26%, respectively. The thin film SQ has a crystallinity comparable to that of the thick film MQS. To better understand the crystal structure of the nylon-11 SQ thin film, we performed a wide angle X-ray diffraction (WAXD) and compared the results with the WAXD diffractograms of the MQS thick film. The nylon-11 MQS film has a polar ÎŽ phase, characterized by a metastable mesophase with randomly oriented hydrogen bonds along the skeleton and between adjacent chains (15, 19). MQS film (Fig. 4A) shows a WAXD peak at 4.79 nm-1, which corresponds to the reflection of the ÎŽ-phase at low angle (001) and is assigned to the smectic arrangement of the amide groups along the polymer chain with a respacing of 1.311 nm (3, 15, 20). The peak at 15.08 nm-1 is wide. To solve the peak, we performed WAXD along the parallel and perpendicular to the stretching direction (see Fig. S8). The scattering angles observed for (100) and (010) show only a small shift, which means that the peaks (100) and (010) are merged and that the peak value corresponds to respacing of 0.417 nm. The peaks (100) and (010) are attributed to the interchain distance along the hydrogen bonds and the inter-leaf distance between the hydrogen-bonded leaves, respectively (15, 20). We note that the two reflections have an almost identical position, indicating a weak intermolecular order with hydrogen bonds randomly oriented along the skeleton and between adjacent chains (3, 19).

Fig. 4 Ferroelectric order in nylon-11 thin film SQ.

(A) WAXD models for the SQ thin film and the MQS reference film. The phase of nylon-11a (001) a low peak q values ​​and a wider peak at a higher q values ​​due to the superposition of reflections (100) / (010). The solid lines indicate the deconvolution of the reflections (100) and (010). The values ​​reported in the literature for phase ή 'are indicated by black bars. (B) Room temperature Fourier transform (FTIR) infrared spectra of the amide I and amide II strips for the SQ thin film and the nylon-11 MQS film. (VS) 1H magic angle rotation (MAS) and (re) 13C: Cross-polarized solid state NMR spectra / MAS (CP / MAS) of the SQ thin film and the MQS film. a.u., arbitrary units.

Nylon-11 SQ thin films crystallize easily in the ή-ferroelectric phase. The WAXD pattern of the SQ film shows reflections similar to those of the MQS film in phase. The peak (100) / (010) however shows a widening and a marginal shift of the position to a higher level. q value of 15.30 nm-1 and a lower respacing of 0.411 nm. We have compared in Table S1 the position of the peaks (100) / (010) and (001) for all the different crystalline forms of nylon-11. L & # 39; observed rethe spacing for the peak (100) / (010) of the thin film SQ can be attributed to the phase γ, γ, or ή #. However, the reflection of the low angle SQ film (001) corresponds to a respacing of 1.311 nm and corresponds exactly to that of the MQS film and to values ​​reported in the literature for the nylon-11 ή phase. The slight shift of the peak (100) / (010) of the thin film SQ is due to a better intramolecular order, the amide groups linked to hydrogen forming two-dimensional sheets (3). Ferroelectric switching in nylon-11 results from the alignment of hydrogen bonds between the sheets. A lower spacing between the sheets of the hydrogen bonds in the thin film SQ with respect to the MQS film gives rise to dipolar interactions between stronger sheets, and therefore to a larger Evs is necessary to change the orientation of the dipoles in the thin film SQ. We note that a similar change in the coercive field to higher values ​​has been reported for thick acid-treated nylon-11 MQS films (21, 22). On the basis of WAXD, the nylon-11 SQ thin film is crystallized in the ή phase but with a better order of the hydrogen bonded amide dipoles.

To further corroborate the hydrogen bond in the SQ thin film, we performed Fourier Transform Infrared (FTIR) spectroscopy and solid state NMR on the MQS film and the SQ thin film. The complete infrared (IR) scanning of the two films is illustrated in FIG. S7. We focus on hydrogen-related peaks, that is, amide I and amide II, with amide I being attributed to the stretching of C═O double bonds while amide I It is due to the bending mode in the plane of N-H and the stretching mode of the central amide bond ─N─CO─ (20, 23, 24). The amide I and amide II peaks for MQS thick films occur at 1636 and 1544 cm-1, respectively. The SQ thin film peaks, however, occur at slightly different wave numbers (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 packing of the nylon chain and the interactions between the amide groups. It has been shown that when the disorder decreases, the amide I and amide II bands move to the lower and upper wave numbers, respectively (25). The observed displacements of the amide I and amide II bands for the SQ thin film compared to the MQS film indicate the formation of a more ordered hydrogen bond in the SQ thin film, in agreement with the WAXD data.

The final confirmation of a better order in the thin film comes from the solid state NMR spectroscopy on the MQS film and the SQ thin film. 1The NMR spectra of magic angle rotation (MAS) H shown in Figure 4C indicate additional signal intensity of low hydrogen bonding. 1The H sites observed around 6 to 7 ppm in the SQ thin film, which is significantly lower 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 spectra (CP-MAS) shown in FIG. 4D. the 13CP / MAS NMR C signals for carbonyl sites observed at 173.5 ppm and both CH2 The units adjacent to the hydrogen bonded amide groups observed at 40.4 and 36.8 ppm correspond perfectly to the MQS film and the SQ thin film. For the aliphatic chain between amide groups, however, we observed significant differences between the two films. The NMR signal of the aliphatic chains is divided into two signals: the signal of the CH2 trans conformation segments at 33.0 ppm and the signal of the left conforming segments at 30.4 ppm. In the film MQS, the CH2 the segments are mainly in the left conformation with only a minor contribution in trans. In contrast, the left-handed distribution in the thin film SQ has a comparable population of both conformations. Whereas the two films have a similar crystallinity and that CH2 chains in noncrystalline regions will preferably adopt left conformations for entropic reasons, we conclude that aliphatic CH2 The chains in the crystalline nylon-11 MQS adopt a crankshaft-like chain structure between adjacent amide units, while the predominant chain structure in the nylon-11 SQ thin film is substantially closer to the all-trans zigzag. The SQ thin film shows a strong conformational difference compared to the MQS film. The peak related to the trans conformation is much stronger, in perfect agreement with the spectroscopies WAXD and FTIR.

Ferroelectric capacitor performance

Destructive reading of a ferroelectric capacitor and the need for a larger field than Evs to reset the data after each reading, a major requirement for polymer stability (polarization) during repeated switching. Thin films of P (VDF-TrFE), the spearhead of ferroelectric polymers, generally exhibit fatigue, that is to say a decrease in polarization as a function of repeated switching cycles (26). Fatigue depends on the amplitude, frequency and profile of the motor electric field (27, 28). To evaluate the performance of the ferroelectric nylon capacitors, we performed a fatigue test and compared the results to those of the P capacitors (VDF-TrFE). The thicknesses of the thin layers of nylon-11 and P (VDF-TrFE) are about the same, and the two capacitors were subjected to stresses under identical conditions by the application of a field of switching of 1.25 ×. Evs. The results of the fatigue tests are shown in Figure 5A. Capacitors consisting of the thin film SQ nylon 11 exhibit a fatigue-free behavior for more than 106 write-erase cycles. On the other hand, capacitors P (VDF-TrFE) are strongly subject to fatigue, with Pr begins to degrade to about 1000 cycles. We note that the fatigue behavior of capacitors P (VDF-TrFE) can be improved by using porous organic electrodes and by introducing a relaxation period between consecutive pulses (decreasing work cycle) (29). Fatigue measurements on thin-film ferroelectric capacitors made of PVDF homopolymer are rare because of the difficulty in achieving the ferroelectric phases in a thin layer. To the best of our knowledge, polarization fatigue has been reported only for thin-film capacitors of ή-PVDF, where the polarization is reduced to 85% of its original value after 106 Nylon 11 SQ ferroelectric capacitors exhibit superior fatigue behavior without any modification of the electrodes or reduction of the working cycle. Table S2 compares the ferroelectric performance of nylon-11 with PVDF and P (VDF-TrFE).

Fig. 5 Performance of ferroelectric thin-film capacitors made of nylon-11.

(A) ±Pr (normalisĂ©s Ă  leur valeur initiale) en fonction du nombre cumulĂ© de cycles. Les condensateurs ferroĂ©lectriques en nylon-11 sont plus performants que P (VDF-TrFE). (B) Conservation des donnĂ©es en fonction du temps mesurĂ© pour une pĂ©riode supĂ©rieure Ă  une semaine. L’encart montre l’histogramme de la Pr valeurs obtenues pour les condensateurs Ă  film mince (240 nm) de diffĂ©rents lots de fabrication. Les films minces de la SQ montrent une distribution Ă©troite Pr. (VS) rĂ©E boucles d'hystĂ©rĂ©sis pour diffĂ©rentes Ă©paisseurs de film en fonction du biais appliquĂ© et du champ Ă©lectrique (incrustĂ©). (rĂ©) L'Ă©volution de Evs et le rendement des condensateurs ferroĂ©lectriques fonctionnels avec l'Ă©paisseur du film. Les lignes pointillĂ©es sont un guide pour les yeux.

Conserver l’état de polarisation est crucial pour toute application des couches minces ferroĂ©lectriques. Nous avons mesurĂ© le temps de rĂ©tention de polarisation des condensateurs ferroĂ©lectriques SQ en nylon-11, comme indiquĂ© sur la figure 5B. La polarisation, mesurĂ©e Ă  la tempĂ©rature ambiante, est restĂ©e stable pendant plus d'une semaine. Nous notons que la dispersion dans Pr les valeurs obtenues Ă  partir de diffĂ©rents condensateurs sont trĂšs faibles. L’encadrĂ© de la Fig. 5B montre l’histogramme de la Pr les valeurs de plus de 80 condensateurs Ă  couche mince ferroĂ©lectriques en nylon-11 obtenus par le traitement SQ. the Pr montre une distribution Ă©troite avec une valeur moyenne de 4,5 ± 0,5 ”C / cm2.

La mise Ă  l'Ă©chelle de l'Ă©paisseur des boucles de dĂ©placement en fonction de la tension est prĂ©sentĂ©e sur la figure 5C pour les condensateurs en faisant varier l'Ă©paisseur du film de 185 Ă  530 nm. Tous les films montrent Pr d'environ 4,5 ÎŒC / cm2. AprĂšs normalisation de toutes les boucles du champ Ă©lectrique, comme indiquĂ© dans l'encadrĂ© de la figure 5C, toutes les courbes d'hystĂ©rĂ©sis sont superposĂ©es, indiquant que le Evs ne dĂ©pend pas de l'Ă©paisseur de la couche. the Evs obtenu en fonction de diffĂ©rentes Ă©paisseurs de couche montre un plateau Ă  210 ± 10 MV / m, comme prĂ©sentĂ© Ă  la Fig. 5D. Nous notons que toutes les Ă©tapes de traitement ont Ă©tĂ© effectuĂ©es en dehors d'une salle blanche, oĂč des particules de poussiĂšre de l'ordre de l'Ă©paisseur du film peuvent provoquer des courts-circuits. Par consĂ©quent, le rendement en condensateurs fonctionnels avec les Ă©paisseurs de film mince SQ infĂ©rieures Ă  100 nm Ă©tait de 50%. Le rendement augmente Ă  90% pour les films d'une Ă©paisseur supĂ©rieure Ă  300 nm. Pour des Ă©paisseurs supĂ©rieures Ă  500 nm, le rendement en condensateurs ferroĂ©lectriques en nylon-11 est remarquablement de 100%. Nous notons que, pour chaque Ă©paisseur, nous avons mesurĂ© plus de 70 condensateurs. Les couches minces de nylon-11 montrent une variation d'Ă©paisseur de seulement 2% sur l'ensemble du substrat. Les films minces sont exempts de trous d'Ă©pingle, de vides et de pores. Par consĂ©quent, les condensateurs de grande surface, d'un diamĂštre de 10 mm, affichent Ă©galement des performances comparables.

MATÉRIAUX ET MÉTHODES

Materials

Le Nylon-11 a été acheté chez Sigma-Aldrich. P (VDF-TrFE) (65 à 35) a été acheté chez Solvay. Le poids moléculaire moyen en poids, Mw, de nylon-5, de nylon-11 et de P (VDF-TrFE) représentaient respectivement 5,8, 56 et 350 kg / mol. Le poids moléculaire moyen en nombre, Mnot, du nylon-5 a également été déterminé sur la base de l'analyse du groupe d'extrémité RMN dans le 1,1,1,3,1,3,3,3-hexafluoro-2-propanol deutéré et s'élevait à 3,4 kg / mol. Le poids moléculaire, Mnotde nylon-11 et de P (VDF-TrFE) s'élevaient à 39,7 et 170 kg / mol, respectivement. Le TFA et l'acétone (99,8%, extra sec) ont été achetés auprÚs de Carl Roth GmbH et Acros Organics, respectivement. Cyclohexanone et acétoneré6 (99,9% atomique D) ont été achetés chez Sigma-Aldrich. Le nylon-5 a été synthétisé à partir de 2-pipéridone par polymérisation en masse par ouverture de cycle anionique en présence de 1 mol% N-acétyl-2-pipéridone comme démarreur et 2-oxopipéridine-1-ide de tétraméthylammonium à la suite du travail de Coutin et de ses collÚgues (25).

Préparation de film mince en nylon

Une solution de nylon a été préparée en dissolvant le polymÚre dans le mélange de TFA et d'acétone (60:40% en moles). Des films minces SQ ont été fabriqués en déposant par centrifugation la solution sur un substrat de verre, puis en trempant la solution en appliquant un vide poussé. L'épaisseur des films a été contrÎlée en modifiant la concentration de la solution. Nous avons également préparé des films épais MQS en pressant à chaud les pastilles de nylon-11 entre deux feuilles d'aluminium à 210 ° C, puis en les trempant dans de l'eau glacée. Les films ont été étirés de maniÚre uniaxiale à la température ambiante avec un rapport d'étirage de 3: 1 à une vitesse de déformation de 5 mm / min en utilisant une machine d'essai universelle (ZwickRoell-Z005). L'épaisseur finale des films épais MQS était de 15 à 20 ”m.

Du P (VDF-TrFE) a Ă©tĂ© dissous dans de la cyclohexanone. Des films minces ayant typiquement une Ă©paisseur de 500 nm ont Ă©tĂ© prĂ©parĂ©s par revĂȘtement par centrifugation.

Caractérisation de la solution

La pression de vapeur absolue des mĂ©langes de solvants a Ă©tĂ© mesurĂ©e Ă  l’aide d’une installation maison Ă©quipĂ©e d’un capteur de pression Ă©lectronique. Un volume fixe de solvant a Ă©tĂ© introduit dans un ballon scellĂ©. La pression Ă  la tempĂ©rature ambiante a Ă©tĂ© mesurĂ©e aprĂšs avoir atteint l'Ă©quilibre. Pour mesurer le point d'Ă©bullition du mĂ©lange de solvants, un fin tube capillaire scellĂ© Ă  une extrĂ©mitĂ© a Ă©tĂ© placĂ© dans le mĂ©lange de solvants avec l'extrĂ©mitĂ© ouverte vers le bas dans le liquide. Un thermomĂštre a Ă©tĂ© fixĂ© au tube Ă  essai avec un Ă©lastique. L'ensemble a Ă©tĂ© immergĂ© dans un bain d'huile. Au fur et Ă  mesure que la tempĂ©rature augmentait progressivement, un dĂ©gagement rapide de bulles commençait Ă  l'extrĂ©mitĂ© du tube capillaire. Le chauffage a Ă©tĂ© poursuivi pendant environ 5 Ă  10 s de plus, puis la source de chauffage a Ă©tĂ© retirĂ©e. La tempĂ©rature Ă©tait enregistrĂ©e chaque fois que les bulles cessaient de sortir du capillaire.

Pour la RMN en solution, le 1Les expĂ©riences de RMN-H ont Ă©tĂ© acquises avec un TXI Ă  triple rĂ©sonance de 5 mm 1H /13C /15Sonde N Ă©quipĂ©e d'un z– dĂ©gradĂ© sur le systĂšme AVANCE III Bruker Ă  850,3 MHz. The spectra were obtained with π/2-pulse lengths of 9 ÎŒs for proton (number of scans, 128; spectral width, 34,000 Hz) and 12 ÎŒs for carbon (scans, 2048; spectral width, 85,000 Hz) at 298 K and a relaxation delay time of 10 s each for 1H-NMR and 13C-NMR. The proton and carbon spectra were conducted in different mixtures of TFA and acetone-rĂ©6, and the spectra were referenced with an external capillary—the residual C2DHCl4 at 5.93 ppm (ÎŽ(1H)) and C2rĂ©2Cl4 at 73.80 ppm (ÎŽ(13C)). The temperature was calibrated with a standard 1H methanol NMR sample using the TopSpin 3.1 software (Bruker).

Microstructure investigation

AFM (NanoScope Dimension 3100, Bruker) was used to analyze the surface morphology of the thin films. Steady-state ultraviolet-visible absorption spectra were measured using a PerkinElmer Lambda 25 spectrophotometer. For haze measurement, we used a “Haze-gard plus” instrument (BYK-Gardner GmbH, Germany) for macroscopic optical properties using white light. Haze is defined as the part of light that deviates from the directly transmitted light at an angle higher than 2.5°. The thickness of the films was measured with a surface profilometer. DSC (DSC3+, METTLER TOLEDO) was performed under N2 atmosphere at a scan rate of 10°C/min. WAXD measurements of the films were performed at the DELTA Synchrotron using beamline BL09 with a photon energy of 13 keV (λ = 0.9537 Å). The beam size was 1.0 mm by 0.2 mm (width by height), and samples were irradiated just below the critical angle for total reflection with respect to the incoming x-ray beam (∌0.1°). The diffracting intensity was detected on a two-dimensional image plate (MAR-345) with a pixel size of 150 ÎŒm (2300 by 2300 pixels), and the detector was placed 523 mm from the sample center. Diffraction data are expressed as a function of the diffraction vector: q = 4π/λ sin(Θ), where Θ is a half the diffraction angle and λ = 0.9537 Å is the wavelength of the incident radiation. Ici, qxy (qz) is a component of the diffraction vector in plane (out-of-plane) to the sample surface. All x-ray measurements were performed under vacuum (~1 mbar) to reduce air scattering and beam damage to the sample. All WAXD data processing and analysis were performed by using the software package Datasqueeze (www.physics.upenn.edu/~heiney/datasqueeze/index.html). The IR spectra of nylon-11 films were recorded at room temperature using a Tensor II FTIR spectrometer with a resolution of 4 cm-1.

Solid-state NMR measurements were performed with a 2.5-mm 1H/X double-resonance CP/MAS probe at a Bruker 700 MHz AVANCE III NMR system. All measurements were taken at MAS speeds of 25 kHz. CP/MAS measurements were acquired with a contact time of 1 ms and 100-kHz radio frequency nutation frequency swept-frequency two-pulse phase modulation high-power composite pulse decoupling. Chemical shifts were referenced to tetramethylsilane using the CH3 group of l-alanine with the 1H peak at 1.3 ppm and the 13C peak at 20.5 ppm as a secondary standard.

Capacitor fabrication

The capacitors were fabricated on a glass substrate on which 50-nm-thick Au bottom electrodes with a 1-nm Cr adhesion layer were thermally evaporated using a shadow mask. After deposition of thin film, gold top electrodes (50 nm) were deposited using shadow mask to form a crossbar pattern with the device area of 0.0016 cm2. For the free-standing MQS thick film, capacitors were fabricated by thermal evaporation of gold (50 nm) on both sides with the device area of 0.74 cm2.

Ferroelectric characterization

Ferroelectric capacitors were characterized in a probe station in a vacuum of 10−5 mbar. rĂ©E hysteresis loops were measured using a Radiant precision multiferroic test system (Radiant Technologies Inc.) equipped with a high-voltage amplifier. Data retention and polarization fatigue were measured using the same setup. The fatigue test was performed using a continuous triangular waveform with the amplitude of 1.25 × Evs, respectively, 250 and 75 MV/m at 100 Hz for nylon-11 and P(VDF-TrFE). After a predefined number of cycles, the remanent polarization was determined by PUND (positive up negative down) measurement using 10-ms-wide pulses. For data retention, a write pulse is followed by two read pulses of the same amplitude but opposite direction. All pulse widths were fixed at 10 ms.

Ferroelectricity in MQS thick films

A bipolar triangular waveform at 10 Hz was applied at room temperature to the samples to study the réE hysteresis loop. As can be seen from fig. S1, when the poling field is below 180 MV/m, a linear dielectric loop is observed for all samples as the field was too low for the dipoles to switch. But when the field was increased to 200 to 250 MV/m, a clear ferroelectric hysteresis loop was observed. The figure below the réE loop shows the current density curve. The values of Pr and Evs are in good agreement with the literature values (3, 15).

Solution NMR

Carbon measurements of the mixture show some additional carbonyl signals compared to pure acetone-rĂ©6. For example, in the mixture 80:20 mol % acetone-rĂ©6:TFA, at least six different signals are detected, with a chemical shift difference of 0.06 ppm (60 parts per billion), which is typical for carbon shift when exchanging a deuterium with a proton next a carbonyl group (CD3C═O to CD2HC═O) (fig. S2A). The exchange of hydrogen and deuterium can be easily explained via the keto-enol tautomerism, in which all deuterium in acetone-rĂ©6 can be substituted with the protons coming from TFA (fig. S2B). the 1H spectra of solution NMR of TFA:acetone mixtures are shown in fig. S3. Figure S4 shows the NMR DOSY measurements for nylon-11 solution in pure TFA and TFA:acetone. The two different diffusion measurements show no change in the diffusion coefficient of the nylon-11 to corroborate that there is no change in the molecular size of the polymer using the TFA:acetone mixture.

Haze measurement

To show the optical quality of the films, we performed haze measurements, as shown in fig. S5 for different film thicknesses. Haze is due to internal or surface scattering due to particles/defects trapped within the film or roughness of the film, respectively (13). The value of haze for the conventionally spin-coated thin film for all thicknesses from 300 nm is 4% and linearly increases to 42% for 900-nm thin films. The SQ thin films however showed substantially lower haze values ranging from 0.3 for 300 nm to 4% for 900-nm thin films. By using solution quenching and high evaporation rate of solvent, thin films of high optical qualities were obtained.

Differential scanning calorimetry

The first heating cycle for the nylon-11 SQ thin film and MQS film is shown in fig. S6. After subtracting heat of cold crystallization from heat of melting, the degree of crystallinity (χvs) was determined using heat of fusion of 100% crystalline nylon-11, which is 225 (J/g) (15). A decrease of 4°C in melting point of the SQ thin film was observed as the film was processed from solution (20).

Fourier transform infrared spectroscopy

Figure S7 shows the FTIR spectra of the nylon-11 SQ thin film and MQS film measured at room temperature. The spectra are normalized to 2852 cm-1, the absorption band of symmetric CH2 stretching (3). The spectra for the SQ thin film were shifted vertically for clarity. The regions 500 to 800 cm-1 consist of the amide VI and V bands and appeared at 586 and 690 cm-1 for the SQ thin film, respectively. Comparing peaks with the MQS film gives an indication of the same phase (ÎŽâ€Č) in both the MQS film and the SQ thin film. The progression bands from the methylene sequence appeared from 1100 to 1400 cm-1, showing no periodic change in the intensities of all films. A shoulder at 1420 cm-1 (CH2 scissoring band) for both films shows same conformations in C─C bond near CO-vicinal CH2 group (24). The hydrogen-bonded N─H stretching was observed at 3292 cm-1 for both the SQ thin film and the MQS film.

Wide-angle x-ray diffraction

WAXD measurements were performed both along the parallel and perpendicular to the stretch direction for the MQS film to better resolve the 100 and 010 peaks. The resulting diffractograms are given in fig. S8 and show resolving of the peak at ~15 nm-1 to two closely spaced peaks. The best fit to the WAXD pattern, as discussed in the main text, was obtained when we placed the peak position of the (100) and (010) reflections at ré-spacing of 0.416 and 0.417 nm, respectively.

Acknowledgments: S.A. and K.A. acknowledge the technical support from the Max Planck Institute for Polymer Research. We acknowledge the beamline 9 of the DELTA electron storage ring in Dortmund for providing synchrotron radiation and technical support for WAXD measurements. We thank V. Maus and F. Keller for technical help and T. Marszalek for fruitful discussion about WAXD measurements. Funding: S.A. and K.A. acknowledge the financial support from the Alexander von Humboldt Foundation (Germany) through the Sofja Kovalevskaja Award. S.A. thanks the National University of Sciences and Technology (Pakistan) for the financial support. P.v.T. acknowledges the Graduate School of Excellence MAINZ for the financial support. Author contributions: S.A. prepared the thin films and ferroelectric capacitors. S.A., H.S.D., M.K., and K.A. performed the electrical, optical, and AFM tests and the following data analysis. S.A., D.P., M.W., R.G., and K.A. performed the NMR measurements and analysis. S.A., W.Z., W.P., and K.A. performed the WAXD measurements and analysis. P.v.T., U.K.-J., and H.F. synthesized the nylon-5. T.L. performed the haze and clarity measurements. K.A. designed the experiments and supervised the work. All authors contributed to data analysis and co-wrote the manuscript. IntĂ©rĂȘts concurrents: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

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