Article pubs.acs.org/JPCB
Electric-Field-Induced Dynamics of Polymer Chains in a Ferroelectric Melt-Quenched Cold-Drawn Film of Nylon-11 Using Infrared Spectroscopy Hayato Isoda and Yukio Furukawa* Department of Chemistry and Biochemistry, Graduate School of Advanced Science and Engineering, Waseda University, Okubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan ABSTRACT: The changes in intensity of the infrared bands of a ferroelectric melt-quenched, cold-drawn film of nylon-11 were measured as a function of a cyclic external electric field of 1.4 MV/cm. The infrared bands assigned to the NH stretching, amide I, NH-vicinal, and CO-vicinal CH2 scissoring modes showed butterfly-shaped hysteresis loops that are characteristic of ferroelectrics; however, the intensity changes of the infrared bands assigned to the CH2 antisymmetric and symmetric stretching modes are small and showed no butterfly-shaped hysteresis loops. These results indicate that the amide groups are inverted, while the methylene groups are not inverted under the external electric field. We propose a new molecular mechanism that explains the ferroelectric properties of nylon11. Only the amide groups in the antiparallel β-sheet structure are inverted by the external electric field to form new hydrogen bonds; these two states form a nearly double-minimum potential.
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INTRODUCTION Ferroelectric polymers are promising materials for piezoelectric sensors, pyroelectric actuators, and ferroelectric memory devices with flexibility and toughness. Poly(vinylidene fluoride) (PVDF) and its copolymers have shown ferroelectric properties and have been widely investigated.1 The ferroelectric properties originate from the inversion of the polymer chains containing polar −CH2CF2− groups. A nylon polymer has an alternating structure of polar amide groups (−NHCO−) and methylene chains, and nylons have exhibited ferroelectric properties.2−6 Nylons form “parallel” or “antiparallel” β-sheet structures.7 Nylon having an odd number of carbon atoms is called oddnumbered nylon. The chemical structure of nylon-11 is shown in Figure 1. The number of carbon atoms in the methylene group is 10, that is, (CH2)10. The repeating unit of nylon-11 has a permanent electric dipole moment originating from the amide
group. Thus, nylon-11 is expected to show ferroelectricity, which depends on its crystal structure. Nylon having an even number of carbon atoms is called even-numbered nylon. The repeating unit of the linear polymer chain of an even-numbered nylon has two alternating amide groups. Thus, the repeating unit of the polymer chain has no permanent electric dipole moment and is not expected to show ferroelectricity in the crystal form; however, odd- and even-numbered nylons show ferroelectricity,5 and thus nylons show interesting ferroelectric properties. Nylon-11 was found to have solid structures such as α, β, γ, δ, and δ′ forms.5,7−11 The thermally stable crystalline α form has a triclinic lattice with the following structural parameters: a = 0.49 nm, b = 0.54 nm, c = 1.49 nm, α = 49°, β = 77°, and γ = 63° (Z = 1), and the space group is P1 (C11).7 In this configuration, the polymer chains form a parallel β-sheet structure; however, no ferroelectric properties were reported for the α form, although the unit cell of the α form is polar. Lee et al.2 reported the ferroelectricity of nylon-11 using films that were uniaxially drawn at low temperatures following meltquenching. As the draw ratio increased, the remanent polarization increased.6 Itoh et al.5 and Zhang et al.10 studied the X-ray diffraction of melt-quenched and melt-quenched, cold-drawn films (δ′ form) of nylon-11 showing ferroelectric properties. A strong, broad diffraction was observed for each of Received: August 20, 2015 Revised: October 6, 2015 Published: October 12, 2015
Figure 1. Polymer structure of nylon-11 and the transition dipole moments of some vibrational modes. © 2015 American Chemical Society
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DOI: 10.1021/acs.jpcb.5b08104 J. Phys. Chem. B 2015, 119, 14309−14314
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the original and stretched films. The δ′ form has a “smectic”type structure.11 Thus, the detailed structures of the δ′ form and that of the melt-quenched, cold-drawn films are unclear. The infrared bands due to the amide I (CO stretching) and amide II (mixture of NH in-plane bending and CN stretching) modes are sensitive to hydrogen bonding and to secondary structures, such as the α-helices and β-sheets of proteins, polypeptides, and polyamides.12 The infrared spectra of various forms of nylon-11 have been reported.11,13,14 Yu and Fina14 reported the polarized infrared spectrum of a melt-quenched, cold-drawn film of nylon-11 that forms a β-sheet structure, and the amide I band was decomposed into bands arising from ordered, disordered, and free amide groups. Thus, the δ′ form has a disordered structure. Infrared spectroscopy is a powerful tool for studying the dynamics of polymer chains induced by an external electric field. Naegele and Yoon15 found that the intensity of an infrared band of ferroelectric PVDF exhibits a butterfly-shaped hysteresis loop as an electric field is cycled between positive and negative values. These butterfly-shaped hysteresis loops were also observed for PVDF and its copolymers.16−18 Yu and Fina19 reported the electric-field-induced infrared spectra of a meltquenched, cold-drawn film of nylon-11. They investigated the dipole reorientations from the infrared absorption bands of the NH stretching and amide I; however, from a molecular structure perspective, the mechanism of ferroelectricity in nylon-11 has not been fully clarified. In this study, we report the electric-field-induced infrared spectrum of a melt-quenched, cold-drawn film of nylon-11. We analyze the changes in the infrared bands of the amide and methylene groups. We propose a new mechanism of dipole inversion from a molecular structure perspective.
RESULTS AND DISCUSSION Polarized Infrared Spectra. The polarized infrared spectra of a uniaxially drawn film of nylon-11 are shown in Figure 2.
Figure 2. Infrared spectra of a melt-quenched, cold-drawn film of nylon-11. The solid and dotted lines correspond to the spectra with perpendicular and parallel polarizations to the drawn direction, respectively.
The solid and dotted lines represent the infrared spectra with perpendicular and parallel polarizations to the drawn direction, respectively. The observed peak positions and assignments of some bands are listed in Table 1, with the assignments based on literature results.20−22 The broad band at 1642 cm−1 is assigned to the amide I mode. According to the study published by Yu and Fina,14 the amide I band is decomposed into four bands, with peak wavenumbers of 1676, 1660, 1646, and 1638 cm−1. The 1676
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EXPERIMENTAL METHODS A melt-quenched, cold-drawn film of nylon-11 (Sigma-Aldrich) was prepared by melt-pressing a pellet between sheets of aluminum foil at 250 °C and under a pressure of 10 ton using a universal film-maker (S. T. Japan), followed by quenching in ice water. After removal of the aluminum foil, the film was uniaxially drawn to a ratio of 3:1 at room temperature. The thicknesses of the drawn films were between 6.5 and 7.5 μm. Then, Au was evaporated as transparent electrodes onto both surfaces of the film. The area of the deposited electrode was ∼100 mm2, and its thickness was 9.0 nm. The infrared absorption spectra were recorded on a Fourier transform infrared spectrometer (Digilab FTS-7000) equipped with a HgCdTe detector at a resolution of 4 cm−1. The observed amide I band was decomposed into five component bands by the least-squares method. A linear combination of the Gaussian and Lorentzian bands was used as a component band. The electric-field-induced spectra were measured with the polarization perpendicular to the drawn direction using a wiregrid polarizer (Specac). Electric fields were applied between the Au electrodes with a DC power supply (Advantest R6161). After poling of 1.4 MV/cm, the electric fields were incrementally applied between the electrodes of the sample for one cycle, starting at 0.0, via steps at −1.4, 0.0, and 1.4 and ending at 0.0 MV/cm. The absorbance spectrum of a sample under an electric field was measured with 512 accumulations, a process that required 3 min. The changes in absorbance were calculated from the observed absorbance spectra.
Table 1. Peak Positions (cm−1) and Assignments of the Observed Infrared Bands of a Melt-Quenched, Cold-Drawn Film of Nylon 11
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peak position (cm−1)
assignments
R
θ
3309 3275 3197 2923 2852 1676 1660 1646 1638 1623 1558 1540 1516 1477 1469 1457 1437 1419 1374 721
NH stretching overtone of amide I combination of amide I and amide II CH2 antisymmetric stretching CH2 symmetric stretching amide I (free) amide I (hydrogen-bonded) amide I (hydrogen-bonded) amide I (hydrogen-bonded) unassigned amide II (hydrogen-bonded) amide II (hydrogen-bonded) amide II (free) NH-vicinal CH2 scissoring CH2 scissoring (in-phase) CH2 scissoring CH2 scissoring CO-vicinal CH2 scissoring amide III CH2 rocking
0.21 0.33 0.26 0.51 0.39 0.58 0.70 0.41 0.25 0.45 1.94 3.42
72 68 70 63 66 62 60 66 71 65 46 37
0.35 0.30
67 69
0.16 1.92 0.33
74 46 68
DOI: 10.1021/acs.jpcb.5b08104 J. Phys. Chem. B 2015, 119, 14309−14314
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The Journal of Physical Chemistry B cm−1 band is assigned to a free amide I mode; the 1660, 1646, and 1638 cm−1 bands are assigned to the hydrogen-bonded amide I modes. The broad band at 1545 cm−1 is assigned to the amide II mode. This band is decomposed into three bands with peak wavenumbers of 1558, 1540, and 1516 cm−1. The former two bands are assigned to hydrogen-bonded amide groups, and the 1516 cm−1 band is assigned to the free amide groups. The bands observed at 1477, 1469, 1457, 1437, and 1419 cm−1 are assigned to the CH2 scissoring modes. The 1477 and 1419 cm−1 bands are assigned to the CH2 scissoring modes of the NH-vicinal CH2 group and to the CO-vicinal CH2 group, respectively.22 The 1469 cm−1 band is assigned to the in-phase mode in which the central CH2 groups, except for the NHvicinal and the CO-vicinal CH2 groups, vibrate in the same phase because the infrared intensity of the band is strong. To investigate the molecular orientation in the uniaxially drawn film, we calculated the dichroic ratio R = A∥/A⊥ for each band and is listed in Table 1. Thus, we can obtain the orientation angle, θ, which is the angle between the drawing direction and the transition moment of each vibration, using the dichroic ratio, R. Under a uniaxial orientation model, the relationship between R and θ is given by the following equation R = 2 cot2 θ
Figure 3. (a) Infrared spectrum of a melt-quenched, cold-drawn nylon-11 film after poling (no electric field was applied) and (b) the electric field-induced infrared difference spectrum of the film. The electric field was 1.4 MV/cm.
The electric field of the infrared light lies in the plane of the polymer surface. The observed decreases in the intensity of the NH stretching band and the amide I band indicate that the NH and CO bonds (z axis) rotate toward the surface normal on average because the infrared intensity is proportional to the square of the inner product of the electric field vector of the infrared light and the transition moment vector of the band. Thus, the external electric field that is perpendicular to the surface forces the electric dipole moment of the amide group (CO bond) to rotate toward the direction parallel to the external electric field, that is, the surface normal. The expanded infrared and electric-field-induced infrared spectra in the amide I range are shown in Figure 4a. The broad amide I band shows a complicated behavior under an external electric field. Under an electric field of 1.2 MV/cm, a negative peak is observed at 1639 cm−1. Under an electric field of 1.4 MV/cm, negative peaks at 1647 and 1639 cm−1 were observed. The absorbance change at 1639 cm−1 was 2.94 × 10−2. These results indicate that the 1639 cm−1 amide band moves faster than the 1647 cm−1 band after the application of the electric field. According to previous assignments,13,14 the amide I band observed at 1639 cm−1 is assigned to ordered groups, whereas the amide I band at 1647 cm−1 is assigned to disordered groups. In agreement with these assignments, the ordered hydrogenbonded amide groups respond quickly to the external electric field. The barrier of inversion for the ordered hydrogen-bonded amide group is probably lower than that for the disordered group. The expanded infrared and electric-field-induced infrared spectra in the amide II range are shown in Figure 4b. A positive peak was observed at 1571 cm−1, whereas a negative peak was observed at 1540 cm−1, which appears to be a derivative-like feature. This feature can be explained by a peak shift of the amide II band, which is induced by a change in the conformation near the amide group. The infrared and electric-field-induced infrared spectra in the CH stretching range are shown in Figure 5a. A derivative-like feature, that is, a positive peak at 2934 cm−1 and a negative peak at 2916 cm−1, is observed in the range of the 2923 cm−1 CH2 antisymmetric stretching vibration. The absorbance change at
(1)
The calculated θ values are listed in Table 1. To analyze the molecular orientation, the relationship between the transition dipole moment of each band and a functional group must be known. The angles between the C O bond and the transition dipole moment of the NH stretching, the amide I, and the amide II modes are 8, 17, and 77°, respectively, as shown in Figure 1.23 The transition dipole moment of the NH stretching mode is nearly perpendicular to the polymer axis (x axis). The transition dipole moment of the amide I mode is nearly perpendicular to the polymer axis, and that of the amide II mode is nearly parallel to the axis. The permanent electric dipole moment vector of the amide group is nearly parallel to the CO bond. The motion of the dipole moments of the NH stretching and the amide I bands reflects the permanent dipole moment of the CO group. The transition moment of the in-phase CH2 symmetric stretching vibration is along the z axis and is perpendicular to the polymer axis, whereas the transition moment of the in-phase CH2 antisymmetric stretching is along the y axis and is perpendicular to the polymer chain. The transition dipole moment of the in-phase CH2 symmetric stretching vibration is nearly parallel to that of amide I. External Electric-Field-Induced Infrared Spectra. The infrared absorption spectrum of the sample with the Au electrodes after poling but without the electric field is shown in Figure 3a. The electric-field-induced infrared absorption spectrum of the sample is shown in Figure 3b, revealing the difference between the spectrum of the sample under an electric field of 1.4 MV/cm and the poled sample without the electric field. The positive peaks indicate increases in intensity under an electric field, whereas the negative peaks indicate decreases in intensity. In Figure 3b, we can observe dramatic decreases in the intensity of the NH stretching and the amide I bands; however, the changes in intensity of the CH 2 antisymmetric and symmetric stretching vibrations were much smaller, indicating differences in the reorientation behaviors of the polar amide groups and methylene units in the nylon-11 polymer chains. 14311
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Figure 4. Expanded infrared spectrum of a melt-quenched, cold-drawn nylon-11 film after poling and the electric field-induced infrared difference spectrum of the film: (a) amide I range and (b) amide II range. The electric field was 1.4 MV/cm.
Figure 5. Infrared spectrum of a melt-quenched, cold-drawn nylon-11 film after poling and the electric-field-induced infrared difference spectrum of the film: (a) CH stretching region and (b) CH2 scissoring region. The electric field was 1.4 MV/cm.
The expanded infrared and electric-field-induced infrared spectra in the CH2 scissoring range are shown in Figure 5b. Negative peaks were observed at 1477 and 1419 cm−1; the electric-field-induced spectrum is different from that of the film without the electric field. The 1477 and 1419 cm−1 bands are assigned to the scissoring modes of the NH-vicinal CH2 group and the CO-vicinal CH2 group, respectively.22 The intensities of these two bands decrease when an electric field is applied. This result indicates that the NH- and CO-vicinal CH2 groups rotate upon the application of the electric field. Thus, conformational changes occur in the C−C bonds near the NH- and CO-vicinal CH2 groups. Infrared Intensity Changes under a Cyclic Stepwise Electric Field. The intensity changes of the NH stretching (3309 cm−1), the 1638 cm−1 amide I, the 1646 cm−1 amide I, the in-phase CH2 symmetric stretching (2853 cm−1), the NHvicinal CH2 scissoring (1477 cm−1), and the CO-vicinal CH2
2916 cm−1 was 7.45 × 10−3. A derivative-like feature, that is, a negative peak at 2853 cm−1 and a positive peak at 2844 cm−1, is observed in the range of the 2852 cm−1 CH2 symmetric stretching vibration. These derivative-like bands indicate that the wavenumbers of the CH2 antisymmetric and symmetric stretching vibrations shift under the external electric field. The amplitudes of the derivative-like features are small. The transition moment of the 2852 cm−1 CH2 symmetric stretching vibration is nearly parallel to that of the amide I. The changes in intensity of the CH2 symmetric stretching band should be similar to those of the amide I band when a nylon-11 polymer rotates as a rigid structure; however, the electric-field-induced change in the 2852 cm−1 CH2 symmetric stretching vibration is different from that of amide I. This result indicates that the methylene chain does not rotate upon the application of the electric field, whereas the amide group does rotate. 14312
DOI: 10.1021/acs.jpcb.5b08104 J. Phys. Chem. B 2015, 119, 14309−14314
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The Journal of Physical Chemistry B scissoring (1419 cm−1) bands under cyclic stepwise electric fields over the range of 1.4 to −1.4 MV/cm are shown in Figure 6a−f, respectively. The changes in intensity of the amide bands,
∼0.6 MV/cm, which are most likely inverted after the inversion of the amide group. Molecular Mechanism of Ferroelectricity. To discuss the electric-field-induced molecular motions associated with ferroelectricity, we used infrared spectroscopy and obtained the electric-field-induced motions of the amide group, the methylene groups connected to the amide group, and the remaining methylene groups (referred to as the central methylene group) separately. The amide group with a large permanent dipole moment is inverted upon the application of an electric field above the coercive field, as revealed by the NH stretching and the amide I bands showing butterfly-shaped hysteresis loops characteristic of ferroelectrics; however, the methylene groups without the NH- and CO-vicinal CH2 groups do not substantially move. The infrared bands of the NH- and CO-vicinal CH2 groups showed butterfly-shaped hysteresis loops, and the dipole moments of the NH- and CO-vicinal CH2 groups are small. Thus, the motions of these groups are caused by the inversion of the amide group. In the case of ferroelectric PVDF, upon applying an external electric field to a polar crystal, the polymer chains are inverted without a change in the molecular structure. In the case of nylon-11, only the amide groups are inverted upon applying the external electric field, and the central methylene chains are not inverted; the molecular structure of nylon-11 is changed. A schematic representation of the inversion is shown in Figure 7. The
Figure 6. Intensity changes versus external electric field: (a) NH stretching (3309 cm−1), (b) amide I (1647 cm−1), (c) amide I (1639 cm−1), (d) in-phase CH2 symmetric stretching vibration (2853 cm−1), (e) NH-vicinal CH2 scissoring vibration (1477 cm−1), and (f) COvicinal CH2 scissoring vibration (1419 cm−1). The dashed lines correspond to the poling process. Figure 7. Schematic representation of the structural inversion due to an external electric field.
the NH stretching, the 1638 cm−1 amide I, and the 1646 cm−1 amide I bands showed butterfly-shaped hysteresis loops that are characteristic of ferroelectrics.15−18 Each hysteresis loop shows a maximum at an electric field of ∼0.6 MV/cm. Under this electric field, the transition dipole moment of each band is inverted. This electric field corresponds to the coercive field of 0.65 MV/cm that was reported from the D−E hysteresis of nylon-11 at 20 °C.2 Above this electric field, the amide groups reorient in opposite directions. The amide I band under an electric field of zero after the inversion is nearly the same as that under an electric field of zero before the inversion. Thus, before and after the inversion, the amide groups form hydrogen bonds. The in-phase CH2 symmetric stretching band at 2853 cm−1 showed small intensity changes and did not exhibit clear butterfly shaped hysteresis loops, as shown in Figure 6d. The in-phase CH2 antisymmetric stretching band at 2916 cm−1 also showed small intensity changes and did not exhibit clear butterfly-shaped hysteresis loops (not shown). These results indicate that the central methylene chains did not move under the application of the external electric field. The NH-vicinal and the CO-vicinal CH2 scissoring bands showed butterfly-shaped hysteresis loops, as shown in Figures 6e,f, respectively. Each hysteresis loop also showed a maximum at an electric field of
infrared results indicate that the amide groups form hydrogen bonds before and after the inversion. An extended polymer chain of nylon-11 can form parallel and antiparallel β-sheet structures. In the antiparallel β-sheet structure, an inverted amide group can form a new hydrogen bond, whereas in the parallel β-sheet structure an inverted amide group cannot form a new hydrogen bond. The extended nylon-11 chains most likely form an antiparallel β-sheet structure.
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CONCLUSIONS We studied the molecular mechanism of ferroelectricity in a ferroelectric melt-quenched, cold-drawn film of nylon-11 using infrared spectroscopy. The infrared spectra of a melt-quenched, cold-drawn film of nylon-11 were measured as a function of a cyclic external electric field of 1.4 MV/cm. The NH stretching band (3309 cm−1) and the amide I bands (1638 and 1646 cm−1) showed butterfly-shaped hysteresis loops characteristic of ferroelectrics. This result indicates that the polar amide groups are inverted by the external electric field; however, the electric-field-induced intensity changes of the CH2 antisymmetric and symmetric stretching bands were small and showed 14313
DOI: 10.1021/acs.jpcb.5b08104 J. Phys. Chem. B 2015, 119, 14309−14314
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Fluoride/Trifluoroethylene Copolymers. Macromolecules 1989, 22, 1092−1100. (18) Isoda, H.; Furukawa, Y. Effect of Electric Field on the Infrared Spectrum of Ferroelectric Poly(vinylidene fluoride-co-hexafluoropropylene) Film. Vib. Spectrosc. 2015, 78, 12−16. (19) Yu, H. H.; Fina, L. J. Electric Field-Induced Dipole Reorientation in Oriented Nylon 11 by In Situ Infrared Spectroscopy. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 781−788. (20) Cannon, C. G. The Infra-red Spectra and Molecular Configuration of Polyamides. Spectrochim. Acta 1960, 16, 302−319. (21) Jakes, J.; Krimm, S. A Valence Force Field for the Amide Group. Spectrochim. Acta, Part A 1971, 27, 19−34. (22) Heidemann, V. G.; Zahn, H. Beitrag zur Deutung des Infrarotspektrums von Nylon 6,6. Makromol. Chem. 1963, 62, 123− 133. (23) Tsubio, M. Infrared Dichroism and Molecular Conformation of α-Form Poly-γ-benzyl-L-glutamate. J. Polym. Sci. 1962, 59, 139−153.
no butterfly-shaped hysteresis loops, indicating that the methylene groups are not inverted by the external electric field. The CH2 scissoring bands of the NH-vicinal and the COvicinal CH2 groups showed butterfly-shaped hysteresis loops, indicating that these groups are inverted following the inversion of the amide group. We propose a new mechanism that explains the ferroelectric properties of nylon-11. The amide groups in the antiparallel β-sheet structure are inverted by the external electric field to form new hydrogen bonds; these two states form a nearly double-minimum potential. Therefore, we can measure the electric-field-induced dynamics of each chemical group of a sample molecule using infrared spectroscopy.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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