Vibrational Sum Frequency Generation (SFG) Analysis of Ferroelectric

Mar 20, 2017 - Gao, Wu, Ma, Robertson, and Nathan. 2017 9 (22), pp 18410–18416. Abstract: This paper presents a flexible graphene/polyvinylidene ...
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Vibrational Sum Frequency Generation (SFG) Analysis of Ferroelectric Response of PVDF-Based Copolymer and Terpolymer Inseok Chae,† Saad Ahmed,‡ Hassene Ben Atitallah,‡ Jiawei Luo,† Qing Wang,§ Zoubeida Ounaies,‡ and Seong H. Kim*,†,§ †

Department of Chemical Engineering and Materials Research Institute, ‡Department of Mechanical and Nuclear Engineering, and Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States §

S Supporting Information *

ABSTRACT: In poly(vinylidene difluoride) (PVDF)-based polymers, the alignment of dipoles of the crystalline β phase with alltrans conformations is responsible for ferroelectricity; however, there is a lack of direct spectroscopic evidence for this phenomenon. We report in situ monitoring of the molecular orientations associated with ferroelectric responses of PVDF-based polymers using vibrational sum frequency generation (SFG) spectroscopy. The orientation of CH2 groups with respect to the −(CH2CF2)n− chain axis of the poled β domains in a PVDF copolymer was determined from the SFG dependence of an uniaxially aligned sample on the laser polarization and azimuthal angle. The relative intensities of CH2 symmetric and asymmetric stretch modes as well as their total intensities revealed the transformation of the all-trans conformation domains during the electrical poling of PVDF based co- and terpolymer films.

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and a relatively high electrostrictive strain (∼4 to 7%).7,9,10 Given the technological importance of the PVDF family of polymers, there is a need for better understanding of the CH2− CF2 molecular orientation and its evolution within the polar β phase as well as in the transition from the nonpolar α phase to the polar β phase. Elucidating the molecular orientation under applied electric field will aid in further improving the electromechanical coupling in PVDF-related structures. The formation and alignment of the β phase PVDF were previously studied with X-ray diffraction (XRD) and infrared (IR) spectroscopy.4,11−13 XRD studies of PVDF and P(VDFTrFE) have shown that the b-axis of the β phase unit cell, which is parallel to the dipole moment responsible for the ferroelectric behavior, is aligned along the poling axis with discrete increments of ∼60°.11,12 In situ IR analysis of a relaxor− ferroelectric terpolymer containing the chlorofluoroethylene system, P(VDF-TrFE-CFE), showed that the polymer assumes mostly the α and γ phases before poling, and the β phase is induced by applying high electric fields.13 Although the β phase dipole of P(VDF-TrFE-CFE) is expected to be aligned along the electric field direction, direct spectroscopic evidence has not been reported. One of the reasons is that in linear spectroscopy such as IR, all phases including the amorphous phase contribute to the observed signals, interfering with the weak signals from

INTRODUCTION Poly(vinylidene difluoride) (PVDF) is a commonly used electroactive polymer, which is both ferroelectric and piezoelectric owing to the presence of a strong −CF2− dipole.1,2 When processed through melt processing or solution casting, PVDF crystallizes predominantly in the nonpolar α phase with a TGTG’ conformation (T = trans, G = gauche), which then can be transformed to the polar β phase with an all-trans (Tm>4) conformation by mechanical stretching.1 Research has shown that introducing a certain level of defects in the polymer structure stabilizes the β phase without the need for mechanical stretching. For example, copolymers of VDF and the trifluoroethylene (TrFE) monomer, P(VDF-TrFE), result in stabilization of the all-trans conformation with a strong ferroelectric behavior.1,3,4 Since the discovery of P(VDF-TrFE) copolymer, many researchers have attempted to increase the electric-field-driven strain response by pursuing this defect-driven β phase formation. Recently, the incorporation of a bulky third monomer (chlorotrifluoroethylene, CTFE) into P(VDFTrFE), forming a P(VDF-TrFE-CTFE) terpolymer, was reported to induce the relaxor ferroelectric behavior.5 The relaxor behavior is believed to originate from the formation of nanopolar domains in the terpolymer system instead of micropolar domains as seen in typical ferroelectric polymers.2,6−8 When electric field is applied, the disordered nanopolar domains change their conformation to all-trans conformation which leads to a high dielectric constant (∼50) © XXXX American Chemical Society

Received: January 24, 2017 Revised: March 6, 2017

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Macromolecules the β phase. Because of this interference, spectral changes upon the β phase formation or reorientation are typically small and often undetectable.13 Nonlinear spectroscopy can circumvent the signal overlapping or interference problem because only the phase or medium in which centrosymmetry is lacking contributes to the nonlinear optical signals.14 For example, second harmonic generation (SHG) spectroscopy has been extensively used to analyze the degree of poling and the orientation of the overall dipole moment of poled polymers.15−20 Among three polymorphs of the crystalline PVDF phases, the noncentrosymmetric β phase is predominantly responsible for the ferroelectric response;1 thus, SHG can selectively detect the dielectric responses of the β phase in the sample. However, the SHG process lacks the molecular specificity and the details of the CH2−CF2 orientation are deduced indirectly from the SHG responses.15,20 In this study, we employed sum frequency generation (SFG) spectroscopy to directly detect and monitor the polar order of the CH2 vibrations of the β phase. Unlike SHG where the input photon (ω1) is not in resonance with vibrational or electronic transition of the functional groups and the detection is the overall dielectric response of the system at the frequency of 2ω1, SFG uses two photons with different frequencies (ω1 and ω2), and the detection is at the frequency (ωSF) of ω1 + ω2.14 If one of the input photons is in the mid-IR range (ωIR), then the SFG process can be enhanced upon resonance with the vibrational excitation of a specific functional group, giving rise to the molecular specificity. The other beam is conventionally called visible (ωVIS) and typically 530 or 800 nm laser pulses. Thus, SFG combines the advantages of the molecular specificity of IR, the selectivity to crystalline phase of XRD, and the sensitivity to the degree of dipole orientation of SHG. The intensity of SFG responses (IωSF), normalized with the intensities of two input beams (IωIR and IωVIS), can be expressed as14,21,22 ISFG =

IωSF IωIRIω VIS

SFG is typically known as a “surface-sensitive” technique;23 however, it should be noted that the surface is only one example which intrinsically provides the noncentrosymmetry needed for SFG. Noncentrosymmetric bulk domains interspersed in amorphous matrices are also SFG-active.21,24−26 In the presence of SFG signals from the bulk, the surface SFG signal becomes negligible or extremely difficult to detect since the bulk signal is dominating. In this paper, we report the SFG responses of the ferroelectric poly(vinylidene fluoride−hexafluoropropylene) (PVDF-HFP) and P(VDF-TrFE) copolymers and the relaxor−ferroelectric P(VDF-TrFE-CTFE) terpolymer. Using the stretched and prepoled P(VDF-HFP) film, the orientations of the CH2 group and its symmetric and asymmetric stretch modes were determined with respect to the −(CH2CF2)n− chain axis of the β phase. By monitoring SFG signals of P(VDFTrFE) and P(VDF-TrFE-CTFE) thin films in situ while sweeping the electrical bias across the sample, we studied how the orientation of the CH2 group, whose molecular axis is parallel to the dipole moment of the β phase, is related to the polarization, charging−discharging capacity, and mechanical strain in response to applied electric field. In the case of relaxor ferroelectric P(VDF-TrFE-CTFE), in situ SFG monitoring of the CH2 group revealed structural information relevant to the transition from paraelectric α phase to ferroelectric β phase.

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EXPERIMENTAL SECTION

Sample Preparation. P(VDF-HFP) (88/12 mol %), P(VDFTrFE) (56/44 mol %), and P(VDF-TrFE-CTFE) (62/30/8 mol %) powders were supplied by the Arkema group (Piezotech, PierreBenite, Cedex). The P(VDF-HFP) powders were dissolved in N,Ndimethylformamide (DMF)/acetone (1:1 v/v) solvent and deposited on a glass slide through a solution-casting process. The film was dried at 135 °C for 12 h under vacuum and then quenched by immersing it in liquid nitrogen for 15 min. It was uniaxially stretched with a draw ratio of 4 through a narrow zone drawing process at 135 °C. The crystalline morphology of the stretched P(VDF-HFP) was analyzed using two-dimensional (2D) wide-angle X-ray diffraction (WAXD; Scintag Cu Kα diffractometer). The stretched film was poled by applying an electric field of 100 MV/m. P(VDF-TrFE) and P(VDFTrFE-CTFE) powders were dissolved in DMF (12 wt %) and mixed for 3 h using a magnetic stirrer at room temperature. The solutions were degassed under vacuum, solution-casted, and then dried at 90 °C under vacuum. These films were annealed for 6 h at 120 °C under vacuum to increase their crystallinity. Vibrational Sum Frequency Generation (SFG) Spectroscopy. A picosecond (ps) scanning SFG system was used for the stretched and prepoled P(VDF-HFP) film analysis. Details of this system are described elsewhere.25 The second harmonic output (532 nm) of a Nd:YAG laser was used as the visible beam. Using an optical parametric generator and amplifier, tunable IR beam was generated. The visible and IR beams were spatially and temporally overlapped on the poled P(VDF-HFP) film. The incident angles of the visible and IR beams were 56° and 60° from the surface normal, respectively. The SFG signal was collected in the reflection mode. It was filtered using a monochromator, detected with a photomultiplier tube, and normalized with the incident 532 nm and IR beam intensities. A femtosecond (fs) broadband SFG system was used for simultaneous monitoring of CH2 symmetric and asymmetric stretch modes while applying E(t) on the P(VDF-TrFE) and P(VDF-TrFECTFE) films. Details of this system are described elsewhere.26 A broadband IR pulse with a center wavenumber of 2990 cm−1 (full width at half maximum of ∼160 cm−1), which covers the symmetric and asymmetric stretch peak regions of CH2, was used. A narrow band of 800 nm pulse was produced by spectral filtering of the fundamental output of the Ti-sapphire amplifier. The incident IR and visible beams, propagating nearly parallel, were focused onto a sample at 45° with

⎛ sin(Δk·ΔL /2) ⎞2 ⎟ ∝ |χ (2) + χ (3) E(t )|2 ⎜ ⎝ Δk·ΔL /2 ⎠ (1)

Here, the first term on the right-hand side of eq 1 is the combined contribution from the second- and third-order susceptibilities (χ(2) and χ(3), respectively). Only noncentrosymmetric media can have nonzero χ(2) terms; χ(2) is zero for all centrosymmetric or random media.14 The χ(3) term does not have noncentrosymmetry requirements; so, χ(3) alone is SFGinactive. But, in the presence of a strong electric field (E(t)), χ(3)E(t) can become SFG-active.14 The sum of these two terms is equivalent to the total polarization of the system. The SFG intensity is expected to be proportional to the square of total polarization. In eq 1, the second term containing the sine cardinal form is the phase synchronization factor. It depends on the sample size (L) and the phase mismatch among the wave vectors of the ⃗ ), visible input (k VIS ⃗ ), and infrared sum-frequency output (k SF ⃗ − (k VIS ⃗ + kIR ⃗ )|.14 The wave ⃗ ) beams, Δk = |k SF input (kIR vector is equal to the wavenumber (ω/c) of light multiplied by refractive index (n) of the medium along the light propagation direction. The phase synchronization term becomes negligible when Δk·L> π; this defines the coherence length of the SFG process to be approximately lc ≈ π /Δk .14,16 B

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Figure 1. Schematic illustration of measuring the field-induced polar ordering of PVDF based polymers using the fs broadband SFG in reflection mode.

Figure 2. (a) Schematic representation of the P(VDF-HFP) repeating units. (b) 2D WAXD image of the mechanically stretched P(VDF-HFP) film. (c, d) ssp and ppp SFG spectra of the stretched and poled 8 μm thick P(VDF-HFP) film at two azimuthal angles: when the laser incidence plane (IP) is aligned (c) parallel and (d) perpendicular to the stretch direction. The spectra were collected with the ps scanning SFG system. The three letters notation (ssp and ppp) represents the polarizations of the output and input beams in the order of SFG, 532 nm, and IR beams. The s- and p-polarized light has the electric vector perpendicular and parallel to IP, respectively. The spectra are normalized with the 2973 cm−1 peak intensity.

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respect to the surface normal. The SFG signal was filtered using a short-pass filter, a monochromator, detected with a CCD camera, and then normalized with the 800 nm intensity, and the IR beam profile was obtained from the nonresonant background of α-quartz.26 Figure 1 depicts the schematic diagram of in situ SFG measurement of polar ordering in PVDF-based polymers as a function of applied electric field. The SFG signal of P(VDF-TrFE) film with a thickness of 25 μm and P(VDF-TrFE-CTFE) film with a thickness of 30 μm were measured while sweeping the electrical bias across the films. The polymer films were coated on the bottom side with a 50 nm thick silver electrode using a EMS 150 RS sputter coater (Quorum Technologies, Laughton, Lewes). The top electrode was an indium tin oxide (ITO)-coated glass. To ensure a good contact between the ITO electrode and the polymer samples, two ring-shaped magnets were used to apply compression on their surface. The samples were dipped in a galden oil for 2 s before SFG measurements in order to prevent breakdown of the thin films. During the SFG measurement, electrical bias was applied using a 33220A function generator (Agilent Technologies, Santa Clara, CA) and high-voltage amplifier (Model 610E) (Trek, Lockport, NY). A sinusoidal electric field with an amplitude of 100 MV/m was applied across the samples. The frequency of the sinusoidal field was 10 mHz and 200 mHz for P(VDF-TrFE) and P(VDF-TrFE-CTFE) measurements, respectively. The fs broadband SFG spectrum was saved every 528 ms for P(VDFTrFE) and 50 ms for P(VDF-TrFE-CTFE). Polarization-Electric Field (P−E) Loop Measurement. The P− E loop of the P(VDF-TrFE) and P(VDF-TrFE-CTFE) films were measured separately using a modified Sawyer−Tower circuit. The electric field was applied across the polymer films, and the current was integrated into charge via a capacitor in series with the samples.

RESULTS AND DISCUSSION In order to investigate the relationship between the CH2 SFG peaks and the β crystal orientation, uniaxially aligned and poled P(VDF-HFP) film (predominantly oriented β phase) was considered first. Figure 2a shows the molecular structure of P(VDF-HFP). In Figure 2b, the 2D WAXD image of the stretched P(VDF-HFP) film indicates that the sample contains both α and β phases (see Figure S1a in the Supporting Information).27 The crystallinity was estimated to be ∼68%. The (110) and (200) planes of the β phase are concentrated on the equator that is perpendicular to the stretching direction (indicated with the arrow in Figure 2b; see Figure S1b for the azimuthal angle distribution). This indicates that the β crystalline domains are uniaxially aligned with their c-axis parallel to the stretch direction. The α phase is SFG-inactive due to symmetry cancellation of dipoles within the TGTG′ conformation. The amorphous phase is also SFG-inactive because it is random. Although the β phase with the all-trans conformation has no inversion symmetry along the b-axis of the unit cell (thus, χ(2) ≠ 0), the stretched P(VDF-HFP) sample does not produce strong SFG signals. This is because the b-axes of the β domains in the stretched sample are randomly distributed around the caxis,11,12 which makes the |χ(2)| averaged over the SFG coherence length (lc) negligible. Upon poling, the b-axes of the β domains rotate along the applied electric field direction, producing a large net dipole over the SFG coherence length. This gives rise to a large |χ(2)| term, and the ISFG signal becomes strong (eq 1). Since the P(VDF-HFP) sample was prepoled C

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Figure 3. (a) SFG spectra of the 25 μm thick P(VDF-TrFE) film at various electric bias voltages. The spectra were collected using the fs broadband system with the ppp polarization combination of the SFG signal, 800 nm pulse, and fs broadband IR pulse. Inset shows the schematic representation of the P(VDF-TrFE) repeating units. (b) Comparison of the ISFG−E and P−E hysteresis curves. In the ISFG−E plot, the ISFG (symbols) is the total area of both d+ and d− SFG signals, and the intensity ratio r = ∫ I(d+)/∫ I(d−) (lines) is shown with a separate scale at the right. In the d− peak, only the components near ∼3016 cm−1 are included. In the P−E plot, the P2−E curve is shown for comparison with the ISFG−E curve. The poling and depoling cycles are shown in red and blue colors, respectively.

sinusoidal electric field with a maximum amplitude of 100 MV/ m was applied across the P(VDF-TrFE) film. The P(VDFTrFE) film is predominantly in the β phase without the need for stretching.31 Since there is no uniaxial alignment of the caxis in the P(VDF-TrFE) film, the CH2 group in the poled sample will have the C∞ symmetry with the b-axis aligned along the symmetric rotation axis. The CH2 ppp-SFG spectra of this sample were measured as a function of E(t) and compared with the P−E hysteresis curve obtained separately. Figure 3a plots the SFG spectra of the ferroelectric P(VDFTrFE) film measured in situ while E(t) varies continuously at a 10 mHz rate. The overall spectral feature is consistent with the P(VDF-HFP) spectra shown in Figure 2. To obtain the integrated intensity of d+ and d−, the SFG peaks were deconvoluted with several components by fitting curves (see Figure S2). When the SFG intensity is small, three components can be identified in the d+ mode: ∼2967, ∼2973, and ∼2984 cm−1. The presence of multiple components in the d+ mode has been reported in the SFG spectra of ethylene glycol, IR spectra of nylon-11, and Raman spectra of polymethylene chain.29,32,33 The split of the d+ band might originate from either subtle differences in molecular conformation or the Fermi resonance with the overtone of the CH2 deformation (δ) mode.29,32 Although the d+ band of SFG spectra at high electric fields appears as a single peak, it could be fitted with the same three components with relatively constant ratios and peak widths (see Figure S2). The d− band also appears to have multiple components over a broader range (see Figure S2). In order to be SFG-active, the band must be Raman-active and IR-active. 34 The d − components near ∼3016 cm−1 are both Raman- and IR-active; however, the components near ∼3050 cm−1 are not detected or very weak in Raman (see Figure S3). So, the origin or dynamics of the high wavenumber components might be different from the components near ∼3016 cm−1. Further details are not known at the moment and beyond the scope of this study. Figure 3b compares the total SFG peak area versus electric field (ISFG−E) plot with the polarization (P−E) hysteresis curve. The ISFG is proportional to the square of the total polarization, |χ(2) + χ(2)E(t)|2, as shown in eq 1. Thus, the

and there was no external E(t) applied to the sample during the SFG measurement, the contribution from χ(3)E(t) in eq 1 is zero. Figures 2c and 2d display the CH2 stretch region of the SFG spectra of the stretched and poled P(VDF-HFP) sample measured at two azimuthal angles: aligning the laser incidence plane parallel and perpendicular to the c-axis of the β domain which is the PVDF chain axis with the all-trans conformation. Two peaks at 2973 and 3016 cm−1 are assigned to the symmetric (d + ) and asymmetric (d − ) stretch modes, respectively, of the CH2 group.28 The strong interactions between the aligned CH2−CF2 dipoles cause the CH2 stretch mode to appear at higher frequencies in PVDF than in typical organic polymers such as polyethylene.28 In the β phase, the transition dipole moment of the CH2 d+ mode is parallel to the b-axis, which is aligned along the poling direction and parallel to the laser incidence plane. The transition dipole moment of the CH2 d− mode is perpendicular to that of the d+ mode and parallel to the a-axis.29 In SFG analysis, when the symmetric stretch mode is aligned along the normal to the sample surface, then its signal is much stronger than the asymmetric stretch mode.30 For the stretched sample, the c-axis of the poled β domains is uniaxially aligned along the stretch direction. When the laser incidence plane is parallel to the stretch direction, then the transition dipole of the d− mode (which is along the a-axis) is perpendicular to the laser incidence plane. Thus, the d− peak is slightly stronger in the ssp spectrum than the ppp spectrum (Figure 2c). The ppp polarization probes electric dipoles only within the laser incidence plane. When the laser incidence plane is perpendicular to the stretch direction (Figure 2d), the transition dipole of the d− mode is within the laser incidence plane; thus, it is stronger in the ppp polarization than the ssp polarization. In order to simultaneously monitor the relative intensities of CH2 symmetric and asymmetric vibration modes (r = ∫ I(d+)/∫ I(d−)) as well as the total intensity of the CH2 peaks (∫ I(d+) + ∫ I(d−)) as a function of the applied electric field (E(t)), the fs broadband SFG system was employed. The D

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Figure 4. (a) The ppp-SFG spectra of the 30 μm thick P(VDF-TrFE-CTFE) film at various electric bias voltages. Inset shows the schematic representation of the P(VDF-TrFE-CTFE) repeating units. (b) Comparison of the ISFG−E and P−E hysteresis curves. In the ISFG−E plot, the ISFG (symbols) is the total area of both d+ and d− SFG signals, and the intensity ratio r = ∫ I(d+)/∫ I(d−) (lines) is shown with a separate scale at the right. In the P−E plot, the P2−E curve is shown for comparison with the ISFG−E curve. The poling and depoling cycles are shown in red and blue colors, respectively.

the poled β domain. This implies that the polar order of most β domains in the P(VDF-TrFE) system remains unchanged and the flipping occurs only near the coercive field (|Ec| − 10 MV/ m < |E| < |Ec| + 10 MV/m). This observation is related to the ferroelectric behavior of the copolymer with the large remnant polarization (Pr) and the low energy release density during the depoling cycle.38 The molecular orientations in the relaxor ferroelectric P(VDF-TrFE-CTFE) film were also studied with the fs broadband SFG system. The presence of large CTFE units in the polymer chain does not allow formation of the large ferroelectric β domains in the P(VDF-TrFE-CTFE) terpolymer system; instead, a number of small ferroelectric and paraelectric domains are formed. In the nonpolar paraelectric phase, the trans and gauche conformations coexist.39,40 As discussed earlier, due to their nanoscopic size, the coercivity of ferroelectric domains is weak and the hysteresis in the P−E plot is small (Figure 4b) compared to the P(VDF-TrFE) copolymer system (Figure 3b). Figure 4a shows the SFG responses of P(VDF-TrFE-CTFE) measured in situ while E(t) varies continuously at a 200 mHz rate. Similar to the P(VDF-TrFE) system, the SFG spectra of P(VDF-TrFE-CTFE) show three components in the d+ band at ∼2967, ∼2973, and ∼2984 cm−1 when the SFG intensity is weak. The large single peak feature at the high-intensity spectrum can also be fitted with the same three components (see Figure S2). The ISFG−E curve deviates more from the P2− E curve (Figure 4b) compared to the P(VDF-TrFE) case (Figure 3b). The deviation might be due to the large Kerr effect of the P(VDF-TrFE-CTFE) terpolymer.35 Compared to the P(VDF-TrFE) system, the r value of P(VDF-TrFE-CTFE) increases gradually over a larger range of |E| from ∼25 to ∼75 MV/m during the poling and the maximum value (∼10) is much larger. The gradual change in r can be attributed to the field-induced conversion of paraelectric (predominantly α phase) domains to the ferroelectric β phase rather than the sudden flip of ferroelectric β domain.13 Since the small β domains are simultaneously aligned while they are formed in the relaxor ferroelectric system under strong electric fields, they do not go through the stepwise rotation process imposed to the larger domains found in the ferroelectric

overall shape of the ISFG−E curve agrees reasonably with the P2−E curve. Also, the r value is the lowest (∼0.8) when the net polarization is zero at the coercive electric field (Ec). At Ec, the polar ordering of the b-axis of the β domains is nulled; thus, the χ(2) term of eq 1 is the smallest or zero. Although very weak, the SFG intensity at Ec shows a finite value; this might be due to a weak contribution from the χ(3)E(t) term of the unpoled sample or incomplete randomization of the polar domains. In any case, the net alignment of the CH2 groups along the poling direction is negligible at Ec; therefore, the enhancement of the d+ mode due to preferential alignment along the electric field is insignificant. In fact, the r value of ∼0.8 at Ec is close to the random orientation of the symmetric mode with respect to the surface normal direction in the ppp SFG spectra of the C∞ symmetry system.30 It is noted that the r value significantly increases from ∼0.8 to ∼2.4 while |E| increases from |Ec| to |Ec| + 10 MV/m. This indicates that the b-axis of the β domains rotates to the poling direction in this narrow range.15,33 The angular distribution of the b-axis in the poled sample is not a delta function, but broad. If it is assumed to be around 10°−15° based on the two-angle WAXD analysis,11,12 we could estimate the average angle between the CH2 molecular axis and the electric field from the theoretically calculated angle dependence of the r value in the ppp SFG spectra of the C∞ symmetry system.30 In this case, the r value of ∼2.4 corresponds to an average d+ dipole angle of 15°−20° tilt from the poling direction. This is comparable to the angle between the dipole moment and the poling direction estimated from previous SHG measurement.15 It is interesting to note that while |E| decreases from the maximum applied value (100 MV/m), the SFG intensity shows a small decrease and the r value does not change until |E| reaches |Ec| − 10 MV/m and then both suddenly drop to the minimum values as |E| approaches from |Ec| − 10 MV/m to |Ec|. The weak field dependence of the SFG signal between |Emax| and |Ec| − 10 MV/m must be due to changes in the χ(3)E(t) term since the r value does not change (i.e., no change in χ(2)). The χ(3)E(t) term is related to the Kerr effect (i.e., changes in dielectric constant at high electric field due to electro-optical effects) of the P(VDF-TrFE) system.35−37 At zero bias, the SFG intensity is purely due to the χ(2) term which scales with E

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Macromolecules P(VDF-HFP) and P(VDF-TrFE) systems discussed above.11,12 The larger the r value means that the net dipole axis is closer to the poling direction and the angular distribution or variance is smaller.30 Unlike the P(VDF-TrFE) case, the SFG intensity of P(VDFTrFE-CTFE) decreases substantially as |E| decreases from the maximum value to zero (Figure 4b). Because the coercivity between domains is weak, the ferroelectric domains become unstable and transform back to the paraelectric phase as the electric field decreases.39,40 These polymorphic transition and structural rearrangement are accompanied by the release of stored charge from the poled P(VDF-TrFE-CTFE) system.38 Unlike the P(VDF-TrFE) copolymer, the r value of the poled P(VDF-TrFE-CTFE) system starts decreasing as soon as the applied E(t) changes its polarity (cross the zero axis); then, it reaches the minimum value (∼2) when |E| approaches the value corresponding to approximately zero polarization (P = 0). At this stage, ferroelectric domains with opposite polarity develop, and the enhancement of the d+ mode due to preferential order is suppressed; thus, r approaches the random orientation value.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Air Force Office of Scientific Research (AFOSR) (grant FA9550-16-1-0062). The fs broadband SFG system used in this study was constructed with the support from the Center for Lignocellulose Structure and Formation (CLSF), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award DESC0001090. The authors acknowledge Dr. Jiezhu Jin for preparation of PVDF-HFP samples for this study and Dr. Laura Bradley for some of preliminary tests in the past.

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CONCLUSIONS This paper reports the use of vibrational sum frequency generation (SFG) spectroscopy as a tool uniquely suited to investigate and monitor ferroelectric domain formation and orientation in PVDF-based polymers. Specifically, it demonstrates the ability of SFG to directly detect and monitor the CH2 vibrations of the noncentrosymmetric β phase; since the molecular axis of the CH2 group is parallel to the dipole moment of the β phase, an in-depth analysis of the SFG spectra of the β phase reveals important fundamental information on ferroelectric and relaxor ferroelectric responses of PVDF-based polymers. In a stretched and poled P(VDF-HFP) copolymer, SFG confirmed the dipolar orientation with respect to the CH2 molecular axis of the β phase which is parallel to the dipole of CF2 group. In situ SFG analysis of a β phase P(VDF-TrFE) copolymer found the onset of poling in the narrow range above the coercive field, Ec, during the poling and the randomization of the β phase as the electric field approaches Ec during the depoling. The in situ SFG monitoring of the CH2 group in relaxor ferroelectric P(VDF-TrFE-CTFE) revealed the gradual transition from paraelectric to ferroelectric phases in P(VDFTrFE-CTFE), which could be ascribed to the nanoscopic size of the domains in the relaxor ferroelectric.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00188. XRD data of the mechanically stretched P(VDF-HFP) film, peak fittings of the d+ and d− SFG bands for P(VDF-TrFE) and P(VDF-TrFE-CTFE) films, and comparison of IR, SFG, and Raman spectra of P(VDFTrFE) copolymer film (PDF)

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REFERENCES

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Seong H. Kim: 0000-0002-8575-7269 F

DOI: 10.1021/acs.macromol.7b00188 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.7b00188 Macromolecules XXXX, XXX, XXX−XXX