Influence of CNTs on crystalline microstructure and ferroelectric

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Influence of CNTs on crystalline microstructure and ferroelectric behavior of P(VDF-TrFE) Min Wang, Lingdong Li, Shenglin Zhou, Rujun Tang, Zhaohui Yang, and Xiaohua Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02392 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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Influence of CNTs on crystalline microstructure and ferroelectric behavior of P(VDF-TrFE)

† Min Wang,† Lingdong Li,† Shenglin Zhou,† Rujun Tang, Zhaohui Yang,*,†,‡,§ Xiaohua

Zhang*,†,‡,§ †

School of Physical Science and Technology, Soochow University, Suzhou 215006, China

‡

Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou 215006, P.R. China

§

Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China

ABSTRACT: We investigate the effect of carbon nanotubes (CNTs) on the crystalline microstructure and ferroelectric behavior of polyvinylidene fluoride-co-trifluoroethylene (P(VDFTrFE)). X-ray analysis suggests that CNT can act as a template and direct the chain orientation of P(VDF-TrFE) crystals. In the presence of CNTs, the molecular chain axis (c-axis) of β-phase crystal is oriented parallel to the long axis of CNTs. Moreover, we find that this templating effect did not cause a polymorph transition. For P(VDF-TrFE)/CNT composites, the crystallinity of P(VDF-TrFE) slightly decreased. The orientation of c-axis induced by the templating effect of CNTs has a significant impact on the ferroelectric behavior of P(VDF-TrFE). As compared to pure P(VDF-TrFE) film, the remnant polarization of P(VDF-TrFE)/CNT composite is enhanced.

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Correspondingly, the piezoelectric property of P(VDF-TrFE)/CNT composite shows a significant enhancement.

INTRODUCTION PVDF and its copolymer are semicrystalline polymers with four well-established polymorphs: non-polar α-phase, ferroelectric β-phase, γ-phase and δ-phase.1 Non-polar α-phase is the most commonly observed crystalline structure. The ferroelectric β-phase is a crystalline structure of interest from an application standpoint due to its piezoelectric and pyroelectric effects. PVDF and its copolymer are being intensively studied because of their application potentials in sensors,2-4 actuator,5-6 transducer7-8 and smart biological scaffold.9 Recent investigations on the biocompatibility of PVDF and its copolymer indicate that PVDF and its copolymer are attractive candidates for electroactive biomaterials.9 Electrical signal created by the piezoelectric effect of PVDF and P(VDF-TrFE) has been considered a viable solution to the mimicking of the bioelectric environment of muscle, nerve and bone regeneration.9-11 Electrical stimuli generated by P(VDFTrFE) have emerged as an effective route to culture neurons10 and to improve bone reconstruction and defect restoration.11 The growth of neural stem cells isolated from mouse olfactory bulb neural cells on PVDF/CNT composite membranes has been investigated by Vicario-Abejón et al.12 Their studies indicate that PVDF composite membranes are promising biomaterials for studying functional neuronal networks due to the biocompatibility of PVDF composite membranes with the generation, differentiation, and maturation of neurons.12 However, a primary challenge with the application potential of PVDF and its copolymer is the control over the crystalline microstructures, which are closely associated with the ferroelectric properties of PVDF and its copolymer. A variety of methods seek to control the crystalline microstructures of PVDF and its copolymer. These methods include solvent annealing,13 electric poling,14 and thermal annealing.15-16 Recently, Ahn et

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al. have reported an interesting method to enhance the ferroelectric properties of PVDF by blending multi-walled CNTs (MWCNTs) into PVDF matrix.17 Due to the excellent properties of CNTs and the controllable growth of CNTs,18-19 polymer/CNTs composites have attracted a lot of attention.20-21 In the presence of single-walled CNTs (SWCNTs), the effect of SWCNTs on the ferroelectric properties of PVDF has also been investigated.22-24 As compared to pure PVDF, PVDF/SWCNT composites exhibit enhancements in pyroelectric and piezoelectric properties. The origin of the enhancement in ferroelectric properties of PVDF is still under investigation. Several reports demonstrated the templating effect of CNT in semicrystalline polymer/CNT blend systems.25-32 CNTs can act as templates and direct the crystal growth of semicrystalline polymers like poly(ethylene oxide), polypropylene, poly(ε-caprolactone) and polyethylene. It is generally appreciated from previous investigations on the influence of CNTs on the crystalline microstructure of semicrystalline polymers. However, incorporating CNTs was not previously emphasized as a means to control the crystalline microstructure of P(VDF-TrFE). Little is known about the influence of CNTs on the crystalline microstructure of P(VDF-TrFE). In this work, we explore the templating effect of MWCNTs in a P(VDF-TrFE)/CNT composite system. Specifically, we focus on the influence of MWCNTs on the crystalline microstructure of P(VDF-TrFE), which is relevant to the ferroelectric properties of P(VDF-TrFE). We are particularly interested in the microscopic origin of the effect of CNTs on ferroelectric properties of P(VDF-TrFE). EXPERIMENTAL SECTION Sample preparation. P(VDF-TrFE) (Piezotech) with a molecular mass of 450 kg mol-1 and a VDF/TrFE mole ratio of 70/30 was obtained from Beijing Xinkailong Biological Technology Co. Ltd.. MWCNT array was grown on a Si wafer with precoated catalyst by a thermal chemical vapor deposition (CVD)


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method. Detailed information about the growth of CNT array (CNT-A) is given in our previous studies.33 MWCNTs with a purity of >97% and a diameter of 10-20 nm were purchased from Shenzhen Nanotech Port Co. Ltd.. P(VDF-TrFE) films were prepared by depositing a 60 mg/ml P(VDF-TrFE) solution in N, N dimethylformamide (DMF) and acetone (1:1) on glass substrates and then drying at room temperature. The thickness of P(VDF-TrFE) film is ca. 20 μm. P(VDFTrFE)/CNT-A composites were prepared by immerging CNT arrays tethered to catalyst substrates in a 60 mg/ml P(VDF-TrFE) solution, picking up and then annealing at room temperature for 12 h. After that, CNT arrays were removed from catalyst substrates. P(VDF-TrFE)/random CNT (P(VDF-TrFE)/CNT-R) composites were fabricated by depositing P(VDF-TrFE) solution on CNT thin film. Firstly, CNT powers were dispersed in ethanol at a concentration of 0.6 mg/ml. CNT dispersion was centrifuged for 5 min. Injecting CNT dispersion to the surface of water leads to the formation of loosely packed CNT thin film on the surface of water. Through a capillary-forceinduced compression assisted by a porous material like commercial porous sponge, a closely packed CNT thin film was formed on the surface of water.34 The CNT thin film was transferred to the surface of mica. P(VDF-TrFE) solution was deposited on the CNT thin film (see Figure S1). After a thermal annealing treatment at 50 oC for 2 h, the solid P(VDF-TrFE)/CNT-R composite films were formed. The thickness of P(VDF-TrFE)/CNT-R film measured by a micrometer is ca. 35 μm. CNT-R/P(VDF-TrFE)/CNT-R film with the thickness of 50 μm was prepared by transferring a closely packed CNT thin film onto the surface of P(VDF-TrFE)/CNT-R composite film (see Figure S1). P(VDF-TrFE)/CNT composites were annealed at 135 oC for 60 min in a vacuum oven. Characterization


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Scanning electron microscope (SEM) images were measured using a Hitachi S-4700 SEM (Hitachi Inc.). Atomic force microscopy (AFM) images were obtained in a tapping mode by a Dimension Icon AFM (Bruker Inc.). Fourier transform infrared spectroscope (ATR-FT-IR, Nicolet 6700(Nicolet Co.)) was utilized to measure the infrared spectrum of sample in an attenuated total reflection mode with a resolution of 2 cm-1 in the range of 600-4000 cm-1. The crystallization enthalpy, crystallization temperature and melting temperature of samples were obtained using a differential scanning calorimeter (DSC2010, TA Instrument, (USA)). The temperature range of DSC measurement is from 25 oC to 180 oC. All samples were measured at a rate of 10 oC/min in a nitrogen atmosphere. Wide-angle X-ray diffraction (WAXD) measurements were carried out to examine the crystalline microstructures of P(VDF-TrFE) using a Bruker D8 Discover diffractometer with Cu Kα radiation (λ=0.154 nm). X-ray incident beam is perpendicular to the long axis of CNT arrays. Two-dimensional WAXD pattern was collected over a 2θ range from 0o to 29o. X-ray diffraction (XRD) measurements were performed using an X-ray powder diffractometer (X`Pert Pro MPD, Netherlands). X-ray wavelength is 0.154 nm. Measurement of Piezoelectric Properties Polarization-electric field (P-E) hysteresis loops were measured at a frequency of 83 Hz using a standard ferroelectric testing system (Multiferroic 100 V, Radiant Technoloies, Inc.). Gold was deposited on the surface of silicon wafer using a vacuum thermal evaporator (JS-16057, Beijing Zhongkekeyi Co., Ltd.). After that, P(VDF-TrFE) solution was dropped on the surface of gold film. P(VDF-TrFE) solid film was formed after a thermal annealing treatment. Gold was again deposited on the top of P(VDF-TrFE) film. For P(VDF-TrFE)/CNT composites, gold was deposited on the surfaces of composites. Pure P(VDF-TrFE) films and P(VDF-TrFE)/CNT composites were poled at an electric field of 10 MVm-1 at room temperature for a period of 30 min. The size of the sample


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used for piezoelectric property measurement was 2.5 cm×1 cm (length×width). The stretch strain is 2%. Piezoelectric output voltages of pure P(VDF-TrFE) films and P(VDF-TrFE)/CNT composites were measured using Precision Multimeter (GDM-8261A, Suzhou Derui Electronics Co., Ltd.). RESULTS AND DISCUSSION Our intent in this work is to investigate the crystallization behavior of P(VDF-TrFE) incorporated in vertically aligned CNT arrays and CNT films. The pristine CNT arrays are fragile. Thus, it is challenging to incorporate P(VDF-TrFE) into pristine CNT arrays without destructing the vertical orientation and integrity of CNT arrays. In order to increase the mechanical property or stiffness of CNT arrays, pristine CNT arrays were densified through a “zipping” effect.35 CNT arrays tethered to catalyst substrates were first immerged in P(VDF-TrFE) solution for 6 h. After that, CNT arrays were picked up and drying at room temperature for 12 h. The surface tension of the liquid, capillary force and van der Waals interactions between CNTs induce carbon nanotubes to “zip” together during immersing and drying. Representative pictures of CNT arrays before and after this densification process are shown in Figure 1. The inner diameter of CNT measured from TEM image (Figure S2) is 13±2 nm. The lateral dimension and area of CNT arrays significantly decrease. The pristine CNT array has the lateral dimension of 1.1 cm and area of 1.21 cm2, and corresponding values for densified CNT array are 0.4 cm and 0.16 cm2, respectively. The average tube-to-tube distance of CNT array, Lo, is calculated from the relation,36 L0=(S·ρπr2h/M)1/2-2r, where S, ρ, r, h and M are CNT-A area, CNT density (1 g/cm3), CNT radius (6.5 nm), CNT-A thickness (1 mm) and total mass of CNT-A (3.2 mg), respectively. The values of Lo for pristine CNT array and densified CNT array are 58 nm and 13 nm, respectively. An overview of our morphological observations on CNT arrays and random CNTs is given in Figure 2. CNT film was


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obtained through a capillary-force-induced compression assisted by a porous sponge.34The orientation and integrity of CNT arrays are retained after this densification process. CNTs in densified CNT array are still vertically oriented. It is natural given the preferred overlap of van der Waals force between highly aligned CNTs.2 The removal of CNT array from catalyst substrate did not have an obvious impact on the orientation of CNTs.

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Figure 1. Schematics of pristine CNT array and densified CNT array (a); the pictures of pristine and densified CNT arrays (b).

Figure 2. SEM images of pristine CNT array (a), random CNT (b) and densified CNT array (c). Figure 3 shows that the representative AFM images of pure P(VDF-TrFE) film and P(VDFTrFE)/CNT composite after a 1 h heat treatment at 135 oC in a vacuum oven. We observe the


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formation of relatively large crystals in pure P(VDF-TrFE) film. The AFM image of P(VDF-TrFE) film in Figure 3 reveals the formation of a needle-like lamella with an average width of 75±7 nm and a length of 220±35 nm. In a needle-like lamella, the c-axis of β-phase is tilted.16 Chain tilting is an energetically favorable chain orientation of needle-like lamella because the tilted growth of lamella facilitates a reduction in steric constraints induced by the exclusion of amorphous chain segments from crystalline region, and also increases the space for excluded amorphous chain segments.37-38 For P(VDF-TrFE)/CNT composite, relatively small lamellae are observed in Figure 3. The width and length of small lamellae are approximately 38±4 nm and 93±14 nm, respectively. For a small lamella morphology, the c-axis of β-phase is parallel to the substrate plane.39 We note that CNTs are laying down on substrates. CNT might act as a physical template to direct the chain orientation of P(VDF-TrFE) during a crystallization process. As pointed out before by Li et al.,26 CNTs can act as nucleation agents to template the crystallization of semicrystalline polymer and lead to the formation of polymer lamellae oriented with lamella plane perpendicular to the long axis of CNT. Our morphological observations on pure P(VDF-TrFE) film and P(VDF-TrFE)/CNT composite indicate that the presence of CNTs has an obvious impact on the morphologies of P(VDF-TrFE) crystals. (b)

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Figure 3. Representative AFM images of (a) pure P(VDF-TrFE) film and (b) P(VDF-TrFE)/ CNTR.


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Figure 4 shows that the FTIR and XRD spectra of pure P(VDF-TrFE) film, P(VDF-TrFE)/CNTR and P(VDF-TrFE)/CNT-A composites. The crystalline form of P(VDF-TrFE) in pure P(VDFTrFE) film is β-phase, which is the crystalline structure of interest from an application standpoint because of its piezoelectric and pyroelectric properties.1 We note that the presence of CNTs did not alter the crystalline forms of P(VDF-TrFE) and β-phase structures in P(VDF-TrFE)/ CNT-R and P(VDF-TrFE)/CNT-A composites are formed. We did not see a transition of β-to-α phase in P(VDF-TrFE)/CNT composites.

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Figure 4. FTIR (a) and XRD (b) spectra of pure P(VDF-TrFE), P(VDF-TrFE)/CNT-A, and P(VDF-TrFE)/ CNT-R. To gain insight into the influence of CNT on the crystallization of P(VDF-TrFE), we conducted DSC measurements on pure P(VDF-TrFE) and P(VDF-TrFE)/CNT composites. Figure 5 shows the heating DSC curves of pure P(VDF-TrFE) and P(VDF-TrFE)/CNT composites after the samples were thermally annealed at different temperatures. The melting temperatures of pure P(VDF-TrFE) film, P(VDF-TrFE)/CNT-R and P(VDF-TrFE)/CNT-A composites are 152 oC±1.5 o

C, 150 oC±1 oC and 148 oC±1.5 oC, respectively. For P(VDF-TrFE)/CNT-R and P(VDF-

TrFE)/CNT-A composites, the melting temperatures of P(VDF-TrFE) crystals are slightly low as


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compared to pure P(VDF-TrFE) samples. The presence of CNTs has a minor impact on the melting temperatures of P(VDF-TrFE) crystals. The P(VDF-TrFE) crystallinity is evaluated from the relation, X = Δhm/Δh0 100%, where Δh0 is the melting enthalpy of a 100% crystalline P(VDFTrFE) (70/30) copolymer. A value of Δh0 = 91.45 J/g is used for the calculation.40 After a thermal annealing treatment at 135oC for 60 min, the crystallinity of P(VDF-TrFE) in pure P(VDF-TrFE) film is 31.94% and the corresponding values for P(VDF-TrFE)/CNT-R and P(VDF-TrFE)/CNTA composites are 29.16% and 28.26%, respectively. The presence of CNTs causes a slight decrease in the crystallinity of P(VDF-TrFE) in P(VDF-TrFE)/CNT composites. No significant decrease in the crystallinity of P(VDF-TrFE) in P(VDF-TrFE)/CNT composites is a positive consequence since the crystalline phase of P(VDF-TrFE) exhibits ferroelectric properties.


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Figure 5. DSC heating curves of (a) pure P(VDF-TrFE), (b) P(VDF-TrFE)/CNT-R and (c) P(VDF-

TrFE)/CNT-A; (d) Crystallinity of pure P(VDF-TrFE), P(VDF-TrFE)/CNT-R and P(VDFTrFE)/CNT-A after a thermal annealing at different temperatures. A two-dimension (2D) X-ray diffraction technique is used to examine the crystalline structures of P(VDF-TrFE) in pure P(VDF-TrFE) and P(VDF-TrFE)/CNT composites. An isotropic scattering ring located at 2θ=19.8o, which arises from (110) and (200) reflections of β-phase, is observed in pure P(VDF-TrFE) film and P(VDF-TrFE)/CNT-R composite. One-dimensional (1D) WAXD intensity profile also shows a signature peak of β-phase at 2θ = 19.8o (see Figures 6d and 6e). The isotropic scattering ring at 2θ=19.8o observed in pure P(VDF-TrFE) film and P(VDFTrFE)/CNT-R composite indicates that β-phase crystalline structures are randomly oriented in sample plane. We take a closer look at 2D WAXD pattern of P(VDF-TrFE) film/CNT-R. An


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isotropic scattering ring at 2θ = 26.2o, which arises from the (002) reflection of the intershell spacing of concentric graphitic shells in MWCNTs, is observed in the WAXD pattern. Onedimensional WAXD intensity profile shows a weak peak at 2θ = 26.2o. It suggests that the long axis of CNTs is parallel to sample plane. Two-dimensional WAXD enables us to determine the molecular orientation of β-phase. For P(VDF-TrFE)/CNT-A, two intense peaks at 2θ = 19.8o associated with β-phase of P(VDF-TrFE) are observed in 2D WAXD pattern at the azimuthal angles of φ = 90 and 270o. At the same azimuthal angles there are two additional weak peaks in 2D WAXD patterns, which are located at 2θ = 26.2o arising from the concentric graphitic shells of MWCNTs. It is indicative of a preferred molecular orientation of β-phase crystal with the molecular chain axis or c axis parallel to the long axis of CNTs. The anisotropic scattering of the concentric graphitic shells of MWCNTs shown in Figure 6c also indicates that MWCNTs are oriented with the long axis of CNT perpendicular to substrate. Figure 6f shows 1D WAXD intensity as a function of scattering vector, which is obtained by integrating over cake-shaped slices with a span of ±27.5o from the axis of φ = 90o in 2D WAXD patterns. The peaks at 2θ = 19.8o and 26.2o in 1D WAXD intensity profile are consistent with the schematic of the morphology of βphase in vertically aligned CNT array in Figure 6i. The azimuthal plot of 2D WAXD pattern between φ= 0o and φ= 180o for P(VDF-TrFE)/CNT-A around 2θ = 19.8o and 26.2o is shown in Figures 6g and 6h. The anisotropic scattering intensities of WAXD are observed. The scattering intensities at φ =0o and φ = 180o are suppressed. It is also indicative of the orientation of molecular chain of P(VDF-TrFE). The c axis of β-phase crystal is preferentially oriented parallel to the long axis of CNT and the a-b plane of β-phase crystal is perpendicular to the long axis of CNT. For P(VDF-TrFE)/CNT-R composite, the long axis of CNT is parallel to substrate. It is possible that the c axis of β-phase is parallel to the long axis of CNTs.


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Figure 6. 2D WAXS pattern of pure P(VDF-TrFE) (a), P(VDF-TrFE)/CNT-R (b) and P(VDFTrFE)/CNT-A (c); 1D WAXS intensities of pure P(VDF-TrFE) (d), P(VDF-TrFE)/CNT-R (e) and P(VDF-TrFE)/CNT-A (f); the azimuthal dependence of intensity of WAXS pattern between φ = 0o and φ = 180o at 2θ = 20 o±5o (g) and 2θ = 26 o±3o (h) for P(VDF-TrFE)/CNT-A composite; (i) the schematic of P(VDF-TrFE)/CNT-A composite. The polarization vs. electric-field (P-E) hysteresis loops of pure P(VDF-TrFE) films and P(VDF-TrFE)/CNT composites are shown in Figure 7. P-E hysteresis loops show the remnant polarization of P(VDF-TrFE)/CNT composites is higher as compared to pure P(VDF-TrFE) film. Under a voltage of 8 V, the remnant polarization for pure P(VDF-TrFE) are 3.08 µCcm-2. The


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corresponding values for P(VDF-TrFE)/CNT-R and CNT-R/P(VDF-TrFE)/CNT-R composites are 3.47 µCcm-2, and 3.78 µCcm-2, respectively. As mentioned above, the c axis of β-phase crystal is preferentially oriented parallel to the substrate. Because both a and b axis can randomly rotate around the c axis, an electric field applied to P(VDF-TrFE)/CNT-R composite leads to the orientation of the polar b axis along to the electric field direction when the electric field is perpendicular to c axis. Thus, P(VDF-TrFE)/CNT-R composites have relatively high remnant polarization. The coercive electric field of P(VDF-TrFE)/CNT composite is ca. 159 kV/m. This value is relatively low as compared to previous reports.41 In these reports, the amplitude of the applied field is relatively high. The coercive electric field is a function of the amplitude of the applied field

42

and increases with the amplitude of the applied field. In our studies, a relatively

low electric field was applied to the samples in order to prevent the samples from electrical breakdown because of the presence of the conductive CNTs in the samples. It might explain the relatively low value of the coercive electric field of P(VDF-TrFE)/CNT composite. In CNT arrays, CNTs are vertically oriented to the substrate. After CNT arrays are removed from the substrate, the orientation of CNTs is retained (see Figure 1). Therefore, in P(VDFTrFE)/CNT-A composites, the c axis of β-phase crystal is aligned along to the direction of CNT growth. The alignment of the c axis of β-phase crystal in vertically oriented CNT arrays should be possible to give rise to variations of the remnant polarization with the direction of applied electric field. Specifically, when the long axis of CNTs is perpendicular to an applied electric field, the polar b axis of β-phase crystal is parallel to the applied electric field and the remnant polarization should be enhanced. If an applied electric field is parallel to the long axis of CNT, the remnant polarization should decrease because the polar b axis that is perpendicular to an external electric field rarely has a response to the electric field. To make this check, we measure the P-E hysteresis


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loops of P(VDF-TrFE)/CNT-A composites in a CNT growth direction and a direction perpendicular to CNT growth. Figure 7b compares the remnant polarizations obtained in electric fields with different directions. When an applied field is perpendicular to the long axis of CNT, the remnant polarization of P(VDF-TrFE)/CNT-A composite are 4.00 µCcm-2. The corresponding values obtained in an electric field parallel to the long axis of CNT are 3.71 µCcm-2. As expected, the remnant polarization of P(VDF-TrFE)/CNT-A composite is enhanced when the polar b axis of β-phase crystal is parallel to an applied electric field. It is worth noting that the conductivity of P(VDF-TrFE)/CNT-A composite in a direction parallel to the long axis of CNT is 2.1×10-4 S/m and the corresponding value obtained in a direction perpendicular to the long axis of CNT is 5.8×10-4 S/m. The presence of CNTs might cause an increase in leakage current, which probably give rise to the slight distortion of P-E loop. P(VDF-TrFE)/CNT-R composites were fabricated by depositing P(VDF-TrFE) solution on CNT thin film (see Figure S1). After a thermal annealing treatment at 50 oC for 2 h, the solid P(VDF-TrFE)/CNT-R composite films were formed. CNT-R/P(VDFTrFE)/CNT-R composite was prepared by transferring a closely packed CNT thin film onto the surface of P(VDF-TrFE)/CNT-R composite film (see Figure S1). Compositions at the top surface and the bottom layer of P(VDF-TrFE)/CNT composite might be slightly different. This difference in the composition at the top surface and bottom layer of composite might also cause the distortion of P-E loop. The neat P(VDF-TrFE) films with the thickness of ca. 20 μm were prepared via dropcoating. As compared to thin P(VDF-TrFE) film prepared via spin-coating, the thickness of dropcoated P(VDF-TrFE) film is less uniform. The minor variations in the thickness of P(VDF-TrFE) films might result in the slight distortion of P-E loop. We find that the P-E loop of the neat P(VDFTrFE) film is slightly distorted at relatively high voltages and the distortion of P-E loop is less


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obvious at low voltages (see Figure 7c). The coercive field, Ec, of 20 μm P(VDF-TrFE) film obtained from the hysteresis loop is ca. 0.2 MV/m, which is lower than the corresponding value of thin P(VDF-TrFE) film. For thin P(VDF-TrFE) films (usually less than 100 nm), the Ec of P(VDFTrFE) is about 50-100 MV/m. The thickness of P(VDF-TrFE) film, d, has a significant influence on the coercive fields.43 Previous studies show that the Ec decreases with an increasing of the thickness of P(VDF-TrFE) film and follows a power law: Ec ~ d-0.58.43 The Ec of thick P(VDFTrFE) film with a thickness of 20 μm can be estimated from this empirical relation using the Ec of thin P(VDF-TrFE) film (for example, Ec=50 MV/m for 40 nm P(VDF-TrFE) film). The estimated value of Ec for 20 μm P(VDF-TrFE) film is ca.1 MV/m. In addition, due to less uniform thickness of drop-coated P(VDF-TrFE) film, the thin regions of P(VDF-TrFE) films probably have the thicknesses of much less than 20 μm. The variations in the thickness of P(VDF-TrFE) films might also result in the relatively low Ec, which is calculated using the thickness of 20 μm.


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(b)

(a) Pure P(VDF-TrFE) P(VDF-TrFE)/CNT-R CNT-R/P(VDF-TrFE)/CNT-R

Polarization(Ccm-2)

Polarization(Ccm-2)

6 4 2 0 -2 -4 -6 -12

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//

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6 3

4V 5V 6V 7V 8V

0 -3 -6 -8

-4

0

4

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Figure 7. P-E hysteresis loops of (a) pure P(VDF-TrFE) films and P(VDF-TrFE)/CNT-R composites, (b) P(VDF-TrFE)/CNT-A composites and (c) pure P(VDF-TrFE) films at different voltages.

The dipole alignment of P(VDF-TrFE) is of great importance for the ferroelectric performance of P(VDF-TrFE)-based device. High dipole alignment of P(VDF-TrFE) can potentially improve the output of P(VDF-TrFE)-based device.44-45 To gain insight into the relationship between the molecular orientation of P(VDF-TrFE) and the piezoelectric properties of P(VDF-TrFE), we measure piezoelectric output voltages of pure P(VDF-TrFE) films, P(VDF-TrFE)/CNT-R and CNT-R/P(VDF-TrFE)/CNT-R composites under a stretch-release mode and a bend-release mode. Figure 8 shows that the piezoelectric output voltages of P(VDF-TrFE)/CNT-R composites are


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higher as compared to pure P(VDF-TrFE) films. The long axis of CNTs in P(VDF-TrFE)/CNT-R composites is perpendicular to the applied electric field. The c axis of β-phase crystal is probably oriented parallel to the long axis of CNTs. The alignment of dipoles along the applied electric field results in the enhancement in the piezoelectric output voltage of P(VDF-TrFE)/CNT-R composites. The piezoelectric property of P(VDF-TrFE)/CNT-R composite is consistent with the observation on the remnant polarization. CNT-R/P(VDF-TrFE)/CNT-R composite, in which the free surface of P(VDF-TrFE)/CNT-R is covered with a CNT film, exhibits higher piezoelectric output voltage as compared to P(VDF-TrFE)/CNT-R composite. It indicates that more dipoles are aligned along the applied electric field in CNT-R/P(VDF-TrFE)/CNT-R composites due to the presence of additional CNT film on the top surface of P(VDF-TrFE)/CNT-R composites.

(b) 35

30

30

25

25 Voltage(mV)

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20 15 10

15 10 5

5 0

20

NT-R FE) NT-R F-Tr E)/C FE)/C P (V D F-Tr -TrF F D D V ( V P( -R/P CNT

0 P(VD

NT-R FE) NT-R E)/C F-Tr E)/C -TrF -TrF F F D D (V P(V -R/P CNT

Figure 8. Piezoelectric output voltages of P(VDF-TrFE) films and P(VDF-TrFE)/CNT-R composites under stretch-release (a) and bend-release (b). CONCLUSIONS We investigate the crystalline microstructures and ferroelectric properties of P(VDFTrFE)/CNT composites. Our results indicate that CNTs play an crucial role in the orientation of P(VDF-TrFE) microstructures. In the presence of CNTs, the c-axis of β-phase crystal is oriented parallel to the long axis of CNT. There is a positive consequence of this templating effect of CNTs.


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CNTs facilitate the orientation of polar β-phase crystal of P(VDF-TrFE). The orientation of polar b-axis of β-phase can be controlled by the direction of CNT long axis. For CNT with its long axis parallel to the substrate, the polar b-axis shows a much higher propensity to align along a direction perpendicular to the substrate. As compared to pure P(VDF-TrFE) films, P(VDF-TrFE)/CNT composite exhibits an enhancement in the remnant polarization. P(VDF-TrFE) with aligned polar b-axis of β-phase is of great interest from an application standpoint. The incorporation of CNTs amplifies the inherent capacity of P(VDF-TrFE), and enhances the ferroelectric property of P(VDF-TrFE). We have further found that a P(VDF-TrFE)/CNT composite with anisotropic ferroelectric property can be obtained by incorporating P(VDF-TrFE) into highly vertically aligned CNT array. Incorporating CNTs might be a promising approach to the fabrication of anisotropic piezoelectric biomaterials. It is possible that P(VDF-TrFE)/CNT composite might be useful as a biosensor for identifying the directionality of bio-signals. ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: TEM image of CNT and the schematic of fabricating P(VDF-TrFE)/CNT-R film and CNTR/P(VDF-TrFE)/CNT-R film. AUTHOR INFORMATION *Corresponding Author E-mail: [email protected]. E-mail: [email protected]; Phone: +86-51265884716 ORCID Xiaohua Zhang: 0000-0002-3996-702X


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Zhaohui Yang: 0000-0003-3329-5311 ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21274103, 51772200). The authors also thank the Specially Appointed Professor Plan in Jiangsu Province (No. SR10800312 and SR10800215). REFERENCES (1) Kepler, R. G.; Anderson, R. A. Ferroelectric Polymers. Adv. Phys. 1992, 41, 1-57. (2) Ikawa, T.; Tabata, H.; Yoshizawa, T.; Utaka, K.; Kubo, O.; Katayama, M. Pressure-Sensing Properties of Single-Walled Carbon Nanotubes Covered with a Corona-Poled Piezoelectric Polymer. Appl. Phys. Lett. 2016, 109, No. 033104. (3) Khan, S.; Dang, W.; Lorenzelli, L.; Dahiya, R. Flexible Pressure Sensors Based on ScreenPrinted P(VDF-TrFE) and P(VDF-TrFE)/MWCNTs. IEEE. T. Semiconduct. M. 2015, 28, 486-493. (4) Lang, C.; Fang, J.; Shao, H.; Ding, X.; Lin, T. High-Sensitivity Acoustic Sensors from Nanofibre Webs. Nat. Commun. 2016, 7, No. 11108. (5) Cho, K. Y.; Cho, A. R.; Lee, Y. J.; Koo, C. M.; Hong, S. M.; Hwang, S. S.; Yoon, H. G.; Baek, K. Y. Enhanced Electrical Properties of PVDF-TrFE Nanocomposite for Actuator Application. Key Eng. Mater. 2014, 605, 335-339. (6) Edqvist, E.; Hedlund, E. Design and Manufacturing Considerations of Low-Voltage Multilayer P(VDF-TrFE) Actuators. J. Micromech. Microeng. 2009, 19, No. 115019. (7) Jeong, J. S.; Shung, K. K. Improved Fabrication of Focused Single Element P(VDF-TrFE) Transducer for High Frequency Ultrasound Applications. Ultrasonics 2013, 53, 455-458. (8) Ohigashi, H.; Koga, K.; Suzuki, M.; Nakanishi, T.; Kimura, K.; Hashimoto, N. Piezoelectric and Ferroelectric Properties of P (VDF-TrFE) Copolymers and Their Application to Ultrasonic


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Table of Contents

The schematic of P(VDF-TrFE)/CNT composite.


Influence of CNTs on crystalline microstructure and ferroelectric

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