Enhanced dielectric and mechanical properties in chlorine-doped continuous CNT sheet reinforced sandwich polyvinylidene fluoride film

Accepted Manuscript Enhanced dielectric and mechanical properties in chlorine-doped continuous CNT sheet reinforced sandwich polyvinylidene fluoride film Zhenchong Zhang, Yizhuo Gu, Shaokai Wang, Qingwen Li, Min Li, Zuoguang Zhang PII:

S0008-6223(16)30443-2

DOI:

10.1016/j.carbon.2016.05.068

Reference:

CARBON 11032

To appear in:

Carbon

Received Date: 20 March 2016 Revised Date:

25 May 2016

Accepted Date: 29 May 2016

Please cite this article as: Z. Zhang, Y. Gu, S. Wang, Q. Li, M. Li, Z. Zhang, Enhanced dielectric and mechanical properties in chlorine-doped continuous CNT sheet reinforced sandwich polyvinylidene fluoride film, Carbon (2016), doi: 10.1016/j.carbon.2016.05.068. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Enhanced dielectric and mechanical properties in chlorine-doped continuous CNT sheet reinforced sandwich polyvinylidene fluoride film Zhenchong Zhanga, Yizhuo Gua,*, Shaokai Wanga, Qingwen Lib,*, Min Lia, Zuoguang Zhanga

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Abstract

The fabrication and performance of a novel continuous carbon nanotube (CNT) sheet reinforced polyvinylidene fluoride (PVDF) film with sandwich structure are firstly reported. Continuous CNT sheet fabricated by means of floating catalyst chemical

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vapor deposition was used. A dispersion-free technique for the sandwich composite film was adopted. For the prepared film with sandwich structure, CNT sheet/ PVDF

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acts as a highly conductive layer to raise dielectric permittivity, while two PVDF films at top and bottom of composite are used as insulating layers to suppress dielectric loss. Thionyl chloride doping treatment was done for CNT sheet and the effects of dopant on chemical composition, structure and properties of CNT sheet and composite film were investigated. Resulted from unique performances of the modified CNT sheet and strong interfacial polarization process, the sandwich composite film possesses excellent

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dielectric and mechanical properties, where dielectric permittivity of 32, dielectric loss of 0.08, tensile strength of 90 MPa and modulus of 3 GPa are shown for optimized chlorine-doping duration time and composite composition. Such composite material and structure have potential applications in hybrid capacitors due to high permittivity,

*a

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low dielectric loss, good flexibility and ease of processability.

Key Laboratory of Aerospace Advanced Materials and Performance (Ministry of Education), School

of Materials Science and Engineering, Beihang University, Beijing 100191, China. Tel & Fax: +86-010-82339800, E-mail: [email protected] b

Suzhou Institute of Nano-Tech and Nano-Bionics, No. 398 Ruoshui Road, Suzhou 215123, China. Tel

& Fax: +86-512-62872577, E-mail:[email protected]

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ACCEPTED MANUSCRIPT 1. Introduction As the overuse of fossil energy is causing severe environmental annihilation and unavoidable resource exhaustion in the imminent future at current consumption rate, electric energy and its storage technology have attracted ever-increasing attentions in

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recent years[1-3]. Polymer composites with high dielectric permittivity, low loss and good mechanical strength have been demonstrated as most promising material in electronic devices, such as structural capacitor, flexible capacitor films, actuators and

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sensors[4-6], due to its combined advantages of high dielectric permittivity from dielectric fillers and good mechanical strength, flexibility and ease of processing from

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polymer matrix[4, 7].

To obtain composites with excellent dielectric property, researchers have adopted many approaches to increase dielectric permittivity and decrease dielectric loss. One intuitive method to endow polymer composites with high dielectric permittivity is to

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homogeneously disperse ferroelectric ceramic particles or fibers (such as BaTiO3[8], SrTiO3[9], CaCu3Ti4O12[10], etc.) with high dielectric permittivity into polymer matrix. However, filler loading of ceramic is usually required no less than 50 vol. %, which

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significantly inhibits applications of these composites where light weight, high

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mechanical strength and ease of processability are needed[4, 11]. An alternative solution, which may address this problem, is to fabricate percolative nanocomposites by introducing conductive fillers, such as metal particles (e.g. Ag[12], Zn[13]), carbon-based materials (e.g. carbon nano fiber[14], graphene[15], reduced graphene oxide[16]), and semiconductor fillers (e.g. SiC[17], Bi2S3[18]) into an insulating polymer matrix. The superiorities of percolative composites lie in their high dielectric permittivity obtained at very low filler loading, in which relatively high mechanical strength can be preserved. Among these conductive fillers, carbon nanotube (CNT) is 2

ACCEPTED MANUSCRIPT more favored as dielectric filler in achieving higher dielectric permittivity of composite due to its larger aspect ratio and higher electrical conductivity compared with spherical and flake shaped fillers. Polymer filled with CNTs has been proven to exhibit high dielectric permittivity and mechanical strength at low filler loading[19]. However,

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progress in composites filled with CNTs is still slow because of several reasons. Firstly, the conductive networks formed by CNTs near percolation threshold usually lead to high dielectric loss and electrical conductivity[19, 20]. Secondly, the percolation

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threshold in these CNTs reinforced dielectric composite is highly unstable and dependent on specific experimental conditions. Moreover, manipulating processing

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condition is also extremely hard due to high sensitivity of performance to slight change in CNT content. Thus, the key issue lies in increasing dielectric permittivity, retaining low dielectric loss while improving mechanical strength under stable, controllable fabrication conditions and composite structure.

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Recently, macroscopic assemblies of CNTs, which mainly include CNT fiber and CNT sheet, show potential applications such as transistors, electrodes, thermal interface and structural materials due to their collective behaviors of individual tubes

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with additional properties arising from tube-tube interactions[21, 22]. Currently, CNT

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assemblies fabricated by floating catalyst chemical vapor deposition (CVD) method, in which continuous, uniform and integrative CNT sheet can be directly assembled and collected, attracts more and more attentions due to its easy fabrication process and promising mass production rate at low cost[23]. Our previous works reported fabricating methods for obtaining light, strong CNT sheet and polymer composite with excellent mechanical, electrical and damping performances[24, 25]. The advantages of floating CVD method is that it allows mass-production (up to the order of tons per year) of CNT sheet with large size (up to the order of meter in both length and width) being 3

ACCEPTED MANUSCRIPT realized through easy fabrication process. Its electrical and mechanical properties can be further improved to meet different requirements by tailoring packing status, chemical doping and resin content[25]. On the other hand, particular structures based on sandwich or multiple layer were proposed to maintain the dielectric loss as low as

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possible, especially for percolative composites with high dielectric loss[26, 27]. Usually, an insulating layer is intercalated to the composites for decreasing dielectric loss. Neat polymers are usually selected as insulating layer attributed to their relatively

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high insulation feature and good mechanical flexibility. Based on above-mentioned ideas, we hypothesize that a sandwich composite comprising a conductive layer (CNT

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sheet/polymer) intercalated between two insulating layers (neat polymer) will be much easier and more efficient to enhance interfacial polarization process and block conductive paths, resulting in excellent comprehensive performances. The resulting dielectric permittivity is expected to be significantly increased and dielectric loss as

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low as the neat polymer[16, 27].

As enabling materials for dielectric applications, the employment of macroscopic CNT sheet to enhance both dielectric and mechanical properties of polymer matrix has

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not been studied. In this paper, we successfully fabricated a novel flexible sandwich

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polyvinylidene fluoride (PVDF) matrix composite film reinforced by CNT sheet, which exhibited both high dielectric permittivity, low dielectric loss and excellent mechanical strength. CNT sheet/PVDF acted as a highly conductive layer to raise dielectric permittivity, while two PVDF films at top and bottom were used as insulating layers to suppress dielectric loss. The significance in choosing CNT sheet as reinforcement is due to its intrinsic macroscopic status, high electrical and mechanical properties and thus allowing an easy dispersion-free fabrication process. Thionyl chloride (SOCl2) doping treatment was done for CNT sheet to improve intrinsic 4

ACCEPTED MANUSCRIPT performance and interfacial adhesion with resin matrix. The effects of chlorine-doping and duration time on composition, microstructure, electrical and mechanical properties of CNT sheet and sandwich film were investigated. Superior dielectric and mechanical performances of chlorinated CNT sheet (Cl-CNT sheet) reinforced sandwich

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composite films were demonstrated in details. 2. Experimental details 2.1 Materials

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All materials in this study are commercially available. CNT sheet with multi-walled CNT content over 90 wt.% were synthesized using floating CVD method and the

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corresponding procedure of preparation is visualized in Fig.1 (a). The mixture of ethanol, ferrocene and thiophene were firstly injected to a heated gas-flow reactor at a feeding rate of 0.15 ml/min. Ar/H2 gas mixture (volume ratio 1:1) was flowed through the reactor tube at a rate of 4000 sccm, and the heating reaction region was about

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1300 °C. The individual nanotubes spontaneously formed into a continuous sock-like aerogel in the gas flow, which can be blown out with the carrying gas. The CNT socks were winded by a rotating roller. Meanwhile, ethanol/water solution was sprayed onto

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the socks and a multi-layer seamless CNT sheet was fabricated after evaporation of the

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solution. The as-prepared CNT sheet with 10 cm ×10 cm is shown in Fig. 1(b). The SOCl2 liquid with chemical purity of 99 % was purchased from Xilong chemical Inc. FR904

PVDF

was

purchased

from

the

Shanghai

3F

Company.

N,N-Dimethylformamide (DMF) solvent with chemical purity of 99% was purchased from Xilong chemical Inc.

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Fig.1 (a) Schematic process of randomly aligned CNT sheet prepared by floating catalyst CVD growth

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method and (b) as-prepared 10 cm×10 cm CNT sheet employed in this work.

2.2 Synthesis of Cl-CNT sheet

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To fabricate Cl-CNT sheet, pristine CNT sheet (P-CNT sheet) was functionalized by dopant of SOCl2. Firstly, SOCl2 was carefully dripped into a watch glass containing DMF solvent with a mass ratio of m(SOCl2):m(DMF)=1:200. Secondly, P-CNT sheet with a proper size was immersed in the mixture. Then, the sealed watch glass was placed on a

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horizontal heating station with a constant temperature of 70 °C for different durations (2 h, 4 h, 6 h, 8 h and 10 h). Eventually, the Cl-CNT sheet was extracted from the solution and heated in a vacuum oven at 70 °C for 1h to vaporize DMF solvent and

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redundant SOCl2. The schematic diagram of the fabrication is shown in Fig. 2(a). 2.3 Preparation of CNT sheet/PVDF

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PVDF was dissolved in DMF solvent with a mass ratio of m(PVDF):m(DMF)=3:97 by

stirring at 60 °C to acquire a transparent diluent. The CNT sheet was directly immersed into watch glass containing 30 ml of transparent PVDF/DMF solution. The watch glass was sealed and placed under room temperature for 3 h. Then, the PVDF-infiltrated CNT sheet was taken out and two pieces of filter paper were attached to both sides of CNT sheet to eliminate abundant resin and solution. The CNT sheet was carefully transferred onto clean glass plate and heated in a vacuum oven at 70 °C for 3 h to fully 6

ACCEPTED MANUSCRIPT remove residual DMF. The obtained PVDF-infiltrated CNT sheet was further hot-pressed at 190 °C under pressure of 5 MPa (prepressed for 30 min at the same temperature, released the press for 15 min, and then repressed for 30 min, followed by cooling to room temperature under the same pressure) to obtain smooth and compact

fabrication is shown in Fig. 2(b). 2.4 Fabrication of sandwich composite film

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CNT sheet/PVDF with average thickness of 10±3 µm. The schematic diagram of the

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Sandwich composite films comprising neat PVDF as top and bottom skins were fabricated through hot press method. Two PVDF films and a CNT sheet/PVDF were

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stacked in the order as P-C-P(P indicates PVDF layer and C indicates CNT sheet/PVDF layer) and placed between smooth molds. The stacked films were hot-pressed into one composite film at 190 °C under 2 MPa (prepressed for 30 min at the same temperature, released the press for 30 min, and then repressed for 30 min,

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followed by cooling to room temperature under the same pressure). The average thickness of the sandwich film was 100±5 µm. The schematic diagram of fabrication

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process and sandwich-structured composite film are shown in Fig. 2(c).

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Fig.2 Schematic diagram of (a) synthesis of Cl-CNT sheet, (b) preparation of CNT sheet/PVDF and (c) fabrication of sandwich composite film.

2.5 Characterization

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Fourier transformed infrared (FTIR, Nicolet Inc. Nexus 470) spectra of all samples were recorded with wavenumber ranging from 400 to 4000 cm-1. X-ray photoelectron

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spectroscopy (XPS, Axis Ultra DLD) was used to characterize the elemental compositions of CNT sheet before and after doping treatment using 150 W Al Kα radiation. Raman spectrum was carried out to characterize the degree of order in CNT structure using a micro-Raman spectroscope (JobinYvon, LabRam HR 800). The morphologies of CNT sheet, CNT sheet/PVDF and sandwich composite film were measured by a scanning electron microscope (SEM, CamScan Inc. Apollo300) under acceleration voltage of 15 kV. A four-point probe facility (Guangzhou Four-point Probes Technology, Co. RTS-9, China) was used to measure electrical conductivities of 8

ACCEPTED MANUSCRIPT CNT sheet and CNT sheet/PVDF. At least 9 positions of every specimen were tested. The mechanical properties were measured using universal testing machine (Instron, Co. Instron 3344) with a load cell of 2 kN at a tensile speed of 2 mm/min at room temperature and the clamping distance was 20 mm. At least six samples were tested for

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each type of CNT sheet, CNT sheet/PVDF and sandwich composite film. The testing samples were cut into 30 mm × 2 mm rectangle strips, and the accurate width was measured by an optical microscope with a calibrated scale bar. The sample thickness

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was measured by micrometer caliper and density measurements were carried out following weighting method. Dielectric permittivity, dielectric loss, AC conductivity of

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the composite were measured by means of precision impedance analyzer (Agilent Technologies, Inc. HP 4294A) at the frequency range of 102-107 Hz at room temperature under applied voltage of 0.5 V. 3. Results and discussion

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3.1 Composition, structure and morphology

It is well known that chemical doping can largely increase the electrical and mechanical properties of CNT sheet due to enhanced interaction among CNTs arising

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from functional groups[28]. Chlorination of CNTs has been studied to improve

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electrical and mechanical properties[29, 30], but its application in dielectric materials is rarely reported. FT-IR measurements were performed to identify doping effect of SOCl2 on CNT sheet. Fig. 3 presents the results of FTIR spectrum of CNT sheet before and after doping treatment. In P-CNT sheet, the peaks at ~1715 cm−1, ~1643 cm−1, ~1245 cm−1, and ~1087 cm−1 can be assigned to C=O stretching motions of carboxylic acid and carbonyl moieties, C=C skeletal vibrations of un-oxidized graphitic domains, C–O stretching vibrations and C–O–C stretching vibrations, respectively[31]. As for Cl-CNT sheet, the absence of peaks at ~1715 and ~1245 cm−1 indicates significant 9

ACCEPTED MANUSCRIPT decrease in functional groups of C=O and C-O. Moreover, a new peak at ~742 cm-1 corresponding to C-Cl appears, confirming the successful doping of Cl on Cl-CNT

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sheet[32].

Fig.3 FTIR spectra of P-CNT sheet and Cl-CNT sheet with duration of 6h.

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The elemental compositions and local functionalities of CNT sheet under different duration time were quantitatively analyzed using XPS and the results are summarized

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in Fig. 4. Fig. 4(a) shows elemental changes in general spectra of CNTs following doping treatment by SOCl2. It is clearly seen that peaks of Cl 2p emerges in Cl-CNT

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sheet after doping treatment, which are absent in P-CNT sheet. The deconvoluted C 1s peaks for both P-CNT sheet and Cl-CNT sheet in Fig. 4(b) present C=C (284.6 eV), C-C (285.1 eV), C-O (286.2 eV), C=O (287.2 eV) and COOH (288.9 eV) groups, it is also noticed that a new peak representing C-Cl (286.5 eV) appears[32-34]. Two different C–Cl bonds, as shown in Fig. 4(c), were quantitatively analyzed according to previous studies[32]. The Cl 2p peaks are split into two pairs where the doublet locating at the lower binding energies corresponds to ionic state of Cl and that at the higher binding energies is covalently-bonded Cl. The peaks appearing at 201.4 and 199.8 eV 10

ACCEPTED MANUSCRIPT represent the Cl–C=O and covalent C–Cl bonds, respectively, while the doublet whose 2p 1/2 peak locating at 198.6 eV is a reflection of ionic bonded chlorine on CNTs. Using the tabulated cross sections for the photo emission process, the concentrations of Cl and S in CNT sheet were studied and presented in Fig. 4(d). The ratio value of (Cl:S)

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listed above the bars varies with duration time, which is different from that in SOCl2 molecules. This clearly indicates chemical reactions between the dopant SOCl2 and CNTs, rather than simple physical absorption intercalation of SOCl2 in CNT sheet. For

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duration time below 6h, both chemical reaction and physical adsorption conduct quickly, since there are sufficient doping/reactive sites in CNTs for SOCl2 molecules to

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interact[32]. This results in a rapid rise of Cl atomic concentration and reaches a maximum value of 0.91 % at duration time of 6h. However, when the available doping/reactive sites are gradually occupied, further increase in duration time results in a reduction in Cl atomic concentration due to the physical desorption and chemical

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dissociation of Cl. An equilibrium between the chemical reaction and the physical adsorption is established and the atomic concentration of Cl maintains around

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0.72~0.8[32].

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Fig.4 XPS results: (a) general XPS spectra of CNT sheet under different duration time; (b) deconvoluted

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spectra of C 1s of P-CNT sheet and Cl-CNT sheet under duration of 6 h; (c) deconvoluted curve fittings in of Cl 2p and (d) atomic concentration of Cl and S with varying duration time.

The Raman spectra of CNT sheet under different duration time are shown in Fig. 5.

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The D-band observed at ~1328.9 cm-1 represents disordered sp3 structure, while

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G-band at ~1582.8 cm-1 corresponds to the first-order scattering of the in-plane B–B stretching (E2g) mode[29, 35]. It is speculated that the structural defects are mainly caused by presence of Cl dopants and concomitant absence of C atoms in the structure of CNT. By analyzing the intensity of G-band with respect to that of D-band (IG/ID), the degree of order in carbon material can be determined qualitatively. It is previously reported that the floating catalyst CVD method allows production of CNT with high purity and order in structure[36, 37]. In our case, the ratio IG/ID of P-CNT sheet reaches 10.71 which indicates very high degree of order and very few defects in CNTs[25]. As 12

ACCEPTED MANUSCRIPT duration time of Cl-doping continues to increase, the ratio IG/ID gradually drops. However, the ratio IG/ID is still 6.6 under duration time of 10h, showing relatively high degree of order in CNTs structure. The XRD results provided in Fig. S1 also suggest the same trend of changing in CNTs structure. It is also worthy noticing that G-band

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position of CNT sheet shifts to higher frequency with increasing duration time. It is well accepted that G-band shifts to higher frequency after p-type doping, while to lower frequency after n-type doping due to different types of charge-transfer complexes. The

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degree of shift using same doping method is a reflection of the extent of doping effect: a more significant doping effect leads to a higher charge carrier concentration, and thus

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a larger shift is observed[32, 35, 38]. As is shown in Fig. 5(b), the G-band shows a maximum shift value of 4.28 cm-1 in Cl-CNT sheet under duration time of 6h, indicating most significant p-type doping effect. This result is consistent with XPS

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results shown in Fig. 4.

Fig.5 Raman spectra of (a) P-CNT sheet and Cl-CNT sheet with different 70°C heat duration and (b) corresponding G-band shifts in Cl-CNT sheet under different doping duration (The black line is a guide line for Raman shift in G-bands).

It is known that both electrical and mechanical properties of CNT sheet are greatly dependent on its packing morphology, structure and density[25]. SEM images and packing density are presented in Fig. 6 to study surface morphology and structure of 13

ACCEPTED MANUSCRIPT CNT sheet under different duration time. The illustration sketches below the corresponding SEM image are presented for better visual understanding of morphology evolution in CNT sheet with increasing duration time. Fig. 6(a) shows that CNTs in P-CNT sheet pack loosely and there are no obvious bundles (assembly of several

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CNTs). After the doping of 2 h and evaporation treatment, it can be seen that large number of bundles are formed in Cl-CNT sheet as shown in Fig. 6 (b), indicating increasing CNT stacking degree[29]. The highest degree of stacking status and most

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remarkably diminished voids in Cl-CNT sheet are observed under duration time of 6h as presented in Fig. 6(c). As the duration time continues to increase, the stacking status

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in Cl-CNT sheet becomes loose due to decreased atomic concentration of Cl, as shown in Fig. 6(d). The packing density of CNT sheet with various duration time is illustrated in Fig. 6(e), which is also consistent with the trend of packing status discussed above. Especially, the packing density in Cl-CNT sheet reaches 0.61 g/cm3 at duration time of

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6h which is 29.8% higher than that of 0.47 g/cm3 in P-CNT sheet. It is known that the chlorine doping on CNTs has two effects: one is to remove amorphous carbon adhering to the outer wall of CNTs, and another is to introduce defect on CNT. Fig. S2

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gives the TEM images of CNT sheet before and after chlorine doping for studying the

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two effects. As revealed in Fig. S2, it is shown that the amorphous carbon adhering on the surface of CNTs can be removed after chlorine doping, while few damages happen to the CNT structure.

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Fig.6 SEM images of surface morphology in (a) P-CNT sheet and Cl-CNT sheet under duration time of (b) 2h, (c) 6h and (d) 10h, and (e) packing density of CNT sheet under different duration time.

The interfacial adhesion and interaction between CNT and PVDF matrix are crucial factors that determine overall electrical, dielectric and mechanical

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performances of composite[32]. SEM images of both surface and cross section in fractured CNT sheet/PVDF are presented in Fig. 7. In Fig. 7(a) and (b), it is seen that PVDF matrix can be uniformly distributed in CNT sheet surface after resin-infiltration

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and hot-press process. However, there are some voids in P-CNT sheet/PVDF. In contrast, Cl-CNT sheet/PVDF shows much fewer voids. Furthermore, Fig. 7 (c) and (d)

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reveal the interfacial adhesion status between CNT and PVDF. It can be noticed that compared with the obvious boundary and poor interfacial bonding between P-CNT and PVDF, the blur and vague interface between Cl-CNT and PVDF indicates improved interfacial adhesion status.

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Fig.7 SEM images of surface morphology in (a) P-CNT sheet/PVDF and (c) its sandwich composite film,

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(b) Cl-CNT sheet/PVDF and (d) its sandwich composite film.

3.2 Electrical and mechanical properties

The above characterizations and analysis have shown that after SOCl2 doping

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treatment, not only are covalent and ionic Cl attached to CNT sheet, but also leads to denser stacking status of CNTs. It is well-known that the dielectric properties in

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conductive filler reinforced polymer composites are closely related to the intrinsic electrical conductivity of filler[39], thus the effect of doping treatment on electrical conductivity in CNT sheet and CNT sheet/PVDF sheet was investigated. The measured electrical conductivity is presented in Fig. 8. The random overlap and joints among CNTs can form a conductive path, which provides the P-CNT sheet with an electrical conductivity of 495.5 S/cm, as shown in Fig. 8(a). Due to formation of CNT/SOCl2 charge/transfer complexes, the increase in width and density of bundles, the electrical conductivity of Cl-CNT sheet firstly increases and then decreases in accordance with 16

ACCEPTED MANUSCRIPT the trend in doping effect and structure change. It is noticed that the highest electrical conductivity value of 2253.1 S/cm is obtained at duration time of 6h. The retention of electrical conductivity in Cl-CNT sheet (duration time of 6h) was measured after heating sample at 70 °C in vacuum for different time. The electrical conductivity shown

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in Fig. 8(b) decreases as duration time increases and then retains at certain value of 1651.3 S/cm. The decrease in property retention rate is mainly due to physical desorption of SOCl2 which further leads to decrease in charge/transfer complex. As for

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the CNT sheet/PVDF shown in Fig.8 (c), the corresponding electrical conductivity is relatively lower than that in CNT sheet due to introduction of insulating PVDF matrix.

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However, a high value of 1309.7 S/cm is still obtained in CNT sheet/PVDF at duration

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time of 6 h.

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Fig.8 (a) Electrical conductivity in CNT sheet under different duration time, (b) electrical conductivity retention of the 6 h- heat duration sample under increasing vacuum heating treating time and (c) electrical

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conductivity in CNT sheet/PVDF under different duration time.

The mechanical properties of CNT sheet, CNT sheet/PVDF and sandwich

composite film are illustrated in Fig. 9. Fig.9(a) and Fig. 9(d) provides typical tensile curves, tensile strength and modulus for CNT sheet before and after doping treatment. The P-CNT sheet has an average tensile strength and modulus of 90.3 MPa and 1.2 GPa. It also exhibits an elongation at break of 23.01 %. As for the Cl-CNT sheet, the average tensile strength and modulus are 118.3 MPa and 1.4 GPa, which are 30.98 % and 18.33 % higher than those of P-CNT sheet. The corresponding elongation at break of 17.6 % 17

ACCEPTED MANUSCRIPT shows a minor decrease by 23.5 %. The enhanced tensile strength and modulus can be mainly attributed to increased bundles, junctions and decreased voids in Cl-CNT sheet confirmed in Fig. 6[25]. The typical tensile curves, tensile strength and modulus for P-CNT sheet/PVDF

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sheet and Cl-CNT sheet/PVDF are visualized in Fig. 9(b) and Fig. 9(e). Both tensile strength and modulus are found to be increased from 134.8 MPa and 4.8 GPa in P-CNT sheet/PVDF sheet to 263.9 MPa and 7.1 GPa in Cl-CNT sheet/PVDF sheet,

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respectively. The significant increases of 95.9 % and 49.9 % in tensile strength and modulus not only benefit from the enhanced intrinsic mechanical properties in Cl-CNT

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sheet, but also due to largely improved Cl-CNT and PVDF interfacial bonding shown in Fig. 7(d). The elongation at break in Cl-CNT sheet/PVDF is 16.7 %, which is 28 % lower than that of 22.8 % in P-CNT sheet/PVDF.

The typical tensile curves, strength and modulus of sandwich composite and

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neat-PVDF films are presented in Fig. 9(c) and Fig. 9(f). The neat-PVDF film under the same hot-press process condition was also tested as comparison. The neat-PVDF film shows a tensile strength of 42.8 MPa, modulus of 1.4 GPa and an elongation at break of

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33.9 %, which indicates excellent mechanical flexibility. By adding CNT sheet as

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macroscopic reinforcement, the tensile strength and modulus of sandwich composite film can be obviously increased. Sandwich composite film with P-CNT sheet exhibits high tensile strength of 60.9 MPa and modulus of 2.4 GPa, which increase by 42.4 % and 70.2 %, respectively. As for the sandwich composite film reinforced with Cl-CNT sheet, tensile strength and modulus show further increase to 90 MPa and 3 GPa, which is 110.3 % and 109.2 % higher than those of neat-PVDF film.

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Fig.9 Typical tensile stress-strain curves, tensile strength and modulus of (a), (d)P-CNT sheet and Cl-CNT sheet, (b), (e) P-CNT/PVDF sheet and Cl-CNT sheet/PVDF, (c), (f)Neat-PVDF film, sandwich film (P-CNT sheet) and sandwich film (Cl-CNT sheet). The duration time is 6 h.

3.3 Frequency dependent dielectric and electrical properties

For the composite, the dielectric permittivity and loss tan can be expressed as

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follows: ε =ε

tanδ = tanδ

+ tanδ

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The symbols of ε

+ε

(1)

+ tanδ

and ε

(2)

represent the dielectric permittivity

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contributed by the interfacial polarization process (dominant in low frequency range) and dipole polarization process (dominant in high frequency range), respectively. The tanδ

, tanδ

and tanδ

represent dielectric loss induced by electron and

charge carrier conduction (dominant in low frequency range), interfacial polarization process (dominant in low frequency range) and dipole polarization process (dominant in high frequency range), respectively[40]. The frequency dependences of dielectric permittivity of sandwich composite films with CNT sheet under different duration time are displayed in Fig.10 (a). The 19

ACCEPTED MANUSCRIPT frequency-dependent dielectric permittivity of neat-PVDF is also included as comparison. It can be clearly seen that the introduction of CNT sheet/PVDF interlayer leads to an increase by 75.5 % in dielectric permittivity from 10.6 (@102 Hz) in neat-PVDF to 18.6 (@102 Hz). An interfacial polarization region can be formed with

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accumulation of negative charges on surface of highly conductive CNT sheet/PVDF interlayer and positive charges in highly insulating neat-PVDF region as illustrated in Fig. 10(d). The charge accumulation phenomenon can induce strong interfacial

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polarization process, thus giving rise to increased dielectric permittivity[16]. As for Cl-CNT sheet reinforced sandwich composite films, the dielectric permittivity firstly

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shows increase with increase of duration time and then decreases. As mentioned before, the electrical conductivity and charge carrier density in Cl-CNT sheet/PVDF can be optimized by tuning duration time. Based on the interfacial polarization theory, the intensity of interfacial polarization process can be enhanced by increasing differences

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in electrical conductivity and charge carrier density between conductive phase and insulating phase[41]. Thus the largely increased electrical conductivity and charge carrier density in Cl-CNT sheet/PVDF lead to significantly enhanced interfacial

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polarization process, which further gives rise to dielectric permittivity. Especially, the sandwich film reinforced with Cl-CNT sheet under duration time of 6 h exhibits a

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maximum dielectric permittivity of 32 (@102Hz), which is nearly three times higher than that in neat-PVDF. The frequency-dependent dielectric loss in sandwich composite film with P-CNT

sheet and Cl-CNT sheet under various duration time are presented in Fig.10 (b). As for neat-PVDF, the dielectric loss of 0.07 (@102 Hz) can be attributed to weak conduction of charge carrier on the boundary between lamellar crystal and interlamellar amorphous region, while the dielectric loss of 0.28 (@107 Hz) is due to dipole polarization process 20

ACCEPTED MANUSCRIPT induced by molecular movements in the crystalline region[42]. For conductive filler reinforced composite, high dielectric loss in low frequency range induced by long-distance charge carrier conduction is usually observed. In our case, the CNT sheet/PVDF interlayer with high electrical conductivity leads to high charge carrier

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accumulation at interface region, while long-distance conduction of charge carrier is strongly inhibited by upper and bottom PVDF insulating layer as illustrated in Fig. 10 (d). Benefitted from the existence of PVDF insulating layer, the dielectric loss in

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sandwich film is successfully retained closely same as that in neat-PVDF. It is also noticed that even with highest dielectric permittivity obtained in sandwich film under

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duration time of 6 h, the dielectric loss is still as low as 0.08 at 100 Hz. The AC conductivity as a function of frequency is displayed in Fig. 10 (c). The AC conductivity in composite is strongly dependent on the intrinsic electrical conductivity, distribution status of reinforcements and long-distance charge carrier conduction. It is

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shown that the addition of highly conductive CNT sheet leads to increase in AC conductivity within whole frequency range. The trend in AC conductivity with duration time is in accordance with the change in electrical conductivity of Cl-CNT sheet. The

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highest frequency dependent AC conductivity of 9.8*10-9 S/m (@102 Hz) is also

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observed in sandwich film under duration time of 6 h. Due to the existence of PVDF insulating layer and its blocking effect on long-distance charge carrier conduction, the good insulation performance is still preserved.

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Fig.10 Frequency dependence of (a) dielectric permittivity, (b) dielectric loss, and (c) AC conductivity in sandwich composite films filled by CNT sheet under different duration time, (d) schematic diagram of mechanism in increasing interfacial polarization and restricting charge carrier conduction.

For comparison with other previously reported polymer composites with high dielectric properties, the corresponding dielectric permittivity, loss, tensile strength,

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modulus, filler content and fabrication method are listed in Table.1[43-51]. It is obviously shown that high dielectric permittivity can be achieved in polymer

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composites filled by BT, CCTO, graphene and CNT. However, the increment in dielectric permittivity is usually accompanied with high filler content in ceramic

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reinforced composites and high dielectric loss in percolative composites. Both high filler content and dielectric loss subject the composites to several inevitable drawbacks, such as poor mechanical flexibility, overheating and low efficiency. Moreover, these composites usually call for fabrication technique such as ultra-sonication or electro spinning, which are both time and energy consuming. It is shown that CNT sheet reinforced sandwich composite film with very low filler content manifests high dielectric permittivity, extremely low loss and enhanced mechanical properties realized through simple fabrication process. Furthermore, the integrative performance of the 22

ACCEPTED MANUSCRIPT composites can be easily tuned for meeting different application requirements. Table 1 Comparisons of dielectric permittivity, loss, tensile strength, modulus and fabrication method for different composites. Dielectric

Dielectric

Tensile

Tensile

Filler

Fabrication

permittivity

loss

strength

modulus

content

technique

90 %

Ultra-sonication,

27.2 (1 kHz)

0.3 (1 kHz)

59.2 MPa

~

(mass)

solution-casting

BT/CEC[43]

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Composites

High temperature

10 %

La-CCTO/PVDF[44]

31 (100 Hz)

0.35 (100 Hz)

43 MPa

1 GPa

extrusion,

(mass)

hot-press

10.1 (100 Hz)

GO-IL/PVDF[47]

γ-MWCNT/PI[48]

MWCNT/PI[49]

32.1 (100 Hz)

145 (1 kHz)

9.8 (100 Hz)

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MWCNT@SiO2/PVDF[50]

13.5 (100 Hz)

162 (1 kHz)

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Gr-CNT/TPU[51]

CNT sheet sandwich

composite in this paper

0.1 (100 Hz)

~

32 (100 Hz)

29.4 (1 kHz)

41.8 MPa

115 MPa

72.5 MPa

0.29 (100 Hz)

1.1 (1 kHz)

0.07 (100 Hz)

0.8 ( kHz)

Ultra-sonication,

(volume)

solution-casting

~

122 MPa

In situ

30 %

4.3 GPa

polymerization, (mass) solution-casting 3%

Ultra-sonication,

(mass)

solution-casting

20 %

Ultra-sonication,

(mass)

solution-casting

5 GPa

~

Ultra-sonication, 14 % 90.2 MPa

~

electrospin, (mass) hot-press

105 MPa

13 MPa

90 MPa

3%

Ultra-sonication,

(mass)

solution-casting

3%

Ultra-sonication,

(volume)

solution-casting

2.2 %

Resin-infiltration,

(mass)

hot-press

2.8 GPa

7.8 MPa

0.08 (100 Hz) 0.03 (1 kHz)

10 %

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0.65 (100 Hz)

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PcLS/PI[46]

49 (100 Hz)

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BT-nw/PVDF[45]

3 GPa

Based on the above results and discussions, it is shown that both mechanical and

dielectric properties of CNT sheet can be significantly enhanced through a sonication-free SOCl2 doping treatment. A dense CNT sheet/PVDF composite with high electrical conductivity and mechanical strength is fabricated through a simple resin-infiltration and hot-press technique. The highly conductive CNT sheet/PVDF can be embedded as an interlayer to largely increase overall dielectric permittivity and 23

ACCEPTED MANUSCRIPT mechanical strength of composite, while the sandwich structure effectively suppresses conduction loss and preserves good insulation property. Considering the dispersion-free fabrication process and high dielectric performance, as well as the excellent mechanical flexibility, this composite is favored to be used as high-k

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materials where both high dielectric and mechanical properties are required. Additionally, the concept of fabricating sandwich structured polymer composite embedded with continuous conductive reinforcements, such as graphene film and CVD

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grown CNT array, is promising to be used as novel high-k materials. Their overall performances can be easily tuned by tailoring structure and chemical property of

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continuous nanomaterial for applications in high technology electronics. 4. Conclusions

In conclusion, a novel sandwich polymer composite with continuous conductive layer (CNT sheet/PVDF) intercalated between insulating layers (neat-PVDF) was

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successfully fabricated. CNT sheet fabricated by floating catalyst CVD method was used, which has unique structure and property. It is found that the doping treatment of SOCl2 leads to p-type doping and benefits the densification of CNT sheet. In addition,

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strong interfacial bonding and the reduction of void defect in CNT sheet/PVDF were

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obtained after chlorination. The combined factors cause large increments in electrical and mechanical properties and optimum performance can be obtained at doping duration time of 6h. The dielectric property of sandwich composite film can be easily tailored with improved mechanical property. The highest dielectric permittivity is obtained at duration time of 6 h which is also in accordance with the trend of doping effect. A mechanism in increasing dielectric permittivity and restricting low dielectric loss is proposed. It is found that the CNT sheet/PVDF can act as an interlayer to increase interfacial polarization process, while neat-PVDF serve as insulating layers to 24

ACCEPTED MANUSCRIPT suppress charge carrier conduction. Furthermore, the incorporation of CNT sheet significantly improves mechanical property of the sandwich structured composite film. Compared with other composites filled by traditional nano fillers, this CNT sheet sandwich composite film allows a dispersion-free fabrication and exhibits high

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dielectric permittivity, low dielectric loss and enhanced mechanical performance at low filler content. Acknowledgments

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This work was supported by funding from the National 863 Program of China (Grant No. 2014AA032801) and National Natural Science Foundation of China (Grant

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No.51403009). The authors would thank Prof. Y Deng for helpful discussions on dielectric measurement and analysis. References

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