Ionic Liquid Composites

Article pubs.acs.org/JPCC

Nanostructured Poly(vinylidene fluoride)/Ionic Liquid Composites: Formation of Organic Conductive Nanodomains in Polymer Matrix Chenyang Xing,†,‡,§ Jichun You,† Yongjin Li,*,† and Jingye Li*,‡ †

College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Road, Hangzhou, 310036, People’s Republic of China ‡ TMSR Research Center and CAS Key Lab of Nuclear Radiation and Nuclear Energy Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800, People’s Republic of China § University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China

Downloaded by TEXAS A&M INTL UNIV on August 27, 2015 | http://pubs.acs.org Publication Date (Web): August 26, 2015 | doi: 10.1021/acs.jpcc.5b05349

S Supporting Information *

ABSTRACT: Nanostructured polymeric composites, based on a homopolymer poly(vinylidene fluoride) (PVDF) and a small molecule, 1-vinyl-3-butylimidazolium chloride [VBIM][Cl], an unsaturated room-temperature ionic liquid (IL) have been fabricated. Our strategy forms organic conductive nanodomains with diameters of 20−30 nm dispersed homogeneously in the PVDF matrix. It is demonstrated that these conductive nanodomains are induced from microphase separation of the IL grafted PVDF (PVDF-g-IL) segments from the neat PVDF, which were produced by using electron-beam irradiation, leading IL molecules to graft onto the amorphous PVDF chains. It is also found that such microphase separation of PVDF-g-IL segments from PVDF matrix occurs only when the grafted IL content exceeds 3 wt %. Furthermore, the formed nanodomains enhance the crystallization rate of the matrix PVDF. The obtained nanostructured PVDF composites show dominant nonpolar α-phase of PVDF crystals and increased crystal long period (L) compared with neat PVDF. Additionally, the resulting nanostructured PVDF composites exhibit enhanced electrical properties, better Young’s modulus and ductility, and improved dielectric performance compared with neat PVDF, making the composites promising for potential use in superthin dielectric capacitors. The intriguing synthesis route will open up new opportunities for fabricating nanostructured polymer composites.

1. INTRODUCTION Room-temperature ionic liquids (IL) are known as a class of organic molten salts, comprised entirely of ions and having melting points below 100 °C.1−7 Ionic structure natures of ILs can be well responsible for their intriguing physicochemical properties such as large polarity, negligible vapor pressure, high ion conductance, high thermal and chemical stabilities.8,9 These advantages, together with their ease of handling, considerably push the utilization of ILs as additives for many polymers.10−19 The ILs may take roles of plasticizers,10,12,14,16,20,21 antistatic agents,10,12,13 lubricants,22,23 and antibacterial agents24 for polymers, especially for miscible polymer/IL blends. On the other hand, polyvinylidene fluoride (PVDF) has attracted considerable attention in both academic and industrial applications because of its unique properties, including high polarity, good mechanical strength, and thermal and chemical stability.25−34 Most investigations have been focused on enhancements of physical properties and polar crystal contents of PVDF with a view to exploring PVDF applications in electric device field. Recently, we have reported multifunctional PVDF composites using an IL as an additive through melt processing.10 We found that the integration of a limidazolium-based IL led to novel PVDF composites having excellent ductility, good optical transparency, and nice permanent © XXXX American Chemical Society

antielectrostatic property. Good miscibility of PVDF with the IL can rationally be responsible for such remarkable improvements.10 On the other hand, in order to enhance the permittivity of PVDF, inorganic nanofillers with high dielectric permittivity were frequently incorporated into the PVDF matrix.35−49 ILs also show high ion electrical conductance1−5 and high dielectric constants50,51 due to nonvanishing electrical dipole moments and their complicated ion pairs and dipolar ion complexes in the bulk state.50,52 Thus, it is feasible to fabricate high-performance dielectric composites by combining PVDF with ILs. However, ILs, at a free state in PVDF matrix by physically mixing, certainly generate serious dielectric loss due to their ion movements under AC electric field. This behavior is detrimental for the dielectric applications of PVDF/IL composites because a large amount of energy stored could be transferred into heat energy, not only reducing electrical energy but also impairing a lifetime of materials. Obviously, immobilizing ILs in PVDF matrix definitely decreases the dielectric loss and improves dielectric performance of PVDF/IL composites. Received: June 4, 2015 Revised: July 26, 2015

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DOI: 10.1021/acs.jpcc.5b05349 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Nanostructured PVDF composite obtained from homopolymer PVDF and a room temperature ionic liquid (IL). (a) Transmission electron microscopy (TEM) image illustrating the homogeneous dispersion of nanodomains (appearing dark, stained with RuO4) in the PVDF matrix (appearing white). (b) High-resolution scanning electron microscopy (HRSEM) image showing the fractured cross-section of the PVDF nanocomposite broke off in the liquid nitrogen. (c) TEM image of PVDF nanocomposite whose solution of DMF was directly dropped onto the copper grid with carbon membranes. (d−f) Elemental mapping images (EMI) showing the signals of F and Cl elements, respectively.

was placed into a polypropylene bag. The absorbed dose adopted was chosen to 45 kGy to initiate the grafting of ILs onto the PVDF chains. 2.2. Preparation of Nanostructured PVDF Composites. Several procedures were used to prepare the nanostructured PVDF composites. First, extraction experiments were performed to remove the ungrafted (excess) IL molecules and/or their homopolymers existing in the above irradiated PVDF/IL blend at 45 kGy. Soxhlet extractor was thus employed and methanol (CH3OH) was chosen as a solvent that can resolve both the IL and its homopolymer poly(ionic liquid). The extraction was performed for 128 h in CH3OH at its boiling point. Second, the above extracted irradiated PVDF/ IL blend was melted by hot-pressing at 210 °C under 10 MPa pressure for at least 20 min and then quenched into room temperature with the aid of running water under the same pressure. The nanostructured PVDF composite was thus obtained. 2.3. Morphology Characterization. Transmission electron microscopy (TEM, Hitachi HT-7700) was performed to observe the nanostructures, where this sample was first ultramicrotomed to a section with a thickness of 70−100 nm at −90 °C and it was subsequently stained with ruthenium tetraoxide (RuO4) for 3 h. An accelerating voltage operated was 80 kV. Elemental mapping image (EMI) analysis (Talos TM F200A) was used to identify the elements in the sample, especially in nanodomains, with an accelerating voltage of 200 kV. The morphologies of the samples were also observed using a high-resolution field emission scanning electron microscopy (HR-FESEM, SEM-JSM 6700). The sample was first fractured in liquid nitrogen and then coated with a thin layer of gold under a completely dry state. An acceleration voltage of 3 kV was used for the observation.

In this work, we have fabricated a nanostructured composite with enhanced dielectric performance based on an unsaturated IL, 1-vinyl-3-butylimidazolium chloride [VBIM][Cl], and PVDF. In more detail, ILs were first chemically grafted onto PVDF amorphous chains (forming PVDF-g-IL molecular chains) by using electron-beam irradiation (EBI) method. This results in the fixing of cations of ILs onto the PVDF chains. The following melting of the electron-beam irradiated PVDF/IL composites leads to microphase separation of PVDFg-IL chains from the PVDF matrix, and thus, a nanostructured PVDF composite with PVDF-g-IL confined in the nanodomains is achieved. Such nanostructured alloys exhibit not only good mechanical performance, but also improved dielectric properties.

2. EXPERIMENTAL SECTION 2.1. Preparation of PVDF/IL Blend and Electron-Beam Irradiation of PVDF/IL Blend. The homopolymer PVDF (KF850, Kureha Chemicals in Japan) has Mw = 209000 and Mw/Mn = 2.0, respectively, and an unsaturated ionic liquid (IL) used is 1-vinyl-3-butylimidazolium chloride [VBIM][Cl]. The PVDF/IL blend was fabricated by means of directly meltblending PVDF with the IL in a Haake Polylab QC mixer at 190 °C. The PVDF/IL blends were processed by mixing with the screw speeds of 20 rpm for 2 min and subsequent 50 rpm for 5 min. The blend was then hot-pressed into a film at 210 °C under 10 MPa pressure and quenched into room temperature with the aid of running water under the same pressure. The hot and cool processing was going through a time of 10 and 2 min, respectively. The electron-beam irradiated PVDF/IL blend was prepared by exposing the above PVDF/IL film upon the electron-beam irradiation in air at room temperature, where the PVDF/IL film B


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Figure 2. Characterization of crystallization behaviors of the nanostructured PVDF composite and related materials, including neat PVDF, the PVDF/IL (100/8) blend, and the irradiated PVDF/IL (100/8) blend at 45 kGy. (a) Differential scanning calorimeter (DSC) cooling curves at 10 °C/min. (b) Corresponding heating curves recorded by the second heating process at 10 °C/min. (c) Wide-angle X-ray diffraction (WAXD) patterns to show the crystal forms of the PVDF crystals in each case in the range of 5−40°. (d) Fourier transform infrared spectroscopy (FTIR) to show the crystal forms of the PVDF crystals in each case in the range of 1600−600 cm−1. (e) Small-angle X-ray scattering (SAXS) patterns to show the correlations between the amorphous and crystalline regions in PVDF.

2.4. Characterization of Crystallization Behaviors. Differential scanning calorimeter (DSC, TA-Q2000) was used to investigate the crystallization and melting behaviors of samples. Each sample with 10−15 mg was first heated to 210 °C and then held there isothermally for 5 min to completely vanish its thermal history. The subsequent cooling process (from 210 °C to −90 °C) and the second heating process (from −90 to 210 °C) were recorded. Both the heating and cooling rates were set to be 10 °C/min and the experiments were carried out under a continuous high purity N 2 atmosphere. Crystal forms of samples were determined by

using wide-angle X-ray diffraction (WAXD, Bruker-D8) and Fourier transform infrared spectroscopy (FTIR, Bruker). The XRD measurement was performed in the range of 2θ of 5−40°, with a scanning speed of 1°/min, an operating voltage of 40 kV and an operating current of 40 mA. The FTIR measurement was performed by an ATR mode with 160 scans and a measurement range of 4000−600 cm−1. The small angle X-ray scattering (SAXS, Shanghai Synchrotron Radiation Facility, China) patterns were obtained using a monochromatic X-ray beam with a wavelength of 1.24 Å. A 1860 mm of sample− detector distance and 200 s of exposed time were used. X-ray C


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The Journal of Physical Chemistry C Table 1. DSC Data and SAXS Data Obtained from Figure 2a, b, and e neat PVDF PVDF/IL blend (100/8) nanostructured PVDF composite

Tca (°C)

Tm,secondb (°C)

ΔHcc (J/g)

ΔHm,secondd (J/g)

Xce (%)

qmaxf (nm−1)

Lg (nm)

141.1 133.7 145.9

173.2 172.4 182.7

47.90 43.04 42.32

40.94 34.96 35.35

39.1 36.1 36.2

0.495 0.423 0.415

12.69 14.85 15.13

Tc: peak temperature in the cooling scanning in the DSC data. bTm,second: peak temperature in the second heating scanning in the DSC data. cΔHc: enthalpy of crystallization from the cooling scanning. dΔHm,second: enthalpy of fusion from the second heating scanning. eXc: PVDF crystallinity calculated from the ΔHm,second using a 104.7 J/g of ΔH°m for 100% crystalline PVDF. fqmax: peak of q value. gL: crystal long period (L = 2π/qmax).


a

(Figure 1e and 1f), but no Cl-signals were observed in the matrix. This result indicates that the grafted cations together with the counterions (Cl−) are fully confined in the nanodomains. The detail analysis will be discussed in the next section. 3.2. Crystallization Behaviors of PVDF in Nanostructured Composites. Figure 2a and b show the DSC cooling and heating curves of neat PVDF, PVDF/IL (100/8) blend, and nanostructured PVDF composites, respectively. The main thermal parameters are displayed in Table 1. In cooling, as shown in Figure 2a, the neat PVDF exhibited a meltcrystallization temperature (Tc) of 141.1 °C, while the blend of PVDF/IL (100/8) had a lower Tc of 133.7 °C. The Tc depression is obviously attributed to the thermodynamic miscibility between the IL molecules and PVDF.10,13 The nanostructured PVDF composite, however, had a Tc of 145.9 °C, about 5 and 12 °C higher than the Tc values of the neat PVDF and PVDF/IL (100/8) blend, respectively. This increase indicates that the PVDF crystallized much faster in the nanostructured PVDF composite than in the neat PVDF. As shown in Figure 2b, the melting point (Tm) of the PVDF crystals in neat PVDF was 173.2 °C, which decreased slightly when incorporating IL to produce the PVDF/IL (100/8) blend. We found a similar phenomenon in previous studies.10,13 The changes of Tm could be ascribed to the two mechanisms. On the one hand, simple incorporation of IL in PVDF leads to the formation of PVDF γ phase (also shown in Figure 2c and 2d), which has higher Tm than α-PVDF phase. On the other hand, the high miscibility between IL and PVDF induces the depression of Tm. In contrast, the Tm of the PVDF crystals in the nanostructured PVDF composite was higher than all the other samples. We attributed the higher Tm in nanostructured PVDF to the higher Tc (in Figure 2a), which leads to the larger crystal long period during the nonisothermal crystallization. The thick lamellae usually exhibit high melting temperatures. The crystal structures and crystal long periods of the PVDF crystals in each sample were characterized by WAXD, FTIR, and SAXS. The results are shown in Figure 2c−e, respectively. As shown in Figure 2c, WAXD patterns indicate that the neat PVDF exhibited the typical nonpolar α form of PVDF crystals. In contrast, the PVDF/IL (100/8) blend exhibited mainly γ crystals coexisting with α crystals, caused by the PVDF interacting with the polar IL molecules.53,54 We found the same behavior in our previous study of the PVDF/[BMIM][PF6] system.10 The PVDF/IL (100/8) blend exhibited no change in crystal structure after being irradiated at 45 kGy, indicating that the γ-PVDF crystals were not influenced by irradiation at this dosage. However, the nanostructure PVDF composites are mainly α-crystals. This means that the γ-PVDF crystals of the irradiated sample transfer into the nonpolar αcrystals during melting and cooling. This behavior may occur because the IL trapped in the nanodomains cannot interact well with the matrix PVDF chains, even in a molten state. Therefore,

scattering intensity patterns were obtained by using a twodimensional array with 2048 × 2048 pixels. 2.5. Physical Property Measurements. An ultrahighresistivity meter was used to measure the electrical properties (Rs, surface resistivity; Rv, volume resistivity) of samples. A URS ring electrode probe with a model of MCP-HT450 was placed on samples under a direct-current voltage of 10 V. An average value was reported for Rs and Rv of each sample when at least five locations on each sample were measured. A 5966-model instron universal material testing system was employed to measure the mechanical properties of the samples. Dumbbell-shaped samples were measured at room temperature with a fixed crosshead speed of 10 mm/min and a gauge length of 18 mm. At lease three samples were measured and an averaged value was reported. The dielectric properties were performed by using Agilent 4294A with 16451B fixture with a frequency of 40 Hz to 110 MHz at room temperature. At least three different areas of each sample were measured.

3. RESULTS 3.1. Formation of Nanostructured PVDF/IL Composites. A homogeneous PVDF/IL (100/8) blend was first prepared by simply melt blending, and it was then subjected to an electron-beam irradiation at room temperature, leading to the grafting of IL molecules onto the amorphous PVDF chains (i.e., PVDF-g-IL chains). The irradiated PVDF/IL (100/8) blend was melted at a temperature higher than the PVDF melting point, which induced microphase separation and led the PVDF-g-IL segments to form fine organic conductive nanodomains homogeneously dispersed in the PVDF matrix. This process generated the nanostructured PVDF composite. Figure 1 shows the morphologies of the nanostructured PVDF composite obtained by above-mentioned procedure. The TEM image (Figure 1a) reveals dark nanodomains with diameters of 20−30 nm homogeneously dispersed in the white PVDF matrix. The calculated interdomain distance was 5−20 nm. The domain size distribution is relative narrow (Figure S1 in the Supporting Information). Similar results can be obtained from the morphologies shown in the SEM image (Figure 1b) of the sample’s fractured surface, which reveals nanostructures with homogeneous nanodomains. To our best knowledge, no reports have shown such a nanostructured PVDF composite obtained from PVDF and a small molecule IL. We also found that these nanosized domains could not be dissolved by DMF (the solvent of neat PVDF), but swelled to increase their diameters in DMF, as shown in Figure 1c. To identify the chemical compositions of these nanodomains, we performed elemental mapping image analysis, as shown in Figure 1d−f. The entire structure uniformly exhibited F-rich signals, (Figure 1d), revealing that both the matrix and nanodomains contains F element. In contrast, the Cl-signals only appeared in nanodomains with diameters of 20−30 nm D


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The Journal of Physical Chemistry C the matrix PVDF in the nanostructured composites crystallized into the normal nonpolar α-crystals. The identification of PVDF crystal forms in each case can be further confirmed by FTIR spectroscopy in Figure 2d. Neat PVDF showed dominant nonpolar α-crystal characteristic absorptions at 615, 764, 796, 855, 976, and 1385 cm−1, respectively.53,54 Incorporation of IL into PVDF (i.e., PVDF/IL (100/8) blend) induced the almost disappearance of α-crystal and generated polar γ-crystals (at 1232 cm−1) as well as β-crystals (at 1275 cm−1).53,54 It can be further concluded that the polar γ-crystal form is dominant in PVDF/IL (100/8) blend in combination with the above XRD results in Figure 2c, which is in good agreement with our previous study.10 In addition, PVDF crystals in nanostructured PVDF composite were also found to be domain α-phase with a trace amount of β-crystal, as shown in Figure 2d. Therefore, the FTIR results are consistent with the XRD results. SAXS patterns (Figure 2e) indicate that the long periods (L) of the PVDF lamellae in neat PVDF and PVDF/IL (100/8) blend are about 12.69 and 14.85 nm, respectively. The increased L in the PVDF/IL (100/8) blend is attributed to the insertion of the IL molecules into the gallery (amorphous region) between the PVDF lamellae. Irradiating the PVDF/IL (100/8) blend at 45 kGy barely changed the L (to 15.14 nm) compared with the PVDF/IL (100/8) blend because the simple grafting of IL onto the PVDF amorphous region does not change the crystal long period of PVDF. The fact that the larger L of PVDF in the nanostructured composites was observed can be attributed to the higher Tc during cooling down from the melts (in Figure 2a). Note that the interdistance of the two neighboring nanodomains is larger than the L of PVDF lamellae. Moreover, the scattering from the nanodomains is diffused scattering. Therefore, we observed much higher scattering intensity at the low q region for the microphase separated sample than that for the blend and irradiated samples. 3.3. Physical Properties of the Nanostructured PVDF Composites. Electrical Property. One of the most intriguing characteristics of our nanostructured PVDF composite is its conductive nanostructures, which makes it distinct from other polymer/polymer nanoblends. Figure 3 and Table 2 show the electrical properties of the neat PVDF, PVDF/IL blend (100/ 8), irradiated PVDF/IL blend, and the nanostructured PVDF composite. The neat PVDF was electrically insulating; its surface resistivity exceeds 1013, which is out of the limit of our instrument. The incorporation of ion-conductive IL into PVDF leads to an enhancement of ionic conductivity. PVDF/IL (100/

Table 2. Electrical Properties of Nanostructured PVDF Composite and Its Counterparts, Including Neat PVDF, PVDF/IL (100/8) Blend, and Irradiated PVDF/IL (100/8) Blend at 45 kGy

neat PVDF PVDF/IL (100/8) blend irradiated PVDF/IL (100/8) blend nanostructured PVDF composite

surface resistivity (Ω/sq)

volume resistivity (Ω·cm)

out of range (≥1013) 2.76 (±1.32) × 107

out of range (≥1013) 2.73 (±1.32) × 106

6.41 (±0.67) × 108

6.35 (±0.67) × 107

7.73 (±1.04) × 1010

7.76 (±1.04) × 109

8) blend exhibits a surface resistivity of 2.76 × 107 Ω/sq, which locates in conductivity of semiconductive materials. In this physically blending sample, the IL molecules (both cations and anions) were fully free and many conductive pathways could be readily formed by these ions migration in electrical field. However, the cations were chemically grafted onto the PVDF chains by irradiation and the surface resistivity increased to 6.41 × 108 Ω/sq in the irradiated PVDF/IL (100/8) blend sample at 45 kGy. Note that the conductivity drops only by 1 order of magnitude because the anions could still migrate freely in this sample. For the nanostructured PVDF composite, the surface resistivity increases considerably to 7.73 × 1010 Ω/sq. The much lower conductivity of the nanostructure composites is attributed to the fact that both the cations and the anions of the IL are trapped in the nanodomains. Note that attention should also be paid to the fact that the surface resistivity of the nanostructured PVDF composite is still lower than that of the neat PVDF by at least 3 orders of magnitude. Mechanical Property. Figure 4a shows the strain−stress curves of the neat PVDF and nanostructured PVDF composite at room temperature. Neat PVDF exhibited typical behaviors of mechanical drawing, including yield and necking phenomena. The yield strength of the nanostructured PVDF composite (53 MPa) was slightly lower than that of neat PVDF (55 MPa), as shown in Table 3. We attribute this decrease to the decreased crystallinity in the nanostructured PVDF composite (Table 1). However, the nanostructured PVDF composite exhibited increased Young’s modulus (from 1.02 to 1.76 GPa) and elongation at break (from 226.7% to 328.4%) in the presence of nanodomains compared with neat PVDF, indicating that the nanodomains were more rigid than the amorphous PVDF surrounding them. The as-prepared nanostructured PVDF composite exhibited excellent ductility, as shown in Figure 4b, which is of great importance for superthin foldable films. Dielectric Property. Figure 5 shows the dependence of dielectric permittivity and loss tangent on frequency for each sample at room temperature. Figure 5a shows that incorporating IL into the PVDF (i.e., PVDF/IL (100/8) blend) significantly increased its dielectric permittivity compared with neat PVDF. We attribute this behavior to the dipole rotation of the IL with high ion-conductance. When the PVDF/ IL (100/8) blend was irradiated at 45 kGy, its dielectric permittivity decreased; this behavior occurred because the IL cations were restricted by the grafting reaction. The permittivity of the nanostructured PVDF composite was modestly larger than that in neat PVDF, though it was lower than those of the as-prepared blend sample and irradiated sample. In more detail, the dielectric permittivity of the nanostructured PVDF composite increased by 124% at 102 Hz and by 38.6% at 103

Figure 3. Electrical properties of PVDF samples including neat PVDF, the PVDF/IL (100/8) blend, the irradiated PVDF/IL (100/8) blend at 45 kGy, and the nanostructured PVDF composite. E


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their use as capacitors because free IL generates serious leakage current. Grafting the IL molecules (i.e., ILs’ cations) onto the PVDF molecules effectively decreased this leakage current, as shown in the irradiated PVDF/IL (100/8) blend at 45 kGy. This was further improved in the nanostructured PVDF composite, wherein IL molecules (i.e., both cations and anions of IL) were successfully trapped in nanodomains; though the Cl anions in these nanodomains were free, they never displaced further than the scale of the nanodomains. These results showing dielectric properties agree well with the electrical properties shown in Figure 3, and both sets of properties correlate closely with the confinement of IL in the PVDF matrix. Compared to neat PVDF, the nanostructured PVDF composite had relatively high dielectric permittivity with excellent ductility; these properties suggest that dielectric capacitors made from this material would have high energy density. The flexibility of a dielectric material plays a crucial role in maintaining its stored energy constant, as expressed in the following equation:35 W=

where W is the energy stored, C is capacitance of the parallelplate capacitor, V is the applied voltage, εr is the relative dielectric permittivity, ε0 is the permittivity of free space (8.85 × 10−12 F/m), Δv is the reduced capacitor volume, and d is the thickness of the dielectric film. This equation shows that, with a given εr, decreasing Δv linearly decreases W, which is a common issue in relatively small capacitors. However, if a capacitor is made from a superthin material, where d is small, then W increases exponentially. This relationship means the flexibility of capacitors is very important. Our nanostructured PVDF composite, made of fully polymeric PVDF with conductive nanodomains, is promising for use in superthin capacitors, considering its excellent ductility and relatively high dielectric permittivity.

Figure 4. Mechanical properties of the neat PVDF and nanostructured PVDF composite at room temperature. (a) Typical strain−stress curves for the neat PVDF and nanostructured PVDF composite, respectively. (b) Macroscopic photo of the nanostructured PVDF composite with a color of brown conferred by the nature color of IL used in this study.

Table 3. Mechanical Properties of Neat PVDF and Nanostructured PVDF Composite

neat PVDF nanostructured PVDF composite

yield strength (MPa)

Young’s modulus (GPa)

strain at break (%)

55 ± 3.6 53 ± 2.7

1.02 ± 0.12 1.76 ± 0.20

226.7 ± 10.5 328.4 ± 7.9

⎛ Δv ⎞ 1 2 1 CV = εrε0V 2⎜ 2 ⎟ ⎝d ⎠ 2 2

4. DISCUSSION 4.1. Microphase Separation of PVDF-g-IL Chains from PVDF Matrix. To illustrate the formation of organic conductive nanodomains in the PVDF matrix, we present a detailed synthesis route in Figure 6. The neat homopolymer

Hz relative to neat PVDF. As shown in Figure 5b, as expected, the dielectric loss of the PVDF/IL (100/8) blend greatly increased in the presence of free IL, which is unfavorable for

Figure 5. Dielectric properties of nanostructured PVDF composite and related materials, including neat PVDF, PVDF/IL (100/8) blend, and irradiated PVDF/IL (100/8) blend at 45kGy. Graphs show frequency dependency of dielectric permittivity (a) and loss tangent (b) for each sample at room temperature. F


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Figure 6. Schematic of synthesis route for producing a nanostructured PVDF composite from homopolymer PVDF and IL. This figure shows the five phase states of the neat PVDF and PVDF/IL composites, changing after each step in the synthesis process. The neat PVDF, a typical semicrystalline polymer, consists of distinct amorphous regions and crystalline regions (state I). The colors of the polymer chains differentiate those in the amorphous region and those crystallized. After being melted with PVDF, IL is introduced to form the PVDF/IL (100/8) blend with a homogeneous dispersion of IL molecules (state II). The limidazolium-based unsaturated IL is shown with a red limidazolium ring and its green counterion of Cl. Note that the IL is only trapped in the amorphous region of the PVDF. The as-formed PVDF/IL (100/8) blend is then irradiated at 45 kGy using electron-beam irradiation in air at room temperature. The limidazolium rings with unsaturated double bonds of IL can be grafted onto amorphous PVDF chains, while the corresponding anions of Cl remain free (state III). The grafting reaction only occurs in the amorphous region of the PVDF. States IV and V are the molten and crystallized states of the nanostructured PVDF composite, respectively. State IV was developed by extracting the irradiated PVDF/IL blend in CH3OH, removing the free/ungrafted IL molecules and their homopolymers, and then melting it at 210 °C for at least 20 min. State V is crystallized from state IV.

PVDF (state I) was first melt-blended with the IL ([VBIM][Cl]) to produce the PVDF/IL (100/8) blend (state II). In this blend, the unsaturated IL molecules were thermodynamically miscible with PVDF. The miscibility of PVDF and the IL originates from the interaction between the CF2 (δ−) in the PVDF and the cation ring (δ+) of the IL.10,11,13 Obviously, the IL molecules were only located in the amorphous region in the blend sample at room temperature because IL cannot be trapped in the crystalline part of PVDF. After the melt blending, the PVDF/IL (100/8) blend was exposed to electron-beam irradiation in air at room temperature (for details, see the Experimental Section), which chemically grafted the cations with double bonds onto the amorphous PVDF chains by radical reaction. The detailed information on the grafting IL onto PVDF chains is shown in Tables S1 and S2 and Figures S3−S5 in Supporting Information. This process created PVDF-g-IL segments, which led to a significant fraction of the grafted chains in the amorphous regions in the irradiated PVDF/IL (100/8) blend (state III). The phase morphology and the crystalline structure of state III was not much different from that of state II (Figure S2 in the Supporting Information). This suggests that both the PVDF/IL blend and the irradiated PVDF/IL sample are homogeneous (as shown in the TEM images in Figure S2) because the grafting reaction of IL onto the PVDF chains occurred in the solid state at room temperature, and the graft reaction only occurs in the amorphous region. In other words, IL molecules are in situ (locally) chemically grafted onto the PVDF molecular chains when the electron beam is being irradiated and no microphase separation occurs in the irradiation stage. However, the IL grafted PVDF (PVDF-g-IL) chains are not miscible with the

neat PVDF chains in the melt state. Therefore, the melting of the irradiated sample leads to the microphase separation of PVDF-g-IL from the neat PVDF chains. The PVDF-g-IL together with the counter Cl anions segregates from the bulk PVDF to form the nanodomains with the size of several 10 nm and all the ions of ILs are then confined within the nanodomains. Therefore, the nanostructured PVDF composites (state V) with the conductive nanodomains are achieved during cooling down from the melts. In addition, we have also calculated the relative fraction of IL molecules grafted onto the PVDF chains based on the crystallinity of the sample, assuming that all the IL molecules were located in the amorphous region, as shown in the Table 4. The calculated number of grafted IL molecules per 100 repeating units (CH2 −CF 2 ) (NIL / N100 units of PVDF) of the PVDF chains (in amorphous state) in the irradiated PVDF/IL (100/8) blend at a 45 kGy dose was 3.79. The value of NIL/N100 units of PVDF is small, indicating that the length of the grafted IL is very short. Additionally, some PVDF molecular chains in amorphous region connecting the neighboring crystal lamellae, so-called “tie molecules”, might also be grafted with ILs during irradiation (please see Figure S7 in the Supporting Information), so the “block-like” molecular chains were synthesized. The existence of these molecules is the main reason that the microphase separation with the nanodomain occurs during the melting of the irradiated sample. The interface between the nanodomains and the matrix is also very strong by such “block-like” molecular chains. 4.2. ILs Grafting Yield Effects on the Microphase Separation. PVDF-g-IL chains have a different configuration from the neat PVDF chains and their microphase separations G


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separation of PVDF-g-IL chains from PVDF matrix induced an increase of Tc for the irradiated PVDF/IL blends at low absorbed doses during cooling from the melts, compared to unirradiated samples, as shown in Figure 8 (the detail DSC

Table 4. Calculated Numbers of Grafted IL Molecules per 100 Units of PVDF in the PVDF-g-IL Chains irradiated PVDF/IL (100/8)-45kGya irradiated PVDF/IL (100/8)-60kGya irradiated PVDF/IL (100/8)-100kGya

WILb (%)

NIL/N100 units of PVDFc

7.066 7.274 7.331

3.79 3.91 3.94


a

Irradiated PVDF/IL (100/8)-45, -60, and -100 kGy: The PVDF/IL (100/8) samples irradiated at low absorbed dose such as 45, 60, and 100 kGy were chosen to calculate these values. This is based on two points. First, the 8 wt % IL was not fully grafted onto the PVDF chains (Table S2 in the Supporting Information). Second, higher absorbed doses (such as 200−1000 kGy) performed on the samples caused the occurrence of serious cross-linking of PVDF chains upon irradiation procedure (Figure S6 in the Supporting Information). bWIL (%): the grafted weight content of IL in the irradiated PVDF/IL blends obtained from the extraction results (Table S2 in the Supporting Information). cNIL/N100 units of PVDF: the calculated numbers of grafted IL molecules per 100 repeating units of PVDF in the PVDF-g-IL chains. The MW of the repeating unit CH2−CF2 and [VBIM][Cl] are 64 and 186.5 g/mol, respectively.

Figure 8. Crystallization temperatures (Tc) of irradiated PVDF/IL blends with 1, 3, 5, and 8 wt % IL loadings as a function of absorbed dose without CH3OH extraction, obtained from Figure S7 in Supporting Information.

occur in molten state (as shown in Figures 1 and 6). However, it was found that the phase separation of PVDF-g-IL chains was much related to IL grafting yield. The IL grafting yield was mainly determined by the IL loadings and absorbed dosage. Figure 7 shows TEM images of samples directly obtained by remelting irradiated PVDF/IL blends at 45 kGy with 1, 3, and 5 wt % IL loadings, respectively. A homogeneous phase was observed in the sample with 1 wt % IL loading (Figure 7a), indicating that PVDF chains grafted by a very small amount of IL molecules were not phase-separated from PVDF matrix. In contrast, a clear phase separation was observed in the samples with 3 and 5 wt % IL, as shown in Figure 7b and c, respectively. Moreover, the size of these nanodomains increases gradually with the increase in IL loadings. This can be explained by the fact that a large IL content can lead to a large grafting yield of IL onto PVDF chains, and high content PVDF-g-IL chains are obtained. It should also be mentioned that 8 wt % IL is saturated for the electron-beam irradiation in PVDF (Table S2 in the Supporting Information). The over loading of IL results in the free IL molecules and their homopolymers without being grafted onto PVDF chains. It should also be noted that too low dosage is not adequate to graft IL onto the PVDF chains. However, the dosage used in this study (above 45 kGy) can fully initiate the IL grafting. 4.3. Nanodomain Effects on the Crystallization Behaivors of PVDF Matrix. We also found that microphase

curves are shown in Figure S8 in Supporting Information). For the irradiated 100/1 blend, no microphase separation was observed after the melting of the blend and the Tc of the PVDF decreases with increasing absorbed dose, as shown in Figure 8. The Tc depression with absorbed dose is attributed to the irregular (branching or cross-linking) structures induced by irradiation. Such PVDF molecular chains showed lower crystallization rate compared with the linear PVDF chains. However, the samples with 3, 5, and 8 wt % IL loadings showed different thermal behaviors with the absorbed doses compared with the 100/1 sample, as shown in Figure 8. An abrupt increase in Tc was observed at 45 kGy for these samples compared with the unirradiated samples. The drastic Tc increase can be attributed to the nucleation effects of the nanodomains formed during the melting. It is clear that the formed PVDF-g-IL segments (or chains) are strongly segregated from the ungrafted PVDF matrix and the nanodomains were obtained in the melt for the irradiated samples with high IL loadings. Such strongly segregated graft polymers could induce PVDF segmental alignment and orientation (relative to chains in bulk) at the interface. These aligned PVDF segments may attach themselves to become nuclei or the precursor of nuclei for crystallization during cooling down from

Figure 7. TEM images illustrating an evolution of organic conductive nanodomains in PVDF matrix with different IL loadings: (a) 100/1; (b) 100/ 3; and (c) 100/5. Note that these samples were nanostructured composites that were obtained by remelting the irradiated PVDF/IL blends with 1, 3, and 5 wt % IL loadings at 45 kGy without CH3OH extraction. H


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the melt state. Therefore, for the all microphase separated samples, we observed an abrupt Tc enhancement at the 45 kGy dose. Further increasing of the absorbed dose led to gradually decreased Tc due to the branching and cross-linking of PVDF chains by irradiation.

5. CONCLUSION In conclusion, we have demonstrated a promising strategy for creating nanostructures from a homopolymer PVDF and a small molecule, an unsaturated IL. This process formed a nanostructured PVDF composite that had organic conductive nanodomains, with diameters of 20−30 nm, composed of PVDF-g-IL segments. Such nanostructured PVDF composites were generated by a microphase separation of PVDF-g-IL chains, formed by irradiating PVDF with IL, from the PVDF matrix due to the reduced entropy of mixing the chains of PVDF grafted IL (PVDF-g-IL chains) with the ungrafted PVDF chains. It was also found that a microphase separation in morphology occurred only when the grafted IL content exceeded 3 wt %. The formed nanodomains were able to nucleate the crystallization of the matrix PVDF. The nanostructured PVDF composites showed dominant nonpolar αphase of PVDF crystals and increased crystal long period compared with neat PVDF. Additionally, the resulting nanostructured PVDF composite exhibited enhanced electrical properties, better mechanical properties, and improved dielectric performance compared with neat PVDF. Our synthesis strategy will open up opportunities to create many new kinds of nanostructured polymer composites.

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ASSOCIATED CONTENT

S Supporting Information *

Size distribution of nanodomains in the nanostructured PVDF composite, morphology of the PVDF/IL blends before and after irradiation, extraction results, NMR spectra of the PVDFg-IL, and the DSC curves of the irradiated PVDF/IL blends (Figures S1−S8 and Tables S1 and S2). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05349. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel.: 86-571-2886-7206. Fax: 86571-2886-7899. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was primarily supported by the National Natural Science Foundation of China (51173036, 21374027) and Program for New Century Excellent Talents in University (NCET-13-0762). The authors thank Prof. G. Chen in Beijing Chemical Technology University for helping the dielectric property measurements.

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