Immobilization of Ionic Liquids onto the Poly(vinylidene fluoride) by

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Immobilization of Ionic Liquids onto the Poly(vinylidene fluoride) by Electron Beam Irradiation Chenyang Xing,†,‡,§ Yanyuan Wang,‡ Cong Zhang,† Linfan Li,† Yongjin Li,*,‡ and Jingye Li*,† †

CAS Center for Excellence on TMSR Energy System, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No. 2019, Jialuo Road, Jiading District, Shanghai 201800, People’s Republic of China ‡ College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Road, Hangzhou 310036, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: An unsaturated room-temperature ionic liquid (IL), 1-vinyl-3-butylimidazolium chloride [VBIM][Cl], has been grafted onto poly(vinylidene fluoride) (PVDF) by electron beam irradiation at room temperature. The structure and physical properties of IL grafted PVDF (PVDF-g-IL) were investigated. Both the extraction experiments and 1H NMR results indicated the successful grafting of IL onto PVDF molecular chains. It was calculated that IL grafting yield was approximately 3.9 per 100 repeating units of PVDF, suggesting very short IL grafting sequences. The melting temperatures (Tm) of the PVDF-g-IL films decreased with absorbed dose, suggesting the occurrence of crystal defects of PVDF caused by the irradiation. However, the morphologies, crystal forms and crystal long periods (L) of PVDF-g-IL films were not significantly influenced by the irradiation. Moreover, the effects of IL grafting and absorbed dose on physical properties of PVDF-g-IL films were investigated. It was found that the irradiation could immobilize IL molecules onto PVDF chains and thus suppressed their migration in electric filed. Therefore, the grafted samples showed lower dielectric loss, electrical conductivity as well as dielectric permittivity compared with the unirradiated blends. Moreover, the elongation at break of the grafted PVDF decreased with the applied irradiation dose, but the Young’s modulus increased. The as-prepared PVDF-g-IL composites exhibited large dielectric permittivity, low dielectric loss and, in particular, excellent toughness, which is promising for use in dielectric capacitor applications. CNTs).17 It was found that the simple melt mixing of IL with PVDF led to intriguing PVDF/IL blends with simultaneously toughened, optically transparent and antistatic physical properties.16 We attributed such intriguing performances to the good miscibility between PVDF and ILs.16,17 Because of high ionic conductivity23−31 and desirable dielectric constants32,33 of ILs, one can expect that PVDF composites incorporated with IL may display enhanced dielectric performance compared with neat PVDF. However, IL molecules in the physically mixed PVDF/IL system are able to migrate freely under electric field, inevitably causing considerable dielectric loss. Clearly, high dielectric loss will consume massive energy stored in dielectric materials in the form of heat energy, which is detrimental for the dielectric devices. In this work, to reduce dielectric loss of PVDF/IL composites in electric filed, an unsaturated IL was chemically grafted onto PVDF chains by electron-beam irradiation and the immobilization of IL was realized in such IL-grafted PVDF dielectric composites (PVDF-g-IL). The effects of IL loading and absorbed dose during irradiation on morphologies, thermal properties, crystal forms and crystal long period of PVDF were investigated. Moreover, the physical properties of PVDF-g-IL films, including electrical properties, dielectric properties and

1. INTRODUCTION Poly(vinylidene fluoride) (PVDF) has gained extensive interest in fabricating flexible polymeric dielectric materials because of its unique molecular structures.1,2 It not only offers superior heat, chemical stabilities, abrasion resistances and mechanical properties but also exhibits relatively high dielectric permittivity due to its polar molecular chain structures. PVDF has also good processability, which makes it an excellent candidate for fabricating flexible dielectric devices with small, robust, thin and light size characteristics. To further enhance the dielectric permittivity, PVDF has been integrated with inorganic nanofillers with high dielectric permittivity, such as ceramics, conductive graphene, carbon nanotubes (CNTs), and metal nanoparticles, etc.3−15 Large volume fractions of nanofillers are frequently required to enhance permittivity of PVDF and this, in turn, causes their poor dispersion within the PVDF matrix due to intrinsic poor compatibility of PVDF with these inorganic fillers. The aggregation of the nanofillers leads to not only the deterioration of the mechanical properties and processability but also the leakage current under electric field, thus resulting in large dielectric loss of the composites. On the other hand, room-temperature ionic liquids (ILs) have been widely used as the modifiers of many polymers by simply compounding due to their low volatility, high chemical stability, high thermal stability, excellent ionic conductance, in particular its ease of handing.16−22 Recently, we have reported the functional modification of PVDF by limidazolium-based ionic liquid (IL)16 and IL-modified carbon nanotubes (IL© 2015 American Chemical Society

Received: Revised: Accepted: Published: 9351

July 31, 2015 September 5, 2015 September 9, 2015 September 16, 2015 DOI: 10.1021/acs.iecr.5b02819 Ind. Eng. Chem. Res. 2015, 54, 9351−9359

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Industrial & Engineering Chemistry Research

The electric properties of samples were evaluated by measuring their surface resistivity (Rs) and volume resistivity (Rv). An ultrahigh-resistivity meter (MCP-HT450) with a ring URS electrode was used. The direct-current voltage and thickness of samples were 10 V and 300 μm, respectively. The dielectric properties of samples were measured by employing Agilent 4294A with 16451B fixture at room temperature and the frequency used was ranging from 40 Hz to 110 MHz. At least five locations of each sample for the two measurements were measured and an average value was reported. The strain−stress curves of samples were obtained by using an instron universal material testing system (Instron, 5966). The dumbbell-shaped samples were stretched with a speed of 10 mm/min at room temperature and at least three samples were tested.

mechanical properties, were also investigated. It was found that the simple electron irradiation at low dosage (45kGy) could effectively chemically graft the cations with double bonds onto PVDF molecular chains and that the as-prepared PVDF-g-IL composites exhibited large dielectric permittivity, low dielectric loss and, in particular, excellent toughness.

2. EXPERIMENTAL SECTION 2.1. Materials. PVDF pellets (KF850) with a Mw of 2.09 × 105 and a MW/Mn of 2.0 were purchased from Kureha Chemicals in Japan. The used unsaturated ionic liquid (IL) is [VBIM][Cl], 1-vinyl-3-butylimidazolium chloride. The solvents such as dimethylformamide (DMF) and methanol (CH3OH) were commercially available and used without any further purification. 2.2. Preparation of Physically Mixed PVDF/IL Blend Films. As reported in our previous study,16 blends, based on PVDF and IL, were first fabricated by melt-compounding at 190 °C in a Haake mixer equipped with a twin roller-rotor (R600). The PVDF/IL blends with 1, 3, 5 and 8 wt % IL loadings were thus prepared. The corresponding PVDF/IL blend films with 300 μm thickness were obtained by a continuous hot-press (10 MPa, 210 °C, 10 min) process and a subsequent quench process with running water (10 MPa, 2 min). 2.3. Preparation of PVDF-g-IL Films by Electron Beam Irradiation. To immobilize the ILs within the PVDF matrix, the above PVDF/IL films were directly exposed upon electron beam at various absorbed doses such as 45, 60, 100, 200, 400, 600, 800 and 1000 kGy at room temperature in air. The PVDFg-IL films were thus obtained and were further characterized in the following procedures. 2.4. Extraction of PVDF-g-IL Films in CH3OH. To determine the extent of the IL grafting within the PVDF matrix under different irradiation conditions, extraction experiments using Soxhlet extractor were conducted in CH3OH. PVDF-g-IL films as well as PVDF/IL blend films were continuously extracted at least for 128 h. 2.5. Characterizations. Field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) was used to investigate the morphologies of samples. An accelerating voltage of 3.0 kV was employed. Before observation, fracture surfaces of samples were coated with a thin gold layer. 1 H nuclear magnetic resonance (NMR, Bruker Avance 500 spectrometer) was performed at room temperature with a frequency of 500 MHz and deuterated DMF as a solvent. The PVDF crystal melting behaviors of samples were characterized by using differential scanning calorimeter (DSC, TA-Q2000). Measured samples with about 10−15 mg were heated from −90 to +210 °C with a heating scanning rate of 10 °C/min under a continuous N2 atmosphere and the first heating process was recorded. The PVDF crystal form and crystal long period of samples were determined by using a wide-angle X-ray diffraction (WAXD, Bruker-D8) and a small-angle X-ray scattering (SAXS, Shanghai Synchrotron Radiation Facility, China), respectively. The WAXD patterns were recorded from 5 to 40° and the operating voltage, current and scanning speed are 40 kV, 40 mA and 1°/min, respectively. The SAXS measurements were performed by using a 1.24 Å wavelength monochromatic X-ray beam and the exposed time, sample− detector distance and pattern pixels are 200 s, 1860 mm and 2048 × 2048 pixels, respectively.

3. RESULTS AND DISCUSSION 3.1. Chemical Structures of Ionic Liquid Grafted PVDF (PVDF-g-IL) Chains. To immobilize IL molecules within the PVDF matrix, melt-mixed PVDF/IL blend films were directly exposed upon electron beam at various absorbed doses at room temperature. This is a typically simultaneous irradiation process and it can be expected that cations of IL in amorphous regions of PVDF can be chemically grafted onto amorphous PVDF chains by radical reaction. At the same time, polymerization of IL may also happen in this procedure.34 Figure 1 shows weight

Figure 1. Weight change of PVDF/IL blend films and their irradiated samples (i.e., PVDF-g-IL films) with different IL loadings (1, 3, 5 and 8 wt %) at various absorbed doses before and after extraction in CH3OH for 128 h.

change of unirradiated (physically mixed PVDF/IL blend films) and irradiated PVDF/IL blend films (i.e., PVDF-g-IL films) before and after extraction experiments in CH3OH for 128 h. Both IL monomer and IL homopolymer are able to be dissolved in CH3OH. It is clearly found that all ILs were extracted in the unirradiated PVDF/IL blend samples. The weight decrease ratios are nearly the same with feed IL ratios. It has been shown in our previous study that ILs are chemically free in PVDF matrix by a common mixing method.16 Thus, they can be readily removed by CH3OH. In contrast, for the irradiated samples with 1, 3 and 5 wt % IL loadings, no more than 0.1% of weight change is observed at as low as 45 kGy, indicating that ILs are fixed in the PVDF matrix and all ILs are grafted onto the PVDF chains. Neither free (ungrafted) IL monomers nor IL homopolymers existed in these irradiated 9352

DOI: 10.1021/acs.iecr.5b02819 Ind. Eng. Chem. Res. 2015, 54, 9351−9359

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Industrial & Engineering Chemistry Research samples. However, for the sample with 8 wt % IL loading, irradiation cannot cause complete grating of ILs, regardless of absorbed dose, indicating that the 8 wt % IL is saturated for the present PVDF/IL system and that there are ungrafted IL monomers and/or their homopolymers in these samples. The saturation occurs at about 7.0 wt % IL loading calculated from the extraction experiments. The possible chemical structure of IL-grafted PVDF and its 1 H NMR spectra are shown in Figure 2, respectively. The H

NIL /N100 units of PVDF =

M unit of PVDF × Wgrafted IL MIL × (1 − Xc)

(1)

where Munit of PVDF and MIL are molecular weights of repeating unit CH2−CF2 (64 g/mol) and IL [VBIM][Cl] (186.5 g/mol), respectively; Wgrafted IL is IL grafted weight in the PVDF-g-IL films at different absorbed doses obtained from Figure 1; Xc is degree of crystallinity of unirradiated PVDF/IL blends obtained by the following equation: Xc (100%) =

ΔHm,1st (1 − WIL) × ΔHm0

× 100% (2)

where ΔHm,first is melting enthalpy of unirradiated PVDF/IL blends in the first heating run; WIL is IL content in PVDF/IL blends; ΔH0m is 104.9 J/g for totally crystalline PVDF crystals. It is calculated that, for the PVDF-g-IL film with 8 wt % IL loading at 45 kGy dose, every 100 CH2−CF2 units of PVDF may contain 3.8 IL molecules in the PVDF-g-IL chains. The grafting yield is slightly increased to a maximum of 3.9 at 100 kGy dose, as shown in Figure 4. The fact that the NIL/ N100 units of PVDF value is quite small suggests the very short IL grafting sequences in the final grafted materials. In other words, each grafting point may contain only one IL molecule. 3.2. Morphologies and Crystallization Behaviors of PVDF/IL and PVDF-g-IL Films. Morphologies. Figure 5 shows the morphologies of unirradiated PVDF/IL blend (8 wt % IL loading) and its irradiated sample at 1000 kGy. A homogeneous phase was found in the PVDF/IL (8 wt % IL), as is shown in Figure 5 A, indicating that the IL used is thermodynamically miscible with the PVDF matrix. The morphology of irradiated sample in Figure 5B was almost same with that of the unirradiated sample (Figure 5A). This suggests the morphology of the PVDF/IL blend film was not influenced by irradiation. In fact, PVDF chains can undergo some possible chemical reactions under irradiation, such as dehydrofluorination, cross-linking, formation of unsaturated bonds and hydroperoxides and main chains scission, etc.;39−41 however, these chemical behaviors cannot significantly affect the phase morphologies of PVDF. On the other hand, as mentioned above, the IL grafting reaction is considered only to occur in the amorphous regions of PVDF at the solid state at room temperature. Such solid state reaction does not induce any micro- or macro- phase separation. Thermal Properties. The thermal behaviors of neat PVDF, PVDF/IL blends and their irradiated samples under different absorbed doses were investigated by using DSC measurements (the detailed DSC curves are shown in Figure S2). The melting temperatures (Tm) of the irradiated samples as a function of absorbed dose is displayed in Figure 6. Neat PVDF samples exhibited a depression of Tm with absorbed dose. This indicates that PVDF crystals had gotten defects under irradiation. It is reported that PVDF crystals can be damaged within their lamellae by irradiation and that these damages may occur not from the surfaces of the crystals but from their cores.42,43 Compared with neat PVDF, a slight increase in Tm was observed in the PVDF/IL blends with 1 and 3 wt % IL loadings and this can be attributed to the formation of γ-PVDF crystals, which usually has a higher Tm than nonpolar α crystals dominant in the neat PVDF (as shown in next section).16,44 Further increases of IL loadings (i.e., 5 and 8 wt %) decreased the Tm of PVDF crystals due to the miscibility of IL with PVDF, which is consistent with our previous study.16,27 Similar

Figure 2. Chemical structure of IL-grafted PVDF (PVDF-g-IL) sample and 1H NMR spectra of neat PVDF and PVDF-g-IL film with 8 wt % IL loading at 45 kGy after extraction in CH3OH, respectively. Note that both ungrafted IL monomers and their homopolymers in the extracted sample were completely removed by CH3OH.

chemical shifts from deuterated DMF solvent as well as CH2 groups of PVDF are shown in Figure S1. The fact that the clear appearance of peaks at 1.10−0.81 (b) and 1.51−1.30 (c) ppm, corresponding to the H signals from −CH3 and −CH2 groups of side chain on IL’s cation in Figure 2, respectively, indicates the presence of IL onto PVDF chains. Figure 3 displays the schematic diagram for the grafting of ILs onto PVDF chains during the irradiation. Similar to our previous study,16 ILs are fully miscible with PVDF in the melt state. However, ILs are expelled out from the crystal lamellae and located in the amorphous regions of PVDF during the crystallization of PVDF upon cooling down from the melt (state II in Figure 3). When PVDF/IL blend films were irradiated at room temperature in air, IL molecules (cations with double bonds) were able to be locally (in situ) grafted onto the PVDF chains. Thus, the ILs were immobilized within the PVDF matrix. This irradiation approach to fix IL is easy and applicable, but different from other technologies to immobilize ILs.35−38 Additionally, according to the above extraction results (in Figure 1), IL molecules can be completely grafted onto PVDF chains in irradiated samples with IL loadings of 1, 3 and 5 wt % IL. However, with 8 wt % IL, excess IL monomers and/ or their homopolymers are also presented in the amorphous region (state III in Figure 3). The average grafting yield can be calculated by the following equation, assuming that the grafting occurs only in the amorphous region: 9353

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Figure 3. Schematic illustrating the formation of PVDF-g-IL chains. Three typical states were present in the entire procedure. State I was the crystalline structure of neat PVDF containing amorphous regions, crystalline regions and their interfaces. Neat PVDF was first melt-blended with ILs to obtain physically mixed PVDF/IL blends, wherein the IL molecules were homogeneously dispersed in the amorphous regions of PVDF (State II). When these films were irradiated by electron beam at room temperature in air, a third state, III, with IL-grafted PVDF chains (PVDF-g-IL chains) in the amorphous region was obtained. In more detail, cations of ILs with CC bonds were grafted onto the amorphous PVDF chains whereas their counterions Cl− were still at a free state to surround them. Note that free IL monomers and/or their homopolymers were still present in state III (IL homopolymers were not shown here for simplification) in the samples with 8 wt % IL loading.

Figure 5. Scanning electron microscopy (SEM) images showing the fractured cross sections of (A) PVDF/IL blend with 8 wt % IL loading and (B) the corresponding irradiated PVDF/IL sample at 1000 kGy. Figure 4. NIL/N100 units of PVDF as a function of Wgrafted IL for the PVDFg-IL film with 8% IL loading. NIL/N100 units of PVDF denotes the ratio of IL molecules grafted to 100 CH2−CF2 units in the PVDF-g-IL chains. Wgrafted IL is IL grafted weight in PVDF-g-IL films at different absorbed doses, obtained from Figure 1.

decreases of Tm shown in Figure 6 with absorbed dose were also found in the irradiated samples, particularly at absorbed doses above 100 kGy, which is attributed to the PVDF crystal defects during electron beam irradiation. Additionally, crystal defects in the irradiated samples with higher absorbed doses led to a tendency toward reduction of degree of crystallinity of PVDF samples, as shown in Figure S3. Crystal Forms. As shown in Figure 7, neat PVDF exhibits typical nonpolar α form crystals, as evidenced by the diffraction peaks of 17.69, 18.4, 19.9 and 26.6° corresponding to α (100), α (020), α (110) and α (021) planes, respectively.44,45 It should be noted that such reflections of α crystal as α (020), α (110) and α (021) planes may overlap closely with γ (020), γ (110)

Figure 6. Melting temperatures (Tm) of samples in the first heating run as a function of absorbed dose.

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Figure 8. PVDF crystal long periods (L) of samples, obtained by using the equation L = 2π/q, as a function of absorbed dose.

increased L value. This can be attributed to the integration of ILs into the gallery of neighboring PVDF lamellae.16 On the other hand, the L values were not significantly influenced by irradiation for all the samples. In fact, the solid irradiation does not influence the crystal morphology of PVDF/IL blends, so the crystal lamellar structure keeps well in the all absorbed doses used in this work. This finding is consistent with previous investigations reported by other authors.43 3.3. Physical Properties of PVDF-g-IL Films. Electrical Conductivity. Figure 9 displays electrical conductivity of both

Figure 7. WAXD patterns of irradiated neat PVDF and PVDF-g-IL films with 5 and 8 wt % IL loadings under various absorbed doses.

and γ (022) planes, respectively.16,44,45 Therefore, much attention should be paid on the identification of neat PVDF crystal forms. According to our previous study,16 the neat PVDF shows dominant α crystal coexisted with very small amount of polar β crystal and without γ crystals. After irradiation, no clear changes in crystal forms were found in irradiated neat PVDF sample, as shown in Figure 7. This result suggested that crystal forms in neat PVDF were not significantly influenced by irradiation. A similar finding was also observed in the PVDF-g-IL samples. The PVDF/IL blends with 5 and 8 wt % IL loadings showed dominant γ crystals coexisted with α crystals, evidenced by the intensity decrease of α crystal characteristic diffraction at 17.69°and the clear presence of diffraction peaks at 18.4, 19.9 and 26.6°. Additionally, a weak shoulder at 20.3° was observed in PVDF/IL blends, indicating the presence of a small amount of β crystals. This result is in good agreement with our previous study.16 The crystals forms of PVDF-g-IL samples were also not influenced by irradiation, regardless of absorbed dose, as is shown in Figure 7. This is rational because the chemical grafting at the amorphous region does not induce any rearrangement of the molecular chain in the crystalline part. It is well-known that PVDF chains have at least three different conformations leading to different crystal forms and that a low-energy conformation (TGTG) can be transformed into high-energy ones (TTTT or TTTG) by external forces. The reported external forces are involved with mechanical drawing at a high temperature,46 ultrasonication47 and electric drawing in solutions,27 static interaction16 and template functions of inorganic nanofillers with large aspect ratio in melts,17 etc. It is noted that such crystal conformation transitions usually occur either at a high temperature (frequently in melts) or in solutions. However, the irradiation procedure for the PVDF samples was performed at room temperature at the solid state and the existing PVDF crystals cannot readily change their conformations due to large packing density of chains in crystalline region, although irradiation can really cause imperfectness of crystals. Crystal Long Periods. The PVDF crystal long periods (L) of irradiated neat PVDF and PVDF-g-IL samples with 5 and 8 wt % IL loadings as a function of absorbed dose are displayed in Figure 8 (the corresponding SAXS profiles are shown in Figure S4). It is clear that incorporation of ILs into PVDF led to an

Figure 9. Electrical properties of PVDF/IL blend films and PVDF-g-IL films with 1, 3, 5 and 8 wt % IL loadings: logarithmic surface resistivity (log Rs) vs absorbed dose.

PVDF/IL and PVDF-g-IL samples. For the samples without irradiation, the order of magnitudes of surface resistivity (Rs) was found to decrease with increasing IL contents from 1 to 5 wt %, indicating the improvement of electrical properties of PVDF samples. This electricity increase phenomenon was ascribed to the high ion conductivity of IL.16 The grafting of IL onto PVDF leads to drastic increase in Rs. In Figure 9, an increase of 2 orders of magnitude of Rs for the PVDF-g-IL samples with 1, 3 and 5 wt % IL loadings at 45 kGy was observed, compared with the same sample without irradiation. Moreover, the values of log(Rs) keep almost constant with increasing absorbed dose. This results are in agreement with the above extraction results shown in Figure 1 and the grafting of IL onto PVDF chains by irradiation leads to the decrease of ion motions for ILs, resulting in the decreased conductivity. Obviously, in the all grafted samples, the motions of cations of ILs are highly constrained while the anions are still free in these samples. The ions transport in the PVDF-g-IL films may be dominated by the Grotthuss mechanism.48 Free anions can leap 9355

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Figure 10. Frequency dependence of dielectric permittivity and dielectric loss of (a and b) neat PVDF and PVDF/IL blends with 1, 3, 5 and 8 wt % IL loadings; (c and d) irradiated PVDF/IL samples (i.e., PVDF-g-IL films) with 5 wt % IL loading; (e and f) irradiated PVDF/IL samples with 8 wt % IL loading, respectively.

to exchange their displacement with the aid of the fixed cations sites and this forms conductive pathways. Dielectric Properties. The dielectric permittivity and loss of PVDF/IL (a and b) and irradiated PVDF/IL (c, d, e and f) samples are presented in Figure 10. From Figure 10a, it can be clearly observed that the dielectric permittivity of PVDF gradually increased with the IL content and sharply decreased with the frequency. The dielectric permittivity increment behavior is due to the IL with high dielectric constant and high ion conductivity leading to enhanced average electric filed as well as conductivity for PVDF matrix. However, free IL molecules movement/motion under electric filed definitely leads to serious leakage current, resulting in an undesirable increase in dielectric loss as shown in Figure 10b. With IL contents over 5 wt %, the loss value is very large that is detrimental for practical application of PVDF/IL dielectric materials. We expected that the chemical immobilization of ILs leads to significantly decrease in the dielectric loss. As shown in Figure 10d,f, the dielectric loss values of irradiated samples with 5 and 10 wt % IL contents were effectively reduced at a dosage of 45 kGy compared with that of samples unirradiated. For the sample with 8 wt % IL at 45 kGy, loss reductions of 59% at 102 Hz and 38% at 103 Hz were obtained when compared with the unirradiated PVDF/IL samples. This means that the strategy to

immobilize IL onto PVDF chains in order to reduce the loss in the PVDF/IL system is applicable and feasible. The reduction of IL molecules motions by the grafting of IL could minish leakage current of IL considerably and constrain the motion of the ions under electric field, and thus decreased the dielectric loss of PVDF/IL samples greatly. It should be noted that the decreased dielectric loss was also accompanied by the decreased dielectric permittivity due to the reduced conductivity, as shown in Figure 10c,e. In spite of this, the as-prepared PVDF-gIL samples still exhibited enhanced dielectric permittivity compared with neat PVDF. The dielectric permittivity of sample with 8 wt % IL at 45 kGy, for example, was increased by 371% at 102 Hz and 171% at 103 Hz, respectively, compared with neat PVDF. Mechanical Properties. Figure 11 shows mechanical properties of PVDF/IL blend and PVDF-g-IL films with 5 wt % IL loading under various absorbed doses. The PVDF/IL blend film with 5 wt % IL exhibited excellent ductility with an elongation at break of as high as 472%, as shown in Figure 11a. This behavior is largely attributed to the plasticizing effect of IL in the PVDF amorphous regions. After irradiation, PVDF-g-IL films showed decreased values of elongation at break. For instance, at lower absorbed doses such as 45, 100 and 200 kGy, PVDF-g-IL films showed 231, 204 and 83% of elongation at 9356

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extraction experiments and 1H NMR, IL was confirmed to be fully grafted onto PVDF amorphous chains at IL loadings of 1− 5 wt % and the saturation occurred at the IL loading of about 7 wt %. The IL grafting length was rather short with a maximum value of 3.9 of the grafted IL molecule numbers per 100 repeating PVDF units in PVDF-g-IL films. The simple irradiation at various absorbed doses did not significantly affect the morphologies, crystal forms and crystal long periods for the PVDF-g-IL samples compared with that of samples unirradiated. However, a decrease of melting temperatures (Tm) was observed with absorbed dose in all the irradiated samples; this has been attributed to the formed defects in PVDF crystals caused by irradiation. The IL grafting onto PVDF chains showed significant effects on the electrical and dielectric properties because it resulted in decreases in electrical conductivity, dielectric permittivity, and, in particular, dielectric loss due to the restricted IL motions in electric field. In addition, irradiation also significantly affected mechanical properties of PVDF-g-IL films. Compared to unirradiated PVDF/IL sample, irradiated samples exhibited decreased elongation at break and increased Young’s modulus. However, the PVDF-g-IL films at lower absorbed doses still exhibited excellent flexibility, which is very important for dielectric capacitors. Moreover, the yield strength of irradiated samples at lower absorbed doses was not much different from the unirradiated sample. The resulting PVDF-g-IL composites with high dielectric permittivity, low loss and excellent toughness are promising for use in dielectric capacitor applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02819. NMR spectra, DSC first heating curves, degree of crystallinity of PVDF samples, SAXS patterns and photos of dissolution in DMF of samples (PDF).

Figure 11. Mechanical properties of PVDF/IL blend and PVDF-g-IL films with 5 wt % IL loading under various absorbed doses: strain− stress curves (a and b); Young’s modulus, elongation at break as well as yield strength as a function of absorbed dose (c).

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break, respectively. With increasing absorbed dose, a further decrease in elongation at break was found, in particular at higher dose, as shown in Figure 11b,c. Two reasons can be responsible for this phenomenon. On the one hand, the grafting of IL on PVDF chains largely decreases plasticizing effect of IL. On the other hand, cross-linking of the PVDF chains occurs at 200 kGy (shown in Figure S5) and the motion of PVDF chains is thus restricted, resulting in a further decrease in elongation at break. It should be noted that the PVDF-g-IL films at lower absorbed doses still exhibit excellent flexibility, which is of great importance for dielectric capacitors.1 Additionally, because of cross-linking of PVDF chains, an increased Young’s modulus to some extent of PVDF-g-IL films was observed compared with the unirradiated sample, as shown in Figure 11c. It should be noted that both cross-linking and main chains scission happened simultaneously in the whole irradiation. Additionally, as shown in Figure 11c, the yield strength of PVDF-g-IL films was not significantly changed compared with that of unirradiated sample.

AUTHOR INFORMATION

Corresponding Authors

*Y. Li. E-mail: [email protected]. *J. Li. E-mail: [email protected]. Tel: +86-571-2886-7206. Fax: +86-571-2886-7899. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51173036, 11175234, 21374027) and Program for New Century Excellent Talents in University (NCET-13-0762). The authors thank Prof. G. Chen and Mr. X. Su in Beijing Chemical Technology University for helping the dielectric property measurements.

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REFERENCES

(1) Dang, Z.; Yuan, J.; Yao, S.; Liao, R. Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage. Adv. Mater. 2013, 25, 6334−6365. (2) Zhu, L.; Wang, Q. Novel Ferroelectric Polymers for High Energy Density and Low Loss Dielectrics. Macromolecules 2012, 45, 2937− 2954.

4. CONCLUSIONS In this work, we reported immobilization of an unsaturated IL within the PVDF matrix via electron beam irradiation method, and the structures and properties of such IL-grafted PVDF dielectric composites were also investigated. From the results of 9357

DOI: 10.1021/acs.iecr.5b02819 Ind. Eng. Chem. Res. 2015, 54, 9351−9359

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DOI: 10.1021/acs.iecr.5b02819 Ind. Eng. Chem. Res. 2015, 54, 9351−9359