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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Crystal Forms and Microphase Structures of Poly(vinylidene fluoride-co-hexafluoropropylene) Physically and Chemically Incorporated with Ionic Liquids Jipeng Guan,†,‡ Jieqing Shen,† Xingru Chen,† Hengti Wang,† Qin Chen,† Jingye Li,‡ and Yongjin Li*,† †

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College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou 310036, People’s Republic of China ‡ Shanghai Institute of Applied Physics, , Chinese Academy of Sciences, No. 2019, Jialuo Road, Jiading District, Shanghai 201800, People’s Republic of China S Supporting Information *

ABSTRACT: The effects of incorporating two imidazoliumtype ionic liquids (ILs), 1-vinyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (IL-TFSI) and 1-vinyl-3-butylimidazolium chloride (IL-Cl), on the crystal forms and phase microstructures of poly(vinylidene fluoride-co-hexafluoropropylene), P(VDF-co-HFP), were investigated. No change in the crystal form was observed with the incorporation of ILTFSI because of the selective interaction between the IL and HFP moieties of the P(VDF-co-HFP) molecular chains. In contrast, the incorporation of IL-Cl induces an α-to-β crystal form transition because of the interactions between IL-Cl and the VDF moieties. Furthermore, the incorporation of IL-TFSI led to distinct phase separation of P(VDF-co-HFP), whereas a homogeneous phase structure was maintained with the incorporation of IL-Cl. Element mapping images reveal that the specific interactions of IL-TFSI with the HFP moieties induce enrichment of the HFP moieties, which results in well-defined HFP/IL-TFSI nanodomains in the P(VDF-co-HFP) matrix. In contrast, a homogeneous morphology was observed in the P(VDF-co-HFP)/IL-Cl blends. The smaller Cl− anion of IL-Cl can interact with VDF moieties and form a thermodynamically miscible state with the P(VDF-co-HFP) matrix. In addition, ILs chemically grafted on P(VDF-co-HFP) were in situ synthesized using electron-beam irradiation of the blends of P(VDF-co-HFP) with ILs at room temperature because both ILs contain double bonds. The phase structures of the IL-grafted copolymers were further studied. After chemical grafting and melting, the original microphase-separated structure of the P(VDF-co-HFP)/IL-TFSI blends was maintained, whereas discrete nanodomains with diameters of 10−20 nm were formed for the original homogeneous P(VDF-coHFP)/IL-Cl blends. The interesting crystal forms and phase structures of P(VDF-co-HFP) physically or chemically incorporated with different ILs demonstrate the effectiveness of this strategy for the design of polymer microstructures using simple blending and radiation-induced grafting processes.

1. INTRODUCTION Polymer materials with well-defined nanostructures are desirable for various applications, such as polymer electrolytes,1−4 proton exchange fuel cells,5−11 and filtration devices.12 A common route to prepare these nanostructured materials involves block copolymers, which can self-assemble into many ordered morphologies such as hexagonally packed cylinders (HEX), lamellae (LAM), or a cocontinuous gyroid.13−19 In addition, inorganic nanofillers such as nanosilica, layered silicates, and carbon nanomaterials are also introduced to further develop the polymer nanocomposites and endow the polymer materials with unique properties.20−25 However, the block copolymers are mainly synthesized using living polymerization techniques, which require strict reaction conditions that restrict their commercial applications.26−29 In addition, the surface modification needed to disperse nanofillers in a polymer matrix increases the difficulty of the © XXXX American Chemical Society

preparation process. There is thus an urgent demand to develop simple and efficient strategies for microstructural design of polymer materials. Very recently, Duchet−Rumeau et al. reported the preparation of nanostructured polytetrafluoroethylene (PTFE) materials by simple physical incorporation of ionic liquids (ILs) into PTFE by tuning the cation−anion interactions and interactions between the ILs and PTFE molecular chains.30−33 They further investigated the morphologies of the physical blends of thermoplastic P(VDF-co-CTFE) and phosphonium-based ILs.30 They found that the chemical structure of the ILs plays a key factor in determining the final structure of the polymer materials. The IL octadecyltriphenylReceived: September 28, 2018 Revised: November 14, 2018

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DOI: 10.1021/acs.macromol.8b02087 Macromolecules XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION

phosphonium iodide (IL-C18) has a lower migration rate and weaker dipolar interaction with the polymer matrix than tributyl(methyl)phosphonium methylsulfate (IL-108); therefore, IL-C18 was located in the rigid amorphous fraction and formed 1D and 2D nanostructures. We also recently prepared well-microphase-separated IL-grafted poly(vinylidene fluoride) (PVDF-g-IL) materials with improved physical and dielectric properties.34−41 The imidazolium-type ILs containing double bonds were thermodynamically miscible with PVDF and homogeneously dispersed in the amorphous region of PVDF. The block grafting copolymers were fabricated by in-situ grafting of the IL molecules onto the PVDF molecular chains using electron-beam (EB) irradiation at room temperature. Various nanostructures including spherical, cylinder, and lamellar can be obtained by subsequent melting processes.35−37 These results can be attributed to the Coulombic interactions of ions, which induce the formation of ionic aggregations from the PVDF-g-IL. In this work, we systematically investigated the crystal forms and microstructures of P(VDF-co-HFP) physically and chemically incorporated with ILs. The copolymerization of hexafluoropropylene (HFP) monomers with vinylidene fluoride (VDF) leads to irregularity of the PVDF molecular chains as well as different electron distributions along the molecular chains. Two imidazolium-type ILs with the same cations but different anions, 1-vinyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (IL-TFSI) and 1-vinyl-3-butylimidazolium chloride (IL-Cl), were physically blended with the P(VDF-co-HFP) matrix to prepare two series of blends with different IL loadings (Scheme 1). IL-TFSI has a big anion

2.1. Materials. The P(VDF-co-HFP) copolymer was purchased from Sigma-Aldrich with a Mw of ∼400000 and Mn of 130000, VDF/ HFP (96/4, mol/mol). The unsaturated ILs, 1-vinyl-3-butylimidazolium chloride ([VBIm] [Cl]) and 1-vinyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide ([VBIm] [TFSI]), were produced by Lanzhou Greenchem and are denoted as IL-Cl and IL-TFSI, respectively. Solvents, including acetone and methanol (CH3OH), were purchased from Sinopharm Chemical Reagent Co. Ltd. and were used without any further purification. 2.2. Sample Preparation. 2.2.1. Preparation of P(VDF-co-HFP)/ IL Physical Blends. First, 10 wt % of P(VDF-co-HFP) solution in acetone was prepared at 60 °C. Different amounts of ILs (IL-Cl or ILTFSI) were added to the polymer solution to prepare the blends. The ratio of IL to P(VDF-co-HFP) was 0.6, 1.0, 3.0, 5.0, and 10 wt %. After stirring at 60 °C for 3 h, the P(VDF-co-HFP)/IL blends were obtained by solution casting on an aluminum substrate at room temperature for 24 h, followed by drying at 80 °C in an air circulation oven for 3 h to remove the residual solvent. To obtain the blend films with 300 μm thickness, the casting films were then hot-pressed at 200 °C at 10 MPa for 10 min and quenched with running water at 10 MPa for 2 min. To clearly identify the samples, the abbreviation P(VDF-coHFP)/IL-TFSI (100/10) was used, which indicates that the mass ratio of IL-TFSI to P(VDF-co-HFP) was 100:10. 2.2.2. Chemically Grafting of Ionic Liquids on the P(VDF-co-HFP) Polymer Chains. To initiate the grafting reaction between the unsaturated ILs and P(VDF-co-HFP) polymer chains, the 1.2 MeV electron beam with an absorbed dose of 45 kGy was used where the dose rate was 23800 kGy/h. Before irradiation, the physically blended films were placed into plastic bags with air at room temperature. After EB irradiation, the samples were extracted with methanol at 95 °C for 24 h to remove the ungrafted monomers and IL homopolymers because methanol is a good solvent for both ILs and IL homopolymers. The grafting efficiency (GE) was calculated using the equation

Scheme 1. Molecular Structures of Two Imidazolium-Type ILs and Fluorine Copolymer in This Study

grafting efficiency (GE) =

m 2 − m0 × 100% m1 − m0

where m0 is the weight of the polymer matrix in the sample, m1 is the weight of the blend, and m2 is the weight of the irradiated sample after extraction with methanol for 24 h. The obtained chemically grafting copolymers were hot-pressed at 200 °C at 10 MPa for 30 min to ensure the formation of nanostructure morphologies in the blend films and quenched with running water at 10 MPa for 2 min. Finally, the microphase-separated blend films were obtained. 2.3. Characterizations. Fourier-transform infrared (FTIR) spectroscopy (Bruker Tensor) was performed to detect the transformation of crystals with a resolution of 2 cm−1 in reflection mode. Differential scanning calorimetry (DSC, TA, Q2000) was applied to detect the thermal behaviors of the samples. A heating/cooling rate of 10 °C/ min was used, and the temperature range was between −30 and 195 °C. Dynamic thermomechanical analysis (DMA, TA Instruments, Q800) was used to characterize the samples in tensile mode in the linear region with 0.05% strain. The dynamic loss (tan δ) was characterized with a frequency of 5 Hz and a heating rate of 3 °C/min as a function of temperature from −70 to 200 °C. Wide-angle X-ray diffraction (WAXD, Bruker D8) was performed to determine the crystal forms in the 2θ range from 5° to 45° with a scanning speed of 2°/min. The nanostructure morphologies of the samples were directly observed using transmission electron microscopy (TEM, Hitachi HT7700) at an acceleration voltage of 80 kV. The samples were ultramicrotomed at −120 °C to obtain sections with thicknesses of approximately 70−100 nm. The samples were then stained with ruthenium tetroxide (RuO4) for 3 h. Elemental mapping images were obtained using a Talos TM F200A with an acceleration voltage of 200 kV. In addition, 19F nuclear magnetic resonance (NMR) measurements (Varian 500 MHz spectrometer) were performed at 23 °C using acetone as the solvent; the chemical internal shifts were

while IL-Cl contains a small anion. It is well-known that the ion−polymer interactions are usually dependent on the radius of the ions. It is therefore expected that various microstructure morphologies were obtained in the polymer blend because of the different specific interactions between the ILs and P(VDFco-HFP) matrix. The physical blends were further irradiated by an EB to induce in-situ IL grafting onto the P(VDF-co-HFP) matrix. Well-defined nanostructures were obtained through phase separation in the melting state of the ILs chemically grafted P(VDF-co-HFP). To the best of our knowledge, this is the first report on the various crystal forms and microphase structures of fluorinated polymers achieved using both physically and chemically incorporated ILs. This work presents a novel strategy for the design of both the crystal forms and microphase structures of fluorine polymers via the incorporation of ILs. B


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Figure 1. WAXD patterns of neat P(VDF-co-HFP) copolymers and P(VDF-co-HFP)/IL blends with different ILs: (A) IL-TFSI and (B) IL-Cl. FTIR spectra of neat P(VDF-co-HFP) copolymers and P(VDF-co-HFP)/IL blends with different ILs: (C) IL-TFSI and (D) IL-Cl. referenced to the VDF−VDF head-to-tail additions (CF2CH2CF2CH2CF2) at −92.4 ppm.

IL-Cl loading, as evidenced by the decreased reflection intensity of the α crystal phase in Figure 1B. These results were confirmed by the FTIR analysis. The incorporation of ILCl in the P(VDF-co-HFP) matrix successfully induced an increase in the polar β crystal phase content, as indicated by the enhanced absorption peaks at 1275 and 840 cm−1 in Figure 1D. In addition, the intensity of the absorption peaks of the α crystal phase at 1213, 976, 796, and 764 cm−1 decreased and finally disappeared at an IL-Cl loading of 10 wt %. 3.1.2. Interactions between P(VDF-co-HFP) and ILs (ILTFSI and IL-Cl). According to our previous study,34−41 the dipolar interactions between ⟩CF2 and imidazolium cationic ions can effectively induce the TTT conformation of PVDF and improve the formation of a polar crystal phase in PVDF/ IL blends.36 In this work, the PVDF-type copolymer, P(VDFco-HFP), was blended with the two ILs with different anions. The dipolar interactions between each IL and the polymer chains were investigated using FTIR spectroscopy, as shown in Figure 2. The characteristic absorption peak at 871.0 cm−1 corresponds to the −CH2−CF2− bending vibration in the amorphous regions.34 The incorporation of IL-TFSI in the P(VDF-co-HFP) matrix did not affect the peak position at 871.0 cm−1 (Figure 2A), which indicates that IL-TFSI has few effects on the molecular chain confirmation of the VDF sequence. However, an obvious blue shift was observed in the P(VDF-co-HFP)/IL-Cl blend films, as shown in Figure 2B. When the IL-Cl loading was 10 wt %, the corresponding peak moved from 871.0 to 875.3 cm−1, which can be attributed to the dipolar interactions between the imidazolium cations of ILCl and ⟩CF2. The different interaction modes of P(VDF-coHFP) with IL-TFSI and IL-Cl well explain the FTIR spectra and WAXD patterns, as shown in Figure 1. In P(VDF-coHFP), only VDF sequences reel in the crystals during the crystallization. Because IL-Cl exhibits strong interactions with VDF sequences, leading to conformation changes of the VDF

3. RESULTS 3.1. Crystal Forms and Phase Structures of the P(VDF-co-HFP) Physically Incorporated with ILs. 3.1.1. Crystal Forms of the P(VDF-co-HFP)/IL Blends. WAXD analysis was performed to determine the crystal forms in the P(VDF-co-HFP)/IL blends with different ILs, as shown in Figures 1A and 1B. Neat P(VDF-co-HFP) exhibited three characteristic diffraction peaks at 18.4°, 19.9°, and 26.6°, which are assigned to the (020), (110), and (021) reflections of the nonpolar α crystal phase, respectively.34−41 When ILTFSI was added into the P(VDF-co-HFP) matrix, the diffraction peaks of the blend did not change obviously. It is well-known that the polar γ crystal phase has similar reflections as the α crystal phase, such as the γ (020), γ (110), and γ (022), and thus, the reflections can be overlapped in WAXD spectra.34 To study the crystal form in detail, FTIR spectra of the P(VDF-co-HFP)/IL-TFSI blends were obtained, as shown in Figure 1C. In neat P(VDF-co-HFP) films, the characteristic absorption peaks of the nonpolar α crystal phase were located at 1213, 976, 796, and 764 cm−1 and are assigned in Figure 1C.34 With increasing IL-TFSI loading, the intensity of the absorption peaks corresponding to the α crystal phase remained almost constant. The absorption peak at 840 cm−1 corresponding to the polar β/γ crystal phases was very weak even with 10 wt % loading of IL-TFSI. These results indicate that the inductive effect of IL-TFSI in the P(VDF-co-HFP) matrix was weak. Notably, completely different behaviors were observed for the P(VDF-co-HFP)/IL-Cl blends, as shown in Figures 1B and 1D. In the WAXD patterns, a diffraction peak appeared at 20.6°, corresponding to the polar β crystal phase, and the reflections became stronger with increasing IL-Cl loading. In addition, the α content decreased with increasing C


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Figure 2. FTIR spectra of neat P(VDF-co-HFP) copolymers and P(VDF-co-HFP)/IL blends with different ILs and loadings: (A) ILTFSI and (B) IL-Cl.

Figure 3. DSC curves of P(VDF-co-HFP)/IL blend films with different ILs and loadings.

sequences, P(VDF-co-HFP) crystallizes into polar crystal forms when physically blended with IL-Cl. No polar crystal form was observed in the blends with IL-TFSI because of the lack of interaction between IL-TFSI and the VDF sequence. 3.1.3. Crystallization Behaviors of the P(VDF-co-HFP)/IL Blends. Figure 3 presents DSC thermograms of the P(VDF-coHFP)/IL blends. As observed in Figure 3A, the melt crystallization temperature Tc of the blends decreased with increasing IL loading. This behavior can be attributed to the interaction of the ILs with the polymer matrix, as the ILs impeded the crystallization of the polymer chains from the melt.41 When 1.0 and 10.0 wt % of IL-Cl were incorporated in the P(VDF-co-HFP) matrix, the Tc of the blends was 2.4 and 13.4 °C lower than that of the neat P(VDF-co-HFP), respectively. The strong dipolar interactions between IL-Cl and the P(VDF-co-HFP) chains and the dilution effect of IL-Cl led to a drastic decrease in Tc of the blends. Different decreasing tendencies were observed in the P(VDF-co-HFP)/ IL-TFSI blends. When 10.0 wt % of IL-TFSI was added into the P(VDF-co-HFP) matrix, the Tc was 104.4 °C, which is much higher than that of the blends with 10.0 wt % IL-Cl added (98.4 °C). This difference can also be attributed to the selective interaction between the ILs and polymer chains. As discussed in Figure 2, IL-TFSI only interacted with the HFP segments in the matrix, which have less effect on the crystallization process of the VDF segments. In contrast, the stronger dipolar interaction between IL-Cl and the VDF segments impeded the melting crystallization of the VDF segments and effectively decreased Tc. Similarly, the melting temperature of IL-Cl-incorporated P(VDF-co-HFP) was lower than that of the IL-TFSI-incorporated polymers. This finding again indicated the drastic effect of IL-Cl on the crystallization

behavior of P(VDF-co-HFP) because of the strong interaction between IL-Cl and the VDF moieties on the molecules. 3.1.4. Microstructures of P(VDF-co-HFP)/IL Blends. Figure 4 presents TEM images and elemental mapping images of the P(VDF-co-HFP)/IL physical blends. All the samples were stained with RuO4 to increase the contrast because ILs are more ready to be stained by RuO4. No phase separation occurred in the neat P(VDF-co-HFP) because of the similar chemical structures between the VDF and HFP segments and the small HFP fraction. The homogeneous morphology of the neat P(VDF-co-HFP) is clearly observed in Figure 4A. When IL-TFSI was incorporated in the P(VDF-co-HFP) matrix, an obvious dark phase was observed in the polymer matrix, indicating a microphase-separated structure. The size of the dark nanodomains slightly increased with increased incorporation of IL-TFSI into the polymer matrix (Figure S1). The domain size was approximately 10−20 nm for the blends containing 10 wt % IL-TFSI (Figure 4B). To analyze the dispersion of the ILs in depth, elemental mapping measurements of nitrogen were performed because IL-TFSI contains nitrogen. In the P(VDF-co-HFP)/IL-TFSI (10.0 wt %) blends, the nitrogen element was only detected in the nanodomains (Figure 4B1). This finding indicates that the TFSI− anions were selectively located in the nanodomains. Combining these results with the FTIR spectra (Figure 2), we propose that the IL-TFSI molecules exhibit strong interactions with the HFP moieties. When the IL-TFSI loading was 10 wt %, the ILs could form nanodomains with the HFP moieties in the polymer matrix, as further confirmed by the in-situ grafting of IL-TFSI onto the P(VDF-co-HFP) molecules in the following section. In contrast, homogeneous dispersion morphologies were observed for the P(VDF-co-HFP)/IL-Cl blends, and no phase separation was observed even with 10 wt % IL-Cl, as D


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Figure 4. TEM images of P(VDF-co-HFP)/IL blend films with different IL loadings: (A) neat P(VDF-co-HFP), (B) 10 wt % IL-TFSI, and (C) 10 wt % IL-Cl. Elemental mapping images of samples showing the signals of nitrogen and fluorine: (A1) neat P(VDF-co-HFP), (B1) 10 wt % IL-TFSI, and (C1) 10 wt % IL-Cl.

Figure 5. DMA analysis of P(VDF-co-HFP)/IL blends with different ILs and loadings: (A, B) loss tangent (tan δ) as a function of temperature with frequency of 5 Hz; (A1, B1) storage modulus as a function of temperature with frequency of 5 Hz.

HFP) were centered at −34.0 and 17.7 °C, corresponding to the motion of chain segments in the amorphous regions (Tg) and amorphous−crystal interface relaxation (T1), respectively.42−44 For the P(VDF-co-HFP)/IL-TFSI blends with 3.0 wt % IL-TFSI, only one relaxation peak was observed at 6.9 °C. The intensity of this new relaxation peak became stronger and the peak position decreased upon further increasing the IL-TFSI content. We considered that the nanodomains of IL-TFSI containing HFP dominated the relaxation spectrum in the DMA curves at large IL-TFSI contents. Different changing tendencies of the relaxation behavior were observed in the P(VDF-co-HFP)/IL-Cl blends. The relaxation at −34 °C moved to higher temperature with

shown in Figure 4C. This result indicate that IL-Cl is thermodynamically miscible with P(VDF-co-HFP) and homogeneously dispersed in the matrix at the molecular level. This behavior can be attributed to the strong interaction between IL-Cl and the major VDF sequence of the P(VDF-co-HFP) matrix. Therefore, physically incorporating ILs into the P(VDF-co-HFP) matrix leads to completely different phase structures of P(VDF-co-HFP). IL-TFSI forms nanodomains together with HFP moieties in the P(VDF-co-HFP) matrix, whereas IL-Cl forms a homogeneous state with the polymer. 3.1.5. Molecular Chain Relaxations of P(VDF-co-HFP)/IL Blends. The molecular chain relaxation of P(VDF-co-HFP) with physical incorporation of two ILs was investigated using DMA, as shown in Figure 5 with both loss tangent (tan δ) and storage modulus (E′) as a function of temperature at a frequency of 5 Hz. Two dissipation peaks of neat P(VDF-coE


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Figure 6. WAXD patterns of neat P(VDF-co-HFP) copolymers and P(VDF-co-HFP)/IL blends with different ILs after electron-beam irradiation: (A) IL-TFSI and (B)IL-Cl. FTIR spectra of neat P(VDF-co-HFP) copolymers and P(VDF-co-HFP)/IL blends with different ILs after EB irradiation: (C) IL-TFSI and (D) IL-Cl.

gradually decreased relaxation intensity with increasing IL-Cl loading. In addition, T1 decreased with increasing IL-Cl loading, as shown in Figure 5B. The homogeneously dispersed IL-Cl molecules averaged the chain segment relaxation and amorphous−crystal interface motions. Note that the storage modulus greatly decreased with the addition of IL-Cl to the polymer matrix; however, this behavior was not observed in the P(VDF-co-HFP)/IL-TFSI blends, as indicated in Figures 5A1 and 5B1. The results also indicated the different dispersion states of the two ILs in the P(VDF-co-HFP) matrix. 3.2. Crystal Forms and Microphase Structures of ILs Grafted onto P(VDF-co-HFP). An electron beam was applied to realize an in-situ grafting reaction between the unsaturated ILs and PVDF chains in the solid state at room temperature.41 The two types of ILs in this work contain double bonds and are readily chemically grafted onto the P(VDF-co-HFP) molecular chains. The P(VDF-co-HFP)/ILs blends were irradiated by EB with an absorbed dose of 45 kGy at room temperature. To verify the grafting reaction of the ILs on the P(VDF-co-HFP) chains, 19F NMR measurements were performed, as shown in Figure S2. In the 19F NMR spectra, signals corresponding to TFSI anions are observed at −78.0 to −78.9 ppm for the CF3 group (−SO2CF3), which also shows the grafting reaction between polymer chains and ILs. The weight method was also used to calculate the grafting efficiency of ILs on the polymer chains, as shown in Figure S3. The results indicate that >80% grafting efficiency was achieved with 10 wt % IL loading. Moreover, IL-Cl had a higher grafting efficiency than IL-TFSI at the same loading. The crystal forms of the as-irradiated P(VDF-co-HFP)/ILs blends were also investigated. Similar to the corresponding P(VDF-co-HFP)/ILs physical blends in Figure 1A, the irradiation (in-situ grafting of ILs on the P(VDF-co-HFP) molecular chains) did not affect the crystal forms. In Figure 6A, three characterization diffraction peaks are observed at 18.4°, 19.9°, and 26.6°, corresponding to the nonpolar α crystal phase in the as-irradiated P(VDF-co-HFP)/IL-TFSI

blends. The results indicate that the grafting reactions between the polymer chains and IL-TFSI did not affect the polymeric structure in the crystalline regions and that the α crystal phase is maintained. Similarly, the as-irradiated P(VDF-co-HFP)/ILCl blends maintained the primary polar β crystal phase after the grafting reactions, as shown in Figure 6B. The FTIR results also show the same phenomenon that the EB did not drastically affect the polymeric crystal forms with almost identical characteristic absorption peaks observed in Figures 6C and 6D as those in Figures 1C and 1D, respectively. The unchanged crystal forms in both cases are rational because the grafting reaction occurred at room temperature, and the small IL molecules were locally grafted onto the molecular chains in either the nanodomains (P(VDF-co-HFP)/IL-TFSI) or the crystal amorphous region (P(VDF-co-HFP)/IL-Cl. Moreover, the irradiation did not affect the phase structure of the P(VDF-co-HFP)/ILs blends. TEM images and the corresponding elemental analysis of the as-irradiated samples are presented in Figure 7. P(VDF-co-HFP)/IL-TFSI maintained the phase-separation structure with a domain size of ∼10 nm. Nitrogen was only located in the nanodomains (Figures 7A and 7A1). These results indicate that the irradiation at room temperature does not affect the phase structure of the original microphase-separated P(VDF-coHFP)/IL-TFSI blends. The grafting only occurs in the nanodomains with the double bonds grafted onto the HFP moieties. Similar to the physical blends of P(VDF-co-HFP) and IL-Cl, the chemically grafted P(VDF-co-HFP)/IL-Cl blends were homogeneous with no phase separation observed, as shown in Figures 7B and 7B1. The element mapping confirms the homogeneous structure of the as-irradiated samples. This homogeneous structure is reasonable because the IL-Cl is thermodynamically miscible with P(VDF-co-HFP), and the local grafting does not induce phase separation in the solid state. 3.3. Crystal Forms and Microphase Structures of ILGrafted Copolymers after Melting. We have previously F


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blends, and the grafted parts were still located in the nanodomains. However, a different situation was observed for the IL-Cl-grafted P(VDF-co-HFP). After melting of the asirradiated samples at 200 °C, numerous discrete domains with sizes of ∼30 nm were observed in the polymer matrix, as shown in Figures 8B and 8B1. This result is similar to that observed in our previous research;35,36,39,40 IL-grafted PVDF segments aggregated due to the Coulombic interactions in the melt and formed nanodomains dispersed in the polymer matrix. In other words, quasi-block grafted copolymers were formed during the irradiation because of the selective location of the ILs in the amorphous region, and the quasi-block grafted copolymers could separate into well-defined nanostructures at the melting state (∼200 °C).40 Figure 9 presents WAXD and FTIR spectra of the chemically grafted P(VDF-co-HFP)/IL-TFSI blends after melting. For the IL-TFSI system, the WAXD pattern and FTIR spectrum of the melted P(VDF-co-HFP)-g-IL-TFSI sample were identical to those of the as-irradiated sample and physical blend sample, as shown in Figures 9A and 9C, respectively. The polymer was composed of a nonpolar crystal phase because of aggregation of IL-TFSI in the domains. However, distinct crystal forms were observed for the IL-Cl grafted P(VDF-co-HFP) system. As observed in Figure 1B, it is clear that the IL-Cl can interact well with VDF moieties and induce the formation of polar crystal forms for the homogeneous dispersion of IL-Cl in the matrix. When the IL-Cl was grafted onto polymer chains and assembled into the nanodomains in the melting state (Figure 8B1), β to α crystal form transitions were observed, as shown in Figures 9B and 9D. The ions were confined to the nanodomains in the melt for the IL-Cl grafted system. Ions in the domains cannot induce the conformation transition of the molecules, and PVDF can only crystallize into the nonpolar α crystal forms.

Figure 7. TEM and elemental mapping (nitrogen) images of P(VDFco-HFP)/IL blend films with different ILs after EB irradiation: (A, A1) 10 wt % IL-TFSI and (B, B1) 10 wt % IL-Cl.

observed the microphase separation of IL-grafted PVDF in the melt.41 In this work, the as-irradiated (chemically grafted) samples were also heated to 200 °C followed by cooling to room temperature. The morphologies of the melted samples were investigated. Figures 8A and 8B show the microstructure

4. DISCUSSION Interestingly, various crystal forms and microphase structures can be achieved by physical and chemical modification of P(VDF-co-HFP) with different ILs. In the physical blends of P(VDF-co-HFP)/IL-TFSI, IL-TFSI undergoes a strong interaction with the minor HFP moieties and forms domains with HFP segments in the polymer matrix. IL-TFSI does not affect the conformation of the VDF sequence; therefore, P(VDF-coHFP) crystallizes into the usually nonpolar α crystals. The EB irradiation of such microphase-separated P(VDF-co-HFP)/ILTFSI blends induces the local grafting of IL-TFSI onto the HFP segments in the nanodomains. The subsequent melting of the chemically grafted P(VDF-co-HFP) does not lead to any structure change because the ions are originally confined in the nanodomains. Therefore, neither crystal forms nor microphase separation was observed in this process from the physical blending to the chemical grafting and the subsequent melting, as shown in Scheme 2A−C. Interestingly, a different situation was observed for the P(VDF-co-HFP)/IL-Cl blending system. IL-Cl exhibits higher affinity with P(VDF-co-HFP) than ILTFSI and is thermodynamically miscible with P(VDF-co-HFP) in the physical blends (Scheme 2D). The interactions between the −CF2− with ions change the molecular conformation of the VDF sequence. Therefore, P(VDF-co-HFP) crystallizes into the polar β crystal forms in the physical blends. The EB irradiation initiates the in-situ grafting of IL-Cl onto the polymer main chains. Such irradiation at room temperature does not affect the phase structure or crystal forms; therefore,

Figure 8. TEM and elemental (nitrogen) mapping images of ILgrafted copolymer films with different ILs after phase separation at 200 °C: (A, A1) 10 wt % IL-TFSI and (B, B1) 10 wt % IL-Cl.

morphologies of copolymers grafted with the two type of ILs, IL-TFSI and IL-Cl, respectively. In the P(VDF-co-HFP)/ILTFSI blend films, nanodomains with sizes of 10−20 nm were observed in the grafted copolymers. This phase structure is almost the same as that of the physical blends (Figure 8A). The nitrogen elemental mapping images also indicated that the grafted ionic liquids (IL-TFSI) nearly aggregated into the nanodomains, as observed in Figure 8A1. We considered that IL-TFSI aggregated into nanodomains and was grafted locally onto polymer chains in the domains. The melting of the irradiated blends had little effect on the phase structure of the G


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Macromolecules

Figure 9. WAXD patterns of neat P(VDF-co-HFP) copolymers and IL-grafted copolymers with different ILs after microphase separation: (A) ILTFSI and (B) IL-Cl. FTIR spectra of neat P(VDF-co-HFP) copolymers and IL-grafted copolymers with different ILs after microphase separation: (C) IL-TFSI and (D) IL-Cl.

Scheme 2. Schematic Diagrams of Microstructural Transformation in P(VDF-co-HFP)/IL Blends with EB Irradiation and Microphase-Separation Processesa

Three microscopic states of films with different types of ILs are shown in the diagrams, corresponding to the physical blends, chemically grafted blends as irradiated, and chemically grafted blends after melting in sequence: (A), (B), and (C) P(VDF-co-HFP)/IL-TFSI and (D), (E), and (F) P(VDF-co-HFP)/IL-Cl. a

the same PVDF crystals and morphological structure were observed for the as-irradiated blends as for the physical blends (Scheme 2E). As the grafting only occurs in the amorphous region during the irradiation, a quasi-grafted block copolymer was obtained. The subsequent melting leads to the separation of the ion-containing block microphase from the polymer matrix. Therefore, numerous nanodomains were observed for the P(VDF-co-HFP) chemically grafted with IL-Cl after melting at high temperature (200 °C). The microphase separation can be attributed to the electrostatic interaction between ionic groups, which would lead the chemically modified segments to aggregate and assemble into ordered

clusters in the matrix. In addition, the aggregation of the ioncontaining blocks in the melt state eliminates the specific interaction between the ions with the VDF sequence. Therefore, nonpolar α crystals were obtained for the chemically grafted P(VDF-co-HFP)/IL-Cl blends after the melting and hot-pressing process, as shown in Scheme 2F.

5. CONCLUSIONS The effects of physically and chemically incorporated ILs with different chemical structures on the crystal forms and phase structures of a P(VDF-co-HFP) matrix were investigated. Tunable microstructure morphologies were obtained for the H


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Macromolecules

Transport in Humidified Phosphonated Peptoid Block Copolymers. Macromolecules 2016, 49 (8), 3083−3090. (6) Irwin, M. T.; Hickey, R. J.; Xie, S.; So, S.; Bates, F. S.; Lodge, T. P. Structure−Conductivity Relationships in Ordered and Disordered Salt-Doped Diblock Copolymer/Homopolymer Blends. Macromolecules 2016, 49 (18), 6928−6939. (7) Jung, H. Y.; Kim, S. Y.; Kim, O.; Park, M. J. Effect of the Protogenic Group on the Phase Behavior and Ion Transport Properties of Acid-Bearing Block Copolymers. Macromolecules 2015, 48 (17), 6142−6152. (8) Schulze, M. W.; McIntosh, L. D.; Hillmyer, M. A.; Lodge, T. P. High-Modulus, High-Conductivity Nanostructured Polymer Electrolyte Membranes via Polymerization-Induced Phase Separation. Nano Lett. 2014, 14 (1), 122−126. (9) Kim, O.; Kim, S. Y.; Ahn, H.; Kim, C. W.; Rhee, Y. M.; Park, M. J. Phase Behavior and Conductivity of Sulfonated Block Copolymers Containing Heterocyclic Diazole-Based Ionic Liquids. Macromolecules 2012, 45 (21), 8702−8713. (10) Kim, S. Y.; Yoon, E.; Joo, T.; Park, M. J. Morphology and Conductivity in Ionic Liquid Incorporated Sulfonated Block Copolymers. Macromolecules 2011, 44 (13), 5289−5298. (11) Kim, S. Y.; Kim, S.; Park, M. J. Enhanced Proton Transport in Nanostructured Polymer Electrolyte/Ionic Liquid Membranes under Water-Free Conditions. Nat. Commun. 2010, 1, 88. (12) Jackson, E. A.; Hillmyer, M. A. Nanoporous Membranes Derived from Block Copolymers: From Drug Delivery to Water Filtration. ACS Nano 2010, 4 (7), 3548−3553. (13) Yin, J.; Chen, Y.; Zhang, Z. H.; Han, X. Stimuli-Responsive Block Copolymer-Based Assemblies for Cargo Delivery and Theranostic Applications. Polymers 2016, 8 (8), 268. (14) Schacher, F. H.; Rupar, P. A.; Manners, I. Functional Block Copolymers: Nanostructured Materials with Emerging Applications. Angew. Chem., Int. Ed. 2012, 51 (32), 7898−7921. (15) Mai, Y.; Eisenberg, A. Self-Assembly of Block Copolymers. Chem. Soc. Rev. 2012, 41 (18), 5969−5985. (16) Schlegel, R.; Duan, Y. X.; Weidisch, R.; Holzer, S.; Schneider, K.; Stamm, M.; Uhrig, D.; Mays, J. W.; Heinrich, G.; Hadjichristidis, N. High-Strain-Induced Deformation Mechanisms in Block−Graft and Multigraft Copolymers. Macromolecules 2011, 44 (23), 9374− 9383. (17) Kim, H.-C.; Park, S. M.; Hinsberg, W. D. Block Copolymer Based Nanostructures: Materials, Processes, and Applications to Electronics. Chem. Rev. 2010, 110, 146−177. (18) Park, M. J.; Balsara, N. P. Phase Behavior of Symmetric Sulfonated Block Copolymers. Macromolecules 2008, 41 (10), 3678− 3687. (19) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Templated Self-Assembly of Block Copolymers: Top-Down Helps Bottom-Up. Adv. Mater. 2006, 18 (19), 2505−2521. (20) Zhang, Z.; Gu, Y.; Wang, S.; Li, Q.; Li, M.; Zhang, Z. Enhanced Dielectric and Mechanical Properties in Chlorine-Doped Continuous CNT Sheet Reinforced Sandwich Polyvinylidene Fluoride Film. Carbon 2016, 107, 405−414. (21) Dang, Z. M.; Zheng, M. S.; Zha, J. W. 1D/2D Carbon Nanomaterial-Polymer Dielectric Composites with High Permittivity for Power Energy Storage Applications. Small 2016, 12 (13), 1688− 1701. (22) Mittal, G.; Dhand, V.; Rhee, K. Y.; Park, S. J.; Lee, W. R. A Review on Carbon Nanotubes and Graphene as Fillers in Reinforced Polymer Nanocomposites. J. Ind. Eng. Chem. 2015, 21, 11−25. (23) Sengupta, R.; Bhattacharya, M.; Bandyopadhyay, S.; Bhowmick, A. K. A Review on the Mechanical and Electrical Properties of Graphite and Modified Graphite Reinforced Polymer Composites. Prog. Polym. Sci. 2011, 36 (5), 638−670. (24) He, F.; Lau, S.; Chan, H. L.; Fan, J. High Dielectric Permittivity and Low Percolation Threshold in Nanocomposites Based on Poly(vinylidene fluoride) and Exfoliated Graphite Nanoplates. Adv. Mater. 2009, 21 (6), 710−715.

IL-selective distributed P(VDF-co-HFP)/IL blends. IL-TFSI assembled into discrete nanodomains and had no effects on the crystal forms of P(VDF-co-HFP) because of the dipolar interactions with HFP moieties in the physical blends. Neither crystal form nor morphology transition was observed during the subsequent grafting and melting because of the confined IL-TFSI in the nanodomains. In contrast, IL-Cl exhibited strong interactions with the VDF moieties and formed a homogeneous state with P(VDF-co-HFP). Therefore, IL-Cl induced obvious crystal form transitions during the physical blending. IL-Cl molecules were then selectively located in the amorphous regions at room temperature, and the grafting induced the formation of quasi-grafting block copolymers, which could be microphase-separated into nanodomains during the melting. Because the selective interactions and dispersion of ILs in the polymer matrix can be finely tuned by changing the structures of the polymer chains and ILs, this strategy provides a new path to build multiple nanostructures for diverse applications, such as those requiring pyro/ferro/ piezo-electric fluorine-based materials.

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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.macromol.8b02087.

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Additional data including TEM, 19F NMR, and elemental mapping of nitrogen (PDF)

AUTHOR INFORMATION

Corresponding Author

*(Y.L.) E-mail [email protected]; Fax +86 57128867899; Tel +86 57128867026. ORCID

Yongjin Li: 0000-0001-6666-1336 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Zhejiang Natural Science Foundation (LD19E030001), the National Natural Science Foundation of China (21674033), the National Key R&D Program of China (2017YFB0307704), and the Zhejiang Provincial Key R&D Program (2018C01038).

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