Colloidal Crystallization and Ionic Liquid Induced Partial β-Phase

Article pubs.acs.org/Macromolecules

Colloidal Crystallization and Ionic Liquid Induced Partial β‑Phase Transformation of Poly(vinylidene fluoride) Nanoparticles Daichi Okada,† Hideki Kaneko,† Katsuhiro Kato,† Seiichi Furumi,‡ Masaki Takeguchi,§ and Yohei Yamamoto*,† †

Division of Materials Science and Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ‡ Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan § Transmission Electron Microscopy Station, National Institute for Materials Science (NIMS), 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan

ABSTRACT: Colloidal crystallization of poly(vinylidene fluoride) (PVDF) nanoparticles (NPs) and its β-phase transformation were studied. The pristine PVDF NPs with an average diameter of 230 nm consist of 46% α-phase and 54% amorphous PVDF. The PVDF NPs were assembled on a quartz substrate by means of vertical deposition method from a tetrahydrofuran dispersion of PVDF NPs with a few volume percentage of n-alkane. The resultant colloidal thin films displayed a pale-greenish structural color with the selective reflection at around 550 nm wavelength due to closely packed PVDF NPs. The colloidal thin films were immersed into an acetonitrile solution containing 2 wt % ionic liquid, subsequently air-dried, and thermally annealed at 140 °C, just below the melting point of the PVDF−IL blends. After annealing, the PVDF NPs partially transformed into its β-phase with the volume percentages of α-, β-, and amorphous phases of 22, 32, and 46%, respectively. The postannealed colloidal films still maintained the face-centered-cubic assembling structure of PVDF NPs, thus displaying the greenish structural color and selective reflection.

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reported for applications to tissue engineering.20 In the present research, we focus our attention on PVDF NPs and attempt a construction of colloidal crystals from the PVDF NPs. Colloidal crystals have attracted interest because of their simple fabrication process to construct 3D periodic structures.21−29 Most polymeric colloidal crystals studied thus far consist of optically and electrically inert polymers such as polystyrene (PS) and poly(methyl methacrylate) (PMMA). If colloidal crystals are constructed from ferroelectric polymers, novel photonic properties can be expected such as confinement of higher order harmonics,30−32 yet colloidal crystals from fluoropolymers are unprecedented. Here, we report on a construction of colloidal crystals from PVDF NPs through vertical deposition of α-phase PVDF NPs. Furthermore, partial β-phase transformation is promoted by coating with ionic liquid (IL) and subsequent thermal annealing at a temperature just below the melting point. The colloidal crystal thin films

INTRODUCTION Poly(vinylidene fluoride) (PVDF) is one of the representative fluoropolymers,1−3 and their unique piezo-, pyro-, and ferroelectric properties are utilized for versatile applications such as sensors, memories, actuators, power storage, and so forth. 4−9 PVDF primarily forms three types of chain conformation in the crystalline state: α-phase with a transgauche+-trans-gauche− (tg+tg−) conformation, β-phase with an all-trans conformation, and γ-phase with an intermediate state of α- and β-phases (tttg+tttg−).10−17 Among these, β-phase PVDF exhibits ferroelectric properties due to its unidirectional polarization.1,16 However, the most thermodynamically stable conformation is the α-phase.1,15 Therefore, methodology for the β-phase transformation is a key issue for utilizing PVDF as ferroelectric materials. Recently, fluoropolymer nanoparticles (NPs) are produced and utilized for enhancing the electronic properties of materials.18,19 For example, by an addition of polarized fluoropolymer NPs into the active layer of π-conjugated polymer photovoltaics, the charge separation is accelerated while the charge recombination is efficiently suppressed.19 In addition, electrosprayed PVDF microparticles were recently © 2015 American Chemical Society

Received: February 16, 2015 Revised: March 31, 2015 Published: April 14, 2015 2570

DOI: 10.1021/acs.macromol.5b00337 Macromolecules 2015, 48, 2570−2575

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Macromolecules

sample of pristine PVDF powder mounted on a Si substrate, showing that the PVDF powder is composed of monodispersed NPs with the average diameter (Dav) of 230 nm. XRD pattern of the PVDF NPs displayed diffraction peaks at 2θ = 18.3, 19.9, and 26.5° (d = 4.84, 4.46, and 3.36 Å, respectively, Figure 1c), originating from the reflection of (020), (110), and (021) planes of the α-phase PVDF. This result indicates that most of the crystalline domains in the NPs consist of α-phase PVDF.15,17 In support of this, reflection FT-IR spectrum of a 1 mm thick pellet of the PVDF NPs showed absorption peaks and shoulders originating from the α-phase PVDF at 613, 761, 795, 974, and 1210 cm−1 with a weak absorption peak at 840 cm−1 due to β- or γ-phase PVDF (Figure 1d).15,17 The degree of crystallinity (χ) of the pristine PVDF NPs was evaluated by the intensity area ratio of the XRD peaks.15,17 As shown in the Figure 1c inset (Lorentz-corrected intensity, Iq2, vs q plot), each diffraction peak is separated into α-phase (red, dotted) and amorphous phase (black, dotted). According to the integrated intensity ratio of the diffraction peaks, χ of α-phase is estimated as 0.46; therefore, the degree of the amorphous phase is 0.54. The value of the α-phase corresponds well with that evaluated from the endothermic peak area in DSC trace of the PVDF NPs (0.48, Figure 2a), calculated from the equation

maintain the morphology and reflection properties due to the periodic structures after the thermal treatment.

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MATERIALS AND MEASUREMENTS

PVDF powder (average molecular weight: 534 000 g mol−1) and IL (1-ethyl-3-methyl imidazolium nitrate, [EMIM]NO3) were purchased from Aldrich Chemical Co., Ltd. All solvents (THF, MeCN, EtOH, nheptane, n-octane, n-decane) were used as received from Kanto Chemical Co. Scanning electron microscopy (SEM) was performed at 25 °C on a JEOL model JSM-5610 scanning electron microscope operating at 20 kV. Silicon was used as a substrate, and 5 nm of Au or Pt was deposited onto the thin films of PVDF NPs. X-ray diffraction (XRD) patterns of thin films or pellets of the PVDF samples were recorded at 25 °C on a Rigaku model Miniflex600 X-ray diffractometer with a Cu Kα radiation source (40 kV and 15 mA). Variable temperature X-ray diffraction patterns of PVDF powder samples were recorded on an Anton Parr model SAXess mc2 X-ray diffractometer with a Cu Kα radiation source (40 kV and 50 mA). Reflection Fourier transform infrared (FT-IR) spectra were recorded on a JASCO model 4700 FT-IR spectrometer equipped with an ATR attachment using pellets of PVDF samples. The resolution, data interval, and integration time are 4 cm−1, 0.964 cm−1, and 100, respectively. Differential scanning calorimetry (DSC) was measured on a SEIKO Instruments model EXSTART7000 differential scanning calorimeter using Al pan under Ar atmosphere, where the temperature was calibrated with In (430 K) and Zn (692.7 K) standards. The scan rate was 10 °C min−1. UV−vis reflection spectra of thin films of PVDF samples were measured on a JASCO model V-570 UV/vis/NIR spectrophotometer equipped with a SLM-468 single reflection unit. Angular dependencies of the reflection spectra were measured on a JASCO model V-670 UV/vis/NIR spectrophotometer equipped with an ARMN-735 absolute reflectivity measurement unit. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS) were performed on a JEOL model JEM-ARM200F aberration corrected transmission electron microscope equipped with an EELS device model Gatan Enfina1000.

χ = ΔHf /ΔHf,PC

(1)

where ΔHf and ΔHf,PC are enthalpy of fusion of PVDF NPs (44.8 J g −1 ) and its perfect crystal (93.1 J g −1 ), respectively.20,33−35

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RESULTS AND DISCUSSION Structural Characterization of Pristine PVDF Nanoparticles. Figure 1a,b displays SEM micrographs of the powder

Figure 2. (a) DSC traces of the first heating of pristine PVDF NPs and their mixture with [EMIM]NO3. The dotted line indicates T = 140 °C. (b) Plots of the temperatures of the endothermic peaks in DSC versus weight percentage of [EMIM]NO3. Open and closed circles at more than 13 wt % of IL indicate the lower and higher temperature of the split peaks. The dotted line indicates T = 140 °C.

Conformational Change of PVDF Nanoparticles with Ionic Liquid. It has recently been reported that spin-cast films of PVDF prepared from a dimethylformamide/acetone solution (1/1 v/v) containing 5 wt % of [EMIM]NO3 form β-phase PVDF through the insertion of nitrate (NO3−) ions around the CH2 groups (δ+) of PVDF via Coulomb interactions.36 The insertion of NO3− ions is thought to affect the PVDF main chains with much extended form (β-phase). We examined if the β-phase transformation of the PVDF NPs occurs upon addition of [EMIM]NO3 and subsequent thermal annealing. Thus, an acetonitrile (MeCN) solution (1 mL) of [EMIM]NO3 was added to the PVDF NPs (300 mg), and after gently stirring, the MeCN was evaporated to dryness under a reduced pressure to obtain PVDF/[EMIM]NO3 blends with [EMIM]NO3 concentrations of 5, 13, 23, and 33 wt %. Figure 2a shows DSC traces of the first heating of PVDF NPs and their blends with

Figure 1. (a, b) SEM micrographs of a powder sample of pristine PVDF NPs. (c) XRD pattern of a pellet of pristine PVDF NPs. Inset shows Lorentz-corrected intensity, Iq2, vs q (= 2π/d) plot of the XRD profiles. The dotted curves are the simulated diffractions of α-phase (red) and amorphous phase (black). (d) Reflection FT-IR spectra of a pellet of pristine PVDF NPs. 2571

DOI: 10.1021/acs.macromol.5b00337 Macromolecules 2015, 48, 2570−2575

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Macromolecules [EMIM]NO3. Pristine PVDF NPs show a single endothermic peak at 159 °C, originating from the melting of PVDF. As the ratio of [EMIM]NO3 to PVDF increased, the melting point was lowered. Notably, the endothermic peak was split when the [EMIM]NO3 content exceeded 13 wt % (Figure 2b), whereas only a single exothermic peak was observed upon cooling from their molten states (data not shown). XRD analysis indicates that the endothermic peak at the lower temperature of the split peaks derives from the partial structural transformation of PVDF NPs from α- to β-phase, while that at the higher temperature is due to the complete melting of the PVDF NPs. Upon heating of the PVDF NPs blended with 13 wt % [EMIM]NO3, a diffraction shoulder at 2θ = 20.5° (d = 4.33 Å) appeared at 120 °C originating from the (200) plane of the β-phase (Figure 3a). The intensity of the shoulder was increased upon further heating to 150 °C. When the temperature was increased beyond 160 °C, diffraction intensities decreased because of the melting of PVDF. The appearance of the β(200) was never observed for the heating of pristine PVDF NPs only (Figure 3b). Figure 3c shows the XRD patterns of PVDF NPs blended with 13 wt % [EMIM]NO3 before and after being annealed at 140 °C. The diffraction intensity of the β(200) peak gradually increased upon being annealed at 140 °C, and after 7 h of annealing, the intensity value reached a plateau (Figure 3d). Diffractions originating from α(020) and α(110) were still observed after annealing (Figure 3c, iv). Judging from the diffraction area ratio of each peak (Figure 3e inset), χ of β-phase was calculated as 0.32, while that of α-phase certainly decreased to 0.22. Generally, it is rather difficult to distinguish β-phase from γphase from XRD because β(200) and γ(110) appear at very close angles (2θ = 20.3 and 20.0° for β- and γ-phase, respectively).17 However, FT-IR spectroscopy shows that the content of γ-phase PVDF is not dominant in comparison with β-phase PVDF. Figure 4 shows the FT-IR spectrum of PVDF NPs blended with 13 wt % [EMIM]NO3 (blue) and then annealed at 140 °C for 3 h (red) along with the spectrum of the pristine PVDF NPs (black). The spectra show that strong absorption peaks derived from β-phase PVDF appeared at 840 and 1275 cm−1 after annealing, with a small enhancement of γphase absorption peaks at 810 and 1231 cm−1. Consistently, the intensity of the peaks derived from α-phase PVDF decreased. HAADF-STEM micrographs clarified that the PVDF NPs, after being annealed at 140 °C, maintain their spherical morphology (Figure 5a). Furthermore, EELS mapping of the NPs, after washing off extra ILs adsorbed on the surface of the NPs with ethanol, showed signals of K-shell electrons of N and O atoms derived from NO3− ions at the surface of the NPs (Figure 5b−g). Considering the results of XRD, FT-IR, and STEM-EELS measurements, PVDF NPs, thermally annealing at 140 °C with [EMIM]NO3, form a mixture of α-/β-phase PVDF, and the β-phase is mainly located on the surface area, while the core part still preserves the α-phase conformation. Assuming that only β-phase PVDF is distributed at the surface of NPs, the thickness of β-phase layer is calculated as 14 nm. Noteworthy, when we heated up with much higher temperature, the PVDF NPs did not maintain its morphology due to the melting. To maintain the spherical morphology, it is important that the core part of α-phase PVDF remained during thermal treatment. The temperature of 140 °C is a necessary and sufficient temperature for inducing β-phase transformation while maintaining the nanospherical morphology.

Figure 3. (a, b) Temperature dependence of XRD patterns of PVDF NPs blended with 13 wt % [EMIM]NO3 (a) and pristine PVDF NPs (b). (c) Powder XRD patterns of PVDF NPs blended with 13 wt % [EMIM]NO3. The XRD patterns were measured at 30 °C (i; before annealing), 140 °C (ii; just after heating), 140 °C (iii; annealed for 7 h), and 30 °C (iv; after annealing at 140 °C for 7 h). (d) Plots of diffraction intensity ratio, β(200)/α(110), at 140 °C versus annealing time. The peak intensities, β(200) and α(110), were baselined according to the background intensity. Plot (i) is the intensity ratio at 30 °C before annealing. (e) XRD patterns of a pellet of PVDF NPs blended with 13 wt % [EMIM]NO3 and then being annealed at 140 °C for 3 h. Inset shows Lorentz-corrected intensity, Iq2, vs q (= 2π/d) plot of the XRD profiles. Dotted curves are simulated diffractions of αphase (red), β-phase (blue), and amorphous phase (black).

Colloidal Crystallization of α-Phase PVDF Nanoparticles by the Vertical Deposition Method. This coatand-anneal treatment with IL works well for the partial β-phase transformation of PVDF NPs while maintaining their spherical morphology. However, the NPs were fused with one another at their contact point after annealing (Figure 5a), which causes difficulty in redispersing these NPs into nonsolvents such as tetrahydrofuran (THF) and MeCN. This is a serious problem for constructing colloidal crystals from the PVDF NPs. We therefore attempted preparation of colloidal crystals from 2572

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Figure 4. Reflection FT-IR spectra of pellets of pristine PVDF NPs (black), that blended with 13 wt % [EMIM]NO3 (blue), and that then being annealed at 140 °C for 3 h (red).

Figure 6. (a, b) SEM micrographs of thin films of PVDF NPs prepared via vertical deposition from a THF-only dispersion (a) and a THF dispersion containing 2.5 wt % n-heptane (b). (c, d) Reflection spectra (c) and photographs (d) of thin films of PVDF NPs prepared by dropcasting from a THF-only dispersion (i, black, broken) and vertical deposition from a THF-only dispersion (ii, black, solid), a THF dispersion containing 2.5 wt % n-heptane (iii, red), n-octane (iv, green), and n-decane (v, blue).

colloidal thin film contains many cracks that most likely occurred in the final drying process of the residual solvent (Figure 6a). To obtain well-ordered colloidal crystals, we attempted colloidal crystallization by adding 2.5 wt % of the higher boiling point (bp) solvents such as n-heptane, n-octane, and n-decane (bp = 98, 125, and 174 °C, respectively) into a THF (bp = 66 °C) dispersion of PVDF NPs. Figure 6c,d shows reflection spectra and corresponding photographs of the colloidal films prepared via the vertical deposition method using the cosolvent dispersion of PVDF NPs. The intensities of the reflection maxima at ∼550 nm were clearly enhanced by the addition of the high-bp solvents into the dispersion. Accordingly, SEM micrograph of the thin film prepared from the cosolvent of THF and n-heptane showed densely packed PVDF NPs with superior periodic structure and fewer cracks (Figure 6b) than that prepared from the THF-only dispersion (Figure 6a). The reflection spectra using polarized light clearly show incident angle (θi) dependencies. Figure 7a−c shows reflection spectra of the colloidal films of PVDF NPs prepared from its THF dispersion containing 2.5 wt % n-decane (Figure 6d, v) using S-, N-, and P-polarized light (polarizing angles; 0, 45, and 90°, respectively, with respect to the substrate surface). As θi and the reflection (detection) angle θr increased from 5 to 60°, the reflection peak position shifted to the shorter wavelength region for all of the polarized light (Figure 7a−d). Assuming that the PVDF NPs pack with a (111) orientation of fcc, the plots were simulated using the equation37,38

Figure 5. (a) HAADF-STEM image of PVDF NPs after being blended with [EMIM]NO3 and annealed at 140 °C. (b−g) STEM-EELS mapping of the PVDF NPs with respect to the K-edges of C (b), O (c), C + O (d), F (e), N (f), and F + N (g).

pristine α-phase PVDF NPs, and then thermal anneal treatment was applied after being dipped into a MeCN solution of ILs. The α-phase PVDF NPs were dispersed in THF (30 mg mL−1), and the dispersion was stirred at 25 °C for 12 h. A quartz substrate was set vertically in the dispersion, and THF was naturally evaporated at 25 °C into the atmosphere (vertical deposition method). After 24 h, a thin film of PVDF NPs was formed on both sides of the quartz substrate. The film showed a pale green structural color originating from the periodicity of the PVDF NPs (Figure 6d, ii). The reflection spectrum of the thin film showed a maximum peak at 539 nm (Figure 6c, ii). Assuming that the colloids are assembled with face-centeredcubic (fcc) packing, the diameter (D) of the colloids was calculated using the equation

λmax = √3nPVDFD

(2)

where nPVDF is the refractive index of PVDF (= 1.42) and λmax is the wavelength of the reflection maximum. The obtained value (219 nm) corresponds well with the Dav of the PVDF NPs (∼230 nm, Figure 1b). However, SEM observation of the PVDF NP film revealed that the NPs were ill-ordered, and the

λmax 2 = (8/3)D2(neff 2 − sin 2 θi) 2573

(3) DOI: 10.1021/acs.macromol.5b00337 Macromolecules 2015, 48, 2570−2575

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Macromolecules

Figure 7. Angular dependencies of the reflection spectra of PVDF NP films with S- (a), N- (b), and P- (c)polarized light with incident and reflected light angles from 5 to 60° with respect to the normal direction of the film surface. (d, e) Plots of the reflection wavelength maxima (d) and intensity maxima (e) versus θi (= θr) with S (blue squares)-, N (green triangles)-, and P (red circles)-polarized incident light. The dotted curve in (d) shows the least-squares fitting of the plots using eq 3. The inset in (e) shows the schematic representation of the polarizing directions of the S-, N-, and P-waves and θi and θr with respect to the colloidal film surface.

Figure 8. (a, b) XRD patterns (a) and reflection spectra (b) of PVDF colloidal films after being immersed in a MeCN solution of [EMIM]NO3 (black), air-dried, and then annealed at 140 °C for 3 h (red). (c) SEM micrograph of PVDF colloidal films after being immersed in a MeCN solution of [EMIM]NO3, air-dried, and then annealed at 140 °C for 3 h.

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CONCLUSION PVDF nanoparticles with an average diameter of 230 nm consist of 46% α-phase PVDF. The PVDF nanoparticles were partly transformed into β-phase PVDF by adding imidazolium nitrate ionic liquid and subsequent thermal annealing just below the melting point of the PVDF/ionic liquid blends. The volume percentages of α-, β-, and amorphous phases in the resultant PVDF NPs were evaluated as 22, 32, and 46% after the thermal annealing. The pristine α-phase PVDF nanoparticles were assembled to form densely packed colloidal crystal films via a vertical deposition from a THF dispersion of PVDF nanoparticles containing a small amount of a high boiling point n-alkane. The nanoparticles were then transformed into α-/β-phase mixture by coating with ionic liquid and subsequent thermal treatment. The postannealed colloidal crystal films maintain their periodic structure with a greenish structural color. Colloidal crystals consisting of fluoropolymers are unprecedented and will be valuable due to their expected photonic properties such as confinement of second harmonic generations and generation of higher-order harmonics,30−32 and novel optical properties are further expected.39−43 Furthermore, piezoelectric effects of the β-phase PVDF NP colloidal crystals will be interesting for applications to sensors and energy generators.4−9

where neff is the effective refractive index of the colloidal film. The best fitting neff and D values were 1.376 and 239.5 nm, respectively. The neff value is slightly lower than nPVDF (= 1.42), which is reasonable because the colloidal film with an fcc structure contains ∼25% of void. The theoretical value of neff is 1.328 using the equation neff = √(nPVDF 2f + n void 2(1 − f ))

(4)

with f = 0.75 and nvoid = 1.00, where f is the filling fraction of PVDF NPs and nvoid is the refractive index of void (air). Notably, the reflection intensity systematically changes with respect to θi, where the Brewster’s angle (θB) was clearly observed at around θi = 50° for the P-polarized light (Figure 7e), which corresponds well with that expected from the theoretical value (θB = 54°; arctangent of neff). The PVDF colloidal film was then immersed in a MeCN solution containing 2 wt % of [EMIM]NO3, air-dried, and then annealed at 140 °C for 3 h under a reduced pressure (