Polymorphic Extended-Chain and Folded-Chain Crystals in Poly

Nov 23, 2015 - (8-11) In the past, most research focuses on how to obtain β ECC .... The melting process of each sample was followed by time-resolved...
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Polymorphic Extended-Chain and Folded-Chain Crystals in Poly(vinylidene fluoride) Achieved by Combination of High Pressure and Ion−Dipole Interaction Yue Li,† Saide Tang,‡ Ming-Wang Pan,§ Lei Zhu,*,‡ Gan-Ji Zhong,*,† and Zhong-Ming Li*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Sichuan, People’s Republic of China ‡ Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States § Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, People’s Republic of China S Supporting Information *

ABSTRACT: Manipulating polymorphism in extended chain-crystals (ECCs), which are commonly achieved by crystallization under high pressures, is important for enriching our understanding of basic polymer crystallization as well as for achieving high performance materials. In this study, the influence of high pressure and ion−dipole interaction on the polymorphism was investigated by comparing neat poly(vinylidene fluoride) (PVDF) and PVDF with 1 wt % cetyltrimethylammonium bromide (CTAB) nonisothermally crystallized from the melt at 210 °C. Under low pressures (≤10 MPa), γ folded-chain crystals (FCCs), rather than α FCCs, were obtained for PVDF/1 wt % CTAB because of the ion−dipole interaction. Under a moderate pressure (100 MPa), pure β FCCs were formed in PVDF/1 wt % CTAB, owing to the synergistic effect of both high pressure and ion−dipole interaction. Under high pressures (≥200 MPa), mixtures of β/γ FCCs and ECCs were obtained for PVDF/1 wt % CTAB. This was different from the neat PVDF, where mixtures of α FCCs and α/γ/β ECCs coexisted when the pressure was between 200 and 400 MPa. The formation mechanisms of various crystalline forms and FCCs versus ECCs during the nonisothermal crystallization are discussed using the T−P phase diagram for PVDF.

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INTRODUCTION Extended-chain crystal (ECC) is a unique type of crystal morphology with improved density, ultrahigh modulus and strength, and higher thermal stability.1,2 From previous studies,3−6 the ECC is mostly obtained under high pressures, and polyethylene (PE) is a model polymer to investigate thermodynamic and kinetic mechanisms of ECC formation because of its unique hexagonal (or rotator) phase above the triple point. As compared with PE, poly(vinylidene fluoride) (PVDF) also has a simple molecular structure but can exhibit numerous polymorphic crystal structures due to strong dipolar interactions.7 This makes PVDF an intriguing polymer for the study of crystallization under high pressures because different crystalline forms (α, β, γ, and δ) and ECC versus folded-chain crystal (FCC) will compete at the same time.8−11 In the past, most research focuses on how to obtain β ECC through the metastable pseudohexagonal phase under high pressure crystallization (by either isothermal crystallization under a constant high pressure or pressure quench) because defect-free β ECCs exhibit the highest performance and thermal stability in piezoelectricity.12 A T−P phase diagram is used to determine the proper temperature and pressure ranges for the growth of β ECC.13 However, the interplay between polymorphism and © XXXX American Chemical Society

ECC/FCC in high pressure crystallization of PVDF still needs further illumination. Polymorphism is a unique behavior for crystalline polymers, and it can lead to drastically different physical properties.14 For example, PVDF can crystallize into five polymorphs, α, β, γ, δ, and ε, which results in different electroactive properties.7 The nonpolar α phase is the most kinetically favored and has two trans−gauche (TGTG′) chains packed antiparallel in a unit cell. The δ phase has exactly the same chain conformation and unit cell as the α phase, but two chains are parallel (polar) in the unit cell. The β phase has the all-trans conformation (TTTT) with two chains packed parallel in a unit cell, which assures the highest ferroelectric and piezoelectric properties. The γ phase has a gauche conformation (G) every fourth repeat unit (T3GT3G′) with two chains packed parallel in the unit cell. The ε phase has the same chain conformation and unit cell as the γ phase, but two chains are antiparallel in the unit cell.15 Some useful methods have been utilized to manipulate the polymorphism of PVDF FCCs and obtain the polar phase,16 Received: August 28, 2015 Revised: October 22, 2015

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DOI: 10.1021/acs.macromol.5b01895 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules including mechanical stretching (β),17,18 solution crystallization (β/γ),19 homogeneously nucleated crystallization (β/γ),20,21 and crystallization in the presence of fillers with surface charge (β/γ).22−26 In recent years, the filler method has received much attention because it is simple and efficient. The fillers can be heterogeneous substances, such as nanoclays,22−24,27,28 ferrites,29,30 onium salts,31 and so on. It is demonstrated that particles with surface charges could have strong ion−dipole interactions with PVDF chains, leading to the appearance of long trans conformation in the PVDF melt. Many factors such as filler type, concentration, and crystallization temperature (Tc) can have a significant effect on the polymorphism of PVDF. Although polymorphism in PVDF FCCs has been widely studied, polymorphism in PVDF ECCs has not been well understood. Generally, there is no difference between the unit cells for ECCs and FCCs, and ECCs can be achieved from the high-pressure hexagonal phase FCCs by lamellar thickening via the sliding diffusion.5,6,32,33 In the present work, we choose to study the synergistic effect of homogeneous filler [e.g., cetyltrimethylammonium bromide (CTAB)] and high pressure on polymorphism in PVDF ECCs and FCCs. First, by applying various pressures (200−400 MPa) to the neat PVDF during nonisothermal crystallization (cooling from the melt at 210 °C to room temperature at −2.5 °C/min), α, γ, and β ECCs with increasing melting temperature (Tm) are observed; Tm(α ECC) = 192 °C, Tm(γ ECC) = 198 °C, and Tm(β ECC) = 201 °C. Second, by addition of only 1 wt % CTAB, pure β FCCs are obtained at a moderate pressure of 100 MPa due to the synergistic effect of ion−dipole interaction and high pressure crystallization. Finally, these experimental findings are explained using the T−P phase diagram of PVDF.

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Figure 1. (a) Schematic of the high-pressure cell (1, guide pillar; 2, mold core; 3, sample; 4, mold; 5, heating jacket; 6, thermocouple) and (b) temperature and pressure protocol. high as 800 MPa, as shown in section I of the Supporting Information. Note that the crystallization of PVDF and its blends with CTAB was a nonisothermal process and was influenced by both high pressure and CTAB. We consider that this could be more practical than the isothermal crystallization. To ensure identical temperature and pressure for precise comparison, neat PVDF and PVDF with 1.0 wt % CTAB were put into two molds of the compression molding. In the following study, the nonisothermally crystallized neat PVDF under different pressures (10, 100, 200, 300, and 400 MPa) was denoted as N10, N100, N200, N300, and N400, respectively. Similarly, the nonisothermally crystallized PVDF samples containing 1.0 wt % CTAB was noted as C10, C100, C200, C300, and C400, respectively. Characterization Methods. The formation of different crystalline modifications under various pressures with and without CTAB was analysis by Nicolet 6700 Fourier transform infrared (FTIR) spectrometer in the ATR mold by averaging 32 scans at a resolution of 2 cm−1 (Thermo Fisher Scientific, Inc., Waltham, MA). A TA Instruments Q2000 differential scanning calorimeter (DSC) was used to study the melting behavior of both neat PVDF and PVDF/CTAB blends crystallized under different pressures. Typically, the sample (5− 10 mg) was heated from 80 to 250 °C at a heating rate of 10 °C/min. Wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurements were conducted at beamline 16B of Shanghai Synchrotron Radiation Facility, Shanghai, China. The wavelength of the monochromatized X-ray was 0.124 nm, and the beam size was ca. 400 μm. Two-dimensional (2D) WAXD and SAXS patterns were collected using a MarCCD detector (model Mar345). An α-form aluminum oxide (α-Al2O3) standard was used to calibrate the scattering angle, and the background of air scattering was subtracted. Linear WAXD and SAXS profiles were obtained from the integrated intensities of the 2D patterns. The melting process of each sample was followed by time-resolved in situ WAXD equipped with a commercial hot-stage (Linkam FTIRSP600 with an accuracy of ±0.1 °C). The sample film (∼200 μm) was fixed on the heating part of the hot stage directly and was heated quickly to 110 °C at a rate of 50 °C/ min, followed by a slower heating (5 °C/min) to slightly above the melting temperatures (Tm). Field-emission scanning electron microscopy (FE-SEM) was performed on a Hitachi S4500 SEM at an accelerating voltage of 5.0 kV. N400 and C400 samples were embedded in standard epoxy, which was cured at 60 °C overnight. The samples were polished at −25 °C using a diamond knife equipped with a water boat, and a mixture of 1:1 (v/v) water/dimethyl sulfoxide was used as the floating liquid. After sputter-coating of 2−3 nm gold on the polished cross sections, FE-SEM images were obtained using a secondary electron detector.

EXPERIMENTAL SECTION

Materials. The PVDF (FR901) used in this study was purchased from 3F Co., Shanghai, China. Its number-average molecular weight (Mn) was 148 000 g/mol, and the polydispersity was 1.93. The melt flow index (MFI) was 10 g/10 min at 230 °C (5 kg force). CTAB was purchased from Chengdu Kelong Chemical Reagent Factory (China). They were used without further purification. Sample Preparation. To obtain the PVDF sample with 1.0 wt % CTAB, a blend with 5 wt % CTAB was prepared first as the master batch using a Haake torque rheometer (XSS-300, Shanghai Kechang Rubber Plastics Machinery Set, Shanghai, P. R. China) at 190 °C with a rotor speed of 60 rpm. After dilution with PVDF using the same melting blending method, the 1.0 wt % CTAB blend sample was obtained. As a control, the neat PVDF sample was also passed through the Haake torque rheometer once using the same temperature and speed. A home-built compression-molding apparatus, which can load a high-pressure stress up to 1 GPa, is schematically shown in Figure 1a. The diameter of the plunger was 20 mm, and the length of the cylindrical channel was 88 mm. Hydrostatic pressure supplied from a hydraulic jack could go up to 1 GPa, and the pressure inside the channel was measured via a modulated pressure meter with an accuracy of ±1 MPa. The molding temperature was monitored via a thermocouple mounted 15 mm away from the channel. The sample inside the channel was heated by an electrical heating sheath, which was controlled by a temperature controller. The sample was heated to 210 °C and kept isothermal for 15 min to melt the crystals and erase thermal history. Afterward, the desired pressure was applied, and the sample was cooled down to room temperature at a cooling rate of −2.5 °C/min. The temperature and pressure protocols are shown in Figure 1b. In this study, to study the polymorphic ECCs (or FCCs) of PVDF in the presence of CTAB, we mainly focused on the pressure in the range of 10−400 MPa because a high amount of β ECCs would form regardless of the presence of CTAB if the applied pressure was as

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RESULTS Polymorphic PVDF Crystals Induced by CTAB under High Pressure. It is known that the similarity of the van der B

DOI: 10.1021/acs.macromol.5b01895 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. WAXD profiles of (a) neat PVDF and (b) PVDF/CTAB 1.0 wt % nonisothermally crystallized under various pressures.

Figure 3. FTIR spectra for (a) neat PVDF and (b) PVDF/CTAB 1.0 wt % nonisothermally crystallized under various pressures.

Waals volumes for fluorine and hydrogen atoms allows conformation flexibility for PVDF chains, which results in rich crystalline polymorphism.7 To study the polymorphic composition in PVDF after crystallization under high pressure with and without the presence of CTAB, WAXD and FTIR were carried out for characterization.7,34,35 Results are shown in Figures 2 and 3, respectively. Note that the polymorphism of neat PVDF has many overlapping X-ray diffraction peaks. For clarity, assignment of the major reflection peaks for α, β, and γ phases is summarized in section II of the Supporting Information. For neat PVDF (Figure 2A), typical α-phase reflections were observed in the WAXD profiles when crystallized under 10 and 100 MPa, and this was confirmed by the absorption bands for the TGTG′ (α phase) conformation in the FTIR spectra in Figure 3A, namely, 763 cm−1 (CF2 bending and skeletal bending), 795 cm−1 (CH2 rocking), 974 cm−1 (CH2 twisting), 1208 cm−1 (CF2 stretching and CH2 wagging), and 1383 cm−1 (CH2 deformation and CH2 wagging). With increasing applied pressure, a new weak reflection peak emerged at 14.70 nm−1 for the N200 sample and became stronger for the N300 and N400 samples. Meanwhile, weak absorption bands for the γ (811, 832, and 1234 cm−1) and β (840 and 1276 cm−1) phases appeared in the FTIR spectra (Figure 3A). Therefore, the new reflection peak at 14.70 nm−1 could be assigned as the mixed (110/200)β/021γ

reflections. From these results, a small amount of β and γ crystals started to grow during the nonisothermal crystallization process for the neat PVDF upon increasing the pressure to 400 MPa. After incorporation of 1.0 wt % CTAB into PVDF, typical γphase reflections were identified in the WAXD profile for C10 under 10 MPa pressure (see Figure 2B). This is confirmed by the characteristic absorption bands of the TTTGTTTG′ conformation in the γ phase, namely, 811 cm−1 (CH2 rocking), 832 cm−1 (CF2 symmetrical stretching and C−C stretching), and 1234 cm−1 (CF2 asymmetrical stretching and rocking). Note that no absorption bands for the TGTG′ conformation for the α phase were observed, indicating a pure γ phase induced by CTAB. This result is consistent with our previous result for the isothermal crystallization of PVDF in the presence of CTAB.36,37 The formation mechanism was attributed to simultaneous growth of γ and α phases from the melt and subsequent α → γ solid phase transformation owing to the ion−dipole interactions between PVDF and CTAB. As the pressure increased to 100 MPa for C100, the reflection peaks for the γ phase (i.e., 020γ, 110 γ, and 022γ) nearly disappeared, while the reflection peak at 14.70 nm−1 remained strong. Meanwhile, the characteristic bands for the long trans Tn conformation at 840 and 1276 cm−1 became obvious, and the characteristic band for the TTTGTTTG′ conformation at 1234 C

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Figure 4. SAXS profiles for (a) neat PVDF and (b) PVDF/CTAB 1.0 wt % after nonisothermal crystallization under various pressures. The inset SEM images are N400 in (A) and C400 in (B).

Figure 5. Ambient pressure DSC melting curves of neat PVDF and PVDF/CTAB 1.0 wt % samples after nonisothermal crystallization under various pressures. The heating rate is 10 °C/min.

cm−1 became much weaker (Figure 3B). From these results, the strong peak at 14.70 nm−1 could be ascribed to the (110/200)β reflection. Comparing N100 to C100, a small amount of γ phase appeared in N100, but a large amount of β phase formed in the presence of CTAB for C100. Therefore, there seems to be a cooperative effect of both CTAB-induced nucleation and moderately high pressure on the formation of a neat β phase in PVDF. This finding will enable a convenient way to fabricate neat β phase PVDF for electroactive applications, e.g., injection molding of PVDF with a small amount of CTAB (1 wt %). Further increasing the pressure to 200−400 MPa, the reflections peaks for the γ phase (i.e., 020γ, 110γ, and 022γ) unexpectedly reappeared in the WAXD profiles for C200− C400 in Figure 2B. At the same time, the absorption bands for the TTTGTTTG′ conformation in the γ phase (832 and 1234 cm−1) also reappeared in the FTIR spectra for C200−C400 samples (Figure 3B), indicating mixed β and γ phases under 200−400 MPa pressure. Lamellar Thickening and Formation of ExtendedChain Crystals under High Pressure. To understand the complex polymorphic behavior for PVDF/CTAB 1.0 wt % under various pressures, i.e., γ at 10 MPa → β at 100 MPa → γ + β at 200−400 MPa, SAXS experiments were carried out. Results for neat PVDF and PVDF/CTAB 1.0 wt % samples

nonisothermally crystallized under various pressures are shown in Figure 4. Under 10 MPa pressure, N10 and C10 samples showed broad scattering peaks at 0.45 nm−1 (14.0 nm) and 0.41 nm−1 (15.1 nm), respectively, indicative of the formation of folded-chain crystals (FCCs) under 10 MPa pressure. When the pressure increased to 100 MPa, still a single broad scattering peak was observed for N100 and C100, respectively, which again could be attributed to FCCs. With further increasing the pressure to above 200 MPa, however, this broad scattering peak around 0.45 nm−1 became fairly weak (more visible in the Lorentz-corrected plots in the inset plots of Figure 4). Instead, an intense scattering peak or shoulder near the beamstop (i.e., 60% and 400 MPa and 240−300 °C), h-FC nuclei will dominate, followed by thickening into h-ECCs, which finally transform into β ECCs. After adding 1 wt % CTAB into PVDF, the situation is changed due to the ion−dipole interaction, and the crystallization pathways are postulated in the bottom panel of Figure 8B. When the pressure is below 10 MPa, no h-FC nuclei are stable and γ- and α-FC nuclei grow simultaneously. Because of the ion−dipole interaction with CTAB, α-FC crystals gradually transform into γ′-FCCs, accompanying the continued growth of the γ-FCCs. Similar result has been reported for crystallization under the ambient pressure in a recent report.37 Under 100 MPa, it is likely that growth of α-FC nuclei become unfavorable because both high pressure and ion−dipole interactions favor the long trans conformation. Meanwhile, the h-FC nuclei are not yet stable for C100. As a result, only γFC nuclei grow under this condition, and they consequently transform into β FCCs under 100 MPa pressure. This is the first report that combined ion−dipole interaction from CTAB and a moderate pressure can induce pure β FCCs. Upon further increasing the pressure to 200−400 MPa, simultaneous growth of γ- and h-FC nuclei becomes possible. Some h-FC nuclei can thicken into h-ECCs, which eventually will transform into mixed β/γ ECCs. The γ-FC nuclei (and any remaining hFC nuclei) will gradually transform into the β FCCs. We speculate that under these high pressures (200−400 MPa) the crystallization kinetics is too fast so that the transformation to the β FCCs is not complete. As a result, mixed β/γ FCCs are obtained with the β FCCs being the majority (>59%). As we can see, the ion−dipole interaction between CTAB and PVDF is effective in inducing the polar γ phase nuclei and subsequent

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DISCUSSION From the above results, the addition of 1 wt % CTAB can alter the polymorphism formation for PVDF crystallization under high pressures by facilitating nucleation of the polar conformation (i.e., the γ phase) and subsequent transformation into the β phase through ion−dipole interactions. Here, it is necessary to review the crystallization behavior for neat PVDF under high pressures.12,13 Figure 8A shows a typical T−P phase diagram for neat PVDF with phase boundary lines among the melt, β-ECC, and the paraelectric pseudohexagonal (hex) phase [which is similar to the paraelectric orthorhombic phase in P(VDF−TrFE)7,40]. The triple point is located around 300 °C and 320 MPa for neat PVDF. γ ECCs exhibit a Tm (198−201 °C) close to that (201−203 °C) for β ECCs at the ambient pressure. In addition, the melting lines for α ECCs and various types of FCCs are shown as dashed lines. Upon cooling for crystallization, the situation becomes more complicated. First, the supercooling for crystallization makes the hex phase metastable at high pressures well below the Tm(β-ECC) and Tc(hex/β-ECC) (i.e., the Curie transition) lines (see the light blue region in Figure 8A). A similar phenomenon is also observed for P(VDF−TrFE) copolymers with 50−82 mol % TrFE, where the paraelectric phase can persist into fairly low temperatures upon cooling from the melt at the ambient pressure.7 Second, nucleation of folded-chain crystals, especially nonpolar α FCCs, predominates at low enough Tcs (see the orange region in Figure 8A). We speculate that there could be some overlapped region, where folded-chain hex and α nuclei may coexist during crystallization. Note that hex-FCC can easily undergo lamellar thickening through the sliding diffusion,5,6,32,33 leading to hex-ECC, due to the enlarged interchain distance and thus enhanced motion along the chain. It is considered that this hex phase is the only pathway for the growth of β ECC in neat PVDF when crystallized under high pressures.12,13 On the basis of this T−P phase diagram, the formation mechanism for various polymorphic PVDF crystals under high pressure nonisothermal crystallization (i.e., cooling from the melt at 210 °C to room temperature at −2.5 °C/min) in this study is postulated in the top panel of Figure 8B. Below 200 G

DOI: 10.1021/acs.macromol.5b01895 Macromolecules XXXX, XXX, XXX−XXX

Macromolecules phase transformation into the β phase during high pressure crystallization. However, it is not effective in the formation of ECCs. Instead, the hex phase is important. Note that the Tc (i.e., lower than 210 °C) is too low to allow complete transformation from h-FC into h-ECC. Either increasing the Tc to ca. 250 °C for PVDF or decreasing the triple point by using P(VDF-TrFE) with 10−18 mol % TrFE will be able to facilitate complete h-FC to h-ECC transition and eventually lead to β ECCs. These crystallization pathways are the best-effort postulation on the basis of the phase diagram in Figure 8A and the above experimental results. To fully understand these crystallization pathways, it is necessary to carry out in situ simultaneous SAXS and WAXD during the high pressure crystallization processes. However, this will require significant modification of our high pressure crystallization apparatus in Figure 1 to be transmissible to X-rays while maintaining the high pressure. This research is currently under development.

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS In this study, we characterized polymorphic crystal formation in a PVDF/CTAB 1 wt % sample crystallized under various high pressures. The goal was to investigate the influence of the ion− dipole interaction on the polymorphic PVDF crystal formation under high pressure crystallization. It was observed that the ion−dipole interaction could effectively lead to pure β FCCs under a moderate pressure of 100 MPa, whereas mixed β/γ FCCs and ECCs were obtained when the pressure increased to 200−400 MPa. These results suggest that the strong ion− dipole interaction can facilitate nucleation of γ-FC nuclei, which eventually grow into β-FCCs under a moderate pressure of 100 MPa. However, this interaction cannot promote the formation of metastable h-FC nuclei under high pressures, which is critical in achieving β/γ ECCs. To better achieve β ECCs at moderate pressures, we propose to incorporate CTAB in P(VDF−TrFE) with 10−18 mol % TrFE, where the combined effects of strong dipole−ion interaction and readily accessible h-FC nuclei under moderate pressures should effectively facilitate the formation of pure β ECCs. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01895. WAXD (a) and DSC (b) profiles of neat PVDF and PVDF/CTAB 1.0 wt % compressed under 800 MPa; the lattice plane and corresponding interplanar spacing, and peak positions of major crystalline forms for PVDF; DSC heating curves for neat PVDF at 5 and 10 °C/min; estimation of β FCC, γ FCC, β ECC, and γ ECC contents in C400 (PDF)

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The authors thank the support from the National Natural Science Foundation of China (Grants 51421061, 51373047, 51528302, and 51473101), Sichuan Department of Science and Technology (Grant 2014TD0002), the Doctoral Program of the Ministry of Education of China (Grants 20130181130012 and 20120181120101), and Project funded by China Postdoctoral Science Foundation (Grant 2014T70868). We acknowledge the National Synchrotron Radiation Laboratory, Shanghai, China, for synchrotron SAXS and WAXD measurements, and State Key Laboratory of Polymer Materials Engineering, Sichuan University (Grant sklpme2014-3-08).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.Z.). *E-mail: [email protected] (G.-J.Z.). *E-mail: [email protected] (Z.-M.L.). Notes

The authors declare no competing financial interest. H

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