Langmuir 2008, 24, 8435-8438
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Enhanced Ferroelectric Phase Content of Polyvinylidene Difluoride Fibers with the Addition of Magnetic Nanoparticles J. S. Andrew* and D. R. Clarke Materials Department, College of Engineering, UniVersity of California, Santa Barbara, California 93106-5050 ReceiVed May 26, 2008. ReVised Manuscript ReceiVed July 2, 2008 Polyvinylidene difluoride (PVDF) fibers with continuously dispersed ferrite (Ni0.5Zn0.5Fe2O4) nanoparticles were prepared by electrospinning from dimethyl formamide (DMF) solutions. The effects of the electrospinning processing conditions and nanoparticle loading on the formation of the R, β, and γ phases of PVDF were studied using infrared spectroscopy and differential scanning calorimetry. The amount of the ferroelectric β and γ phases present in the fibers was found to increase with increased nanoparticle loading. We have shown that the formation of PVDF phases with extended chain conformations can be enhanced by the addition of a well-dispersed nanoparticle phase. At increased nanoparticle loadings, the R phase is completely converted to the more extended β and γ phases.
1. Introduction Magnetic composite fibers, where a fibrous polymer matrix is filled with magnetic nanoparticles, have a wide range of potential applications. The addition of magnetic nanoparticles to the polymer fiber provides a magnetic-field-tunable phase, adding additional degrees of material design freedom. Electrospun polymeric fibers show the potential for biomedical applications, where they can be used for tissue engineering, drug delivery, and gene therapy.1–3 The addition of magnetic nanoparticles can enhance their usefulness in biomedical applications4 while also allowing the composite fiber to be used in electronic and, specifically, multiferroic applications.5 In this letter, we will focus on magnetic composite fibers for possible multiferroic applications to see the effects of adding ferrite nanoparticles to a ferroelectric polymer matrix. Polyvinylidene difluoride (PVDF) is an unusual polymer in that it can exhibit piezo-, pyro-, and ferroelectric behavior. PVDF is also notable for its polymorphism, having at least three regular conformations with similar energies; the all-trans (t), tg+tg– (trans gauche + trans gauche–), and tttg+tttg–, which are referred to as the β, R, and γ phases, respectively.6 The all-trans β phase is the most polar, and the polymer chains stack in the unit cell such that their respective polarizations are aligned in the same direction. In the R phase, the chains stack in alternating directions such that their respective polarizations cancel out, resulting in paraelectric behavior. The β phase can be obtained directly by high-pressure quenching from a melt or by casting from dimethyl acetamide (DMAc), a strongly polar solvent.7 Electric field poling or drawing can be used to transform the more readily obtained R and γ phases into the β phase.7 The γ phase can be formed by slow cooling from a melt, solution casting from dimethyl * Corresponding author. E-mail:
[email protected]. (1) Liang, D.; Hsiao, B. S.; Chu, B. AdV. Drug DeliVery ReV. 2007, 59, 1392– 1412. (2) Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramakrishna, S. Comput. Sci. Technol. 2003, 63, 2223–2253. (3) Chew, S. Y.; Wen, Y.; Dzenis, Y.; Leong, K. W. Curr. Pharm. Design 2006, 12, 4751–4770. (4) Wang, M.; Singh, H.; Hatton, T. A.; Rutledge, G. C. Polymer 2004, 45, 5505–5514. (5) Andrew, J. S.; Mack, J. J.; Clarke, D. R. J. Mater. Res. 2008, 23, 105–114. (6) Lovinger, A. J. Science 1983, 220, 1115–1121. (7) Nalwa, H. S. Ferroelectric Polymers: Chemistry, Physics and Applications; Marcel Dekker Inc.: New York, 1995; pp 63-181.
formamide (DMF),7 or high-temperature annealing of the R or β phase. Electrospinning is a simple technique that can be used to form the ferroelectric β phase of PVDF directly from solution.8 This technique can be easily adapted to form nanoparticle composite fibers when the nanoparticles are suspended in the polymer solution.5 Nanoparticles are used because their size allows for homogeneous incorporation within the ultrathin fibers produced via electrospinning. In electrospinning, a viscous polymer solution or melt is uniaxially stretched in an electric field because of electrostatic repulsions between surface charges along the jet. In a typical electrospinning apparatus, a syringe with a metal tip is connected to a high-voltage dc power supply. When a voltage is applied, the pendant droplet at the syringe tip is distorted into a conical shape as it becomes electrified. This distortion is known as the Taylor cone and is caused by the electrostatic repulsion between surface charges and the Coulombic force exerted by the external field.9 Once a critical voltage is reached, the electric repulsion between surface charges will overcome the surface tension of the solution. At this critical voltage, an electrified jet of polymer solution is ejected from the syringe tip. As the jet extends, the solvent evaporates, producing a solid polymer fiber. Vibrational spectroscopy is a powerful tool for gathering information on the crystalline polymorphs of PVDF present within a given sample.8–11 The vibrational spectrum of PVDF is characterized by many peaks that result from absorptions from all three phases (R, β, and γ). However, there are several peaks that are unique to each of these phases and can be used to identify and quantitatively determine the relative amounts of the three polymorphs. The paraelectric R phase is characterized by vibrational bands at 532 cm-1 (CF2 bending), 612 and 763 cm-1 (CF2 bending and skeletal bending), 796 cm-1 (CH2 rocking), and 854, 870, and 970 cm-1. The ferroelectric all-trans β phase has vibrational bands at 509 cm-1 (CF2 bending), 839 cm-1 (CH2 rocking), and 1273 cm-1 (trans band). Lastly, three vibrational bands at 812, 882, and 1234 cm-1 correspond to the presence of the γ phase.10 However, the γ-phase band near 1234 (8) Andrew, J. S.; Clarke, D. R. Langmuir 2008, 24, 670–672. (9) Li, D.; Xia, X. AdV. Mater. 2004, 16, 1151–1170. (10) Salimi, A.; Yousefi, A. A. Polym. Test. 2003, 22, 699–704. (11) Kobayashi, M.; Tashiro, K.; Tadokoro, H. Macromolecules 1975, 8, 158– 171.
10.1021/la801617q CCC: $40.75 2008 American Chemical Society Published on Web 07/22/2008
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cm-1 also appears in molten samples and can be attributed to the amorphous phase.7 In our previous work, we outlined the effects of the electrospinning processing conditions on the morphology of the electrospun fibers. Here we report on the relationship between overall crystallinity and the amount of β phase present as a function of electrospinning processing conditions and nanoparticle loading. We also show that the addition of a well-dispersed nanoparticle inclusion phase enhances the amount of β phase present in electrospun PVDF fibers.
2. Experimental Methods Materials. Polyvinylidene difluoride (Mw ) 687 000, PDI ) 1.2) was obtained from Solvay Solexis, Inc. (Thorofare, NJ). Iron(III) nitrate, nickel nitrate, sodium hydroxide, tetraethyl ammonium hydroxide (TEAH, 20 wt % in water), pure ethanol, nitric acid, and dimethyl formamide were obtained from Fisher Scientific. 2-[Methoxy(polyethyleneoxy)propyl]trimethoxysilane was obtained from Gelest, Inc. Magnetic Nanoparticle Synthesis. Ferrite nanoparticles were synthesized using a modified Massart method,12 which is described in detail elsewhere.5,13 Briefly, Ni0.5Zn0.5Fe2O4 nanoparticles were formed by the aqueous coprecipitation of the metal nitrates in an alkaline medium. After synthesis, the particles were collected with a magnet and washed and dialyzed in deionized water to remove any excess salts. After dialysis, tetraethylammonium hydroxide (TEAH) was added to create a stable alkaline ferrofluid. To create composite fibers, it was first necessary to functionalize the nanoparticle surface such that they could form a ferrofluid in DMF. This was achieved by functionalizing the surface of the ferrite nanoparticles with 2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane, which herein will be referred to as PEO-silane, using modified Sto¨ber conditions.14,15 PEO-silane surface functionalization was performed in a water/ethanol solution with an ammonia concentration of 0.5 M. After the silane coating, the Ni0.5Zn0.5Fe2O4 nanoparticles were washed and dried. Electrospinning Composite Fibers. PVDF powders were dissolved in DMF at various concentrations ranging from 10 to 20 wt % by sonication at room temperature. Simultaneously, various amounts of PEO-silane nanoparticles were dispersed in DMF. Once the PVDF was dissolved, the two solutions were combined and mixed via sonication. The samples were sonicated until the solutions were uniform. We employed the electrospinning setup that is described in our previous work.5 Fibers were spun at flow rates ranging from 0.4 to 1.5 mL/h and voltages between 7 and 20 kV. A grounded copper plate covered with aluminum foil was used as the collector, and a distance of 15 cm was maintained between the syringe tip and the collector. Characterization Methods. X-ray diffraction (XRD) was used to characterize the Ni0.5Zn0.5Fe2O4 nanoparticles and the PVDFNi0.5Zn0.5Fe2O4 fibers. The average particle size was calculated from the peak broadening using Scherrer’s formula, whereas transmission electron microscopy was used as verification and to obtain size distribution information. PVDF-ferrite solution viscosities were measured using a Brookfield digital viscometer (model RDV-II+ Pro) with a small sample adapter at room temperature. Scanning electron microscopy (SEM) was used to explore the diameter and morphology of the electrospun fibers. Adobe Photoshop CS3 was used to measure the diameter of the fibers from SEM images, and 75 measurements were made per sample. Transmission electron microscopy (TEM) was used to examine the dispersion of the ferrite nanoparticles within the electrospun fibers. (12) Tourinho, F. A.; Franck, R.; Massart, R. J. Mater. Sci. 1990, 25, 3249– 3254. (13) Naughton, B. T.; Majewski, P.; Clarke, D. R. J. Am. Ceram. Soc. 2007, 90, 3547–3553. (14) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (15) Philipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92–99.
Figure 1. Schematic surface functionalization reaction between 2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane and the surface of Ni0.5Zn0.5Fe2O4.
A Fourier transform infrared spectrometer (FTIR, Nicolet Magna 850) was used to obtain the vibrational spectra of the electrospun composite fiber mats. A total of 32 scans were collected for each sample. Differential scanning calorimetry (DSC, TA Instruments DSC 2920) with a heating rate of 10 °C/min was used to measure the overall crystallinity of the samples. The overall degree of crystallinity was calculated by comparing the heat of fusion of the electrospun composite fibers to that of a fully crystalline polymer.16 The same heat of fusion (104.6 J/g) is assumed for all crystalline forms of PVDF.
3. Results and Discussion X-ray diffraction revealed an average particle size of 8-10 nm for the Ni0.5Zn0.5Fe2O4 nanoparticles. Transmission electron microscopy verified this average size and revealed a size distribution that could be fit by a log-normal function with a standard deviation of 0.3. To electrospin composite fibers it was necessary to form a stable ferrofluid of the Ni0.5Zn0.5Fe2O4 nanoparticles in DMF. This was accomplished by capping the Ni0.5Zn0.5Fe2O4 nanoparticles with 2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane (PEO-silane) using a modified Sto¨ber method. The silane capping occurs in an alkaline medium (pH ∼10), above the isoelectric point (IEP, pH ∼8) of the nanoparticles where their surface is characterized by the presence of OH groups. These OH groups react with the PEO-silane following the schematic shown in Figure 1, where n ) 6-9 for the silanes used in this study. PEO-silane-functionalized nanoparticles could be dispersed to form stable ferrofluids in DMF along with a range of polar solvents. A transmission electron micrograph of the composite fibers is shown in Figure 2; this micrograph is representative of the entire fibrous mat. The PEO-silanefunctionalized nanoparticles are well dispersed both radially and axially within the fiber for nanoparticle loading up to 10.43 vol %. X-ray diffraction for the electrospun mats revealed X-ray peaks at 2θ ) 20.6 ° indicative of β-phase (200) and (110) peaks (Figure 3). Peaks at 18.4 and 26.9° can be attributed to the R/γ phases.17 Upon nanoparticle addition, the peaks at 18.4 and 26.9° decrease in intensity and disappear, respectively, indicating a reduction in the R-phase content of the fibers. The X-ray diffraction pattern for the as-received powder is shown in Figure 3 and reveals peaks characteristic of the R phase. Many of these R phases are missing in diffraction data for the electrospun fibers, and those that remain can also be indicative of the γ phase. The X-ray diffraction data shows no significant change upon the addition of nanoparticles to the electrospun fibers. (16) Benz, M.; Euler, W. B. J. Appl. Polym. Sci. 2003, 89, 1093–1100. (17) Hasegawa, R.; Takahashi, Y.; Chatani, Y.; Tadokoro, H. Polymer 1972, 3, 600–610.
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Langmuir, Vol. 24, No. 16, 2008 8437
Figure 4. IR spectra for electrospun PVDF composite fibers as a function of nanoparticle loading, where the R, β, and γ phases are labeled as R, β, and γ, respectively.
Figure 2. Transmission electron micrograph of a PVDF-Ni0.5Zn0.5Fe2O4 composite fiber.
Figure 3. X-ray diffraction data for the as-received PVDF powder and samples electrospun with 0, 2.10, and 9.71 vol % Ni0.5Zn0.5Fe2O4.
Figure 4 shows IR spectra for PVDF-ferrite composite fibers with a range of nanoparticle loadings that were prepared by electrospinning. The IR spectra appeared to be independent of the electrospinning processing conditions, with the most significant effects resulting from variations in solids loading. The characteristic absorption bands of the β phase at 509, 839, and 1273 cm-1 are apparent in each of the spectra and appear to increase in relative intensity with respect to those for the spectra with increased nanoparticle loading. However, the corresponding absorption bands for the R phase (532, 612, 763, and 970 cm-1) diminish with increased nanoparticle loading. With increased nanoparticle loading, a peak appears at 1234 cm-1 that can be attributed to the γ phase. The appearance of the γ phase indicates that the R-phase material is converting to the more stretched out β and γ phases. This increase in relative intensity of the β-phase peaks and the appearance of the γ-phase peak is observed only when the nanoparticles are well dispersed within the polymer matrix. In contrast to previous work,8 the increase in the relative intensity of the β-phase peak is independent of fiber diameter
and processing conditions but instead increases with nanoparticle loading. This increase in the formation of the more extended β and γ phases on nanoparticle addition can be explained by the expanded Flory mixing theory developed by Mackay et al.,18 where the enthalpic effects of polymer-nanoparticle mixing were included. Mackay et al. showed that dispersed nanoparticles caused the polymer to swell when the radius of the nanoparticles was less than the radius of gyration, Rg, of the polymer. The radius of gyration of PVDF used in this study was calculated on the basis of the freely jointed chain model, where Rg ) 1/6 nL2C∞. This calculated Rg is 27.5 nm, which is greater than the average diameter of the ferrite nanoparticles. As we add nanoparticles to the PVDF fiber the polymer swells, increasing its radius of gyration. This increase in Rg, or the chain expansion of PVDF, is manifest by PVDF transforming to its more extended forms, the all-trans β phase or the tttg + tttg γ phase. Therefore, the increase in the overall fraction of β phase observed in PVDFferrite composite fibers occurs as the nanoparticles increase the radius of gyration of the polymer, promoting the extension of the chain into the all-trans β phase. Previous researchers have also observed changes in the structure of PVDF on the addition of inorganic nanoparticles. Several groups have demonstrated an enhancement in the β phase on addition of a hydrated salt to PVDF, proposing that the hydrogen bonding between the water and the polar C-F bond is responsible for this enhancement.19,20 The ferrite nanoparticles are coated with a PEO-silane, where the hydrophilic polyethylene oxide could interact with the PVDF matrix in a similar manner. Therefore, it is possible that the addition of a hydrophilic moiety onto the surface of a magnetic nanoparticle promotes the observed increase in the β phase. The hydrophilic surface coating on the nanoparticles ensures that they can be dispersed in DMF, which is necessary for their homogeneous incorporation within the PVDF matrix. Therefore, this makes it difficult to test the counterhypothesis. Differential scanning calorimetry (DSC) was used to determine the enthalpy of mixing of the composite samples. The overall degree of crystallinity of the electrospun composite samples was calculated by comparing this melting enthalpy to that of fully crystalline PVDF. Figure 5 shows the effects of nanoparticle loading and electrospinning voltage on the overall crystallinity (18) Mackay, M. E.; Tuteja, A.; Duxbury, P. M.; Hawker, C. J.; Van Horn, B.; Guan, Z.; Chen, G.; Krishnan, R. S. Science 2006, 311, 1740–1743. (19) Benz, M.; Euler, W. B.; Gregory, O. J. Macromolecules 2002, 35, 2682– 2688. (20) Chen, S.; Yao, K.; Tay, E. H.; Liow, C. L. J. Appl. Phys. 2007, 102, 1041081–1041087.
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Ni0.5Zn0.5Fe2O4 composites would be to employ an ultradrawing technique similar to the one used to obtain highly oriented polyethylene,21 as well as other techniques to obtain dense composite materials.
4. Conclusions
Figure 5. Overall crystallinity of the electrospun PVDF-Ni0.5Zn0.5Fe2O4 composite fibers as a function of nanoparticle loading for different electrospinning voltages.
of the PVDF in the composite fibers. The voltage has very little effect on the crystallinity. However, the overall crystallinity decreases slightly with the addition of ferrite nanoparticles. The crystallinity range observed is between 46 and 58%, whereas the range observed for PVDF fibers in our previous work is 49-58%,8 where the crystallinity was independent of processing conditions. Therefore, the addition of a well-dispersed nanoparticle phase does not significantly affect the overall crystallinity of the sample. The melting temperature, Tm, determined from DSC is approximately 170 °C for all electrospun composite samples, with the onset of melting occurring at approximately 160 °C. In our previous work8 we showed that the amount of β phase was enhanced in electrospun fibers predominantly because of stretching and was due only minimally to electric field poling. Therefore, the next step toward realizing multiferroic PVDF-
A thorough study of the effects of nanoparticle addition on the structural properties of electrospun PVDF-ferrite composite fibers has been performed using XRD, FTIR, and DSC. The amount of R phase in the fibers decreases with increased nanoparticle loading, and at higher loading,s the R-phase peaks seem to disappear. One mechanism for this could arise because the radius of the ferrite nanoparticle is less than the polymer radius of gyration (Rg) as the polymer swells, increasing its Rg. This increase in Rg favors the formation of PVDF in its more extended polymorphs, the all-trans β phase and the tttg + tttg γ phase. Other mechanisms include the interaction of the hydrophilic PEO capping agent with PVDF. The addition of well-dispersed nanoparticles allows the formation of electrospun fibers where the overall crystalline fraction is made up solely of the ferroelectric β and γ phases, while maintaining overall crystallinity in the range of 49-58%. Electrospinning has once again been shown to be a useful technique for forming PVDF fibers with enhanced properties. Acknowledgment. This work was supported by the Ceramics program of the National Science Foundation GOALI program under grant number DMR-0203785. This work made use of MRL Central Facilities supported by the MRSEC Program of the National Science Foundation under award no. DMR00-80034. LA801617Q (21) Griswold, P. D.; Zachariades, A. E.; Porter, R. S. Polym. Eng. Sci. 1978, 18, 861–863.
