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Langmuir 2008, 24, 670-672
Effect of Electrospinning on the Ferroelectric Phase Content of Polyvinylidene Difluoride Fibers J. S. Andrew* and D. R. Clarke Materials Department, College of Engineering, UniVersity of California, Santa Barbara, California 93106-5050 ReceiVed NoVember 13, 2007. In Final Form: December 26, 2007 Polyvinylidene difluoride (PVDF) fibers were prepared by electrospinning from dimethyl formamide (DMF) solutions. The effects of the electrospinning processing conditions on the formation of the R and β phases of PVDF were studied using infrared spectroscopy and differential scanning calorimetry. We have shown that β-phase PVDF fibers can be electrospun directly from a dimethyl formamide (DMF) solution with a maximum fraction of β phase, F(β)max, of 0.75. The fraction of β phase is found to be greater for smaller-diameter fibers and those spun at an increased voltage.
1. Introduction Polyvinylidene difluoride (PVDF) has been studied extensively because of its unique nature. Not only does PVDF exhibit piezo-, pyro-, and ferroelectric properties, which are rare in polymeric materials, but it is also notable for its polymorphism. PVDF has 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.1 Although each PVDF polymer chain has an effective molecular dipole moment, only the β and γ phases have a net crystalline dipole moment. The all-trans β phase is the most polar, with the polymer chains stacking with their respective polarizations aligned in the same direction. In the R phase, the chains stack with their respective polarizations in alternating directions, resulting in paraelectric behavior. The β phase can be formed directly by high-pressure quenching from a melt or by casting from a strongly polar solvent such as dimethyl acetamide (DMAc).2 This phase can also be created from the R or γ phase by electric field poling or drawing.2 The γ phase can be formed by slow cooling from a melt, solution casting from dimethyl formamide (DMF),2 or through high-temperature annealing of the R or β phases. Electrospinning is a simple technique to form the ferroelectric β phase of PVDF directly from solution. This technique allows for solution processing under an applied electric field, combining solution casting and electric field poling into one step. Previous work suggests that PVDF nanofibers can be formed by electrospinning solutions of PVDF dissolved in DMF.3-6 The electrospinning process involves the uniaxial stretching of a viscous polymer solution or melt in an electric field due to electrostatic repulsions between surface charges along the jet. A typical electrospinning apparatus consists of a syringe with a metal tip connected to a high voltage dc power supply. When a voltage is applied the pendant droplet at the syringe tip becomes electrified, and is distorted into a conical shape. This distortion, known as the Taylor cone, is due to the electrostatic repulsion between the surface charges and the Coulombic force exerted * Corresponding author. E-mail:
[email protected]. (1) Lovinger, A. J. Science 1983, 220, 1115-1121. (2) Nalwa, H. S. Ferroelectric Polymers: Chemistry, Physics and Applications; Marcel Dekker: New York, 1995; pp 63-181. (3) Andrew, J. S.; Mack, J. J.; Clarke, D. R. J. Mater. Res. 2008, 23, 105-114. (4) Ren, X.; Dzenis, Y. Mater. Res. Soc. Symp. Proc. 2006, 920, 55-61. (5) Koombhongse, S.; Liu, W.; Reneker, D. H. J. Polym. Sci., Part B 2001, 39, 2598-2606. (6) Zhao, Z.; Li, J.; Yuan, X.; Li, X.; Zhang, Y.; Sheng, J. J. Appl. Polym. Sci. 2005, 97, 466-474.
by the external electric field.7 When a sufficiently high voltage is applied the electrostatic repulsion between surface charges overcomes the surface tension of the solution, and an electrified jet of polymer solution is ejected from the syringe tip. As the jet extends, the solvent evaporates leaving a solid fiber. The vibrational spectrum of PVDF can be used to gain information regarding the crystallinity and structure of PVDF.8,9 Though many of the peaks in the IR spectrum of PVDF result from overlapping absorptions of the R, β, and γ phases, several peaks are unique to one phase, and can be used to identify and quantify the relative amounts of the phases. The paraelectric R phase has vibration 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 corresponding vibration bands at 509 cm-1 (CF2 bending) and 839 cm-1 (CH2 rocking). Vibrational bands for the γ phase are common with some of the β-phase bands; however, two bands at 812 and 882 cm-1 can be used to characterize the γ phase.8 In this study we show that electrospinning of PVDF fibers is a straightforward technique for forming the β phase of PVDF directly from solution. We also report on the relationship between the overall crystallinity and the amount of β phase present in the fibers with the electrospinning processing conditions. 2. Experimental Methods Materials. Polyvinylidene difluoride (Mw ) 687 000) was obtained from Solvay Solexis, Inc. (Thorofare, NJ). Dimethyl formamide (DMF) was obtained from Fisher Scientific. Electrospinning PVDF Fibers. As received PVDF powders were dissolved in DMF at various concentrations, ranging from 10 to 20 wt %, by sonication at room temperature. A standard electrospinning apparatus was employed, using a syringe as the solution reservoir with a metal syringe needle. A positive high voltage supply (Gamma High Voltage, Inc. ES40) was attached to the syringe needle, making it the cathode. The syringe was connected to a metering pump (KD Scientific, KDS100) to maintain a constant flow rate and solution at the syringe tip. Fibers were spun at flow rates ranging from 0.8 to 1.5 mL/h, with an applied voltage between 7 and 20 kV, and were collected on a grounded copper plate covered with aluminum foil. On the basis of our previous work, the distance between the syringe tip and the collector was maintained at 15 cm.3 (7) Li, D.; Xia, X. AdV. Mater. 2004, 16, 1151-1170. (8) Salimi, A.; Yousefi, A. A. Polym. Test. 2003, 22, 699-704. (9) Kobayashi, M.; Tashiro, K.; Tadokoro, H. Macromolecules 1975, 8, 158171.
10.1021/la7035407 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/12/2008
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Figure 1. Infrared spectra for PVDF fibers of different diameters. The R and β phase are labeled as (n) and (s), respectively. Characterization Methods. The viscosities of the PVDF solutions were measured at room temperature using a Brookfield digital viscometer (model RDV-II+ Pro) with a small sample adapter. The diameter and morphology of the electrospun fibers were examined using scanning electron microscopy (SEM). Fiber diameters were measured from SEM images using Adobe Photoshop CS3. A total of 75 measurements were made per sample. Infrared Spectra of the electrospun fiber mats were obtained using a Fourier transform infrared spectrometer (FTIR, Nicolet Magna 850) in the 500-1000 cm-1 range. The fraction of β phase present in each sample was determined by the characteristic absorption bands of the R and β phases at 532 and 839 cm-1, respectively. Assuming that these absorbencies follow the Beer-Lambert law and that their absorption coefficients are KR ) 6.1 × 104 and Kβ ) 7.7 × 104 cm2/mol at 532 and 839 cm-1, respectively,8 the fraction of β phase, F(β), can be calculated using the following equation F(β) )
Xβ Aβ ) XR + Xβ 1.26AR + Aβ
(1)
where XR and Xβ are the crystalline mass fractions of the R and β phase and AR and Aβ are their absorption bands at 532 and 839 cm-1, respectively. The overall crystallinity of the electrospun samples was measured using differential scanning calorimetry (DSC, TA Instruments DSC 2920), with a heating rate of 10 °C/min. The degree of crystallinity was calculated by comparing the heat of fusion of the electrospun fibers to that of a fully crystalline polymer.10 Assuming the same heat of fusion for all crystalline forms of PVDF, a heat of fusion of 104.6 J/g was used.10
3. Results and Discussion Figure 1 shows the IR spectra for PVDF fibers with a range of diameters that were prepared by electrospinning from PVDF solutions. As described in our previous work, the fiber diameter is controlled by the viscosity of the spinning solution, with higher viscosity solutions producing fibers with larger diameters.3 The characteristic absorption bands of the β phase at 509 and 839 cm-1 along with those of the R phase at 532, 612, 763, and 970 cm-1 are observed in each spectrum, as shown Figure 1. The intensity of the characteristic absorption peaks of the R phase relative to those of the β phase increases with increasing diameter of the electrospun fibers. To quantify these results for the electrospun fibers, the fraction of β phase in each sample was calculated using eq 1, and the results are presented in Figure 2. The fraction of β phase decreases with increasing fiber diameter for fibers spun at lower voltages, e10 kV, whereas for samples spun at 15 and 20 kV the fraction of β phase increases with increasing fiber diameter. (10) Nakagawa, K.; Ishida, Y. J. Polym. Sci. 1975, 11, 2153-2171.
Figure 2. Fraction of β phase as a function of the average diameter for electrospun PVDF fibers.
These results can be qualitatively understood in terms of the electrospinning process. As the electrified jet travels from the syringe tip to the collector, it undergoes a bending and stretching process,11 during which the electrified jet and therefore the resultant polymer fiber is elongated. During this time the fiber is subject to two competing forces: the electrostatic force acts to elongate the fiber, while the viscoelastic force dampens this elongation. Lower viscosity solutions produce smaller diameter electrospun fibers; these solutions have a decreased viscoelastic restoring force and therefore exhibit an increased elongation as they travel from the syringe to the collector.3 We believe that the increased β-phase content observed in these smaller diameter electrospun fibers (Figure 2) is a direct result of this increased elongation and is linked to the well-known fact that an enhancement of the β phase is seen after the drawing or stretching R-phase PVDF films at temperatures below 90 °C.1,2,8 When stretching occurs at higher temperatures the chains are able to slide past one another without changing their conformation. However, at temperatures below 90 °C the polymer is stiffer than at higher temperatures, and stretching forces the molecular chains into the most extended, all-trans, β phase.1 It is also important to note, that the viscosity is controlled by the amount of PVDF dissolved in DMF, and the duration of the elongation process is linked to the amount of time it takes for the solvent to evaporate. Therefore, finer diameter fibers that are produced from low-viscosity solutions undergo an elongation process longer in duration than fibers produced from solutions with higher viscosities due to an increased solvent content. Figure 3 shows the effect of the applied electrospinning voltage on the fraction of the β phase present in samples spun from solutions of varying viscosities, ranging from 391 to 4860 cP. It is important to note that the applied voltage has much less of an effect on the fraction of β phase than the fiber diameter. The fraction of β phase increases with increasing voltage for sample spun from solutions of higher viscosities, with the applied voltage having less of an effect on the low viscosity solutions. At low voltages the stretching process outlined above is primarily responsible for the formation of the β phase in the electrospun fibers. As the voltage is increased the polymer jet undergoes further elongation due to an increased number of charges. However, this additional voltage also induces a larger electric field between the syringe tip and the collector and increases the electric field along the radial direction caused by the surface charges along the jet. These two fields act as poling fields, further increasing the fraction of the β phase that crystallizes in the electrospun fiber. These fields can be better understood by (11) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87, 4531-4547.
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Figure 3. Fraction of β phase as a function of the electrospinning voltage for PVDF fibers.
Figure 4. DSC thermogram of sample electrospun from a PVDF solution.
considering the path taken as the electrified jet travels from the syringe tip to the collector. This path is defined by two regions. In region 1 the jet travels in a straight line that is parallel to the applied field, whereas in the second region the jet undergoes a bending instability causing the jet to travel nearly perpendicular to the applied field.11 As the polymer fiber is stretched, it is assumed that the chain axis is parallel to the fiber axis, and thus the dipole moment of PVDF is perpendicular to the fiber axis. In region 1 the electric field poling of the jet, is likely due to this poling in the radial direction due to surface charges along the jet, whereas in region 2 the electric field poling can be caused by a combination of poling from the electric field in the radial direction and also by the electric field formed between the syringe tip and the collector. Therefore, the enhanced formation of β phase at higher applied voltages can be attributed to these increased poling fields that act on the polymer jet as it travels to the collector. The overall degree of crystallinity for each of the electrospun samples was calculated by measuring the melting enthalpy of the electrospun samples using differential scanning calorimetry (DSC). This calculated melting enthalpy was then compared to that for fully crystalline PVDF. A typical DSC thermogram is shown in Figure 4, where the onset temperature of the melting peak is approximately 160 °C, and the melting temperature, Tm, measured at the melting peak is approximately 172 °C. This broad endothermic peak can be described as a superposition of the melting peaks for the lower-melting R phase and the highermelting β phase. This endothermic peak is weighted toward higher temperatures, which is indicative of the relative amount of the β phase to R phase in these samples. Overall crystallinities in the range of 49-58% were measured for all samples, with changes
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in processing conditions having no significant effect. The melting temperature, Tm, was similarly independent of the processing conditions, with one exception. A slight decrease in Tm was observed with increased fiber diameter, this decrease corresponds to the decrease in the amount of β phase observed in larger diameter fibers. In PVDF, and in most semicrystalline polymers, the melting temperature has been linked to the lamellar thickness, with lower melting temperatures resulting from thinner lamellae.10,12 However, we conjecture that this decrease in melting temperature is mainly a result of the change in the relative amounts of the R and β phases. The subsequent decrease in the melting temperature occurs as the endothermic peak shifts to lower temperature due to an increased contribution from the lower melting R phase. To fully realize the advantages of electrospun PVDF fibers, we compare our results to those for PVDF films prepared by solution techniques. Benz and Euler reported overall crystallinities of 52-60% with F(β)max ) 0.53 for PVDF films that were prepared by spin coating from an acetone/DMF solution and subsequently stretched uniaxially.13 Salimi et al. reported a F(β)max ) 0.74 and an overall crystallinity of 42.6% for melt-quenched stretched PVDF films.8 Salimi et al. also found that they could increase F(β)max up to 80% by stretching PVDF films that were solution cast from N,N-dimethylacetamide (DMAc) solutions.14 However, the stretching of these films led to a reduction in their overall crystallinity. Each of these techniques requires a minimum of two distinct processing steps, first the preparation of a PVDF film and second stretching the film in order to obtain the β phase. By electrospinning we have been able to form fibers with F(β)max ) 0.75 and overall crystallinities in the range of 49-58% in a simple one-step process. Thus, in contrast to the film routes electrospinning enables PVDF to have both a high crystallinity and an increased fraction of the ferroelectric β phase.
4. Conclusions An in-depth study on the effects of electrospinning on the structural characteristics of electrospun PVDF fibers has been performed by FTIR and DSC. The β phase forms while the polymer jet elongates as it travels from the syringe tip to the collector. The amount of the β phase formed in these fibers can be tuned by varying the processing parameters. The maximum β-phase content fibers are obtained by electrospinning from low viscosity solutions or under a higher applied voltage for high viscosity solutions. The increase in the formation of the β phase for high viscosity solution spun at increased voltages can also be attributed to electrical poling by the increased electric field that forms between the syringe tip and the collector. Electrospinning provides a simple one-step technique to form highly crystalline PVDF fibers primarily in the β phase. 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. LA7035407 (12) Sencadas, V.; Lanceros-Me´ndez, S.; Mano, J. F. Thermochim. Acta 2004, 424, 201-207. (13) Benz, M.; Euler, W. B. J. Appl. Polym. Sci. 2003, 89, 1093-1100. (14) Salimi, A.; Yousefi, A. A. J. Polym. Sci., Part B 2004, 42, 3487-3495.
