COMMENT pubs.acs.org/JPCB
Comment on “Preparation and Characterization of Silver Poly(vinylidene fluoride) Nanocomposites: Formation of Piezoelectric Polymorph of Poly(vinylidene fluoride)” Dipankar Mandal,*,† Karsten Henkel,‡ and Dieter Schmeisser‡ † ‡
Department of Physics, Jadavpur University, Kolkata 700032, India Brandenburgische Technische Universit€at Cottbus, Angewandte Physik-Sensorik, K,-Wachsmann-Allee 17, 03046 Cottbus, Germany
’ COMMENT Poly(vinylidene fluoride) (PVDF) is a versatile polymer due to its chemical resistive properties and five different crystalline polymorphs (R, β, γ, δ, and ε).1 The β crystalline phase is the main focus of interest due to its effective dipole moment. In the commented article,2 the main emphasis is on the formation of the piezoelectric active β phase in silver PVDF nanocomposite. From the available results (Figure 6 and 7 in the commented article), it is easy to understand that there is no existence of the β phase formation, rather it is mainly dominated by the γ phase in the silver PVDF nanocomposite. The FT-IR results (Figure 7 in the commented article) do not show the main characteristic peak of the β phase around 1279 cm 1 (see the concerning Table 2 in the commented article) whereas the main characteristic peak of the γ phase around 1234 cm 1 is clearly visible (not listed in Table 2 in the commented article). Moreover, samples PNC 11 and PNC 0.5 contain six and five peaks, respectively, regardless of the consistency of their position and intensity, within the spectral region between 1260 and 1100 cm 1 (Figure 7 in the commented article). This is due to the saturation of the absorption intensity (deviation of the Lambert Beer’s law of absorption), which can be recognized by the very sharp multiple peaks and their partial separation by horizontal straight lines. This inconsistency can be further clarified by the FT-IR data of the sample PNC 2.5 of the commented paper, where the saturation of the absorption does not take place. Here, it can be realized that the γ phase exists (band around 1234 cm 1), while there are hints for the nonexistence of the R phase (absence of the 764 cm 1 band as well as 2θ ∼ 18.4° reflection peak, shown in Figure 6 of the commented article) and the nonexistence of the β phase (absence of the 1279 cm 1 band). Furthermore, in our previous work we have found that such a multiple peak-like appearance (1260 1100 cm 1 frequency region) in the commented article cannot be attributed to the intrinsic property of the β phase containing silver nanoparticle embedded PVDF films.3,4 It is very difficult to recognize the specific phase of PVDF on the basis of the IR peaks at 840 and 510 cm 1, because these peaks are dual characteristic peaks corresponding to the β phase as well as to the γ phase.5 9 Therefore, the appearance of these bands at 840 and 510 cm 1 is a prerequisite to identify the β phase in PVDF, but it is never conclusive if the 1279 cm 1 vibrational band does not exist in the FT-IR spectra. The authors of the commented article used another band at 483 cm 1 for the identification of the β phase. We state that this is also not correct. It can be clarified by refs 9 11. Due to the existence of the TTT structure in the chain confirmation of both β and γ phases of PVDF, r 2011 American Chemical Society
most of the vibrational bands of these two phases appear at the same or very similar frequencies in the FT-IR spectra. Recently, some of these issues have been mentioned by Cebe and co-workers.9 Due to its very close interplanar spacing (dβ021 = 0.431 nm, dγ200/110 = 0.427 nm), regular laboratory based wide angle X-ray diffraction (WAXD) is not very useful to distinguish β and γ phases; however, the γ phase has another interplanar spacing (dγ111 = 0.39 nm), but that is not always detectable.12 It has been demonstrated that synchrotron based high resolution WAXD is necessary to explicitly identify these phases.6,9 Therefore, the diffraction peak at 2θ = 20°, mentioned in the commented article (based on laboratory based WAXD), is not useful for the argumentation of the β phase existence, especially when the main characteristic vibrational band of the β phase (around 1279 cm 1) is absent. Another wrong argument has been mentioned in the text of the commented paper. There the melting temperature of the β phase is higher than that of the R phase, whereas it is quite well established that the β phase is the least thermally stable phase among R, β, and γ phases.6,8,13 The main source of the melting point confusion is based on differential scanning calorimetry (DSC) studies. In the case of fast thermal scanning rates (10 °C/min, employed in the commented paper) multiple melting of the phases may occur, in particular when the difference of the melting temperatures of the β and R phases is around 10 15 °C.6,14 Therefore, a low thermal ramp is required (i.e., 2 °C/min) within the thermal scanning measurement methods to avoid this kind of confusion.4,14 Therefore, the words “Formation of Piezoelectric Polymorph” within the title of the commented article do not reflect the primary content. Nevertheless, it has been proven in our recent work that the desirable amount of silver nanoparticle doping in PVDF and suitable experimental conditions can induce the β phase.3,4
’ GENERAL REMARK ON THE β, R, AND γ PHASE IDENTIFICATION Our literature survey indicates that the phase identification in PVDF is often done inaccurately, which leads to misinterpretations. In this context, we are extending our discussion with experimental evidence to avoid similar mistakes in the future.15 Received: February 10, 2011 Revised: May 10, 2011 Published: August 15, 2011 10567
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(4) Afterward, we have casted the films on clean glass slides followed by vacuum drying at 130 °C for 1 week. We choose this temperature because we have seen that a lower temperature (i.e., 60 °C) favors the γ phase formation rather than the β phase. (5) After the lift-up of the films from the glass slides, we have checked the existence of silver nanoparticles by FE-SEM and UV vis absorption spectroscopy. We have also assured the existence of silver nanoparticles by TEM from the solution state sample preparation method. (6) TR-FTIR characterizations (with Bio-Rad FTS-60A FTIR spectrophotometer at a resolution of 4 cm 1 and 100 number of scans) were carried out with special care to avoid the saturation absorption.
Figure 1. TR-FTIR spectra of neat PVDF and silver nanoparticle embedded PVDF film: (A) 1600 400 cm 1 region; (B) region 1400 1100 cm 1 of (A).
’ EXPERIMENTAL SECTION In this work, we have prepared silver nanoparticle embedded PVDF films and for reference we have also prepared neat PVDF films under the same conditions. Our preparation condition (solvent casting method) is similar to that of the commented paper, except the following conditions: (1) We used 6 wt % PVDF (Aldrich, Germany)/DMF (Sigma-Aldrich, Germany) (w/v) solutions (which concentration is very much suitable for casting (40 μL) on clean glass slides as well as lift-up the films for characterizations). (2) Different concentrations of silver nanoparticles were prepared by dissolving different amounts of AgNO3 (2.5, 4.5, 7.0, 10.5 mM) in 10 mL of each 6 wt % PVDF/ DMF solutions. The samples are indexed regarding their AgNO3 (Sigma-Aldrich, Germany) concentrations as PVDF-Ag2.5, PVDF-Ag4.5, PVDF-Ag7.0, and PVDF-Ag10.5. (3) We did not stir for a long time (i.e., 10 days or so) at room temperature, because we have observed that long time stirring at room temperature (due to the probably slow reduction rate of AgNO3 and to the fact that PVDF-DMF solvent is not a good stabilizing agent for silver nanoparticles) may lead to silver metal coating on the containers as well a solution casted film surface, which severely shows the saturation absorption during the FT-IR measurement in transmission mode (TR-FTIR). Therefore, we have shortened the stirring time (12 h) and the stirring was carried out at 60 °C.
’ RESULTS AND DISCUSSION From the Figure 1A, it is confirmed that the neat PVDF sample contains R and γ phases, the positions are shown by the dotted lines. In contrast, all other samples show the appearance of 1275 and 445 cm 1 bands and significant improvements of the 840 and 510 cm 1 band intensities, confirming the formation of the β phase in silver nanoparticle embedded PVDF samples. The presence of a small intense band at 764 cm 1 in the samples PVDF-Ag2.5 and PVDF-Ag4.5 indicates a small amount of the R phase. However, other R phase characteristic vibrational bands (i.e., 1213, 1150, 976, 855, 796, 614, 531 cm 1) are not present. The spectra of the samples PVDF-Ag7.0 and PVDFAg10.5 clearly exhibit the existence of predominantly β and γ phases. To understand the behavior of the vibrational bands existing approximately between 1310 and 1150 cm 1, we have plotted TRFTIR spectra in the 1400 1100 cm 1 region (shown in Figure 1B) and marked peaks as 1 (β phase), 2 (γ phase), 3 (R phase), and 4 (νa(CF2) + ω(CH2)/νa(CF2) F(CF2) + F(CH2)). From our results and the data in the refs 9, 11 13, 16, and 17 we can conclude that there should not exist more than four intense bands in the mentioned region. However, this number reduces when the PVDF contains only β or R or γ phases or combinations of only two of these phases. In the commented article, the samples PVF2 (number of bands: 3) and PNC 2.5 (number of bands: 2) (see Figure 7 of the commented article) are quite consistent with our observations. But, in contrast, the other two samples, PNC 0.5 and PNC 11, show five and six bands with inconsistent positions and intensities (these effects may be extrinsic, not related to the property of PVDF). In addition, some of the peaks are clearly visible separated by horizontal straight line (around the 1182 cm 1 region), which is an indication of the saturation of absorption (deviation of the Lambert Beer’s law of absorption). We have performed an experiment to show how the saturation absorption may produce multiple peaks regardless its position; see Supporting Information, Figure S1. Therefore, the peaks at 1260 and 1254 cm 1, usually also a shoulder in this region according to the γ phase PVDF,13 cannot be assigned to the existence of the β phase, especially when no significant improvement of the shoulder at 1279 cm 1 (clearly visible in all samples in Figure 7 of the commented article, including sample PVF2) and a less intense band at 840 cm 1 can be observed.16 To give more insight about the behavior of band 4 (marked in Figure 1B) as a function of combination of the β, R, and γ phases, TR-FTIR results are enclosed as Supporting Information (Figures S2 S5). 10568
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’ CONCLUSION We have discussed the correct way of the phase identifications in PVDF and pointed out the typical associated mistakes. Misinterpretations are based on (i) dual characteristic peaks at 840 and 510 cm 1 of the β and γ phases in FTIR studies, (ii) multiple melting of the phases at rapid thermal ramps in DSC measurements, and (iii) the limitation of the resolution and the very close interplanar distance of the main reflection peaks of the β and γ phases in laboratory based WAXD investigations. ’ ASSOCIATED CONTENT
bS
Supporting Information. TR-FTIR discussion and spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by German Research Foundation (DFG) within the priority program SPP 1157 (grant No. Schm 745/11-1-2). ’ REFERENCES (1) Lovinger, A. J. In Developments in Crystalline Polymers 1; Bassett, Ed.; Applied Science Publishers: London, 1981; p 195. (2) Manna, S.; Batabyal, S. K.; Nandi, A. K. J. Phys. Chem. B 2006, 110, 12318. (3) Mandal, D.; Yoon, S.; Lee, J. S.; Kim, K. J. Fall Meeting of the Polymer Society of Korea, 2010, Abstract No. 1PS-295. (4) Mandal, D.; Henkel, K.; Philip, S.; Schmeisser, D. Manuscript under prepration. (5) Hasegawa, R.; Kobayashi, M.; Tadokoro, M. Polym J. 1972, 3, 591. (6) Ramsundaram, S.; Yoon, S.; Kim, K. J.; Park, C. J. Polym. Sci. B: Polym. Phys. 2008, 46, 2173. (7) Benz, M.; Euler, E. B. J. Appl. Polym. Sci. 2003, 89, 1093. (8) Osaki, S.; Ishida, Y. O. J. Polym. Sci., Polym. Phys. Ed. 1975, 13, 107. (9) Ince-Gunduz, B. S.; Alpern, R.; Amare, D.; Crawford, J.; Dolan, B.; Jones, S.; Kobylarz, R.; Reveley, M.; Cebe, P. Polymer 2010, 51, 1485. (10) Tashiro, K.; Kobayashi, M. Phase Trans. 1989, 18, 213. (11) Bachmann, M. A.; Gordon, W. L.; Koenig, J. L.; Lando, J. B. J. Appl. Phys. 1979, 50, 6106. (12) Buckley, J.; Cebe, P.; Cherdack, D.; Crawford, J.; Ince, B. S.; Jenkins, M.; Pan, J; Reveley, M.; Washington, N.; Wolchover, N. Polymer 2006, 47, 2411. (13) Tashiro, K.; Kobayashi, M.; Tadokoro, H. Macromolecules 1981, 14, 1757. (14) Mandal, D.; Philip, S.; Henkel, K.; M€uller, K.; Schmeisser, D. Polymer 2011manuscript submitted for publication. (15) We appreciate the suggestions of one of the reviewers to include the discussion of phase identifications in PVDF with our experimental observation. (16) Boccaccio, T.; Bottino, A.; Capannelli, G.; Piaggio, P. J. Membr. Sci. 2002, 210, 315. (17) Kobayashi, M.; Tashiro, K.; Tadokoro, H. Macromolecules 1975, 8, 158.
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