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The Influence of Preparation Conditions on the Surface Morphology of Poly(vinylidene fluoride) Films Marcel Benz,† William B. Euler,*,† and Otto J. Gregory‡ Department of Chemistry, 51 Lower College Road, University of Rhode Island, Kingston, Rhode Island 02881, and Department of Chemical Engineering, Room 110 Crawford Hall, University of Rhode Island, Kingston, Rhode Island 02881 Received August 21, 2000. In Final Form: October 19, 2000 The R phase (or form II) of poly(vinylidene fluoride), PVF2, produced from an acetone/N,Ndimethylformamide (DMF) solution gave different surface morphologies, depending on the solvation temperature of the PVF2 solution, the DMF concentration, and the relative humidity when deposited onto a smooth silicon substrate. Solutions prepared at less than 30 °C always gave rise to transparent films. However, solutions prepared at temperatures greater than 50 °C resulted in a rough and opaque, white surface when depositing at high humidity and in a transparent film when depositing at low humidity. The aging behavior of the polymer solutions as measured by the viscosity revealed an enormous dependence on the DMF concentration. Optical light and atomic force microscopies and infrared spectroscopy characterized the differences of these film surfaces.
Introduction Poly(vinylidene fluoride) (PVF2) is a very well documented polymer.1-8 The interest in PVF2 lies especially in its remarkable piezoelectric and pyroelectric properties but also in its complicated polymorphism.9-13 Among the four main crystal structures, R (nonpolar), Rp, β, and γ (all polar) of PVF2, the β phase receives most attention due to its ferroelectric properties.14-19 However, the most common polymorph is the R phase, the form obtained most often by crystallization from the melt or precipitation from solution. It is known that casting from acetone at room temperature gives the disoriented R phase.20,21 While preparing films of PVF2 from acetone/DMF solutions, we noticed that sometimes the films were transparent and other times * To whom correspondence should be addressed. E-mail: weuler@ chm.uri.edu. † Department of Chemistry. ‡ Department of Chemical Engineering. (1) Gregorio, R., Jr.; Cestari, M. J. Polym. Sci. B 1994, 32, 859. (2) Davis, G. T.; McKinney, J. E.; Broadhurst, M. G.; Roth, S. C. J. Appl. Phys. 1978, 49, 5042. (3) Beaulieu, R.; Lessard, R. A.; Chin, S. L. J. Appl. Phys. 1996, 79, 8038. (4) Marchetti, S.; Giorgi, M.; Simili, R. Infrared Phys. Technol. 1996, 37, 239. (5) Tashiro, K.; Itoh, Y.; Kobayashi, M.; Tadokoro, H. Macromolecules 1985, 18, 2600. (6) Kaura, T.; Nath, R.; Perlman, M. M. J. Phys. D 1991, 24, 1848. (7) Crowe, R.; Badyal, J. S. J. Chem. Soc. 1991, 14, 958. (8) Toci, G.; Mazzoni, M.; Mazzinghi, P. Rev. Sci. Instrum. 2000, 71, 1635. (9) Glennon, D.; Cox, P. A.; Ewen, R. J. J. Mater. Sci. 1998, 33, 3511. (10) Marand, H.; Stein, R. S. J. Polym. Sci., Part B 1989, 27, 1089. (11) Prest, W. M., Jr.; Luca, D. J. J. Appl. Phys. 1975, 46, 4136. (12) Hsu, C. C.; Geil, P. H. J. Appl. Phys. 1984, 56, 2404. (13) Prest, W. M., Jr.; Luca, D. J. J. Appl. Phys. 1978, 49, 5042. (14) McFee, J. H.; Bergman, J. G., Jr.; Crane, G. R. Ferroelectrics 1972, 3, 305. (15) Miller, R. L.; Raisoni, J. J. Polym. Sci., Part B 1976, 14, 2325. (16) Hsu, C. C.; Geil, P. H. J. Mater. Sci. 1989, 24, 1219. (17) Okuda, K.; Yoshida, T.; Sugita, M.; Asahina, M. Polym. Lett. 1967, 5, 465. (18) Southgate, P. D. Appl. Phys. Lett. 1976, 28, 250. (19) Kaura, T.; Bharti, V. J. Polym. Sci. 1994, 11, 295. (20) Ferroelectric Polymers; Nalwa, H. S., Ed.; Marcel Dekker: New York, 1995. (21) Kobayashi, M.; Tashiro, K.; Tadokoro, H. Macromolecules 1975, 8, 158.
the films were opaque white. IR spectroscopy indicated that the bulk phase was the same for both types of films. As reported here, our investigations showed that the different visual appearances arise because of the surface structure of the films. Transparent films have smooth surfaces and white films have rough surfaces that strongly scatter visible light. There are two contributing factors to the formation of the rough surfaces: the temperature at which the dissolution occurs and the humidity of the surrounding atmosphere during film formation. Experimental Section PVF2 powder was obtained from Aldrich (Mw 534,000). HPLC grade acetone and spectranalyzed grade DMF were obtained from Fischer-Scientific. The solvent was prepared as a v/v composition of DMF/acetone ranging from 5/95 to 50/50. Nominal polymer concentrations ranged from 25 g/L to 70 g/L. The polymer was dissolved at two different temperatures. Some solutions were dissolved at low temperature by sonication in an ultrasonic waterbath at 50 °C). In some experiments, the initial preparation was as in the low-temperature method, followed by equilibrating the solution at a given temperature (between 32 °C and 46 °C) in a waterbath for 1 h. The solutions were sealed and allowed to sit for 4 days at room temperature before any measurements were done in order to settle any undissolved polymer. The viscosity of the polymer solutions were measured with a Brookfield viscometer, model RFT with a UL adapter for low viscosity. Before each viscosity measurement, the samples were equilibrated in a water bath at 25 °C for 15 min. Free-standing thin films were obtained by either solution casting or spincasting a polymer solution onto highly polished single-crystal silicon wafers, followed by drying at room temperature or in a temperature-controlled oven. The surfaces of the silicon wafers were supplied with about 300 Å of the oxide and were RCA (rinses with acetone, methanol, deionized water, and then a N2 blow dry) cleaned prior to use. Spincasting onto the silicon wafers was done with a microprocessor-controlled spin coater from Laurell Technologies Corporation (model WS400), typically using a spinning speed between 500 and 2000 rpm and an acceleration rate of 1245 rpm/s. Solution casting was done by adding approximately 0.5 mL of solution to the silicon wafer and allowing the solvent to evaporate. The thickness of a
10.1021/la001206g CCC: $20.00 © 2001 American Chemical Society Published on Web 12/13/2000
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Benz et al. Table 1. Solution Lifetimes Prior to Gel Formation for Different PVF2 Solutions
Figure 1. Time dependence of the solution viscosity: filled circles, 5% DMF, low-temperature preparation; open triangles, 10% DMF, low-temperature preparation; filled squares, 15% DMF, low-temperature preparation; open diamonds, 20% DMF, low-temperature preparation; filled triangles, 2.5% DMF, hightemperature preparation. All solutions are 50 g/L PVF2. solution cast film from a 50 g/L PVF2 solution varied from about 4 µm in the middle to about 14 µm at the edge of the film. A spincast film was more uniform in thickness, varying somewhat between 1 and 2 µm. The relative humidity during deposition was measured using a Fisher Scientific Thermo-Hygro hygrometer. Film thicknesses were measured with a Sloan, Dektak IIA profilometer. The characterization of the resulting polymer films were performed by means of reflective light microscopy (Nikon, type 104), IR spectroscopy (Perkin-Elmer, 1600 series FTIR), and atomic force microscopy (Park Scientific Instruments, Autoprobe cp). The same type of microlever was used for all the measurements with the AFM in order to compare the individual roughness of the surface structure (type ML06D for contact mode from Park Scientific Instruments). Surface roughness was determined from the AFM images using the instrumentation software as the root-mean-square roughness from the standard definition:
Rrms )
x
N
∑(z
n
- zm)2
n)1
N-1
where Rrms is the surface roughness, zn is the height at point n, zm is the mean height, and N is the number of data points.
Results and Discussion Solution Properties. Polymer films produced from a 50 g/L PVF2 solution were mechanically stable and robust enough to be completely free-standing. PVF2 concentrations below this level resulted in deposited films that were too thin and could not be easily removed from the silicon wafers. The lifetimes of the solutions were limited, in some cases, by the formation of a gel. The viscosities of several PVF2 solutions were measured as a function of time, and the results of the rheological measurements are shown in Figure 1. All the polymer solutions displayed nonnewtonian, pseudoplastic fluid behavior. The decrease in viscosity with increasing shear rates is common in many polymer solutions. In addition, neither thixotropic nor dilatent behavior of the solutions was observed. It was discovered that solutions prepared at low temperature with small amounts of DMF or high PVF2 concentrations coagulated after a short period of time. Solutions prepared at high temperature had significantly longer lifetimes (defined as the time prior to gelation), even at the lowest DMF concentration. For example, the viscosities of a 70 g/L PVF2 (10% DMF) solution or a 50 g/L PVF2 (2.5% DMF) solution (both prepared at low temperature) started to increase with respect to time,
[PVF2] (g/L)
volume % DMF
solvation temperature
gelation time (days)
50 50 50 50 50 60 70 50 50
2.5 5 10 15 20 10 10 2.5 10
low low low low low low low high high
10 22 100 120 >120 23 5 >120 >120
Table 2. Effect of Solution Preparation Conditions on PVF2 Thin Film Appearance deposition solvation drying relative condition temperature temperature humidity 1 2 3 4 5 6 7 8
low low low low high high high high
low low high high low low high high
low high low high low high low high
film appearance transparent transparent transparent transparent transparent white transparent white
hence began to coagulate, in less than 10 days. In contrast, no significant viscosity increase in the first 120 days occurred in solutions prepared at high temperature, even with only 2.5% DMF, or in solutions prepared at low temperature with at least 20% DMF in solution. Table 1 gives the gelation times of the different polymer solutions. As expected, the solvation of the PVF2 was affected by both temperature and solution composition. Presumably, when DMF becomes dissolved in the long-chain macromolecules of the polymer (via interaction of the CdO dipole with the CH2CF2 dipole or by limited hydrogen bonding), it disrupts the dipolar and van der Waal’s interactions that hold the polymer chains together, thereby creating more room for chain motion.22 This disruption of the intermolecular polymer bonds occurs only to a limited extent in the case of a polymer solution with low DMF concentration and at low temperature but is enhanced in the case of elevated temperature, so even with low DMF concentrations, the interior of the polymer chain can be solvated. In either case, once the chain is disentangled, the increased mobility prevents gel formation. Thin Film Morphology. Different morphologies of the polymer films were obtained, depending on the solution preparation temperature, the DMF concentration, and the relative humidity. The temperature used to dry the films and the film preparation method (i.e., spincasting or solution casting) had no effect on the film surface properties. Table 2 shows the effect of the different deposition conditions on the visual appearance of the PVF2 films. Solutions prepared at low temperature always produced transparent films. However, films deposited from solutions prepared at high temperature resulted in opaque white surfaces when dried under high relative humidity conditions. Even though much faster desolvation occurred at higher drying temperatures, this had no effect on the surface morphology. In addition, white opaque samples were much less mechanically robust and exhibited less plasticity. Films prepared from solutions with more disentangled polymer chains (high DMF or high temperature) might be expected to lead to rougher surfaces since the more (22) Csernica, J.; Brown, A. J. Chem. Educ. 1999, 76, 1526.
Morphology of Poly(vinylidene fluoride) Films
Figure 2. Optical micrographs of films prepared from a PVF2 solution (50 g/L PVF2, 10% DMF) at high temperature and high humidity (A, deposition condition 6) and low humidity (B, deposition condition 5). In A, the polymer film appears opaque white and has a visually rough and inhomogeneous surface, whereas in B the polymer film is transparent and smooth.
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Figure 4. AFM image of a film from a 50 g/L, 10% DMF PVF2 solution using deposition condition 1. The transparent polymer film has a generally smooth morphology. The occasional features on the surface are due to incomplete dissolution of the PVF2 powder.
Figure 5. AFM image of a film from a 50 g/L, 10% DMF PVF2 solution using deposition condition 6. The opaque white polymer film has a rough morphology.
Figure 3. Optical micrographs of films prepared with different solvation temperature and DMF concentrations: A, deposition condition 3, 2.5% DMF; B, deposition condition 3, 10% DMF; C, deposition condition 3, 50% DMF; D, deposition condition 7, 2.5% DMF; E, deposition condition 7, 10% DMF; F, deposition condition 7, 50% DMF. The concentration of PVF2 is 50 g/L in all cases.
mobile polymers can disrupt smooth surface formation. The role of water in the gas phase is less clear. Perhaps during desolvation the hydrogen bonds from the water to the DMF near the surface slow the release of the solvent, thereby preventing collapse of the polymer chains into a dense smooth surface. Optical Microscopy. Reflection light microscopy showed that the difference in the visual appearance of the white and transparent films was due to the surface morphology, as demonstrated in Figure 2. The white polymer film (deposition condition 6) had a much rougher and more irregular surface structure. In contrast, the surfaces of transparent films (5) were smooth. Figure 3 shows the effects of solvation temperature and DMF concentration on the surface structure of transparent films examined by optical microscopy. As the DMF concentration increases, the surface roughness also increases, as shown in Figure 3 for films solution cast from solutions prepared at both high and low temperature (7 and 3, respectively). At low DMF concentration, the films were smooth, but they became increasingly rough as the DMF concentration was increased. Despite this increased roughness, the critical roughness necessary to obtain an
opaque film was never exceeded and the films were always transparent. Thus, the visual transparency was independent of the solvation temperature and whether the film was solution-cast or spin-cast. Polymer-solvent (DMF) complexation23,24 in the solution state is a likely cause for these structural modifications. As shown in micrographs C and F of Figure 3, samples with crystallite sizes of 5-10 µm are still transparent. This exceeds the wavelength of visible light, yet causes insufficient scattering for the film to be opaque to the eye. Thus, the particle size in the transverse direction of the film is not responsible for the visual appearance of the film. In contrast, the solvating power strongly influences the morphology. In good solutions (high DMF or high temperatures), where the polymer is more effectively solvated, there are fewer nucleation sites, so larger crystallites can be formed. At poor solvation conditions, there are more nucleation sites, which prevents particle growth and leads to films with smaller crystallites, as in A or B of Figure 3. There is some increase of surface roughness with particle size but not enough to lead to an opaque film. Atomic Force Microscopy. The surface structure, along with the average roughness of each sample was determined using AFM. Figure 4 shows an AFM image of a smooth, transparent film (deposition condition 1). The large artifacts observed (typically 15 × 15 × 1.5 µm) were likely due to undissolved polymer particles (these large features completely disappear for dissolution temperatures above 40 °C, although the overall roughness increases). The surface roughness of the relatively smooth areas was estimated to be 0.07 µm. In contrast, Figure 5 shows an AFM image of a white opaque film (deposition condition 6). Here, the surface topography was much rougher, 0.43 µm, than that shown in Figure 4. The film (23) Dikshit, A. K.; Nandi, A. K. Macromolecules 2000, 33, 2616. (24) Cheng, L. P. Macromolecules 1999, 32, 6668.
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Figure 6. Average surface roughness of polymer films as a function of the DMF concentration in solution. Circles: 50 g/L PVF2, deposition condition 3; triangles, 50 g/L PVF2, deposition condition 7.
Figure 7. Infrared spectra of films deposited from 50 g/L PVF2, 10% DMF solution. Film 1 uses deposition condition 5 and has a visually transparent appearance. Film 2 uses deposition condition 6 and has an opaque white appearance. The major difference in the spectra is the scattering between 4000 and 1500 cm-1.
side that was facing the wafer during casting was very smooth in all the cases, with average roughness varying from 0.02 to 0.06 µm. Figure 6 shows a plot of the average surface roughness of transparent films as a function of the DMF concentration. The minimum roughness was observed at concentrations of 10% DMF in solution. Infrared Spectroscopy. Figure 7 shows the mid-IR spectra of two PVF2 films, one transparent (deposition condition 5) and one white (deposition 6). The two films have the same average thickness and the infrared shows the same bulk spectra for both samples (R phase). 13,21,25-27 However, there are several differences in the IR spectra among the transparent and the white films. The major difference is the baseline slope between 4000 and 1500 cm-1. This is associated with scattering from the polymer surface. This slope of the baseline was plotted against the relative humidity and is shown in Figure 8 for both spincast and solution-cast films. The change in morphology from a transparent to a white face was commensurate with a sharp change in the IR baseline slope. For spincast, films this transition occurs at 35% relative humidity and for solution-cast films, the transition occurs at 23% relative humidity, as long as the solutions are prepared at temperatures greater than 50 °C. None of the samples contained spectral features associated with water, which would be observed in the IR spectra in the region of 3600-3200, 1635, and 700 cm-1. This was surprising, given the importance of the relative humidity on the surface morphology when depositing a polymer solution. After sufficient drying time (up to several
Benz et al.
Figure 8. Slope of the IR scattering profile between 1500 and 4000 cm-1 as a function of relative humidity. Circles, solutioncast; triangles, spincast. In both cases, the films were prepared from 50 g/L PVF2, 10% DMF, deposition condition 5 to 6. The sharp rise in each plot corresponds to the change from a transparent to white film. This occurs at ∼23% relative humidity for the spincast films and ∼35% relative humidity for the solution-cast films.
Figure 9. Plots of surface roughness measured by AFM (left axis) and IR baseline slope (right axis) as a function of dissolution temperature. Open circles, surface roughness, 11% relative humidity; open squares, surface roughness, 45% relative humidity; open triangles, surface roughness, 69% relative humidity; filled circles, IR baseline slope, 11% relative humidity; filled squares, IR baseline slope, 45% relative humidity; filled triangles, IR baseline slope, 69% relative humidity. All films were solution-cast at room temperature using 10% DMF and 50 g/L PVF2 and room-temperature drying conditions.
days), there was no evidence for any residual DMF or acetone trapped in the films, which would be easily detected by the presence of the carbonyl band near 1600 cm-1. Although all samples were primarily deposited as the R phase,13,21,25-27 there was evidence in the IR spectra that different amounts of the γ phase was found in films deposited from solutions prepared at high temperature. The peaks found at 1427, 1232, 950, 838, 812, and 510 cm-1 corroborate this conclusion. Figure 9 shows a plot of the surface roughness and the IR slope as a function of dissolution temperature for different relative humidity conditions employed during deposition. Both determinations of the surface quality show a sharp rise at about 42 °C, indicating that this is the temperature at which a significant change in the solution structure must be occurring. The magnitude of the surface roughness also depended upon the relative humidity during deposition but is constant, within experimental error, up to the sharp rise. In contrast, the (25) Cortili, G.; Zerbi, G. Spectochim. Acta 1967, 23, 285. (26) Wentink, T.; Willwerth, L. J.; Phaneuf, J. P. J. Polym. Sci. 1961, 55, 551. (27) Tashiro, K.; Kobayashi, M.; Tadokoro, H. Macromolecules 1981, 14, 1757.
Morphology of Poly(vinylidene fluoride) Films
Figure 10. Plot of surface roughness measured by AFM against relative humidity (right axis, triangles) and IR baseline slope (left axis, circles). Films were deposited from a 50 g/L PVF2 10% DMF solution using deposition conditions 5 to 6. The dashed line shows the linear correlation between the surface roughness and the relative humidity.
slope of the IR spectrum baseline was independent of the relative humidity as long as the casting solution was prepared below the 42 °C transition. Finally, the roughness of the polymer film prepared from solutions dissolved at high temperature, as measured by AFM, was plotted against the IR baseline slope and the relative humidity (Figure 10). At a surface roughness of about 0.31 µm, which corresponds to a film cast at 35% relative humidity, there was a dramatic rise in the baseline slope. However, the plot of the relative humidity against the surface roughness was essentially linear and did not exhibit a significant change when the appearance of the polymer films turned from transparent to opaque white. Conclusion The solution properties of PVF2 in acetone/DMF are strongly dependent upon preparation conditions. The time to gel formation depends on both the concentration of DMF and the dissolution temperature. The optimal solution preparation for the longest lifetime is achieved by dissolving PVF2 at high temperature with a concentration of 50 g/L in a solvent that is composed of 10% DMF/90% acetone. Interestingly, even though the solution lifetime varies with preparation conditions, the initial viscosities of all solutions of a given concentration are the same. This suggests that the complexation between the DMF and the polymer is relatively slow. However, the complexation can be promoted by increasing the DMF concentration or
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by increasing the temperature, as evidenced by the increasing gelation time. Finally, whatever the structure of the DMF-PVF2 complex, it causes a disruption of the long-range order required to form the gel. The nature of a PVF2 thin film formed either by solution casting or spincasting also depends on the solution properties and the relative humidity. All films reported here were primarily R phase, but the visual appearance and mechanical strength of each film changed with deposition conditions. Opaque white, mechanically unstable films were formed from solutions prepared at high temperature and deposition at high relative humidity. The white appearance arises because of light scattering from rough surfaces. The surface roughness of the films, as estimated by AFM, depended upon DMF concentration, the dissolution temperature, and relative humidity. Infrared spectra showed a strong scattering profile in the 1500-4000 cm-1 region that was used as a marker of the surface roughness and showed a sharp transition between transparent and white films. Spincasting provided a more uniform film thickness compared to solution casting, but otherwise there were few other differences associated with the deposition method. Since the size of the surface roughness increased with DMF concentration, this suggested that the DMF-PVF2 complex was critical in determining the structure of the deposited film. Only those solutions that contained a large amount of the complex (high temperatures and high DMF) yielded rough surfaces. Further, high relative humidity was also required to attain a surface roughness visible to the naked eye. The role of the gas-phase water in promoting a rough surface was not obvious. One possible mechanism is that the water abruptly assists in breaking up the DMF-PVF2 complex at the final stages of desolvation, perhaps via hydrogen bonding to the DMF. Investigation of this hypothesis will be the subject of future work. Acknowledgment. Financial support for this research project was provided by the National Science Foundation (Grant 9729819), Teltron Technologies, Inc., and the URI Sensors and Surface Technology Partnership. Cherry Semiconductor donated the silicon wafers used in this work, and Rene´e Mallen was responsible for oxidation of the wafers. We also thank Dr. Everett Crisman for many helpful discussions. LA001206G
