Crystalline Phase Formation of Poly(vinylidene fluoride)

Journal of Macromolecular Sciencew, Part B: Physics, 47:434–449, 2008 Copyright # Taylor & Francis Group, LLC ISSN 0022-2348 print/1525-609X online DOI: 10.1080/00222340801954811

Crystalline Phase Formation of Poly(vinylidene fluoride) from Tetrahydrofuran/ N,N-dimethylformamide Mixed Solutions WENZHONG MA,1 JUN ZHANG,1 SHUANGJUN CHEN,1 AND XIAOLIN WANG2 1

College of Materials Science and Engineering, Nanjing University of Technology, Nanjing, P. R. China 2 Department of Chemical Engineering, Tsinghua University, Beijing, P. R. China The present work focused on the effect of the interactions between poly(vinylidene fluoride) (PVDF) chains and solvent molecules on the structure and crystallization behavior of PVDF in films obtained by solution casting. In a single solvent system, the film cast from the good solvent of N,N-dimethylformamide (DMF), showed dominantly b-phase crystals with the highest PVDF crystallinity (50.6%) and the largest spherulite size, about 4 mm, at the top surface. The samples deposited from good swelling agents, such as tetrahydrofuran (THF) and methyl ethyl ketone (MEK), exhibited mainly the original a phase with some amount of b-phase crystals; the crystallization behavior and the morphology of the surface were similar to the original PVDF resin, because of the only partially dissolved PVDF chains in these two solvents. In a mixed solvent system (THF/DMF), the b phase formation linearly increased as the DMF component increased, determined by Fourier transform infrared spectroscopy (FTIR) techniques, owing to increased interactions between PVDF chains and DMF molecules. The film surface consisted of b spherulites with average size of about 3 mm, which were smaller than those grown from pure DMF, because of the increased crystallization rate in the mixed solvent. Keywords solution

poly(vinylidene fluoride) (PVDF), crystallization, crystalline phase,

Introduction Poly(vinylidene fluoride) (PVDF) is a semicrystalline polymer with at least four crystalline phases, a, b, g, and d, which differ both in lattice type and chain conformation in the lattice.[1,2] The b phase has an orthorhombic structure and an all-trans (TTTT) molecular conformation (a planar zigzag structure). The a phase has a monoclinic lattice with trans-gauche (TGTG0 ) conformation; the g phase has a TTTGTTTG0 conformation with an orthorhombic lattice; and the d phase corresponds to the polar form of the a phase but has an orthorhombic lattice. Because of the strong electronegativity of fluorine atoms compared to those of hydrogen and carbon, each PVDF chain possesses a dipole Received 3 October 2007; Accepted 27 January 2008. Address correspondence to Jun Zhang, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing, P. R. China. E-mail: [email protected]

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moment perpendicular to the polymer chain.[2] If the polymer chains pack in crystals with parallel dipoles, the crystal possesses a net dipole moment, as in the polar form b, g, and d phases; whereas, with antiparallel chain dipoles, the net dipole moment vanishes as in the nonpolar a phase.[2] Among these phases, the b phase contains the largest piezoelectric and pyroelectric coefficient.[1] Its piezoelectricity and pyroelectricity properties,[3,4] which provide possibilities for many technological applications, have attracted much attention and has been extensively investigated over the past several decades. Currently, its copolymer—poly(vinylidene-fluoride – trifluoroethylene) [P(VDF-TrFE)] has been effectively applied as an electroactive polymer; its ferroelectricity arises from the all-trans conformation.[5,6] As to obtaining each phase, the a phase is the most common, and is usually obtained by cooling the melt,[7] the g phase by annealing the a phase crystals or isothermal crystallization at high temperature,[8] and the d phase by applying an electric field to the a phase.[9] Direct methods for preparing the b phase in PVDF homopolymer include blending with small amounts of poly(methyl methacrylate) (PMMA)[10,11] or poly(omethoxyaniline) (POMA),[12] quenching, and then annealing[13] and crystallizing from solution under appropriate conditions.[14 – 16] Indirect methods (i.e., phase transition, which can increase the fraction of the b phase during processing, accompanied by an increase in the piezoelectric constant[2]) include tensile deformation and uniaxial compression deformation of the a phase,[17,18] and applying a strong electric field.[19] In all these methods for preparing the b phase, solution deposition, especially solution casting, is a convenient way to prepare b-phase PVDF films under controlled conditions. It is known that PVDF films prepared from various solvents at different temperatures yield different crystal phases.[20] However, explanation about how the b phase develops from the solution casting has not been clarified. In this work, solutions prepared in good swelling agents (THF, MEK) and a good solvent (DMF) were used to investigate the effect of solvent properties on the morphology and crystallization behavior of PVDF films obtained by solution casting. Mixed solvents (THF/DMF) with various mass ratios were used to examine the mechanism of b phase formation during the process. The analysis was carried out by Fourier transform infrared spectroscopy (FTIR), wide angle x-ray diffraction (WAXD), differential scanning calorimetry (DSC), and scanning electron microscope (SEM) techniques.

Experimental Materials PVDF (Kynar K-761) (powder) was supplied by Elf Atochem of North America Inc. (USA). N,N-dimethylformamide (DMF) was obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Methyl ethyl ketone (MEK) and tetrahydrofuran (THF) were purchased from Ling Feng Chemical Reagent Co., Ltd. (China). Preparation of Samples The original PVDF resin (powder) without any treatment was partially dissolved in good swelling agents such as MEK and THF at 608C[21] with stirring for 24 h; whitish, opaque suspensions were obtained mainly due to the original PVDF particles, which were not fully dissolved. Subsequently, samples were formed by depositing the suspensions onto a clean glass substrate at 508C for evaporation. Then the cracked and not continuous samples were

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obtained. A good solvent (DMF)[21] and mixed solvents with different mass ratio (THF/DMF ¼ 9:1, 8:2, and 5:5) were also used to dissolve the original PVDF resin at 508C. After stirring for 24 h, the transparent solutions were cast onto glass substrates; then, the solution evaporation was carried out at 508C. Free-standing flat films (thickness 30 mm) were formed in the end when the solvent completely evaporated. The initial polymer concentration of the solution was 10 wt.% PVDF.

Characterization Techniques Fourier transform infrared spectroscopy spectra of the films (obtained from DMF and the mixed solvents), the original powder, and the cracked samples (obtained from THF and MEK solvents) in a KBr pellet were obtained on a FTIR spectrometer (Bruker Vector22) with resolution of 4 cm21. Wide angle x-ray diffraction (WAXD) of the films, the original PVDF powder sample, and the cracked samples were done in reflection in a diffractometer (Shimadzu XRD-6000) at a scanning velocity of 48/min (Cu Ka radiation ˚ ), 40 kV, and 30 mA). Calorimetric analyses were carried out using differen(l ¼ 1.542 A tial scanning calorimetery (DSC) (Perkin-Elmer DSC-7C), at a heating rate of 108C/min. Micrographs (SEM) were obtained in a scanning electron microscope (JEOL JSM-5900) with an accelerating voltage of 15 kV after coating with Au in vacuum.

Results and Discussion Crystallization from the Single Solvents Figure 1 presents the FTIR spectroscopy (region 3100– 2900 cm21 and 1000 – 400 cm21) results for the original PVDF resin and the samples prepared from various organic

Figure 1. FTIR spectra for (a) the original PVDF resin, and for samples obtained from different solutions at 508C: (b) THF, (c) MEK, (d) DMF.


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solvents. The spectra of the original PVDF resin and the samples obtained from THF and MEK contain the same defined absorption bands for crystalline phases (as marked); i.e., these three samples have similar crystalline structures. To the contrary, different absorption bands are obtained from the DMF film. In the region 3100 –2900 cm21, the two bands for each sample are very close and may correspond to the a, b, or g phase. The band at 3022 cm21, refers to the antisymmetric stretching of CH2,[20] while, the band at 2982 cm21, which refers to the CH2 vibration (type of segment TTT or TGT), is independent of the PVDF phases.[22] In the region 1000 – 400 cm21, the bands at 975, 796, 764, 614, 532, and 408 cm21 are characteristic of the a phase and the band at 510 cm21 is characteristic of the b phase.[15] However, the g phase and b phase can coincide or are very close to each other near 510 cm21, owing to the TTT segment conformations in these two phases. So further identification of the crystalline phase of PVDF should be done by other techniques (WAXD). Whatever the crystalline phase exists in the original PVDF, an additional crystalline phase can be induced to different levels in the various solvents. In Fig. 1 there is a change in absorbance of the band at 510 cm21. This implies that the THF and MEK have an effect on the TTTT segment conformations to some extent. To identify, the absorbance ratio of the band at 510 cm21 to that observed at 2982 cm21 was employed. The absorbance ratio increased to 5.4 and 5.2, respectively, for the THF and MEK cast films, in comparison with the original PVDF resin (3.8); i.e., some g phase or b phase forms in the cast films. It has been demonstrated that g phase formation needs high temperature (.1608C).[14,15,23,24] Therefore, the partially dissolved PVDF chains in the suspensions, when prepared from THF and MEK at 508C, mostly crystallize into b-phase crystals. As for the DMF cast film, only one absorbance band for the TTT segments conformations at 510 cm21 is observed; all of the a-phase absorbance bands disappear, which means that the b phase is predominant in the DMF cast film. The absorbance ratio of the band at 510 cm21 to that observed at 2982 cm21 is 2.6, which is lower than that obtained from THF and MEK suspensions. This may be due to the different experimental methods used when doing the FTIR measurements, the DMF film being cast on the KBr while the other samples were ground with the KBr and then pellitized. In addition, the band at 659 cm21 is characteristic of the O55C-N bending modes in DMF and the band at 675 cm21 may be assigned to the O55C-N bending modes in the complex, which was formed as a result of interaction between DMF and PVDF chains.[25] The weak band at 433 cm21, which refers to the g phase, indicates that the segment conformation of the g phase was detected by FTIR. The identification of the crystalline phase in the bulk cast films was carried out by WAXD, discussed below. To obtain further information of the crystal structure, WAXD experiments were performed. The WAXD profile for the original PVDF resin (Fig. 2a) reveals three strong peaks of the crystalline phase at the diffraction angles 2u ¼ 18.21, 19.97, and 26.628, and four weak peaks at 2u ¼ 33.00, 35.91, 38.82, and 41.178. These peaks can be associated with the reflections of the (020), (110), (021); and (130), (200), (002), and (201) planes, respectively, typical for the monoclinic cell of PVDF in its crystalline phasea.[15,19,26] Thus, the a phase was the predominant crystalline phase in the original PVDF resin. Similar to the FTIR analysis, the WAXD results of the cast films can be divided into two classes. One includes samples from THF and MEK (partially dissolving PVDF resin); the other is DMF cast film (completely dissolving PVDF resin). The WAXD profiles for the THF and MEK samples are shown in Figs. 2b and c, respectively, which are both very similar to those obtained from the original PVDF resin (Fig. 2a). As denoted by arrows in Fig. 2, the peak at 19.978 in the original PVDF resin shifts to a larger angle

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Figure 2. X-ray diffractograms of (a) the original PVDF resin, and for samples obtained from different solutions at 508C: (b) THF, (c) MEK, (d) DMF.

(about 20.268, which is the sum of the (110) a reflection and the (200) reflections characteristic of the b phase[15,26]) in the THF and MEK samples; and the intensity of the peak at this position is enhanced compared with the original PVDF resin, suggesting an increased total crystallinity of PVDF in the THF and MEK samples.[27] This indicates that small amounts of b phase of PVDF formed in the THF and MEK samples, which agrees well with the FTIR analysis. As for the film cast from DMF, only the sum of the (110) and (200) reflections at 20.358 is observed (Fig. 2d), which means that the b phase of PVDF was the predominant crystalline structure in this sample and no g phase is illustrated in the WAXD trace. The g phase crystals may be so few that they can just be detected by FTIR measurement. The type of solvent dissolving PVDF at 508C has an impact on the DSC results, as shown in Fig. 3. Analyzed by FTIR and WAXD, the original PVDF resin mainly possesses the a phase, and the films cast from DMF are predominantly b phase; while the mixture of these two phases are present in the THF and MEK samples. As reported, the melting peak of a- and b-PVDF are so close that the difference between them is only 38C (the a-PVDF has the lower melting peak temperature)[28] or they are very close.[14,15] Therefore, the melting peak at the lower temperature in Figs. 3a – c is for the a phase of PVDF; the one for DMF cast film is in correspondence with the b phase. The melting of the small amount of b phase formed in THF and MEK samples is overlapped by the major a phase in the samples (Figs. 3b, c). Table 1 lists the data obtained from Fig. 3, which illustrates the influence of the solvent type on PVDF crystallization. Here, DHm is the heat of fusion of the sample; Xc is the relative crystallinity of PVDF, calculated as in our previous work.[11] The melting temperatures, Tmon, Tmp, and Tmf, observed in the melting curves are the onset melting temperature, peak melting temperature, and final melting temperature, respectively. The values for THF and MEK samples are close to those of the original PVDF resin. This means the original PVDF crystalline phase (a) was not obviously changed by THF and MEK. The values of Tmp and Tmf obtained from DMF are the highest in the samples obtained from the three solvents and the original PVDF resin. According to FTIR and WAXD results,


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Figure 3. DSC curves of (a) the original PVDF resin, and for samples obtained from different solutions at 508C: (b) THF, (c) MEK, (d) DMF.

the b phase is mainly formed from DMF solution; it has a melting peak temperature of 169.78C. The values DTm (DTm ¼ Tmf 2 Tmon) for the samples fluctuate with different solvents, suggesting that various interactions between PVDF chains and solvent molecules gave rise to different crystallization behaviour of PVDF. The effect of solvent type on the crystallinity of PVDF is significant. For THF and MEK, crystallinity (Xc) of PVDF was 38.1% and 40.0%, respectively, which is higher than that in the original PVDF resin (37.1%). The highest crystallinity of PVDF was obtained for the film cast from DMF with Xc ¼ 50.6%. This may be attributed to the various solubilities of PVDF in each solvent. Both THF and MEK just partially dissolve the PVDF resin, resulting only in swelling; only a small extra b phase exists in the deposited samples compared with the original PVDF resin. However, DMF is a good solvent for PVDF and the amorphous and crystalline regions were completely dissolved. After being dissolved in DMF at 508C, the PVDF chains can crystallize into

Table 1 DSC melting results of the original PVDF resin and samples obtained from different solvents Solvent Original PVDF resin THF MEK DMF

Tmon (8C)

Tmp (8C)

Tmf (8C)

DTm (8C)

DHm (J . g21)

Xc (%)

156.7

165.8

170.7

14.0

38.8

37.1

156.6 156.3 157.8

168.5 166.7 169.7

173.3 170.6 174.5

16.7 14.3 16.7

39.8 41.8 52.9

38.1 40.0 50.6

Tmon: onset melting temperature of PVDF; Tmp: peak melting temperature of PVDF; Tmf: final melting temperature of PVDF; DTm ¼ Tmf 2 Tmon; DHm: melting enthalpy; Xc: crystallinity of PVDF.

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b phase, which then prevails in the film; thus, this sample yielded the highest crytallinity of 50.6%. In a good solvent (DMF), PVDF chains can be expanded and become flexible; as crystallization occurs, these chains can be arranged into the lattice easily resulting in the high crystallinity of PVDF. In addition, the crystallinity obtained in the MEK sample was higher than that from THF. This may be ascribed to the fact that MEK can swell PVDF better than THF does during the dissolution procedure; and more b phase of PVDF formed in the MEK sample. The original morphology of the PVDF resin (powder) is shown in Fig. 4. The original PVDF resin is full of small particles (about 0.3 mm) or aggregates of these particles, which may be the as-polymerized particles. As demonstrated by FTIR and WAXD in previous sections, predominantly a phase is present in these particles. Figure 5 gives the surface morphology of the deposited or cast samples with distinct surface structures. As expected, the morphologies of the samples depend on the solvents. For THF and MEK, both of which just partially dissolve the PVDF resin, on the top surface of the samples (grown at the air– solution interface) it seems that the swollen particles solidified with substantial amorphous regions (Figs. 5a, b). The particles of PVDF resin, which did not undergo dissolution, aggregated at the bottom of the films, as shown in Fig. 5d, e, which is similar to that of the original PVDF resin (Fig. 4). However, the one cast from DMF has discernable PVDF spherulites with average diameter about 5 mm. In the DMF solution, PVDF resin can be completely dissolved; the chains are expanded and become flexible in the solution. The increased mobility of the chains can favor nucleation and spherulite growth when crystallizing,[29] leading to large spherulites on the top surface of the film (Fig. 5c). Additionally MEK swells PVDF powder better than THF, so on the top surface the original polymerization particles cannot be seen clearly. This is consistent with the results that the crystallinity obtained from MEK is higher than that obtained from THF. These results indicate that THF and MEK just swell and partially dissolve PVDF resin, resulting in mainly a and, to a lesser extent, b phase; while DMF can completely dissolve PVDF and predominantly b phase is obtained. As reported, there are interactions between the DMF molecules and the PVDF chains via the C55O dipole with the CH2CF2 dipole or by limited hydrogen bonding.[30] This interaction disrupts the dipolar and van der Waal’s interactions that hold the polymer chains together, thereby creating more room for

Figure 4. SEM micrograph of the original PVDF resin.


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Figure 5. SEM micrograph of the cast films from (a, d) THF, (b, e) MEK, (c, f) DMF solutions at 508C; a, b, c for the top surface (grown from the air – solution interface) and d, e, f for the bottom surface of the films (grown from the solution – substrate interface).

chain motion.[31,32] On the other hand, the molecular interactions between polymer chains and solvent also play an important role during the crystallization process. In a solution, crystallization begins with the formation of liquid-like clusters of PVDF molecules and the rate-limiting organization of such clusters into crystal nuclei.[32] According to the model proposed by Abraham,[33] dipoles of the molecules in the nuclei have an influence on the crystallization of the solute in the solution. He and his coworker pointed out that the dipolar interaction and hydrogen bonding between a PVDF chain and DMF molecules can preferentially lead to trans conformation packing of CH2-CF2 dipoles, which is the b phase. Therefore, DMF can favor the b phase of PVDF when crystallizing at 508C. In order to confirm the effect of the interface between the PVDF nucleus and DMF molecules on the b phase formation at low temperature (508C), THF/DMF mixed solvents with different mass ratios were used, as discussed in the following section.

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Crystallization from the Mixed Solvents With allowance for the presence of different crystallographic phases, an attempt was made to predict the possibility of a solvent-induced b phase of PVDF theoretically. In this section, we assume the formation of b phase was induced by the DMF in the mixed solvent. The mixed solvents with various mass ratios, with m/m composition of THF/DMF at 9/1, 8/2, and 5/5, were used to dissolve the original PVDF resin at 508C; homogeneous transparent solutions were obtained. Then they were crystallized at 508C in the oven to form the cast films. Figure 6 shows the FTIR spectra in the region 3100 –2900 cm21 and 1000 –400 cm21 for the solution-cast films derived from the mixed solvents at 9/1, 8/2, and 5/5. In the region 3100– 2900 cm21, two bands at 3022 and 2982 cm21 refer to the CH2 vibration are observed.[22] These peaks in curve c (Fig. 6c) are low due to a higher baseline attributed to residual DMF. In the region 1000 –400 cm21, the absorption bands at 976, 855, 796, 764, 614, 532, and 410 cm21 correspond to the a phase; while the bands at 510 cm21 refer to the b phase.[14,15] It is clear from the figure that both a and b phases existed in the films when the mass ratio of THF/DMF is 9/1 and 8/2. As the DMF content increased further (THF/DMF ¼ 5/5), most absorption bands for the a phase disappeared or decreased in intensity; and there was an increase in the bands characteristic of the b phase (510 cm21). This implies that for the THF/DMF ¼ 5/5 sample, the presence of DMF in the solution favored the b phase formation. On the other hand, the band at 659 cm21, characteristic of the O55C-N bending modes in DMF, is enhanced as DMF content increased and the band at 675 cm21, which is formed as a result of interaction between DMF and PVDF chains,[25] is more obvious with more DMF in the mixed solvent. This suggests that as DMF increased in the mixed solvent the interactions between PVDF chains and DMF molecules were enhanced. In order to find the relationship between b phase formation and DMF content in mixed solvent, the absorbance ratio of the band at 510 cm21 to that observed at 2982 cm21 (independent on the crystalline phase of PVDF[22]) was calculated. In Fig. 7, the absorbance

Figure 6. FTIR spectra of PVDF films by casting from the solution with various mass ratios of THF/DMF: (a) 9/1, (b) 8/2, (c) 5/5.


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Figure 7. Absorbance ratio, a measure of b phase in the cast film, as a function of the DMF content in the mixed solvent.

ratio is shown as a function of the DMF content in the mixed solvent. As expected, the b phase of PVDF obtained from mixed solvent increased linearly with the increasing DMF content. In addition, as the DMF content rises up to 20 wt.% (THF/DMF ¼ 8/2), a small absorption band at 431 cm21, characteristic of g phase, can be observed; at THF/ DMF ¼ 5/5, two small absorption bands at 431 and 812 cm21 are present in the cast film. This implies that highly polar solvents (i.e., the DMF composition increased) may induce the b phase (TTTTTT) formation as well as a smaller amount of g phase (TTTGTTTG0 ) formation. Further identification of the crystalline phase in the samples was done by the WAXD measurements as follows. Figure 8 shows the wide angle x-ray diffractograms of the films cast from the mixed solvents. The films cast from THF/DMF ¼ 9/1 and 8/2 have peaks at 2u ¼ 18.5, 26.8, and 39.08, corresponding to reflections from the planes (020), (021), and (002) respectively, all characteristic of the a phase of PVDF.[14,19] In addition, both of them have a weak peak at 2u ¼ 20.38, corresponding to the sum of the reflections from the (110) and (200) planes, characteristic of the b phase.[14,19] The intensity of the peaks at 2u ¼ 20.3 and 39.08, characteristic of the b phase and a phase, respectively, are enhanced for THF/DMF ¼ 8/2, in comparison with the result obtained for THF/ DMF ¼ 9/1. This may be due to the fact that the mixed solvent of THF/DMF ¼ 8/2 enhances b phase formation and the growth of the (002) plane. With respect to the case for THF/DMF ¼ 5/5, the main peak at 2u ¼ 20.58, which represents the b phase, is predominant; while the (020) and (021) peaks assigned to a phase completely disappear. Compared with the cast film from THF/DMF ¼ 8/2, the peak at about 398, corresponding to the (002) a reflection, is weaker. This can be interpreted by the enhanced interactions between PVDF chains and DMF molecules. As THF/DMF ¼ 5/5, stronger dipolar interactions between PVDF and DMF (i.e., hydrogen bonds C55O...H-C) favor the local packing of CH2-CF2 dipoles when PVDF is crystallizing. So mainly b phase is present, while a phase is hindered in the film cast from THF/DMF ¼ 5/5. Table 2 lists values of 2u and the interplanar spacing, d, corresponding to peaks observed in the diffractograms of the films cast from mixed solvents with different

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Figure 8. WAXD diffractograms for PVDF films by casting from the solution with various mass ratios of THF/DMF: (a) 9/1, (b) 8/2, (c) 5/5.

Table 2 WAXD data of PVDF films cast from mixed solvents hkl

˚) dref (A

THF/DMF

2u (8)

˚) d (A

˚) L (A

020

4.82a,b

10/0 9/1 8/2 5/5

18.42 18.80 18.55 —

4.82 4.72 4.78 —

42.62 42.64 39.79 —

110, 200

4.38a

10/0 9/1 8/2 5/5

20.27 20.55 20.28 20.50

4.38 4.32 4.38 4.33

53.37 59.86 61.90 67.98

021

3.34a,b

10/0 9/1 8/2 5/5

26.81 27.29 26.83 —

3.32 3.26 3.32 —

60.60 50.86 78.98 —

002

2.31b

10/0 9/1 8/2 5/5

38.84 39.36 38.95 —

2.32 2.29 2.31 —

45.04 59.58 53.30 —

a

From literature.[15] From literature.[19] —: Not observed. u: Bragg angle; d: the interplanar distance; L: the lateral crystal size. b


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compositions of THF and DMF in (Fig. 2b and Fig. 8). The Bragg equation and Scherrer equation[34] were applied to calculate the d value and the lateral crystal size Lhkl in the direction perpendicular to the (hkl) crystal plane, based on the (hkl) reflection peak in the WAXD profile, as reported in previous work.[11,35,36] The calculated lateral crystal size L(110, 200) for the b phase increased with increasing DMF content (Table 2). For even a small amount of DMF in the mixed solvent (THF/DMF ¼ 9/1), the value of ˚ , in comparison with that for a pure THF L(110, 200) increased from 53.37 to 59.86 A sample. This confirms that the stronger dipolar interaction between PVDF chains and the solvent molecules favors growth of the (110, 200) planes as DMF content increased. Thus, the completely dissolved PVDF chains can crystallize into the b phase preferably, as DMF increased in the mixed solvent. To the contrary, the L value of the a phase decreased, except for the crystalline (021) plane. It seems that for THF/DMF ¼ 8/2, the growth of the crystalline (021) plane was favored, resulting in the highest value of L(021). For the crystallization in a single solvent, a prevailing a phase formed in the THF and MEK samples; conversely, the predominant b phase was present in the film cast from DMF. This indicates that mainly one phase is observed in the films, resulting in the one melting peak in the DSC curves (Fig. 3). As for crystallization in the mixed solvent with various mass ratios of THF/DMF, different observations were found. In Fig. 9 it is observed that the films obtained from solution with THF/DMF ¼ 9/1 and 8/2 have two endothermic peaks (curves a, and b, respectively). The lower temperature endotherm (at circa 1628C) corresponds to the melting of a phase and the higher one refers to the b phase.[14,15,18] As DMF content increased to the level THF/DMF ¼ 5/5, only one peak emerged in the DSC trace (Fig. 6c) and the a phase endotherm disappeared. This can be attributed to the fact that primarily b phase formation occurred for this condition and few a phase crystals formed. This result fits well to that obtained by FTIR and WAXD analysis. In addition, the melting peak of the b phase shifted to a higher temperature, as shown in Fig. 8. This may be due to the more perfect b crystal structure formed in the mixed solvent at higher DMF content. Therefore, it is confirmed that when DMF is low in the mixed solvent (THF/DMA ¼ 9/1 and 8/2) both a and b phase of PVDF formed in the films. However, predominantly b phase of PVDF is promoted by more DMF in the mixed solvent (THF/DMA ¼ 5/5), resulting in a sharp melting peak (Fig. 3c).

Figure 9. Melting curves of DSC trace for PVDF films by casting from the solution with various mass ratios of THF/DMF: (a) 9/1, (b) 8/2, (c) 5/5.

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W. Ma et al. Table 3 DSC melting results of PVDF films cast from mixed solvents

Mixed solvent (THF/DMF)

Tmon (8C)

Tmp (8C)

Tmf (8C)

DTm (8C)

DHm (J . g21)

Xc (%)

9/1 8/2 5/5

151.3 155.5 158.6

165.6 166.9 168.7

169.5 173.6 173.9

18.2 18.1 15.3

55.4 57.9 57.8

53.0 55.4 55.3

Tmon: onset melting temperature of PVDF; Tmp: peak melting temperature of PVDF; Tmf: final melting temperature of PVDF; DTm ¼ Tmf 2 Tmon; DHm: melting enthalpy; Xc: crystallinity of PVDF.

Table 3 lists the values obtained from the melting curves in Fig. 9. Compared with data for THF in Table 1, the melting temperatures Tmon, Tmp, and Tmf are higher for THF/ DMF ¼ 9/1; simultaneously, the melting enthalpy DHm for THF/DMF ¼ 9/1 increased from 38.8 to 55.4 J . g21 resulting in an increase in the crystallinty from 37.1% to 55.3%. Thus it can be deduced that even a small amount of the DMF mixed with THF can induce formation of the b phase and enhance crystallinity. As DMF increased in the mixed solvent, all the melting temperatures and the crystallinity of PVDF increased. This result agrees well with the FTIR and WAXD analysis. The values DTm (DTm ¼ Tmf 2 Tmon) decreased with increased DMF content in the mixed solvent, suggesting an increasing homogeneity of the PVDF crystals from solution crystallization. Additionally, the crystallinity of PVDF obtained from the mixed solvents was higher than that obtained from pure DMF. This suggests the mixed solvents of THF/ DMF can promote PVDF crystallization. The surface topography of the cast films can clearly be seen in the SEM photographs, as shown in Fig. 10. Films cast from THF/DMF ¼ 9/1 and 8/2 have a similar surface structure with irregular crystalline structure on the top surface (grown at air– solution interface), in which the a phase of PVDF prevails, corresponding to FTIR and WAXD results. Similar morphology of a PVDF sample crystallized from the melt was reported in Lovinger’s work,[7] and it is characteristic of a phase crystals. Films cast from THF/ DMF ¼ 5/5 show a very different feature on the top surface, consisting, we suggest, of distinct spherulites that correspond to the b crystals of PVDF.[11,15,24] This indicates that b phase formation is favored at THF/DMF ¼ 5/5. In Fig. 10, smoother bottom surfaces (grown at solution/substrate interface) are observed in the mixed solvent cast films (Figure 5d –f). It is observed that for the film cast from THF/DMF ¼ 50/50, the spherulite size formed on the top surface is about 3 mm, which is smaller that those obtained from pure DMF cast film. This is attributed to the difference in the crystallization rate in these two samples. The evaporation rate of the mixed solvents is faster than DMF because of the existence of THF (it is more volatile than DMF). As reported in our previous work,[36] the growth of spherulite includes nuclei formation and the growth of PVDF crystals. In comparison with the one crystallizing from DMF, when PVDF crystallized in the mixed solvent (THF/DMF ¼ 5/5) at 508C, not only was the formation of nuclei quick, but also the restrained polymer chains in solution had less time to contribute to the growth of PVDF spherulites due to the fast evaporation rate. So the spherulite size obtained in THF/DMF ¼ 5/5 sample is smaller than that obtained from the pure DMF film. As for the mixed solvents (THF/DMF ¼ 9/1, 8/2), the faster evaporation rate of


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Figure 10. SEM micrograph of the cast films from the mixed solvents (THF/DMF) at 508C: (a, d) 9/1, (b, e) 8/2, (c, f) 5/5. a, b, c for the top surface and d, e, f for the bottom surface of the films.

solvent also applies. This may also be the cause of the absence of the spherulitic structure on the top surface of the films.

Conclusion In conclusion, PVDF films deposited from the single organic solvents, such as THF, MEK, and DMF, at 508C showed different crystallization behavior and morphology. The solvent DMF, which was good for PVDF, induced cast films with predominantly b-phase crystals, with the highest crystallinity of 50.6% and the largest spherulite size about 4 mm at the top surface. Samples deposited from the good swelling agents (THF, MEK) exhibited mainly original a phase with small amounts of extra b-phase crystals; the crystallization behaviors detected by DSC and the morphology of the surfaces were similar to the original PVDF resin.

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In a mixed solvent system (THF/DMF), a small addition of DMF can improve the solubility for PVDF; and both a and b phases were yielded in the films. It was interesting to find that b phase formation increased as the DMF component increased. By calculation using FTIR techniques, the b phase of PVDF obtained from the mixed solvent increased linearly with the increased DMF content. The surface of the film cast from THF/ DMF ¼ 5/5 consisted of b spherulites with an average size of about 3 mm, which were smaller than those grown from pure DMF. We attribute this to the increased crystallization rate in the mixed solvent. For the other two mixed solvents (9/1, 8/2), no obvious spherulitic crystals were observed from SEM images. It is apparent that the DMF content in mixed solvent has a definite effect on the crystalline phase of PVDF and the resulting morphology of the films. This was attributed to the dipolar interaction and hydrogen bonding between PVDF chains and DMF molecules. This study provides a new approach for preparation of the b phase of PVDF and in doing so, points the way toward a novel approach for the design of new mixed solvents.

Acknowledgments This research is supported by the Key Project of BMSTC (D0406003040191).

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