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Structural Study of Conformational Phases in Poly(viny1idene Fluoride) by Far-Infrared Spectroscopy M. LATOUR, Laboratoire de Physique Mole'culaire et Cristalline, A. MONTANER and M. GALTIER, Laboratoire de Spectroscopie Infrarouge, and G. GENEVES, Laboratoire de Physique Molbculaire et Crystalline, Universite' des Sciences et Techniques du Languedoc, 34060 Montpellier Cedex, France Synopsis Polarized and unpolarized far-infrared transmission spectra of poly(viny1idene fluoride) powders and films in the range 40-400 cm-l are examined in terms of the a, @, and y forms of this polymer. The bands which appear allow us to discuss various theoretical models. An interpretation of the polymer's configurationalong and between chains includingdefects is proposed. Relations between the crystal form and treatments applied to the sample during its manufacture are demonstrated. In the same way the tttgtttg' conformation previously reported by some authors for the y form is corroborated. INTRODUCTION Poly(viny1idene fluoride) (PVF2) exists in three crystalline forms a, p, and y (also denoted 11, I, 111). A great number of measurements by near-infrared spectroscopy confirmed by a few in the far infrared allow characterization of these phases. In fact, the description of the actual samples by the amounts of the different phases is very crude. We show that far-infrared spectroscopy allows a description in terms of defects or order. In this paper we report the far-infrared spectra of various PVFz samples (with polarized and unpolarized light) and compare them with previous work.1>2 Commercial films supplied by different manufacturers as well as samples prepared in our laboratory, using methods previously described, have been used. The low-frequency oscillatory motions of the carbon chain are very sensitive to structural modifications induced by variations in manufacturing conditions or subsequent physical treatment. Theoretical models are discussed on the basis of the experimental results. APPARATUS AND SAMPLES Far-Infrared Spectrometers Most measurements were performed in our laboratory on a Beckman RIIC 620 Fourier-transform spectrometer, in the region from 40 to 400 cm-l. Two other spectrometers were used: a Nicolet 8000 HV kindly placed a t our disposal by Nicolet Instruments in Madison and a Bruker IFS 113V apparatus. The resolution was always better than 4 cm-l. Journal of Polymer Science: Polymer Physics Edition, Vol. 19,1121-1129 (1981) 0 1981 John Wiley & Sons, Inc. CCC 0098-1273/81/071121-09$01.00

Structural study of conformational phases in poly(vinylidene fluoride) by far-infrared spectroscopy

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Page 1: Structural study of conformational phases in poly(vinylidene fluoride) by far-infrared spectroscopy

Structural Study of Conformational Phases in Poly(viny1idene Fluoride) by Far-Infrared

Spectroscopy

M. LATOUR, Laboratoire de Physique Mole'culaire et Cristalline, A. MONTANER and M. GALTIER, Laboratoire de Spectroscopie

Infrarouge, and G. GENEVES, Laboratoire de Physique Molbculaire et Crystalline, Universite' des Sciences et Techniques du Languedoc, 34060

Montpellier Cedex, France

Synopsis

Polarized and unpolarized far-infrared transmission spectra of poly(viny1idene fluoride) powders and films in the range 40-400 cm-l are examined in terms of the a, @, and y forms of this polymer. The bands which appear allow us to discuss various theoretical models. An interpretation of the polymer's configuration along and between chains including defects is proposed. Relations between the crystal form and treatments applied to the sample during its manufacture are demonstrated. In the same way the tttgtttg' conformation previously reported by some authors for the y form is corroborated.

INTRODUCTION

Poly(viny1idene fluoride) (PVF2) exists in three crystalline forms a, p, and y (also denoted 11, I, 111). A great number of measurements by near-infrared spectroscopy confirmed by a few in the far infrared allow characterization of these phases. In fact, the description of the actual samples by the amounts of the different phases is very crude. We show that far-infrared spectroscopy allows a description in terms of defects or order. In this paper we report the far-infrared spectra of various PVFz samples (with polarized and unpolarized light) and compare them with previous work.1>2 Commercial films supplied by different manufacturers as well as samples prepared in our laboratory, using methods previously described, have been used. The low-frequency oscillatory motions of the carbon chain are very sensitive to structural modifications induced by variations in manufacturing conditions or subsequent physical treatment. Theoretical models are discussed on the basis of the experimental results.

APPARATUS AND SAMPLES

Far-Infrared Spectrometers

Most measurements were performed in our laboratory on a Beckman RIIC 620 Fourier-transform spectrometer, in the region from 40 to 400 cm-l. Two other spectrometers were used: a Nicolet 8000 HV kindly placed at our disposal by Nicolet Instruments in Madison and a Bruker IFS 113V apparatus. The resolution was always better than 4 cm-l.

Journal of Polymer Science: Polymer Physics Edition, Vol. 19,1121-1129 (1981) 0 1981 John Wiley & Sons, Inc. CCC 0098-1273/81/071121-09$01.00

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1122 LATOUR ET AL

Samples

We used commercial resins and films supplied by different manufacturers, resins from Kureha (KF-1100), Solvay (Powder and grains), Dynamit Nobel, and Pennwalt Co. (Kynar 881), and Industrial films from Kureha Chemical, Solvay (Solef ZOOO), Bemberg, Ugine Kulmann, Dynamit Nobel, Freudenberg. Film samples were prepared in our laboratory from the resins by conventional methods described in the literature. Film thicknesses varied from 25 to 100 p.

RESULTS

We review the various methods of preparation for each phase. These methods have been collected from many sources.

Form a (or 11)

The following preparation methods for unoriented films are found in the lit- erature: (i) Melting powders or grains, and applying a pressure for a few seconds a t temperatures between 160 and 280OC. Films of different thicknesses are prepared by varying the pre~sure.~ (ii) Melting and cooling the melt very slowly to room temperature. (iii) Casting films from acetone solution at about 5OoC and removing the solvent.13 (iv) Dissolution of a mixture of monochlorobenzene and dimethylformamide (volume ratio 90/10), and slow cooling.6

Mechanically oriented films are prepared by stretching and rolling cast films about three times their original length at a temperature (16OOC) just below the melting p ~ i n t . ~ - ~ Annealing for 6 h a t 12OOC seems to stabilize the dimensions of the samples in further thermal treatment.g

Far-infrared Transmission Spectra

The results in Figure 1 were obtained on films prepared by method (i) for unoriented films, in our laboratory, from Pennwalt resin (Kynar 880); Figure 2 concerns Kureha Chemical cast films; Figure 3, Solvay (Solef 2000) commercial

I 100 200 300 400) cm-'

Fig. 1. Infrared spectrum of PVFz film a phase obtained in the laboratory from Pennwalt basic resin (Kynar 881) melted and pressed for a few seconds at 15OOC. The same spectrum is obtained by melting and slow cooling of the same resin.

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: 400 cm-' 100 200 300

Fig. 2. Spectrum of PVFz cast film from Kureha Chemical (a phase).

.

.

film; Figure 4, Bemberg commercial film; and Figure 5 deals with compacted Pennwalt powder (Kynar 881).

The near-infrared transmission spectrum was obtained for each sample in order to obtain the content of the different phases.1°-13

1 100 200 300 400 Em-'

Fig. 3. Spectrum of commercial PVFz films from Solvay (Solef 2000) (a phase).

T(o'o) 100 t

1 100 200 300 460 Em-'

Fig. 4. Spectrum of commercial PVFp film from Bemberg (a phase),

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LATOUR ET AL

Fig. 5. Spectrum of PVF2 sample obtained by compaction of Pennwalt powder (Kynar 881) (a phase).

Table I lists the absorption peaks of the samples. We give for comparison the absorption frequencies obtained by Kobayashi et a1.l Mechanically oriented films were studied with polarized light. The frequencies reported in Table I are assigned to directions parallel or perpendicular to the chain, though they appear in both spectra since film orientation is only partial. On the whole, the observed frequencies confirm Kobayashi's resu1ts.l Nevertheless we have not observed the band at 208 cm-l (which concerns a mode polarized in the chain direction), but rather one at 214 cm-l. All samples show an absorption band at 372 cm-', which is not consistent with Kobayashi's mode1.l This mode is probably due to the head-to-head arrangement of neighboring units.

The most interesting results appear when the spectra of different samples are compared. The band intensities at 54,72,100, and 175 cm-l clearly vary from one sample to another. The band at 71 cm-l, which appears only in the spectrum in Figure 3, arises from a non-negligible proportion of the 0 phase ( this result is confirmed by the 500-550 cm-l region of the spectrum). On the other hand, the differences of intensity or width of the other bands can be interpreted by variations of crystallinity or some kind of disorder along the chain. According to Kobayashi's model, the vibration at 54 cm-l appears only in the crystal. The mode at 175 cm-l is a torsional one, and the mode at 100 cm-l is a combination

TABLE I Far-Infrared Absorption Frequencies for Different Samples of PVFz with the a Conformation

Figure Origin Sample

1 Pennwalt fused under

2 Kureha commercial

3 Solvay commercial 1 film /I

4 Bemberg commercial 1 film 11

5 Pennwalt pellet Kobayashi et al.' I

II

pressure

film

Absorption frequencies (cm-')

54 101 176 216 250 286 315 355 372

100 215 287 355 372

71 104 150 214 355

100 175 218 355 150 214 287 326 373

215 286 372 54 103 176 215 286 355 372 53 100 175 215 355

144 208 288 370

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FAR-INFRARED ABSORPTION IN PVF2 1125

of the torsional mode, angular deformation of the chain, and wagging of CF2. The absence of the absorption band at 54 cm-' is explained by poor crystalli- zation (Figs. 2 4 , commercial films).

The peaks at 100 and 175 cm-l should be sensitive to order both along and between the chains, defects of interchain order affecting especially the torsional mode at 175 cm-l. Assuming this interpretation, we can conclude that the sample giving Figure 3 has the poorest order. In the sample giving Figure 2, the order along the chain seems good while the interchain order is very poor.

Wide-angle x-ray diffraction patterns (transmission scan) confirm that the degree of crystallinity remains rather low (50%) in the commercial samples (corresponding to Figs. 2-4). For the samples corresponding to Figure 1, the x-ray diagram permits a more accurate evaluation. In this case the intensities of the major reflections can be measured easily by densimetry.*

Phase f l (or I)

In this form (the piezoelectric one) the molecular configuration is a planar zigzag. The methods described in the literature to obtain this phase are the following:

Unoriented Films

(i) Evaporation from solution of the initial powder or grains in dimethylsul- foxide or dimethylformamide.6 Our trials with this method always lead to unoriented y films. The same result is obtained using diethylformamide as solvent. Nevertheless it seems that dimethylacetamide or hexamethylphos- phoramide allow one to get the /3 phase.lI2 (ii) Copolymerization of difluo- roethylene with tetrafluoroethylene at a mole ratio of 1:5.14 (iii) Solidification of fused material: the crystallization being produced by a quick increase of pressure (1 to 6 bar in 0.1 s). (iv) An a to /3 transition induced by a strong elec- trostatic field (E = lo6 V/m).9-11 For lower values of the field it seems possible to obtain another piezoelectric phase, ap9

Mechanically Oriented Films

These are mainly obtained by stretching a or y films to three or five times their initial length at temperatures between 50 and 100"C,4>5 then annealing at 150°C for 12 h.9J4 This annealing seems important to obtain the maximum concen- tration of the 0 form (the stretching can be uniaxial or biaxial). It must be noted that anisotropic stretching leads to low strength in the direction perpendicular to the direction of maximum elongation.

Far-Infrared Spectra

Infrared spectroscopy of commercial films prepared by anisotropic stretching (Freudenberg and Kureha) are shown in Figures 6 and 7, respectively. For both films we observed a strong band at 70 cm-l with polarization perpendicular to the film, a very broad band centered at 200-210 cm-l and two sharp, partially

* Results of x-ray measurements will be published separately.

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1126 LATOUR ET AL

50 .

L 100 200 300 4;)o Fm-’

Fig. 6. Spectrum of commercial PVFz film from Freudenberg ( p phase) prepared by anisotropic stretching.

polarized bands at 350 and 370 cm-i. The spectrum of the Freudenberg film exhibits one additional band at 287 cm-’.

Group theoretical predictions for a planar zigzag chain allow only one mode in this region, clearly the 70 cm-l band associated with the libration of the chain. The absorption at 210,287, and 370 cm-l could be due to a residual amount of the a phase (this is consistent with the infrared absorption in the range 500-550 cm-l). X-ray measurements show the classical pattern of the p unit cell. We do not detect indices of reflection modes corresponding to the a form.

The parameters used in this work are a = 4.96, b = 9.64, and c = 4.62 A for the a form, and a = 8.58, b = 4.91, and c = 2.56 A for the

The residual a form cannot be understood as a proportion of chains among (3 chains, but rather as short a sequences within the chain. This is attested by the broadening of the 210 cm-l band and the vanishing of the vibrations at 54, 100, and 176 cm-l of the a chain.

This explanation cannot hold for the band at 350 cm-l, which is too sharp to be confused with the 355 cm-l line in the a structure. It seems reasonable, however, to suppose that it is associated with the same kind of molecular motion (CFZ twisting). Such a mode, forbidden for an ideal chain, can be activated by

form.

L 100 200 300 A o Em-’

Fig. 7. Spectrum of commercial film from Kureha (capacitor grade) prepared by anisotropic stretching ( p phase).

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FAR-INFRARED ABSORPTION IN PVF2 1127

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defects. Two possibilities a t least can be suggested: one is a head-to-head de- fect, the other is a conformation defect on the chain (for instance one g bond between two zigzag chains). In both cases, if CF2 twisting modes become infrared active, the same holds for CHZ twisting. A weak line appearing at 950 cm-l on these samples is suitable to this type of mode. It is not easy to choose between the two kinds of defects, but taking into account the strong activity of the mode, conformational defects seem more likely.

-

Phase y (or 111)

This phase has been described by several a ~ t h o r s l ~ ? ~ ~ ; the methods reported to obtain it are the following.

Unoriented Films

(i) Evaporation of a solution of PVFz (powder or grains) in dimethylsulfoxide at 85OC, in dimethylformamide at 6OoC, or in dimethylacetamide at temperatures between 60 and 65°C.10J7*21 We have also obtained this phase with diethyla- cetamide (as already pointed out). (ii) Reheating a film at 185OC for1 20 h at high pressure. (iii) Control of the recrystallization from the melt in the presence of an external agent such as a siloxane-oxyalkylene block c~polymer.~

Mechanically Oriented Films

It is difficult to obtain an oriented film by stretching unoriented y film since the 0 phase is the usual result.

Far-Infrared Spectra

Figures 8 and 9 show transmission spectra of y films obtained from dimeth- ylsulfoxide solutions at 85" C using, respectively, Pennwalt and Dynamit Nobel powders. (In every case, the solvent extraction is monitored by near-infrared spectroscopy.)

Fig. 8. Spectrum of PVFz (y phase) film obtained in this laboratory from Pennwalt (Kynar 881) powder solution in dimethyl sulfoxide at 85OC.

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LATOUR ET AL

100 200 300 4 w &I-’

Fig. 9. Spectrum of PVFz (y phase) film obtained in this laboratory from Dynamit Nobel powder

In both spectra a strong absorption band appears at 90 cm-I and weaker ones at 175, 204, 300, 334, 370, and 380 cm-l. Kobayashi found a similar result. Assuming a planar zigzag chain (the same as for the ,b phase), he concluded that 90 cm-l is the only fundamental frequency. However, even if the 350 and 370 cm-l bands could be due to residual a phase, there is no doubt that the others are characteristic of the y phase. Moreover, there is a close similarity between the a and y absorption maxima, the 355 and 372 cm-l bands for the (Y phase corresponding to the 334 and 380 cm-l bands of the y phase. This result seems to us to be in good agreement with one of the structures proposed by Tripathy et a1.,18 who proposed on the basis of energy calculations, and x-ray data that the configuration is tttgtttg or tgtg’tg’tg. The first structure allows us to predict vibrational modes close to the a modes for t segments (but nearly inactive) and close to the ,b modes for g segments, but of course with much weaker intensity. This is actually a good qualitative description of the observed result. The tgtg’tg’tg configuration is not consistent with the low intensity of the observed lines.

solution in dimethylsulfoxide at 85OC.

CONCLUSION

Our results show that far-infrared spectroscopy is a useful tool for character- izing the different polymorphs of PVFz and obtaining information about order and defects in the chain. Using this method we hope to be able to obtain infor- mation on the phase transition mechanism, under stretching or in an electric field, taking into account the method of preparation of the material.

References

1. M. Kobayashi, K. Tashiro, and H. Tadokoro, Macromolecules, 3,158 (1975). 2. J. P. Luongo, J. Polym. Sci. A-2, 10,1119 (1972). 3. J. Scheinbeim, C. Nakafuku, B. A. Nenman, and K. D. Pae, J. Appl. Phys., 50, 4399

4. R. Hasegawa, M. Kobayashi, and H. Tadokoro, Polym. J. , 3,591 (1972). 5. R. Hasegawa, Y. Takahashi, Y. Chatoni, and H. Tadokoro, Polym. J., 3,600 (1972). 6. S. Enomoto, Y. Kawai, and N. D. Sugita, J. Polym. Sci. A-2,6,861 (1969). 7. W. A. Prest, M. Abkowitz, D. J. Luca, and G. Pfister, presented at the 175th American Chemical

Society Meeting, Anaheim, CA, 1978; Org. Coatings Plast. Chem., 38,334 (1978).

(1979).

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FAR-INFRARED ABSORPTION IN P V F 2 1129

8. J. I. Scheinbeim, K. T. Chung, K. D. Pae, and B. A. Newman, J. Appl. Phys., 50, 6101

9. G. T. Davis, J. E. Mackinney, M. G. Broadhurst, and S. C. Roth, J. Appl. Phys., 49,4998 (1979).

(1978). 10. D. K. Dasgupta, K. Doughty, and D. B. Shier, J. Electrost., 7,267 (1979). 11. M. Latour, presented at the International Symposium on Electrets and Dielectrics, Sao Carlos,

12. M. Latour, J. Electrost., 2,241 (1976). 13. M. Latour, Polymer, 8,278 (1977). 14. P. T. A. Klase and J. Van Turnhout, in Organization for Industrial Research TNO Publication

15. G. Natta, G. Allegra, I. W. Bassi, D. Sianesi, G. Caporiccio, and E. Torti, J. Polym. Sci. Part

16. W. W. Doll and J. B. Lando, J. Macromol. Sci. Phys., 2,205 (1968). 17. G. Cortili and G. Zerbi, Spectrochim. Acta, 23-A, 285 (1967). -18. S. K. Tripathy, R. Potenzone, Jr., A. J. Hopfinger, N. C. Banik, and P. L. Taylor, Macromol-

19. W. L. Gordon and J. Welch, J. Polym. Sci. A-2, 14,1683 (1976). 20. M. A. Bachmann, W. L. Gordon, J. L. Koenig, and J. B. Lando, J. Appl. Phys., 50, 6106

21. N. C. Banik, P. L. Taylor, S. K. Tripathy, and A. J. Hopfinger, Macromolecules, 12, 1015

22. W. M. Prest and D. J. Luca, J. Appl. Phys., 46, (1975). 23. Y. E. L. Gal 'perin, B. P. Kosmynin, ard R. A. Bychkov, Vysokomol. Soedin. Ser. B , 12,555

24. S. Osaki and Y. Ishida, J. Polym. Sci. Polym. Phys. Ed., 13,1071 (1975).

(unpublished).

P79/30 (1979, unpublished), p. 4 [IEE Conf. Publ. (to be published)].

A , 3,4263 (1965).

ecules, 12,656 (1979).

(1979).

(1979).

(1970).

Received May 1,1980 Accepted February 4,1981