Makromol. Chem. 190, 2203 -2210 (1989) 2203

Structural characterization of vinylidene fluoride/vinyl fluoride copolymers

Gaetano Guerra, Gabriele Di Dino, Roberto Centore, Vittorio Petraccone

Dipartimento di Chimica, Universita di Napoli, Via Mezzocannone 4, Napoli, Italy, 1-801 34

Jan Obrzut, Frank E. Karasz? William J MacKnight

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA

(Date of receipt: October 1 I , 1988)

SUMMARY The crystalline structure and the melting behavior of vinylidene fluoride/vinyl fluoride (VF)

copolymers are reexamined. For copolymers containing even a small mole fraction of VF units (= 12%) a structure similar to the p-form of poly(viny1idene fluoride) and to the crystalline structure of poly(viny1 fluoride) is stabilized. Combined wide-angle X-ray scattering, differential scanning calorimetry and dielectric measurements show that in contrast to other vinylidene fluoride copolymers, no crystal-crystal transition below the melting region is present.

Introduction

Since the discovery of the piezoelectric and pyroelectric properties of poly(viny1idene fluoride) (PVDF) a large number of papers describing the structure of the various polymorphs (a, 0, y, 6) of this polymer have been published (reviews are presented in refs. ' s ')). Many studies have defined the thermomechanical histories which produce the widely used piezoelectrically active 0 modification of PVDF

The isomorphic placement of comonomeric units in the crystal lattice of copolymers of vinylidene fluoride (VDF) with vinyl fluoride (VF) 3 3 4 ) , trifluoroethylene or tetra- f l~oroethylene~) , which modify the relative stabilities of the different PVDF modifications was also discussed at an early point and detailed structural studies of copolymers of VDF with trifluoroethylene 5 - 7 ) and tetrafluoroethylene*-") have been reported. It has been clearly shown that the presence of the comonomeric units stabilize the piezoelectrically active 0-modification; moreover these studies revealed that these copolymers of suitable composition exhibit a first-order phase transition below the melting temperature which involves a transition from a ferroelectric to a paraelectric crystal habit (a Curie transition).

Copolymers of VDF and VF are identified as possible materials for piezoelectric application in the patent literature ' ' 3 12) as well as in a technical report 1 3 ) . However, the only paper which presents a structural characterization of these copolymers 3, was presented before the discovery of the peculiar electric properties of PVDF.

In this paper the crystalline structure and the melting behavior of VDF-VF copolymers are reexamined. From these studies and from dielectric measurements the possible occurrence of crystal-crystal transformations below the melting region was also investigated.

0025-1 16X/89/$03.00

2204 G. Guerra, G. D. Dino, R. Centore, V. Petraccone, J. Obrzut, F. E. Karasz, W. J. MacKnight

Experimental part

The homopolymer and copolymer samples (listed in the caption of Fig. 1) were supplied by Springborn Laboratories.

Viscosity measurements in N,N-dimethylformamide (DMF) at 125 "C gave intrinsic viscosity values of 0,05 dL/g for poly(viny1idene fluoride) (PVDF) and 0,lO dL/g for poly(viny1 fluoride) (PVF), while for the copolymers intermediate values were obtained.

The copolymer compositions were evaluated by carbon and fluorine elemental analysis and are discussed throughout in terms of mole fractions.

A 19F NMR study of the microstructure showed a substantially random distribution of the comonomeric units and gave an estimate of the abundance of the head-to-head: tail-to-tail defects of 3,3% in PVDF, of 10% in PVF and intermediate values for the respective copolymers.

Samples with three different thermal/solvent histories were used for the structural and differential scanning calorimetry (DSC) measurements: the "as-prepared powders", comgl;ession- molded samples and powders obtained by dissolution in DMF at 110 "C followed by precipitation in a methanol/water (50/50) bath.

The compression-molded samples were obtained by heating at temperatures 30 "C above the melting temperature followed by cooling under pressure to room temperature; the cooling rate was 10 "C/min. The results of compression molded samples obtained by cooling at rates ranging from 1 to 40 " C h i n were similar.

The X-ray diffraction patterns at different temperatures were obtained with an automatic Philips powder diffractometer (Ni-filtered CuK, radiation), with a temperature control of k0,5 "C.

The DSC scans were carried out in a Mettler TA3000 calorimeter, in a flowing N, atmosphere, and at a heating rate of 10 "C/min.

The samples for the dielectric measurements were cast from DMF at 70°C, and then dried at 90 "C under vacuum for 7 days, to obtain 80- 120 pm thick films. Circular gold electrodes 33 mm in diameter were vacuum deposited onto both surfaces of the films. The relative permittivities (E ' ) and damping factors (tan 6) of the films were measured with a Polymer Laboratories Dielectric Thermal Analyzer.

Results and discussion

X-ray diffraction analysis

The X-ray diffraction patterns, a t room temperature, for the "as-prepared" powders of the two homopolymers (PVDF and PVF) and of several copolymers of different compositions are reported in Fig. 1. The patterns for PVF and for all the copolymers having a VDF content up to 85 mol-%, are very similar (curves c-g in Fig. 1). The pattern of PVDF (curve a) is typical of the a-form of this polymer'4,'5). The most intense reflections of the a-form (26' = 18,4" and 26' = 20,O") are also present in the pattern of the 88 mol-To VDF copolymer (curve b in Fig. 1 ) together with an intense reflection at 2 6' = 20,6". This last reflection is at the approximate position of the most intense reflection typical of all the other copolymers and of PVF itself (curves c-g in Fig. 1).

As early studies have shown3), a high degree of crystallinity for these copolymers over the entire composition range is clearly present. The degrees of crystallinity, measured using the method described in ref. 1 6 ) , are in fact all within the range 41 - 51 To. The maximum of the amorphous halo was located for each composition by the extrapolation at room temperature of its position in partially or completely melted samples.

Structural characterization of vinylidene fluoridehinyl fluoride copolymers 2205

Fig. 1 . X-ray diffrac- tion patterns of "as- prepared" powders of: (a) poly(viny1idene fluoride) (PVDF), (b) 88% VDF copolymer, (c) 84% VDF copoly- mer, (d) 51 To VDF copolymer, (e) 43% VDF copolymer, (f) 3% VDF copolymer, (g) poly(viny1 fluoride) (PVF). (All copolymer compositions in mol-Vo VDF). The intensities in the high 2 8 region are amplified with respect to the intensities in the low 2 8 region

I I

A A-

&-

._ 1- - I I I 1

100 20" 30° L O O 50" 2

Different treatments (precipitation from solution, crystallization from the melt) do not substantially change the X-ray diffraction patterns of PVF and of the copolymers with a VDF content lower than 88 mol-%. Major changes of the X-ray diffraction patterns are, however, observed for the PVDF and for the 88% copolymer; in particular the patterns for samples dissolved in DMF and then precipitated in methanol/water (volume ratio 50/50) and for the compression-molded samples are shown in Figs. 2 A and 2B, respectively. As is well known, PVDF precipitated from DMF solution is in the y-form14s'7*18) (curve a of Fig. 2A) while when compression-molded it is in the a -

(curve a of Fig. 2B). By comparison of curve b of Fig. 1 with curves b of Figs. 2 A and 2 B, it is apparent that for the 88 mol-Yo VDF copolymer as a consequence of the treatments, the reflections typical of the a-form of PVDF disappear and the pattern becomes similar to those of the other copolymers (Fig. I).

The spacings and the halfwidths of the main reflection of the precipitated powders are reported as a function of copolymer composition in Figs. 3 A and 3 B, respectively.

2206 G. Guerra, G. D. Dino, R. Centore, V. Petraccone, J. Obrzut, F. E. Karasz, W. J. MacKnight

I x u) C W

C

c .-

c

-

1-

1

1.30 i

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a

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1 I 1

I II - l . - I - - - l l 1.00 loo 20° 30' LOo S O 0 IPVFI

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2 0 Copolymer mole f rac t i on

Fig. 2. Fig. 3.

Fig. 2. X-ray diffraction patterns of samples (A) precipitated from DMF (a) poly(viny1idene fluoride) (PVDF), (b) 88% VDF copolymer; (B) obtained by compression molding: (a) PVDF, (b) 88% VDF copolymer. (All copolymer compositions in mol-%). The intensities in the high 28 region are amplified with respect to the intensities in the low 28 region

Fig. 3. d-Spacings (A) and halfwidths A20 (B) of the most intense reflection for samples precipitated from DMF as a function of copolymer composition

A sudden change in d-spacing as the composition is changed from pure PVDF (d = 4,38 A, typical of the y-form'5120)) to the 88 mol-Vo VDF copolymer (d = 4,30 A) is shown. The reported data suggest that, for all the copolymers, a structure similar to the structure of PVF (d = 4,30 A) and to the 0-form of PVDF (d = 4,27 A "1) (both with a zig-zag planar conformation of the chains) is present. This is also confirmed for the copolymer samples by the absence of the broad peak centered at 28 = 39", typical of the y-form (curve (a) of Fig. 2A). For the 88 mol-Yo VDF copolymer, in the "as- prepared" powder, the crystalline fraction is a mixture of two different forms: one similar to the a-form of pure PVDF and another one similar to the structure observed for the other copolymers. This latter form is the only one which is present after the described treatments (precipitation from solution, crystallization from the melt).

Structural characterization of vinylidene fluoridehinyl fluoride copolymers 2207

Fig. 4. different temperatures for the 43 mol-% VDF copolymer: (a) 50 "C, (b) 70 "C, (c) 90"C, (d) llO"C, (e) 13OoC, (f) 150"C, (g) 170 "C. (VDF = vinylidene fluoride)

X-ray diffraction patterns at

h

-I--

l C o 20° 30' 20

Minimum values of halfwidth (Fig. 3B), corresponding to maximum sizes of the domains in which an ordered arrangement of the chain axes is maintained, are observed for the nearly equimolar copolymers, that is for the most constitutionally disordered copolymers. A similar phenomenon has been observed in ethylene-tetrafluoroethylene copolymers2'). For all the copolymer samples, including the 88 mol-% VDF, for which structural changes are involved, both the described thermalholvent treatments leave the degree of crystallinity substantially unchanged.

The X-ray diffraction analysis of the present copolymers at different temperatures, up to the melting region, did not reveal the occurrence of crystal-crystal transitions similar to those reported for other VDF copolymers6, lo). X-ray diffraction patterns for the representative 43 mol-To VDF copolymer, for several temperatures, in the 2 8 range 10"-30", are reported in Fig. 4.

2208 G. Guerra, G. D. Dino, R. Centore, V. Petraccone, J. Obrzut, F. E. Karasz, W. J. MacKnight

Differential scanning calorimetry

The DSC scans, at 10"C/min, for the homopolymer and copolymer samples, precipitated from DMF, are reported in Fig. 5 . The enthalpy of fusion for the entire composition range is nearly constant and is in the range of 68 - 72 J/g, again suggesting a roughly constant degree of crystallinity and hence the complete isomorphism of the comonomeric units3). The peak positions of the melting endotherms are reported as a function of copolymer composition in Fig. 6. A sudden change is again observed between the 88 mol-Vo copolymer and PVDF, possibly related to the presence in the PVDF of the y-form and in the copolymers of a structure similar to the (3-form of PVDF. In contrast to other VDF copolymers no additional endotherms were observed, again suggesting the absence of crystal-crystal transitions.

I I I 1

I I I I

50 100 150 200 Tempera ture in O C

Fig. 5.

1

~

1.00 0.50 1.00 IPVF) IPVDF) Copolymer mole fraction

Fig. 6.

Fig. 5. DSC scans of samples precipitated from DMF for: (a) poly(viny1idene fluoride (PVDF), (b) 88 mol-Vo VDF copolymer, (c) 84 mol-Vo VDF copolymer, (d) 51 mol-Vo VDF copolymer, (e) 3 mol-Vo VDF copolymer, (f) poly(viny1 fluoride) (PVF)

Fig. 6 . Melting temperatures 1, (data from Fig. 5) as a function of copolymer composition

Structural characterization of vinylidene fluoride/vinyl fluoride copolymers

0

Fig. 7. Relative permittivity E' (-) and damping factor tan6 (------) at 1 kHz, as a function of tem- perature t: (a) poly(viny1idene fluoride) (PVDF), (b) 43 mol-Vo VDF copolymer, (c) poly(viny1 fluoride) (PVF)

20

10

t 0

'41 10

0 p-" C I

-100 0 100 t/"C

Dielectric measurements

Fig. 7 shows E' and tan 6, at 1 kHz, as functions of temperature for cast samples of the two homopolymers and of the copolymer with 43 mol-Vo VDF. The data for the homopolymers is similar to that r e p ~ r t e d ~ ~ . ~ ~ ) . The dielectric response of the copolymers is approximately intermediate to that of the homopolymers, as shown for instance for the 43 mol-To VDF copolymer (Fig. 7B). High values of E' for the vinylidene fluoride/trifluoroethylene copolymers compared with those of both the homopolymers have been pointed out 5 ) ; a maximum in E' is indeed present for those copolymers at around 70 " C , which is related to the ferroelectric-paraelectric transition 2 2 9 2 4 ) . The present dielectric measurements do not show evidence for analogous transitions below the melting region in the VDF-VF copolymers.

Final remarks

X-ray diffraction analysis and differential scanning calorimetry on VDF-VF copoly- mers indicate, in agreement with previous work 3,4), the occurrence of isomorphic

2210 G. Guerra, G. D. Dino, R. Centore, V. Petraccone, J. Obrzut, F. E. Karasz, W. J. MacKnight

replacement of the two monomeric units in the crystal lattice. For a small mole fraction of VF units (= 12%) a structure similar to the (3-form of PVDF and to the known crystal structure of PVF is stabilized. Combined wide angle X-ray scattering (WAXS), DSC and dielectric measurements show that, in contrast to other VDF-containing copolymers, no crystal-crystal transition below the melting region is present. The stabilization of the piezoelectric modification of PVDF, the moderate piezoelectric effect already present in PVF25,26) and the absence of a Curie temperature below the melting region (which lowers the thermal stability of the other VDF copolymers2’)) render these copolymers particularly interesting candidates as piezoelectric materials.

We thank Prof. P Corradini and Dr. C. De Rosa of the University of Naples for useful discussion. This work was supported by the Ministry of Public Education (Italy) and by AFOSR 88-001 (FEK).

I ) A. J. Lovinger, Developments in Crystalline Polymers-I, edited by D. C. Bassett, Appl. Sci.

2, G. T. Davis, The Applications of Ferroelectric Polymers, edited by T. T. Wang, J. M. Herbert

3, G. Natta, G. Allegra, I. W. Bassi, D. Siamesi, G. Caporiccio, E. Torti, J. Polym. Sci., Part A,

4, W. W. Doll, J. B. Lando, J. Macromol. Sci., Phys. B4, 897 (1970)

6 , Y. Tajitsu, A. Chiba, T. Furukawa, M. Date, E. Fukada, Appl. Phys. Lett. 36, 286 (1980) ’) K. Tashiro, K. Takano, M. Kobayashi, Y. Chatani, H. Tadokoro, Polymer 22, 1312 (1981) 8, A. J. Lovinger, Macromolecules 16, 1529 (1 983) 9, A. J. Lovinger, G. E. Johnson, H. E. Bair, E. W. Anderson, J. Appl. Phys. 56, 2412 (1984)

lo) a) A. J. Lovinger, D. D. Davis, R. E. Cais, J. M. Kometani, Macromolecules 19, 1491 (1986); b) H. Tanaka, A. J. Lovinger, Macromolecules 20, 2638 (1981)

I * ) British Pat. 1349860, N. Murayama, Kureka Kagaku Kogyo KK, Piezoelectric Element- Production by Treating a Vinylidene Fluoride Copolymer with Direct Current (1971)

12) US. Pat. 4 204 135, N. Murayama, Kureka Kagaku Kogyo KK, Piezoelectric Elements of High Molecular Weight Tetrafluoroethylene and Vinylidene Fluoride Copolymers (1 980)

13) H. Stefanou, National Technical Information Service Report No. ADA083002, (1 980) 14) Ye. L. Gal’perin, B. P. Kosmynin, V. K. Smirnov, Vysokomol. Soedin., Ser. B: 12, 555 (1970) I s ) R. Hasegawa, Y. Takahashi, Y. Chatani, H. Tadokoro, Polym. J. 3, 600 (1972) 16) Ye. L. Gal’perin, B. P. Kosmynin, V. K. Smirnov, Vysokomol. Soedin., Ser. A: 12, 1880 (1970) 17) T. A. Bychkov, Ye. L. Gal’perin, A. A. Konkin, Vysokomol. Soedin., Ser. A: 13, 1156 (1971) I s ) G. Guerra, F. E. Karasz, W. J. MacKnight, Macromolecules 19, 1935 (1986) 1 9 ) J. B. Lando, H. G. Olf, A. Peterlin, J. Polym. Sci., Part A-I, 4, 941 (1966) 20) S. Weinhold, M. H. Litt, J. B. Lando, J. Polym. Sci., Polym. Lett. Ed. 17, 585 (1979) 2’) M. Iuliano, C. De Rosa, G. Guerra, V. Petraccone, P. Corradini, Makromol. Chem. 190, 827

22) T. Furukawa, M. Ohuchi, A. Chiba, M. Date, Macromolecules 17, 1384 (1984) 23) S. Osaki, S. Uemura, Y. Ishida, J. Polym. Sci., Polym. Phys. Ed. 9, 585 (1971) 24) T. Furukawa, G. E. Johnson, J. Appl. Phys. 52, 940 (1981) 25) R. J. Phelan Jr., J. R. Mahler, A. R. Cook, Appl. Phys. Lett. 19, 337 (1971) 26) Y. Wada, R. Hayakawa, Jpn. J. Appl. Phys. 15, 2041 (1976) 27) F. Micheron, Makromol. Chem., Macromol. Symp. 1, 173 (1986)

Publishers, London 1982, chapter 5

and A. M. Glass, Blackie, Glasgow 1988, chapter 4

3, 4263 (1965)

T. Yagi, M. Tatemoto, J. Sako, Polym. J. 12, 209 (1980)

(1989)