Conductive poly(vinylidene fluoride) reticulate doped with the CT complex TTF-TCNQ

Synthetic Metals, 37 (1990) 181 - 188

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CONDUCTIVE POLY(VINYLIDENE FLUORIDE) RETICULATE DOPED WITH THE CT COMPLEX TTF-TCNQ C. H. CHOIa, A. TRACZb, J. K. JESZKAb, G. BOITEUX%G. SEYTRE~, J. ULAI~ISKIc and M. KRYSZEWSKIb ~Laboratoire d'Etudes des Matdriaux Plastiques et des Biomatdriaux, URA-CNRS 507, Universitd Lyon I, 69 622 Villeurbanne Cddex (France) bCenter of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90- 363 L6d~ (Poland) CInstitute of Polymers, Technical University of L6d~, 90- 924 L6d~ (Poland)

Abstract The preparation and properties of conductive PVDF films obtained by a reticulate doping technique using TTF-TCNQ as the additive are described. It is shown that PVDF can be made conductive by reticulate doping with CT complexes and that such additives can have an important influence also on its crystalline structure. In the presence of dendrites of the CT complex, crystallization of PVDF in the (fl + 7) phase is favored, especially at high temperatures, and spherulite-like structures are observed. Films showing exceptionally high conductivity as compared with other reticulate doped systems (1 S/cm at the additive content 2 wt.% only) were obtained.

Introduction Reticulate doped polymers, i.e. heterogeneous conductive systems obtained by crystallization of conductive charge-transfer complexes in s i t u , during film casting show many interesting properties. The most important is the surprisingly high efficiency of doping. It has been shown that a percolation threshold below 0.003 can be obtained and conductivities of the films approach the upper limit of the Clausius-Mossotti approximation. Another interesting feature of this technique is the possibility of controlling the properties of the prepared materials by modification of casting parameters and the choice of the polymer matrix [1- 4]. When semicrystalline polymers are used as polymer matrices, crystallization of the polymer and of the additives are interrelated. It has been shown for polyolefins that the systems in which crystallization of the CT complex begins first are more homogeneous and exhibit higher conductivity [4]. In the present work we report studies on the poly(vinylidene fluoride) (PVDF)-TTF-TCNQ system. This system is interesting because exceptionally high conductivities of the films can be obtained and also because PVDF 0379-6779/90/$3.50

© Elsevier Sequoia/Printed in The Netherlands

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presents different crystalline phases and morphologies depending on the casting conditions. Therefore, formation of the conductive network can also have an effect on the crystalline structure of the PVDF matrix.

Experimental The conductive films were obtained by casting a 4% polymer solution in dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) onto a glass plate in the temperature range 350-390 K at ambient or reduced pressure (in the case of DMSO). In most cases films of non-doped PVDF were cast at the same time under the same conditions for comparison. Film thickness was 1030 ~m. Poly(vinylidene fluoride) supplied by Kureha Chemicals (Japan) or synthesized at the pilot plant of ATOCHEM, France (PVDF-P896) was used (without additional purification) and no significant difference was found. Tetrathiofulvalene (TTF) and tetracyanoquinodimethane (TCNQ) were purified by recrystallization or vacuum sublimation. The molar ratio of donor and acceptor was always 1:1. The crystallinity of PVDF was calculated from the enthalpy of melting determined by using differential scanning calorimetry, assuming 25 cal/g for a 100% crystalline sample [5]. The contents of the a and (fl + T) phases were determined from Fourier-transform infrared spectra, according to the method described in ref. 6 (from the relative intensity of the absorption bands at 485, 510 and 530 cm-1). Electrical conductivity measurements were performed using a fourprobe method and equipment described in ref. 7. The frequency dependence of the conductivity was measured on Hewlett-Packard 4274A and 4275A RCL meters. Scanning electron microscope pictures were taken using a JEOL 1200EX electron microscope operating in the scanning mode at 80 keV.

Results and discussion

Morphology of the films In general, the conductive network of TTF-TCNQ complex crystallites observed under the optical microscope shows a dendritic structure (Fig. 1). The size of the dendrites depends on the casting conditions (i.e. temperature, pressure and film thickness) and can vary from 3 to 50 pm. An example of the submicroscopic structure of the conductive network in the upper surface layer of the film revealed by scanning electron microscopy is presented in Fig. 2. It is different from the almost random whisker network with very few branchings found for TTT-TCNQ in amorphous polymer matrices [8]. Oriented growth and branching of microcrystallites correspond relatively well to the dendritic structure observed with the optical microscope. Comparison of

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(a) (b) Fig. 1. Optical micrographs of PVDF films reticulate doped with 2 wt.% of TTF-TCNQ (cast from DMF at 80 °C): (a) in non-polarized light; (b) with the film placed between crossed polarizers. Scale bar indicates 100 #m.

Fig. 2. Scanning electron micrograph of the PVDF film reticulate doped with 2 wt.% of TTFTCNQ cast from DMSO at 90 °C. It shows the structure of the conductive network near the bottom (glass) surface of the film. S E M p i c t u r e s of b o t h sides of the film s u g g e s t t h a t the d e n d r i t e s g r o w p r e f e r a b l y f r o m t h e b o t t o m t o w a r d s t h e top of the film. W h e n t h e c a s t i n g c o n d i t i o n s a r e n o n - h o m o g e n e o u s in t h e p l a n e of t h e films (e.g. due to c o n t r a c t i o n of the c a s t s o l u t i o n d u r i n g s o l v e n t e v a p o r a t i o n or n e a r the edges), the d e n d r i t e s a r e n o t c i r c u l a r b u t elongated; this h a s no significant effect on the e l e c t r i c a l p r o p e r t i e s b u t seems to f a v o r crystallizat i o n of t h e p o l y m e r in t h e (fl + ~) form. U n d e r specific c o n d i t i o n s ( c o n c e n t r a t i o n g r a d i e n t in t h e p l a n e of the film, p r o p e r e v a p o r a t i o n rate), films s h o w i n g a fibrous s t r u c t u r e of the c o n d u c t i v e n e t w o r k are obtained. M i c r o p h o t o g r a p h s of s u c h a film a r e p r e s e n t e d in Fig. 3.

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Fig. 3. Morphological structure of the fiber-like conductive TTF-TCNQ network in PVDF film cast from DMSO at 80 °C under vacuum. Scale bar indicates 100 #m.

Electrical properties Temperature dependences of the electrical conductivity of films with dendritic and fibrous structures of the conductive network are compared in Fig. 4 with that reported for TTF-TCNQ single crystals. One can see that the highly conductive sample with a fibrous network (curve b) exhibits maximum conductivity at about 250 K, while the conductivity of the sample with a dendritic network (curve a) is thermally activated in the entire investigated temperature range. In spite of this, and in spite of c. two orders of magnitude

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difference in conductivity, the temperature dependence of the low temperature part is exactly the same in both cases with low activation energy ( < 20 meV), i.e. lower than that determined for single crystals of TTF-TCNQ in this temperature range. It is worth mentioning that a very similar ~(T) dependence was found also for another system (polyethylene reticulate doped with TTF-TCNQ), while systems doped with different CT complexes exhibit different temperature dependences of conductivity [10]. The conductivity maximum around 200 K for PE reticulate doped with TTF-TCNQ was found in d.c. as well as in a.c. [11] and microwave measurements [12]. It should be emphasized that the conductivity of PVDF samples with a fibrous structure of the TTF-TCNQ network (c. 1 S/cm at room temperature) is about two orders of magnitude higher than reported for other reticulate doped systems. This effect cannot be explained by the morphology of the network alone, expecially when compared with anisotropically conductive systems [13]. The frequency dependence of the a.c. conductivity of representative samples is presented in Fig. 5. It can be seen that the a.c. conductivity is frequency independent up to at least 10 MHz, which is in agreement with our previous results on other reticulate doped polymers with dendritic networks

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Fig. 5. Frequency dependence of the electrical conductivity of PVDF films reticulate doped with TTF-TCNQ with additive concentration: (A) 1.2 wt.% cast from DMF at 80 °C; (B) 1.2 wt.% cast from DMF at 105 °C; (C) 1.2 wt.% cast from DMF at 125 °C; (D) 2.0 wt.% cast from DMF at 80 '~C; (E) 2.0 wt.% cast from DMSO at 80 °C; under vacuum (fiber.like structure).

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[11]. This also means that in PVDF the conducting network in continuous down to the submicroscopic level and any discontinuity cannot be attributed to interdendrite boundaries (cf. discussion in ref. 10).

Effect of the CT complex on the crystallinity of P V D F It is well known that PVDF can crystallize in at least three crystalline phases, referred to as ~, fl and ~ [14]. The non-polar, spherulitic ~ phase is generally dominant in films cast above 100 °C from common solvents (except hexamethylphosphoramide), while the non-spherulitic (fl + ~) phase dominates in films cast below 100 °C [15]. We have found that the presence of the network of TTF-TCNQ crystallites has an influence on the crystallization of the PVDF matrix. In Table 1 we compare the crystallinity and the content of the (fl + ~) phase for doped and non-doped PVDF films cast under exactly the same conditions. It can be seen that the presence of the CT complex crystallites has no significant effect on the crystallinity of PVDF, which is about 50% in all cases. It has, however, a significant effect on the content of the (fl + y) phase. This effect is most pronounced for higher temperatures. Below 80 °C the content of the (fl + ~) phase in the films cast from DMF and DMSO is high even without a CT complex, so the increase can hardly exceed 20% (it is higher for higher additive concentrations). On the other hand, at temperatures higher than 110 °C the ~ phase dominates and formation of a conducting network is more difficult, so higher additive concentrations are necessary to obtain conductive films. The higher content of the (fl + ~) phase seems to be due to preferential crystallization of this phase on the crystallites of the additive (the CT complex crystallizes first in the systems in which this effect is observed). This

TABLE 1 Crystallinity and the content of the (fl + ~) phase for reticulate doped and non-doped PVDF films cast at different temperatures at ambient pressure Solvent

TTF-TCNQ content (wt.%)

Casting temperature

Crystallinity

Content of (fl + 7) phase

(°C)

Pure (wt.%)

Pure (wt.%)

Doped (wt.%)

Doped (wt.%)

Increase on doping

(%) DMF

1.2 2.0 1.2 1.2

80 80 105 125

49 49 47 51

49 52 46 46

70 70 25 15

85 >90 40 18

21 29 60 20

DMSO

2.0 2.0 2.0

80 94 125

54 51 54

56 52 51

70 60 15

>90 80 45

29 33 200

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effect is clearly seen in the case of larger additive crystallites in non-conductive films. The consequence of this interesting effect is that the (fl + ?) phase, which in the pure polymer is non-spherulitic, in the presence of dendrites of the CT complex forms spherulite-like structures showing well-known Maltese cross between crossed polarizers. These spherulites of the (fl + 7) phase are always superimposed on dendrites of TTF-TCNQ, as can be seen in Fig. 1 comparing the microphotographs of the same film between crossed polarizers and without polarizers.

Conclusions It is concluded that PVDF can be made conductive by reticulate doping with CT complexes and that such additives can have an important influence also on its crystalline structure. Under appropriate conditions it is possible to obtain films with a fibrous structure of the conductive network which exhibit exceptionally high conductivity (1 S/cm) at the additive content of 2 wt.% only. The presence of CT complex crystallites induces important changes in the crystalline structure and morphology of the polymer. Generally, crystallization of the (fl + 7) phase is favored especially at high temperatures and the content of the ~ phase is reduced. The (fl + 7) phase, which is nonspherulitic in the pure polymer, forms spherulite-like structures in the presence of dendrites of the CT complex.

Acknowledgements The authors are indebted to Professor R. Deltour from the Universit6 Libre de Bruxelles for the use of a cryostat for the conductivity measurements and to G. Debrue and I. Glowacki for technical assistance. This work was partially supported by Projects CPBP 01.12 and 01.14 of the Polish Academy of Sciences.

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