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The effect of dopant molecules on the molecular order of electrically-conducting films of polypyrrole
Synthetic Metals, 26 (1988) 247 - 257
247
THE E F F E C T OF DOPANT MOLECULES ON THE MOLECULAR ORDER OF ELECTRICALLY-CONDUCTING FILMS OF POLYPYRROLE G. R. MITCHELL*, F. J. DAVIS and C. H. LEGGE J. J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading RG6 2AF (U.K.) (Received July 7, 1988; accepted July 20, 1988)
Abstract X-ray scattering curves have been measured for a range of electrochemically-prepared conducting polypyrrole films employing a variety of counterions in aqueous solutions. Films containing counterions based on aromatic rings exhibit an anisotropic molecular organization. The degree of anisotropy is enhanced through the use of highly planar counterions. The electrical conductivity of such films is also improved if the charge/volume ratio of the counterion is maintained at a high level. Polypyrrole films prepared using 'spherically' shaped counterions such as SO42- do not display such anisotropic molecular organizations, and exhibit lower electrical conductivities. The competing structural roles of the counterions within these molecular composites are discussed.
1. Introduction Stable, flexible and electrically-conducting polymer films may be prepared from pyrrole in a one-step electro-oxidation process [ 1 - 2]. Films prepared in this manner have electrical conductivities ranging from 10 to 1000 S cm 1 and require no additional dopants to achieve these conductivity levels. This electrochemical method of preparation of polypyrrole and other polymers based on similar heterocyclic units has been widely studied, and it may be performed in aqueous or organic solvent-based electrolytes. The process of oxidation of the polymer chains is accompanied by the inclusion, in the growing polymer film on the electrode, of counterions that act as dopant molecules. Typical levels of ' d o p a n t ' concentration are in the range of one counterion for three to four pyrrole units. A considerable variety of counterions have been used in an aqueous environment; units such as toluene sulphonate, perchlorate and tetrafluoroborate are among the most widely *Author to whom correspondence should be addressed.
248 studied [3 - 8]. In the case of p-toluene sulphonate, typical levels of doping result in the counterions comprising some 50% by volume of the conducting film. It is therefore not surprising that these counterions have a significant effect upon the quality and mechanical properties of the polymer films [9 11], in addition to that expected for the electrical transport properties. Moreover, it has been suggested that the counterions employed in the electrochemical preparation have a direct impact on the molecular organization of the resultant films [12, 13]. The molecular organization of polypyrrole films is highly disordered. The exact chemical configuration of the polypyrrole chains is unknown, but spectroscopic techniques indicated that a substantial proportion of the polymerization occurs through the ~ - ~ sites [ 14, 15]. There is also evidence to suggest the presence of highly defective chains with crosslinking and polymerization through the fl positions [ 16]. We have recently shown using both X-ray [17, 18] and neutron [19] scattering techniques that the polypyrrole films prepared using toluene sulphonate as the counterion exhibit an anisotropic molecular organization, in which the planes of the pyrrole units and of the toluene sulphonate ions lie preferentially parallel to the electrode surface. Moreover, by utilizing isotopic substitution, the neutron scattering measurements [ 19] demonstrated that the counterions lie between the planes of the polypyrrole chains, although the structure is highly disordered. It was found that the electrical conductivity and the level of order and anisotropy in these films prepared from aqueous solution, with toluene sulphonate as the counterion, depended strongly on the temperature and voltage used in the electrochemical preparation [18]. This contribution is concerned with the structural role of the counterion in these molecular composite films. We have set out to determine whether the anisotropic structure that is formed with the toluene sulphonate counterions is promoted by the planar nature of the dopant unit, and hence whether spherical counterions will disrupt such structures. Equally, will larger aromatic counterions possessing greater geometric anisotropy initiate a higher level of preferred orientation within the structures? A number of polypyrrole films were prepared electrochemically using a variety of counterions but otherwise identical conditions. These films were examined using quantitative wide-angle X-ray scattering procedures and the levels of order and anisotropy evaluated.
2. Experimental The electrochemical synthesis was performed in a standard threec o m p a r t m e n t cell using a Sycopel Potentiostat 801 under computer control. The working electrode was a conductive coating deposited on glass (Baltracon Z l 0 ) , the counterelectrode was a carbon rod, and the anodic potential was measured v e r s u s a saturated calomel electrode (SCE). No attempt was made to exclude oxygen from the electrochemical cell. Following
249
previous work [3,, 18], aqueous solutions (double deionized water), containing 0.10 mol 1-1 of the sodium salt of the appropriate counterion (used as supplied by Aldrich) and 0.25 mol 1-1 pyrrole were electrolysed at 20 °C under constant voltages ( 0 . 8 - 2 V v e r s u s SCE) until a fixed charge (8 or 16 C) had passed. Toluene sulphonate, naphthalene and anthraquinone sulphonates and disulphonates (1,5 and 2,6), C1Oa-, BF4- and SOa 2 were used as the dopant units. Films 20 × 20 mm in area were easily prepared from most of these counterions. Films prepared from anthraquinone sulphonates were very fibrous, and those containing tetrafluoroborate were of poor quality. The films were washed in doubly deionized water for four hours and then dried in a vacuum oven overnight. D.c. electrical conductivities were measured in air using the standard four-probe technique and the geometrical corrections of Valdes [20]. Table 1 summarizes the range of samples prepared and indicates both the scale of electrical conductivity and the nature of the films. In a previous study [ 18] of polypyrrole prepared from aqueous solution with p-toluene sulphonate as the counterion, the resultant electrical conductivity increased with increasing anodic potential used in the electrochemical preparation until a plateau was reached, for that example, at an anodic potential of ~ 1 . 2 V v e r s u s SCE. For samples prepared in this study similar effects were observed for polypyrrole films containing naphthalenebased counterions, although the plateau point was at a higher voltage. Films prepared using anthraquinone-based sulphonates showed little variation in electrical conductivity following different anodic voltages. TABLE 1 A summary of the polypyrrole films prepared Counterion
Range of d.c. conductivitya at 20 °C
Type of molecular organization
Physical appearance
Smooth, coherent Smooth coherent Very smooth coherent Patchy fibrous Patchy fibrous Patchy Fairly smooth Uneven surface
(S cm-1) Toluene sulphonate Naphthalene sulphonate Naphthalene disulphonate Anthraquinone sulphonate Anthraquinone disulphonate Tetrafluoroborate Perchlorate Sulphate
10
30
Anisotropic
I
10
Anisotropic
10
50
Anisotropic
1
2
Anisotropic
2
5
Anisotropic
1 2 1- 2
Isotropic Isotropic
1- 2
Isotropic
aMeasured in air using four-probe method. Range corresponds to range of the anodic potential used in the preparation of the films.
250 x
\ \\ x
\ \
Fig. 1. Definition of the geometry used for the X-ray scattering measurements, k0 and k 1 are the incident and scattered X-ray wavevectors. The scattering vector $ = k 0 - - k l , s = 4~" sin O])t. The symmetrical arrangement with the sample rotating by 0 and the detector by 20 ensures that the angular relationship X between the scattering vector and the plane of the sample is maintained over the complete scattering angle range.
X-ray scattering measurements were performed using a diffractometer operating in symmetrical transmission mode, which allowed a fixed angular relationship (×) between the plane of the sample and the scattering vector to be maintained over the whole scattering angle (20) range (Fig. 1). Thus, for example, X = 0° gives an arrangement in which the scattering vector lies within the plane of the sample, as in a conventional arrangement, whereas with X = 90 °, the scattering vector is normal to the plane of a film sample. The use of a copper targeted X-ray source and an incident beam graphite m o n o c h r o m a t o r (k = 1.54178 A) gave a useful scattering vector (s*) range of 0.2 to 6.5 A - ' . The angle X could be varied continuously from 0 to 360 °. Samples for X-ray scattering measurements were prepared by cutting the conducting polymer film into narrow sections. These sections were stacked on t o p of each other preserving the relative orientations. Using this approach, satisfactory X-ray scattered intensity curves could be obtained easily. Such measurements were made under normal laboratory conditions; no attempt was made to exclude oxygen. Intensity measurements were made in stepscan mode using fixed counting times. For the scattering peaks of interest the statistical noise is ~ 1%. The data displayed below are as recorded and uncorrected for experimental factors such as absorption and polarization. 3. Results The measured X-ray scattering intensity curves I(s) for samples of the polypyrrole films prepared using different counterions are shown in Fig. 2. In all cases the scattered intensity peaks are diffuse and are typical of highly *s = 4~ sin 0/k, where 20 is the angle between the incident and scattered X-ray paths and k is the wavelength of the incident X-radiation.
251
1
2
3 s(~:') 4
5
6
7
(a)
(c)
1
2
3
(b)
I
I
1
2
I
1
I
I
I
I
3 S(~,_,)4
5
6
7
1
2
l
i
4
s(~")
i
3 s(~_L) 4
5
6
i
5
?
A
6
?
(d)
Fig. 2. The X-ray scattered intensity curves I(s) measured with X = 0 ° ( - - - - - - ) and X = 90 ° ( ) for thin films of p o l y p y r r o l e prepared electrochemically at r o o m t e m p e r a t u r e f r o m a q u e o u s solutions with various counterions. (a) Toluene sulphonate; (b) naphthalene disulphonate; (c) perchlorate; and (d) sulphate.
disordered non-crystalline polymers [21]. For each sample two curves are displayed. One is for a scattering pattern r e c o rded with X = 0 °, in which case the scattering vector lies parallel to the original electrode surface and arises from structural correlations within the plane of the film. The second curve is measured with X = 90°, and hence the scattering vector lies normal to the electrode surface and probes the organization through the thickness of the film. For an isotropic sample, i.e., one in which the molecular organization in these two directions is similar, these two curves should be identical. Of course, there will be variations in experimental geom et ry for the two values of ×, which lead to differing modifications of the scattered intensity. We will consider this factor in m or e detail presently; however, it is clear from an examination of Fig. 2 that some samples display similar curves for X = 0 and
252 90 °, and others exhibit markedly different curves. In essence, some of the polypyrrole-based films possess an isotropic distribution of molecular organization, others an anisotropic structure. From Fig. 2 we see that polypyrrole films prepared using toluene- or naphthalene-based counterions have an anisotropic structure and those prepared using sulphates or chlorates an isotropic organization. The principal features of these diffuse scattering patterns are the peaks in the region 1 - 2 A -1, which arise from spatial correlations on the scale of 3.5 - 6 A. If we consider first the scattering pattern from polypyrrole films prepared using toluene sulphonate, then from earlier work [17, 18] we may associate the sharper peak {{Fig. 2(a)) at s ~ 1.8 A - l with disordered layers of aromatic rings, either belonging to the polypyrrole rings or the counterions. This peak is more intense for the curve recorded for × = 90 °. In contrast, the curve recorded for × = 0 ° contains a broader more diffuse peak at s ~ 1.3 A -1, which arises from the interactions between polypyrrole chain segments lying in the same plane. The presence of these two components in the scattering pattern indicates anisotropy on a local scale. That these two components are observed for separate angular relationships between the polymer film and the scattering vector indicates in addition t h a t this anisotropy is global in nature, i.e., the planes of all aromatic rings lie preferentially parallel to the electrode surface. The level of this anisotropy was found to be related to the preparation conditions of the polypyrrole films [18]; greater anodic potential and lower temperatures were found to promote a greater degree of preferred orientation. Similar behaviour was found for the films prepared in this study employing other aromatic-based counterions. As an example, Fig. 2(b) shows the scattering for a film grown using naphthalene disulphonate as the counterion. It shows the basic features described above. In other words, this film also exhibits a preferred orientation of the aromatic rings parallel to the film surface. In all cases, films prepared using either naphthalene sulphonate or naphthalene disulphonate or the anthraquinone-based salts show some level of anisotropy. The precise level of anisotropy was heavily dependent upon the preparation conditions. In contrast, films of polypyrrole prepared electrochemically using either perchlorate or sulphate as the counterion did not exhibit such anisotropy. In each case {{Figs. 2(c) and (d) respectively) the X-ray scattering curves recorded for × = 0 ~ and × = 90 ° were very similar. To emphasize the differences between the two types of sample, Fig. 3 shows the scattering recorded as a function of × with s = 1.8 A -1 for samples of polypyrrole prepared using naphthalene disulphonate and sulphate. The variation in the scattered intensity as a function of × provides a direct measure of the level of anisotropy. That for the film prepared using the naphthalene-based salt is clearly substantially greater than t h a t for the film containing the simple sulphate ion as the dopant unit. It is appropriate now to consider the effect of the sample geometry upon the variation of the scattered intensity as a function of ×. There are two factors to take into account: the variation in absorption due to the dif-
253
+40
[~
.20
I
0 -20 -40 0
90
180
~c
Fig. 3. The X - r a y scattered i n t e n s i t y f u n c t i o n s
270
;(X)
360 measured at s = 1.8 ~- 1 f o r samples
of polypyrrole prepared using naphthalene disulphonate (full line) and sulphate (broken line) counterions.
fering path length, and the changes in the sample volume. The latter factor may be minimized by cutting the sections of the film to a size close to that of the incident X-ray beam. Estimates of the former factor gave a variation in the absorption correction of a maximum of 20%. In fact, the competition between the two factors would decrease that maximum and the variation in the absorption correction A (X) would not necessarily 'peak' at × = 0 ° or 90 °. The variation of the scattered intensity I(×) for the film containing sulphate as the counterion is seen as arising from the sample geometry effects. That for the film containing the naphthalene salt clearly is too great to arise from absorption effects and therefore must arise from an anisotropic molecular organization. This conclusion is confirmed by the differences in positions of the diffuse maxima in the curves for × = 0 ° and 90 °, both for peaks arising from interchain and intrachain correlations. In summary, films prepared using counterions containing an aromatic ring structure exhibit an anisotropic molecular organization, while those containing perchlorate, sulphate and tetrafluoroborate counterions do not. A similar observation was made on tetrafluoroborate as the counterion for films of polypyrrole prepared in organic solvents [22]. These polypyrrole films prepared using different counterions of course showed other differences in addition to variations in their X-ray scattering patterns. As Qian and coworkers have recorded [6, 7], films containing toulene sulphonate and sulphate as the counterions were smooth and dense. In contrast, films prepared with perchlorate counterions were of poor quality and exhibited about 50% space filling. We observed similar effects and in addition we note that films containing naphthalene-based sulphonates were also dense and smooth, while those involving anthraquinone derivatives were of poor quality. These observations are summarized in Table 1. We also list in
254 Table I the range of conductivities recorded for the variety of polypyrrole films prepared in this study. The highest conductivities recorded were those for the disulphonated naphthalene. Films containing non-aromatic-based counterions exhibited lower conductivities than those for aromatic-based systems. The exception to this rule were those films prepared using anthraquinone derivatives, which also displayed low electrical conductivities.
4. Discussion Variation of the choice of counterion in the preparation of conducting films of polypyrrole clearly has several effects. There are changes both at a macroscopic level to the morphology of the composite and at the microscopic or molecular levels, as indicated b y the X-ray scattering analysis. These structural changes significantly affect both the magnitude of the electrical conductivity as detailed in Table 1, and other properties of the resultant films such as mechanical [10] or environmental stability [23]. The role of the counterions in those and other properties and interactions is obviously complex. Here we will consider some of the properties of the counterion units that may relate to the resultant structural organization. Shape o f counterion unit The X-ray scattering analysis reported here and the previous X-ray [17, 18] and neutron [19] scattering studies show unequivocally that the organization of polypyrrole films prepared with counterions containing aromatic rings is anisotropic. We suggest that a major factor in the promotion of this anisotropic structure is the anisotropic shape of the counterion. Films prepared using 'spherical' counterions, such as tetrafluoroborate or sulphate, do not exhibit this preferred orientation of the pyrrole rings to the electrode surface. Although the precise level of preferred orientation observed in the toluene- or naphthalene-based systems was dependent upon the particular temperature and anodic potential employed, Figs. 2(a) and (b) indicate a more organized structure for the films containing naphthalene derivatives as the counterions. In other words, a more anisotropic counterion leads to a higher level of preferred orientation in the film. The anisotropy required of the counterion is of course its degree of planarity rather than as an extended rod molecule. Extension of this degree of planarity to three aromatic units does not lead to superior films. This may result from the disruptive effect such a large unit would have if not perfectly located in the molecular organization. We are presently conducting studies to determine the effects of the counterions and this anisotropy upon the conformation of the polypyrrole chains themselves. Size o f counterion unit Large counterions will naturally have a restricted mobility. This may have advantages in terms of stability after the formation of the film, but disadvantages during the growth of the polymer on the electrode. Limited
255 mobility during film growth may lead to restricted doping levels and hence electrical conductivity. The counterions also act as separators of the polymer chains, or at least sections of the polymer chains. In fact if the spherical counterions such as C104- and SO42- do intercalate polypyrrole layers as observed in the toluene sulphonate-based material [19], then the separation of those polymer layers must be ~ 5 - 6 A given the size of the counterions and the position of the maximum in the X-ray scattering pattern. This would be in contrast to the spacing of 3.4 - 4 A for the aromatic-based counterion systems. However, these larger spacings between polypyrrole chains need not necessarily lead to a reduction in the electrical conductivity, as the work of Wernet et al. [12] with n-alkylsulphonates as counterions showed. Particular counterions may p r o m o t e a more organized polymer chain and hence enhance electrical conductivity. Charge to v o l u m e ratio o f c o u n t e r i o n unit
The role of the counterion in promoting an anisotropic structure has already been discussed. However, there is little to be gained from enhancing the anisotropy through the inclusion of large rings, if at the same time the level of charge on the counterions effectively drops because of the increase in volume of the counterion unit. This effect may be deduced from Table 1. Changing from toluene sulphonate to naphthalene sulphonate results in a drop in electrical conductivity and the charge/volume ratio is halved, although the level of anisotropy rises. However, employing the disulphonated naphthalene unit, i.e., returning the charge/volume ratio to approximately that of toluene sulphonate, leads to an enhancement of electrical conductivity. In fact, the conductivity is higher than observed with toluene sulphonate, since the degree of preferred orientation has been raised and the charge/volume ratio of the counterion maintained. This would suggest that the spherically-shaped counterions and single atom ions should yield higher doping levels and hence electrical conductivity. However, such units do not p r o m o t e an organized or specific molecular structure. A n iso tropic grow t h
The production of a polymer film with a global preferred orientation not only requires anisotropic local packing, but also the maintenance of a smooth growth face throughout the film preparation time. The factors controlling the morphology of the growing surface of the film will be many and varied, and the particular role of the counterion is not altogether clear. The polypyrrole chains themselves are intrinsically planar except where defective through crosslinking or non-aromatic bonding (other conformational defects maintain planarity [18, 24] ). The adherence of such a planar molecule to the electrode will depend upon an even distribution of charge. The inclusion of spherically-shaped counterions can only lead to a disruption of a smooth growth face, while an aromatic-based unit will possibily aid its retention. The role of the counterion in the final package of properties of the resultant electrochemically-grown films of polypyrrole is far from straight-
256 forward. However, we can isolate some particular effects. Enhancement of electrical conductivity in these disordered materials m a y be achieved through the use of a planar-type counterion with a high charge/volume ratio. It would appear that low preparation temperatures and high anodic potentials are also an advantage. Films prepared in this manner are dense and smooth with good mechanical properties. In Fig. 4 we show a schematic representation of the organization at a molecular level in such films. In addition to the counterion units acting in a non-disruptive manner during film growth, it is possible that they also m o d i f y the polypyrrole chain conformations, and work is in progress to extract such information from the X-ray scattering curves using computer-generated molecular models.
po~mer layers
Fig. 4. Schematic representation of the molecular organization of a polypyrrole film containing aromatic based counterions. The planar polypyrrole chains lie in disordered sheets separated by the "dopant" units.
5. Summary The type of counterion used in the electrochemical preparation of conducting films of polypyrrole has a significant impact upon the resultant molecular organization and hence its bulk properties, including the level of electrical conductivity. Counterions involving aromatic rings promote a high level of preferred orientation of the pyrrole rings parallel to the electrode or growth surface. The greater the degree of planarity of the counterion, the higher the level of preferred orientation and the electrical conductivity displayed by the polymer film. Films prepared from spherically-based counterions do not exhibit this anisotropic molecular organization and display lower levels of conductivity.
Acknowledgements This work was supported by the Science and Engineering Research Council (GR/E 04240) and by the University of Reading (Fenowship to F.J.D.). Part of this work was performed as a final year project for a B.Sc. degree of the University of Reading (C.H.L.).
257
References 1 A. F. Diaz, K. K. Kanazawa and G. Gardini, J. Chem. Soc. Chem. Commun., (1979) 635. 2 G. B. Street, in T. A. Skotheim (ed.), Handbook o f Conducting Polymers, Marcel Dekker, New York, 1986, p. 265. 3 M. Satoh, K. Kaneto and K. Yoshino, Synth. Met., 14 (1986) 289. 4 M. Satoh, K. Kaneto and K. Yoshino, Jpn. J. Appl. Phys., 24 (1985) L423. 5 A. S. N. Murthy, S. Pal and K. S. Reddy, J. Mater. Sci. Lett., 3 (1984) 745. 6 R. Qian and J. Qiu, Polym. J., 19 (1987) 157. 7 R. Qian, J. Qiu and B. Yan, Synth. Met., 14 (1986) 81. 8 X. Bi, Y. Yao, M. Wan, P. Wang, K. Xiao, Q. Yang and R. Qian, Makromol. Chem., 186 (1985) 1101. 9 K. J. Wynne and G. B. Street, Macromolecules, 18 (1985) 2361. 10 A. F. Diaz and B. Hall, IBMJ. Res. Develop., 27 (1983) 342. 11 D. Bloor, R. D. Hercliffe, C. G. Galiotis and R. J. Young, in H. Kuzmany, M. Mehring and S. Roth (eds.), Electronic Properties o f Polymers and Related Compounds, Springer, Berlin, 1981, p. 179. 12 W. Wernet, M. Monkenbusch and G. Wegner, Makromol. Chem. Rapid. Commun., 5 (1984) 157. 13 L. F. Warren and D. P. Anderson, J. Electrochem. Soc., 134 (1987) 101. 14 G. B. Street, T. C. Clarke, T. M. Krounbi, K. K. Kanazawa, Y. V. Lee, P. Pfluger, J. C. Scott and G. Weiser, Mol. Cryst. Liq. Cryst., 83 (1982) 253. 15 F. Devreux, G. Bidan, A. A. Syed and C. J. Tsintavis, J. Phys.(Paris), 46 (1985) 1595. 16 P. Pfluger and G. B. Street, J. Chem. Phys., 80 (1986) 544. 17 G. R. Mitchell, Polym. Commun., 12 (1986) 346. 18 G. R. Mitchell and A. Geri, J. Phys. D: Appl. Phys., 20 (1987) 1346. 19 G. R. Mitchell, F. J. Davis, R. Cywinski and W. S. Howells, J. Phys. C: Solid State Phys., 21 (1988) L411. 20 L. B. Valdes, Proc. Inst. Radio Eng., 42 (1954) 420. 21 G. R. Mitchell, in S. E. Keinath, R. L. Miller and J. K. Rieke (eds.), Order in the A m o r p h o u s State o f Polymers, Plenum, New York, 1987, p. 1. 22 B. Kaye, A. E. Underhill and G. R. Mitchell, unpublished work. 23 M. A. Druy, Synth. Met., 15 (1986) 243. 24 G. R. Mitchell, in preparation.
247
THE E F F E C T OF DOPANT MOLECULES ON THE MOLECULAR ORDER OF ELECTRICALLY-CONDUCTING FILMS OF POLYPYRROLE G. R. MITCHELL*, F. J. DAVIS and C. H. LEGGE J. J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading RG6 2AF (U.K.) (Received July 7, 1988; accepted July 20, 1988)
Abstract X-ray scattering curves have been measured for a range of electrochemically-prepared conducting polypyrrole films employing a variety of counterions in aqueous solutions. Films containing counterions based on aromatic rings exhibit an anisotropic molecular organization. The degree of anisotropy is enhanced through the use of highly planar counterions. The electrical conductivity of such films is also improved if the charge/volume ratio of the counterion is maintained at a high level. Polypyrrole films prepared using 'spherically' shaped counterions such as SO42- do not display such anisotropic molecular organizations, and exhibit lower electrical conductivities. The competing structural roles of the counterions within these molecular composites are discussed.
1. Introduction Stable, flexible and electrically-conducting polymer films may be prepared from pyrrole in a one-step electro-oxidation process [ 1 - 2]. Films prepared in this manner have electrical conductivities ranging from 10 to 1000 S cm 1 and require no additional dopants to achieve these conductivity levels. This electrochemical method of preparation of polypyrrole and other polymers based on similar heterocyclic units has been widely studied, and it may be performed in aqueous or organic solvent-based electrolytes. The process of oxidation of the polymer chains is accompanied by the inclusion, in the growing polymer film on the electrode, of counterions that act as dopant molecules. Typical levels of ' d o p a n t ' concentration are in the range of one counterion for three to four pyrrole units. A considerable variety of counterions have been used in an aqueous environment; units such as toluene sulphonate, perchlorate and tetrafluoroborate are among the most widely *Author to whom correspondence should be addressed.
248 studied [3 - 8]. In the case of p-toluene sulphonate, typical levels of doping result in the counterions comprising some 50% by volume of the conducting film. It is therefore not surprising that these counterions have a significant effect upon the quality and mechanical properties of the polymer films [9 11], in addition to that expected for the electrical transport properties. Moreover, it has been suggested that the counterions employed in the electrochemical preparation have a direct impact on the molecular organization of the resultant films [12, 13]. The molecular organization of polypyrrole films is highly disordered. The exact chemical configuration of the polypyrrole chains is unknown, but spectroscopic techniques indicated that a substantial proportion of the polymerization occurs through the ~ - ~ sites [ 14, 15]. There is also evidence to suggest the presence of highly defective chains with crosslinking and polymerization through the fl positions [ 16]. We have recently shown using both X-ray [17, 18] and neutron [19] scattering techniques that the polypyrrole films prepared using toluene sulphonate as the counterion exhibit an anisotropic molecular organization, in which the planes of the pyrrole units and of the toluene sulphonate ions lie preferentially parallel to the electrode surface. Moreover, by utilizing isotopic substitution, the neutron scattering measurements [ 19] demonstrated that the counterions lie between the planes of the polypyrrole chains, although the structure is highly disordered. It was found that the electrical conductivity and the level of order and anisotropy in these films prepared from aqueous solution, with toluene sulphonate as the counterion, depended strongly on the temperature and voltage used in the electrochemical preparation [18]. This contribution is concerned with the structural role of the counterion in these molecular composite films. We have set out to determine whether the anisotropic structure that is formed with the toluene sulphonate counterions is promoted by the planar nature of the dopant unit, and hence whether spherical counterions will disrupt such structures. Equally, will larger aromatic counterions possessing greater geometric anisotropy initiate a higher level of preferred orientation within the structures? A number of polypyrrole films were prepared electrochemically using a variety of counterions but otherwise identical conditions. These films were examined using quantitative wide-angle X-ray scattering procedures and the levels of order and anisotropy evaluated.
2. Experimental The electrochemical synthesis was performed in a standard threec o m p a r t m e n t cell using a Sycopel Potentiostat 801 under computer control. The working electrode was a conductive coating deposited on glass (Baltracon Z l 0 ) , the counterelectrode was a carbon rod, and the anodic potential was measured v e r s u s a saturated calomel electrode (SCE). No attempt was made to exclude oxygen from the electrochemical cell. Following
249
previous work [3,, 18], aqueous solutions (double deionized water), containing 0.10 mol 1-1 of the sodium salt of the appropriate counterion (used as supplied by Aldrich) and 0.25 mol 1-1 pyrrole were electrolysed at 20 °C under constant voltages ( 0 . 8 - 2 V v e r s u s SCE) until a fixed charge (8 or 16 C) had passed. Toluene sulphonate, naphthalene and anthraquinone sulphonates and disulphonates (1,5 and 2,6), C1Oa-, BF4- and SOa 2 were used as the dopant units. Films 20 × 20 mm in area were easily prepared from most of these counterions. Films prepared from anthraquinone sulphonates were very fibrous, and those containing tetrafluoroborate were of poor quality. The films were washed in doubly deionized water for four hours and then dried in a vacuum oven overnight. D.c. electrical conductivities were measured in air using the standard four-probe technique and the geometrical corrections of Valdes [20]. Table 1 summarizes the range of samples prepared and indicates both the scale of electrical conductivity and the nature of the films. In a previous study [ 18] of polypyrrole prepared from aqueous solution with p-toluene sulphonate as the counterion, the resultant electrical conductivity increased with increasing anodic potential used in the electrochemical preparation until a plateau was reached, for that example, at an anodic potential of ~ 1 . 2 V v e r s u s SCE. For samples prepared in this study similar effects were observed for polypyrrole films containing naphthalenebased counterions, although the plateau point was at a higher voltage. Films prepared using anthraquinone-based sulphonates showed little variation in electrical conductivity following different anodic voltages. TABLE 1 A summary of the polypyrrole films prepared Counterion
Range of d.c. conductivitya at 20 °C
Type of molecular organization
Physical appearance
Smooth, coherent Smooth coherent Very smooth coherent Patchy fibrous Patchy fibrous Patchy Fairly smooth Uneven surface
(S cm-1) Toluene sulphonate Naphthalene sulphonate Naphthalene disulphonate Anthraquinone sulphonate Anthraquinone disulphonate Tetrafluoroborate Perchlorate Sulphate
10
30
Anisotropic
I
10
Anisotropic
10
50
Anisotropic
1
2
Anisotropic
2
5
Anisotropic
1 2 1- 2
Isotropic Isotropic
1- 2
Isotropic
aMeasured in air using four-probe method. Range corresponds to range of the anodic potential used in the preparation of the films.
250 x
\ \\ x
\ \
Fig. 1. Definition of the geometry used for the X-ray scattering measurements, k0 and k 1 are the incident and scattered X-ray wavevectors. The scattering vector $ = k 0 - - k l , s = 4~" sin O])t. The symmetrical arrangement with the sample rotating by 0 and the detector by 20 ensures that the angular relationship X between the scattering vector and the plane of the sample is maintained over the complete scattering angle range.
X-ray scattering measurements were performed using a diffractometer operating in symmetrical transmission mode, which allowed a fixed angular relationship (×) between the plane of the sample and the scattering vector to be maintained over the whole scattering angle (20) range (Fig. 1). Thus, for example, X = 0° gives an arrangement in which the scattering vector lies within the plane of the sample, as in a conventional arrangement, whereas with X = 90 °, the scattering vector is normal to the plane of a film sample. The use of a copper targeted X-ray source and an incident beam graphite m o n o c h r o m a t o r (k = 1.54178 A) gave a useful scattering vector (s*) range of 0.2 to 6.5 A - ' . The angle X could be varied continuously from 0 to 360 °. Samples for X-ray scattering measurements were prepared by cutting the conducting polymer film into narrow sections. These sections were stacked on t o p of each other preserving the relative orientations. Using this approach, satisfactory X-ray scattered intensity curves could be obtained easily. Such measurements were made under normal laboratory conditions; no attempt was made to exclude oxygen. Intensity measurements were made in stepscan mode using fixed counting times. For the scattering peaks of interest the statistical noise is ~ 1%. The data displayed below are as recorded and uncorrected for experimental factors such as absorption and polarization. 3. Results The measured X-ray scattering intensity curves I(s) for samples of the polypyrrole films prepared using different counterions are shown in Fig. 2. In all cases the scattered intensity peaks are diffuse and are typical of highly *s = 4~ sin 0/k, where 20 is the angle between the incident and scattered X-ray paths and k is the wavelength of the incident X-radiation.
251
1
2
3 s(~:') 4
5
6
7
(a)
(c)
1
2
3
(b)
I
I
1
2
I
1
I
I
I
I
3 S(~,_,)4
5
6
7
1
2
l
i
4
s(~")
i
3 s(~_L) 4
5
6
i
5
?
A
6
?
(d)
Fig. 2. The X-ray scattered intensity curves I(s) measured with X = 0 ° ( - - - - - - ) and X = 90 ° ( ) for thin films of p o l y p y r r o l e prepared electrochemically at r o o m t e m p e r a t u r e f r o m a q u e o u s solutions with various counterions. (a) Toluene sulphonate; (b) naphthalene disulphonate; (c) perchlorate; and (d) sulphate.
disordered non-crystalline polymers [21]. For each sample two curves are displayed. One is for a scattering pattern r e c o rded with X = 0 °, in which case the scattering vector lies parallel to the original electrode surface and arises from structural correlations within the plane of the film. The second curve is measured with X = 90°, and hence the scattering vector lies normal to the electrode surface and probes the organization through the thickness of the film. For an isotropic sample, i.e., one in which the molecular organization in these two directions is similar, these two curves should be identical. Of course, there will be variations in experimental geom et ry for the two values of ×, which lead to differing modifications of the scattered intensity. We will consider this factor in m or e detail presently; however, it is clear from an examination of Fig. 2 that some samples display similar curves for X = 0 and
252 90 °, and others exhibit markedly different curves. In essence, some of the polypyrrole-based films possess an isotropic distribution of molecular organization, others an anisotropic structure. From Fig. 2 we see that polypyrrole films prepared using toluene- or naphthalene-based counterions have an anisotropic structure and those prepared using sulphates or chlorates an isotropic organization. The principal features of these diffuse scattering patterns are the peaks in the region 1 - 2 A -1, which arise from spatial correlations on the scale of 3.5 - 6 A. If we consider first the scattering pattern from polypyrrole films prepared using toluene sulphonate, then from earlier work [17, 18] we may associate the sharper peak {{Fig. 2(a)) at s ~ 1.8 A - l with disordered layers of aromatic rings, either belonging to the polypyrrole rings or the counterions. This peak is more intense for the curve recorded for × = 90 °. In contrast, the curve recorded for × = 0 ° contains a broader more diffuse peak at s ~ 1.3 A -1, which arises from the interactions between polypyrrole chain segments lying in the same plane. The presence of these two components in the scattering pattern indicates anisotropy on a local scale. That these two components are observed for separate angular relationships between the polymer film and the scattering vector indicates in addition t h a t this anisotropy is global in nature, i.e., the planes of all aromatic rings lie preferentially parallel to the electrode surface. The level of this anisotropy was found to be related to the preparation conditions of the polypyrrole films [18]; greater anodic potential and lower temperatures were found to promote a greater degree of preferred orientation. Similar behaviour was found for the films prepared in this study employing other aromatic-based counterions. As an example, Fig. 2(b) shows the scattering for a film grown using naphthalene disulphonate as the counterion. It shows the basic features described above. In other words, this film also exhibits a preferred orientation of the aromatic rings parallel to the film surface. In all cases, films prepared using either naphthalene sulphonate or naphthalene disulphonate or the anthraquinone-based salts show some level of anisotropy. The precise level of anisotropy was heavily dependent upon the preparation conditions. In contrast, films of polypyrrole prepared electrochemically using either perchlorate or sulphate as the counterion did not exhibit such anisotropy. In each case {{Figs. 2(c) and (d) respectively) the X-ray scattering curves recorded for × = 0 ~ and × = 90 ° were very similar. To emphasize the differences between the two types of sample, Fig. 3 shows the scattering recorded as a function of × with s = 1.8 A -1 for samples of polypyrrole prepared using naphthalene disulphonate and sulphate. The variation in the scattered intensity as a function of × provides a direct measure of the level of anisotropy. That for the film prepared using the naphthalene-based salt is clearly substantially greater than t h a t for the film containing the simple sulphate ion as the dopant unit. It is appropriate now to consider the effect of the sample geometry upon the variation of the scattered intensity as a function of ×. There are two factors to take into account: the variation in absorption due to the dif-
253
+40
[~
.20
I
0 -20 -40 0
90
180
~c
Fig. 3. The X - r a y scattered i n t e n s i t y f u n c t i o n s
270
;(X)
360 measured at s = 1.8 ~- 1 f o r samples
of polypyrrole prepared using naphthalene disulphonate (full line) and sulphate (broken line) counterions.
fering path length, and the changes in the sample volume. The latter factor may be minimized by cutting the sections of the film to a size close to that of the incident X-ray beam. Estimates of the former factor gave a variation in the absorption correction of a maximum of 20%. In fact, the competition between the two factors would decrease that maximum and the variation in the absorption correction A (X) would not necessarily 'peak' at × = 0 ° or 90 °. The variation of the scattered intensity I(×) for the film containing sulphate as the counterion is seen as arising from the sample geometry effects. That for the film containing the naphthalene salt clearly is too great to arise from absorption effects and therefore must arise from an anisotropic molecular organization. This conclusion is confirmed by the differences in positions of the diffuse maxima in the curves for × = 0 ° and 90 °, both for peaks arising from interchain and intrachain correlations. In summary, films prepared using counterions containing an aromatic ring structure exhibit an anisotropic molecular organization, while those containing perchlorate, sulphate and tetrafluoroborate counterions do not. A similar observation was made on tetrafluoroborate as the counterion for films of polypyrrole prepared in organic solvents [22]. These polypyrrole films prepared using different counterions of course showed other differences in addition to variations in their X-ray scattering patterns. As Qian and coworkers have recorded [6, 7], films containing toulene sulphonate and sulphate as the counterions were smooth and dense. In contrast, films prepared with perchlorate counterions were of poor quality and exhibited about 50% space filling. We observed similar effects and in addition we note that films containing naphthalene-based sulphonates were also dense and smooth, while those involving anthraquinone derivatives were of poor quality. These observations are summarized in Table 1. We also list in
254 Table I the range of conductivities recorded for the variety of polypyrrole films prepared in this study. The highest conductivities recorded were those for the disulphonated naphthalene. Films containing non-aromatic-based counterions exhibited lower conductivities than those for aromatic-based systems. The exception to this rule were those films prepared using anthraquinone derivatives, which also displayed low electrical conductivities.
4. Discussion Variation of the choice of counterion in the preparation of conducting films of polypyrrole clearly has several effects. There are changes both at a macroscopic level to the morphology of the composite and at the microscopic or molecular levels, as indicated b y the X-ray scattering analysis. These structural changes significantly affect both the magnitude of the electrical conductivity as detailed in Table 1, and other properties of the resultant films such as mechanical [10] or environmental stability [23]. The role of the counterions in those and other properties and interactions is obviously complex. Here we will consider some of the properties of the counterion units that may relate to the resultant structural organization. Shape o f counterion unit The X-ray scattering analysis reported here and the previous X-ray [17, 18] and neutron [19] scattering studies show unequivocally that the organization of polypyrrole films prepared with counterions containing aromatic rings is anisotropic. We suggest that a major factor in the promotion of this anisotropic structure is the anisotropic shape of the counterion. Films prepared using 'spherical' counterions, such as tetrafluoroborate or sulphate, do not exhibit this preferred orientation of the pyrrole rings to the electrode surface. Although the precise level of preferred orientation observed in the toluene- or naphthalene-based systems was dependent upon the particular temperature and anodic potential employed, Figs. 2(a) and (b) indicate a more organized structure for the films containing naphthalene derivatives as the counterions. In other words, a more anisotropic counterion leads to a higher level of preferred orientation in the film. The anisotropy required of the counterion is of course its degree of planarity rather than as an extended rod molecule. Extension of this degree of planarity to three aromatic units does not lead to superior films. This may result from the disruptive effect such a large unit would have if not perfectly located in the molecular organization. We are presently conducting studies to determine the effects of the counterions and this anisotropy upon the conformation of the polypyrrole chains themselves. Size o f counterion unit Large counterions will naturally have a restricted mobility. This may have advantages in terms of stability after the formation of the film, but disadvantages during the growth of the polymer on the electrode. Limited
255 mobility during film growth may lead to restricted doping levels and hence electrical conductivity. The counterions also act as separators of the polymer chains, or at least sections of the polymer chains. In fact if the spherical counterions such as C104- and SO42- do intercalate polypyrrole layers as observed in the toluene sulphonate-based material [19], then the separation of those polymer layers must be ~ 5 - 6 A given the size of the counterions and the position of the maximum in the X-ray scattering pattern. This would be in contrast to the spacing of 3.4 - 4 A for the aromatic-based counterion systems. However, these larger spacings between polypyrrole chains need not necessarily lead to a reduction in the electrical conductivity, as the work of Wernet et al. [12] with n-alkylsulphonates as counterions showed. Particular counterions may p r o m o t e a more organized polymer chain and hence enhance electrical conductivity. Charge to v o l u m e ratio o f c o u n t e r i o n unit
The role of the counterion in promoting an anisotropic structure has already been discussed. However, there is little to be gained from enhancing the anisotropy through the inclusion of large rings, if at the same time the level of charge on the counterions effectively drops because of the increase in volume of the counterion unit. This effect may be deduced from Table 1. Changing from toluene sulphonate to naphthalene sulphonate results in a drop in electrical conductivity and the charge/volume ratio is halved, although the level of anisotropy rises. However, employing the disulphonated naphthalene unit, i.e., returning the charge/volume ratio to approximately that of toluene sulphonate, leads to an enhancement of electrical conductivity. In fact, the conductivity is higher than observed with toluene sulphonate, since the degree of preferred orientation has been raised and the charge/volume ratio of the counterion maintained. This would suggest that the spherically-shaped counterions and single atom ions should yield higher doping levels and hence electrical conductivity. However, such units do not p r o m o t e an organized or specific molecular structure. A n iso tropic grow t h
The production of a polymer film with a global preferred orientation not only requires anisotropic local packing, but also the maintenance of a smooth growth face throughout the film preparation time. The factors controlling the morphology of the growing surface of the film will be many and varied, and the particular role of the counterion is not altogether clear. The polypyrrole chains themselves are intrinsically planar except where defective through crosslinking or non-aromatic bonding (other conformational defects maintain planarity [18, 24] ). The adherence of such a planar molecule to the electrode will depend upon an even distribution of charge. The inclusion of spherically-shaped counterions can only lead to a disruption of a smooth growth face, while an aromatic-based unit will possibily aid its retention. The role of the counterion in the final package of properties of the resultant electrochemically-grown films of polypyrrole is far from straight-
256 forward. However, we can isolate some particular effects. Enhancement of electrical conductivity in these disordered materials m a y be achieved through the use of a planar-type counterion with a high charge/volume ratio. It would appear that low preparation temperatures and high anodic potentials are also an advantage. Films prepared in this manner are dense and smooth with good mechanical properties. In Fig. 4 we show a schematic representation of the organization at a molecular level in such films. In addition to the counterion units acting in a non-disruptive manner during film growth, it is possible that they also m o d i f y the polypyrrole chain conformations, and work is in progress to extract such information from the X-ray scattering curves using computer-generated molecular models.
po~mer layers
Fig. 4. Schematic representation of the molecular organization of a polypyrrole film containing aromatic based counterions. The planar polypyrrole chains lie in disordered sheets separated by the "dopant" units.
5. Summary The type of counterion used in the electrochemical preparation of conducting films of polypyrrole has a significant impact upon the resultant molecular organization and hence its bulk properties, including the level of electrical conductivity. Counterions involving aromatic rings promote a high level of preferred orientation of the pyrrole rings parallel to the electrode or growth surface. The greater the degree of planarity of the counterion, the higher the level of preferred orientation and the electrical conductivity displayed by the polymer film. Films prepared from spherically-based counterions do not exhibit this anisotropic molecular organization and display lower levels of conductivity.
Acknowledgements This work was supported by the Science and Engineering Research Council (GR/E 04240) and by the University of Reading (Fenowship to F.J.D.). Part of this work was performed as a final year project for a B.Sc. degree of the University of Reading (C.H.L.).
257
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