Structural properties of a conducting polymer: Doped (CH)x

Physica 127B (1984) 158-I64 North-Holland, Amsterdam

STI~UCTURAL PROPERTIES 'OF A CONDUCI'ING POLYMER: DO~'ED (CI-I)~ J.P. P O U G E T * , P, ROBIN~, R. C O M E S ¢, H,W. GIBSON$, A.J. EPSTE]N:[: and D. B I L L A U D § * Laboratoir~ de. Physiqne des Solides and LURE Univerzit~ Paris-Sad, F.91405 Or'say, France ~"Laboratoire eenlral de Reeherches, Thomson C,S.F., F,91401 Orsav, France ;~Xerox Webster Research Center, W114, Webster, N,Y, 14580 USA § Laboratoire de Chimle du Sollde-Min:.ral, Universiff: Nancy I.F,54506 Vandoeuvre les Nanc),, France

Structural properties o[ pure and doped [CH]~, pr~:pared by the method tff Shirakawa. are reviewed. MaterlaV, are charucteT'iT.edby their structural featur~.~(discn-der cr~,~tallinity,homogeneity of the doping) which can influence clcctr;cat properties. Cis to trt~ns isomerization proces,,,ob~rvcd either upon thermal annealing or upon doping is pr~semed on structural basis. New s|ructures in which dopant kr.tercalatcs in coI,umns or layers within the (CH), host lattice are also considered.

1. Introdaction Polyacetylene, (CH).., has been subject to recent intense theoretical and experintental studies (1], This is due to the fact that this simplest conjugated polymer has an electr['~al conductivity which can be increased by more than eight orders of magnitude through expo.~ttre to the yapours of aceeptor or donor chemical species [2]. In addition it is a potential material for applications (batteries, cells, electrodes) because of its fibrillar morphology which provides a great specific area. It has been eslablished that electronic properties of conducting polymers depend stro:ngly o[ the molecular structure and of the supramolecular organization [3], ( C H } . which can be obtained under higl~ crystallinity, offers a unique possibility to study the relationship between electrical conduction and structural properties of polymers. T h e aim of this paper is to review here the struOurat properties of pure and doped (CHL.

7.. Stngcture ol undoped (CH)~ (CI-I)~ can be synthesized in either a cis or a thermodynamically more stable trafl~ isonler. In

the crystalline state these two isomers fo:m chains of different symmetry (fig. 1) and repeat distances: 4.36 A, (4 CH units) and 2.46 A, (2 C H units) respectively. Pure (CH) x is a semiconductor due to a commensurate Peierls distc¢tion leading to alternation in bond lengths. T r a a s (CH)~ has one of two degenerate ground states dep,~nding on the phase of the bond alternatiotr,, while phase reversal ol the bond alternation i~a cis (CH)~ interchanges the: non degenerate :;,, transoid and trans cisoid structures. 2.1. Morphology

(CH)~ prepared by the method of Shirakawa [4] are obtained under the form of films, Scanning electron micrographic studies of these films show a mat of fibrils randomly oriented, with a fibrillar diameter varying from 100 to 5(10 ~ . However, fibrils can be aligned pr,efereutially along a given direction by stretchinl? or rolling the film. X-ray studies of stretched (CH)~ as welt as electron diffraction and transmission electron microscc~py studies have shown that (CH)~ chains are directed along the fibril axis [5] [19]. Debye-Scherrer diffraction patterns obtained with (CH)~ films are composed of a weal,: and broad halo and of a series of sharp.~.r lines (fig. 3) wlfieh have been attributed respectively to X-ray diffraction of 'amorphous' and crystalline

0378-4363/84/$03.00 ~1 Elsevier Science Publishers B.V. (North-Holhmd Physics Publishing Division)

J.P, Pouget et al. / Doped (CH)~

~---==x..~j

cls-transoid

~----~,~

trans-clsoid

i

r(p.al dlstanca It)

159

(rn~ t i

Pg9

Ir~nS-trahso~d

lilt

Fig. 1. Structures and symmetries of c[s (CH},. [¢is-transoid, and trans-c~soid] ~nd Irans (CI..I)~ [trans-tran,sold] chains.

regions in th,z fibrils. From the integrated crystalline and amorphous intensities tl'Je X-ray crystallinity is estimated in the range 75-90% for both cis and trans films [6-8]. The obse'rvation of the diffuse halo at a d spacing 6i about 4 A, very close to the d spacing of the two strongest reflections of the D e b y e Scherrer rings (3.81 A for cis and 3.72-3.56 A for trans) shows that the 'average' nearest neighbour packing arrangement in 'amorphous' regions does not differ substantially from that of c~3,stalline regions. In the 'amorphous' regions disorder is believed to be caused by chain ends, chain hack ~olding, changes in the chain configuration and sp 3 hybridized C atoms [7]. Orientational disorder ot C - C bonds is essential to explain the stre~ch~,ng of (CH)~ films under modest external fo~'ces. 'Amorphous' regions may also influence the electrica] conductivity, the speed and the homogeneity of doping processes. Debye-Scserrer rings coming from the crystalline parts o; the fibers are in fact broader than the experirr~ental resolution, which means that the order is spatially limited. However. its extent does n~t d..:pend appreciably of the fibril dimension, fez' samples having 200 ,~ and 500 A as average diameter [13]. In a crud<, model assuming the fibril .:omposed of crystallit~,s of finite size Lj and LII in directions p e t e r , ~leular a~d parallel to the chain axis, respectively., the width of the reflections at low 0 Bragg angle~;, gives the val!ues L.~ ~ 70-90/~ [711] and Eli ~.~100-I30 A [9~10], according to the Seherrer formula [I2]. VahJes of L j, identical for cis and trans samples, are several times smaller than the fibrillar diameter. Debye-Waller factors

Fig. 2. Projection along lhc chain axis of the structure of cis and trans (CH) r The symmetry of the corrt.~ponding 2D space group and the a and b directions are also indicated,

found in structural refinements show that a considerable disorder exists in the 2D lattice of the chains projection [6, 9, 14]. In addition, fluctuations of lateral distances (due to fluctuations of the setting angle • - See fig. 2) might contribute to the loss of transverse coherence. Further studies of the broadening of (hk0) equatorial reflections with 0 increasing are necessary to clarify this aspect of the disorder. Values of LI are much smaller than the estime.ted chain length ( 5 0 0 - 5 0 0 0 A - s e e [5] and references therein). Such a difference is probably due to conformational defects changing locally the conjugation sequence. More explicitely, from the. broadetdng of the width of two (001) meridional reflections, it was concluded that no long range order exist:; in chain direction in trans (CH)~ [9]. This disorde'.r of the 2nd kind might be due, for example, to the presence of random cis units (estimated at about 7% [15]) in the trans backbol~te. 2.2. Structure

With this morphology, on the basis of the observation of about 10 well defined crystalline reflections and the results of packing arrangement calculations, structures of cis a,~d trans (CH)~ [obtained by thermal isomerization of cis (CH)~] were derived. Fig. 2 shows the projection of cis and trans structur~:s along the chain direction. They both have the Pgg 2D space group with 2 chains per unit cell (structural refinements gives a setting angle • in the range 50-60 ° for the two isomers). X-ray patterns similar to that shown in fig, 3 give lattice constants a =7.6 A, b--~4.4 ,~ for eis (CH)., [1l, l,l., 16-18] and a = 7.4A, b=4-.l A for trans (CH), [11,18, t9]. Slightly differem lattice constants a - - 7 . 3 A and

160

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cls

{c~l,

I

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l

12'°J0} l:

SO't*I~ANI]

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Fig. 3. X-ray Duhye-Scherrer pallor*is of cis ¢cr:tl., 5o% cis-SB% '*ra~ (CH)~ and trans (CH)~. Miller indi:us of the strOngcsl rellections are ~,idi¢iLled(th~ a, b, e directions are th,)se o[ ti,~, 1 and 2). Tho broad "amorphous" halo appears a~:~a weak slmtdder on the left side of the strongcsl reflection. b = 4.25/I, Iv,re been found in trans (CH), when the doublet (2. O, 0)-(1, 1,0) (see fig. 3) was not resolved [9, 20]. It is also interesting to remark that qui~J different lattice periodicities were found in trans (CH)~ prepared by other methods [21,22]. Cis (CH). belongs to the 3D orthorhombic space groul5 Pnam (using lattice directions of figs. 1 and 2) [14, 17]. Since the formation of bond alternations in trans (CH). chain.% loses the mirror symmetry, the 3D space group of trans (CH)~ is of lower symmetry. From the packing ef trans (CH)~ segments in diphenylpolyrnes, it was sugg~:sted that wans (CH)~ ha,~ a monocliniic symmetry, with the long axis, a, as the b~nary t~xis [19]. Two space groups were proposed P2~j, [19, 24] c,r P2,,,[9] depending on whether the bowLd alternation on the two chains per unit cetl is either ia phase or out of phase. Further work

is necessary to differentiate between these two symmetries. Finally, structural and ClaN1VIAR studies have shown tha~t the dJmerization distortion of trans (CH), is of the order of 0.03/~, in chain direction [9, 23, 24], |ending to single bond and double bond of 1.44 .~ and 1.36/~, respectively, very close to standard values. By thermal annealing cls (CH)x can be convetted continuously imp trans (CH)~ (see [1,5] and references therein). Structural data [11] has shown that this process keeps oanstant the degree of crystallinity and the apparent lateral crysta~lite size (see fig. 3). Furthermore, this figure shows that equatorial (h, k, 0) reflections shift continuously from their position in cis (CH)~ to that in t r a ~ (CI-I')~ [11, 18, 25, 26]. This means that the isomerization occurs hc,mogeneously and randomly throughout the polymer and not in isolated amorphous or selected crystalii'te regions. The continuous decrease o~ a and b lateral periodicities reflects a progressive increa,~e o1~ lateral compaeity with the trans content, which is locally allowed by the same lateral arrangement of chains (see fig. 2) in the eis and trans isomers. However, as the cis and trans chains are of different symmetries and periodicities, the chala must be reconstructed during the isomerization process in the 0resence of an overa[t lattice structure. Elastic constraints from neigbbouring chains are thus certainly very important for this process. They must be taken into account in any mode] of the kinetics of isomerization.

3. St~uctu,e o~ doped (CH),. The conductivity at room temperature of (CH), increases rapidly from I0-9(~cm) -~ or 10-~(Ctcm) -j for the cis and trans isomers respectively, when doped with accepter such as Is, AsF.~ or CIO.~ or donors such as IA, to 1 (~crn) -1 at about 1% doping concentration. With fuzther dopant, it increases more :;lowly to about 103 (~Qcm)-1. On the contrary magnetic susceptibility stays small to about 5% of dopant, then it increases to values usually found in rnetals. However the exact value of these physical quantities varies from sample to sample.

161

J,P, pouget ~:t aL / Doped (CH)x

Dopants ~re believed to produce charged soliton states in the middle of the gap. Between about 1% and 5% of dopant these states progressively broaden into a band. This leads to a nearly metaltlic conductivity, with a very small density of state at Fermi level as deduced from magnetic measureJments. Above about 5% of dopant, the Peierls gap is dosed leading to a metallic like conductivity and to a Pauli magnetic susceptibility (for a critical and recent review of these aspects see [1]). However, in addition to the formation of charged solitons, the dopants modify the electronic properties by the disorder and the structural changes introduced in the fibers. In particul~, in the nearly metallic regime, by smearing the band edges, the disorder can explain the small but finite density of state fo,und by magnetic measurements.Under cert~fin conditions, it may also dominate the charge conduction process [27]. In the metallic like regime, disorder appears also to be essential to understand the low temperatare Fermi glass behaviour. Characterization of samples used in other physical studies shows generally that the doping: - is inhomogeueous -reduces the crystalline order -induces a cis-trans isomerization in the undopecl parts -leads to new erystaUine structures by its intercalation either between (CH)~ planes of the prima~ lattice or within channels delimited by a new (CH'.)~ host lattice. However, these structural i~eatures vary with the initial polyacetylene state (cis-trans content, erystallinity, fibril diameter, erosslinking, ctc), doping conditions (vapor pressure, eleetrochemiez~i potential, temperature, ete,) and nature of the doT"ant (dopant size, shape, reactivity, etc.). Deoye-Scherrer patwrns of (CH)~ doped with more trmu a few percent of electron accepters (I;[8, 28-30]AsFg[38], CIO2[13], FeCI2[13]) or electron donors (alkali metals [31]) show in addition to the reflect:ions from pure (CH),, broader rel~etions a~ different Bragg angles (see fig. 4). Thi~ show,'; that, even at low level the dopant is not randomly dist~ributed in pelyacetyilene, but tends to be ordere~ with (CH)~ chains into a flew

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e

20

30

gO

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Fig. 4. X-ray Debye--Seherrer patterns of a) [CH(Iz)o.m,t]~, b) [CH(CtO.0o.j~; and e) [CH(FeCI4)o.oT],. The single arrows point towards the reflections of trarts (CH)~ for the former two compounds and o17(CH)~ with 70% trans content for the latter compound. The double arrows point toward the

new re~leetiom at ~oaeings 8=7.95, 3.95. 3.05A for [CH(I~)u.m.~]~, d=8.30 5.4, 4.0/~ [or [C.H(CIO4)o.06~; and d =9.3, 5.2, 4.65.3.4A for [CH(FeCI4)o.o-z~~.

structure. However, the observation of the superposition of this structure with ~hat of pure (CH)x, shows clearly that, at th+.'se levels, the doping is not homogeneous. Due to the lo0g diffusion time of the dopant into the fibers, the outer parts of the fibers are doped prefereutia/ly at low amounts of dopant. However, lhe observation of a cis-trans isomerization in the undoped part of several samples ~8], shows that the

]62

J.P. Pouget et al. I Doped (CH).~

doping is able to trig additional features in these regions situated mostly near the center of the fiber, It is olaly for the highest amounts of dopant that, from the non observation of pure (CH):~ reflections, ;he fibers seem to be unif0.rmly doped [10, 29]. The observation of a new set of well-defined reflections, as ,;cell as the decrease of the intensity of pure (CH)~ reflections with the amount of doping [8. 29], show clearly th~tt the dopam enters into crystalline regions. It has been recently suggested [32] that the dopant induced strain energy Ca,cots aggregation of dopant into columns (directed along the chain axis) or arrays of columns 0ayers). Structural analysis of these new reflections (see below) shows that the effect occurs for all the dopants introduced (at the exception of Li t~31]) above a threshold of a few percent concentrations. With the angular width of the new reflections, one can deduce, fro~fnthe Schel~rer formula, that dopant orders on, distances varying from 20 ~ to 60 A. This value seems to depend of the nature of ~he dopant and o1' the conditions of doping. But for the same conditions of doping it does not change appreciably with the amount o[ dopant and the form, cis or trans, of the starting material [13]. However, the actual dimensions of the domains c a n be higher than that c3timate~ by the rough procedure quoted above, because disto:xions can cause an additional broadening of the reflections. In acceptor doped polyacetylenc, a common feal:ure of the new reflections is the presence of a long interplanar spacing. This distance is of about 7.95 ~, for I~-, 8.3 ~ for C107~ and 9.3 for FeCI~ in (CH)~, for the data shown in fig. 4. It can be interpreted as the distance between two planes of dopant molecule, or two dopant rich planes separated by a close packed plane of polyacetylene chains. [14, 19, 37]. Informations on the other two directions of the intercalated structure can be obtained using Bragg reflections found at higher angles. In particular, it,dine doped polyacetylene reflections give two d spacings at 3.95 ,~k and 3.05 ~ in lie, 4. They can be related, respectively, (a~ either to the other lateral distance in the layers of close packed iodine chains {of Van der Weal diameter 3.97.~) or

(CH)~ columns (pa~ki,ig distance of 4.1-3.7 .~ in trans (CH)~) or to half of the long interplanar spacing and (b) to the average, dL,;tance between iodine atoms forming I~ or and 1F units in chain direction [28, 30, 8]. In the case of FeC1S~/(CH)~ structure, the d spacing of 5.2.~ represents the packing distance between FeC!~ tetrahedra, and that at 4.65 ~, may con'espond to half of the long interplanar spacing. The structure we described in iodine doped (CH)~ correspond to a first stage intercalated structure. It has been suggested recently [28], that (CH)~ lightly doped with iodine may present a third stage iodine complex where columns of anions partially displace the (CH)~ chains on ever3, fourth close packed layer of these chains. Metallic complexes of (CH)~ with alkali metals were found [23] to have a different structure in which the dopant molecules pack in linear arrays in channel within the polymer host lattice; this lattice being diffe.rent from that of pure (CH)~. One of the most surprising results of the structural study of doped materials was to find that nndoped regions of the initially cis (CH)~ isomerizes to the trans (CH).~ structure during the doping process with iodine [8] or ClO7, [13]. ~-Iowever the degree of isomerization seems to depend from the t~Lature and from the amount of dopant inserted in the fibers. This point requires further studies. Isomerization of (CH)~ chains has been observed by spectroscopic measurements [33-~36]. The dopant induced isomerization of the undoped regions may be due to the removal of an electron from the ¢r orbital of the cis form, enabling easier hand rotation. Alternatively, the crystals which make up the outer layers of fibrils should elongate parallel to the fiber direction upon doping. This stress induces a strain parallel to the chain axis in undoped inner crystal As trarts (CH)~ chains are about 10% longer than cis (CH)~ chains, th;s ~;tress may induce isomerizatifm. In addition to flaermal anne;,fiing, doping appears as another method to ir~duce an irreversible cis-trans isomerization. F~,g. 5 shows, from the observation of lateral (hk0) reflections and of the in chain 4002) reflection ot trans (CH}~, that cis (CH)~ can be transformed into trans (CH)~ by

J,P. Pouget et a L / Doped (CH)., --J

I~1t~ans tCH~w

C

(b) c(s (CHI~ dope{I ~hen ur~'op ed

'"

?,~t.s~2~

Fig. 5. Comparisorlbetween the X-ray D,:byc-Schcrrer palt,zrns of a) pure trnns (CH)~: and b) eis (CH)~eleetroeh,.'miealll¢ doped with 3.2% of Cl(r.~ ihen dedoped to about L1%

163

used for other physical studies, but that they can bring by themselves physical informations of primary importance like the magnitude of the symmetry breaking distortion of trans (CH)~, the cis to trans isomerization processes, lhe way by which dopants intercalate in the (CH)z host lattice e t c . . . Although the presence of poorly oriented samples prevent a detailed analysis of these structural features, large areas have not been explored. For example it might be interesting to check if a direct relationship exists between the fonnation of dopant/(CH)s structures and the cross over from the semi conducting to nearly metallic or metallic like properties of doped (CH)~. The connection between the various structures of trans (CH')~ and the methods of preparation has to be done. In situ studies of structural aspects of the catalyst and doping processes of (CH)~ should further be performed.

of C~O~.

Acknowledgeme~t eiecxrochemical doping and dedoping with CIO~. However at t!he difference of the annealing treatmeat, the dolpiag and dedoping method increase the disorder in the fibers. From standard values of pure (CH) x (see section 2.1) it is found that in this experiment, the cry~,tallinity has doped to about 60% and that the apparent lateral crystal size Lz i~as decreased to about 45 ,~. The drop of the crystalline coherence length and the increase of the intensity of the tiiffuse halo at d = 4 A might be due to the distribu'Jon of lateral distances (disorder in the settling angles) as well as bending and twisting of chai~,s, due to the penetration of the dopant inside crystalline regions during doping and dedopiog processes. Such a degradation of the erystallinity ",~ith line doping and '~he dedoping of the sample is an important parameter for electrochemical applications of polyaeet,ylene.

,~, Conclusion Struclural studies of (CH)~ have shown that the?/are not only unique to characterize samples

This work was supported in part by the US National Foundation Grant D M R 82 1802 1.

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J.P. Pouget et z,L / Doped(CHL

164

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[10] C, Reikel, H.W. Hasslin, K, Menke a~l:[ S. Roth, J. Chem. Phys, 77 (1982) 425,L [:ill P. Robin, J.P. Pouget, R, Comes, H.'~;. Gibson and A.J. E~teln, Phys. Ray. B27 (1983) 3938; and ref. 1, pp. C3=77. The values of Lz given in these referene.~s must be divided by two. [12] A. Ouinler, Th6orie et Technique de la Radiocristailol;raphie (Dunod, Paris 1956L [13] P. Robin, J.P. Pouget, unpublished results. [14] R.H. Baughman, S.L, Hsu, G.P. Fez and A,J. Signorelli. J. Chem. Phys. 68 (1978) 5405. [1511 H.W. Gil'con. RJ. Weagley, W.M. Prest, R. Mosher and S, Kaplan in ref. 1, p. C3-123. [16"1 G, Lieser, G. Wagner. W. Muller and V. Enkelmann, Makromol. Chem. tiapld Cornmun. 1 (1980) 621. [17~ J.C. Chien, F.tL ]~r~LsZ and K. Shimamura, Macromoleeules 15 (1982) 1012. [18]' G, Perego, G, Lugii, U. Pedretti and E, Cern[a in ref. 1, p. C3-93. [19] R.H, Baughman. S.1.. l-lsu, L.R, Anderson, G.P. Pez and A.J. Signorelli, in: 'Molecular Metal'*, N A T O Confercac¢ Series, W.E. Hatfield, ed, (Plenum, New York. I979) p. 187. [20] K, Shimanurll, F.I~, Karasz. J.A, Hirsch and J.C.W. Chien. M.'~kromoL Chet~. Rapid, Commua. 2 (1981) 473. [21] G. Lieser, G, Wegner~ W. Muller anti V. Enkelmarm. Makromol. Chcm, Rapid Commun. 1 (1980) 62% [22] Y, Cap. R, Oian, F, Wang and X. Zhso. Makromol. Chem. Rapid Commun. 3 (1982) 687. [2,'3,] T.C. Clarke, R.D. Kendriek and C.S. Yannonl in re~'. 1, p. C3-.369.

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