Carbon Allotropy and Carbon Black

ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS

Carbon Á Allotropy Á Fullerenes Á Carbynes Á Fullerene-like structures Á Carbon Allotropy and Carbon Carbon black Á Polymer-filler interaction Black In the light of the new carbon allotropes recently discovered: fuller- F. Cataldo, Rome (Italy) ene, nanotubes, onion-like carbon and carbyne, it is discussed the possible presence in carbon black of some sites which could be struc- turally analogous to the mentioned carbon allotropes. The effects on the rubber-to-filler interaction and In the latest years our knowledge on car- tures in graphite [13, 14]. We have docu- on the bound rubber are discussed bon allotropy has been radically and un- mented the formation of hexagonal dia- in detail. expectedly changed due to a series im- mond microdomains, glassy carbon portant discoveries which have involved and onion-like carbon [13, 14]. Previously Allotrope Modifikation des the identification of fullerenes in 1985 Galvan and his colleagues [15] have [1] and the subsequent discovery of car- shown that c radiation induces the rota- Kohlenstoffs und Ruû bon nanostructures such as multiple- tion of graphite planes. These results are Kohlenstoff Á Allotrope Modifikation Á walled [2] and single-walled carbon na- not surprising because Banhart and co- Fullerene Á Carbyne Á FullerenaÈ hnliche notubes [3]. These new molecules and workers [16] have shown that under cer- new structures were initially produced tain circumstances which involve electron Strukturen Á Ruû Á Polymer-FuÈ llstoff- by laser ablation of graphite targets [1] irradiation and annealing, the carbon Wechselwirkung and later by a simple resistive heating onions collapse to form ultradisperse dia- of graphite or graphite vaporization in mond. This transformation is not irrever- Im Lichte der kuÈ rzlich entdeckten neuen allotropen Kohlenstoffmodifi- an electric arc under Helium atmosphere sible and it is possible to transform ultra- kationen: Fullerene, Nanotubes, [4]. Fullerenes formation has been de- disperse diamond back to onion-like car- zwiebelartiger Kohlenstoff und Car- tected also in low pressure premixed bon. flames [5] and even in carbon blacks [6]. This rather incomplete list of experi- byne, wird das moÈ gliche Vorhan- Ugarte reported the mechanism mental facts shows that in the textbooks densein von strukturellen Analoga through which fullerene black (which of general and inorganic chemistry [17± zu den oben erwaÈ hnten Kohlen- morphologically seems comparable to a 19] , the chapter where carbon allotropy stoffmodifikationen an bestimmten certain degree to carbon black) under- is discussed needs now a deep revision. Stellen im Ruû diskutiert. Der Ein- fluû auf die Polymer-FuÈ llstoff-Wech- goes a transformation by thermal treat- Now it is time to make ourselves this selwirkung in vernetztem Kautschuk ment, into a polyhedral particle having a question: what was and what is the im- wird im Detail besprochen. structure of a closed-shell nanotube pact of all these discoveries in the field and by electron or ion bombardment of rubber industry and in our understand- into an onion-like graphitic particle [7]. ing of carbon black reinforcing effect? By HRTEM (High Resolution Transmis- Donnet and his coworkers have pub- sion Electron Microscope) microscopy it lished a fundamental article which has has been shown that arc-discharge pro- documented that fullerene-like structures duced soot shows a tendency to form are present in commercial carbon black curved surfaces after heat treatment [6]. These results have been confirmed and electron irradiation [8]. Evidences very recently [20] by using premixed ben- of the formation of fullerene onions and zene/oxygen flames to produce soot curved sheets were observed in electron which was studied by HRTEM. Highly irradiation of soots, carbon blacks and curved surface were observed especially chars [9]. Curved layers which should in the samples collected relatively far from contain pentagonal sites like in fullerenes the burner [20]. have been observed also in diesel engine In a previous paper we have discussed soot [10]. Curling of graphite sheets have the consequence of the presence of full- been observed by heavy ion bombard- erene-like sites on carbon black surface ment and electron irradiation of graphite for the interaction between carbon black [11], but it has also been reported even and diene polymers [21a]. The purpose of the formation of nanodiamonds [12]. this paper is to present an updated brief By Raman spectroscopy we have review of carbon allotropy. The new dis- shown that even c radiation is able to in- coveries in carbon allotropy are broaden- duce the formation of new nanostruc- ing and will broaden our knowledge on

22 KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001 Carbon Allotropy and Carbon Black

carbon black science and carbon black- polymer interaction. In this paper we would like to discuss just these new implications and new possible point of view.

Carbon allotropy: a short and updated survey

Graphite, diamond and amorphous carbon All the textbooks on general and inorganic chemistry report that there are two carbon allotropes graphite and diamond [17±19]. Graphite is made by sp2 hybridized carbon atoms while diamond is made by sp3, tetrahedrally coordinated, carbon atoms (see Fig. 1). Graphite is a weak and lubricating material and is made by parallel planar layers. In the nor- mal a (or hexagonal) graphite the layers are arranged in the sequence -ABAB-, whereas in b (or rhombohedral) graphite, the stacking sequence is -ABCABC-. In both forms, the carbon-carbon distance within the basal plane is 0.142 nm and the interplanar spacing is 0.335 nm (see Fig. 1). Each graphite plane has a two-dimensional development into the so-called graphene sheet which is a single layer of carbon atoms made up of polyaromatic hexagonal nets of atoms. There is the problem of the dangling bonds at the end of each sheet, the smal- ler are the graphene sheets and the higher should be the concentration of the carbon dangling bonds. This is the reason why for instance carbon black is a para- magnetic solid. Of course not all the carbon atom at the end of the graphene sheet have dangling bonds, in fact part Fig. 1. Carbon allotropes (interatomic distancesare given in Angstrom) of them are saturated by adventitious im- purities. It is possible to introduce different de- linked tetrahedra make up the cubic unit made only by allenic sp2 hybridized car- grees of disorder in graphite and also to cell of eight carbon atoms. This peculiar bon [22a] reduce the dimension of crystallite sheets architecture confers to diamond an ex- CCCCCCCCC of graphite. Amorphous carbon is com- ceptional hardness (see Fig. 1). n posed of fragments of carbon sheets or Thermodynamically, we have shown graphene layers arranged with no long- [22b] that carbyne is considerably less Carbyne range order and carbon black shows a stable than graphite and diamond and turbostratic distribution of the mentioned After graphite and diamond there is an- between the two isomers the acetylenic layers. other carbon allotrope which however (called also a-carbyne) is more stable On the other hand, each diamond has not been isolated in pure form till than the allenic (called b-carbyne). This crystal is a unique macromolecule whose now: carbyne. Carbyne is a carbon chain can be one of the reasons of the difficulty carbon atoms are covalently bonded in a made by sp hybridized carbon atoms of its isolation in pure form. three-dimensional network. Each carbon [22a]: We have used several synthetic ap- atom is tetrahedrally surrounded by four proaches in order to synthesize carbyne CCCCCCCCCC equidistant neighbours (see Fig. 1). The n [23±31] and our research work is still in C-C distance is 0.154 nm and four inter- it has also been hypothesized an isomer progress in this field. Of course, many

KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001 23 Carbon Allotropy and Carbon Black

The root of fullerene instability resides on the fact that two adjacent pentagons introduce a so-called pentalene site [33]. Pentalene is an unstable and beautiful molecule which has never been isolated at room temperature because it is ex- tremely unstable and undergoes easily a dimerization reaction. Therefore fuller- Ê enes with a pentalene site should be re- Fig. 2. Model of the C60-based 10 A tube with zigzag configuration active and could be trapped as organo- metallic complexes [33]. other efforts on carbyne synthesis have according to the following relationship: Another feature of C60 fullerene (and its been done by other researchers [22a]. 2 10 X N8 carbon atoms. Hence homologues) is that the double bonds The presence of carbyne domains in the smallest fullerene that can be imag- present in this molecule are localized the carbonaceous matter produced by ined is C20 when X 0. Starting from and weakly conjugated. This is due to the Glaser reaction has been confirmed C20 any even-membered carbon cluster, the strong deviation from planarity of by Raman, FT-IR and solid state except C22 can form at least one fullerene the curved surfaces of these molecules. 13C-NMR spectroscopy [27, 28]. In these structure. With increasing X the number The deviation from planarity interrupts solids, carbyne occurs in combination of possible fullerene isomers rises drama- or reduces the conjugation. In C60 all with diamond-like carbon and disordered tically, from only 1 for X 0 to 20 000 for bonds at the junction of two hexagons graphite [27, 28]. Acetylenic carbyne is X 29 and so on. The reason why the (6,6) are shorter (1.38 A) than the bonds relatively stable especially if its chain is smallest stable and most studied fuller- at the junction of an hexagon and a pen- end-capped for instance by copper ene is C60 fullerene is linked to a simple tagon (5,6), which measures 1.45 A. atoms [22a]. It is remarkable to report rule, the isolated pentagons rule, which Thus the double bonds are localized at here that low molecular weight oligomers requires that the pentagonal sites cannot the 6,6 junction. of carbyne called cyanopolyynes have be adjacent but must be fully annealed by In the case of graphite sheets, all dou- been identified in the interstellar clouds hexagonal sites. Only under these cir- ble bonds are conjugated and this ac- especially near carbon-rich stars [22a]. cumstances a stable fullerene molecule counts for the high electrical and thermal Another interesting result on carbyne is can exist. This rule phases out all the full- conductivity as well as the anisotropy of that it shows a tendency to crosslink erene homologues having adjacent pen- these properties in this carbon allotrope. and the resulting materials are dia- tagonal sites. All the homologues having Carbon nanotube (Fig. 2) and fullerene mond-like carbon and disordered gra- less than 60 carbon atoms are unstable onions (Fig. 3) have different morpholo- phite [28]. We have obtained encoura- and do not exist. All the isomers of C60 gies and involve a different number of car- ging results [29±31] in carbyne synthesis have adjacent pentagons and are un- bon atoms, they all share a remarkable by dehydrohalogenation of halogenated stable, thus C60 fullerene is a unique mo- cage-like polyhedral geometry that is carbyne precursors. The resulting pro- lecule and does not have isomers. the consequence of arranging sp2- ducts show a relatively good stability. bonded carbon atoms in pentagonal

Fullerene, carbon nanotubes and onion-like carbon: the concept of ªcurved surfaces in carbon nano- structureº

C60 fullerene has been discovered in 1985 but only in 1991 it became available in bulk quantities to study its chemistry on lab scale [32]. Fullerenes are a series of carbon molecules having a close shell or cage-like structure. The most studied fullerene has 60 carbon atoms (Fig. 1) and possesses an highly symmetric structure of truncated icosahedron. On the surface of C60 fullerene we can detect 12 pentagonal sites which are fully annealed by hexagonal sites. The building principle of fullerene serie is described by the Euler's theorem according to Fig. 3. Onion-like graphitic particles formed by three concentric layers (C60, C240, C540): poly- which only 12 pentagons are needed hedral (marked P) and spheroidal (marked S) structures. For clarity, only a half part of each and X hexagon to built a fullerene cage shell is shown

24 KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001 Carbon Allotropy and Carbon Black

of the number of pentagonal sites present.

Fullerene polymers

When C60 fullerene is irradiated with ultra- violet light, it is compressed at very high pressure, or it is treated with high intensity ultrasound in solution, it is transformed into a new material where all the fullerene cages are still existing but are chemically linked together in irregular chains of three-dimensional networks [35±38]. These fullerene polymers are actually Fig. 4. Types of disclination points for a 2D hexagonal lattice creating: (a) a five-fold ring, associated with positive local curvature; (b) a seven-fold ring, associated with negative local another carbon allotrope and it has curvature been claimed that certain phases of fullerene piezopolymers are harder than diamond. and hexagonal rings to form a closed deep or even approaching closure as shell. Regardless of the number of carbon the number of pentagons increases or atoms in a closed-shell fullerenic struc- approaches 12. The variety of morpholo- Mechanochemistry: the case of ture, the cage is always comprised of ex- gies of curved layers is very wide, given polymer-filler interaction actly 12 pentagons distributed over the the many different possible arrangements surface of the cage with the remaining of pentagons [34]. When polydienes are mixed with carbon carbon atoms arranged in hexagons Fig. 4 shows that the presence of a black at high temperature (up to 180 8C) that complete the cage. pentagon introduces a positive local cur- under strong shear forces, in air, several The presence of the pentagonal rings vature in a graphene sheet, while an hep- complex phenomena occurs and can in a sheet of benzenoid carbon causes tagon introduces a negative local curva- be summarized as follows [21a, 21b, strain in the sheet that is relieved by the ture [34]. The presence of pentagons may 21c]: introduction of curvature at each penta- induce the formation of even more exotic * Polymer chains undergo mechano- gon [34]. The pentagons provide suffi- nanostructures including carbon cones chemical degradation which involves cient curvature for a sheet of otherwise (Fig. 5±7). Particularly interesting in rela- carbon-carbon bond rupture with the hexagonally bound carbon atoms to tion to carbon black surface roughness consequent formation of free radicals form a closed shell, the shape of which [21] are the toroidal structures of Fig. 5 * Reaction of polymer free radicals with is determined by the arrangement of where a combination of pentagonal and the surface carbon black radicals. the pentagons around the shell. If the heptagonal sites create the mentioned * Disproportionation of polymer macro- sheet contains fewer than 12 pentagons structures. Similarly, the conical struc- radicals with formation of unsaturated the layer is curved in an open shell, the tures of Fig. 6 should be present on car- and saturated chain ends. depth of which can range from shallow bon black surface and the sharpness of * Recombination reaction among the if only a few pentagons are present to the tip is directly related to the amount polymer macroradicals has been proved to be of minor importance. * Free radicals chain transfer: the polymer chains macroradicals may ab- stract the allylic hydrogen atom at any adjacent polymer segment creating double-bond stabilized radicals. * Shear forces under mixing lead to the formation of additional new radicals on carbon black surface also by making available radical sites which were orig- inally sterically hindered. * Air oxygen always present in the mixing processes oxidizes rubber especially by attacking allylic radicals with formation of hydroperoxides and transferring the radical to another chain segment. Oxygen take-up reaction is typical un- Fig. 5. Optimized shallow toroidal structures: the subscripts indicate the number of the car- der these conditions unless some anti- bon atoms in the tours; pentagons and heptagons are shaded oxidant is present.

KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001 25 Carbon Allotropy and Carbon Black

these sites act exactly as free radicals sponges (see next paragraph and ref. 21c and 39). Moreover these sites will act as dienophiles if conjugated double bonds are formed on the dienic polymer chains [21a, 39]. Therefore, in addition to the free radical sites on carbon black surface due to the dangling bonds at the end of each graphene sheet, we should now consider the fullerene-like sites.

Bound rubber and fullerene-like sites on carbon black In the context of this paper, we would like to point out the extreme reactivity of fullerenes with free radicals. For instance, when molecular chlorine is irradiated with UV or high intensity visible light, it undergoes a scission into chlorine atoms or

radicals. In presence of these radicals C60 fullerene reacts promptly to give highly chlorinated derivatives [40, 41]. Similarly

C60 reacts promptly also with alkyl radicals [32]. An interesting experiment has shown that remarkably stable radical adduct can be formed and detected when

C60 is reacted with a bulky R Á radical, with Fig. 6. The possible R Á being for instance a t-butyl group. The tip structure with persistent R-C60 Á adduct can be de- cone shape, in tected by ESR spectra (see Fig. 8). In which the pentagons are included. the mentioned adducts the unpaired As a function of the electron is not delocalized in the fullerene number of penta- cage but remains in proximity of the site of gons, the cone addition and it is stabilized by resonance shape changes.

* Polymer chain segments containing oxygen in radicalic or peroxidic form can be linked to the surface of carbon black by an oxygen bridge. * At the initial stages of mixing a preva- lence of mechanochemical degradation can be observed. However, at higher mixing temperature the thermo-oxidative degradation becomes Fig. 7. Helically coiled form C : dominant. 360 one pitch contains a Thanks to the presence of fullerene-like torus C360. pentagonal sites on carbon black sur- (a) coil face, as discussed in the previous para- length 12.9 AÊ and, (b) coil graph, now we can adjust the current length 13.23 AÊ . model covering the interaction between The tiling pattern of rubber and carbon black, especially for heptagons in the the chemical contribution to the phenom- inner ridge line is changed, though enon called ªbound rubberº [21a± the pattern of pen- 21c, 39], by saying that the polymer tagons in the outer macroradicals once formed will react ea- ridge line remains upon changing the sily with the pentagonal sites because coil length

26 KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001 Carbon Allotropy and Carbon Black

_ Fig. 8. ESR spectra of t-BuC60 in benzene at _ 13 80 8C: (A) CH33CC60 shows the C hyper- Fig. 9. The major fine satellites and 0.17 G hyperfine splitting canonical reso- due to the 9 methyl protons. (B) nance structures _ 13 1 _ CD33CC60 shows the C satellites of RC60 as shown in Fig. 9. The highest spin den- tion of mechanical hysteresis of a rubber black with c radiation or other type of par- sities are located on the carbon atom compound. ticle bombardment (electrons, ions, a- number 2, number 4 and 11 and number particles). With these treatments it will 6 and 9 of Fig. 9 [32]. In the case of diene Outlook on future developments be possible to modify freely the surface rubber the radical added in proximity of a roughness of a carbon black due to the pentagonal site in carbon black is of The new development in carbon black radiation damage which is dependent course a polymer chain macroradical. technology should involve the ability to in- to the radiation dosage and it will be pos- Of course, multiple macroradical addi- troduce selectively the fullerene-like sites sible by selecting the appropriate radia- tions are possible, although some sterical in carbon black and other sites as dis- tion and conditions to selectively intro- hindrance between adjacent macroradi- cussed in the introduction. One possible duce the type of the desired site. As ex- cals could occur. However, it has already tool is the radiation treatment of carbon plained above many different sites can be been shown [32] that up to 8 bulky benzyl groups are added to C60. A scheme for the multiple addition to a fullerene-like site is shown in Fig. 10. Recently we have started a study on the addition of diene rubber radical on

C60. We have used squalene as model compound for diene rubber to study it addition on C60 fullerene [39]. Our (prelimi- nary) study [39] has shown that about

2,5 molecules of C60 become chemically bound to each squalene molecule under thermo-oxidative conditions simulating part of the mixing cycle between rubber and carbon black. Further work is in progress. The implications of this result throw new light in the explanation of the phe- nomenon known as ªbound rubberº (especially for the amount of chemi- sorbed polymer on filler surface after mixing), as well as in the explanation of the reinforcement effects observed by filling rubber with carbon black and the reduc- Fig. 10. A scheme for the multiple addition to a fullerene-like site

KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001 27 Carbon Allotropy and Carbon Black

introduced; each site will be linked to a [6] J.B. Donnet, T.K. Wang, C.C. Wang, M. Month- [27] F. Cataldo, Carbon, 37 (1999) 161. certain carbon allotrope. ioux, M.P. Johnson, D.T. Norman, R.W. Wans- [28] F. Cataldo and D. Capitani, Mater. Chem. Phys., borough and P. Bertrand, Kautschuck Gummi 59 (1999) 225. Particularly interesting could be car- Kunststoffe, 52 (1999) 340. [29] F. Cataldo, Die Angewandte Makromol. Chem., byne-type or polyyne sites which could [7] D. Ugarte, Carbon, 33 (1995) 989. 264 (1999) 65. [8] M. Miki-Yoshida, R. Castillo and S. Ramos, Car- [30] F. Cataldo, Journal Mater. Sci., 35 (2000) 2413. be very reactive with diene rubber and bon, 32 (1994) 231. [31] F. Cataldo, Journal Macromol. Sci. Part A, Pure hence could play a comparable role to [9] A.P. Burden and J.L. Hutchinson, Carbon, 36 Appl. Chem., A 37 (2000) 881. the fullerene-like sites. (1998) 1167. [32] A. Hirsch, The Chemistry of the Fullerenes, G. [10] T. Ishiguro, Y. Takatori and K. Akihama, Com- Thieme Verlag, Stuttgart, (1994). We have already demonstrated [42] bust Flame, 108 (1997) 231. [33] F. Cataldo, Fullerene Science & Technology, 7 that the most simple, safe and economic [11] M. Takeuchi, S. Muto, T. Tanabe, H. Kurata and (1999) 289. K. Hojou, J. Nucl. Materials, 271& 272 (1999) [34] S. Elliott, The Physics and Chemistry of Solids, approach to modify carbon black and its 280 and 285. p.140 ±150, J. Wiley & Sons, New York, (1998). performances involves the use of c radia- [12] T.L. Daulton, R.S. Lewis, L.E. Lehn and M.A. [35] F. Cataldo, Polymer International, 48 (1999) 143. tion. We have detected [13] the growth of Kirk, Mat. Res. Symp. Proc., 540 (1999) 189. [36] F. Cataldo, Fullerene Science & Technology, 7 [13] F. Cataldo, Carbon, 38 (2000) 623. (1999) 725. certain kind of defects after this treatment [14] F.Cataldo, Fullerene Science & Technology, ac- [37] F. Cataldo, European Polym. J., 36 (2000) 653. in line with the results of other investiga- cepted for publication. [38] F. Cataldo, Fullerene Science & Technology, 8 tors (see the introduction of this work). [15] D.H. Galvan, I.L. Garzon, P.Santiago and M. (2000) 39. JoseÁ -Yacaman, Fullerene Science & Technol- [39] F. Cataldo, Fullerene Science & Technology, 8 Additionally we have measured a signifi- ogy, 6 (1998) 867. (2000) 153. cant increase (20 %) in the reinforcing [16] F. Banhart and P.M. Ajayan, Nature, 382 (1996) [40] F. Cataldo, Fullerene Science & Technology, 4 433. (1996) 1041. effect of carbon black treated with c ra- [17] R.B. Heslop and P.L. Robinson, Inorganic [41] F. Cataldo, D. Heymann, R. Fokkens, N.M.M. diation in comparison to an untreated Chemistry, A Guide to Advanced Study, Elsevier, Nibbering and R.D. Vis, Fullerene Science & sample [42], in a study on a standard rub- Amsterdam (1967). Technology, 7 (1999) 159. [18] F.A. Cotton and G. Wilkinson, Advanced Inor- [42] F. Cataldo, Intern. J. Polym. Mater., in press [Ti- ber compound. In the same study [42], ganic Chemistry, J.Wiley & Sons, New York, tle: Effects of c radiation treatment on the rein- we have also found a mechanical hyster- (1988). forcing properties of carbon black in rubber esis reduction of 5 % on the compound [19] N.N. Greenwood and A. Earnshaw, Chemistry compounds]. of The Elements, Pergamon Press, London, sample filled with carbon black treated (1984). with c radiation. [20] W.J. Grieco, J.B. Howard, L.C. Rainey and J.B. The authors The first results look promising and we Vander Sande, Carbon, 38 (2000) 597. [21a] F. Cataldo, Fullerene Science & Technology, 8 Dr. Franco Cataldo is the responsible for the Re- are just at the beginning. (2000) 105. search and Development on Materials Compounds [21b] G. Kraus, Reinforcement of Elastomers, Wiley and Labs in Trelleborg Wheel Systems spa / Pirelli, (Interscience), New York, (1965). Tivoli, Rome, ITALY. He is also Contract Professor References [21c] W.F. Watson, Ind. Eng. Chem., 47 (1955) 1281. of Polymer Chemistry at the 2nd University of [22a] R.B. Heimann, S.E. Evsyukov and L.Kavan, Rome. He has published more than 85 papers on in- [1] H.W. Kroto, J.R. Heath, S.C. O'Brien, R.F. Curl Carbyne and Carbynoid Structures, Kluwer ternational journals of polymer chemistry and materi- and R.E Smalley, Nature, 318 (1985) 162. Academic Publishers, Dordrecht, (1999). als science. He is author of 20 patents. [2] S. Iijima, Nature, 354 (1991) 56. [22b] F. Cataldo, Fullerene Science & Technology, 5 [3] S. Iijima and T. Ichihashi, Nature, 363 (1993) (1997) 1615. Corresponding author 603. [23] F. Cataldo, Eur. J. Solid State Inorg. Chem., 34 Franco Cataldo, Trelleborg Wheel Systems / Pirelli, [4] W. Kratschmer, L.D. Lamb, K. Fostiropoulos and (1997) 53. Via Casilina 1626/A, 00133, Rome, ITALY. D.R. Huffman, Nature, 347 (1990) 354. [24] F. Cataldo, Polymer International, 44 (1997) 191. [5] J.B. Howard, J.T. McKinnon, Y. Makarovsky, [25] F. Cataldo, Eur. J. Solid State Inorg. Chem., 35 A.L. Lafleur and E.M. Johnson, Nature, 352 (1998) 281 and 293. (1991) 139. [26] F. Cataldo, Polymer International, 48 (1999) 15.

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