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 inter- action Black In the light of the new carbon allo- tropes 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 im- plications and new possible point of view. Carbon allotropy: a short and updated survey Graphite, diamond and amorphous carbon All the textbooks on general and inorgan- ic chemistry report that there are two car- bon allotropes graphite and diamond [17±19]. Graphite is made by sp2 hy- bridized 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 sin- gle 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 high- er should be the concentration of the car- bon dangling bonds. This is the reason why for instance carbon black is a para- magnetic solid. Of course not all the car- bon 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.