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POSSIBILITIES OF CARBON-METALCONTAINING TUBULENES OBTAINING FROM POLYMERS CONTAINING FUNCTIONAL GROUPS IN MELTS AND SOLUTIONS OF LAMELLAR MINERAL ACTIVE MEDIA
V.I. KODOLOV, O.A. NIKOLAEVA, L.G. MAKAROVA, A.YU. VOLKOV*, E.G. VOLKOVA*
Basic research-high educational center of Chemical Physics and Mesoscopy, Udmurt scientific center, Ural Branch, Russian Academy of Sciences ♦Institute of Metal Physics, Ural Branch, Russian Academy of Sciences
ABSTRACT. The possibility of the formation of carbon-metal containing tubulenes from polyvinyl alcohol in lamellar mineral media is studied. Polyphosphoric acid, polyvanadium acid and chlorides of transition 3d-metals are used as such media. X-ray photoelectron spectroscopy, transmission electron microscopy and microdiffraction were used for the investigation of the samples obtained. It is found out that the composition of carbonization products depends on different factors being changed during synthesis, in particular, on temperature mode and medium composition. In accordance with X-ray photoelectron spectroscopy data, in products obtained in the medium of metal chlorides, product composition depends on d-shell filling of a transition metal in medium. While investigating the samples by transmission electron microscopy, tubular and small-crystal structures have been observed.
INTRODUCTION
The possibilities of carbon-metalcontaining tubulenes obtaining from aromatic hydrocarbons in active mineral media have already been considered [1,2]. Besides, there are methods of carbon structures obtaining from polymers, which contain functional groups in side branches [3,4]. The processes of carbon fibers formation from polyacrylonitrile, polyvinylchloride, polyvinyl alcohol are well known. So, it is expedient to study the possibility of carbon-metalcontaining tubulenes obtaining in lamellar mineral media, which stimulate carbonization of such polymers.
Such oxide systems as polyphosphoric and polyvanadium acids belong to these media. Dehydropolycondensation reactions of aromatic amines in vanadium-oxide systems with tubulenes formation are known [5,6]. Therefore, it is of interest to investigate the possibility of tubulenes obtaining in such media, taking polyvinyl alcohol as an example.
It is known that due to phosphorilated polyvinyl alcohol introduction into the items during polyethylene processing, the basic material combustibility is sufficiently decreased [7]. It is connected with the fact that a polymer introduced migrates to polyethylene surface layers
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during the processing forming net structures. When the fire affects the material obtained the polymer promotes the formation of a surface carbonized layer. A carbon layer is formed due to PVA stimulated carbonization in the presence of polyphosphoric acid being formed
EXPERIMENTAL
Products of polyvinyl alcohol heat-treatment in active mineral media are investigated. Chlorides of transition 3d-metals, polyvanadium acid and its derivatives containing Cr, Mo, polyphosphoric acid, and their mixtures are used as carbonization stimulators.
The synthesis is carried out in two ways. When metal chlorides and polyphosphoric acid are used as an active medium, the mixture of fine powders of polymer and mineral phase are heated at 300 °C during 4-12 hours. To remove the excess of non-reacted medium and salts hydrolysis products, the reaction mass is washed by hydrochloric acid and distilled water. The products obtained are fine black powders insoluble in water, acids and organic solvents.
When polyvanadium acid and its derivatives are used as a medium, PVA is combined with acid solution. This mixture is placed between carbon electrodes and treated by electric field (voltage 12 V). After that the reaction mass is heated up to 100°C.
The products obtained are investigated by X-ray photoelectron spectroscopy, transmission electron microscopy and microelectron diffractometry. The spectra are obtained at X-ray photoelectron magnetic spectrometer with AlKa-radiation. The vacuum in spectrometer chamber equals 10"5 Pa. The device resolution is 1,2 eV, measuring accuracy of peaks location is 0,2 eV. The samples for XPE obtaining are applied to copper base. Electron microphotographs and microdiffractograms of the samples are obtained at transmission electron microscope JEM-200CX with accelerating voltage 160 kV.
RESULTS AND DISCUSSION
While investigating the samples obtained from PVA in the medium of metal chlorides, the following structure peculiarities are found. In accordance with the review spectra, the intensive peaks for carbon and oxygen are observed.
Cls spectrum of PVA heat-treatment product in manganese chloride medium is shown in Fig. 1. The spectrum consists of four main components, which reflect different surroundings of carbon atom. The main maximum (ECH=285 eV) corresponds to C-H bonds. The second maximum (Eb = 282 eV) reflects the content of carbon bound with manganese. The third component (Eb = 284 eV) corresponds to carbon in graphite-like structures. The fourth peak corresponds to carbonyl and carbonate groups by bond energies (Eb = 286-287 eV).
Fig. 2 shows Cls spectrum of the sample synthesized in nickel chloride medium. The spectrum also consists of four components, corresponding to Ni-C bonds (282.5 eV), C-C
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279 280 281 282 283 284 285 286 287 288 289 290 29
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Fig. 2. X-ray photoelectron spectrum (Cls) of the sample obtained from PVA in nickel chloride medium
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279 280 281 282 283 284 285 286 287 288 239 290
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Fig. 1. X-ray photoelectron spectrum (Cls) of the sample obtained from PVA in manganese chloride medium
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(284 eV), C-H (285 eV) and C02 (287 eV). If we compare this spectrum with the one in Fig. 1, the decrease of the component corresponding to C-C bonds is observed. The peak intensity corresponding to C-C bonds is approximately 2.5 times higher, than C-H. Carbide component for this sample is also larger, than for the sample obtained in manganese chloride medium.
The increase of carbon bonds content in the presence of nickel ions can be explained on the basis of PVA carbonization mechanism.
The carbonization process of polyvinyl alcohol in the medium of transition metal chlorides can be presented via polyene chain formation due to dehydration. As the reaction proceeds in active mineral medium, the winding of polymer chains into a spiral is preferable from energetical point of view. As there is a higher electron density inside the spirals owing to electrons derealization at conjugated system of bonds, metal ions interact with double bonds of polyene chain. The formation of C-Met bonds weakens the strength of C-H bonds. The breaking of protons and their transfer into medium become easier due to this. Polymer dehydration rate depends on the energy of interaction of metal ions with conjugated 7i-bonds system. As C-Ni bond is stronger than C-Mn, correspondingly C-H bond energy is decreased. Consequently, the presence of nickel ions facilitates proton breaking-off and stimulates PVA carbonization process.
As a result of dehydration the cross-linking between the branches of spirals takes place and the formation of carbon tubulenes including metal is possible.
The presence of metals in the shell of tubulenes obtained in metal chlorides medium is also confirmed by microelectron diffractometry data. According to diffractograms for samples obtained in manganese chloride medium, the basic state is fine-crystal. Diffractograms show a large number of point reflexes which form concentric circles (Fig. 3). The estimate of grid parameters, carried out on these rings, shows that the reflections observed belong to graphite and diamond and this is characteristic of nanotubes [8].
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Fig. 3. Microdiffractogram of the sample obtained from PVA in manganese chloride medium
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Mixed state is registered in samples obtained in polyphosphoric acid medium. One of them is amorphous. But a fine structure is more frequently found. It produces microdiffraction pictures consisting of bright and weak spots, which form the pictures like "six-pointed stars" (Fig. 4). Such complex reflexes are seen from quasi-crystals consisting of the combination of pentagons and hexagons.
In accordance with electron microscopy data for the sample, obtained in chloride medium, carbon-metal containing structures obtained are defective to a great extent (Fig. 5). It is connected with the fact that the temperature of PVA melting (about 230 °C) is lower than a melting temperature of chlorides being used. Therefore, the formation of carbon particles from polymer melt takes place in the area between unmelted salt particles. Probably, the introduction solutions are formed on the surface of salt particles and carbonization process proceeds in a thin chemisorbed layer. Therefore, the defectiveness of carbon formations formed is determined by crystals structure and medium dispersity. So, the method of tubulenes obtaining in melts or solutions with lamellar structure is more perspective.
While the sample, obtained in the solution of polyvanadium acid intercalated by Mo, is investigated by transmission electron microscopy cylindrical branched structures resembling dendrites are found (Fig. 6). The formation of such structures can be connected with the influence of active mineral medium. Lamellar structure of polyvanadium acid leads to the acceleration of tubulene formation processes and sufficient number of iheir growth centers. This leads to the formation of defective structures consisting of tubes intergrown in different directions. Some tubes are formed as scrolls (Fig. 7). The similarity of these formations with nanotubes structure obtained in the paper [9] is observed. However, the size of structures observed by us is by an order larger. The formation of such extended structures with the diameter up to several mem can be explained by the fact that PVA macromolecules form lamellar aggregates. Strong intermolecular interaction of polar OH-groups facilitates the laying of polymer chains into packs. When PVA is dehydrated, intermolecular interaction is weakened; as a result, macromolecules laid into packs can bend. The reconstruction of inner
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Fig. 4. Microdiffractogram of the sample obtained from PVA in polyphosphoric acid melt
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Fig. 5. Microphotograph of structure obtained in the medium of PVA in manganese chloride medium (x 150000)
Fig. 6. Microphotograph of dendrite-like structure obtained in the medium of polyvanadium acid intercalated by molybdenum (x75000)
Fig. 7. Microphotograph of tubular structure obtained in the medium of polyvanadium acid intercalated by molybdenum (xl 50000)
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formations, taking place with the decrease of surface energy, leads to the formation of sufficiently elongated tubular structures.
The defectiveness of carbon-metal containing structures can have the advantages when introducing them into polymer composition materials and when using them as additions decreasing combustibility in intumescent fireproof polymer coatings.
REFERENCES
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5. F. Krumeich, H.J. Muhr et.al. J. Am. Chem. Soc., v. 121, №36, p. 8324-8333.
6. H.J. Muhr, F. Krumeich et.al. Adv. Mater., v. 12, №3, p.231-236.
7. V.I. Mikhailov, S.G. Bystrov et al. Comp. polymer materials, iss. 46 (1990), p.49-52 (in Russian).
8. S.Iijima. Nature, 354 (1991), 56.
9. Solid. State. Com., 1998, v.105, №3, p.145-149.
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