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Processing and structural properties of in situ polymerized poly(tetrafluoroethylene)/graphite fluoride composites.

INTRODUCTION

Poly(tetralluoroethylene) (PTFE) is a thermoplastic polymer possessing unique properties as a record low friction, chemical inertness, not moistened with water, and most organic solvents. These wonderful qualities are preserved in a wide temperature range from-100 to +250 [degrees]C (1-3). As a consequence. PTFE has found diverse applications in nodes of the "dry" friction, as a sealant and protection products operating in chemically aggressive environments, in the interior pipe coatings reducing energy costs for transportation of petroleum products, etc. A broader application of PTFE is restricted by the low wear resistance, continuous creep, and extremely high viscosity making impossible processing of this polymer by the conventional methods of liquid-phase extrusion or injection molding.

A noticeable decrease in creep has been obtained by the exposure of PTFE to ionizing radiation at melting temperature of the polymer crystals. Such exposure leads to the detachment of fluorine atoms or rupture of polymer chains. This favors the formation of terminal and median fluoropolymer macroradicals which in its turn stimulates the development of the branched and side chains (4-8) leading to reduction of polymer creep. Certain improvement in PTFE mechanical properties can be achieved by means of the introduction of solid microparticles as graphite, coke, oxides, carbon and glass fibers, etc. (9-11). This may be got under the condition of homogeneous mixing of the well-dispersed agglomerates of microparticles. However, the uniform distribution of nanoparticles is impeded due to their strong liability to agglomeration. This deficiency leads to significant deviations of physical and mechanical properties of composites from the expected ones. Particularly, one of the consequences of nanoparticles agglomeration is the increase in the percolation threshold which prevents improving mechanical properties of a composite material at small volume fraction of nanofiller. As applied to the PTFE-based nanocomposites, this problem is of especially important. Indeed, owing to impossibility of the liquid-phase extrusion, the preliminary blending of PTFE powders and nanoparticles is often carried out before the material sintering (12), (13). Under such conditions, it is hard to attain the complete desagglomeration of nanoparticles.

Mechanical properties and wear resistance of the composite materials crucially depend on the adhesion quality between inclusions and polymer matrix. Poor adhesion of filler particles with polymer matrix leads to a significant reduction in elastic moduli and strength of composites. Due to chemical inertness of PTFE, adhesion of this polymer with filler particles is caused solely by physical interactions. Particularly, use of polar ultrafine particles of ceramics or aluminum oxide results in some increase in the strength of PTFE-based composites in tension (14), (15). It is obvious that covalent bonds between polymer matrix and inclusions should provide a more substantial enhancement of mechanical properties of nanocomposites. Due to the fact that the bonds play a part of effective crosslinks or branch points of polymer chains, they may sufficiently decrease in a material creep as well.

The latter problem finds a solution in polymerization in situ of macromolecules grafted to the particles surface. Such polymerization was shown to be promising for producing polymer nanocomposites based on polyolefins (16-23), poly(methyl methacrylate) (24), (25), and polyurethanes (26), (27). Specially, it was shown that intercalation of the catalyst and monomer into the interlayer space of the clay nanoparticles during in situ polymerization results in the effective exfoliation of nanoparticles. This method results in a homogeneous distribution of filler particles while their exfoliation to the separate layers leads to the true nanocomposites.

As for PTFE, it was found that it may be produced by the in situ gaseous-phase polymerization of tetrafluorethylene (TFE) which is initiated on the graphite fluoride [fluorographene (FG)] particles under the ambient conditions (28). In this case, polymerization proceeds via free-radical mechanism which is able to synthesize composite material with mass fraction of PTFE up to 95-98% [29]. The resulting polymer was shown to possess ultrahigh molecular weight of [M.sub.w] ~ [10.sup.6]-[10.sup.7]. It was found that in situ polymerization of FITE in the presence of fluorinated petroleum coke is characterized by the long induction period followed by the exponential growth of the polymer yield rate. Such polymerization kinetics was assumed to be accompanied by the polymerization exfoliation of FG particles into separate blocks due to growing of polymer chains. This assumption is implicitly supported by the data of electron paramagnetic resonance of fluorinated coke showing that in addition to chain radicals, the novel centers of chemical reaction appear. They reveal themselves as anisotropic singlets caused by the formation of peroxide radicals (29). These radicals were suggested to appear due to a part of paramagnetic centers of the graphite fluoride that become accessible to oxygen molecules due to the polymerization exfoliation of the particles. In turn, the FG polymerization exfoliation could lead to a sharp increase in the number of radicals needed for the extra synthesis of polymer chains.

These assumptions, however, need to be proved by a direct investigation of the composite structure. This was a main goal of this article, where we present experimental studies of polymerization kinetics and structure of the PTFE/FG composites synthesized by means of gaseous-phase in situ polymerization.

EXPERIMENT

Materials

The graphite fluoride was supplied by Institute of Electrocarbon Products (Elektrougli, Moscow Region, Russia) as a finely dispersed powder of fluorinated petroleum coke. The fluorine weight content of the powder was close to 60% and corresponds to chemical composition C[F.sub.0.95] which is close to the carbon monolluoride CF1.0. The decomposition temperatures of FG in air and argon are 760 and 810 K, respectively. The carbon monolluoride is a true-layered compound. Its structure comprises of the regular layers of quasitwo-dimensional corrugated sheets of graphite coated with the covalently attached fluorine atoms at both sides. C[F.sub.1.0] stoichiometric compound is an insulator (the resistivity is of [10.sub.13-14] Ohm cm) crystallized to the hexagonal lattice with constant a = b = 2.54-2.57 A. The intersheet spacing c is strongly dependent of the type of the ingoing graphite and fluorination regime. It was reported that depending on reaction temperature, c takes on values between 5.85 and 9.0 A (30).

TFE was produced by the pyrolytic decomposition of PTFE and rectified at low temperatures. The purity of the obtained monomers was no less than 99.9%. As reference materials, we also investigated the structural properties of a graphite powder (G; supplied by SGL Group under the reference Ecophit-G) and a neat PTFE material (supplied in the form of rod by Goodfellow).

Polymerization Procedure and Kinetic Measurements

Polymerization was performed in the reactor at constant monomer gas pressure in the range from 4.0 to 86.5 KPa and temperature from 273 to 353 K. Fluorinated coke powder of ~0.1 g was spread out in the reactor to a thin layer of about 1 mm thick. This allows avoiding the nonexothermicity of processes related to the high heat of TFE polymerization. The kinetic setup was composed of reaction and feeding parts separated by an automatic valve. As soon as monomer pressure in the reaction part decreased as a result of polymerization, the valve operated and the desired constant pressure was restored by feeding in the monomer. The kinetics was monitored by recording pressure drop in the feeding part. A reduction in pressure by 133 Pa (1 mm Hg) corresponded to the formation of 8X[10.sup.-4] g of PTFE. The kinetic data measured as a function of the monomer consumption were recalculated to the polymer yield function g(t) expressed in grams per gram of the starting carbon fluorinated material. Polymerization kinetics at the initial stage was reported by measuring the pressure drop in the reaction part using a differential silphon gage. This procedure increased the sensitivity of the detection system by more than an order of magnitude and allowed measurements of Instantaneous polymerization rates.

Defluorination of Fluorinated Coke Inclusions

After the in situ polymerization of PTFE on the fluorinated petroleum coke, part of the samples were treated to the defluorination procedure which resulted in removing fluorine atoms from the graphite fluoride particles and reducing the initial carbon material (31). The defluorinat ion was carried out by means of interaction of ethanolamine with PTFE/FG. composites in the temperature range between 80 and 100C during about 3 h. Defluorination degree was examined by means of calorimetric analysis of the reaction heat which was found to be approximately equal to 100%.

Scanning Electron Microscope

Microstructures of the materials were characterized by means of the scanning electron microscope (SEM) FEI FEG Quanta 200 equipment (FEI Europe By, Eindhoven, The Netherlands). The electron beam was generated at 10 kV and we utilized the low vacuum mode with a water pressure of 150 Pa. Images were recorded with the gaseous analytical detector to have chemical contrast micrographs. SEM observations were directly performed on the specimen (as-polymerized paste-like sample or powder regarding graphite or fluorinated graphite) without any particular preparation procedure.

Wide-Angle X-Ray Diffraction

The crystalline properties of the materials were investigated by wide-angle X-ray diffraction (WAXD) using a Panalytical X'Pert Pro MPD diffractometer (Panalytical B.V., Almelo, The Netherlands). The utilized radiation was the K[alpha] radiation of cupper ([lambda] = 0.154 nm) generated at 45 mA and 40 kV. Detection was performed by means of the ID detector PIXcel (Panalytical By., Almelo, The Netherlands). A spinning procedure of the specimen (revolution of 4 s) was used to put the maximum of crystal in diffraction position and avoid any loss of information due to preferential orientation. The analysis of the diffractograms was done using the software HighScore Plus (Panalytical B.V., Almelo, The Netherlands) and the database PDF 4+ (ICDD. Newtown Square).

RESULTS AND DISCUSSION

Kinetics of In Situ Polymerization

The ability of fluorocarbon materials to initiate the polymerization of TFE and other vinyl monomers was first described in the patent (28). As revealed by quantum chemical calculations (32), (33), the carbon monofluoride can exist in three conformation states: "armchair", "bed", and "washboard". The ground state energies of these conformations are very close. However, the barriers between them exclude the possibility of spontaneous transitions from one conformation to another. As a consequence, within a single graphite fluoride sheet, one can meet domains of different conformations. Boundaries between these domains contain linear defects. They are important owing to the following reasons: (i) the active centers of initiation of polymerization are likely originated by these defects and (ii) they could significantly facilitate diffusion transport of TFE monomers between graphite fluoride monolayers (sheets).

Location of the active centers of initiation of polymerization should significantly influence both on polymerization kinetics as well as final composite structure. Indeed, if the active centers are associated with boundaries of domains of different conformation, the major portion of the active centers should be located away from a sheet edge. This causes the interlayer diffusion of monomer molecules to be the limiting stage of polymerization. In this case, the most active centers are located within a stack of monolayers and their number is proportional to mass of the original powder. However, because the polymerization proceeds via free-radical mechanism, diffusion restriction should be reduced due to an approach of the end of the growing polymer chain to the edge of the monolayer thus resulting in opening of the interlayer space. In this case, polymerization rate should be accelerated even if the initially active centers are equally accessible. On the other hand, if the active centers are located at the edges of graphite fluoride sheets, their number will be proportional to the mass of graphite monofluoride to the power 2/3 and will be decreased with degree of polymerization due to blocking of the growing molecules by the product chains. Anyway, the important factor is that the macromolecules are covalently linked to the carbon monofluoride in any location of the active centers. Hence, the described process allows synthesis of composite material with chemically bounded PTFE matrix and FG particles which results in their strong adhesion.

Figure 1 represents polymerization kinetics of TFE in the presence of the fluorinated coke in terms of dependence of polymer yield q on the process time. This is characterized by the long induction period followed by the explosive growing. The kinetic curves remind the radical polymerization in the presence of a strong inhibitor. However, this is only an apparent similarity. Actually, polymer yield in the nonstationary parts of the kinetic curves rises exponentially as q(t) = [q.sub.0]exp(At). The polymerization rate w(t) = dq/dt = [w.sub.0]exp(At) grows exponentially as well which is demonstrated in Fig. 2 in semilogarithmic coordinates. This indicates an unusual mechanism of initiation of in situ polymerization of TFE under the influence of fluorocarbon materials. Afterward the process becomes stationary with the almost unchangeable polymerization rate (29).

The original FG-layered structure possesses a number of the active centers capable to interact with TFE mers and initiate chain growing. Polymerization starts with a small amount of such centers located in the areas initially accessible for the monomers or in that environment where the reaction becomes easier. Growth of polymer chains and their interaction with the layered FG may lead to the deformation or exfoliation of graphite mono-fluoride stacks of sheets. These processes may involve the previously isolated centers of initiation of polymerization and remove restrictions in monomers diffusion. This may lead to breaking of bonds in fluorocarbon particles followed by formation of new free radicals. The increase in the number of growing chains should intensify their impact in the structure of fluorocarbon particles and likely cause exponential acceleration of polymerization.

Structure of As-Polymerized Composites S

EM micrographs of the initial graphite fluoride and the composites PTFE/FG (11/89), (20/80), and (98/2; values are in weight percent) are shown in Fig. 3. It is seen that FG particles are characterized by a marked layered structure. They exhibit noticeable flaws and defects and can be easily split into smaller stacks with an average thickness of several hundred nanometers. This kind of graphite monofluoride structure excludes any diffusion or steric hindrance associated with particles agglomeration. The micrograph of the composite containing a low amount of PTFE (Fig. 3b) confirms that polymeric chains of PTFE grow on FG particles. Particularly, it appears that PTFE molecules are generated at the surface of FG particles and propagates in the interlayer spaces. When increasing the amount of PTFE (PTFE/FG (20/80)), graphite particles are totally coated by PTFE, and a fragmentation mechanism of graphite fluoride sheets into submicronic-thick fragments is noted (Fig. 3c). For the composite containing the highest concentration of the polymer, PTFE/FG (98/2), we observed imbricated microsized domains having the same dimensions and chemical contrast (Fig. 3d). However, some of them have an important size and may be graphite residues coated by PTFE that may arise from a gradual fragmentation of the initial graphite particles with increasing the amount of PTFE.

At a lower magnification (Fig. 4), we can observe that PTFE/FG (98/2) contains some voids that appear due to a weak consolidation of the polymerized medium. We also note the presence of flat and long domains that are probably FG lamellae residue. The latter are homogeneously distributed within the polymer matrix, which is suitable for a polymer-based composite.

Results of WAXD investigation of the reference graphite (G), FG, and PTFE/FG composites are displayed in Fig. 5. We also studied by WAXD a composite that was defluorinated after the in situ polymerization. When comparing FG with G, the (001) crystalline peak of FG is wider and is positioned at a lower angular place than (002) crystalline peaks of G (both belonging to hexagonal phase of graphite and fluorinated graphite). This demonstrated that the intersheet distance of FG is higher and is more heterogeneous than that of G (average intersheet distance 0.675 nm vs. 0.35 nm). When increasing the amount of PFTE from 0 to 20%, we do not see any difference in the position and width of the crystalline peak (001) of FG, demonstrating no overall intercalation or destruction of crystalline order by exfoliation. Therefore, fragmentation phenomena observed by SEM only involved some limited domains of graphite particles. Regarding the composite containing the higher amount of PTFE (PTFE/FG (98/2)), it can be seen that (001) crystalline peak of FG is not visible, whereas crystalline peak (200) of FG is present. The presence of this peak demonstrates that the crystalline order was not destroyed, and confirms what was observed for the other composites. The fact that the peak (001) peak is not visible is explained by the occurrence of an amorphous bump of PTFE ranging between 13 and 22[degrees] that has a much larger intensity than the crystalline peak. Regarding the defluorination process, it causes an increase in the angular position of (001) crystalline peak and a broadening of this peak compared to the (001) peak of G. Defluorination, that is desirable to increase electrical conductivity of the composite, decrease the intersheet distance from 0.675 to 0.456 nm (near to that of the reference graphite) and increases the crystalline disorder.

Coupling of Polymerization Kinetics and Structure

The overall scheme of polymerization mechanisms of PTFE/FG composites is illustrated in Fig. 6. The long induction period noted in the kinetics study (Figs. 1 and 2) is linked to the diffusion time of TFE within the interlayer space to react with the active centers located on the linear defects of FG monolayers (Fig. 6a). Subsequently, polymerization starts near the surface of graphite particles (higher concentration of monomer). Formation of polymer in the interlayer space followed by wedging creates more favorable conditions for nucleation and growth of polymer chains. This leads to an explosive rise of polymer yield and explains the exponential growth rate. During this step, a gradual fragmentation of the graphite particles occurred (Fig. 6b). At the end of the polymerization, we obtained a composite with graphite fragments coated by PTFE homogeneously distributed within the polymer matrix (Fig. 6c). Such a distribution is due to the fact that polymer yield rate becomes constant at the stationary phase of the polymerization process.

CONCLUSIONS

This study demonstrates that the gaseous-phase in situ polymerization of TFE on the fluorinated coke particles can be used for the manufacture of PTFE/FG composites with different polymer concentration. A covalent bonding is obtained between the polymer matrix and the graphite filler, while a homogeneous distribution of the filler particles is ensured by the process of the in situ polymerization. These two characteristics are suitable for a polymer-based composite.

In situ polymerization of PTFE leads to an extremely high polymerization degree of Mw 106-107. Synthesis of macromolecules with such a high molecular weight indicates kinetic stability of the polymerization process and the almost complete absence of the inhibitory factors leading to chain termination. Polymerization kinetic is satisfactorily explained in terms of assumption of polymerization in the intersheet space of graphite fluoride particles which results in their fragmentation as shown by SEM and WAXD. However, this process did not reach destruction of crystalline order of the graphite fluoride that would lead to a perfect exfoliation of FG sheets, and hence, to a nanocomposite with enhanced properties. Increasing the exfoliation process of fluorinated graphite during in situ polymerization of PTFE will be one of the subject of a next research work. It is also to be mentioned that there is a possibility of chemical modification that is the almost complete defluorination of the original particles, at any concentrations of fluorinated coke. This may create the composites of different electrical properties.

Correspondence to: Dr. Stanislav A. Patlazhan; e-mail: sapat@yandex.ru

DOI 10.1002/pen.23505

Published online in Wiley Online Library (wileyonlinelibratry.com).

[c] 2013 Society of Plastics Engineers

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V. P. Mel'nikov, (1) F. Addiego, (2) V.V. Smirnov, (1) D. Ruch, (2) A.A. Berlin, (1) S.A. Patlazhan (1), (2)

(1) Semenov Institute of Chemical Physics of Russian Academy of Sciences, 4, Kosygin Street, 119991 Moscow, Russia

(2) Public Research Centre Henri Tudor, Advanced Materials and Structures Department, 29, Avenue J.F. Kennedy, L-1855 Luxembourg
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Author:Mel'nikov, V. P.; Addiego, F.; Smirnov, V.V.; Ruch, D.; Berlin, A.A.; S.A. Patlazhan
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EUNE
Date:Dec 1, 2013
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