ISSN 20751133, Inorganic Materials: Applied Research, 2015, Vol. 6, No. 2, pp. 179–186. © Pleiades Publishing, Ltd., 2015. Original Russian Text © G.S. Baronin, V.M. Buznik, G.Yu. Yurkov, D.O. Zavrazhin, D.E. Kobzev, V.V. Khudyakov, Yu.V. Mescheryakova, A.S. Fionov, E.A. Ovchenkov, A.A. Ashmarin, M.I. Biryukova, 2014, published in Perspektivnye Materialy, 2014, No. 7, pp. 50–61.
METHODS OF MATERIALS PROPERTIES ANALYSIS
Study of Structure and Properties of Polymer Composites Based on Polytetrafluoroethylene and Cobalt Nanoparticles G. S. Baronina, b, V. M. Buznikc, d, G. Yu. Yurkovc, d, D. O. Zavrazhina, D. E. Kobzeva, V. V. Khudyakova, Yu. V. Mescheryakovaa, b, A. S. Fionove, E. A. Ovchenkovf, A. A. Ashmarinc, and M. I. Biryukovac a
Tambov State Technical University, Tambov, Russia Research and Educational Center SolidPhase Technologies, Moscow, Russia cBaikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russia d AllRussian Scientific Research Institute of Aviation Materials (VIAM), Moscow, Russia eKotel’nikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, Russia fMoscow State University, Moscow, Russia email: baronin
[email protected],
[email protected],
[email protected], zavrazhin
[email protected],
[email protected],
[email protected],
[email protected] b
Received March 31, 2014
Abstract—This work discusses the study of structure and properties of composites based on polytetrafluoro ethylene (PTFE) and filler, which is a composite material of cobalt nanoparticles and ultrafine polytetraflu oroethylene (UF PTFE). Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Xray phase analysis (XRF) are applied for complex investigation into the properties of the obtained composite materials, such as specific energy absorption rate, thermal resistance and internal orientational stresses, thermophysical properties, and dielectric permeability. It is demonstrated that the obtained compos ite materials are superior to the initial matrix—PTFE. The magnetic study of the composite materials con firms the occurrence of metal nanoparticles and their interaction with the polymer matrix. Keywords: nanocomposite materials based on fluoropolymer, cobalt nanoparticles, tribological and thermo physical properties DOI: 10.1134/S2075113315020057
INTRODUCTION One of the promising trends of development of materials science is polymer composite materials (PCM), including those containing nanosized fillers. Numerous properties of PCM, including performance characteristics, depend on the distribution of filler in the composite. A promising polymer matrix for com posite materials is PTFE owing to some of its proper ties: chemical and thermal resistance, hydrophobicity, low coefficient of friction, and others. At the same time, its insolubility and high viscosity of melt make it impossible to apply liquid phase technologies used for other polymers within production of composite mate rials with a homogeneous distribution of nonagglom erated nanoparticles of filler. Taking into consider ation this reason, in order to produce PTFEbased composite material, it was required to develop dedi cated technological methods which would make it possible to obtain materials with nanosized fillers [1]. The simplest method includes mechanical activation of the mixture of ultrafine PTFE powders and inor ganic substances in a planetary mill [1, 2]. Therefore,
inorganic particles with size of several tens of microns are produced, capsulated with a fluoropolymer coat ing with thickness of a few microns. An alternative method is proposed in [3, 4]; it includes pyrolysis of the mixture of fluoropolymer with easily subliming ammonium fluorides. One more method is thermo chemical production of composite materials based on ultrafine PTFE powder, which was implemented in [5–7], including the use of the solution of metalcon taining compounds (MCC): carbonyls, formates, ace tates of metals. Application of the solution on micro particles of PTFE powder and appropriate thermal processing leads to generation of nonagglomerated metalcontaining nanoparticles with size less than 10 nm on the surface of polymer particles; these nanopar ticles have complex chemical, crystalline, and mag netic structures [8]. Magnetic nanoparticles in the PCM composition are of interest from academic and practical points of view [9]. Fundamental interest is related to establish ment of regularities of their generation and investiga tion into the structure and properties of the composite materials. The practical value is due to the possibility
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of applying such materials in the problems of electro magnetic compatibility. The most attractive magnetic PCM are the systems containing cobalt nanoparticles, since they are characterized by the highest coercive force and saturation magnetization in comparison with other composite materials [10]. For practical application, the interest is attracted by bulk products of these PCM, which possess not only appropriate magnetic and electrophysical properties but also cer tain physicomechanical characteristics. This work is aimed at development of the method of production of bulk polymer materials based on PTFE and cobalt nanoparticles, synthesis of speci mens, and study of their properties and structure. EXPERIMENTAL Cobaltcontaining composite materials were pro duced by thermal decomposition of cobalt nitrate solution on the surface of UF PTFE microgranules. The main concept of the technology lies in the ther molysis of metalcontaining compounds in a fluidized bed of UF PTFE above the surface of preheated ther mostable organic liquid. As a consequence of thermal decomposition of MCC, metal nanoparticles are gen erated, which in interaction with the surface of fluo ropolymer microgranules are stabilized in the form of single (nonagglomerated) particles. The synthesis is described in detail elsewhere [11–13]. In order to produce a bulk item, the synthesized nanocomposite material was used as a disperse addi tive to suspended PTFE powder (GOST (State Stan dard) 1000780). In this way, it is possible to vary the concentration of cobaltcontaining particles and to obtain composite materials with a low concentration of the disperse additive, which are of the highest con cern for production of bulk items. The specimens were produced as follows: mixing in an electromagnetic mixer for 30 min at rotation frequency n = 1000 rpm; pelletizing in a plunger mold at pressure P = 100 MPa for 60 s; sintering with the filler content of 0.05, 0.1, 0.5, 1, and 5 wt % with regard to suspended PTFE at temperature T = 638–643 K for 60 min without exces sive pressure. The obtained bulk composite materials are known as PTFE + CoFP. The content of C, H, and F in the composite mate rials was determined by a conventional method: burn ing in oxygen with the use of a CHF analyzer. The mea surement accuracy was ±0.1%. Quantitative determi nation of metals was carried out using a VRA20 XRF analyzer with accuracy up to 1%. The average size of the obtained nanoparticles and the homogeneity of their distribution in the composite material were determined by TEM using a JEOL JEM1011 microscope. The analysis was performed as follows: the powdered composite material was applied onto a copper grid sequentially coated with polyvinyl formal and carbon. The size distribution of cobalt par
ticles was determined by smallangle Xray scattering (KPM1 camera). The topography of the microgranule surface was studied by SEM using a ZEISS Ultra 5.5 microscope. The phase composition of the materials was ana lyzed using a Shimadzu XRD 6000 diffractometer at ambient temperature in monochromatic copper radi ation (Cu Kα radiation, λ = 1.54056 Å). Crystalline phases were identified by the ICDD2003 database. The structure of PCM was studied also by IR spec troscopy using an IK20 spectrometer. The magnetic properties of the composite material were studied using an EG&G PARC155 vibrating magnetometer with a sensitivity of 5 × 10–5 G cm3 at 300 K in a magnetic field up to 0.5 T. The performance characteristics of the produced bulk composite materials were estimated on the basis of the following experiments: ⎯Measurement of the specific energy absorption rate using a modified differential scanning calorimeter based on a DSC2 device, recording thermal effects within linear variation of the specimen temperature. ⎯Determination of the thermal resistance and internal orientational stresses by plotting isothermal heating diagrams [14]. ⎯Determination of the thermal conductivity and thermal diffusivity using a data measuring system of nondestructive control of thermophysical properties of solids. ⎯Measurement of the dielectric permeability of composite materials by a noncontact method in a plane capacitor (diameter of measuring electrode is 38 mm). The capacitance was measured using an Agi lent E4980A precision LCR meter. ⎯Estimation of the wear resistance in the mode of abrasive wear using a MIR1 friction test machine. RESULTS AND DISCUSSION The study of the initial UF PTFE revealed that the shape of granules of the initial matrix is close to spher ical [15]; their dimensions are not higher than several microns (Fig. 1). The composition and structure of the granules can be described by the nucleus–shell model, where the nucleus is high molecular PTFE and the shell is formed by layers of low molecular fluoroparaf fins. The structure of microparticles of ultrafine PTFE is described in detail elsewhere [16, 17]. According to XRF and IR spectroscopy data the UF PTFE fractions include low molecular fluorinecon taining oligomers [18] with the size of up to ~10 units. Their presence makes it possible to efficiently stabilize metal particles on the polymer surface, which prevents agglomeration of nanoparticles. At the same time, within thermal processing, there occurs chemical interaction between metal nanoparticles and the matrix accompanied by formation of metal fluorides: cobalt in this case [7].
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Fig. 1. UF PTFE granules: (a) SEM; (b) TEM.
The cobalt concentration in the composite mate rial, determined by elemental analysis, is 9.73 wt % in the initial composite concentrated material. The initial powdered UF PTFE was separated by a pneumocircu lating centrifuge, which facilitated separation of gran ules with the sizes in the range from 100 to 150 nm. The size of cobaltcontaining nanoparticles is determined by TEM; it has been established that the average size of nanoparticles does not exceed 5 nm (Fig. 2); the nanoparticles are uniformly distributed over the surface of UF PTFE microgranules. The typical Xray pattern of synthesized initial composite material based on UF PTFE is illustrated in Fig. 3; for the sake of interpretation of the metalcon taining component in the composite material, the intense reflexes peculiar to PTFE were removed. According to the results, it is possible to identify the following cobaltcontaining phases in the specimen: Co (101), Co (002), CoO (111), and CoO (200). It should be mentioned that the highlighted crystalline maxima have a low intensity comparable with the noise level. In addition, in the range of 16°–26°, a halo can be observed, which indicates a nanosized state of the filler. The Xray pattern does not contain maxima corre sponding to the crystalline phase of cobalt fluoride; a similar situation was observed in [7], which could be attributed to its low content. INORGANIC MATERIALS: APPLIED RESEARCH
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Xray emission spectroscopy (XRES) was used in order to detect interaction between cobalt nanoparti cles and PTFE. In XRES spectra of CoKβ5, several components can be observed which confirm interac tion of metal valence p electrons with valence orbitals O(2s) and F(2s), as well as metal–metal interaction in the particles. Lowenergy components of the spectra indicate the existence of metal fluoride in the speci men, which agrees with the previous results in [7, 12]. Additional confirmation of the existence of metal lic cobalt in the specimen was obtained by nuclear magnetic resonance (NMR) of broad lines in 57Co No. 2
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nuclei. In the NMR spectrum, a single signal with maximum at 227 MHz is observed, which is typical of cobalt in a nanosized state of HCP structure, which also agrees with the data in [19]. A peculiar feature of the molecular structure of the composite materials based on UF PTFE and cobalt nanoparticles is the existence of low and highmolec ular fractions of PTFE. The first is represented by oli gomer molecules with the size of several tens of units (CF2)n. Additional bands are observed in the spectra at 986 and 1786 cm–1 not characteristic of initial PTFE and corresponding to trifluoromethyl groups (CF3–) and finite olefin groups (–CF=CF2). On the basis of the acquired data, it is possible to state that the synthesized composite material is com posed of several metalcontaining components; in addition to the metallic nucleus, there are the oxide and halogenide shell and direct metallopolymer bonds with PTFE matrix similar to metalorganic com pounds. The Xray pattern of suspended PTFE used as matrix is characterized by the existence of sharp reflexes corresponding to the polymer crystalline phase and amorphous halo with gravity centers of 17°and about 40° by 2θ, probably corresponding to the Xray amorphous fraction (CF2)n (Fig. 4b) of disor dered phases. It should be mentioned that in the dif fraction pattern of bulk metal–polymer composite material filled with ultrafine fraction of PTFE, modi fied with cobalt nanoparticles, reflexes peculiar to the metal phase were not detected, more probably, owing to a low content of cobaltcontaining compounds (Fig. 4a). It was also mentioned that addition of modifying agents to the nanocomposite material in the amount of 5 wt % is accompanied by variation of the intensity ratio of reflexes of crystalline and amorphous phases of polymer matrix; in the angle range lower than 18°, an additional halo appears, which corresponds to occur rence of the amorphous phase with the nearest disor dering. It is possible to assume that, upon transfer to composite materials, the fraction of amorphous and partially disordered phases increases. On the basis of the acquired results, it is possible to assume that the complex structure of the disperse filler CoFP used in this work can influence the physical and thermal properties of the bulk material. The influence of the filler on the matrix structure was studied using differential calorimetry: tempera ture dependences of the specific energy absorption rate of the composite materials were obtained [20]. Figure 5 illustrates the results of study of the ther mal effects and energy absorption rate in the range of 50–350°C of bulk PTFE filled with CoFP, acquired using a DSC2 device. The maximum values of specific energy absorption rates of composite material PTFE + CoFP at the filler concentration of 0.05 wt % CoFP per 100 wt % PTFE
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and comparison of surface area under abnormalities in the melting region for composite material PTFE + 0.05 wt % CoFP and initial PTFE are evidence that nanocomposite material with such content of CoFP filler has the highest interchain coupling owing to for mation of a high amount of intermolecular bonds between the polymer chain and active portions of nanofiller surface. Figure 6 illustrates the concentration dependences of maximum energy absorption rate W by the speci men in the melting region for bulk composite materi als PTFE + CoFP in comparison with other physico chemical properties. The absolute maximum value of W for all systems is achieved in the CoFP concentra tion range of 0.05 –0.5 wt %. The observed decrease in W with increase in the filler content occurs owing to aggregation of nanoparticles with increase in nano filler concentration above 0.05 wt % related to decrease in activity of the filler surface layers. On the basis of these thermophysical studies of the energy state of bulk combined nanocomposite based on PTFE, it is possible to conclude that CoFP disperse filler with concentration up to 1.0 wt % sharply increases interaction in the boundary layer polymer— INORGANIC MATERIALS: APPLIED RESEARCH
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filler and makes the structure more rigid owing to for mation of a high amount of bonds between the poly mer matrix and active surface parts of nanosized filler. The difference in energy state of the combined nanocomposite in comparison with initial PTFE causes an increase in the overall set of system proper ties: decrease in thermal conductivity λ and increase in heat distortion temperature THD and wear resistance under conditions of abrasion Ip (Fig. 6). The dielectric permeability of PTFE + CoFP spec imens was measured at frequencies of 20, 50, 100, 200, 500 Hz, 1, 2, 5, 10, 20, 50, 100, 200, 500 kHz, 1 and 2 MHz, overlapping the operating range of the LCR meter. Figure 7 illustrates dielectric permeability (ε) of specimens for the considered polymer systems as a function of concentrations of modifying agents in the range from 100 Hz to 1 MHz. Extreme concentrations of composite materials ε as a function of concentra tion of disperse filler are highlighted. Here, no fre quency dependence of ε for all considered composite materials was revealed. The dependence of ε of PTFE + CoFP composite material on concentration of modifying agent (Fig. 7) is nonmonotonic. One can observe a maximum of No. 2
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Fig. 8. Magnetization curves of cobaltcontaining nano composite materials: (1) composite material CoFP; (2) bulk composite material PTFE + CoFP.
dielectric permeability at low (0.01–0.05 wt %) con centrations and its drop upon a subsequent increase in the filler concentration. These variations in the range of the considered concentrations are not higher than 0.05 in comparison with ε of unfilled matrix and can be caused by “modulation” of material density upon addition of filler (for instance, due to variation of the fraction of crystalline and amorphous phases). Such dependence of ε on concentration of modifying agent correlates with the values of other physicochemical properties of fluoropolymer nanocomposite materials (Fig. 6).
Quantitative estimates of the state of cobalt parti cles in the filler CoFP were obtained by approximation of magnetization curve using the following equation: mμ 0 H⎞ (1) M ( H ) = M s L ⎛ , ⎝ kB T ⎠ where Ms is the saturation magnetization and L(x) is the Langevin function. Equation (1) describes the field dependence of magnetization, measured at the tem perature T, of particles with magnetic moment equal ing to m Bohr magnetons and overall saturation mag netization Ms. As a consequence of the approximation, we obtained the value of m of about 3000 and Ms of about 0.1 (A m2)/kg. If it is considered that the cobalt mag netization in these particles is the same as in metal, then the particles consist of about 1500 atoms; that is, their size is not greater than 3 nm. The concentration of such particles in the bulk composite material is close to 0.05 wt %. On the basis of the results of thermophysical stud ies, relaxation properties in the annealing mode of ori ented specimens obtained by ram solid phase extru sion [21, 22], and the results of investigation into dielectric, magnetic, and tribotechnical properties of composite materials, it was demonstrated that a corre lation exists between the maximum energy absorption rate Wmax, thermal conductivity, heat distortion tem perature, dielectric permeability, and wear resistance of polymer composite materials PTFE + CoFP. Therefore, it is possible to control the properties of molecular nanocomposite materials both with regard to composition and to their processing into items of various functional purposes (high frequency insula tors, thermotechnical items, tribotechnical items, and others). The variation pattern of the set of properties of the polymer system in the region of minor additions of the filler (CoFP) and existence of an extreme point are
Magnetic properties of disperse filler CoFP (1) and bulk composite material PTFE + CoFP (2) were stud ied. Magnetization curves for the considered com pounds measured at ambient temperature are illus trated in Fig. 8. As can be seen in the illustrated data, the bulk material PTFE + CoFP (Fig. 8, curve 2) obtained by pressing has no noticeable magnetization in the considered range of fields. This indicates that cobaltcontaining nanoparticles in this composite material exist in the form of paramagnetic and antifer romagnetic phases (cobalt oxides and fluoride). Mag netization of the disperse filler (Fig. 8, curve 1) is very low, which is characteristic of ferromagnetic nanopar ticles (metallic cobalt), and on the other hand, minor values of magnetization can be a consequence of an uncompensated moment on the surface of antiferro magnetic particles (cobalt oxide (II)). Taking into consideration the absence of magnetization in the PTFE + CoFP specimen, it would be reasonable to assume that the source of magnetization in disperse filler CoFP is composed of ferromagnetic metallic cobalt particles, since transformation of the metallic cobalt phase into cobalt fluoride upon pressing is more probable than transformation of cobalt oxide. A simi lar phenomenon consisting of interaction between nanoparticles of d metals with UF PTFE was men tioned previously in [7].
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evidence of the transition of the system from a single phase structure to doublephase structure (spinodal decomposition) and vice versa [23]. At the extreme point, the polymer system is in the metastable state, where the highest dispersity of the modifying agent is detected in the polymer matrix and, hence, the highest homogeneity of the system. It should be mentioned that similar mechanisms of structure formation were detected for polymer mixtures and alloys, the compo nents of which are in nanostructured state. Upon the transition from a single to doublephase system (stratification), the depositing phase is in a highly dis perse state and forms a thermodynamically stable sys tem with a particle size not higher than several tens of nanometers. The interface layer in such polymer sys tem, owing to proximity to critical conditions, has sig nificant thickness and the interface surface area is high. The highly developed interface surface in the transitional regions of the boundary layer of polymer in nanocomposite materials and existence of particles of nanometer size lead to extreme modification of physicochemical properties of the polymer system in this concentration region; that is, these are the deter mining factors. CONCLUSIONS We have discovered the possibility to control the structure and properties of polymer composite materi als based on PTFE by addition of minor amounts of disperse metal–polymer nanocomposite material based on UF PTFE and cobalt nanoparticles. The technology has been developed which makes it possi ble to apply the known method of doping of organic and inorganic materials to the production technology of molecular composite polymers based on PTFE. The main physiochemical properties of composite materials based on PTFE matrix have been detected. ACKNOWLEDGMENTS This work was supported by the Ministry of Educa tion and Science of the Russian Federation within the scope of the Program of Fundamental Research by the Presidium of the Russian Academy of Sciences P8 and the Russian Foundation for Basic Research, grant no. 130312168 ofi_m. REFERENCES 1. Buznik, V.M., Fomin, V.M., Alkhimov, A.P., Metal lopolimernye nanokompozity (MetalPolymer Nano compounds), Novosibirsk: Sibir. Otd. Ross. Akad. Nauk, 2005. 2. Lomovsky, O.I., Politov, A.A., Dudina, D.V., Korcha gin, M.A., and Buznik, V.M., Mechanochemical methods of metalceramicPTFE composite production, Chem. Sustain. Develop., 2004, vol. 12, pp. 601–607. 3. Kantayev, A.S., Dyachenko, A.N., and Buznik, V.M., RF Patent 2469056, Byull. Izobret., 2012, no. 34. INORGANIC MATERIALS: APPLIED RESEARCH
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