ISSN 19950780, Nanotechnologies in Russia, 2013, Vol. 8, Nos. 1–2, pp. 69–80. © Pleiades Publishing, Ltd., 2013. Original Russian Text © S.V. Pol’shchikov, P.M. Nedorezova, A.N. Klyamkina, V.G. Krashenninikov, A.M. Aladyshev, A.N. Shchegolikhin, V.G. Shevchenko, E.A. Sinevich, T.V. Monakhova, V.E. Muradyan, 2013, published in Rossiiskie Nanotekhnologii, 2013, Vol. 8, Nos. 1–2.
Composite Materials Based on Graphene Nanoplatelets and Polypropylene Derived via In Situ Polymerization S. V. Pol’shchikova, P. M. Nedorezovab, A. N. Klyamkinaa, V. G. Krashenninikova, A. M. Aladysheva, A. N. Shchegolikhinb, V. G. Shevchenkoc, E. A. Sinevichc, T. V. Monakhovab, and V. E. Muradyand a Semenov
Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia cEnikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, ul. Profsoyuznaya 70, Moscow, 117393 Russia dInstitute of Problems of Chemical Physics, Chernogolovka Branch, Russian Academy of Sciences, pr. Ak. Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia email:
[email protected]
b
Received April 6, 2012; in final form, October 10, 2012
Abstract—In recent years researchers have paid considerable attention to the creation of multifunctional polymer composite materials bearing nanoscale fillers. The introduction of nanoparticles into polymer matri ces in relatively small concentrations (reaching 2.5 vol %) makes it possible to produce materials possessing better properties than the initial matrix polymers and conventional dispersionfilled composites. Carbon nanostructures are promising fillers for polymer nanocomposite materials: fullerenes, carbon nanotubes, nanofibers, and graphene nanoplatelets. A combination of structural, physicomechanical, and electrophysical properties of these fillers upon their introduction into polymer matrices affords the creation of composite materials possessing improved stress–strain, electro and thermophysical characteristics, and noncombustibility. DOI: 10.1134/S1995078013010114
Uptodate carbon nanotubes (CNTs) held a dom inant position among nanosized carbon fillers used in the creation of polymer composite materials. How ever, the effective use of CNTs is impeded by the com plexity of their dispersal in a polymer matrix and high cost. Nowadays, researchers focus much attention on graphene, a new material which has become a very popular object of inquiry [1]. As is known, graphene is a twodimensional carbon modification formed by a layer of atoms one atom in thickness connected via sp2 bonds into a hexagonal twodimensional crystalline lattice. For a small number of layers in a stack, the terms fewlayer graphene or graphene nanoplatelets are used. Compared to CNTs, graphene nanoplatelets seem more promising for the production of polymer nanocomposites with improved strength and func tional characteristics [2, 3].
and an investigation into composite materials based on isotactic PP and graphene nanoplatelets. EXPERIMENTAL Graphene nanoplatelets (GNP) were obtained by a method based on the chemical oxidation of graphene followed by its reduction [11]. Graphite oxide was syn thesized by the modified Hammers method [12] via the oxidation of graphite under the action of KMnO4 in concentrated H2SO4 [13]. Graphite oxide (GO) in the form of an aqueous suspension was reduced by hydrazine hydrate under ultrasonic treatment at 70°C for 4 h followed by refluxing for 2 h. GNP powder was reduced, washed with bidistilled water, and dried via cryodrying and then heated under an argon atmo sphere at 900°C for 1 h.
One known method for producing composites with nanocarbon fillers is the method of polymerization filling or in situ polymerization. The polymerization method of introducing various types of fillers into a polyolefin matrix was suggested in [4, 5] and gained further development for the production of PP compo sites with graphite [6, 7], multilayer carbon tubes [8], graphene nanoplatelets [9], and graphite oxide [10].
The Xray diffraction pattern of the GNP powder for Xray phase analysis was recorded on an ADP1 diffractometer (CuKα monochromatic radiation). An interlayer distance was defined by the Wolf–Bragg equation: d002 = λ/2 × sinθ002, where λ is the wave length of CuKα radiation and θ is the diffraction angle. The crystallite sizes in the direction of the c axis was determined according to the following formula Lc = 0.94λ/β002 × cosθ, where β002 is the angular halfwidth of (002) lines in radians.
This work is devoted to synthesis via in situ poly merization using a homogeneous metallocene catalyst 69
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The Raman spectra were obtained with a Senterra dispersion Raman microscope (Bruker) equipped with laser excitation with a wavelength of 785 nm and ther moelectrically cooled to –53°C with a silicon Idus PZS detector (Andor). The spectra were registered in an optical geometry of 180° in the range of Raman shifts from 100 to 3200 cm–1 at an optical resolution of 3–5 cm–1 using 1–50 mW laser power for the Raman excitation of various samples. The digital resolution of the Raman signal comprised 0.5 cm–1. The composite materials were synthesized in bulk propylene in the presence of a highly effective homo geneous catalytic system based on ansazirconocene racMe2Si(2Me4PhInd)2ZrCl2 activated with MAO according to the method described in [14]. This cata lyst is characterized by high isospecificity and activity in propylene polymerization and provides for the pro duction of isotactic polypropylene (IPP) with a high molecular weight [15]. Propylene polymerization was carried out at 60°C and a pressure of about 2.5 MPa in a steel reactor with a volume of 0.2 L equipped with a highspeed rapid stirrer (3000 rpm). The nanocomposites were synthe sized via different methods: (1) the initial GNP pow der preliminarily evacuated at 200°C was introduced into the reactor, the reactor was filled with liquid pro pylene (100 mL), and the required amount of MAO and metallocene catalyst was supplied; (2) prior to the application in synthesis, a suspension of GNP powder in toluene was prepared and ultrasonic treated (US) for 10 or 30 min; then the required amount of MAO was added and the ultrasonic impact was continued for another 10 or 30 min, respectively. The US radiant power comprised 50 W. After this, the suspension was introduced into the polymerization reactor filled with propylene and supplied with a catalyst. After the ter mination of polymerization, the powder of composite materials was unloaded from the reactor and rinsed from the residues of catalytic system components with a mixture of ethanol and HCl (10% aq. solution); then it was multiply washed with alcohol and dried to a con stant weight under vacuum at 60°C. The infrared (IR) spectra of IPP samples and IPP/GNP compositions in the form of films 100 μm in thickness derived via hot pressing were registered on a Bruker Vertex 70 FTIR spectrometer. The stereo regularity of IPP (macrotacticity) was determined by the ratio of intensities of absorption bands at 973 and 998 cm–1 (D998/D973), characterizing the part of pro pylene units in isotactic sequences with a length of more than 11–13 monomeric units [16]. The stress–strain characteristics of the resulting polymeric materials were studied on an Instron 1122 tensile testing machine with blade samples (0.5 × 5 × 35 mm) and a rate of tension of 50 mm/min. The films for tests were prepared by pressing at 190°C and a pres sure of 10 MPa at a rate of melt cooling of 16 K/min.
The test result was considered an average value among 5–6 measurements; the measurement accuracy com prised 5–10%. The composite materials were studied by scanning electron microscopy (SEM) using a JSM5300LV microscope (Jeol) and transmission electron micros copy (TEM) using a LEO912AB electron micro scope. The SEM method was used to analyze low temperature chips of the film samples which were fro zen in liquid nitrogen and then delicately broken. For the TEM analysis of composites, ultrafine sections of samples with thicknesses of 50 nm were prepared using a microtome with a diamond knife. The thermophysical characteristics (temperature and melting and crystallization enthalpies) of the nanocomposites were determined by differential scan ning calorimetry (DSC). The DSC thermograms were obtained with a DSC7 apparatus (PerkinElmer) at the heating/cooling rate of 10 K/min; the measure ment accuracy was 5–8%. The gas permeabilities of the IPP films and com posites with a thickness of about 100 μm were defined by the volume of gas that passed through the film at the given difference in pressure for the known period of time at the constant flow rate. The tests were per formed at room temperature. The working part of a membrane had a diameter of 35 mm. The selectivity coefficient α(O2/N2) was taken to be equal to the ratio of permeability coefficients of neat gases. Thermal oxidation was studied with composite samples in a kinetic mode in the temperature range of 130–160°C and at an oxygen pressure of 300 mm Hg [17]. The kinetics of oxygen absorption was studied on a highly sensitive manometer unit. Solid KOH was used as an absorber for volatile products. A thermogravimetric analysis (TGA) of the sam ples was carried out on a TG 209 F1 Iris microbalance of Netzsch production (Germany) under dynamic conditions of heating in air. The samples were 5–8 mg. The analysis was performed at a heating rate of 10°C/min to 700°C. The temperature calibration was performed by the thermal effects of standard com pounds: In, Sn, Bi, Zn Al, and Ag. The specific electric resistance ρv of the composites was measured via a doubleprobe method using circu lar electrodes. The dielectric properties of the nano composites in the SHF range were studied using a res onator method on KSVN units of P2 series and rect angular resonators with the working mode of H01n. The working range of frequencies was 3.2–30 GHz. The dielectric properties were also measured in the fre quency range of 50–106 Hz in a cell with circular elec trodes using a Fluke PM6306 automated LCRmea suring tool. The resonator method is based on the measurement of the change in resonance frequency Δf and change in the quality of a resonator (1/Q –1/Q0) upon the placement of the analyzed material sample in
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Table 1. Effect of the polymerization conditions on the activity of the racMe2Si(4Ph2MeInd)2ZrCl2/MAO catalytic sys tem. Tpol = 60°C; volume of the liquid phase 0.1 L GNP, g
Zr, 10–7 mol
–
1.1
0.019 0.106 0.091 0.106
3.4 4.5 5.0 4.4
0.05 0.1
5.3 3.6
Al/Zr
Time, min
Yield, g
IPP 24 23 IPP/GNP (initial) 10720 15 18 9260 15 15 6930 10 4 8550 6 2.1 IPP/GNP (ultrasonic processing for 20 min) 6500 23 9 7010 30 5.4 20000
A
GNP in compos ite, wt % (vol %)
380
–
213 134 70 94
0.1 (0.05) 0.6 (0.3) 2.3 (1.1) 5 (2.5)
44 30
0.6 (0.3) 1.9 (0.9)
A is activity, PP kg/Zr mmol h.
a resonator cavity. The samples for measurements rep resented rectangular platelets with sizes of 15 × 1 × 0.5 mm. The measurement accuracy ranged from 10 to 15%. The reflection coefficient of the materials on a metallic substrate was measured using antenna horns bound to the waveguide of KSVN measuring tools. The sample sizes were somewhat greater than the antenna aperture. RESULTS AND DISCUSSION The initial GNP samples were studied by Raman spectroscopy. As is known, Raman spectroscopy makes it possible to estimate the ratio of ordered crys talline and nonordered carbon polymorphs. The so called G line in the Raman spectrum of ordered graph ite corresponds to the lattice vibrations of E2g symmetry in planes of graphene layers with sp2hybridized valence bonds. For disordered carbon polymorphs, the Raman spectra contain an additional peak which is called a Dline. This line is usually associated with the small sizes of the ordering regions and the presence of clear boundaries of crystallites, which disturb the selection rule by the wave vector upon Raman scattering. Figure 1 depicts the Raman spectrum of the GNP sample and the division of the envelope G line (1640–1540 cm–1) into components with maxima divided by 10 cm–1 and more. The data of Raman spectra is in good agreement with the dependence of the frequency of the G line on the number of layers in carbon particles, which was suggested in [18], and testifies that the GNP used in the study contains an essential part of the 2 to 5lay ered graphene. This is indicated by the presence of a GNP of a rather broad expressed mode of Raman scattering with a maximum at about 1586.5 cm–1 in the envelope G line (Fig. 1b). As for the high fre NANOTECHNOLOGIES IN RUSSIA
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quency components, for example, at 1597, 1607, and 1617 cm–1, their occurrence is connected with the presence of structural defects within a graphene layer that lead to the process of double Raman resonance and an appearance in the composition of the G line of a shoulder at 1605–1620 cm–1 (the socalled D1 line) [20]. The D1 line is inevitably present in the composi tion of the G line in the Raman spectra of graphenes derived via the reduction of GO with hydrazine hydrate. The ratio of intensities of the G and D lines in the Raman spectrum of the GNP used in the present study was used to define the size of crystallites along the a axis, which comprised 45 nm. The following formula was used in calculations: La = 2.4 × 10–10λ4IG/ID [19], where λ is the laser wavelength (nm). The XRD analysis of the starting GNP powder made it possible to estimate the crystal lattice param eters. According to the Xray patterns, d002 = 0.47 nm, Lc = 1.13 nm; i.e., the crystallites consist of 4 ± 1 graphene layers. Taking into account the data derived from the Raman spectra, the ratio of crystallite sizes along a and c axes was defined and comprised 40. The IPP/GNP composites were obtained via in situ polymerization in the medium of liquid propylene using an isospecific metallocene catalytic system. To create a composite material of the required composi tion, the amount of resulting polymer was controlled by varying both the filler amount and polymerization time and catalyst. The MC concentration composed (3–5) × 10–6 mol/L; Al/Zr was 13000–15000. The polymerization time ranged from 15 to 30 min. Table 1 shows data on the activity of the racMe2Si(2Me 4PhInd)2ZrCl2/MAO catalytic system in the synthesis of IPP/GNP nanocomposites. 2013
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D
(a)
600 G
400
200
0 3000
2500
2000 1500 1000 Raman shift, 1/cm
500
0
Raman intensity, arb. units (b)
1586.5
1597
1617
1607.5
GNP
1640
1620
1600 1580 Raman shift, 1/cm
1560
1540
Fig. 1. Raman spectra of the initial graphene nanoplatelets: (a) whole Raman spectrum and (b) Gline frequencies in the Raman spectrum.
The introduction of GNPs leads to a reduction in activity of the catalytic system. From the table it is obvious that the application of GNPs treated with ultrasound affords a more pronounced drop in activity reaching 30–40 kg of PP per Zr mmol in 1 h. IR spectroscopy showed that the introduction of the filler into the reaction medium practically does not affect the stereoregularity of IPP synthesized based on the catalytic system in use. For homopolymer and a sample with a GNP content of 0.05 vol %, a degree of polymer macrotacticity was determined which almost
does not change upon the introduction of GNPs and comprises 89–92%. The character of the filler distribution in nanocom posites with graphene platelets, both initial ones and those treated with US for 20 and 60 min, was analyzed via transmission microscopy. Figure 2 depicts TEM micrographs of films of IPP/GNP composites. TEM data demonstrated that the filler is rather uniformly distributed over the polymer matrix. From Fig. 2b it is apparent that, during 20 min of ultrasoni
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(a)
500 nm
(c)
(d)
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500 nm
100 nm
Fig. 2. TEM micrographs of IPP/GNP composites: (a) initial GNP (1.1 vol %), (b) GNP (0.9 vol %) treated with US for 20 min, and (c, d) GNP (0.1 vol %) treated with US for 60 min.
cally processing the starting GNP particles, their sizes essentially shrink. However, a further increase in the time of US impact to 60 min (Figs 2c, 2d) leads to the agglomeration of the filler particles and the formation of groups consisting of small particles divided by the polymer. The character of the GNP particle distribution in the resulting IPP/GNP composite materials can be also judged from SEM micrographs of lowtempera ture chips of the composites (Fig. 3). As follows from the presented micrographs, the composite samples obtained with GNP treated with US are rather uni form material; therewith, the most uniform distribu tion of nanoparticles in the polymer matrix is achieved upon US processing of the filler for 20 min (Fig. 3b). THERMOPHYSICAL PROPERTIES Table 2 presents data on the temperature and heat of the first and second melting and crystallization. It is obvious that the introduction of GNP almost does not affect the melting point of IPP. The melting enthalpy during the first melting changes insignificantly, but NANOTECHNOLOGIES IN RUSSIA
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during the second melting it increases for samples based on GNP treated with US (60 min), which indi cates a higher ordering of PP crystals after crystalliza tion. For the samples based on GNP treated with US for 20 min, another pattern is observed: the reduction of the melting enthalpy of the polymer, which is likely caused by the growth in the defectiveness of a crystal line structure of PP on more uniformly distributed particles of the filler. An increase in the filler content in all cases leads to the extension of exothermic curves, which implies that the GNP particles hamper the for mation of large PP crystallites. The introduction of GNPs, as was shown previ ously for composites based on PP and MWCNTs [8], has a nucleating effect, leading to the growth of the polymer crystallization point from 105.5 to 123.4°C. As can be seen from Table 2 and Fig. 4, the intro duction of graphene nanoplatelets preliminarily treated with US into the polymer matrix leads to a greater increase in the IPP crystallization point, which is likely connected with the reduction of GNP sizes in the composite material. 2013
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100 μm
(a)
100 μm
(b)
100 μm
(c)
Fig. 3. Micrographs of chips of IPP/GNP films derived via SEM (magnification ×100): (a) initial GNP (1.1 vol %), (b) GNP (0.9 vol %) treated with US for 20 min, and (c) GNP (1.3 vol %) treated with US for 60 min.
MECHANICAL PROPERTIES The stress–strain properties of the nanocomposites were studied upon quasistatic tension. From σ–ε dia grams, the values of elasticity modulus E, yield point σy, and ultimate strength and strain at break were deter mined. It was shown that a small filler content in the polymer matrix leads to an increase in the elasticity
The introduction of the filler leads to a drastic drop in the strain properties of the composites, which is more pronounced for composites with the initial GNPs (Fig. 5a). It is interesting that composites with the initial GNPs, the value of σy falls by 15% upon the introduction of 1.25 vol % of the filler, whereas upon the application of GNPs subjected to ultrasonic impact, σf practically does not change depending on the filler content in composite (Fig. 5b), which testifies to the change in the character of deformation upon the filling of PP with particles of higher dispersity and, consequently, a higher specific surface area [21].
Temperature of crystallization, Tcr, °C 2 120 1
114
108 0
modulus upon extension. Upon the introduction of 1–1.5 vol % of the GNPs, an increase in the elasticity modulus by 25–35% compared to the initial matrix polymer takes place. The dependences of the relative elongation upon breakage and yield points on the GNP content, both initial ones and those treated with US, are presented in Fig. 5.
BARRIER PROPERTIES 1 2 3 4 5 Filler content in composite, vol %
6
Fig. 4. Temperature of crystallization of IPP/GNP compos ites: (1) initial GNP and (2) GNP treated with US (60 min).
It is known that the application of layered carriers can complicate the gas permeability of composites on their base [22].To estimate the barrier properties of the IPP/GNP composites, their gas permeability towards O2 and N2 was measured. The corresponding coeffi
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Table 2. Melting points (Tm) and enthalpies (ΔHm) and crystallization points (Tcr) and enthalpies (ΔHcr) of IPP/GNP composites GNP content in composite, vol %
T m (°C)
ΔH m (J/PP g)
–
159.8
108.8
0.05 0.3 1.1 2.5
159.5 158.7 158.2 157.0
107.3 107.0 104.0 102.0
0.1 1.3 5.8
160.9 159.0 158.3
107.0 107.0 110.0
0.25 0.9 4.6
157.2 158.6 157.2
102.0 102.0 96.0
1
1
IPP 105.5 IPP/GNP (initial) 107.8 108.9 121.1 117.7 IPP/GNP (US 60 min) 113.5 122.5 123.4 IPP/GNP (US 20 min) 114.4 119.6 120.4
cients of permeability for films of nanocomposites with variable GNP contents of are presented in Table 3. It is obvious that, as the nanofiller content increases, the gas permeability falls. Interestingly, the selectivity coefficients α of the composites practically do not change compared to the unfilled PP. As is obvi ous from Fig. 6, an increase in GNP content to 4.6 vol % reduces the IPP permeability almost 3 times. The improvement of barrier properties is apparently closely related to the fact that the introduction into the poly mer matrix of filler platelets with high characteristic ratios leads to an increase in the effective length of the gasmolecule free path upon diffusion through a com posite film. INVESTIGATION OF THERMOOXIDATIVE DESTRUCTION VIA OXYGEN ABSORPTION It is known that different carbon polymorphs such as carbon black [23] and graphite [24] decelerate poly mer oxidation as a result of the cleavage of the kinetic chains on the filler surface. Therefore, the effect of GNPs on thermooxidative destruction of IPP in the composites was elucidated. Figure 7 demonstrates the absorption curves of oxygen at 130оC for composites obtained upon the application of GNPs, both the initial one and that treated with US. It is obvious that the method for filler production essentially affects the kinetics of oxygen absorption: both the induction period and maximal rate of oxidation vary. Table 4 lists data on the induction period and max imal oxidation rate for the resulting composites. NANOTECHNOLOGIES IN RUSSIA
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ΔHcr (J/PP g)
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Nos. 1–2
2
2
T m (°C)
ΔH m (J/PP g)
102.0
154.6
100.8
97.4 97.8 102.5 96.3
157.9 158.6 158.2 155.5
99.1 97.8 93.2 102.1
92.4 95.4 105.1
156.4 157.8 158.3
106.3 95.6 110.8
94.3 96.0 85.5
155.5 156.2 154.4
96.0 97.0 91.2
As can be seen from Table 4, the most stable to oxida tion with oxygen are composite samples with GNPs treated with US for 20 min. The effect of the carbon filler on the PP oxidation kinetics can be explained both by physical (change in the polymer structure) and chemical (participation in the chain initiation and ter mination) factors. It is known that, at the initial stage of noninhibited oxidation of polyolefins, the oxygen absorption kinet ics can be described by the parabolic law [17] N O2 = b 2(t − τ)2,
(1)
Table 3. Gas permeability towards N2 and O2 and selectiv ity coefficients of the nanocomposites with different GNP content GNP content P(N2) × 1017, P(O2) × 1017, α (O2/N2) in composite, mol m/m2 s Pa mol m/m2 s Pa vol % – 0.3 1.15 0.25 0.9 4.6 2013
IPP 5.35 30.0 IPP/GNP (initial) 4.85 25.9 4.04 21.3 IPP/GNP (US 20 min) 5.01 27.0 4.10 21.9 1.88 10.2
5.60 5.34 5.27 5.40 5.33 5.40
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Yield point σy, MPa 40
Pc/P0 1.00
(a)
38 2
36
0.75
34
1
32 30
0.50
28
1
26 0
0.25 0.50 0.75 1.00 1.25 Filler content in composite, vol % Relative elongation at break, ε/ε0
1.50
0
1.0 (b)
2
0.25
1 2 3 4 Filler content in composite, vol %
5
Fig. 6. Effect of GNP content on the relative gas perme ability of nanocomposites for (1) N2 and (2) O2. Pc and P0 are the permeabilities of composite and unfilled IPP, respectively.
0.5
linear termination, the kinetic curve anamorphosis is shifted along the t axis by the value of τ. The value of b represents the tangent of a slope of linear anamorpho sis of the kinetic curve in the coordinates of equation (3); τ can be found as a separation of the linear line on the t axis. The induction period is a main characteristic of oxidation that is directly connected with the rate con stant of linear termination k by equation [17]
2
0
1
0
0.2 0.4 0.6 Filler content in composite, vol %
0.8
Fig. 5. Dependence of relative yield point σy (a) and elon gation at break ε/ε0 (b) of composites on the filler content: (1) initial GNP and (2) GNP treated with US (20 min).
τ = k / ασ k2 k4[RH ]2 .
where N O2 is the amount of absorbed oxygen, t and τ are the time and induction period of oxidation, and b is the kinetic parameter, which is equal to
b = (ασk22k4[RH ]3 /8k6 )1/2 ,
(2)
where k2 and k6 are the rate constants of oxidation chain growth and termination, k4 is the rate constant of the hydroperoxide decomposition, α is the yield of hydroperoxide per mole of absorbed oxygen, and σ is the probability of the degenerated branching of the kinetic oxidation chains. To analyze the oxidation kinetics, the kinetic data for the IPP/GNP samples (US for 20 min) were calcu lated according to Eq. (3)
(N O2 )1/2 = b(t − τ).
(3)
In the absence of a linear cleavage, τ = 0, and an anamorphosis of the kinetic curve represents a linear line starting from the reference point. In the case of
(4)
The calculated values of b and τ are presented in Table 5. As can be seen from this table, an increase in temperature leads to an increase in the oxidation rate and parameter b, being the ratio of the constants of chain growth and termination. As the content of GNP grows, this parameter reduces, which indicates the participation of the carbon filler in the reactions of kinetic chain growth and termination. THERMOGRAVIMETRIC ANALYSIS The data on the temperature of the maximal rate of weight loss for all series of studied IPP/GNP materials are presented in Table 6. As can be seen from this table, according to TGA data obtained in air at small addi tives of the filler, an increase in the temperature of the maximal rate of weight loss is observed when com pared to analogous data for the initial IPP. An increase in the thermal stability of IPP upon the addition of variable types of nanofillers was observed in a number of works [21, 25, 26]. Usually this is associated with
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77
Weight, % 1
300
100
(a)
(a)
75
200
2 50
100
1 2 25
3
3
0 0 100
200 400 Oxidation time, min
200
Weight, % Amount of absorbed oxygen, mol/kg
600
700
100 (b)
2
3
300 400 500 Temperature, °C
3
75
(b) 4
2
50 1
1
5
25 3 0 100
0 0
200 400 600 800 10001200140016001800 Oxidation time, min
Fig. 7. Oxygen absorption curves upon the thermooxida tive destruction of IPP/GNP composites (130°C, O2 – 300 Hg mm): (a) GNP treated with US (60 min). Content of GNP, vol %: (1) 0, (2) 1.2, and (3) 5.6; (b) GNP treated with US (20 min). Content of GNP, vol %: (1) 0, (2) 0.1, (3) 0.23, (4) 0.85, and (5) 4.6.
the deceleration of the thermooxidative decomposi tion of the polymer due to the reduction of the rate of oxygen diffusion in the presence of the filler with the radical termination on the filler surface. An increase in the GNP content in the composites (to 2.5–5.8 vol %) leads to the reduction of the temperatures of ther mooxidative destruction. It can be assumed that, in the presence of the carbon filler, a thermally induced desorption of oxygencontaining groups from the sur face takes place which is more pronounced at higher contents of the carbon filler. Moreover, in the compos ites, as the content of the filler increases, the thickness of the polymeric layer on the particle surface substan tially reduces, which might facilitate oxygen access to polymer molecules and, as a result, the thermooxida tive destruction of the polymer can start earlier. Figure 8 shows the curves of weight loss according to the TGA data obtained in air and in argon for IPP and compos ites based on GNPs treated with US for 20 min. Inter estingly, according to the results derived in argon, an NANOTECHNOLOGIES IN RUSSIA
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1 400 500 Temperature, °C
2 600
700
Fig. 8. TGA results for composites with GNP treated with US for 20 min. GNP content, vol %: (1) 0, (2) 0.4, and (3) 4.6; (a) measurements in air and (b) measurements in argon.
increase in the GNP content in the composites does not lead to the reduction of the composite tempera tures of thermal destruction. ELECTROPHYSICAL PROPERTIES For all composites synthesized, electric conductiv ity at the constant current σdc and dielectric properties in alternative fields were studied both in the range of lower frequencies (in the region of 102–106 Hz) and in the microwave range (in the interval of 3 × 109–3 × 1010 Hz), depending on the GNP content. For com posites with the initial GNPs, a marked level of the electric conductivity of 1.9 × 10–7 (O cm)–1 possess materials with 2.4 vol % of the filler. The composites with GNPs treated with US have an electric conduc tivity of 1.5 × 10–10 and 1 × 10–6 (O cm)–1 at a GNP content of 2.9 and 5.6 vol %, respectively. This means that the value of percolation threshold comprises no less than 2–3 vol %, which is essentially lower than for the polymerizationfilled IPP with graphite [7]. Therewith, the IPP/GNP composites are charac terized by high values of dielectric permeability (ε') 2013
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Table 4. Data on the induction period and oxidation rate for IPP/GNP composites (130°C, O2 –300 Hg mm) GNP content in Induction period, composite, vol % min – 0.3 2.5 0.25 0.9 4.6 0.1 1.3 5.8
IPP 130 IPP/GNP (initial) 75 100 IPP/GNP (US 20 min) 420 530 790 IPP/GNP (US 60 min) 165 110 190
Oxidation rate, 104 mol/(kg s)
GNP content in composite, vol %
4.6
T = 130°C Oxidation rate, 103 mol/(kg s)
4.3 2.8
Induction period τ, min b
3.7 1.8 0.13
– 0.05 0.3 2.5 0.1 1.3 5.8 0.1 0.5 0.9 4.6
Tmax (air), °C
Oxidation rate, 103 mol/(kg s) Induction period τ, min
3.3 1.5 0.9
b
Tmax (argon), °C
IPP 363.6 IPP/GNP (initial) 363.3 366.2 242.2 IPP/GNP (US 60 min) 399.0 402.4 266.3 IPP/GNP (US 20 min) 395.2 396.6 396.7 226.2
0
0.9
4.6
0.46
0.18
0.013
130
530
790
0.012
0.0057
0.0004
0.74
1.1
0.37
T = 150°C
Table 6. TGA data on the temperatures of the maximal rate of weight loss (Tmax) GNP content in composite, vol %
Table 5. Results of thermooxidative destruction of the IPP/GNP (US 20 min) samples at different temperatures (O2 ⎯300 Hg mm)
469
50
140
200
0.026
0.017
0.005
Table 7. Coefficients of reflection (R) from the samples of IPP and IPP/GNP composites at different frequencies GNP content in composite, vol %
R, dB (26 GHz)
R, dB (30 GHz)
R, dB (35 GHz)
IPP
–
–
95%
IPP/GNP (initial)
– – – – – –
0.05
–0.3 (93%)
–0.2 (95.5%) –0.5 (89%)
0.35
–0.6 (87%)
–0.9 (81%)
1.1
–1.3 (74.1%) –2.0 (63.1%) –3.0 (50.1%)
–0.8 (83%)
IPP/GNP (US 20 min)
469 470 478 470
and dielectric losses (ε'') in the SHF range. The dielec tric permeability and dielectric losses grow with an increase in the degree of polymer filling, especially for the composites with the initial GNPs (Fig. 9). In the whole range of studied frequencies (3.2– 11 GHz), the dielectric permeability of the compos ites with GNPs subjected to ultrasonic impact is sub stantially lower than in the case of the application of the initial GNP which was not treated with US. The dependences of ε' on the GNP concentration were analyzed using a mathematical model [3]. The calcu lations showed that the effective form factor of parti cles of the initial GNP in the composite is equal 112; during grinding under US action, the form factor reduces to 48 (20 min) and further to 39 (60 min). The
1.0
–0.7 (85.1%) –0.9 (81.3%) –1.5 (70.8%)
4.6
–7.1 (19%)
–7.5 (18%)
–5.2 (30%)
analysis of the results makes it possible to conclude that the GNP in the composite is situated in the form of extended anisotropic particles (these can include the particle aggregates, since the model in use makes it possible to define the ratio between the maximal and minimal particle sizes rather than their absolute sizes). Probably just a change in the filler particleform coef ficient causes the differences in the properties of com posites based on GNPs subjected to US impact. A GNP as a filler possessing high electric conduc tivity imparts the polymer composites with the ability to absorb the highfrequency electromagnetic field. Therewith, an important factor is the fact that the per colation threshold has a relatively high value and the local electric conductivity is combined with the absence of essential reachthrough conductivity. This significantly increases the dielectric losses (Fig. 10a),
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COMPOSITE MATERIALS Dielectric permittivity, ε' 30
1 (a)
20
2
10 3
0
1 2 3 4 Filler content in composite, vol % Dielectric losses, ε'' 1 6
5
79
Dielectric permittivity, ε' 1 3.5 3.0 3 (a) 4 2.5 2 2.0 1.5 1.0 0.5 0 −0.5 −0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Filler content in composite, vol % Dielectric losses, ε'' 2 3 1 4
10 8
(b)
(b)
6 3 2
4 2
0 0
3
0 −0.5
1 2 3 4 Filler content in composite, vol %
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Filler content in composite, vol %
Fig. 9. Concentration dependences of dielectric properties at a frequency of 4.8 GHz for samples: (1) IPP/GNP, (2) IPP/GNP treated with US (60 min), and (3) IPP/GNP treated with US (20 min); (a) dielectric permittivity ε' and (b) dielectric losses ε''.
Fig. 10. Concentration dependences of dielectric proper ties of composites with GNP treated with US for 60 min at different frequencies ((1) 3.2 GHz, (2) 4.8 GHz, (3) 6.6 GHz, and (4) 11 GHz); (a) dielectric permittivity, ε'; (b) dielec tric losses, ε''.
and the dielectric permeability (Fig. 10b) still remains considerably lower than in the presence of reach through conductivity. For IPP and IPP/GNP composites, the values of the coefficients of reflection for electromagnetic waves of the SHF region from 26 to 35 GHz from the sam ples placed on a metallic substrate were determined (Table 7). Taking into account the fact that the film thickness was 300 μm, these data show that the resulting com posites possess good electrodynamic characteristics and seem to be promising materials for the creation of an electromagnetic irradiation shield of an absorbing type. The treatment of a GNP in the US field changes the effective form factor of the filler particles, which defines the frequency and width of the maximum of dielectric losses. Therefore, such a processing of the filler makes it possible to control the parameters of the absorption band of the electromagnetic irradiation shield (its width and median frequency). Hence, the production of composite materials based on PP and nanocarbon fillers via in situ poly
merization can provide a rather uniform distribution of the filler and obtain materials with a complex of improved characteristics. The comparatively low val ues of dielectric permeability and high dielectric losses in the SHF region allow one to consider the applica tion of the resulting materials as shields and filters for the electromagnetic irradiation of the corresponding range, as well as the semiconducting layers in power cables, rather promising [27, 28].
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ACKNOWLEDGMENTS This work was carried out using equipment of the Center for the Combined Usage of Moscow Institute of Physics and Technology (MIPT) and REC (Research and Education Center) Nanotechnology, MIPT. REFERENCES 1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Fir sov, Science 306 (5696), 666–669 (2004). 2013
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