ISSN 19950780, Nanotechnologies in Russia, 2010, Vol. 5, Nos. 9–10, pp. 647–655. © Pleiades Publishing, Ltd., 2010. Original Russian Text © R.A. Dvorikova, L.N. Nikitin, Yu.V. Korshak, M.I. Buzin, V.A. Shanditsev, A.A. Korlyukov, I.S. Bushmarinov, S.S. Abramchuk, A.L. Rusanov, A.R. Khokhlov, 2010, published in Rossiiskie nanotekhnologii, 2010, Vol. 5, Nos. 9–10.
ARTICLES
FerroceneContaining Polyphenylenes as Precursors of Magnetic Nanomaterials R. A. Dvorikovaa, L. N. Nikitina, Yu. V. Korshakb, M. I. Buzina, V. A. Shanditseva, A. A. Korlyukova, I. S. Bushmarinova, S. S. Abramchuka, A. L. Rusanova, and A. R. Khokhlova a
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia b Mendeleev University of Chemical Technology of Russia, Miusskaya pl. 9, Moscow, 125047 Russia email:
[email protected] Received August 13, 2010
Abstract—New magnetic nanomaterials based on ferrocenecontaining polyphenylenes were obtained. The reaction of polycyclocondensation of 1,1'diacetylferrocene in the presence of triethyl orthoformate and p toluenesulfonic acid within 20–200°C under common conditions and in supercritical carbon dioxide (scCO2) was carried out for the first time to give highly branched ferrocenecontaining polyphenylenes as precursors of magnetic nanomaterials. The heating of the polyphenylenes in the range of 200–750°C resulted in the crosslinking of macromolecules and the formation of crystalline ironcontaining magnetic nanoparti cles in a carbonized polymer matrix. The magnetization of these nanomaterials was found to reach 32 G cm3/g in a magnetic field of 2.5 kOe. Transmission electron microscopy (TEM) showed the average size of magnetic nanoparticles to be from 6 to 22 nm. Xray diffraction studies of the materials revealed a complex composition of the ironcontaining magnetic nanoparticles (Fe, Fe3C, Fe2O3, Fe3O4, etc.). DOI: 10.1134/S1995078010090090
INTRODUCTION The increased interest that researchers have in nanomaterials in recent years is a result of their unusual physical and chemical properties when com pared with bulk samples. Of their physical properties, special attention is centered on the magnetic proper ties of nanomaterials. Currently, the unique properties of magnetic nanoparticles are being studied very much [1–9]. Magnetic nanomaterials are used in data recording and storage systems, permanent magnets, magnetic cooling systems, as magnetic sensors, etc. [1, 2]. Bio medical applications for magnetic nanoparticles, including drug delivery, are of great importance [10]. New applications of magnetic nanoparticles were revealed in magnetic resonance tomography; cell sort ing and target cell isolation; and in the processes of bioseparation, probing, enzyme immobilization, immunoassay, catalytic reactions, etc. [11–14]. The stabilization of nanoparticles in polymer matrices is a key factor; it provides one possible method for designing magnetic nanomaterials. In this case, composites are prepared by introducing mag netic particles into an oligomer or polymer matrix [1, 2]. Recently, we have suggested a new approach to pre paring nanosized composites by the structurization of ferrocenecontaining polymers with terminal reactive groups upon heating [15]. We obtained and character ized soluble ferrocenecontaining polyphenylenes
derived from 1,1'diacetylferrocene whose thermal treatment within 150–350°C gave rise to the forma tion of network structures and the appearance of iron containing nanoparticles and magnetic order. The synthesis of the highly branched ferrocene containing polyphenylenes was carried out for the first time by the cyclotrimerization of 1,1'diacetylfer rocene in a liquid and supercritical carbon dioxide (scCO2) as an environmentally friendly solvent [16]. Supercritical carbon dioxide as a “green solvent” for chemical processes is widely used for the synthesis and modification of polymers [17–24]. New approaches to the design of metal–polymer and, in particular, magnetic nanocomposites are attracting special inter est [25–33]. The work [16] reported the use of ptoluene sulfonic acid (pTSA) or a mixture of SiCl4 with C2H5OH as a catalyst in the presence of triethyl ortho formate as a ketalizing agent. The reaction was carried out in a liquid and scCO2 at a pressure of 20 MPa and a temperature of 20 or 50°C. Under these conditions, the yield of the polymers was 15–20% when pTSA was used as a catalyst. Heating at 300°C led to the crosslinking of the polymers to give crystalline iron containing magnetic nanoparticles with a magnetiza tion of up to 13 G cm3/g in a magnetic field of 2.5 kOe. The average size of nanoparticles was about 10–40 nm. The thermogravimetric study of the samples heated in inert atmosphere showed that 5% weight loss occurred
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Table 1. Properties of FPs prepared in the presence of pTSA
Sample FP1 FP2 FP3 FP4 FP5 FP6 FP7** FP8 FP9** FP10
Tempera ture, °C
Reaction time and keeping time, h
Catalyst amount, %
Yield, %
70 70 100 100 100 100 110 120 130 140
2.5 + 40 5 + 20 2+0 2+0 2 + 15 2 + 20 2 + 40 2+0 2 + 20 2 + 20
10 10 10 20 10 10 10 10 10 20
24 25 16 24 45 66 18 19 19 71
Elemental analysis, %* C
H
Fe
S
66.51 65.64 64.43 61.51 68.81 62.85 64.22 64.95 50.14 67.32
4.86 4.91 4.56 4.34 5.45 5.05 4.21 4.73 4.15 5.51
20.69 20.02 12.83 13.10 18.55 19.37 14.88 12.81 26.67 20.20
0.80 0.62 2.11 3.18 – 0.65 1.65 2.11 2.00 –
σHmax, G cm3/g (at tempera ture, °C) 9 (500°C) 10 (500°C) 21 (750°C) 21 (500°C) 13 (500°C) 26 (500°C) 14 (500°C) 18 (1000°C) 21 (750°C) 32 (750°C)
Notes: * Calculated for C126H102Fe9O6 (%): C, 66.88; H, 4.54; Fe, 22.21. ** The synthesis was carried out in ditolylmethane solution at the ratio of 1.2 mol of triethyl orthoformate per 1 acetyl group.
at approximately 400°C, while the weight of carbon ized residue was about 80% at 750°C. In this work we have studied the reaction of polycy clocondensation of 1,1'diacetylferrocene in solution and in supercritical medium within extended temper ature range for the first time with the aim of searching for optimal conditions for the preparation of the poly mers in higher yield and with improved magnetic properties. EXPERIMENTAL Ferrocenecontaining polymers (FPs) were obtained by the traditional procedure described in the [15] or in scCO2 (FPSCs) at a pressure of 20 MPa and a temperature of 70–200°C using a facility described in [16]. 1,1'Diacetylferrocene (1 g, 0.0037 mol), 3 mL (0.018 mol) of triethyl orthoformate, and 0.1 g of p toluenesulfonic acid was magnetically stirred in a two necked flatbottom flask equipped with a thermome ter and a reflux condenser at 70°C for 2.5 h and kept at ambient temperature for 40 h. The resultant dark brown precipitate was separated by filtration; washed with ethyl alcohol, water to pH = 6–7, and ethyl alco hol; and dried in a vacuum. Yield 0.34 g (24%). When the reaction temperature increased to 140°C, the yield grew to 71%. The properties of the resultant polymers are presented in Table 1. The synthesis of the polyphenylenes in scCO2 was carried out in a highpressure reactor with an internal volume of 10 cm3. After loading the monomers, the reactor was purged with CO2 and heated to the pre scribed temperature (±0.5°C). Carbon dioxide was delivered in the reactor with the aid of a High Pressure Equipment handpowered press (United States) and
the pressure was taken to the desired value (20 MPa), then magnetic stirring was started. The reaction time varied from 2.5 to 5 h. After the completion of the reaction and the cooling of the autoclave, stirring was stopped and pressure was decreased. The resultant polymer was further isolated; washed with ethyl alco hol, water to pH = 6–7, and ethyl alcohol; and dried. The maximum yield was 98% (Table 2). Magnetic nanomaterials were obtained by heating the samples of FPs and FPSCs in quartz ampules in the measuring cell of a magnetometer at different tem peratures (200–750°C) or by heating in tubes with a side arm in an argon flow at 250–500°C. The emergence of magnetic order in the course of thermal treatment of the polymers was studied with the use of a Fonertype vibration magnetometer. Electron micrographs of nanocomposites were obtained by transmission electron microscopy (TEM) with a LEO 912AB OMEGA instrument. In each case, the size distribution of nanoparticles was determined by the statistical treatment of data for 50–100 parti cles. An Xray diffraction study was performed using a Bruker D8 Advance diffractometer (λ[CuKα] = 1.54184 Å) equipped with a secondary monochroma tor within a 2θ angle range of 2°–90° with a step of 0.02° and an exposure time of 10 s per step at ambient temperature. The phase composition was determined with the aid of a DIFFRAC EVA program [34] and a PDF2 powder data base [35]. Thermogravimetric (TGA) studies were conducted on a MOM DerivatographC instrument (Hungary) using ~15mg samples at a heating rate of 10°C/min in argon atmosphere and in air.
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Table 2. Properties of FPSCs
Sample FPSC1 FPSC2 FPSC3 FPSC4 FPSC5 FPSC6 FPSC7
Tempera ture, °C
Reaction time and keeping time, h
Catalyst amount, %
Yield, %
20 50 70 140 140 160 200
0.5 + 60 0.5 + 60 2.5 + 20 2 + 20 2 + 20 2 + 20 2
10 10 10 10 60 10 10
20 18 32 31 98 37 92
Elemental analysis, %*
RESULTS AND DISCUSSION
C
H
Fe
S
σHmax, G cm3/g (at 500°C)
68.33 67.39 67.43 58.98 53.67 62.09 63.43
4.6 4.77 5.17 4.40 4.68 4.44 4.95
22.90 20.59 20.76 16.47 23.36 16.04 16.81
– – 0.25 2.73 0 2.01 0.80
12 6 6 15 12 12 14
pTSA as a catalyst. The reaction was conducted by the scheme reported in [16] (Scheme 1). The reaction scheme is idealized because the synthesis process also results in defect dipnone (βmethylchalcone), polyvi nylene, and other fragments [36] (Scheme 2).
We have studied the reaction of polycycloconden sation of 1,1'diacetylferrocene in the presence of a catalyst and a ketalizing agent with the aim of search ing for optimal conditions for preparing polymers with a higher yield and with improved magnetic properties. The ferrocenecontaining polymers were obtained both under common conditions and in scCO2 using
We have studied the effect of reaction conditions (temperature, reaction time, catalyst amount, and exposure time of the reaction mixture without stirring
O O
Fe
Fe
H+
O
T°C HC(OC2H5)3
Fe Fe
Fe
Fe
Scheme 1.
n
Fe
Fe
O
Scheme 2. NANOTECHNOLOGIES IN RUSSIA
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Specific magnetization, G cm3/g 30
Specific magnetization, G cm3/g 40 1
35
25 30 20
25
2 3
15
20
4
10
15
5
10 5
5
0
100
200 300 400 500 Heating temperature, °C
600
Fig. 1. Magnetization curves for ferrocenecontaining polymers obtained at different temperatures: (1) 100°C (FP6); (2) 130°C (FP9); (3) 110°C (FP7); (4) 120°C (FP8); (5) 70°C (FP1).
at ambient temperature) on the yield and properties of resultant polymers. Tables 1 and 2 show reaction con ditions and properties of prepared FPs and FPSCs. The reaction was carried out in the presence of triethyl orthoformate as a ketalizing agent, which also served as a solvent, and pTSA as a catalyst in the temperature range from 20 to 200°C. Ditolylmethane was used as a solvent in some cases. The polymers are dark brown powders partly soluble in organic solvents (dioxane, methylene chloride, and benzene). Table 1 shows that the yield of FP increased from 24 to 66% when the reaction temperature was enhanced up to 100°C. The content of Fe in the poly mers, as a rule, coincides with the calculated value. According to elemental analysis data, all polymer samples contain ~2% sulfur, which seems to result from the chemical binding of pTSA to the polymer, which explains the reduced content of carbon and iron. The maximum yield of FP was obtained at a reac tion temperature of 100°C and a catalyst concentra tion of 10%, whereas at catalyst content of 20%, the maximum yield was observed at 140°C. For the poly mers obtained at 100°C, the increase in the keeping time of the reaction mixture at ambient temperature leads to a considerable increase in yield from 16 to 66% (Table 1). This table shows that an increase in the catalyst content from 10 to 20% also enhances the yield of the polymer. The synthesis of FPs under scCO2 conditions (FPSCs) is of special interest. Table 2 shows the results of the synthesis. It is seen from this table that the yield of the polymers obtained in the presence of 10% pTSA grows from 18 to 92% when the reaction tem
0 400
450 500 550 600 650 700 Heating temperature, °C
750
800
Fig. 2. Dependence of magnetization of FP10 on heating temperature.
perature is elevated from 20 to 200°C. When the cata lyst amount is enhanced from 10 to 60%, the iron con tent in the polymer increased considerably and the yield of FPSC grew to quantitative. The IR spectra of the ferrocenecontaining poly mers show a band of C–C stretching vibrations at 1595 cm–1 typical for the 1,3,5substituted benzene ring. The IR spectra also show bands typical for fer rocene fragments (the band of C–H stretching vibra tions of medium intensity at 3090 cm–1, the band of C–H outofplane deformations of the substituted Specific magnetization, G m3/g 20 1, 2 15 3 10
4 1 5
5 2
0
100
200 300 400 500 Heating temperature, °C
600
Fig. 3. Magnetization curves for FPSCs obtained at differ ent temperatures: (1) 140°C (FPSC4); (2) 160°C (FPSC6); (3) 200°C (FPSC7); (4) 140°C, 60% pTSA (FPSC5); (5) 70°C (FPSC3).
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(а)
651
100 nm (c)
100 nm
Fig. 4. TEM micrographs: (a) FPSC7 after heating in argon at 250°C for 2 h, average particle size is 8 nm; (b) FP8 after heating in argon at 350°C for 2 h, average particle size is 13 nm; (c) FP6 after heating in argon at 500°C for 1 h, average particle size is 22 nm.
100 nm
(а)
100 nm
(b)
Fig. 5. TEM micrographs of FP10: (a) after heating in magnetometer cell at 500°C for 1 h, average particle size is 10.6 nm; (b) after heating at 700°C for 1 h, average particle size is 6.3 nm.
Cp ring in the region of 820–830 cm–1, and the band of doubly degenerate antisymmetric Fe–Cp stretching vibration at 484 cm–1). Intense absorption bands at 1275 cm–1 correspond to the symmetrical C–C stretching vibrations in disubstituted Cp rings, while the intense absorption bands in the region of 1670 and 1700 cm–1 may be attributed to the C=O stretching vibrations of dipnone fragments and terminal acetyl groups. A comparison of the IR spectra of a model compound, 1,3,5triferrocenylbenzene [16], with the polymers indicates that the characteristic bands of the model compounds are also detected for the polymer samples, as a rule, as broadened lines, which can be explained by the growth of molecular weight.
The heating of the polymers at 250°C and higher temperatures led to a decrease in the content of defec tive βmethylchalcone fragments and terminal acetyl groups, which was evidenced by the decrease in the intensity of bands at 1660 and 1700 cm–1 and an increase in the fraction of 1,3,5substituted benzene rings (the growth of intensity of the band of C–C stretching vibrations at 1600 cm–1). The simultaneous structurization of the polymers and the appearance of a phase of crystalline ironcontaining magnetic nano particles took place. The mechanism of formation of nanoparticles includes their selforganization at the nanolevel, which is determined by the structure of the initial polymer. A comparison of the magnetic proper ties of FPs upon heating (Fig. 1) shows that magneti zation is dependent on the temperature of synthesis
Table 3. Size distribution of particles depending on heating temperature Heating temperature, °C Average size of particles, nm Maximal size of particles, nm Minimal size of particles, nm
500 10.63 13.63 8.55
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675 6.66 22.07 2.09
700 6.32 20.56 2.14
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100
80 2 60
40 1 200
400
600
T, °C
Fig. 6. TGA curves for FP10 (1) in air and (2) in argon atmosphere at a heating rate of 5°C/min.
and thermal treatment. Figure 1 shows that the forma tion of magnetically ordered phase for FP6 prepared at 100°C occurs even at 200°C and maximum magne tization is 26 G cm3/g. For FP10 obtained at 140°C, the formation of a magnetically ordered phase begins upon heating in a magnetometer above 500°C, with magnetization reaching maximum value of 32 G cm3/g at 750°C (Fig. 2). Figure 3 shows the dependence that sample mag netization has on heating temperature for FPSCs pre
Intensity 2000
pared at 70–200°C. The data indicate that the forma tion of magnetically ordered phase begins at 200°C for samples FPSC4 and FPSC6 obtained at 140 and 160°C, respectively. The maximum magnetization (~15 G cm3/g) is observed for polymers FPSC4, FPSC6, and FPSC7 (Table 2) prepared at 140, 160, and 200°C; however, FPSC7 obtained at 200°C begins to show magnetic properties only at 300°C. The lowest magnetization was observed for the sample obtained at 70°C (FPSC3, Table 2), like under regu lar conditions. The samples obtained in scCO2 exhib ited lower magnetic properties than FPs, which may be caused by the higher thermal stability of FPSCs. According to TEM data, the heated samples con tain ironcontaining nanoparticles with average sizes of 6–22 nm, depending on the preparation conditions and heating temperature, that are uniformly distrib uted in the polymer matrix. It should be noted that an increase in the heating temperature from 250 to 500°C for polymers obtained in the presence of 10% pTSA led to a growth in nanoparticle size from 8 to 22 nm. Figures 4a and 4b show the electron micrographs of FPSC7 and FP8 samples heated at 250 and 350°C in argon for 2 h. The size of nanoparticles increased from 8 to 13 nm when the temperature rose. Figure 4c dis plays the electron micrograph of FP6 heated at 500°C in argon flow for 1 h; the average size of nano particles is 22 nm. The dimension of nanoparticles for FP10 obtained with the use of 20% of the catalyst and heated at 500°C was onehalf the size, 10.6 nm
a Fe3O4 331
b
1800
c
1600
Fe3O4 440 Fe3O4 511
Fe3O4 220
1400
Fe3O4 400
1200 1000 800 600 400 200 0 10
20
30
40 2θ,°
50
60
70
Fig. 7. Diffractograms of (b) FPSC2 and (c) FPSC1 samples heated in a magnetometer cell at 300°C for 5 h and (a) FP6 sam ple heated in argon at 500° for 1 h with a subtracted background. Diffraction maxima of Fe3O4 are denoted. NANOTECHNOLOGIES IN RUSSIA
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(Fig. 5a), the growth of the temperature to 700°C; i.e., in the region of magnetization growth, the average size of nanoparticles decreased to 6.3 nm (Table 3, Fig. 5b). Table 3 exhibits data on the variation in nanoparti cle size within a narrow temperature range for an FP 10 sample that was heated at 500, 600, 650, 675, and 700°C. The average size of particles decreased with the heating temperature. The spread of the particle size within the sample heated at 500°C was negligible, whereas it was about 15 nm at heating temperatures above 500°C. The thermal and thermaloxidative stability of the obtained ferrocenecontaining polymers were studied by TGA. The main features of thermal and thermal oxidative destruction, such as the temperature range of thermal transformations and the higher extent of deg radation upon TGA in air than in inert atmosphere, were found to be the same for all the ferrocenecon taining polymers we studied. As an example, Fig. 6 shows TGA curves for FP10 polymer whose degrada tion products contained the largest amount of mag netically ordered phase. It is seen that weight loss is ~70% when TGA is performed in air, whereas in an inert atmosphere the sample loss is much smaller, only 30% of initial weight. The beginning temperatures of weight loss by samples in air and in argon almost coin cide (190°C), however, as the temperature rises, the processes of thermal degradation in air slow down and the sample begins to decompose rapidly only at 400°C. This behavior may result from the formation of a dense spatial network in the polymer matrix in the beginning stage of thermaloxidative degradation, which hampers the escape of gaseous degradation products. In an inert atmosphere, the sample loses weight slowly as the temperature rises in a wide range up to 700°C, after which the weight remains constant. An Xray diffraction study of FP6, FPSC2, and FPSC1 samples showed that iron in FPSC2 and FP 6 samples is present only as Fe3O4 [37] (Fig. 7), whereas the crystalline phase of FPSC1 sample has a rather complex composition: 14.6% of Fe3C, 43.3% of Fe3O4, 36.6% of graphite 2H, and 5.5% of elemental Fe. The sizes of Fe3O4 crystallites determined by the Sherer formula are 10.6 or 14 nm in FPSC1 and FPSC2 or FP6, respectively. The decrease in the share of Fe3O4 agrees well with the decrease in the magnetic susceptibility of FPSC1 sample. Figure 8 shows diffractograms of FP10 samples heated at 600 and 675°C in a magnetometer cell. The samples are composed mainly of magnetite Fe3O4, cementite Fe3C, and wustite Fe0.97O. The sample heated at 600°C contained 43% Fe3O4, 25% Fe3C, and 32% Fe0.97O, while the sample heated at 675°C was made up of 11% Fe3O4, 64% Fe3C, and 25% Fe0.97O. It was impossible to determine the crystallite size in NANOTECHNOLOGIES IN RUSSIA
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Intensity 500 450 Fe3C310
400 350 300 250 200 150 100 50 0 10
20
30
40 2θ,°
50
60
70
Fig. 8. Diffractograms of FP10 samples heated in a mag netometer cell at (b) 600°C and (a) 675°C with a sub tracted background. The most intense cementite line is denoted; other lines correspond to wustite and magnetite.
these samples because of strong overlapping of lines, but it was less than 10 nm for all components. CONCLUSIONS Highly branched ferrocenecontaining polymers were prepared by the polycyclocondensation of 1,1' diacetylferrocene in solution at 70 to 140°C, and the best conditions were revealed for their synthesis with the highest yield up to 71%. When the reaction was carried out in scCO2, the yield of the polymers increased to quantitative. Nanocomposites with magnetization up to 32 G cm3/g in a magnetic field of 2.5 kOe were obtained for the first time by the structurization of the obtained polymers upon heating within 250 to 750°C. According to TGA data, the temperature of the onset of degradation coincides with the temperature at which the magnetic properties of the polymers begin to appear. The average size of magnetic nanoparticles involved in polymer matrices was determined by TEM. The fundamental possibility of controlling the size of nanoparticles by varying the preparation condi tions and temperature of structurization of the poly mers has been shown. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research (project no. 08–03–00294), the Russian Academy of Sciences (the complex pro gram of the Division of Chemistry and Material Sci ence, Russian Academy of Sciences, “Design of New 2010
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