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Article

Hybrid Electromagnetic Nanomaterials Based on Polydiphenylamine-2-carboxylic Acid

by
Sveta Zhiraslanovna Ozkan
1,*,
Aleksandr Ivanovich Kostev
1,
Galina Petrovna Karpacheva
1,
Petr Aleksandrovich Chernavskii
1,2,
Andrey Aleksandrovich Vasilev
1 and
Dmitriy Gennad’evich Muratov
1
1
A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky prospect, 119991 Moscow, Russia
2
Department of Chemistry Lomonosov Moscow State University, 1-3 Leninskie Gory, 119991 Moscow, Russia
*
Author to whom correspondence should be addressed.
Polymers 2020, 12(7), 1568; https://doi.org/10.3390/polym12071568
Submission received: 18 June 2020 / Revised: 4 July 2020 / Accepted: 12 July 2020 / Published: 15 July 2020
(This article belongs to the Special Issue Hybrid Polymeric Materials II)
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Abstract

:
Hybrid ternary nanomaterials based on conjugated polymer polydiphenylamine-2-carboxylic acid (PDPAC) (poly-N-phenylanthranilic acid), Fe3O4 nanoparticles and single-walled carbon nanotubes (SWCNT) were prepared for the first time. Polymer–metal–carbon Fe3O4/SWCNT/PDPAC nanocomposites were synthesized via in situ oxidative polymerization of diphenylamine-2-carboxylic acid (DPAC) by two different ways: in an acidic medium and in the interfacial process in an alkaline medium. In an alkaline medium (pH 11.4), the entire process of Fe3O4/SWCNT/PDPAC-1 synthesis was carried out in one reaction vessel without intermediate stages of product extraction and purification. In an acidic medium (pH 0.3), to prepare the Fe3O4/SWCNT/PDPAC-2 nanocomposites, prefabricated magnetite nanoparticles were deposited on the surface of obtained SWCNT/PDPAC-2. The phase composition of the nanocomposites does not depend on the synthesis reaction medium pH. The influence of the reaction medium pH on the structure, morphology, thermal, magnetic, and electrical properties of the obtained ternary nanocomposites was studied.

Graphical Abstract

1. Introduction

Modern technology demands the creation of new generation materials with a range of required properties. In recent decades, polymer nanocomposites have become a prominent area of current research promoted by rapid developments in both polymer science and nanotechnology [1,2,3,4,5,6]. Polymer nanocomposites have unique physicochemical properties owing to synergistic performance derived from each component. Such materials are of growing interest to researchers due to their great potentials for a wide range of applications [7,8,9,10,11,12,13,14,15,16]. Polymer–metal–carbon nanocomposites containing conjugated polymers, magnetic nanoparticles and carbon nanomaterials take a special place in this class of polymer nanocomposites [17,18]. Such ternary nanocomposites are superparamagnetic [19,20,21,22] and can effectively absorb electromagnetic radiation [20,21,22,23,24,25,26,27]. Due to the combination of their electrical and magnetic properties, these electromagnetic nanocomposites can be used for medical applications (immobilization of trypsin in a nanocomposite for protein digestion) [28]; to produce sensors [29]; supercapacitors [30,31,32]; as cathode materials for rechargeable batteries and fuel cells [33,34,35]; and as anticorrosive coatings [36].
Polyaniline, polypyrrole, polythiophene, and their derivatives are examples of conjugated polymers used to synthesize ternary nanocomposites. Graphene [30,31,32], reduced graphene oxide (RGO) [25,27,29,30,36,37], and carbon nanotubes (CNT) [19,21,26,35,38,39,40] can be used as carbonaceous nanofillers. The choice of carbon nanomaterials is explained by their excellent structural and functional properties [41,42]. Ternary nanocomposites use Fe3O4 [19,21,26,27,36,38,39,40], Co3O4 [25,30,32], Fe2O3 [29,37], FeCoO [35], and CoFe2O4 [31] as magnetic nanoparticles.
There are two most common approaches to the preparation of polymer–metal–carbon nanocomposites. The first way is the in situ oxidative polymerization of aromatic amines in the presence of magnetic nanoparticles immobilized on the surface of carbon nanomaterials [19,20,23,29,39,40]. The second approach provides the in situ oxidative polymerization of aromatic amines in the presence of carbon nanomaterials followed by the deposition of magnetic nanoparticles on the polymer surface [21,22,25,38]. Depending on the synthesis way, prefabricated magnetite nanoparticles uniform in size and shape were used to synthesize the Fe3O4/MWCNT and Fe3O4/RGO nanocomposites [19,20,23]. Graphene oxide was reduced with hydrazine [20,22,25]. Multi-walled carbon nanotubes (MWCNT) were pretreated with concentrated acids to functionalize their surface [21,23]. Modified Fe3O4 nanoparticles were added to the reaction solution containing MWCNT/PANI [21].
Earlier, we obtained hybrid nanomaterials based on poly-3-amine-7-methylamine-2-methylphenazine (PAMMP), single-walled carbon nanotubes (SWCNT), and Fe3O4 nanoparticles via in situ chemical oxidative polymerization of 3-amine-7-methylamine-2-methylphenazine hydrochloride (Neutral Red) in the presence of prefabricated Fe3O4/SWCNT nanocomposites [43]. The Fe3O4/SWCNT/PAMMP nanomaterials are superparamagnetic.
In the present study, ternary nanocomposites based on polydiphenylamine-2-carboxylic acid (PDPAC), Fe3O4 nanoparticles, and SWCNT were prepared for the first time. The Fe3O4/SWCNT/PDPAC nanomaterials were synthesized under different conditions of oxidative polymerization: in the interfacial process in an alkaline medium (pH 11.4) and in a 5 M solution of sulfuric acid (pH 0.3). A comparative analysis of the structure, morphology, thermal, magnetic, and electrical properties depending on the synthesis conditions was done.

2. Experimental

2.1. Materials

Diphenylamine-2-carboxylic acid (N-phenylanthranilic acid) (C13H11O2N) (analytical grade), sulfuric acid (reagent grade), aqueous ammonia (reagent grade), and chloroform (reagent grade), as well as iron (II) sulfate (high-purity grade) and iron (III) chloride (high-purity grade) were used as received. Ammonium persulfate (analytical grade) was purified by recrystallization from distilled water. The aqueous solutions of reagents were prepared with the use of distilled water. SWCNT from Carbon Chg, Ltd. (Moscow, Russia) were produced using the electric arc discharge technique with a Ni/Y catalyst (d = 1.4–1.6 nm, l = 0.5–1.5 µm).

2.2. Synthesis of Fe3O4/SWCNT

The synthesis of Fe3O4 nanoparticles immobilized on the SWCNT surface was carried out via hydrolysis of iron (II) and (III) salts mixture with a molar ratio of 1:2 in a solution of ammonium hydroxide in the presence of SWCNT at 60 °C. For that, 0.86 g of FeSO4·7H2O and 2.35 g of FeCl3·6H2O were dissolved in 20 mL of distilled water (Table 1). 0.03 g of SWCNT were added to the resulting solution and heated to 60 °C, then 5 mL of ammonium hydroxide NH4OH were added. The resulting suspension was heated on a steam bath to 80 °C and stirred for 0.5 h. The suspension was cooled at ambient temperature under intensive stirring for 1 h. The obtained Fe3O4/SWCNT nanocomposite was filtered off, washed with distilled water until neutral reaction of the filtrate, and vacuum-dried over KOH to constant weight.

2.3. Preparation of Fe3O4/SWCNT/PDPAC Nanocomposites

2.3.1. Preparation of Fe3O4/SWCNT/PDPAC Nanocomposites in the Interfacial Process in an Alkaline Medium

The following method was used to prepare the Fe3O4/SWCNT/PDPAC nanocomposite in the interfacial process in an alkaline medium. First, the synthesis of Fe3O4 nanoparticles of the required concentration, immobilized on the surface of SWCNT, was carried out as described in 2.2 at 55 °C. The amount of carbon nanotubes is CSWCNT = 3 and 10 wt. % relative to the monomer weight. Diphenylamine-2-carboxylic acid (DPAC) solution with the required concentration (0.15 mol/L, 1 g) in a mixture of an organic solvent—chloroform (60 mL) and NH4OH (5 mL)—was added to the obtained aqueous alkaline suspension of the Fe3O4/SWCNT nanocomposite. The process was conducted at 55 °C with continuous intensive stirring for 0.5 h. The suspension was cooled at room temperature with vigorous stirring for 1 h. After that, the suspension was thermostated under continuous stirring at 0 °C, and the aqueous solution of ammonium persulfate (0.3 mol/L, 1.96 g) was added. The solutions of the organic and aqueous phases were mixed in one shot without gradual dosing of the reagents. The volume ratio of organic and aqueous phases was 1:1 (Vtotal = 120 mL). The polymerization reaction continued for 3 h under intensive stirring at 0 °C. When the synthesis was completed, the reaction mixture was precipitated in a threefold excess of 1 M H2SO4. The obtained product was filtered off and washed repeatedly with distilled water until neutral reaction of the filtrate. Since the product was in a neutral basic form, it was vacuum dried over alkali (KOH) to constant weight. The yield was 1.058 g for the nanocomposite prepared at CFeSO4⋅7H2O = 0.86 g and CFeCl3⋅6H2O = 2.35 g, CSWCNT = 0.03 g (3 wt. % relative to the monomer weight).
Nanocomposites based on PDPAC, SWCNT, and magnetite nanoparticles obtained in an alkaline medium were marked as Fe3O4/SWCNT/PDPAC-1. PDPAC was synthesized under the same conditions [44] to compare the structure and properties of nanocomposites and polymer, and marked as PDPAC-1.
The Fe3O4/SWCNT/PDPAC-1 nanocomposites depending on synthesis conditions in alkaline medium were marked as FxCyP-1, where F—Fe3O4, C—SWCNT, P—PDPAC, x—the iron content according to ICP-AES data, and y—the SWCNT content relative to the monomer weight (Table 1).

2.3.2. Preparation of Fe3O4/SWCNT/PDPAC Nanocomposites in an Acid Medium

The following method was used to prepare the Fe3O4/SWCNT/PDPAC nanocomposite in an acidic medium. First, the SWCNT/PDPAC nanocomposite was synthesized. For that, SWCNT were added to the solution of DPAC (0.1 mol/L, 0.64 g) in 5 M H2SO4 and stirred in an ultrasonic bath at room temperature for 0.5 h. The mixture was heated to 25 °C. The amount of carbon nanotubes was CSWCNT = 3 and 10 wt. % relative to the monomer weight. Then, for the oxidative polymerization of DPAC in the presence of SWCNT, an aqueous solution of ammonium persulfate (0.2 mol/L, 1.368 g) in the same solvent (1/4 of total volume, Vtotal = 30 mL) was added dropwise to the suspension previously thermostated at 0 °C under continuous stirring. The synthesis was carried out during 3 h under intensive stirring at 0 °C. When the reaction was completed, the mixture was precipitated in 200 mL of distilled water. The obtained SWCNT/PDPAC nanocomposite was filtered off and washed repeatedly with distilled water until there was neutral reaction of the filtrate. The synthesis of Fe3O4 nanoparticles was carried out by a known method via hydrolysis of iron (II) and (III) salts mixture in a solution of ammonium hydroxide [45]. To synthesize Fe3O4 nanoparticles of the required concentration, FeSO4·7H2O and FeCl3·6H2O with a molar ratio of 1:2 were dissolved in 20 mL of distilled water and heated to 60 °C, then 5 mL of ammonium hydroxide NH4OH were added. The reaction mixture was heated on a steam bath to 80 °C and stirred for 0.5 h. Freshly prepared SWCNT/PDPAC nanocomposite was added immediately, without preliminary drying, to the obtained Fe3O4 suspension. The reaction mixture was stirred in an ultrasonic bath at ambient temperature for 0.5 h; then, to remove the liquid phase, it was heated to 60 °C in a crystallizing pan until precipitation. The obtained product was filtered off, washed repeatedly with distilled water to an uncolored filtrate, and to avoid dedoping, vacuum dried over CaCl2 to constant weight. The yield was 0.82 g for the nanocomposite prepared at CFeSO4⋅7H2O = 0.43 g and CFeCl3⋅6H2O = 1.175 g, with CSWCNT = 0.019 g (3 wt. % relative to the monomer weight).
Nanocomposites based on PDPAC, SWCNT, and magnetite nanoparticles obtained in an acidic medium were marked as Fe3O4/SWCNT/PDPAC-2. To compare the structure and properties of nanocomposites and polymer, PDPAC was synthesized under the same conditions [46,47] and marked as PDPAC-2.
The Fe3O4/SWCNT/PDPAC-2 nanocomposites depending on synthesis conditions in acidic medium were marked as FxCyP-2, where F—Fe3O4, C—SWCNT, P—PDPAC, x—the iron content according to ICP-AES data, and y—the SWCNT content relative to the monomer weight (Table 1).

2.4. Preparation of Suspensions for Magnetic Fluids

The suspensions of the Fe3O4/SWCNT and Fe3O4/SWCNT/PDPAC nanocomposites in ethanol were prepared. For this, 0.01 g of the nanocomposite was added to 5 mL of ethanol, mixed thoroughly, and left in a closed vessel to prevent evaporation of the solvent.

2.5. Characterization

The molecular mass of diphenylamine-2-carboxylic acid polymers was measured by GPC using Water’s 150C chromatograph (San Diego, CA, USA) equipped with PLgel 5 μm MIXED-C GPC columns (Santa Clara, CA, USA), using N-methylpyrrolidone as an eluent at 60 °C. The eluent flow rate was 1 mL/min. Volume of the injected sample was 150 mL. Calibration was performed on polystyrene. RI-detector was used. Accuracy of MM definition is ~5%.
The metal content in the nanocomposites was measured quantitatively by an inductively coupled plasma atomic emission spectroscopy method (ICP-AES) using a Shimadzu ICP emission spectrometer (ICPE-9000).
FTIR spectra were measured in air on a Bruker IFS 66v FTIR spectrometer (Karlsruhe, Germany) in the range of 400–4000 cm−1 and analyzed with Soft-Spectra software. The samples were prepared as KBr pressed pellets. Attenuated total reflection (ATR) FTIR spectra of the samples in the attenuated total reflectance mode were recorded using a HYPERION-2000 IR microscope (Bruker, Karlsruhe, Germany) coupled with the Bruker IFS 66v FTIR spectrometer in the range of 600–4000 cm−1 (150 scans, ZnSe crystal, resolution of 2 cm−1).
An X-ray diffraction study was performed in ambient atmosphere using a Difray-401 X-ray diffractometer (Scientific Instruments Joint Stock Company, Saint-Petersburg, Russia) with Bragg–Brentano focusing on CrKα radiation, λ = 0.229 nm. The results of X-ray diffraction analysis were used to calculate the size distribution of the coherent scattering regions of crystallites [48] in magnetic nanoparticles.
An electron microscopic study was performed using a LEO912 AB OMEGA transmission electron microscope (Bioz Inc., Los Altos, CA, USA) and a Hitachi TM 3030 scanning electron microscope (Hitachi High-Technologies Corporation, Fukuoka, Japan) with magnification up to 30,000 and 30 nm resolution. The size of nanoparticles is determined using the EsiVision software (eVision Software, The Hague, The Netherlands).
A vibration magnetometer was used to study the magnetic characteristics of the systems. The cell of the vibration magnetometer was designed as a flow quartz microreactor, which made it possible to study chemical transformations in the in situ mode [49]. Specific magnetization depending on the magnetic field value was measured; magnetic characteristics of the samples at room temperature were determined.
The AC conductivity was measured with a 6367A precision LCR-meter (Microtest Co., Ltd., New Taipei City, Taiwan) in the frequency range of 0.1 Hz–1.0 MHz.
Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/DSC1 (Giessen, Germany) in the dynamic mode in the range of 30–1000 °C in air and in the argon flow. The weight of the samples was 100 mg, the heating rate was 10 °C/min, and the argon flow velocity was 10 mL/min. Calcined aluminum oxide was used as a reference. The samples were analyzed in an Al2O3 crucible.
Differential scanning calorimetry (DSC) was performed on a Mettler Toledo DSC823e calorimeter (Giessen, Germany). The samples were heated at the rate of 10 °C/min in the nitrogen atmosphere, with the nitrogen flow rate of 70 mL/min. The measurement results were processed with the service program STARe supplied with the device.

3. Results and Discussion

3.1. Synthesis and Characterization of Nanomaterials

Synthesis methods were developed to prepare polymer–metal–carbon nanocomposites based on polydiphenylamine-2-carboxylic acid (PDPAC) first synthesized by the authors [44,46], single-walled carbon nanotubes (SWCNT), and magnetite nanoparticles (Fe3O4). Hybrid Fe3O4/SWCNT/PDPAC nanomaterials were synthesized via in situ oxidative polymerization of diphenylamine-2-carboxylic acid (DPAC) in an acidic medium and in the interfacial process in an alkaline medium. Table 1 presents the conditions for synthesis of the nanocomposites. For comparison, polymers of diphenylamine-2-carboxylic acid were synthesized under the same conditions: in a NH4OH solution in the presence of chloroform (PDPAC-1) and in 5 M H2SO4 (PDPAC-2). According to the GPC, the molecular mass of PDPAC-1 reached Mw = 2.6 × 104, the degree of polymerization was more than 120, and the polydispersity index was 2.2 [44]. The PDPAC-2 molecular mass achieved Mw = 1.1 × 104, the polymerization degree was above 50, and the index of polydispersity was 2.0 [46]. The oxidative polymerization in the heterophase system in the presence of chloroform reduces the probability of oxidative hydrolysis and macromolecules destruction owing to separation of reaction products and the oxidizer. This makes it possible to increase the molecular mass of PDPAC-1 by a factor of nearly 2.5 as compared to the homogeneous process.
Figure 1 shows the sequence of synthesis steps of the ternary nanomaterials.
In an alkaline medium (pH 11.4), the entire process of Fe3O4/SWCNT/PDPAC-1 synthesis was carried out in one reaction vessel without intermediate stages of disengagement and purification. The synthesis of Fe3O4 nanoparticles; their immobilization on the surface of SWCNT with the formation of Fe3O4/SWCNT nanocomposite; the anchorage of a monomer to Fe3O4/SWCNT by adding a DPAC solution in a mixture of chloroform and NH4OH; and in situ interfacial oxidative polymerization in the presence of ammonium persulfate were performed stepwise (Figure 1a).
In an acidic medium (pH 0.3), to prepare the Fe3O4/SWCNT/PDPAC-2 nanocomposites, the in situ oxidative polymerization of DPAC in the presence of SWCNT was performed followed by the deposition of prefabricated magnetite nanoparticles on the surface of obtained SWCNT/PDPAC-2 (Figure 1b).
The formation of Fe3O4/SWCNT/PDPAC nanocomposite materials was confirmed by X-ray diffraction (XRD), FTIR spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM).
The data presented in Figure 2 allow us to conclude that the phase composition of the nanocomposites does not depend on the synthesis reaction medium pH. According to the XRD data, irrespective of the synthesis method, diffractograms of Fe3O4/SWCNT/PDPAC identify reflection peaks of Fe3O4 in the range of scattering angles 2θ = 46.1°, 54.3°, 66.8°, 84.8°, 91.2°, and 102.2° (CrKα radiation) [43]. These diffraction peaks correlate to Miller indices (220), (311), (400), (422), (511), and (440), respectively, and refer to the cubic structure of Fe3O4 (JCPDS 19-0629) [50]. As can be seen in Figure 3, electron diffraction confirms the crystalline nature and phase composition of Fe3O4 nanoparticles. The diffraction rings correspond to Fe3O4 crystals. Since the single plane of SWCNT does not give a diffraction pattern, there is no carbon phase reflection peak at 2θ = 39.8°. An amorphous halo in the range of scattering angles 2θ = 20°–43° in the diffraction patterns of nanocomposites characterizes the polymer component (Figure 2b). The small wide diffraction peaks in the range of 2θ = 20°–43° characterize the crystalline DPAC oligomers contained in the polymer fraction of nanomaterials obtained in the acidic medium (Figure 2c), which is also confirmed by the DSC data. Removal of the crystalline low molecular weight fraction by washing nanocomposites with acetone can lead to a dedoping of the polymer component.
Table 2 demonstrates the volume fraction of materials. The fraction of amorphous and crystalline phases was calculated from the ratio of the areas of reflection peaks of the corresponding phases using standard samples. This technique is described in detail in [51]. As can be seen, the volume fraction of the crystalline low molecular weight fraction is negligible. Volume fraction of Fe3O4 in the nanocomposites reaches 62–95%.
We have previously shown that, during the hydrolysis of iron (II) and (III) salts mixture in the solution of ammonium hydroxide in the presence of SWCNT, the synthesis of Fe3O4 nanoparticles and their immobilization on the surface of SWCNT with the formation of the metal–carbon Fe3O4/SWCNT nanocomposite occur simultaneously [43]. The XRD, TEM, and SEM data confirm the formation of Fe3O4/SWCNT (Figure 2a, Figure 4a, and Figure 5a). According to the FTIR data, during the Fe3O4/SWCNT/PDPAC-1 synthesis in the interfacial process in an alkaline medium, the polymer interacts with the Fe3O4/SWCNT surface by binding the carboxylate ion and iron to form the Fe–OOC bond. The FTIR spectra of Fe3O4/SWCNT/PDPAC-1 nanomaterials (Figure 6a) show a hypsochromic shift of the absorption band at 561 to 578 cm−1, which corresponds to stretching vibrations of the νFe–O bond. The intensity of this band grows with the increase of Fe3O4 in the Fe3O4/SWCNT/PDPAC-1 nanocomposites. There is also a long-wavelength shift in the absorption band of the stretching vibrations of νC=O bonds in the carboxyl group to 1672 cm−1 compared to the location of this band in the polymer at 1682 cm−1 (Figure 7a).
It is shown that the size and shape of Fe3O4 nanoparticles do not depend on the pH of the synthesis reaction medium as well. Spherical Fe3O4 nanoparticles are clearly visible in TEM and SEM images of the Fe3O4/SWCNT/PDPAC nanocomposites (Figure 4 and Figure 5). However, the presence of an organic solvent (chloroform) in an alkaline medium leads to the formation of a polymer morphology with distinct cavities (Figure 5b). These cavities appear in places of chloroform droplets during the precipitation of the Fe3O4/SWCNT/PDPAC-1 nanocomposites in a sulfuric acid solution. According to the TEM data, the size of Fe3O4 nanoparticles in the nanomaterials is within the range of 2 < d < 14 nm irrespective of the obtaining method (Figure 4). The size distribution of coherent scattering regions in the Fe3O4 nanoparticles was calculated using the XRD data (Figure 8). In the Fe3O4/SWCNT/PDPAC nanocomposites, about 80–90% of Fe3O4 crystallites are up to 5 nm in size. According to the ICP-AES, depending on the synthesis conditions, the content of iron is CFe = 6.2–34.7%. In the obtained ternary hybrid nanomaterials, the problem of agglomeration of nanocomposite components is solved by the fact that the polymer structure contains functional groups that interact both with magnetite nanoparticles and with carbon nanotubes, preventing their aggregation.
Thus, regardless of the synthesis method, the Fe3O4/SWCNT/PDPAC-1 and Fe3O4/SWCNT/PDPAC-2 nanocomposites have the same phase composition: they contain PDPAC, carbon nanotubes, and Fe3O4 nanoparticles. However, the Fe3O4/SWCNT/PDPAC-1 nanomaterials prepared in the interfacial process in an alkaline medium are metal–carbon Fe3O4/SWCNT nanocomposites coated with PDPAC-1, whereas the Fe3O4/SWCNT/PDPAC-2 nanocomposites obtained in an acidic medium are SWCNT dispersed in a polymer matrix with Fe3O4 nanoparticles immobilized on the SWCNT/PDPAC-2 surface (Figure 4 and Figure 5).
Comparison of FTIR spectra of polymers and nanocomposites prepared under the same conditions (Figure 6 and Figure 7) shows that all the main bands characterizing the chemical structure of PDPAC are retained in the FTIR spectra of Fe3O4/SWCNT/PDPAC regardless of the obtaining method. As can be seen in Figure 6 and Figure 7, the chemical structure of the polymer component depends strongly on the pH of the synthesis reaction medium. It was established that during the synthesis of Fe3O4/SWCNT/PDPAC-1 nanocomposites in an alkaline medium (pH 11.4), the polymer chain grows via the C–C bonding of phenyl rings in 2- and 4-positions in relation to nitrogen (δC–H = 828 and 753 cm−1). This differs significantly from the type of C–C bonding that takes place during the synthesis of Fe3O4/SWCNT/PDPAC-2 nanocomposites in an acidic medium (pH 0.3), when the polymer chain grows via the C–C bonding of phenyl rings in the para position relative to nitrogen (δC–H = 892 and 803 cm−1). The chemical structure of the corresponding polymer components is shown in Figure 1. This dependence of the polymer matrix chemical structure on the reaction medium pH was observed during the synthesis of SWCNT/PDPAC nanocomposites in the absence of Fe3O4 nanoparticles [52].
As the content of carbon nanotubes increases, ATR FTIR spectra of Fe3O4/SWCNT/PDPAC-1 nanocomposites (Figure 7a) demonstrate a bathochromic shift in the PDPAC-1 skeletal vibration frequencies at 10–16 cm−1, which indicates the π–π* interaction of aromatic units of the polymer with the Fe3O4/SWCNT surface in an alkaline medium (stacking effect). The polymer formation on the Fe3O4/CNT surface provides the charge transfer, which is manifested in the shift of skeletal oscillation frequencies of the polymer [53,54,55].
As the content of carbon nanotubes grows, the FTIR spectra of Fe3O4/SWCNT/PDPAC-2 nanocomposites show the shift of absorption bands at 1659 and 1226 cm−1, associated with stretching vibrations of νC=O in COOH groups, as well as the shift of the band at 561 to 583 cm−1, corresponding to stretching vibrations of the νFe–O bond (Figure 6 and Figure 7). This indicates the interaction of carboxyl groups of SWCNT/PDPAC-2 with Fe3O4.

3.2. Electrical Characterization of Nanomaterials

Frequency dependence on the AC conductivity of Fe3O4/SWCNT/PDPAC nanocomposites was studied in the frequency range of 0.1 Hz–1.0 MHz (Figure 9). According to [56,57,58], the frequency dependence on the conductivity (σac) is described by evaluation:
σac = σdc + Aωn,
where ω = 2πf is the angular frequency,
  • σdc—the frequency independent (dc) part of conductivity,
  • n—the exponential parameter (0 ≤ n ≤ 1),
  • A—the thermally activated quantity.
  • A and n depend on the temperature and the volume fraction of the conducting component.
Table 3 lists the conductivity values of materials. It can be seen that the conductivity (σac) of nanocomposites depends on the pH of the synthesis reaction medium.
At low frequencies, the Fe3O4/SWCNT/PDPAC-1 nanocomposites as well as Fe3O4/SWCNT are characterized by weak frequency dependence on electrical conductivity. At the same time, the electrical conductivity of nanocomposites is several orders of magnitude higher than that of PDPAC-1 (3.1 × 10−12 S/cm) and amounts to 3.5 × 10−9 S/cm (F34C3) and 8.1 × 10−9 S/cm (F34C3P-1) at 0.1 Hz (Figure 9a). Apparently, the conductivity of Fe3O4/SWCNT/PDPAC-1 is contributed significantly by the metal–carbon Fe3O4/SWCNT nanocomposite, i.e., the increase in electrical conductivity of nanomaterials is due to the presence both of SWCNT and magnetite nanoparticles. As frequency grows, the electrical conductivity of F34C3P-1 increases gradually by two orders of magnitude to 8.7 × 10−7 S/cm.
The electrical conductivity of Fe3O4/SWCNT/PDPAC-2 nanocomposites at low frequencies is significantly higher than the conductivity of Fe3O4/SWCNT/PDPAC-1. This is due to the doping of the polymer matrix that occurs during the synthesis of nanocomposites in an acidic medium. As can be seen in Figure 9b, the electrical conductivity of neat PDPAC-2 does not depend much on frequency and amounts to (1.4–2.2) × 10−5 S/cm. The F7C3P-2 nanocomposite with a low magnetite content demonstrates an increase in its conductivity by an order of magnitude compared to PDPAC-2 up to 2.1 × 10−4 S/cm. Conductivity of carbon nanotubes contributes to the growth in electrical conductivity as well. This also explains the fact that conductivity (σac) does not depend on frequency at low magnetite content. However, with the increase of Fe3O4 content in the F14C3P-2 and F34C3P-2 nanocomposites, the electrical conductivity drops to 4.6 × 10−6 S/cm and 3.1 × 10−7 S/cm, respectively. An important role in reducing the conductivity of nanocomposites belongs to the electrical conductivity of Fe3O4 that has a significantly lower conductivity (~10−10 S/cm) compared to the doped polymer and CNT. At the same time, the frequency dependence on the electrical conductivity is observed. As frequency increases, the electrical conductivity of F14C3P-2 and F34C3P-2 rises to 3.1 × 10−5 S/cm and 3.6 × 10−6 S/cm, respectively.
As can be seen in Figure 9c, the electrical conductivity of the nanocomposites depends on carbon nanotubes concentration. With the growth in SWCNT content from 3 to 10 wt. %, the electrical conductivity of Fe3O4/SWCNT/PDPAC-1 increases by two orders of magnitude from 3.6 × 10−10 S/cm (F7C3P-1) to 1.2 × 10−8 S/cm (F7C10P-1). At high frequencies, the electrical conductivity of Fe3O4/SWCNT/PDPAC-1 nanocomposites reaches (1.2–9.6) × 10−7 S/cm, depending on the content of SWCNT. With the increase in SWCNT content from 3 to 10 wt. %, the conductivity of Fe3O4/SWCNT/PDPAC-2 rises from 2.1 × 10−4 S/cm (F7C3P-2) to 3.2 × 10−3 S/cm (F7C10P-2). The main contribution to the growth of electrical conductivity is made by carbon nanotubes, which is confirmed by the independence of the σac conductivity on frequency when Fe3O4 content is low.
Thus, as can be seen in Table 3, for all nanocomposites, the exponential parameter n lies in the range of 0 ≤ n ≤ 1, which is typical for disperse systems with a hopping mechanism of charge transfer [43,56,57,58,59,60]. Since the frequency dependence on the AC conductivity of nanocomposites shows an increase with the growth in current frequency, it can be argued that the effect of tunneling is minimal. At low frequencies, the DC conductivity plays an important role, the growth of which leads to a high frequency shift of the region of sharp increase in electrical conductivity.

3.3. Thermal Properties of Nanomaterials

TGA and DSC methods were used to study thermal stability of the nanocomposites depending on the preparation process. Figure 10 shows TGA thermograms of Fe3O4/SWCNT/PDPAC compared to PDPAC at heating up to 1000 °C in argon flow and in air. Table 4 presents the main thermal properties of the materials. As can be seen, the weight loss curves of the obtained materials have a stepwise pattern. Weight loss at low temperatures is associated with moisture removal, which is also confirmed by the DSC data. Figure 11 shows DSC thermograms of Fe3O4/SWCNT/PDPAC. There is an endothermic peak at ~105 °C on the DSC thermograms of nanocomposites. A small endothermic peak at ~273 °C in the Fe3O4/SWCNT/PDPAC-2 is associated with the decomposition of low molecular weight oligomers. When re-heated, this peak is absent. X-ray diffraction data also confirm the content of the crystalline low molecular weight fraction (0.75–2.35%) in the nanocomposites obtained in an acidic medium (Figure 2c, Table 2).
As seen in Figure 10, both in the PDPAC-1 and PDPAC-2 polymers, the weight loss at ~170 °C is connected with the removal of COOH groups [52]. In this temperature range, the DSC thermograms of polymers have an exothermic peak associated with decomposition [44,46]. The absence of weight loss in the Fe3O4/SWCNT/PDPAC nanomaterials in this temperature range is explained by the fact that the carboxylate groups of the polymer interact with either the Fe3O4 nanoparticles or the aromatic structures of SWCNT, depending on the method of nanocomposites preparation. The DSC thermograms of nanomaterials presented in Figure 11 do not show thermal effects in this temperature range.
The resulting nanomaterials are characterized by high thermal stability that exceeds the thermal stability of PDPAC significantly. Regardless of the preparation method, the Fe3O4/SWCNT/PDPAC nanocomposites lose half of their initial weight in an inert atmosphere at temperatures above 900 °C. At 1000 °C, both in an inert atmosphere and in air, the residue is ~50% (Table 4).

3.4. Magnetic Properties of Nanomaterials

Magnetic properties of Fe3O4/SWCNT/PDPAC nanomaterials were studied and the values of their main magnetic parameters were measured. The dependence of the magnetization on the magnitude of the applied magnetic field at room temperature is shown in Figure 12. Table 5 lists the values of the main magnetic properties of nanocomposites.
It can be seen that, regardless of the preparation method, the Fe3O4/SWCNT/PDPAC nanocomposites are superparamagnetic; this is due to the small size and high dispersion of magnetic nanoparticles [49,61]. The squareness ratio of the hysteresis loop is кS = MR/MS = 0. The saturation magnetization MS depends on the content of magnetite nanoparticles and reaches 31.6–39.4 emu/g depending on the synthesis method of nanocomposites. An increase in the concentration of SWCNT to 10 wt. % has little effect on the main magnetic parameters of the nanomaterials (Figure 12b).
The obtained hybrid nanomaterials can be promising as active components of magnetic fluids that are stable suspensions of magnetic nanoparticles in water or an organic solvent. Suspensions based on magnetic nanocomposites in ethanol were prepared. It was found that suspensions of Fe3O4/SWCNT/PDPAC-1 in ethanol remain stable for more than 8 months, whereas the Fe3O4/SWCNT/PDPAC-2 nanocomposites as well as Fe3O4/SWCNT dispersed in ethanol start sedimenting from the first minutes (Figure 13).
This behavior of the Fe3O4/SWCNT/PDPAC-1 and Fe3O4/SWCNT/PDPAC-2 nanocomposites is associated with the different chemical structures of the polymer component. The stability of the Fe3O4/SWCNT/PDPAC-1 suspension in ethanol is explained by the presence of a positive charge in the polymer shell structure of nanocomposites, which prevents the aggregation of magnetic nanoparticles. A positive charge can be formed both due to the charge transfer during the π-π* interaction of aromatic polymer units with the surface of Fe3O4/SWCNT, and due to the electronic interaction of delocalized π-electrons with the free d-orbitals of an iron atom.

4. Conclusions

Ternary nanomaterials based on polydiphenylamine-2-carboxylic acid (PDPAC), single-walled carbon nanotubes (SWCNT), and magnetite nanoparticles in an acidic medium and in the interfacial process in an alkaline medium were obtained for the first time. The influence of the reaction medium pH on the structure, morphology, phase composition, thermal, electrical and magnetic properties of the obtained hybrid nanocomposites was studied. It was shown that the phase composition of Fe3O4/SWCNT/PDPAC as well as the size and shape of Fe3O4 nanoparticles do not depend on the reaction medium pH. The size of spherical Fe3O4 nanoparticles is within the range of 2 < d < 14 nm. Regardless of the preparation method, the nanomaterials are superparamagnetic (кS = MR/MS = 0). The Fe3O4/SWCNT/PDPAC nanomaterials lose half of their initial weight in an inert atmosphere at temperatures above 900 °C. Electrical conductivity of the nanomaterials depends strongly on the synthesis method as well as on the concentration both of carbon nanotubes and of Fe3O4 nanoparticles. At the same time, at low frequencies, the electrical conductivity of Fe3O4/SWCNT/PDPAC-2 nanocomposites is significantly higher than the conductivity of Fe3O4/SWCNT/PDPAC-1 due to the polymer matrix doping in an acidic medium. It was shown that, unlike Fe3O4/SWCNT/PDPAC-2, the Fe3O4/SWCNT/PDPAC-1 nanomaterials prepared in an alkaline medium form magnetic fluids in ethanol; this is connected with differences in the chemical structure of the polymer component. Therefore, the results indicate that prepared hybrid electromagnetic nanomaterials have potential applications in the manufacture of sensors, supercapacitors, rechargeable batteries, anti-corrosion coatings, materials absorbing electromagnetic radiation, etc. The ability of powders of the prepared ternary hybrid nanocomposites to form suspensions in water and organic media allows to obtain the nanocomposite-based coatings on smooth and rough surfaces. Furthermore, these ternary nanocomposite-based suspensions can be applied for the impregnation of porous supports. These processing methods are important at fabrication of electrodes for chemical current sources and energy storage, electromagnetic shielding, sensor sensitive elements, anti-corrosion protection, etc.

Author Contributions

S.Z.O. proposed synthesis methods for novel hybrid nanomaterials, conducted the experiments, analyzed the data, interpreted the results, and prepared the manuscript paper. G.P.K. revised and edited the manuscript. A.I.K. conducted the experiments, analyzed the data, and prepared the figures. P.A.C. and A.A.V. conducted the investigation of nanocomposites by magnetometry and X-ray diffraction. D.G.M. measured the AC conductivity. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

This work was carried out within the State Program of TIPS RAS. This work was performed using the equipment of the Shared Research Center «Analytical center of deep oil processing and petrochemistry of TIPS RAS».

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Synthesis sequence of Fe3O4/SWCNT/PDPAC in alkaline (a) and acidic media (b).
Figure 1. Synthesis sequence of Fe3O4/SWCNT/PDPAC in alkaline (a) and acidic media (b).
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Figure 2. X-ray diffractograms of Fe3O4/SWCNT (a), PDPAC, and Fe3O4/SWCNT/PDPAC, prepared in alkaline (b) and acidic media (c).
Figure 2. X-ray diffractograms of Fe3O4/SWCNT (a), PDPAC, and Fe3O4/SWCNT/PDPAC, prepared in alkaline (b) and acidic media (c).
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Figure 3. Fe3O4 diffraction images of Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b).
Figure 3. Fe3O4 diffraction images of Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b).
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Figure 4. TEM images of Fe3O4/SWCNT (a) and Fe3O4/SWCNT/PDPAC, prepared in alkaline (b) and acidic media (c).
Figure 4. TEM images of Fe3O4/SWCNT (a) and Fe3O4/SWCNT/PDPAC, prepared in alkaline (b) and acidic media (c).
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Figure 5. SEM images of Fe3O4/SWCNT (a) and Fe3O4/SWCNT/PDPAC, prepared in alkaline (b) and acidic media (c).
Figure 5. SEM images of Fe3O4/SWCNT (a) and Fe3O4/SWCNT/PDPAC, prepared in alkaline (b) and acidic media (c).
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Figure 6. FTIR spectra of PDPAC, Fe3O4/SWCNT, and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b).
Figure 6. FTIR spectra of PDPAC, Fe3O4/SWCNT, and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b).
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Figure 7. Attenuated total reflection (ATR) FTIR spectra of PDPAC and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b).
Figure 7. Attenuated total reflection (ATR) FTIR spectra of PDPAC and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b).
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Figure 8. Fe3O4 crystallites size distribution in Fe3O4/SWCNT and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b).
Figure 8. Fe3O4 crystallites size distribution in Fe3O4/SWCNT and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b).
Polymers 12 01568 g008aPolymers 12 01568 g008b
Polymers 12 01568 g009a 550Polymers 12 01568 g009b 550
Figure 9. Frequency dependence of the conductivity for PDPAC, Fe3O4/SWCNT, and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a,c) and acidic media (b,c).
Figure 9. Frequency dependence of the conductivity for PDPAC, Fe3O4/SWCNT, and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a,c) and acidic media (b,c).
Polymers 12 01568 g009aPolymers 12 01568 g009b
Polymers 12 01568 g010 550
Figure 10. TGA thermograms of PDPAC and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b), at heating up to 1000°C in the argon flow and in air.
Figure 10. TGA thermograms of PDPAC and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b), at heating up to 1000°C in the argon flow and in air.
Polymers 12 01568 g010
Polymers 12 01568 g011 550
Figure 11. DSC thermograms of Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b), at heating in the nitrogen flow to 350°C (1—first heating, 2—second heating).
Figure 11. DSC thermograms of Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b), at heating in the nitrogen flow to 350°C (1—first heating, 2—second heating).
Polymers 12 01568 g011
Polymers 12 01568 g012 550
Figure 12. Magnetization of Fe3O4/SWCNT and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b), as a function of applied magnetic field at room temperature.
Figure 12. Magnetization of Fe3O4/SWCNT and Fe3O4/SWCNT/PDPAC, prepared in alkaline (a) and acidic media (b), as a function of applied magnetic field at room temperature.
Polymers 12 01568 g012
Polymers 12 01568 g013 550
Figure 13. Suspensions of Fe3O4/SWCNT (a) and Fe3O4/SWCNT/PDPAC, prepared in alkaline (b) and acidic media (c) in ethanol. (a,c)—in 5 min, (b)—in 8 months.
Figure 13. Suspensions of Fe3O4/SWCNT (a) and Fe3O4/SWCNT/PDPAC, prepared in alkaline (b) and acidic media (c) in ethanol. (a,c)—in 5 min, (b)—in 8 months.
Polymers 12 01568 g013
Table
Table 1. Conditions for synthesis of Fe3O4/SWCNT and Fe3O4/SWCNT/PDPAC nanocomposites.
Table 1. Conditions for synthesis of Fe3O4/SWCNT and Fe3O4/SWCNT/PDPAC nanocomposites.
Nanomaterials AbbreviationsFe, % *Preparation Method** Amount of SWCNT, gAmount of DPAC, g*** Amount of Iron Salts, g
Fe (II)Fe (III)
F34C3 ****61.2Synthesis of Fe3O4/SWCNT in
an alkaline medium		0.0300.862.35
F34C3P-133.5Polymerization of DPAC in the interfacial process in an alkaline medium
with Fe3O4/SWCNT		0.031.00.862.35
F18C3P-117.90.431.18
F14C3P-112.00.220.59
F7C3P-16.40.110.29
F7C10P-18.40.1
F34C3P-234.7Precipitation of Fe3O4 onto the surface of the SWCNT/PDPAC, prepared in
an acidic medium		0.0190.640.431.18
F18C3P-217.80.270.73
F14C3P-213.40.110.29
F7C3P-27.60.0540.15
F7C10P-26.20.064
* According to ICP-AES data. ** CSWCNT = 3 and 10 wt. % relative to the monomer weight. *** FeSO4·7H2O and FeCl3·6H2O. **** Fe3O4/SWCNT.
Table
Table 2. Volume fraction of materials.
Table 2. Volume fraction of materials.
MaterialsVolume Fraction, %
Amorphous
Polymer		Crystalline
Polymer		Fe3O4
PDPAC-1100.00--
F14C3P-137.69-62.31
F34C3P-15.24-94.76
F34C3 *--100.00
PDPAC-295.604.40-
F14C3P-228.992.3568.66
F34C3P-26.210.7593.05
* Fe3O4/SWCNT.
Table
Table 3. The conductivity values of materials.
Table 3. The conductivity values of materials.
Materials* σac, S/cmσdc, S/cmnA
PDPAC-13.1 × 10−121.1 × 10−72.8 × 10−120.758.5 × 10−12
F7C3P-13.6 × 10−101.2 × 10−72.4 × 10−100.821.7 × 10−13
F7C10P-11.2 × 10−89.6 × 10−71.1 × 10−80.961.9 × 10−12
F34C3P-18.1 × 10−98.7 × 10−76.8 × 10−90.995.4 × 10−13
F34C3 **3.5 × 10−92.5 × 10−62.3 × 10−91.009.7 × 10−13
PDPAC-21.4 × 10−52.2 × 10−51.0 × 10−50.456.9 × 10−9
F7C3P-22.5 × 10−42.7 × 10−42.1 × 10−40.313.0 × 10−7
F7C10P-23.7 × 10−33.3 × 10−33.2 × 10−30.512.0 × 10−8
F14C3P-24.6 × 10−63.1 × 10−54.3 × 10−60.649.0 × 10−11
F34C3P-23.4 × 10−73.6 × 10−63.1 × 10−70.741.3 × 10−9
* σ—The AC conductivity at 0.1 Hz and 1.0 MHz. ** Fe3O4/SWCNT.
Table
Table 4. Thermal properties of materials.
Table 4. Thermal properties of materials.
Materials* T5%, °C** T25%, °C*** T50%, °C**** Residue, %
PDPAC-1182/205400/324522/6630/19
PDPAC-2104/102232/243517/3960/35
F34C3P-1238/201403/628> 1000/91053/48
F34C3P-2180/225388/677542/ > 100048/52
* T5%, ** T25%, *** T50%—5, 25 and 50% weight losses (air/argon), **** residue at 1000 °C (air/argon).
Table
Table 5. Magnetic properties of nanomaterials.
Table 5. Magnetic properties of nanomaterials.
NanomaterialsFe, % *HC, OeMS, emu/gMR, emu/gMR/MS
F7C3P-16.404.800
F10C3P-18.4011.300
F14C3P-112.0015.700
F18C3P-117.9022.500
F34C3P-133.5031.600
F34C3 ** 61.2647.30.450.009
F7C3P-27.605.300
F7C10P-26.204.500
F14C3P-213.4012.100
F18C3P-217.8026.100
F34C3P-234.7039.400
* According to ICP-AES data, ** Fe3O4/SWCNT. HC—coercive force, MS—saturation magnetization, MR—residual magnetization.

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Ozkan, S.Z.; Kostev, A.I.; Karpacheva, G.P.; Chernavskii, P.A.; Vasilev, A.A.; Muratov, D.G. Hybrid Electromagnetic Nanomaterials Based on Polydiphenylamine-2-carboxylic Acid. Polymers 2020, 12, 1568. https://doi.org/10.3390/polym12071568

AMA Style

Ozkan SZ, Kostev AI, Karpacheva GP, Chernavskii PA, Vasilev AA, Muratov DG. Hybrid Electromagnetic Nanomaterials Based on Polydiphenylamine-2-carboxylic Acid. Polymers. 2020; 12(7):1568. https://doi.org/10.3390/polym12071568

Chicago/Turabian Style

Ozkan, Sveta Zhiraslanovna, Aleksandr Ivanovich Kostev, Galina Petrovna Karpacheva, Petr Aleksandrovich Chernavskii, Andrey Aleksandrovich Vasilev, and Dmitriy Gennad’evich Muratov. 2020. "Hybrid Electromagnetic Nanomaterials Based on Polydiphenylamine-2-carboxylic Acid" Polymers 12, no. 7: 1568. https://doi.org/10.3390/polym12071568

APA Style

Ozkan, S. Z., Kostev, A. I., Karpacheva, G. P., Chernavskii, P. A., Vasilev, A. A., & Muratov, D. G. (2020). Hybrid Electromagnetic Nanomaterials Based on Polydiphenylamine-2-carboxylic Acid. Polymers, 12(7), 1568. https://doi.org/10.3390/polym12071568

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