Magnetic tetraethylenepentamine-functionalized graphene oxide to prepare new methyl rich bisphenol-based soluble and heat-resistant polyamide ether nanocomposites: Synthesis and characterization

Abstract

The new magnetic and heat-resistant polyamide ether (PAE)/graphene oxide (GO) nanocomposites were prepared by blending PAE with modified GO. Furthermore, a novel PAE containing flexible ether, methyl, and triphenylmethane moiety in the main chain of PAE was prepared by direct polycondensation reaction of a diamine and 4,4′-(butane-1,4-diylbis(oxy))dibenzoic acid (9). The prepared PAE was characterized using various techniques. The obtained PAEs were readily soluble in various aprotic polar solvents at room temperature. The incorporation of bulky aryl pendant groups and flexible ether linkage into the backbones of polyamide may provide beneficial effects for solubility, as this approach produces a separate chain of the polymer, which makes weaker of hydrogen bonds. GO, which synthesized by the Hummer method, was modified with tetraethylenepentamine (GO-TEPA) and sequentially magnetized with Fe₃O₄ nanoparticles via chemical coprecipitation (GO-TEPA@Fe₃O₄). The PAE/GO/TEPA@Fe₃O₄ nanocomposite films, PAE/GO-TEPA@Fe₃O₄, were also synthesized as a heat-resistant and superparamagnetic nanocomposites, which were characterized using different analyses, such as field emission scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray, thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), and Fourier transform infrared techniques. TGA of nanocomposites exhibit higher degradation temperature than that of neat PAE, which is an indication of high levels of interfacial interaction and dispersion.

Keywords

Polyamide ether magnetic nanocomposite heat resistant graphene oxide bisphenol

Introduction

Over the last years, composite materials consisting of polymeric matrix have been a topic of scientific research and industrial development. The nanocomposite materials are defined as particle-filled polymers in which at least one dimension of the dispersed particles is in the nanometer range.^1–3 Polymeric nanocomposites are commonly used due to having different properties, such as thermal, mechanical, and barrier properties and flame retardancy.^4–6 Specifically, aromatic polyamides exhibit excellent thermal stability and good mechanical properties, which makes them useful for their study as high-performance polymers.^7,8 One of the major problems with high-temperature polymers is their poor process-ability caused by low solubility in organic solvents. The general procedure for improving the solubility and process-ability of aromatic polyamides includes the incorporation of aliphatic and heteroaromatic monomers into the polymeric backbone.^9–15 Polymer-based magnetic nanocomposites have received increasing attention due to their potential applications in a wide range of industrial fields, such as cell separation and magnetic resonance imaging, magneto-optical solid devices, RNA and DNA purification, and targeted drug delivery.^16–21 Carbon nanostructures have received considerable attention because of their excellent properties.^22–24 Single-layer graphene was produced by mechanical exfoliation of bulk graphite²⁵ and by chemical vapor deposition.²⁶ These ways, though preferred for accurate device assembly, are less effective for bulk-scale manufacturing.²⁷ The most common approach to graphite exfoliation is the use of chemical oxidants as a driving force for exfoliation.^28,29 Thus, incorporation of graphene to a polymer matrix has achieved a number of improved properties with potential applications in many areas, such as biosensor, antifouling coatings, supercapacitor, and medicine.^30–34 In addition, loading of various functional groups into the surface of the graphene oxide (GO) could be useful to obtain better compatibility between the GO and polymer.^35,36 Among the chemical groups, the amine group has high reactivity and can react with many chemicals including polymers.³⁷ Many researchers functionalized the GO with amine groups and found that amine-modified graphene possessed adsorption performance and excellent catalytic activity.^38,39 One of the main objects of this research was to prepare a new type of heat-resistant and magnetic nanocomposite containing GO-TEPA@Fe₃O₄ nanoparticles in the polymer matrix. Since the addition of inorganic particles into polymers leads to improved thermal properties, we used graphene, which is the equivalent of inorganic materials in terms of increasing heat-resistance effect. Furthermore, to improve the dispersion stability of nanoparticles in the polymer, the modification of graphene surface was investigated. Interesting findings in improvement of thermal properties of the polyamide ether (PAE)/GO-TEPA@Fe₃O₄ nanocomposites are discussed.

Experimental

Materials

Triphenyl phosphite (TPP), 4-chlorobenzaldehyde, 2,5-dimethylphenol, p-toluenesulfonic acid (p-TSA), 1-fluoro-4-nitrobenzene, potassium carbonate (K₂CO₃), palladium charcoal (Pd/C) 10%, hydrazine monohydrate 80%, 4-hydroxybenzoic acid, 1,4-dibromobutane, sodium hydroxide, sodium nitrate (NaNO₃), potassium permanganate (KMnO₄), hydrogen chloride, graphite powder, sulfuric acid (H₂SO₄), hydrogen peroxide (H₂O₂), thionyl chloride (SOCl₂), TEPA, ferric chloride (FeCl₃ 6H₂O), ferrous chloride (FeCl₂ 4H₂O), aqueous ammonia 25%, and calcium chloride (CaCl₂) were purchased from Sigma-Aldrich Chemical Co (Milwaukee) and Merck Chemical Co (Germany). All chemicals were used as received without any further purification. Pyridine (Py), N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), and dimethylformamide (DMF) were purchased from Merck Chemical Co and dried with molecular sieves before use.

Measurements

Fourier transform infrared (FTIR) spectra were recorded on a Galaxy series FTIR 5000 spectrophotometer over the range of 400–4000 cm⁻¹. Nuclear magnetic resonance (NMR) spectrum was recorded at room temperature using Bruker (Germany) Avance spectrometer operating at 300 MHz for proton nuclear magnetic resonance (¹H NMR) and at 75 MHz for carbon-13 nuclear magnetic resonance (¹³C NMR) using deuterated dimethyl sulfoxide (DMSO-d₆). Melting points were measured using the capillary tube method with an Electrothermal 9200 apparatus. X-ray diffraction (XRD) patterns were recorded on Philips Xpert MPD X-ray (Germany) at 40 mA (copper K_α radiation). A Hitachi S-4700 (Japan, operating voltage; 15 kV) scanning electron microscope (SEM) was used to measure the morphologies of samples. Thermal gravimetric analysis (TGA) and differential thermal gravimetric (DTG) analysis were conducted with a Mettler TA4000 system, and experiments were carried out on approximately 10 mg of samples in flowing nitrogen atmosphere at a heating rate of 10°C min⁻¹. The UV-Vis absorption spectrum was obtained via a Perkin-Elmer Lambda 15 (USA) spectrophotometer. The magnetic measurements were performed using Vibrating Sample Magnetometer Model 7300 VSM system (Lake Shore Cryotronic, Inc., Westerville, Ohio, USA).

Monomer synthesis

Synthesis of 4,4′-((4-chlorophenyl)methylene)bis(2,5-dimethylphenol) ( 3 )

4-Chlorobenzaldehyde (0.98 g, 7 mmol), 2,5-dimethylphenol (2.13 g, 17.5 mmol), and p-TSA (0.13 g, 0.7 mmol) were stirred at 90°C for 2 h under thermal solvent-free condition. The reaction mixture was cooled to room temperature. Then, the precipitate was collected by filtration, washed thoroughly with the mixture of EtOH-H₂O(3:1), and dried in a vacuum oven (2.23 g, 87%). Melting point: 202–205°C.

Synthesis of 5,5′-((4-chlorophenyl)methylene)bis(1,4-dimethyl-2-(4-nitrophenoxy)benzene) ( 5 )

A mixture of 4,4′-((4-chlorophenyl)methylene) bis (2,5-dimethylphenol) (3) (1.83 g, 5 mmol), K₂CO₃ (1.38 g, 10 mmol), 1-fluoro-4-nitrobenzene (2.11 g, 15 mmol), and DMF (30 mL) were refluxed for 48 h. After cooling to room temperature, the yellow solid was collected by filtration and washed with hot ethanol and dried under vacuum oven to afford the compound (5) (2.70 g, 89%). Melting point: 225–229°C.

Synthesis of 4,4′-((((4-chlorophenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline ( 6 )

Diamine (6) was synthesized by the catalytic reduction of dinitro compound 5. In a 100-mL round-bottom flask, 5,5′-((4-chlorophenyl)methylene)bis(1,4-dimethyl-2-(4-nitrophenoxy)benzene) (5) (2.47 g, 4 mmol) and 0.19 g of Pd/C 10% were suspended in 30 mL of ethanol. Over a period of 0.5 h, 5 mL of hydrazine monohydrate 80% was added to the above mixture. Subsequently, the reaction was maintained at reflux temperature for another 2 h. The solution was filtered to remove the catalyst, and the filtrate was poured into distilled water. The precipitated product (6) was collected by filtration and dried under vacuum. The yield was 1.91 g (87%). Melting point: 177–181°C.

Synthesis of 4,4′-(butane-1,4-diylbis(oxy))dibenzoic acid ( 9 )

The 4,4′-(butane-1,4-diylbis(oxy))dibenzoic acid was prepared according to the reported literature.⁴⁰

Polymer synthesis

Into a 100-mL round-bottom flask equipped with a magnetic stirrer, the equal mole of diamine (6) 1.48 g (2.7 mmol) and diacid (9) 0.89 g (2.7 mmol), some dried CaCl₂, TPP (7.2 mL), Py (3 mL), and NMP (7.5 mL) were placed, respectively. The reaction mixture was heated to refluxing at 140°C. After cooling, the viscous solution was poured into 500 mL of methanol. The precipitated polymer was collected by filtration, washed with hot methanol, and dried under vacuum to afford 1.88 g (80%) of PAE (10). Analysis calculated for C₅₃H₄₇ClN₂O₆: C, 75.48; H, 5.62; N, 3.32, found: C, 75.04; H, 5.76; N, 3.24.

Synthesis of nanostructures

Synthesis of GO nanosheets

GO was synthesized from graphite powder using a modified Hummer’s method.⁴¹ Firstly, 1 g of graphite powder and 0.5 g of NaNO₃ were mixed together followed by the addition of 23 mL of concentrated H₂SO₄. Under vigorous agitation, KMnO₄ (3 g) was added gradually to keep the temperature of the suspension lower than 10°C. The mixture was stirred at 35°C for 2 h, and distilled water was added. Later, 30% H₂O₂ solution was added to stop the oxidation process. The GO formed was washed with distilled water and absolute ethanol, respectively, followed by filtration and drying, GO sheets were thus obtained.

Synthesis of tetraethylenepentamine-functionalized graphene oxide nano-sheets (GO-TEPA)

A suspension of GO (500 mg) in 10 mL of SOCl₂ was heated to 70°C for 24 h under ultrasonic irradiation, and then, the solvent was evaporated under reduced pressure. In a 100-mL round-bottom flask equipped with a stirring bar, the obtained GO-Cl, 30 mL of THF and 400 mg of TEPA were added. The solution was stirred at 60°C for 2 h under ultrasonic irradiation again. After cooling to room temperature, the precipitate was filtered off and washed thoroughly with ethanol and dried at 60°C under vacuum.

Synthesis of magnetic tetraethylenepentamine-functionalized graphene oxide nanocomposite (GO-TEPA@Fe₃O₄)

GO-TEPA (100 mg) was dispersed in deionized water using ultrasonic to produce a homogeneous suspension. Then, 35 mg of FeCl₂·4H₂O and 50 mg of FeCl₃·6H₂O were dissolved in 50 mL GO-TEPA aqueous suspension. The mixture was heated under nitrogen (N₂) at 90°C for 1 h. Meanwhile, 1 mL of concentrated ammonia (25%) was mixed quickly to produce a stock solution. The solution was stirred under N₂ for another 1 h and then cooled to room temperature. Afterward, the precipitate was separated using a magnet, washed with ethanol several times, and dried under vacuum.

Preparation of PAE/GO-TEPA@Fe₃O₄ nanocomposites

PAE/GO-TEPA@Fe₃O₄ nanocomposite films (polyamide ether nanocomposite (PAEN) 2% and PAEN 5%) were synthesized via a solution mixing method as follows: Into a three-necked flask equipped with mechanical stirring and ultrasonic irradiation, PAE was dissolved in THF at 25°C. After dissolution completely, a calculated amount of GO-TEPA@Fe₃O₄ was added to the PAE homogenous solution. To control the dispersibility of GO-TEPA@Fe₃O₄ in the PAE matrix, constant stirring was applied at 25°C for overnight. To remove the THF solvent, the solutions were poured into petri dishes and uniformly heated. Then, PAE/GO-TEPA@Fe₃O₄ nanocomposite films were formed.

Results and discussion

Monomer synthesis

The new aromatic symmetrical diamine monomer 6, 4,4′-((((4-chlorophenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline was successfully synthesized using 4-chlorobenzaldehyde as a starting material, as shown in Figure 1. Firstly, 4,4′-((4-chlorophenyl)methylene) bis (2,5-dimethylphenol) (3) was synthesized using a one-pot condensation reaction of 4-chlorobenzaldehyde (1) with 2,5-xylenol (2) in the presence of p-TSA under solvent-free conditions according to the reported literature.⁴² Then, nucleophilic fluorodisplacement of 1-fluoro-4-nitrobenzene (4) with the phenolate of 4,4′-((4-chlorophenyl)methylene)bis(2,5-dimethylphenol) (3) gave 5,5′-((4-chlorophenyl)methylene)bis(1,4-dimethyl-2-(4-nitrophenoxy)benzene) (5), which was subsequently reduced to 4,4′-((((4-chlorophenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline (6) with hydrazine hydrate as the reducing agent and Pd/C as the catalyst. The chemical structure of diamine monomer 6 was characterized using FTIR, ¹H NMR, and ¹³C NMR techniques.

Figure 1.

The synthetic pathway of 4,4′-((((4-chlorophenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline (6).

Figure 2 shows the FTIR spectra of diamine monomer 6. FTIR spectrum exhibited absorption peaks at 3300–3500 cm⁻¹ (NH₂ asymmetric and symmetric stretching vibrations), 3022 cm⁻¹ (aromatic C–H stretching vibrations), 1263 cm⁻¹ (C–N stretching vibrations), 1213 cm⁻¹ (C–O–C stretching vibrations of ether units) and 827 cm⁻¹ (C–Cl stretching vibrations). Figure 3 shows the ¹H NMR spectra of the diamine 6, in which its chemical structure was clearly identified. The ¹H NMR spectra confirm that the nitro groups were completely converted into the amino groups (H_c), this chemical shift appeared at 4.85 ppm. Two singlet peaks at 2.07 and 1.97 ppm are characteristics of the methyl groups (H_e and H_f). Singlet peak at 5.54 ppm is ascribed to the protons of triphenylmethane group (H_d). Also, the absorption peaks at 6.49–7.38 ppm were assigned to the aromatic ring protons.

Figure 2.

FTIR spectrum of 4,4′-((((4-chlorophenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline (6).

Figure 3.

¹H NMR spectrum of 4,4′-((((4-chlorophenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline (6).

As to the ¹³C NMR spectra shown in Figure 4, the resonance signals of all carbon atoms of diamine monomer (6) showed 17 peaks. The carbon of aromatic rings appeared in the range of 115.4–155.03 ppm. The peaks at 155.1, 148.2, and 146.4 ppm could assign to the C_a, C_b, and C_c, respectively. Also, the presence of single peak at 48.2 ppm (C_d) related to C–H carbon of triphenylmethane group and two single peaks at 19.3 ppm (C_e) and 16.3 ppm (C_f) for carbon of methyl groups. The results indicate that the synthesis of monomer diamine 6 should be successful in this work.

Figure 4.

¹³C NMR spectra of 4,4′-((((4-chlorophenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline (6).

Polymer synthesis and characterization

General procedure for the preparation of PAE 10 was shown in Figure 5. The structures of the PAE (10) were characterized by FTIR and ¹H NMR. Figure 6 shows FTIR spectrum of this PAE. They exhibited characteristic absorption bands of the amide groups around 3425 cm⁻¹ and the absorption bands at 2874 and 2924 cm⁻¹ were attributed to the symmetric and antisymmetric vibrations of methyl groups, carbonyl stretching vibration (C=O) at 1662 cm⁻¹, C=C phenyl ring stretching vibration at 1404 and 1606 cm⁻¹, and the very strong peak at 1213 cm⁻¹ related to C–O stretching vibration.

Figure 5.

Synthesis route of PAE (10).

Figure 6.

FTIR spectra of the PAE (10).

Figure 7 shows ¹H NMR spectrum of PAE (10). The appearance of amide peak for the monomer diamine 6 at 10.23 ppm (H_a) suggests that polymerization has been carried out efficiently. The presence of singlet peak at 5.61 ppm (H_e) corresponds to the proton of triphenylmethane moiety and at 2.08 and 2.86 ppm correspond to the protons of methyl groups (H_h and H_g, respectively) supported the structure of the PAE backbone. Furthermore, two singlet peaks at 4.10 (H_f) and 1.87 ppm (H_i) were assigned to the aliphatic protons of the methylene groups and three signals observed in the region of 7.01–7.86 ppm are assigned to the aromatic protons H_b, H_c, and H_d related to the ortho protons of C=O, NH, and Cl groups, respectively. Moreover, the appeared absorption peaks above 7 ppm are related to protons of aromatic rings of PAE backbone.

Figure 7.

¹H NMR spectra of the PAE (10).

The organosolubility behavior of the PAE (10) was tested qualitatively in various solvents by dissolving 0.01 g of polymeric sample in 2 mL of solvent at room temperature. The PAE exhibited good solubility in highly polar solvents, such as DMSO, N,N-dimethyl acetamide (DMAc), DMF, and even in less polar THF, but practically insoluble in methanol, ethanol, and water. The incorporation of monomer with flexible linkages, such as ether and bulky groups into the polymer backbone that led to this polymer, has good solubility in various solvents, especially organic aprotic solvents.⁴³

Synthesis and characterization of nanostructures

The synthesis process of GO-TEPA@Fe₃O₄ nanocomposite was schematically illustrated in Figure 8. Synthesis of GO was achieved by placing graphite in concentrated acid (H₂SO₄) in the presence of an oxidizing agent (KMnO₄). To improve its dispersion in the polymer matrix, GO was modified with TEPA. Then, the magnetic tetraethylenepentamine-functionalized graphene oxide (GO-TEPA@Fe₃O₄) prepared via homogeneous suspension of GO-TEPA in deionized water and then alkaline hydrolysis of the Fe³⁺ and Fe²⁺ ions.

Figure 8.

Preparation of GO-TEPA@Fe₃O₄ nanocomposite.

The characteristic FTIR spectrum of GO, GO-TEPA, and GO-TEPA@Fe₃O₄ is depicted in Figure 9. In all curves in Figure 9, the absorption bands at around 3400 and 3440 cm⁻¹ can be attributed to the O–H group vibrations stretching of the GO. The absorption peak at 1739 cm⁻¹ and the strong peak between 1028 cm⁻¹ and 1107 cm⁻¹ related to C=O and C–O stretching, respectively. Compared with GO, two absorption peaks at about 2852 and 2926 cm⁻¹ are assigned to the C–H bonds stretching vibrations, which confirm the presence of TEPA on the surface of GO-TEPA nanosheets (Figure 9(b) and (c)), and GO-TEPA@Fe₃O₄ shows a characteristic absorption of Fe-O bond at about 592 cm⁻¹, which confirms the presence of Fe₃O₄ nanoparticles (Figure 9(c)). Considering results, the surface modification of GO-TEPA@Fe₃O₄ nanocomposite was successful.

Figure 9.

The comparative FTIR spectra for (a) GO, (b) GO-TEPA, and (c) GO-TEPA @Fe₃O₄ nanosheets.

The EDX spectrum of GO-TEPA@Fe₃O₄ (Figure 10) shows iron, nitrogen, carbon, and oxygen. The obvious attendance of Fe signal in the EDX spectrum demonstrates that GO-TEPA@Fe₃O₄ have been successfully functionalized by Fe₃O₄ nanoparticles.

Figure 10.

EDX spectrum of GO-TEPA@Fe₃O₄ nanocomposite.

The synthesized GO-TEPA@Fe₃O₄ nanocomposite was observed with SEM (Figure 11). FE-SEM images indicates that the Fe₃O₄ nanoparticles on the surface of modified GO (GO-TEPA) are approximately spherical and the average diameter of the nanoparticles is 40 nm. The results obtained for GO-TEPA@Fe₃O₄ nanocomposites express that the agglomeration of Fe₃O₄ nanoparticles has occurred. The agglomeration role in the interfacial properties of the matrix is much important because it reduces the interfacial area between matrix and nanoparticles, which lastly result in the poor mechanical properties of samples.

Figure 11.

FE-SEM images of GO-TEPA@Fe₃O₄ nanocomposite.

The PAE/GO-TEPA@Fe₃O₄ 5% and PAE/GO-TEPA@Fe₃O₄ 2% nanocomposite films were prepared according to the Yamazaki method.⁴⁴ Then, the PAEN 5% and PAEN 2% were produced by solution intercalation method, in which different amounts of GO-TEPA@Fe₃O₄ were mixed with appropriate amounts of PAE solution to yield particular nanocomposite concentrations. The resultant nanocomposites (PAE/GO-TEPA@Fe₃O₄ 5% and PAE/GO-TEPA@Fe₃O₄ 2%) were characterized using FE-SEM, XRD, UV-Vis spectrometry, TGA, and VSM.

To investigate the quality of the dispersion of GO-TEPA@Fe₃O₄ nanocomposite in the polymer matrix and the morphology of the nanocomposites, FE-SEM images of the surface of the PAE/GO-TEPA@Fe₃O₄ 5% nanocomposite have been utilized (Figure 12). From the SEM results, it can be seen that GO-TEPA@Fe₃O₄ have been well dispersed within the PAE matrix with very small aggregations. Also, spherical Fe₃O₄ nanoparticles are observed on the nanocomposite surface.

Figure 12.

FE-SEM images of PAEN 5% nanocomposites.

The crystallinity of the GO-TEPA@Fe₃O₄, PAEN 2%, and PAEN 5%, was examined by XRD characterization (Figure 13). Seven diffraction lines were observed in the representative XRD pattern of GO-TEPA@Fe₃O₄ at 2θ = 30°, 36°, 43°, 54°, 57°, 63°, and 74°, which assigned to the (220), (311), (400), (422), (511), (440), and (553) reflections, respectively, of the pure cubic spinel crystal structure of Fe₃O₄ with cell constant a = 8.39 A° (JCPDS card no. 19-0629). The average crystallite size for GO-NH₂@Fe₃O₄ nanoparticle was also determined from the linewidth broadening of the XRD peak corresponding to (311) reflection, using the Debye–Scherrer equation.⁴⁵ The value of crystal size is 35.5 nm, which is smaller than the size determined by FE-SEM (Figure 12) observation. The reflection peak positions and relative intensities of both the nanocomposites are similar to the XRD patterns of GO-TEPA@Fe₃O₄ nanoparticles in Figure 13(a). Both the magnetic nanocomposites show a broad peak at θ = 25°C due to the polymer network. Additionally, the sharpness of the Fe₃O₄ peak for magnetic nanocomposites increases with Fe₃O₄ concentration.

Figure 13.

XRD curves of (a) GO-TEPA@Fe₃O₄, (b) PAEN 5%, and (c) PAEN 2% nanocomposites.

UV-Vis spectra of PAE, PAEN 2%, and PAEN 5% were shown in Figure 14, PAE exhibits strong UV-Vis absorption bands at 250 and 290 nm, assignable to π–π* transitions. Also, the PAEN 5% and PAEN 2% nanocomposites exhibit UV absorption bands at 250 and 295 nm and 252 and 300 nm, respectively. A redshift is observed at the maximum wavelength of both nanocomposites, which can be attributed to the strong interactions between filler nanoparticles and polymer matrix. According to the results, PAENs containing magnetic nanoparticles have optical properties, such as PAE.

Figure 14.

UV-Vis spectra of PAE and their magnetic nanocomposites.

To analyze the thermal stability of the PAE and PAEN films, we carried out thermogravimetric analysis (TGA) and DTG characterizations under nitrogen atmosphere, which are described in Figure 15 and summarized in Table 1. It can be observed that the PAE and their nanocomposites were a mass loss ranging from about 240°C and continuing until about 480°C.

Figure 15.

TGA and DTG curves of PAE and their magnetic nanocomposites.

Table 1.

Thermal characteristic data of the PAE and nanocomposites.

Polymer	T₅(°C)^a	T₁₀(°C)^b	T_max(°C)^c	Char yield^d	LOI^e
PAE	292	356	411	50.4	37.6
PAEN 2%	304	369	455	54.8	39.4
PAEN 5%	295	361	450	51.4	38.1

PAE: polyamide ether; LOI: limiting oxygen index.

^a The temperature at which 5% weight loss occurs.

^b The temperature at which 10% weight loss occurs.

^c The maximum weight loss temperature.

^d The residual weight percentage at 550°C.

^e Calculated by the equation of Van Krevelen and Hoftyzer.

The thermostability of the polymer and corresponding nanocomposites (PAE/GO-TEPA@Fe₃O₄ 2% and 5%) were evaluated by TGA under nitrogen atmosphere at a heating rate of 10°C min⁻¹. The corresponding weight loss temperatures of 5% (T₅) and weight loss temperatures of 10% (T₁₀) were all determined from original curves and listed in Table 1. The T₅% and T₁₀% values of them stayed within 292–304°C and 356–369°C, respectively. Maximum decomposition temperature (T_max) of the samples, which was obtained from the first derivative of TGA curves, was around 411–455°C, and also, the weight of materials remained was about 50.4–54.8%. The weight retained by these samples at 550°C is roughly proportional to the amount of GO-TEPA@Fe₃O₄ in the nanocomposites. In addition, PAENs 2% and PAENs 5% compared with the thermal decomposition behavior of neat PAE shows a slightly higher 10 wt% weight loss temperature and char yields. Because of appropriate crosslinking between the PAE matrix and GO-TEPA@Fe₃O₄ nanoparticles. The increasing GO-TEPA@Fe₃O₄ content from 2 wt% to 5 wt%, because of agglomeration of GO-TEPA@Fe₃O₄ in the polymer matrix, causes char yield to decrease.

The limiting oxygen index (LOI), which is the minimum concentration of oxygen gas in an atmosphere consisting of a mixture of oxygen and nitrogen, is employed to compute the flammability of a material. Char yield can be used as criteria for evaluating LOI of the polymers in accordance with Van Krevelen and Hoftyzer equation⁴⁶

LOI = 17.5 + 0.4 CR

The PAE, PAEN 2%, and PAEN 5% having LOI values were calculated from their char yield at 550°C. Therefore, the samples can be classified as self-extinguishing materials (Table 1).

To further confirm the thermal stability trend observed from TGA and DTG thermograms, an analysis of the heat-resistance index is performed on the PAE and nanocomposites. The heat-resistance index (T_HRI) is a measurement of the ability of a material to resist a heat flow⁴⁷ and is given by the equation

T_{HRI} = 0.49 [T_{5} + 0.6 (T_{30} - T_{5})]

Using the thermograms shown in Figure 15, the heat-resistance index was calculated (Table 2) for PAE and nanocomposites. From the T_HRI value, the addition of GO-TEPA@Fe₃O₄ can improve the thermal stability of nanocomposites. In addition, PAEN 2% because of uniform dispersion exhibited a higher heat-resistance index than PAEN 5%.

Table 2.

Temperature values at 5% and 30% weight loss and heat-resistance index of the PAE and nanocomposites.

Polymer	T₅(°C)	T₃₀(°C)	T_HRI(°C)^a
PAE	292	439	186
PAEN 2%	304	457	194
PAEN 5%	295	449	189

PAE: polyamide ether.

^aT_{Heat-resistance index} = 0.49[T₅ + 0.6 (T₃₀ − T₅)]. T₅ and T₃₀ are corresponding decomposition temperatures of 5 and 30 wt% weight loss, respectively.

Figure 16 shows the magnetization (M) versus the applied magnetic field (H) (M-H curves or hysteresis loops) of PAEN 5% and PAEN 2% measured at room temperature for the PAENs with different loading amounts. The hysteresis curve allows determination of the saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc). The magnetization of samples could be completely saturated at high fields of up to ±8000.0 Oe and the specific Ms of the PAEN 2% and PAEN 5% nanocomposites were about 1.33 and 2.34 emu g⁻¹, respectively, which increased with the increased loading amounts of GO-TEPA@Fe₃O₄. It has been reported that the ferromagnetic nanoparticles exhibit superparamagnetic behavior when the particle size decreases to below a critical value.⁴⁸ The nanocomposite samples PAEN 2% and PAEN 5% illustrate very small Mr and Hc as magnetic nanocomposites; Mr = 0.18 and 0.34 emu g⁻¹, and Hc = 74 Oe for both of them, indicate that PAEN 2% and PAEN 5% have superparamagnetic behavior.

Figure 16.

Magnetic hysteresis loops of magnetic nanocomposites (a) PAEN 5% and (b) PAEN 2% samples.

Conclusions

We have prepared heat-resistant and magnetic PAE/GO-TEPA@Fe₃O₄ 2% and PAE/GO-TEPA@Fe₃O₄ 5% nanocomposites. The new PAE derived from 2,5-dimethylphenol (2) was synthesized from diacid (9), and diamine (6) by direct polycondensation reaction. Also, GO nanosheets were produced by Hummer’s method from graphite powder in concentrated sulfuric and NaNO₃. To improve the compatibility between the matrix and filler, GO nanoparticles were modified with TEPA and sequentially with Fe₃O₄ nanoparticles. The modified GO was characterized by various analytical methods, such as FTIR, EDX, and SEM. The properties of PAE and the effects of GO-TEPA@Fe₃O₄ nanoparticles on its properties were examined using SEM, XRD, UV-Vis, TGA-DTG, and VSM analyses. As a result, thermal and magnetic properties of PAE/GO-TEPA@Fe₃O₄ 2% and PAE/GO-TEPA@Fe₃O₄ 5% nanocomposites showed enhanced effect on the thermal stability and the char yields in comparison to the neat PAE. Also, the TGA results showed that PAE/GO-TEPA@Fe₃O₄ 2% have better thermal properties than PAE/GO-TEPA@Fe₃O₄ 5%. Furthermore, alternating gradient force magnetometer shows the characteristics of superparamagnetism.

Footnotes

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Hassan Moghanian

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Magnetic tetraethylenepentamine-functionalized graphene oxide to prepare new methyl rich bisphenol-based soluble and heat-resistant polyamide ether nanocomposites: Synthesis and characterization

Abstract

Keywords

Introduction

Experimental

Materials

Measurements

Monomer synthesis

Synthesis of 4,4′-((4-chlorophenyl)methylene)bis(2,5-dimethylphenol) ( 3 )

Synthesis of 5,5′-((4-chlorophenyl)methylene)bis(1,4-dimethyl-2-(4-nitrophenoxy)benzene) ( 5 )

Synthesis of 4,4′-((((4-chlorophenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline ( 6 )

Synthesis of 4,4′-(butane-1,4-diylbis(oxy))dibenzoic acid ( 9 )

Polymer synthesis

Synthesis of nanostructures

Synthesis of GO nanosheets

Synthesis of tetraethylenepentamine-functionalized graphene oxide nano-sheets (GO-TEPA)

Synthesis of magnetic tetraethylenepentamine-functionalized graphene oxide nanocomposite (GO-TEPA@Fe3O4)

Preparation of PAE/GO-TEPA@Fe3O4 nanocomposites

Results and discussion

Monomer synthesis

Polymer synthesis and characterization

Synthesis and characterization of nanostructures

Conclusions

Footnotes

Funding

ORCID iD

References

Synthesis of magnetic tetraethylenepentamine-functionalized graphene oxide nanocomposite (GO-TEPA@Fe₃O₄)

Preparation of PAE/GO-TEPA@Fe₃O₄ nanocomposites