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Fe3O4 magnetic nanofibers with solvent resistant properties
Composites Science and Technology 133 (2016) 97e103
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
One-pot preparation of polyimide/Fe3O4 magnetic nanofibers with solvent resistant properties Chao Luo a, Xiuxing Wang a, Jianqiang Wang b, Kai Pan a, * a b
Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China Department of Civil Engineering, The University of Hong Kong, Pokfulam 999077, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2016 Received in revised form 18 July 2016 Accepted 23 July 2016 Available online 25 July 2016
A series of solvent resistant magnetic polyimide/Fe3O4 (PI/Fe3O4) nanofibers were prepared by electrospinning the polyamide acid/ferric acetylacetonate (PAA/Fe(acac)3) mixed solutions and followed by a thermal treatment procedure. Samples were fully characterized in terms of FTIR, SEM, TEM, XRD and vibrating sample magnetometer (VSM). PI/Fe3O4 nanofibers structure and performance can be tuned by varying the thermal treatment procedure and Fe(acac)3 loading amount as well. Results revealed the complete transformation from PAA to PI after thermal treatment procedure, and the Fe3O4 particles dispersed relatively well, and mainly in the outer sphere of PI nanofibers under optimal conditions. VSM results showed that the saturation magnetization of PI/Fe3O4 nanofibers with a Fe(acac)3 loading amount of 21% can reach up to 10.46 emu/g. The one-pot prepared magnetic PI/Fe3O4 nanofibers were stable in most common organic solvents. This methodology can be scaled up and the obtained nanofibers could be an encouraging candidate for the template of adsorption, separation and catalysis, especially in the organic solvent phase. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Electrospinning Polyimide Nanofiber Magnetic Solvent resistant
1. Introduction In recent years, polymer nanofibers modified by different kinds of inorganic particles were widely studied for the intrinsic advantages of the polymers such as flexibility, light weight and easy machinability, and also the customizable functions brought from the hybridization with inorganic particles [1e7]. Those customizable functions include excellent electrical, magnetic, optical properties, etc. The related composite nanofibers can be widely used in sensors [8e10], energy storage devices [11e13], catalysts [14e18] and tissue engineering [19,20]. To fabricate these composite nanofibers, a feasible low cost and common technology named electrospinning was often used. In general, the most immediate way to fabricate the hybrid nanofiber is to electrospin the polymer/inorganic particles mixed spinning dope. For example, Wang et al. [21] fabricated the fluorescent poly(vinyl pyrrolidone) nanofiber embedding CdTe nanoparticles by electrospinning PVP/ CdTe mixed spinning dope. Although this method is relatively simple, it suffers the problem of aggregation and poor dispersion in nanofiber which may weaken the functionality significantly.
* Corresponding author. E-mail address: [email protected] (K. Pan). http://dx.doi.org/10.1016/j.compscitech.2016.07.021 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
Compared to the addition of dispersant, combining the electrospinning of polymer and/or inorganic particle nanofiber precursors with a proper post-processing is more effective. Lu et al. [22e24] prepared a variety of finely dispersive PVP/sulfide nanofibers by introduce a gas-solid reaction to the as-prepared PVP/metal ions nanofiber precursors. Ji et al. [25] fabricated a-Fe2O3 nanoparticleloaded carbon nanofiber composites via electrospinning FeCl3$6H2O salt-polyacrylonitrile (PAN) precursors and a subsequent carbonization in inert gas. The prepared a-Fe2O3 nanoparticles with an average size of about 20 nm had a homogeneous dispersion along the carbon nanofiber surface and the nanofiber composites can be used as anode materials for rechargeable lithium-ion batteries. Polyimide (PI) is famous for the advantages of high thermal stability, superior chemical resistance, and excellent mechanical properties. The preparation and application of functional PI-based composites has attracted more and more attention [26e32]. Especially in the field of nanofiber obtained by electrospun, Zhang et al. [33] prepared ultra-fine polyimide nanofibers containing silver nanoparticles though the electrospinning of polyamide acid(PAA)/soluble silver(I) salt and followed by a thermal curing treatment. The obtained PI/Ag nanofibers have a potential value in conductive and antibacterial materials. Metal oxide nanoparticles
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sieves (5A). Ferric acetylacetonate [Fe(acac)3, 99%] was provided by Sinopharm Chemical Reagent Co. Ltd. All other reagents used were of analytical grade and used as received.
Table 1 Thermal treatment procedures for PAA/Fe(acac)3 nanofiber. No.
Thermal treatment proceduresa Stage one
I II III IV V a
200 C, 1.5 h
Stage two
300 C, 1.5 h
Stage three 360 400 450 450 500
C, C, C, C, C,
10 h 3h 1.5 h 3h 3h
Under the protection of nitrogen, with a ramping rate of 5.0 C/min.
2.2. Preparation of polyamide acid solution Polyamide acid solution precursors were synthesized by a polycondensation reaction between PMDA and ODA in DMF solvent. The exhaustive method was similar to that reported in our previous work [35]. Typically, the predetermined amount of ODA was thoroughly dissolved in DMF solvent, and a corresponding amount of PMDA was then added. The solution was stirred in a three-necked bottle placed in an ice water bath till an obvious Weissenberg effect appeared, which means a viscous solution of PAA was formed. All procedures were carried out in a nitrogen atmosphere. The content of PAA in the solution was fixed at about 10%.
2.3. Electrospinning PAA/Fe(acac)3 nanofibers
Fig. 1. The effect of the PAA content (or viscosity) on the diameter of the resulting nanofiber.
as a kind of dopants were used to reinforce PI nanofibers. By electrospinning the soluble PI 5218 spinning dope blended with core-shell FeeFeO nanoparticles, the FeeFeO nanoparticles reinforced PI magnetic nanocomposite fibers were obtained [34]. Compared with the pure PI nanofibers, the thermal properties of the nanocomposite fibers such as glass transition temperature (Tg) and metaling temperature (Tm) increased significantly. However, the FeeFeO nanoparticles reinforced PI magnetic nanocomposite fibers were soluble which may limit their applications. What's more, the fabrication of solvent resistant polyimide magnetic nanofiber with a well dispersive magnetite is rarely reported. Inspired by the above work, a one-pot method was used to prepare the PI/Fe3O4 magnetic nanofibers, which not only avoided the dispersion problem of functional nanoparticles, but also given consideration to the excellent solvent resistance of PI. In this work, we electrospun the PAA/ferric acetylacetonate [PAA/Fe(acac)3] solutions, and then followed by a thermal treatment procedure to obtain solvent resistant PI/Fe3O4 magnetic nanofibers. The thermal treatment procedures were studied in detail to find an optimum for both the thermal imidization of PAA and the generation of magnetic nanoparticles.
2. Experimental 2.1. Chemicals Pyromellitic dianhydride (PMDA, 97%) and Bis(4-aminophenyl) ether (ODA, 98%) were purchased from Alfa Aesar and used after being purified through vacuum sublimation above their gasification temperature. Dimethyl formamide (DMF) was dried with Molecular
The PAA/Fe(acac)3 DMF solutions with the Fe(acac)3 loadings range from 7% to 21% (vs. PAA, hereinafter all the Fe(acac)3 loadings mean vs. PAA) were prepared by adding a certain amount of Fe(acac)3 solutions (DMF as the solvent) to the pristine PAA solutions with an overnight magnetic stirring. And the PAA/Fe(acac)3 nanofibers were prepared by electrospinning at room temperature. The PAA/Fe(acac)3 nanofibers were collected on the aluminum foil at a high voltage of 10e15 kV. The solution feed speed was 1.2 mL/h and the spinneret diameter was 0.7 mm. The distance between the collector and the spinneret was fixed at 10e14 cm. A rotating metal drum (diameter: 10 cm, rotating speed: 80 rpm) was used to collect the deposited PAA/Fe(acac)3 nanofibers during the electrospinning process. The fibers are dried at 60 C in a vacuum oven.
2.4. Thermal treatment of PAA/Fe(acac)3 nanofibers To find a better thermal treatment procedure for both the imidization of PAA and the formation of magnetic particles, the PAA/Fe(acac)3 nanofibers were thermal treated in a nitrogen atmosphere by different thermal treatment procedures with a ramping rate of 5.0 C/min. And the detailed thermal treatment procedures are shown in Table 1. Finally, the samples were cooled to room temperature for further using.
2.5. Apparatus and instrumentations The viscosity properties of PAA/Fe(acac)3 solutions were detected by a rotary viscosimeter (NDJ-8S) at room temperature. The FTIR spectra were recorded by Fourier transform infrared (FTIR) spectrometer using a Perkin-Elmer spectrum RXI with a resolution of 4 cm 1. The morphologies of the nanofibers were studied by Hitachi S-4700 scanning electron microscopy (SEM) and Hitachi800 Transmission electron microscope (TEM). X-ray diffraction (XRD) patterns of the as-prepared magnetic PI nanofibers were acquired using Cu-Ka photo from an advanced wide-angle X-ray diffractmeter (Bruker D8) operated at 40 kV, 40 mA with 2Q ranging from 20 to 70 . Magnetizations of PI/Fe3O4 nanofibers obtained by different thermal treatment procedures were studied using room temperature magnetization measurements in a Princeton Applied Research Model 155 Vibrating Sample Magnetometer (VSM) unit with a maximum magnetic field of 1.0 T and a sensitivity of 10 5 emu.
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Fig. 2. SEM images of PAA/Fe(acac)3 nanofiber obtained at different operational conditions:10 KV, 10 cm, a, c); 14 KV, 14 cm, b, d).
3. Results and discussions 3.1. Parameter optimization for electrospinning It is well known to us that the diameter and morphology of electrospinning nanofiber were affected by operational parameters, such as viscosity of spinning solution, tip-to-target distance and applied electrical voltage [36,37]. For the electrospinning of PAA/ Fe(acac)3 nanofibers, the PAA solutions with different concentrations and a fixed Fe(acac)3 loading (7%) were used (10 kV and 10 cm). SEM images of the PAA/Fe(acac)3 nanocomposite nanofibers clearly indicated the formation of nanofibers, which means the solution viscosity was suitable. As shown in Fig. 1, the rotary viscosities of PAA/Fe(acac)3 solutions increased linearly with the increasing of PAA contents (inset fig. e). For the resulting PAA/ Fe(acac)3 nanofibers, the average diameters of nanofibers increased from ~200 nm to ~550 nm. It is suggested that, with the increasing
of PAA contents, the surface tension of electrospinning solution increased, which resulted in the obvious increase of fiber diameter. As reported by the literature [38], shorter tip-to-collector distance and low applied voltage both contribute to a lager diameter. In our experiments, the distances (10 cm and 14 cm) are fixed in paralled with voltages (10 kV and 14 kV) while maintaining two kinds of PAA solutions with different contents (6.32% and 8.62%) and a fixed Fe(acac)3 loading (7%). Fig. 2 showed the morphology and specific size of nanofibers. Fig. 2a and c showed the microstructures of the nanofibers prepared with an applied constant electrical voltage of 10 kV and a working distance of 10 cm and the PAA content was 6.32% and 8.62%, respectively. For Fig. 2b and d, parameters became 14 kV, 14 cm and the PAA content was 6.32% and 8.62%, respectively. It can be clearly seen that from the comparison of Fig. 2a and b (and that of Fig. 2c and d), the diameters of the obtained nanofibers were similar, that means the antagonistically changing of parameters eliminates the influence. Therefore,
Fig. 3. SEM images of PI/Fe3O4 nanofibers obtained from different thermal treatment procedures: a-e belong to procedure IeV, respectively; f: the TEM image of cross-section of PI13%-III.
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Fig. 4. XRD diffraction patterns of PI/Fe3O4 nanofiber obtained at different thermal treatment procedures.
Fig. 6. Hysteresis loops of PI/Fe3O4 nanofiber obtained at different thermal treatment procedures.
nanofibers with almost the same average diameter were obtained.
3.2. Thermal treatment Although the thermal imidization procedure of PAA synthesized from the poly-condensation reaction between PMDA and ODA was studied in detail [35,39], to determine the optimal thermal treatment procedure in this work needed to coordinate the following factors: imidization degree of PI and transformation of Fe3O4 from Fe(acac)3. In order to find a better thermal treatment procedure for the transformation of both PAA and Fe(acac)3, a series of thermal treatment procedures were studied (as shown in Table 1). Fig. 3 showed the SEM and TEM images of the nanofibers obtained after the thermal treatment procedure. From Fig. 3 we can see the surface of PI magnetic nanofibers were more coarse than that of pristine PAA/Fe(acac)3 nanofibers (as shown in Fig. 2). Moreover, some nanoparticles were observed on the surface of nanofibers. It's worth to see that in Fig. 3f (TEM image of the cross-section of PI13%-III), nanoparticles generally existed in outer sphere of the
Fig. 5. IR spectrum of PAA nanofibers with different Fe(acac)3 loadings and the corresponding PI nanofibers: PAA-0%, a); PAA-13%, b); PI-0%-III, c); PI-13%-III, d).
Table 2 Saturation magnetization data of PI/Fe3O4 nanofibers obtained at different thermal treatment procedures. Sample
PI-13%-I
PI-13%-II
PI-13%-III
PI-13%-IV
PI-13%-V
PI-21%-III
Ms (emu/g)
1.52
3.89
6.40
4.35
3.54
10.46
nanofiber. This may due to that the type of phase separation is spinodal type during the electrospinning process of PAA/Fe(acac)3, and the concentration fluctuations occur in a wave-like fashion resulted in the uniform dispersion of polymer rich and polymer poor areas throughout the solution [40]. So in the polymer rich areas (outer sphere), Fe(acac)3 is rich to generate the most nanoparticles. Although literature [41,42] had reported the pyrogenic decomposition of Fe(acac)3 under the temperature of 300 C, to find an optimized and detailed thermal treatment procedure was necessary both for the pyrogenic decomposition of Fe(acac)3 and the imidization of PAA in our experiments, especially in a nitrogen atmosphere. We proceeded different thermal treatment procedures, and found that the generated nanoparticles were different in size and distribution. In Fig. 3a, few nanoparticles were found. That difference may be the main reason that even treated at a temperature of 360 C for 10 h, just few Fe(acac)3 decomposed. With the increase of processing temperature, more Fe(acac)3 decomposed, therefore many small nanoparticles were observed in Fig. 3b. From Fig. 3c, the nanoparticles became bigger. While in Fig. 3d and e, nanoparticles became small again. The size changes of the resulting particles could be explained like that more Fe(acac)3 decomposed resulted in the formation of bigger particles with the increasing of temperature. While, once the Fe(acac)3 decomposed completely, the mount of resulting particles was fixed, and the further increasing of temperature or treated time leaded to the resulting particles recrystallized to be small size and perfect crystallite shaped particles. In general, the thermal decomposition product of Fe(acac)3 in air was Fe2O3 under certain thermal conditions [42,43]. In this experiment, the decomposition was done in a nitrogen atmosphere, so the decomposition product might be Fe3O4 with a low oxidation. To confirm that, magnetic nanofibers obtained from different thermal treatment procedures were analyzed by XRD, and the results were
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Fig. 7. SEM images of PI/Fe3O4 nanofiber before and after soaking in solvents: PI-7%-IV, a, b, c) and PI-13%-IV, d, e, f).
shown in Fig. 4. It is clear to see that XRD diffraction patterns differed with the different thermal treatment procedures and the Fe(acac)3 loading. In samples of PI-13%-I and PI-13%-II (samples from thermal treatment procedure I and II, and the loading of Fe(acac)3 was 13%), no obvious peaks were observed. Under the thermal treatment procedure I and II, the decomposition degree of Fe(acac)3 is low and the amount of the generated magnetic particles are too less to be detected. With the increasing of the temperature and the treated time, Fe(acac)3 furtherly decomposed. In samples of PI-13%-III, PI-13%-IV and PI-13%-V, obvious peaks were appeared at the 2Q of 35.5 and 62.5 . Especially, when the loading of Fe(acac)3 increased to 21%, the crystal peaks appeared at the 2Q of 30.1 (220), 35.5 (311), 43.1 (400), 57.0 (511) and 62.5 (440). Compared with the standard PDF card, the peaks were coinciding with those of magnetite, which confirmed that the magnetic particles in this experiment were Fe3O4. FTIR spectra of PAA and PI nanofibers were recorded to confirm the accomplishment of the imidization under the thermal treatment III (0% means no Fe(acac)3 was added). Results were shown in Fig. 5. By comparing Fig. 5a,b to c,d, new peaks appeared at 1776 cm 1, 1718 cm 1, 1371 cm 1 in PI-13%-III (PI-III) nanofibers, which assigned to C]O symmetric stretching vibration, C]O asymmetric stretching vibration and CeNeC stretching vibration, respectively, indicated that PI was prepared. And the disappearance of PAA characteristic peaks (appeared at 1648 cm 1, 1604 cm 1 and 1409 cm 1, assigned to C]O stretching vibration, NeH vibration
and CeN stretching vibration, respectively) meant PAA transformed to PI completely after the thermal treatment procedure. Therefore, after the thermal treatment procedure, both PI and Fe3O4 were prepared at the same time. Fig. 6 showed the room temperature magnetic hysteresis loops of the magnetic nanofibers obtained from different thermal treatment procedures. The data of the corresponding max saturation magnetization was shown in Table 2. From Table 2, we can see the max saturation magnetization of the sample of PI-21%-III is up to 10.46 emu/g. When the Fe(acac)3 loading decreased to 13%, the corresponding max saturation magnetization linearly changed to 6.40 emu/g. That is to say, the magnetization of the PI/Fe3O4 nanofiber is due to the magnetic Fe3O4 particles. What's more, under a certain thermal treatment, the difference of Fe(acac)3 loading do not affect the decomposition. For samples with a Fe(acac)3 loading fixed at 13%, the different thermal treatment procedures differed the max saturation magnetization of the resulting
Table 3 Saturation magnetization data of PI/Fe3O4 nanofibers before and after soaking in organic solvents. Sample
Ms (emu/g)
PI-7%-IV PI-13%-IV
3.54/before soaking 1.62/before soaking
3.46/DMF 1.57/C6H5CH3
3.51/THF 1.49/CHCl3
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magnetic nanofiber. The max saturation magnetization of the resulting magnetic nanofiber climbed up firstly and then declined from thermal treatment procedures I to V. That change tendency of the max saturation magnetization is similar to that of the magnetic particles size. Under the thermal treatment procedure I and II, the meager and small sized particles led to a weak magnetism. When the thermal treatment procedure changed to IV or V, the Fe(acac)3 decomposed completed, however, the size of the particles became small. For Fe3O4 particles, the magnetism varies with the size. In general, the smaller size the lower magnetism [44]. Hence, the max saturation magnetization of samples from procedure IV and V decreased. In a word, conclusion can be drawn that the best thermal treatment procedure in this experiment is procedure III. 3.3. Solvent resistance test In some harsh situations, materials may be used under the presence of organic solvents. To test solvent resistance performance of the obtained PI/Fe3O4 magnetic nanofibers, samples with different Fe(acac)3 loadings were soaked in one certain organic solvent for one month. The morphologies of PI/Fe3O4 magnetic nanofibers before and after soaking were compared. As shown in Fig. 7, samples of PI-13%-IV were soaked in methylbenzene and chloroform, and that PI-7%-IV were DMF and THF, respectively. After the soaking, the morphologies of PI/Fe3O4 magnetic nanofibers almost kept it the same. Moreover, the magnetism changes were negligible (as shown in Table 3). That means the obtained PI/ Fe3O4 magnetic nanofibers were stable in most organic solvent. Although the saturation magnetization of the obtained PI/Fe3O4 magnetic nanofibers wasn't as high as other magnetic PI fiber (10.46 vs. 30.56 emu/g) [34], the extra solvent resistance may endow the obtained PI/Fe3O4 magnetic nanofibers more potential application possibility, especially in the presence of organic solvents. 4. Conclusions Solvent resistant magnetic polyimide (PI/Fe3O4) nanofibers were prepared by the electrospinning of polyamide acid/ferric acetylacetonate [PAA/Fe(acac)3] solution and followed by a thermal treatment. Both the spinning parameter and the thermal procedure were optimized. The Fe3O4 particles dispersed well mainly in outer sphere of the nanofiber. The excellent magnetism and solvent resistance are attributed to the best heat treatment process which catered to the formation of magnetic particles and the thermal imidization. In this experiment the optimal thermal treatment procedure was procedure III: 200 C for 1.5 h, 300 C for 1.5 h and 450 C for 1.5 h with a ramping rate of 5.0 C/min in a nitrogen gas atmosphere. The obtained solvent resistant nanofiber with high magnetism is very promising as an encouraging candidate for the template of adsorption, separation and catalysis, especially in the organic solvent phase. Acknowledgments The project is supported by the Beijing Science and Technology Project (Z141100000914001). References [1] Y. Cai, F. Huang, Q. Wei, E. Wu, W. Gao, Surface functionalization, morphology and thermal properties of polyamide6/O-MMT composite nanofibers by Fe2O3 sputter coating, Appl. Surf. Sci. 254 (2008) 5501e5505. [2] Y. Lin, W. Cai, H. He, X. Wang, G. Wang, Three-dimensional hierarchically structured PAN@geAlOOH fiber films based on a fiber templated hydrothermal route and their recyclable strong Cr(vi)-removal performance, RSC Adv. 2
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
One-pot preparation of polyimide/Fe3O4 magnetic nanofibers with solvent resistant properties Chao Luo a, Xiuxing Wang a, Jianqiang Wang b, Kai Pan a, * a b
Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China Department of Civil Engineering, The University of Hong Kong, Pokfulam 999077, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2016 Received in revised form 18 July 2016 Accepted 23 July 2016 Available online 25 July 2016
A series of solvent resistant magnetic polyimide/Fe3O4 (PI/Fe3O4) nanofibers were prepared by electrospinning the polyamide acid/ferric acetylacetonate (PAA/Fe(acac)3) mixed solutions and followed by a thermal treatment procedure. Samples were fully characterized in terms of FTIR, SEM, TEM, XRD and vibrating sample magnetometer (VSM). PI/Fe3O4 nanofibers structure and performance can be tuned by varying the thermal treatment procedure and Fe(acac)3 loading amount as well. Results revealed the complete transformation from PAA to PI after thermal treatment procedure, and the Fe3O4 particles dispersed relatively well, and mainly in the outer sphere of PI nanofibers under optimal conditions. VSM results showed that the saturation magnetization of PI/Fe3O4 nanofibers with a Fe(acac)3 loading amount of 21% can reach up to 10.46 emu/g. The one-pot prepared magnetic PI/Fe3O4 nanofibers were stable in most common organic solvents. This methodology can be scaled up and the obtained nanofibers could be an encouraging candidate for the template of adsorption, separation and catalysis, especially in the organic solvent phase. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Electrospinning Polyimide Nanofiber Magnetic Solvent resistant
1. Introduction In recent years, polymer nanofibers modified by different kinds of inorganic particles were widely studied for the intrinsic advantages of the polymers such as flexibility, light weight and easy machinability, and also the customizable functions brought from the hybridization with inorganic particles [1e7]. Those customizable functions include excellent electrical, magnetic, optical properties, etc. The related composite nanofibers can be widely used in sensors [8e10], energy storage devices [11e13], catalysts [14e18] and tissue engineering [19,20]. To fabricate these composite nanofibers, a feasible low cost and common technology named electrospinning was often used. In general, the most immediate way to fabricate the hybrid nanofiber is to electrospin the polymer/inorganic particles mixed spinning dope. For example, Wang et al. [21] fabricated the fluorescent poly(vinyl pyrrolidone) nanofiber embedding CdTe nanoparticles by electrospinning PVP/ CdTe mixed spinning dope. Although this method is relatively simple, it suffers the problem of aggregation and poor dispersion in nanofiber which may weaken the functionality significantly.
* Corresponding author. E-mail address: [email protected] (K. Pan). http://dx.doi.org/10.1016/j.compscitech.2016.07.021 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
Compared to the addition of dispersant, combining the electrospinning of polymer and/or inorganic particle nanofiber precursors with a proper post-processing is more effective. Lu et al. [22e24] prepared a variety of finely dispersive PVP/sulfide nanofibers by introduce a gas-solid reaction to the as-prepared PVP/metal ions nanofiber precursors. Ji et al. [25] fabricated a-Fe2O3 nanoparticleloaded carbon nanofiber composites via electrospinning FeCl3$6H2O salt-polyacrylonitrile (PAN) precursors and a subsequent carbonization in inert gas. The prepared a-Fe2O3 nanoparticles with an average size of about 20 nm had a homogeneous dispersion along the carbon nanofiber surface and the nanofiber composites can be used as anode materials for rechargeable lithium-ion batteries. Polyimide (PI) is famous for the advantages of high thermal stability, superior chemical resistance, and excellent mechanical properties. The preparation and application of functional PI-based composites has attracted more and more attention [26e32]. Especially in the field of nanofiber obtained by electrospun, Zhang et al. [33] prepared ultra-fine polyimide nanofibers containing silver nanoparticles though the electrospinning of polyamide acid(PAA)/soluble silver(I) salt and followed by a thermal curing treatment. The obtained PI/Ag nanofibers have a potential value in conductive and antibacterial materials. Metal oxide nanoparticles
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sieves (5A). Ferric acetylacetonate [Fe(acac)3, 99%] was provided by Sinopharm Chemical Reagent Co. Ltd. All other reagents used were of analytical grade and used as received.
Table 1 Thermal treatment procedures for PAA/Fe(acac)3 nanofiber. No.
Thermal treatment proceduresa Stage one
I II III IV V a
200 C, 1.5 h
Stage two
300 C, 1.5 h
Stage three 360 400 450 450 500
C, C, C, C, C,
10 h 3h 1.5 h 3h 3h
Under the protection of nitrogen, with a ramping rate of 5.0 C/min.
2.2. Preparation of polyamide acid solution Polyamide acid solution precursors were synthesized by a polycondensation reaction between PMDA and ODA in DMF solvent. The exhaustive method was similar to that reported in our previous work [35]. Typically, the predetermined amount of ODA was thoroughly dissolved in DMF solvent, and a corresponding amount of PMDA was then added. The solution was stirred in a three-necked bottle placed in an ice water bath till an obvious Weissenberg effect appeared, which means a viscous solution of PAA was formed. All procedures were carried out in a nitrogen atmosphere. The content of PAA in the solution was fixed at about 10%.
2.3. Electrospinning PAA/Fe(acac)3 nanofibers
Fig. 1. The effect of the PAA content (or viscosity) on the diameter of the resulting nanofiber.
as a kind of dopants were used to reinforce PI nanofibers. By electrospinning the soluble PI 5218 spinning dope blended with core-shell FeeFeO nanoparticles, the FeeFeO nanoparticles reinforced PI magnetic nanocomposite fibers were obtained [34]. Compared with the pure PI nanofibers, the thermal properties of the nanocomposite fibers such as glass transition temperature (Tg) and metaling temperature (Tm) increased significantly. However, the FeeFeO nanoparticles reinforced PI magnetic nanocomposite fibers were soluble which may limit their applications. What's more, the fabrication of solvent resistant polyimide magnetic nanofiber with a well dispersive magnetite is rarely reported. Inspired by the above work, a one-pot method was used to prepare the PI/Fe3O4 magnetic nanofibers, which not only avoided the dispersion problem of functional nanoparticles, but also given consideration to the excellent solvent resistance of PI. In this work, we electrospun the PAA/ferric acetylacetonate [PAA/Fe(acac)3] solutions, and then followed by a thermal treatment procedure to obtain solvent resistant PI/Fe3O4 magnetic nanofibers. The thermal treatment procedures were studied in detail to find an optimum for both the thermal imidization of PAA and the generation of magnetic nanoparticles.
2. Experimental 2.1. Chemicals Pyromellitic dianhydride (PMDA, 97%) and Bis(4-aminophenyl) ether (ODA, 98%) were purchased from Alfa Aesar and used after being purified through vacuum sublimation above their gasification temperature. Dimethyl formamide (DMF) was dried with Molecular
The PAA/Fe(acac)3 DMF solutions with the Fe(acac)3 loadings range from 7% to 21% (vs. PAA, hereinafter all the Fe(acac)3 loadings mean vs. PAA) were prepared by adding a certain amount of Fe(acac)3 solutions (DMF as the solvent) to the pristine PAA solutions with an overnight magnetic stirring. And the PAA/Fe(acac)3 nanofibers were prepared by electrospinning at room temperature. The PAA/Fe(acac)3 nanofibers were collected on the aluminum foil at a high voltage of 10e15 kV. The solution feed speed was 1.2 mL/h and the spinneret diameter was 0.7 mm. The distance between the collector and the spinneret was fixed at 10e14 cm. A rotating metal drum (diameter: 10 cm, rotating speed: 80 rpm) was used to collect the deposited PAA/Fe(acac)3 nanofibers during the electrospinning process. The fibers are dried at 60 C in a vacuum oven.
2.4. Thermal treatment of PAA/Fe(acac)3 nanofibers To find a better thermal treatment procedure for both the imidization of PAA and the formation of magnetic particles, the PAA/Fe(acac)3 nanofibers were thermal treated in a nitrogen atmosphere by different thermal treatment procedures with a ramping rate of 5.0 C/min. And the detailed thermal treatment procedures are shown in Table 1. Finally, the samples were cooled to room temperature for further using.
2.5. Apparatus and instrumentations The viscosity properties of PAA/Fe(acac)3 solutions were detected by a rotary viscosimeter (NDJ-8S) at room temperature. The FTIR spectra were recorded by Fourier transform infrared (FTIR) spectrometer using a Perkin-Elmer spectrum RXI with a resolution of 4 cm 1. The morphologies of the nanofibers were studied by Hitachi S-4700 scanning electron microscopy (SEM) and Hitachi800 Transmission electron microscope (TEM). X-ray diffraction (XRD) patterns of the as-prepared magnetic PI nanofibers were acquired using Cu-Ka photo from an advanced wide-angle X-ray diffractmeter (Bruker D8) operated at 40 kV, 40 mA with 2Q ranging from 20 to 70 . Magnetizations of PI/Fe3O4 nanofibers obtained by different thermal treatment procedures were studied using room temperature magnetization measurements in a Princeton Applied Research Model 155 Vibrating Sample Magnetometer (VSM) unit with a maximum magnetic field of 1.0 T and a sensitivity of 10 5 emu.
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Fig. 2. SEM images of PAA/Fe(acac)3 nanofiber obtained at different operational conditions:10 KV, 10 cm, a, c); 14 KV, 14 cm, b, d).
3. Results and discussions 3.1. Parameter optimization for electrospinning It is well known to us that the diameter and morphology of electrospinning nanofiber were affected by operational parameters, such as viscosity of spinning solution, tip-to-target distance and applied electrical voltage [36,37]. For the electrospinning of PAA/ Fe(acac)3 nanofibers, the PAA solutions with different concentrations and a fixed Fe(acac)3 loading (7%) were used (10 kV and 10 cm). SEM images of the PAA/Fe(acac)3 nanocomposite nanofibers clearly indicated the formation of nanofibers, which means the solution viscosity was suitable. As shown in Fig. 1, the rotary viscosities of PAA/Fe(acac)3 solutions increased linearly with the increasing of PAA contents (inset fig. e). For the resulting PAA/ Fe(acac)3 nanofibers, the average diameters of nanofibers increased from ~200 nm to ~550 nm. It is suggested that, with the increasing
of PAA contents, the surface tension of electrospinning solution increased, which resulted in the obvious increase of fiber diameter. As reported by the literature [38], shorter tip-to-collector distance and low applied voltage both contribute to a lager diameter. In our experiments, the distances (10 cm and 14 cm) are fixed in paralled with voltages (10 kV and 14 kV) while maintaining two kinds of PAA solutions with different contents (6.32% and 8.62%) and a fixed Fe(acac)3 loading (7%). Fig. 2 showed the morphology and specific size of nanofibers. Fig. 2a and c showed the microstructures of the nanofibers prepared with an applied constant electrical voltage of 10 kV and a working distance of 10 cm and the PAA content was 6.32% and 8.62%, respectively. For Fig. 2b and d, parameters became 14 kV, 14 cm and the PAA content was 6.32% and 8.62%, respectively. It can be clearly seen that from the comparison of Fig. 2a and b (and that of Fig. 2c and d), the diameters of the obtained nanofibers were similar, that means the antagonistically changing of parameters eliminates the influence. Therefore,
Fig. 3. SEM images of PI/Fe3O4 nanofibers obtained from different thermal treatment procedures: a-e belong to procedure IeV, respectively; f: the TEM image of cross-section of PI13%-III.
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Fig. 4. XRD diffraction patterns of PI/Fe3O4 nanofiber obtained at different thermal treatment procedures.
Fig. 6. Hysteresis loops of PI/Fe3O4 nanofiber obtained at different thermal treatment procedures.
nanofibers with almost the same average diameter were obtained.
3.2. Thermal treatment Although the thermal imidization procedure of PAA synthesized from the poly-condensation reaction between PMDA and ODA was studied in detail [35,39], to determine the optimal thermal treatment procedure in this work needed to coordinate the following factors: imidization degree of PI and transformation of Fe3O4 from Fe(acac)3. In order to find a better thermal treatment procedure for the transformation of both PAA and Fe(acac)3, a series of thermal treatment procedures were studied (as shown in Table 1). Fig. 3 showed the SEM and TEM images of the nanofibers obtained after the thermal treatment procedure. From Fig. 3 we can see the surface of PI magnetic nanofibers were more coarse than that of pristine PAA/Fe(acac)3 nanofibers (as shown in Fig. 2). Moreover, some nanoparticles were observed on the surface of nanofibers. It's worth to see that in Fig. 3f (TEM image of the cross-section of PI13%-III), nanoparticles generally existed in outer sphere of the
Fig. 5. IR spectrum of PAA nanofibers with different Fe(acac)3 loadings and the corresponding PI nanofibers: PAA-0%, a); PAA-13%, b); PI-0%-III, c); PI-13%-III, d).
Table 2 Saturation magnetization data of PI/Fe3O4 nanofibers obtained at different thermal treatment procedures. Sample
PI-13%-I
PI-13%-II
PI-13%-III
PI-13%-IV
PI-13%-V
PI-21%-III
Ms (emu/g)
1.52
3.89
6.40
4.35
3.54
10.46
nanofiber. This may due to that the type of phase separation is spinodal type during the electrospinning process of PAA/Fe(acac)3, and the concentration fluctuations occur in a wave-like fashion resulted in the uniform dispersion of polymer rich and polymer poor areas throughout the solution [40]. So in the polymer rich areas (outer sphere), Fe(acac)3 is rich to generate the most nanoparticles. Although literature [41,42] had reported the pyrogenic decomposition of Fe(acac)3 under the temperature of 300 C, to find an optimized and detailed thermal treatment procedure was necessary both for the pyrogenic decomposition of Fe(acac)3 and the imidization of PAA in our experiments, especially in a nitrogen atmosphere. We proceeded different thermal treatment procedures, and found that the generated nanoparticles were different in size and distribution. In Fig. 3a, few nanoparticles were found. That difference may be the main reason that even treated at a temperature of 360 C for 10 h, just few Fe(acac)3 decomposed. With the increase of processing temperature, more Fe(acac)3 decomposed, therefore many small nanoparticles were observed in Fig. 3b. From Fig. 3c, the nanoparticles became bigger. While in Fig. 3d and e, nanoparticles became small again. The size changes of the resulting particles could be explained like that more Fe(acac)3 decomposed resulted in the formation of bigger particles with the increasing of temperature. While, once the Fe(acac)3 decomposed completely, the mount of resulting particles was fixed, and the further increasing of temperature or treated time leaded to the resulting particles recrystallized to be small size and perfect crystallite shaped particles. In general, the thermal decomposition product of Fe(acac)3 in air was Fe2O3 under certain thermal conditions [42,43]. In this experiment, the decomposition was done in a nitrogen atmosphere, so the decomposition product might be Fe3O4 with a low oxidation. To confirm that, magnetic nanofibers obtained from different thermal treatment procedures were analyzed by XRD, and the results were
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Fig. 7. SEM images of PI/Fe3O4 nanofiber before and after soaking in solvents: PI-7%-IV, a, b, c) and PI-13%-IV, d, e, f).
shown in Fig. 4. It is clear to see that XRD diffraction patterns differed with the different thermal treatment procedures and the Fe(acac)3 loading. In samples of PI-13%-I and PI-13%-II (samples from thermal treatment procedure I and II, and the loading of Fe(acac)3 was 13%), no obvious peaks were observed. Under the thermal treatment procedure I and II, the decomposition degree of Fe(acac)3 is low and the amount of the generated magnetic particles are too less to be detected. With the increasing of the temperature and the treated time, Fe(acac)3 furtherly decomposed. In samples of PI-13%-III, PI-13%-IV and PI-13%-V, obvious peaks were appeared at the 2Q of 35.5 and 62.5 . Especially, when the loading of Fe(acac)3 increased to 21%, the crystal peaks appeared at the 2Q of 30.1 (220), 35.5 (311), 43.1 (400), 57.0 (511) and 62.5 (440). Compared with the standard PDF card, the peaks were coinciding with those of magnetite, which confirmed that the magnetic particles in this experiment were Fe3O4. FTIR spectra of PAA and PI nanofibers were recorded to confirm the accomplishment of the imidization under the thermal treatment III (0% means no Fe(acac)3 was added). Results were shown in Fig. 5. By comparing Fig. 5a,b to c,d, new peaks appeared at 1776 cm 1, 1718 cm 1, 1371 cm 1 in PI-13%-III (PI-III) nanofibers, which assigned to C]O symmetric stretching vibration, C]O asymmetric stretching vibration and CeNeC stretching vibration, respectively, indicated that PI was prepared. And the disappearance of PAA characteristic peaks (appeared at 1648 cm 1, 1604 cm 1 and 1409 cm 1, assigned to C]O stretching vibration, NeH vibration
and CeN stretching vibration, respectively) meant PAA transformed to PI completely after the thermal treatment procedure. Therefore, after the thermal treatment procedure, both PI and Fe3O4 were prepared at the same time. Fig. 6 showed the room temperature magnetic hysteresis loops of the magnetic nanofibers obtained from different thermal treatment procedures. The data of the corresponding max saturation magnetization was shown in Table 2. From Table 2, we can see the max saturation magnetization of the sample of PI-21%-III is up to 10.46 emu/g. When the Fe(acac)3 loading decreased to 13%, the corresponding max saturation magnetization linearly changed to 6.40 emu/g. That is to say, the magnetization of the PI/Fe3O4 nanofiber is due to the magnetic Fe3O4 particles. What's more, under a certain thermal treatment, the difference of Fe(acac)3 loading do not affect the decomposition. For samples with a Fe(acac)3 loading fixed at 13%, the different thermal treatment procedures differed the max saturation magnetization of the resulting
Table 3 Saturation magnetization data of PI/Fe3O4 nanofibers before and after soaking in organic solvents. Sample
Ms (emu/g)
PI-7%-IV PI-13%-IV
3.54/before soaking 1.62/before soaking
3.46/DMF 1.57/C6H5CH3
3.51/THF 1.49/CHCl3
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magnetic nanofiber. The max saturation magnetization of the resulting magnetic nanofiber climbed up firstly and then declined from thermal treatment procedures I to V. That change tendency of the max saturation magnetization is similar to that of the magnetic particles size. Under the thermal treatment procedure I and II, the meager and small sized particles led to a weak magnetism. When the thermal treatment procedure changed to IV or V, the Fe(acac)3 decomposed completed, however, the size of the particles became small. For Fe3O4 particles, the magnetism varies with the size. In general, the smaller size the lower magnetism [44]. Hence, the max saturation magnetization of samples from procedure IV and V decreased. In a word, conclusion can be drawn that the best thermal treatment procedure in this experiment is procedure III. 3.3. Solvent resistance test In some harsh situations, materials may be used under the presence of organic solvents. To test solvent resistance performance of the obtained PI/Fe3O4 magnetic nanofibers, samples with different Fe(acac)3 loadings were soaked in one certain organic solvent for one month. The morphologies of PI/Fe3O4 magnetic nanofibers before and after soaking were compared. As shown in Fig. 7, samples of PI-13%-IV were soaked in methylbenzene and chloroform, and that PI-7%-IV were DMF and THF, respectively. After the soaking, the morphologies of PI/Fe3O4 magnetic nanofibers almost kept it the same. Moreover, the magnetism changes were negligible (as shown in Table 3). That means the obtained PI/ Fe3O4 magnetic nanofibers were stable in most organic solvent. Although the saturation magnetization of the obtained PI/Fe3O4 magnetic nanofibers wasn't as high as other magnetic PI fiber (10.46 vs. 30.56 emu/g) [34], the extra solvent resistance may endow the obtained PI/Fe3O4 magnetic nanofibers more potential application possibility, especially in the presence of organic solvents. 4. Conclusions Solvent resistant magnetic polyimide (PI/Fe3O4) nanofibers were prepared by the electrospinning of polyamide acid/ferric acetylacetonate [PAA/Fe(acac)3] solution and followed by a thermal treatment. Both the spinning parameter and the thermal procedure were optimized. The Fe3O4 particles dispersed well mainly in outer sphere of the nanofiber. The excellent magnetism and solvent resistance are attributed to the best heat treatment process which catered to the formation of magnetic particles and the thermal imidization. In this experiment the optimal thermal treatment procedure was procedure III: 200 C for 1.5 h, 300 C for 1.5 h and 450 C for 1.5 h with a ramping rate of 5.0 C/min in a nitrogen gas atmosphere. The obtained solvent resistant nanofiber with high magnetism is very promising as an encouraging candidate for the template of adsorption, separation and catalysis, especially in the organic solvent phase. Acknowledgments The project is supported by the Beijing Science and Technology Project (Z141100000914001). References [1] Y. Cai, F. Huang, Q. Wei, E. Wu, W. Gao, Surface functionalization, morphology and thermal properties of polyamide6/O-MMT composite nanofibers by Fe2O3 sputter coating, Appl. Surf. Sci. 254 (2008) 5501e5505. [2] Y. Lin, W. Cai, H. He, X. Wang, G. Wang, Three-dimensional hierarchically structured PAN@geAlOOH fiber films based on a fiber templated hydrothermal route and their recyclable strong Cr(vi)-removal performance, RSC Adv. 2
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