Preparation and characterization of polyindole nanofibers by electrospinning method

Synthetic Metals 162 (2012) 2069–2074

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Preparation and characterization of polyindole nanofibers by electrospinning method Cai Zhijiang a,b,∗ , Zhang Ruihan a , Shi Xingjuan a a b

School of Textiles, Tianjin Polytechnic University, Tianjin 300160, China Key Laboratory of Advanced Textile Composites, Ministry of Education of China, Tianjin 300160, China

a r t i c l e

i n f o

Article history: Received 16 May 2012 Received in revised form 25 September 2012 Accepted 27 September 2012 Available online 9 November 2012 Keywords: Polyindole Nanofibers Electrospinning

a b s t r a c t Polyindole nanofibers with diameter ranging from 770 nm to 250 nm were firstly fabricated by an electrospinning method. Chemically synthesized polyindole was dissolved in acetonitrile to make polymer solution under ultrasonification. Electrospinning of polyindole was then carried out under electrical field strength of 1.0 kV/cm. The electrospun polyindole nanofibers exhibited smooth surface and the diameter of the fibers was ranged from 768 nm to 255 nm. The specific surface areas of polyindole nanofibers were ranged from 32 to 65 m2 /g, which is significantly higher than that of the powder with same volume. The electrical conductivity of the polyindole nanofibers can reach 0.24 S/cm, which is much higher than that of the polyindole film. The polyindole nanofibers showed high thermal stability with glass transition temperature (Tg ) around 132 ◦ C and melting point (Tm ) around 239 ◦ C. The crystallinity of polyindole nanofibers was higher than that of polyindole film due to the formation of ordered molecule chains during the electrospinning. Cyclic voltammetry test results revealed that the doping and de-doping processes of BF4 − ions were reversible and polyindole nanofibers had high electronically activity in the electrolytic solution containing LiBF4 . © 2012 Elsevier B.V. All rights reserved.

1. Introduction Conducting polymers nanofibers with high specific surface areas are excellent candidates for many applications such as microsensors, actuator and supercapacitor, etc. As the most effective method to fabricate nanofibers, electrospinning technology has gained much attention because it can provide a simple and unique technique for preparation of fibers with the diameters ranging from the nano- to micro-meter scale [1]. The electrospun fibers have exceptionally long length, uniform diameter, high porosity, interconnectivity, interstitial and large surface to volume ratio. Nowadays, fabrication of nanofibers made of conductive polymers has been reported in the design and construction of electrochemical devices [2]. The nanofibers with high specific surface area are advantageous as electrodes for the energy storage devises such as batteries and capacitors [3,4]. The high specific surface area electrodes bring high utilization of the electrochemical active materials and high charge–discharge rate to batteries and capacitors. Polyindole is a conducting electroactive polymer, which can be obtained by electrochemically oxidation of indole in various

∗ Corresponding author at: No. 399 BingShuiXi Street, XiQin District, Tianjin 300387, China. Tel.: +86 22 83955240; fax: +86 22 83955187. E-mail address: [email protected] (C. Zhijiang). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.09.019

electrolytes and chemical polymerization by oxidants [5]. Polyindole has fairly good thermal stability [6], high-redox activity and stability [7], slow degradation rate in comparison with polyaniline and polypyrrole [8], and an air stable electrical conductivity close to 0.1 S/cm in the doped state [9]. Although polyindole possess the properties of both poly(para-phenylene) and polypyrrole together, polyindole and its derivatives have only scarcely been investigated among various aromatic-compound-based conducting polymers. By now, electrospinning of polyaniline [10,11], polypyrrole [12,13] and polythiophene [14] have been investigated. However, few literature about polyindole nanofiber has been reported. In the present study, we report for the first time a process to produce polyindole nanofibers by electrospinning. The characteristics and electrical performance of polyindole nanofibers are investigated.

2. Experimental 2.1. Materials The materials used in this investigation and their sources are as follows: indole was obtained from Aldrich Chemical Co., LTD; chloroform and ferric chloride were obtained from Sigma Chemical Co., LTD and used as received. All other chemicals employed were of analytical reagent grade.

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Table 1 Electrospinning parameters and properties of polyindole nanofibers membrane. Sample code

Concentration (%)

Viscosity (cP)

Electrical field strength (kV/cm)

Fiber diameter (nm)

Specific Surface area (m2 /g)

PI-1 PI-2 PI-3

2.2 3.5 5.0

900 ± 40 1700 ± 85 2600 ± 110

1.0 1.0 1.0

255 ± 60 414 ± 85 768 ± 120

65.27 49.11 32.65

2.2. Synthesis of polyindole The synthesis of polyindole was carried out by the following procedures. The reactor used was a 4-necked 500 ml round-bottom flask, provided with a stirrer and was placed in a thermostatic bath. It was purged by a cycle comprising of placing under vacuum 3 times and rinsing 3 times with pure and dry nitrogen. 180 ml of chloroform, previously degassed with nitrogen, were introduced into this round-bottom flask, which was kept at 25 ◦ C under nitrogen, and 25 g of anhydrous ferric chloride were then filled with 5 ml of demineralized water and the reservoir with a tap was filled with 20 ml of chloroform and 3.6 g of degassed indole. The water, using a syringe, and the chloroform and indole, using the reservoir with a tap, were then introduced in parallel into the flask in the course of 10 min. The molar ratio of ferric chloride to indole was 5.0. The flask was then kept at 15 ◦ C for 5 h, with stirring; the pH of the reaction mixture is 1.50 ml of water were then introduced into the flask, which was kept at 15 ◦ C, in the course of 45 min. The product, which at this stage was in the form of a suspension, was filtered under air at 20 ◦ C. The product obtained was washed 4 times with 100 ml of water at 20 ◦ C and then dried overnight under vacuum at 20 ◦ C under 2670 Pa.

NETZSCH DSC 200 F3, USA. Samples were heated from room temperature to 400 ◦ C at the heating rate of 10 ◦ C/min in nitrogen atmosphere (flow rate, 20 ml/min). Thermogravimetric analysis (TGA) was carried out with a NETZSCH STA 409 PC/PG system. All analyses were performed with a 10 mg sample in aluminum pans under a dynamic nitrogen atmosphere from room temperature to 800 ◦ C. X-ray diffraction pattern (XRD) was recorded on an X-ray diffractometer (D/MAX-2500, Rigaku), by using Cu K( radiation at 40 kV and 30 mA. The diffraction angle ranged from 5◦ to 40◦ . Electrical conductivity measurement for polyindole nanofibers membrane (bulk) was performed by the four-point probe method using a Bekktech conductivity cell. At least three measures were taken for each sample and average values were reported in this paper. The I–V curve of polyindole single fiber was recorded on an Agilent 4156 semiconductor analyzer at room temperature, from which the conductivity of polyindole single fiber was calculated. The cyclic voltammogram was performed with a Solartron electrochemical interface (model 1287 Solartron UK) connected to a PC through the serial port. The experiment was carried out in a three-electrode single-compartment cell using Pt gauge and a saturated calomel electrode (SCE) as counter and reference electrode, respectively.

2.3. Elelctrospinning of polyindole

3. Results and discussion

Polyindole solution was prepared by dissolving polyindole powder in acetonitrile under ultrasonification and then filtered through Teflon membrane filter. Electrospinning of polyindole was carried out as the following procedures. The polyindole solutions were filled into a glass syringe terminated by a stainless steel needle whose inner diameter is 0.30 mm. The syringe was placed in a automatic pump and polyindole solution was extruded out at a constant speed of 1.0 ml/h under the electrical field strength of 1.0 kV/cm. The polyindole nanofibers were collected by conductive paper of carbon fiber. The experiment was done in a environmental chamber with constant temperature at 25 ◦ C and the relative humidity (RH) at 35%. Electrospinning conditions and sample code is listed in Table 1.

In the electrospinning process, a suitable viscous solution is key point to make polyindole nanofibers. Polyindole with high molecular weight is insoluble. To make polyindole nanofibers, polyindole with low molecular weight of 5100 was synthesized by controlling the parameters of synthesis process. At viscosities lower than 500 cP, the polyindole solution is not viscous enough to electrospin a uniform fibrous structure. This is in the transition zone between electrospinning and electrospraying, which results in what is commonly referred to as a “bead-on-a-string” structure. At viscosities over 3000 cP, the polyindole solution is too viscous to flow through the nozzle. In the present work, polyindole solution with viscosities ranged from 900 to 2600 cP is used for electrospinning by controlling the concentration of polyindole solutions from 2.2% to 5.5%. Fig. 1 shows the surface morphology of polyindole nanofibers prepared by electrospinning method. The porous structure made by polyindole nanofibers can be observed and the polyindole nanofibers exhibit smooth surfaces, round shape and are randomly oriented. As we known, fiber diameter is dependent on many electrospinning variables including electrical field strength and polymer concentration. By varying electrical field strength and polymer concentration, it is possible to modify the average fiber diameter of the electrospun nanofibers. In the present work, the electrical field strength is fixed at 1.0 kV/cm and the polyindole solution concentration is ranged from 2.2% to 5.5%. The diameter of the electrospun nanofibers is measured by computer analysis using five SEM images for each sample. With the concentration of polyindole solution increasing from 2.2% to 5.5%, the average fiber diameter increases from 255 nm to 768 nm. The distribution of the diameter of the electrospun polyindole fibers is plotted and shown in Fig. 2. The fiber diameters mainly lay in the distribution range of 100–400 nm for PI-1 and the variation of diameter of the fibers is in very small range. With the concentration of polyindole solution increasing, the fibers diameter distribution range becomes broad

2.4. Characterization Viscometer (Brookfield Corporation DV-II) was used to measure the viscosity of polyindole solution prior to electrospinning. The morphological and phase analysis for the obtained polyindole nanofibers were characterized by scanning electron microscopy (SEM, Model S-4200, Hitachi, Japan), a transmission electron microscope (TEM, JEM-1200EX, Japan) and Fourier transformation infrared (FT-IR) spectroscopy (Perkin Elmer Spectrum RX-I). The 1 H NMR spectrum was recorded on a JEOL GAM-ECP600 NMR spectrometer and CDCl3 was used as the solvent. The molecular weight of polyindole before and after electrospinning was determined by Gel Permeation Liquid Chromatography (GPLC) method (Shimadzu, Japan). The Brunauer–Emmett–Teller (BET) surface area was measured by using a surface area analyzer (SAA: Sorptomatic 1990, ThermoFinnigan Co.). The samples were degassed overnight in a vacuum at 100 ◦ C, and N2 gas was used. A relative pressure range, P/P0 , of 0.05–0.3 was used for calculating the BET surface area. Differential scanning calorimetry (DSC) test was performed using

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70

PI-1

Percentage (%)

60 50 40 30 20 10 0 0

100

200

300

400

500

600

700

800

Diameter (nm) 60

PI-2

Percentage (%)

50 40 30 20 10 0 0

100

200

300

400

500

600

700

800

900

Diameter (nm) PI-3

50

Percentage (%)

40

30

20

10

0 Fig. 1. Morphology of polyindole nanofibers membrane prepared by electrospinning under different conditions.

0

100

200

300

400

500

600

700

800

900 1000

Diameter (nm) Fig. 2. Distribution of the diameter of the electrospun polyindole nanofibers.

and the variation of diameter of the fibers also becomes bigger. The average diameter as well as specific surface areas is summarized in Table 1. The specific surface areas are in the range of 32–65 m2 /g, depending on the fiber diameter. With the diameter of polyindole nanofiber decreasing, the specific surface area increases. The electrospinning process also has influence on the molecular weight of polyindole. The average molecular weight of polyindole decreases to 3800 after electrospinning. The reason might be attributed to sonication in acetonitrile and high electrical strength. The research is ongoing in our lab and we will make a deep investigation. A TEM image for the nanofibers is shown in Fig. 3. It can be seen that the polyindole nanofiber shows a regular round shape structure with diameter about 250 nm. The surface of the nanofiber is smooth and there are no pores on it.

Fig. 4 presents the FT-IR spectrum of polyindole nanofibers obtained by electrospinning. The broad peak at 3409 cm−1 observed in the spectrum of polyindole nanofibers membrane is a characteristic absorption of the N H bond. This band together with the band at 1560 cm−1 can be ascribed to be the stretching and deformation vibrations of the N H bond, respectively. The band at 1366 cm−1 is related to modes involving the C8 N C2 C3 group. The band at 741 cm−1 indicates that the benzene ring is not affected during the polymerization process of indole. The single peak located at 1459 cm−1 is assigned to the stretching of the benzene ring. The band at 1636 cm−1 can be ascribed to the C C vibration on indole ring. The characteristic peaks are similar in both the cases for PI-1, PI-2 and PI-3, which are showing the same type of polymer growth

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Fig. 5.

Fig. 3. TEM image of polyindole nanofiber (PI-1) obtained by electrospinning.

and no difference in the monomers linkages. The peak at 3400 cm−1 assigned to the N H stretching indicating nitrogen species are not the polymerization sites, therefore the only possibility of the polymerization is through 2 and 3 positions of the monomers. Fig. 5 displays the 1 H NMR spectrum of indole and polyindole nanofiber obtained from electrospinning. The spectrum of indole monomer (Fig. 5a) shows five groups of protons: ı6.6(H-3), ı7.5(H-2), ı7.6(H-6 and H-7), ı8.18(H-4 and H-5) and ı11.7(H1), respectively. Three peaks around ı7.4, ı8.7 and ı11.9 can be observed in the polyindole nanofiber spectra as shown in Fig. 5b, which represent the H-6 and H-7, H-4 and H-5, H-1 proton lines arisen from the benzene ring of indole units, respectively. The proton lines of polyindole are much broader than the corresponding proton lines of indole monomer and the peaks location is shifted to high value region due to the wide molar mass distribution of polyindole. Based on NMR and FT-IR spectra and literature reported [15–18], the polymerization reactions of polyindole should be occurred at C2 and C3 positions of the monomers. Thermal stability of the polyindole before and after electrospinning was studied by differential scanning calorimetry (DSC) up to 400 ◦ C. The results are shown in Fig. 6. Before electrospinning, polyindole shows glass transition temperature (Tg ) at 134 ◦ C and

Fig. 4. FT-IR spectrum of polyindole nanofibers obtained by electrospinning.

1

H NMR spectra of indole (a) and polyindole nanofiber PI-1 (b).

melting point (Tm ) at 240 ◦ C. After electrospinning, the glass transition temperature (Tg ) and melting point (Tm ) are both slightly shifted to lower temperature region, which is about 130 ◦ C and 238 ◦ C. The reason might be attributed to molecular weight degradation during the electrospinning process. However, in both case, the polyindole show high thermal stability with glass transition temperature (Tg ) around 132 ◦ C and melting point (Tm ) around 239 ◦ C. Thermo-gravimetric analysis (TGA) is a continuous process, involving the measurement of sample weight in accordance with increasing temperature in the form of programmed heating. Since TGA provides better understanding of thermal decomposition behavior, the TGA test is performed for polyindole before and after electrospinning and the results are given in Fig. 7. For both case, the TGA curves of polyindole show an initial weight loss and a prominent weight loss. The initial weight loss is about 3% at temperatures below 70 ◦ C before electrospinning and 5% at temperatures below 100 ◦ C after electrospinning. This small weight loss could be due to a loss of moisture trapped in the polymer. After electrospinning, the moisture trapped in the nanofiber is higher than that of before electrospinning. The reason should be due to high specific surface areas of nanofiber which can absorb more moisture and trap moisture harder. The prominent weight loss happens in the range of temperature between 425 and 650 ◦ C before electrospinning and in the range of temperature between 420 and 520 ◦ C

Fig. 6. DSC curves of polyindole (a) and polyindole nanofiber PI-1 (b).

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Fig. 9. Electrical conductivity of polyindole film and polyindole nanofibers.

Fig. 7. TGA curves of polyindole (a) and polyindole nanofiber PI-1 (b).

after electrospinning. This prominent weight loss is inferred to be due to the degradation of the skeletal polyindole backbone chain structure. Compared with two TGA curves before and after electrospinning, the thermal degradation starting temperature is very close while the thermal degradation termination temperature has big difference. The thermal degradation rate of polyindole after electrospinning is quite higher than before electrospinning. Since the thermal stability is strongly effect by molecular structure, this difference in thermal performance should be due to the molecular weight degradation during the electrospinning process. However, the thermal stability of polyindole only slightly decays after electrospinning which will not affect its applications. X-ray diffraction studies of polyindole before and after electrospinning, i.e. polyindole powder and polyindole nanofibers are shown in Fig. 8. Polyindole shows crystalline characteristics. For polyindole powder (Fig. 8a), two peaks located at 19.10◦ and 26.30◦ (2) can be observed. Polyindole nanofibers show highly crystalline nature of the polymer (Fig. 8b–d) in comparison to polyindole powder. However, both show much more crystallinity than electrochemically formed polyindole. Crystalline nature of the polyindole nanofibers is supported by various sharp peaks at 11.4◦ , 17.5◦ , 20.10◦ , 23.4◦ , 26.4◦ (2). The characteristic peaks of polyindole nanofibers with different diameter (PI-1, PI-2, PI-3) are almost same. The only difference lies in the scattering intensity. With the diameter increasing, the scattering intensity decreases, which

Fig. 8. XRD patterns of polyindole (a) and polyindole nanofibers: PI-1 (b), PI-2 (c), PI-3 (d).

means the less crystallinity. As previous studies have shown that the strong stretching forces associated with electrospinning may induce orientation of polymer chains along the long axis of a fiber [19]. The finer fibers have more ordered molecule chains and have a higher degree of crystallization. Fig. 9 shows the electrical conductivities of polyindole films obtained from the polymer solution casting and polyindole nanofibers prepared by the polymer solution electrospinning. From Fig. 9, we can see that the electrical conductivity of the polyindole nanofibers is higher than that of the polyindole film. The electrical conductivity of polyindole nanofibers increases almost linearly with the average fiber diameter decreasing. The electrical conductivity of polyindole nanofibers increases from 0.16 S/cm to 0.24 S/cm when the average fiber diameter decreases from 768 nm to 255 nm. This result indicates that the electrical conductivities measured by the four-point probe method are affected not only by the intrinsic properties of the materials but also by the geometric structure of the fibers. The reason might be explained as following. Under high electrical strength the electrostatic force is higher which makes the fibers more highly stretched. In the results, the polymer chains are more compacted in the fibers, leading to a decrease in diameter. The chains within the fibers would then have better interactions with their neighbors, the lack of which is known to be the main limiting parameter to electrical conductivity. The polymer chain compaction should then lead to a better charge transport inside the fibers [20]. At the same time, the decrease in fiber diameter allows the deposition of more fibers in a given volume. Since the measured conductivity is a volumic one, more fibers can provide more electronic paths usable for the conduction. Fig. 10 shows the I–V curves for polyindole single nanofiber that have a average diameter ranged from 768 nm to 255 nm. The I–V curves show a linear relationship and the calculated conductivity for polyindole nanofiber, PI-1, PI-2 and PI-3 is 0.136 S/cm, 0.093 S/cm and 0.076 S/cm, respectively. These values represent the intrinsic conductivity of the polyindole nanofiber. It indicates that it is a very efficient method by decreasing the diameter of polyindole nanofiber to increase the conductivity. It is because that the finer fibers have much more compact as analyzed above. This a little more compact in some place along the long axis of fiber can lead to a higher conductivity. Since the polyindole nanofibers show significantly higher specific surface areas together with higher electrical conductivity, it is expected to be more useful in various potential applications such as sensor, actuator, and electrode materials in the future. Fig. 11 shows CV curves of the polyindole nanofibers in the EC solution of LiBF4 at room temperature between 0.2 and 1.3 V with a

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4. Conclusions

Fig. 10. I–V curves for polyindole nanofibers with different diameter.

Polyindole, which was synthesized by chemical oxidation using anhydrous ferric chloride as oxidants, is firstly electrospun into nanofibers using acetonitrile as solvent. Electrospinning parameters such as viscosity, electrical field strength, flow rate and environmental condition are important parameters to successfully prepare polyindole nanofibers. In the present work, electrical field strength of 1.0 kV/cm is applied and the nanofibers with diameter ranged from 768 nm to 255 nm are fabricated. The FT-IR and 1 H NMR spectrum confirm the chemical structure of polyindole nanofibers. SEM and TEM images show electrospun polyindole nanofibers have smooth surface and round shape. The electrical conductivity of the polyindole nanofibers can reach 0.24 S/cm, which is much higher than that of the polyindole film. The specific surface areas of polyindole nanofibers are significantly high ranged from 32 to 65 m2 /g. The polyindole nanofiber shows high thermal stability and higher degree of crystallization than that of polyindole film due to the formation of ordered molecule chains during the electrospinning. Cyclic voltammetry test results reveal that the doping and de-doping processes of BF4 − ions are reversible and polyindole nanofiber has high electronically activity in the electrolytic solution containing LiBF4 . This electrospun polyindole nanofibers is promising in some potential applications especially as electrode materials for the rechargeable battery. This kind of work is ongoing in our lab and the results will be reported later. Acknowledgement This work was supported by Tianjin Municipal Natural Science Foundation under the contract of 11JCYBJC02500. References [1] [2] [3] [4]

Fig. 11. Cyclic voltammogram of polyindole nanofibers in the EC solution of LiBF4 at room temperature (scan rate: 3 mV/s).

scan rate of 3 mV/s. A multi-step redox process seems to be operating for polyindole nanofiber with anodic peaks at 1.1 V and catholic peak at 0.9 V. It is known that the conductivity of a conducting polymer can be improved greatly by doping other ions. In this study, polyindole is in a state that BF4 − ions are doped into it in the EC solution. Upper peak at 1.1 V corresponds to the oxidation (BF4 − doping) and lower peak at 0.9 V corresponds to the reduction (BF4 − de-doping). The CV curves follow same pattern with slight difference in path, showing that the diameter of polyindole nanofiber might have some effect on the effective doping and de-doping process of BF4 − during cyclic voltammetry test which may be due to less electrical conductivity and specific surface area of polyindole nanofibers. It is well known that the plot of I–E on the cyclic voltammogram is equivalent to that of current vs. time [21]. That is, the area of the cyclic voltammogram represents the quantity of electricity. Based on the areas of curves in Fig. 11, PI-1 shows the highest electrochemical activity. This result is in accordance with the electrical conductivity test results.

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