Polyaniline Electrospun Composite Nanofibers Reinforced with Carbon Nanotubes

2.1. Main Experimental Materials and Instruments

PANI powder was purchased from TCI Chemical Industry Development Co., Ltd. (Shanghai, China); camphor sulfonic acid powder (CSA) was purchased from Tokyo Chemical Industry Co., Ltd. (Japan); CNT powder was purchased from Beijing Boyu High-tech Co., Ltd. (China); nanometer (10~20 nm diameter) CB powder was purchased from Tianjin Damao Chemical Reagent Factory (China), and PAN powder with a molecular weight of 150,000 was purchased from Tangshan Xiangsheng Plastic Products Co., Ltd. (China). The above reagents were of analytically pure grade and were used directly without any treatment. An ultrasonic cleaner (CD-4800, Changzhou Ruipin Precision Instrument Co., Ltd., China), a high-voltage DC power supply (DW-P503-1ACDF, Dongwen High Voltage Power Supply Tianjin Co., Ltd., China), and a multi-channel microinjection pump (LSP10-18, Baoding Lange Constant Flow Pump Co., Ltd., China) were used in the experiments.

2.2. Experimental Scheme

PAN powders with mass fractions of 8%, 10%, and 12% were added to the appropriate amount of DMF solution, and then were magnetically stirred at 50 °C until the PAN powder was dissolved. After ultrasonic dispersion for 30 minutes, magnetic stirring was carried out overnight to prepare a spinning solution. The electrospinning process was used to prepare a PAN electrospun nanofiber membrane. PANI and CSA powder were mixed in equimolar mass for reserve. CSA-doped PANI with 1% and 2% mass fractions, respectively, was added into PAN solutions of three concentrations. After ultrasonic dispersion for 30 min, magnetic stirring was carried out overnight to prepare a spinning solution. The electrospinning process was carried out to prepare a CSA-PANI/PAN electrospun nanofiber membrane. The mass fractions of PAN and CSA-PANI were determined according to the electrospinning process and the quality of film formation. After determining that the mass fractions of PAN and CSA-PANI were 10% and 2%, respectively, nanometer CB powders with the mass fractions of 1%, 2%, 3%, 4%, 5%, 6%, and 7% were added to a DMF solution of PAN and CSA-PANI. After ultrasonic dispersion for 30 min, magnetic stirring was carried out overnight to prepare a spinning solution. The electrospinning process was carried out to prepare a CSA-PANI/CB/PAN electrospun nanofiber membrane. After determining that the mass fractions of PAN and CSA-PANI were 10% and 2%, respectively, hydroxylated multi-walled CNTs (MWCNTs-OH) and carboxylated multi-walled CNTs (MWCNTs-COOH) with the mass fractions of 0.5%, 1%, 1.5%, 2%, and 2.5% were added to the DMF solutions of PAN and CSA-PANI. After ultrasonic dispersion for 30 min, magnetic stirring was carried out overnight to prepare a spinning solution, and the electrospinning process was carried out to prepare a MWCNTs/CSA-PANI/PAN electrospun nanofiber membrane. The distance between the needle nozzle and the drum receiving device of the in-house electrospinning device was 15 cm, three needles were used, the flow rate of the injection pump was 1 mL/h, the voltage was 30 kV, the drum rotation speed was 60 r/min, and the spinning time was 12 h.

3.1. Scanning Electron Micrograph Analysis

Figures 1(a)–(c) show the scanning electron micrographs (SEMs) of the electrospun nanofibers membranes with mass fractions of 8%, 10%, and 12% PAN. The average fiber diameters were 120.99, 173.47, and 299.24 nm, respectively. A higher PAN content in the electrospun solution corresponded to larger average fiber diameter, and for all three mass fractions, the PAN fiber surface was very smooth and uniform, and no obvious spindle fibers appeared. Although the average diameter of the PAN nanofibers with a mass fraction of 8% was smaller, its fiber irregularity was the highest.

Figure 1.

SEM image of the pure PAN electrospun nanofibers membrane. (a) 8% PAN; (b) 10% PAN; (c) 12% PAN).

Figures 2(a)–(h) show the SEMs of the electrospun nanofibers membrane prepared by adding 0%, 1%, 2%, 3%, 4%, 5%, and 6% CB (CB) with the mass fractions of 10% PAN and 2% CSA-PANI, respectively. The average diameter of the fibers is shown as a function of concentration in Fig. 3. Under the same mass fraction of PAN and CSA-PANI, the average diameter of the fibers increases with the amount of the added CB. Because the CB is insoluble in the DMF solution of PAN and PANI, it is directly attached to the fibers in the form of nanoparticles with diameters in the 10–20 nm range. This will directly increase the diameter of the fibers, and when the mass fraction of CB increases to 5%, the average fiber diameter shows an abrupt increase. Compared to the diameter of pure PAN electrospinning fiber (173.47 nm) with a PANI mass fraction of 10%, the average fiber diameter decreased after adding 2% CAS-PANI. This is because the small molecule CAS-PANI mixed in the large-molecular-weight PAN reduces the binding force of the PAN molecular chain, making it easier for the fiber to be elongated and stretched in the electric field. No obvious spindle-type fibers were found in all of the images presented in Fig. 2, but it was clear that upon addition of CB, the surface of the fibers started to exhibit pronounced burr-like bulges. The burrs on the surface of the prominent fibers were formed by the aggregation of CB on the surface of the fibers. When the CB mass fraction reached 5% and 6%, the convex burrs were increasingly distributed on the surface of the fiber. In the SEM images, the surface of the fibers appeared to be covered with dense scales with many tiny holes between the scales. Upon CB addition, the diameter irregularity of a single fiber was greatly increased, but the surface of the fiber showed a large number of scaly protrusions and micropores. This structure may be valuable for applications in the fields of high adsorptive materials and superhydrophobic (lotus leaf effect) materials. Despite the fact that the mean fiber diameters of both papers were significantly different, the fibers all showed a rougher surface compared to the obtained fibers with low CB concentrations [51]. The difference in mean fiber diameter depended on the concentration of polymer and added material.

Figure 2.

SEM images of CSA-PANI/CB/PAN electrospun nanofibers membranes. (a) 10%PAN+2%CSA-PANI; (b) 10%PAN+2%CSA-PANI+1%CB; (c) 10%PAN+2%CSA-PANI+2%CB; (d) 10%PAN+2%CSA-PANI+3%CB; (e) 10%PAN+2%CSA-PANI+4%CB; (f) 10%PAN+2%CSA-PANI+5%CB; (g) 10%PAN+2%CSA-PANI+6%CB).

Figure 3.

Average diameter of CSA-PANI/CB/PAN nanofibers at varying CB concentrations

Figures 4(a)–(e) show the SEM micrographs of electrospun nanofibers membrane prepared by adding 0.5%, 1%, 1.5%, 2%, and 2.5% MWCNTs-COOH with mass fractions of 10% PAN and 2% CSA-PANI, respectively. The average diameter of the fibers as a function of MWCNTs-COOH concentration is shown in Fig. 5. Under the same mass fraction of PAN and CSA-PANI, the average diameter of the fibers decreases with increasing CNT concentration. Carboxylated CNTs can be more uniformly dispersed in the polymer matrix. Under the tensile action of a high-voltage electric field, electrospinning increases the interaction between the CNTs and polymer molecular chains, leading to the observed reduction of the fiber diameter. When the mass fraction of CNTs is 2.5%, some obvious bumps appear on the fibers. This is because CNTs tend to gather on the fiber surface in order to reduce the surface energy of MWCNTs. A high CNT content is likely to give rise to agglomeration in the fibers, leading to fiber protrusion. The SEM images of MWCNTs-COOH/PAN fiber membrane were cylindrical and uniform with a diameter between ~1 and 1.5 μm [52]. Compared with MWCNTs-COOH/PAN fiber membrane, the diameter of MWCNTs-COOH/CSA-PANI/PAN fiber membrane decreased after the addition of CSA-PANI, which was mainly because the conductivity of the spinning dope was higher after the addition of the CSA-PANI, so that the electrostatic field tensile force to which the fiber was subjected was reduced to increase the diameter of the fiber.

Figure 4.

SEM image of MWCNTs-COOH/CSA-PANI/PAN electrospun nanofibers membrane. (a) 10%PAN+2%CSA-PANI+0.5%MWCNTs-COOH; (b) 10%PAN+2%CSA-PANI+1% MWCNTs-COOH; (c) 10%PAN+2%CSA-PANI+1.5% MWCNTs-COOH; (d) 10%PAN+2%CSA-PANI+2% MWCNTs-COOH; (e) 10%PAN+2%CSA-PANI+2.5% MWCNTs-COOH).

Figure 5.

Average diameter of MWCNTs-COOH/CSA-PANI/PAN nanofibers at varying MWCNTs-COOH concentrations.

Figures 6 (a)–(e) show the SEM micrographs of the electrospun nanofibers membranes prepared by adding 0.5%, 1%, 1.5%, 2%, and 2.5% MWCNTs-OH with mass fractions of 10% PAN and 2% CSA-PANI, respectively. The average diameter of the fibers as a function of the MWCNTs-OH concentration is shown in Fig. 7. Under the same mass fraction of PAN and CSA-PANI, the average diameter of the fibers decreases strongly with increasing MWCNTs-OH concentration. For a given CNT concentration, the average diameter of the MWCNTs-OH nanofibers is smaller than that of the MWCNTs-COOH nanofibers. The SEM figure shows the rough and irregular surface morphology of PAN hard yarn waste/MWCNT-OH nanofibrous composite mats, which is due to the increased addition of MWCNT-OH in the nanofibrous composites, and the maximum fiber diameter distribution was observed in the range of 51 to 60 [53]. The maximum diameter is small because the concentration of polymer material is very low.

Figure 6.

SEM images of MWCNTs-OH/CSA-PANI/PAN electrospun nanofibers membranes. (a) 10%PAN+2%CSA-PANI+0.5%MWCNTs-OH; (b)10%PAN+2%CSA-PANI+1% MWCNTs-OH; (c) 10%PAN+2%CSA-PANI+1.5% MWCNTs-OH; (d) 10%PAN+2%CSA-PANI+2% MWCNTs-OH; (e) 10%PAN+2%CSA-PANI+2.5% MWCNTs-OH).

Figure 7.

Average diameter of MWCNTs-OH/CSA-PANI/PAN nanofibers at varying MWCNTs-OH concentrations.

3.2. Strength Test and Analysis

Strength tests were performed using a 3365-INSTRON Strength Tester and rectangular samples with the dimensions of 1 cm×10 cm. The fabric thickness was tested using a fabric thickness tester 10 times and the average strength of the fibers was calculated according to

where σ is the fracture strength (Pa), P is the breaking strength (N), and A is the section area (m²).

Figure 8 shows the fracture strength curve of the electrospun nanofiber membranes with CB and CNTs. Figure 8(a) shows that the fracture strength of the electrospun nanofiber membrane reaches the highest values for the CB fraction of 1% compared to 2% CSA-PANI in 10% PAN. The fracture strength of the fiber membrane decreases with the further addition of CB. The fracture strength with 4% CB is lower than that without CB. This is because CB is a nanospherical structure with a small aspect ratio. Thus, when added in a small amount, CB enhances the strength, while when a large amount of CB is added, the continuity of the polymer macromolecule is broken, leading to increasing brittleness and decreased fracture strength. Figure 8(b) shows that the fracture strength of the fiber membrane is enhanced by the addition of CNTs. Even for the CNTs’ mass fraction of 2.5%, both hydroxylated and carboxylated CNTs improve the fracture strength of the fibrous membrane compared to the membranes with no CNT additive. However, it is observed from the figure that with increasing addition of CNTs, the fracture strength of the fiber membrane first increases and then decreases. It can be speculated that with the continuous addition of CNTs, the fracture strength of the fiber membrane will decrease to a value lower than that of the membrane without CNTs. It is also observed that the rates of increase and decrease in the fracture strength of the fiber membrane obtained by the addition of hydroxylated CNTs are different from those obtained by the addition of carboxylated CNTs. The variation in the fracture strength of the fiber membrane is greater for the carboxylated CNTs than for the hydroxylated CNTs, and the maximum fracture strength is nearly three times higher. The optimal CNTs concentrations are also different for the carboxylated and hydroxylated CNTs, which is related to the structure of the CNTs after the surface treatment. Overall, the addition of CNTs leads to a greater improvement in the fracture strength of the electrospun nanofiber membrane than the addition of CB. Tensile testing of the prepared electrospun neat polyurea films demonstrated an average tensile strength of 14.1 MPa, and the analysis further confirmed the significant increase in tensile strength after adding 0.2% and 0.4% of MWCNTs with P values of less than 0.001. The tensile strength significantly decreased with a 1% MWCNT addition [54], which could be attributed to the nonuniform distribution, agglomeration of MWCNTs, and disruption in the hard domains due to the infiltration of MWCNTs in between the polymer chains.

Figure 8.

Fracture strength curves of electrospun nanofibers membranes at different concentrations. (a) CSA-PANI/CB/PAN; (b) MWCNTs/CSA-PANI/PAN).

3.3. Electromagnetic Shielding Test and Analysis

The electromagnetic shielding performance of the electrospun nanofiber membranes was tested using a DR-913G fabric anti-electromagnetic radiation performance tester and a sample with the dimensions of 18 cm×18 cm. The electromagnetic shielding values were calculated according to

where Se is the shielding amplitude (dB), S_B is the transmission, and S_A is reflection.

A lower negative value of the shielding amplitude, that is, a larger absolute value of the amplitude, corresponds to a smaller amount of transmission and better electromagnetic shielding performance. Figures 9(a)–(c) show the electromagnetic shielding amplitudes of CSA-PANI/CB/PANI, MWCNTs-COOH/CSA-PANI/PAN, and MWCNTs-OH/CSA-PANI/PAN electrospun nanofiber membranes, respectively, at 1, 30, and 300 MHz as a function of the additive concentration. It is observed that the electromagnetic shielding performance of fibrous membranes shows the same trend with change in the additive concentration under different radiation frequencies. Compared to the CSA-PANI/PAN fiber membranes, the electromagnetic shielding performance of the CB and CNTs fiber membranes showed the trend of first increasing and then decreasing, while the electric field shielding performance decreased to a lower level than that of CSA-PANI/PAN, and the decrease for the CSA-PANI/CB/PANI fiber membranes was more pronounced after the addition of CB. After adding 5% CB, no shielding was obtained under irradiation at 300 MHz, and no shielding was obtained under irradiation at 1 and 30 MHz after the addition of 6% CB. It is observed from Fig. 9(a) that the best electromagnetic shielding performance of CSA-PANI/CB/PANI fiber membrane was obtained for the addition of 4% CB. Figure 9(b) shows that the best electromagnetic shielding performance of the MWCNTs-COOH/CSA-PANI/PAN fiber membrane was obtained after the addition of 1% carboxyl CNTs. Figure 9(c) shows for the MWCNTs-OH/CSA-PANI/PAN electrospun nanofiber membrane, the best electric field shielding performance was obtained for the hydroxylated CNTs content of 2%. Electrospun electrically conductive nanocomposite fibers were prepared with PANI, PAN, and MWCNTs, and the reflection loss value of PANI/PAN/MWCNTs nanocomposite fibers was increased from 4.6 to 5.9 dB by increasing the thickness of composites films [54]. Combining MWCNTs with PANI is an effective strategy to improve the conductivity property for potential broadened applications.

Figure 9.

Absolute amplitude value of electrospun nanofibers membrane at different additive concentrations. (a) CSA-PANI/CB/PANI; (b) MWCNTs-COOH/CSA-PANI/PAN; (c) MWCNTs-OH/CSA-PANI/PAN).