Electromagnetic interference shielding with polyaniline nanofibers composite coatings.

INTRODUCTION

Since the findings by Huang at al. [1] that polyaniline (PANI) nanofibers can be facilely prepared by interfacial polymerization in 2003, PANI nanofibers has been extensively investigated worldwide in both the synthesis and application areas. In the interfacial polymerization, aniline and ammonium peroxydisulfate (APS) were dissolved in an organic solvent and an acidic aqueous solution, respectively, and the polymerization occurred only at the interface formed between the two solutions. The primarily formed PANI nanofibers were drawn into the water phase owing to their hydrophilicity and, thus, the secondary growth of the primary PANI nanofibers was prevented [1]. By selecting the acid species and controlling their concentrations, diameters of the PANI nanofibers can be roughly controlled according to the same group [2]. Latterly, an even simpler method was reported by Huang and Kaner [3] and Chiou and Epstein [4], in which the acidic aqueous solutions of aniline and APS were mixed rapidly and left without agitation. Because all the aniline and APS molecules reacted instantly, the secondary growth of the primary PANI nanofibers was depressed, leading to formation of PANI nanofibers. However, the aniline concentration in this method was very low, which is disadvantageous for the industrial production of PANI nanofibers. Although several other template-free methods, chemically and electrochemically [5], were reported for the preparation of PANI nanofibers in the following years, the interfacial polymerization is still deemed as the most facile and general one [1, 2].

PANI nanofibers have relatively higher specific surface area and capacitance, and can be very promising candidate materials for gas sensing and energy storages. For instance, on exposure to HCl and N[H.sub.3] gases with concentration of 100 ppm, the resistance response of the PANI nanofibers film was much faster than that of the traditional PANI film [1]. Besides, the response sensitivity of the PANI nanofibers film was independent on the film thickness, which is completely different from the traditional PANI film. In addition to the acidic or basic gases, PANI nanofibers were also tested for neural gases like methanol [6], water [6], and hydrogen [7], and faster responses and good repeatability were observed. In the field of energy storage, a much higher capacitance value was observed for the HC1 doped PANI nanofibers [8]. Recently, PANI nanofibers were also employed as cathode material for rechargeable battery and higher specific energy and good recycleability were demonstrated [9]. Furthermore, PANI nanofibers showed good promising in the field of flash welding [10], field emission [11], and nonvolatile memory [12]. Another important characteristic of PANI nanofibers is their excellent dispersibility. For example, PANI nanofibers can be easily dispersed in water without use of any stabilizer, and PANI nanofibers films can be prepared by casting the dispersion [13]. The good dispersibility of PANI nanofibers is beneficial for preparation of PANI nanofibers based composites, a promising way to solve the processing problem of PANI [14, 15].

Electromagnetic interference (EMI) is of any unwanted, conducted or radiated signals [16] and caused not only unacceptable degradation of systems or equipment performances, but also some health concerns [17]. Although metallic materials exhibit good shielding performance of EMI, these materials suffer both the weight and the corrosion penalties. Therefore, novel candidate materials like carbon [18], conducting polymers [19], especially PANI [20, 21], etc. have been evaluated for EMI shielding owing to their lightness and excellent corrosion resistance. For example, Trivedi and Dhawan [20] found that the grafted PANI fabrics exhibit shielding effectiveness of 16-18 dB at the frequencies range of 0.1 MHz to 1 GHz, and more than 40 dB at lower frequencies. The same group [22] also studied the shielding performance of free standing PANI films. PANI was also extrusion blended with acrylonitrile-butadiene-styrene and tested for EMI shielding with different loadings, and the best shielding effectiveness, larger than 60 dB, was achieved with PANI loading of 50% [23]. Later, Dhawan et al. [24] reported the similar results on PANI/polystyrene composites, and the best shielding effectiveness with PANI loading of 50% in the composite was 58.59 dB, lower than that for the ABS composite [23]. Conductive PANI/silicone rubber composite with different PANI loadings was also studied for EMI shielding in the low frequency range of 3-1500 MHz, and shielding effectiveness from 16 to 19.3 was demonstrated by the composite at 100 mass ratio loading of PANI [25]. Furthermore, the conjugated polymers exhibit a dominant shielding characteristic of absorption rather reflection like metals [26], which is more preferred in military uses like camouflage and stealth technology [27]. The preparation of CSA doped PANI based EMI shielding coating by using bead milling approach was also reported by the author [28].

In this study, PANI nanofibers were prepared by an interfacial polymerization, and conductive PANI nanofibers composite coatings were prepared and suggested for EMI shielding. UV-vis spectroscopy, Fourier transformed infrared (FTIR) spectroscopy, scanning electronic microscopy (SEM), four-probe conductivity, and EMI shielding effectiveness measurement were employed to characterize the structure and properties of the PANI nanofibers and the conductive composite coatings.

EXPERIMENTAL

Materials

All reagents were analytical grade from Xi'an Chemical Reagent Factory and were used as received except for aniline, which was doubly distilled under reduced pressure before use. Polyacrylate resin (methacrylate and acrylate) was industrial materials from Wuxi Resin Factory. Deionized water was used always.

Synthesis of PANI Nanofibers

PANI nanofibers were synthesized by an interfacial polymerization. In a typical procedure, 4.65 g of aniline was dissolved in a 500 ml beaker containing 100 ml tetrachloride and the solution was brought to 20[degrees]C. In the same time, 5.70 g of APS was dissolved in 250 ml hydrochloric acid (1 mol/l) with mechanical stirring. After brought to 20[degrees]C, the aqueous APS solution was gently transferred to the beaker containing the organic solution of aniline, and the mixture was left for polymerization without any agitation. With 8-h reaction, the aqueous phase was filtered and washed with hydrochloric acid (1 mol/l) until the filtrate became colorless. The product isolated was mixed with 300 ml acetone by mechanical stirring for 30 min and then filtered. After dried in vacuum (45[degrees]C) for 8 h, the hydrochloric acid doped PANI nanofibers were obtained. Dedoped PANI nanofibers were prepared by treating the doped samples with ammonia water.

Preparation of Conductive Coatings

A specific amount of the as-prepared hydrochloric acid doped PANI nanofibers were mixed with 30 ml cyclohexanone and mechanically stirred for 1-2 h to form a homogeneous dispersion. Meanwhile, 5 g of polyacrylate powders were dissolved in 25 ml cyclohexanone to form a solution. The PANI nanofibers dispersion and the polyacrylate solution were then mixed and mechanically stirred for 1 h. The green or dark green colored dispersions were sprayed on poly (vinyl chloride) coupons (90 x 90 x 2.0 [mm.sup.3]), which were precleaned with ethanol and water, and dried at room temperature in advance, to prepare the conductivity and EMI shielding effectiveness measuring specimens. The thickness of the composite coatings was kept at 100 [+ or -] 5 [micro]m with the help of a micrometer screw gauge.

Characterization

UV-vis spectrum was recorded on a U-2001 UV-vis spectrophotometer (Hitachi). Fourier transform infrared (FTIR) spectra were recorded on an FTIR spectrophotometer AVATARTM 360 FT-IR (Thermo Nicolet). Solid specimens were prepared by mixing the PANI nanofibers with KBr and then pressed into round pellets. Morphological studies were performed on a JSM-6460 (JEOL) scanning electron microscope (SEM). The electrical conductivities of the pressed pellets were measured with a four-probe technique using a Keithley 2001 electrometer. EMI shielding effectiveness was measured according to the method specified by American Military Standard MIL-STD-285 [29].

RESULTS AND DISCUSSION

Synthesis of PANI Nanofibers

In consideration of the volatility and toxicity of the organic solvents like benzene, toluene, and xylene, tetrachloride was selected as the organic solvent for aniline in that its density is larger than water and can be covered with the solution of APS during the reaction. In the interfacial polymerization reported by Huang et al. [1], the concentration of aniline (0.32 mol/l), the molar ratio of APS/aniline (1/4), and the polymer yield (6-10%) are very low. To enhance the yield of the polymerization, the concentration of aniline was increased to 0.5 mol/l, and the molar ratio of APS/aniline was increased from 1/4 to 1/2, so that more PANI can be formed. The yield of the hydrochloric acid doped PANI nanofibers can be as high as 23%, which is two times higher than that reported by Huang et al. [1].

With transfer of the aqueous acidic solution of APS to the organic solution of aniline, green color was firstly observed at the interface, indicating the formation of the primary PANI nanofibers. With proceeding of the reaction, similar phenomenon as that reported by Huang et al. [1], i.e., diffusion of the green color from the interface into aqueous phase, was observed. In about 2 h, the whole water phase was filled with the green color. To ensure the complete of the polymerization, the product was isolated after 8-h reaction. It is shown in morphology study that most of the product is in the form of nanofibers (Fig. 1). In addition, a small amount of irregular shaped PANI particles and PANI nanofibers with relatively larger diameter were also presented in the product, which was probably resulted from the relatively higher concentration of aniline. With diffusion of the primary PANI nanofibers formed at the interface into the aqueous phase, aniline molecules were also dissolved slowly from the organic phase into the acidic aqueous phase. It has been reported that the polymerization of aniline would be catalyzed by PANI [30], as well as surfaces or interstices [31, 32]. The polymerization of the aniline molecules dissolved in the acidic aqueous solution occurred preferentially on the surfaces of the PANI nanofibers, leading to the formation of the PANI nanofibers with larger diameters, as well as few irregular shaped PANI particles.

[FIGURE 1 OMITTED]

Characterization of PANI Nanofibers

As reported by other researchers [5], PANI nanofibers have excellent dispersibility and can be easily dispersed in water. By ultrasonic processing, ca. 0.01 g of the PANI nanofibers, either in doped or dedoped state, can be dispersed in 300 ml cyclohexanone, obtaining a light green or blue colored dispersion, with which the UV-vis spectra of the PANI nanofibers were recorded (Fig. 2). The spectrum of the dedoped nanofibers is similar to the bulk sample [28], with two absorption bands located at ca. 328 and 610 nm, representing the [pi]-[pi]* transition in the benzenoid structure and the exciton formation in the quinonoid rings, respectively. For the doped nanofibers, the first band located at ca.382 nm, which might be resulted from the polaron-[pi]* transition of doped PANI; the second band at ca. 850 nm is related to the polaron of the doped PANI.

[FIGURE 2 OMITTED]

FTIR spectrum of the dedoped PANI nanofibers was similar to that reported in literature and is not presented here. The bands of 1590 and 1498 [cm.sup.-1] correspond to the quinoid and benzenoid ring-stretching modes of dedoped PANI molecules, respectively. The bands at 1510, 1580, and 1608 [cm.sup.-1] are assignable to the aromatic C=C stretch; and an 820 [cm.sup.-1] band to an aromatic C-H out-of-plane bending.

Conductivity

The pellet conductivity of the hydrochloric acid doped PANI nanofibers is 4.7 S/cm, which is in the same order of magnitude as the traditionally prepared hydrochloric acid doped PANI. By incorporating the PANI nanofibers into the insulating polyacrylate coating, the composite coatings become electrically conductive. The conductivity of the coatings increased with the loadings of the PANI nanofibers and a percolation behavior was demonstrated (Fig. 3). The percolation threshold of the PANI nanofibers composite coatings is ca. 0.2, which is lower than the 0.3 for the previous study on PANI powder composite coatings [28]. The reason can be due to the higher aspect ratio of the PANI nanofibers than that of the PANI powders, which is favorable for the formation of conducting paths insider the composite coatings and hence a relatively lower percolation threshold. Furthermore, it can be seen from Fig. 3 that at higher loadings of PANI (larger than 15 wt%), the conductivity of the PANI nanofibers composite coatings is significantly higher than that of the PANI powder composite coatings. The reason lies in that, on the one hand, the higher conductivity of hydrochloric acid doped PANI than that of CSA doped PANI; on the other hand, as mentioned above, the higher aspect ratio of PANI nanofibers. While at lower loadings of PANI (less than 10 wt%), the conductivities of the two composite coatings are nearly the same (Fig. 3), indicating that no effective conducting paths were formed. Morphological studies revealed that PANI nanofibers were uniformly distributed in the composite irrespective of their loadings (Fig. 4). While the higher the PANI nanofibers loadings, the more the contacts formed between the nanofibers and the higher the conductivity of the composites.

[FIGURE 3 OMITTED]

EMI Shielding Effectiveness

Shielding effectiveness of the composite coatings increased with increasing of the PANI nanofibers loadings in the coating (Table 1). As long as the PANI nanofibers loadings is high enough (e.g., higher than 35%), shielding effectiveness in the range of 38-63 dB can be achieved with coatings thickness of ca. 100 [micro]m, which is higher than the 30 dB for common commercial application requirement. Furthermore, as compared with the previous study [28], the shielding effectiveness of the PANI nanofibers composite coatings was relatively higher, which can be attributed to, on the one hand, the better dispersing state of the PANI nanofibers in the composites as mentioned above, and on the other hand, the higher conductivity of the hydrochloric acid doped PANI nanofibers than that of the CSA doped PANI. For a given coatings, similar trends of relatively higher shielding effectiveness at low frequencies and slightly lower shielding effectiveness at higher frequencies were observed as reported in the previous study [28], suggesting that the coatings were more effective in shielding electromagnetic waves of low frequencies. The coatings demonstrate lightness and corrosion resistance as compared with the metal based shielding materials and can be promising choice for EMI shielding.

[FIGURE 4 OMITTED]

CONCLUSIONS

PANI nanofibers were synthesized by an interfacial polymerization with relatively higher monomer concentrations. The polymer yield can be as high as 23%, though a small amount of irregular shaped PANI particles and PANI nanofibers with relatively larger diameter were also obtained. Pellet conductivity of the PANI nanofibers is 4.7 S/cm. The conductivity of the PANI nanofibers composite coatings increased with the PANI nanofibers loadings and demonstrated a percolation threshold of 0.2. The shielding effectiveness of the PANI nanofibers coatings was in the range of 38-63 dB in the frequency range of 100 kHz to 10 GHz with PANI nanofibers loadings higher than 35%, which poses great importance in common commercial applications of the materials.

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Yuhua Niu

School of Vocational Technology, Shaanxi University of Science and Technology, Xi'an 710016, People's Republic of China

TABLE 1. EMI shielding effectiveness of the PANI nanofibers composite
coatings (dB).

                    F
L (%)  200 k  600 M  1 G  5 G  10 G

25     23     35     41   26   22
35     45     55     47   44   38
45     50     63     52   46   43

F and L denote the frequency of the electromagnetic wave in Hz and the
PANI nanofibers loadings in the composite coatings, respectively.