Polyaniline/montmorillonite nanocomposites for adsorption purposes

JOURNAL OF SCIENCE OF HNUE Natural Sci., 2008, Vol. 53, N ◦ . 5, pp. 104-109 POLYANILINE/MONTMORILLONITE NANOCOMPOSITES FOR ADSORPTION PURPOSES Vu Quoc Trung, Nguyen Van Thang Pham Van Hoan and Nguyen Duc Chuy Hanoi National University of Education Abstract. Nanocomposites based on polyaniline (PANI) and montmoril- lonite (MMT) were prepared by in-situ polymerization of aniline monomers using FeCl3 as oxidant in the presence of MMT. X-ray diffraction patterns (XRD) showed that the obtained materials were in the nanostructures. The chemical structure of PANI in nanocomposites were characterized by FT- IR and Raman spectra. The thermal analysis (TGA) showed that PANI in the nanocomposites was stable until 6000C. Adsorption properties of the nanocomposites were investigated. 1. Introduction Conducting polymers are novel organic semiconducting materials with great promise because of their wide range of potential technological applications [1]. This includes their applications in electrochemical batteries, electrochromic devices, light emitting diodes, non-linear optics, photovoltaic devices, FET circuits and LEDs [1], as a corrosion inhibitor [2] and as a material for the electromagnetic interference shielding [3]. The understanding of the nature of these polymers is of utmost impor- tance for developing electrochemical devices. Among the conducting polymers, PANI has been studied extensively due to the commercial availability of the monomer, its easy synthesis, well-behaved electrochemistry, good environmental stability, high conductivity and multiple redox and protonation states [1,4]. Normally intractable PANI can be made processable either by making it sol- uble by doping it with organic sulfonic acids [4] or by the preparation of polyaniline in colloidal form [5]. Polymeric as well as non-polymeric surfactants have been used to make colloidal polyaniline. The presence of a surfactant or polyelectrolyte sig- nificantly modifies both the microscopic and macroscopic properties of the final polymer. Recently, the electrical and electrochemical properties of conducting poly- mer/clay nanocomposites in which clay as organic surfactants have been studied and nanocomposite materials have mainly been performed as corrosion inhibitors [6]. In this paper, the doped PANI/MMT nanocomposites were prepared by in- situ polymerization using iron trichloride as an oxidant. Properties of the obtained 104 Polyaniline/montmorillonite nanocomposites for adsorption purpose material were characterized by XRD, TGA, FT-IR and Raman spectra. The elec- tromagnetic interference shielding of this nanocomposite was investigated. 2. Content 2.1. Experiments 2.1.1. Preparation of PANI/MMT nanocomposites PANI/MMT nanocomposites were chemically prepared as the procedure de- scribed in [5,7]. Firstly, a dispersion was prepared by mixing (for 30 min.) of 10.0 g Na + -MMT (prepared from bentonite Dilinh-Vietnam as described in [7]) and 2.0 ml aniline monomers in a mixture of 80.0 ml distilled water and 20.0 ml isopropanol. Chloride acid was added into the suspension to obtain a medium of pH = 3. Then 7.5 g FeCl3 (water-free, Fluka Chemie) was added to the oxide particle dispersion during stirring. The colour of the mixture was changed from grey to green black. After 2 hours of stirring, the particles were cleaned by distilled water, filtered and dried at 40 - 50 0 C for several days under low pressure. 2.1.2. Characterization of PANI nanocomposites Thermal gravimetric analysis (TGA) was done by Ghimashu-50 H with scan rate of 10 0 C/min in atmospheric condition. The X-ray diffraction patterns of MMT, monomer-absorbed MMT and PANI/MMT nanocomposites were done by SIMENS D-5005. The chemical structure of the nanocomposites was characterized by Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. FT-IR spectra were performed by GBC Cintra 40-Nicolet Nexus 670 FT-IR. Raman spectra were measured by a Laser Raman Spectrophotometer (Ramalog 9I, USA). The electro- magnetic shielding features of the nanocomposites were performed by HP8720D Net- work Analyzer (USA). 2.2. Results and discussion 2.2.1. FT-IR and Raman scattering spectra Table 1. Wavenumbers and assignments of the Raman bands of PANI/MMT nanocomposites Wavenumber (cm −1 ) Soluble PANI [8] Oxidized PANI/MMT State Assignment 1626 - 1630 1620 (shoulder) I C-C ring stretching 1585 - 1600 1582 (strong) II C-C ring stretching 1506 - 1516 absence II C=N stretching 1480 - 1486 1490 (shoulder) II C=N stretching 1339 - 1349 1336 III and IV C-N + stretching 105 Vu Quoc Trung, Nguyen Van Thang, Pham Van Hoan and Nguyen Duc Chuy 1257 - 1266 1263 (weak) I C-N stretching 1190 - 1197 absence I C-H in-plane bending 1171 - 1174 1168 II C-H in-plane bending 885 890 (weak) I in-plane bending deformation 830 - 836 absence II in-plane bending deformation 800 - 815 819 II C-H out-of-plane bending 712 - 724 absence I C-H out-of-plane deformation 685 - 698 685 (weak) II C-H out-of-plane deformation 636 610 I in-plane ring deformation 511 - 526 510 out-of-plane C-N-C torsion 412 - 420 414 II out-of-plane C-H wag PANI is usually produced by the anodic oxidation of aniline in acidic aqueous solution (electrochemical polymerization), but can also be prepared by chemical oxidation (chemical polymerization) [4]. Hence, it is not surprising that the oxidation of PANI is pH-dependent. It is generally accepted that there are different, possibly coexisting forms of PANI, including (I) benzoid form with free amine groups, (II) quinoid form with imine groups, (III) protonic amines (bipolarons) and (IV) radical cationic state (polarons) (Figure 1). Figure 1. Structures of PANI [4] Figure 2. Raman spectra of PANI/MMT (nanocomposites measured at 514 nm with power of 1 mW) The typical Raman spectra of PANI presents in a range of 1000-1700 cm −1 [8]. Peaks corresponded to pure leucoemeraldine base (I) are at 1620, 1263, 890 and 610 cm −1 (Figure 2). While all peaks related to the oxidized emeraldine and pernigraniline base (II) are very clear and strong such as a peak at 1582 cm −1 (C-C ring stretch- ing) and 1168 cm −1 (C-H in-plane bend- ing). In addition, a peak at 1336 cm −1 corresponds to emeraldine salt (III and IV). These evidences show that PANI in 106 Polyaniline/montmorillonite nanocomposites for adsorption purpose nanocomposites presents in both an oxidized state and a neutral one (Figure 2). Ta- ble 1 gives the assignments of some typical Raman bands and it is in comparison with the frequencies collected on soluble oxidized PANI [8]. Figure 3. FT-IR spectra of PANI/MMT nanocomposites The principal absorption bands observed in the FT-IR spectra of PANI/MMT powder (in KBr) is given in Figure 3. In the region 1650 - 1400 cm −1 to the aromatic ring breathing, stretching N-H deforma- tion and C-N are observed. Bands at 1563 and 1479 cm −1 are the charac- teristic bands of nitrogen benzenoid and quinoid form and are present due to the conducting state of PANI [4]. The band at 1154 cm −1 is assigned to be present due to the charge delo- calisation on the polymer backbone. The band at 3434 cm −1 belongs to the N-H bonds and O-H groups adsorbed in the material. The presence of the strong band at 1092 cm −1 is attributed to MMT. 2.2.2. Thermal analyses Figure 4. TGA curve of PANI/MMT nanocomposites Thermal analyses of PANI/MMT nanocomposites are shown in Figure 4. Under 120 0 C, the weight reduction originates from water inside samples. The reduction (5%) in this tempera- ture range can be explained by the hydrophilic property of MMT and the oxidized state of PANI. It is also the source of the wide band between 3700 and 3000 cm −1 in the FT-IR spectra. In the range of 120 - 250 0 C, the weight reduction is very small, corresponding to the decomposition of redundant monomers, oligomers. At higher temperatures (350 - 700 0 C), the change of weight (20%) is attributed to the decomposition of PANI. 2.2.3. X-ray diffraction patterns The XRD patterns of the materials before and after polymerization are shown in Figure 5. At first, Na + -MMT was mechanically stirred for 30 min as reference. 107 Vu Quoc Trung, Nguyen Van Thang, Pham Van Hoan and Nguyen Duc Chuy Figure 5. X-ray diffraction patterns of MMT (A), monomer-absorbed MMT (C) and PANI/MMT nanocomposites (B) However, there are no changes in the XRD patterns of MMT before and after stirring. Therefore, the stirring does not affect the crystalline of the MMT itself. The diffraction peak of Na + -MMT was observed at 2θ = 8.70, thus, the basal spacing of Na + -MMT was 1.10 nm (Figure 5A). The intercalation of aniline monomers into MMT is shown in Fig- ure 5B. The basal spacing increased from 1.10 nm to 1.54 nm , indicating the expansion of the interlayer space (d-expansion) by 0.44 nm; and the successful intercalation by the mechanical intercalation method. The diffraction peaks of the products after polymerization were shifted to a higher angle than those before polymerization as shown in Figure 5C, indicating the synthesis of PANI in the clay layers. As a result, the basal spacing of monomer-absorbed MMT changed from 1.54 nm to 1.42 nm . They are in agreement with other publications [8]. 2.2.4. Shielding effectiveness measurements Shielding effectiveness is measured as the ratio of the field strength before and after attenuation and is expressed in decibels (dB) calculated according to formula as [6]: SE = 10 log Pt Pi (1) where SE is the shielding effectiveness; p is the power in watt (i stands for incident wave, t for transmitted wave). Figure 6. SE data obtained with the samples of PANI/MMT coated fabrics 108 Polyaniline/montmorillonite nanocomposites for adsorption purpose The shielding effectiveness as measured by coaxial transmission line method from 8 to 12 GHz was studied. The result reveals that on using PANI/MMT nanocom- posite - coated fabric, a shielding effectiveness of around  10 dB (99.9%) is obtained (Figure 6). It can be explained by the shielding of the layered MMT in the com- posites. However, for industrial applications, it was of modest value. Therefore, the shielding effectiveness of these nanocomposites must be improved for a best perfor- mance of possible applications 3. Conclusion In this paper, the PANI/MMT nanocomposites were prepared by chemical polymerization. X-ray diffraction patterns show that the composites are obtained with nanostructure. The chemical structure of PANI in nanocomposites were charac- terized by FT-IR and Raman spectra. They showed the same signals in comparison with that of the soluble PANI prepared by chemical polymerization. The thermal analysis showed that PANI in the nanocomposites was stable at around 600 0 C. The shielding effectiveness of the PANI/MMT nanocomposite - coated fabric measured in the range of 8 - 12 GHz, was in the order of -10 dB. However, for industrial applications, it was of modest value. Therefore the shielding effectiveness of these nanocomposites must be improved for possible applications. New results of these materials will be updated in the next publication. REFERENCES [1] A. G. Mac Diarmid, 2001. Synthetic metals: A novel role for organic polymers. Synthetic Metals, Vol. 125, N ◦ . 1, pp. 11. [2] U. Rammelt, P. T. Nguyen, W. Plieth, 2001. Protection of united steel by modi- fication with thin films of polymethylthiophene. Electrochimca Acta, Vol. 46, pp. 4251. [3] E. Hakansson, A. Amiet , A. Kaynak, 2006. Electromagnetic shielding properties of polypyrrole/polyester composites in the 118 GHz frequency range. Synthetic Metals, Vol. 156, N ◦ . 14-15, pp. 917. [4] J. Heinze, 1990. Topics in Current Chemistry. Springer-Verlag Berlin Heidelberg, Vol. 152, pp.1. [5] T. T. K. Thu, N. D. Chuy, H. V. Hung, 2005. Study on synthesis of nanocom- posite polyaniline-H2SO4/clay from bentonite Di Linh  Vietnam. Vietnamese Journal of Chemistry and Application, Vol. 1, pp. 32, (in Vietnamese). [6] J-M. Yeh, S-J. Liou, C-Y. Lai, and P-C. Wu, 2001. Enhancement of Corrosion Protection Effect in Polyaniline via the Formation of Polyaniline-Clay Nanocomposite Materials. Chemistry of Materials, Vol. 13, N ◦ . 3, pp. 1131. [7] L. H. Thuy, N. D. Chuy, 2005. Study on synthesis of nanocomposite polypyrrole/ Na + - Montmorillonite from bentonit Di Linh  Vietnam. Journal of Sciences of Hanoi National University of Education, Vol. 4, pp. 71, (in Vietnamese). [8] S. Shreepathi and R. Holze, 2005. Spectroelectrochemical Investigations of Soluble Polyaniline Synthesized via New Inverse Emulsion Pathway. Chemistry ofl Materials, Vol. 17, N ◦ . 16, pp. 4078. 109