Light induced electropolymerization of aniline

Transcription

1 Indian Journal of Chemical Technology Vol. 12, March 2005, pp Light induced electropolymerization of aniline Ishwar Das a*, Sujeet Kumar Gupta a & R S Lall b a Chemistry Department, DDU Gorakhpur University, Gorakhpur , India b Chemistry Department, St. Andrew s College, Gorakhpur , India Received 23 February 2004; revised received 7 January 2005; accepted 24 January 2005 Electropolymerization of aniline has been carried out in the presence of light. The kinetics of polymer growth has been studied in terms of weight of the aggregate as a function of time in the absence and presence of ZnSO 4 and found that zinc sulphate inhibited the polymer growth by incorporating itself in the polymer network. An empirical equation w = ce mt was obeyed in acid-aniline system while a different equation w 2 1 = m 1 t + c was obeyed when zinc sulphate was admixed. It indicated that a different mechanism is operative for the polymerization of aniline in the presence of zinc sulphate in acidaniline system. TG studies reveal that zinc sulphate increases the stability of the polymer. The aggregates were characterized by IR, FAB mass, TG and XRD studies. Analysis of mass spectra shows the possibility of cross linked fragments in the polymer. Oscillatory characteristics have been studied in HCl-aniline system and its dependence on light of different wavelengths, zinc sulphate, applied potential, aniline, HCl and acetonitrile concentrations has also been studied. Polymerization was found to be reduced by 70% in complete darkness. Keywords: Electropolymerization, aniline, TG, XRD, mass spectra. IPC Code: C07C211/46; C08F2/58 There is considerable scientific and technological interest in conducting polymers. Semiconducting nature of polyaniline was first reported in However, study of its use in electrodes for secondary batteries only began in 80's. Among conducting polymers polyaniline has wide-spread applications such as in solar energy conversion, batteries, electronic sensors and corrosion etc. Polyaniline is expected to be a good material for practical secondary batteries since they are stable and have good electrochemical characteristics 2. At present, polyaniline is the only conducting polymer used in battery industry as a button type lithium rechargeable battery with organic liquid electrolyte, and as positive electrode. Polyaniline attracted the interest of researchers due to its satisfactory environmental stability, high degree of electrical conductivity with reasonable mechanical properties and its ease of processing 3. Polyaniline is a peculiar conducting polymer because its electrical properties not only depend on its oxidation state but also on its protonation state and hence the acidity of electrolyte medium 4. Polyaniline is structurally unique among the known conducting polymers. It is the only known conducting polymer with nitrogen atom occupying the *For correspondence ( ishwar_das@rediffmail.com; Fax : +91(551) ) bridging position in its backbone structure. Nitrogen atoms are the protonation sites. A review concerning electroconductive polyaniline and conductive polyaniline blends and composites has appeared recently 5. In this paper, results on the electrochemical polymerization of aniline in the complete darkness as well as in the presence of light, growth kinetics, characterization of the product by IR, FAB mass, XRD, TG are reported. Results of potential changes during polymerization of aniline recorded as a function of time in presence of light of different wavelengths, ZnSO 4 and acetonitrile are also reported. Experimental Procedure Aniline (AR, Qualigens), HCl (AR, Ranbaxy), ZnSO 4 (AR, Qualigens), and acetonitrile (s.d. Fine Chem. Ltd. India) were used as such. Electrochemical synthesis The electrochemical synthesis of polyaniline aggregate was carried out at room temperature with the help of an experimental setup shown in Fig. 1. The two platinum electrodes were cleaned carefully. These electrodes were attached to a rheostat and a battery to supply the DC current. Experiment was carried out in the presence of light of intensity 40 Lux. Intensity was measured with the

2 DAS et al.: LIGHT INDUCED ELECTROPOLYMERIZATION OF ANILINE 199 help of ELVOS LM 1010 LUXMESSER (Germany). Electropolymerization was also carried out in the complete darkness and in the light of different wavelengths by using different coloured transparent papers. Polyaniline was synthesized by mixing 3 M aqueous HCl with aniline in the ratio 16:1 (v/v). Cathode was immersed into the solution while anode was kept on the surface of the solution. Separation between these electrodes was 2.5 cm. The above solution was taken in a petridish. A constant potential of 1.5 volt was applied across the electrodes. Electropolymerization was carried out at moderate potential to prevent the oxidative decomposition of the solvent, electrolyte and the polymer. As soon as the potential was applied, synthesis of polyaniline started. Polyaniline aggregates were collected after different intervals of time. The aggregates were washed with distilled water, dried in an oven at 50 C and weighed. Polyaniline samples were also synthesized in the presence of zinc sulphate. For this Fig. 1Experimental setup for electrochemical synthesis and monitoring the potential changes with time during polymerization P 1, P 2 = platinum electrodes, B = D.C. battery, R = rheostat, C = calomel electrode. purpose zinc sulphate (0.094 M) was added to the hydrochloric acid-anline system before applying the electric field. The kinetics of polymerization was studied by recording the weight of the aggregates at different time intervals. The results are given in Table 1. The synthesized polyaniline aggregates were characterized by the following techniques: Infrared spectra of polymer samples were recorded, using Perkin-Elmer RX 1 FT-IR spectrophotometer. The IR spectral results are given in Table 2. Thermogravimetric studies of polymer samples were carried out with the help of a METLER TOLEDO STAR TG analyser. The TG measurements were carried out in the temperature range of C. The thermogravimetric data at two conditions are compared and results are summarized in Table 3. Mass spectra of polymer samples obtained at different times of polymerization were taken, using Jeol SX 102/DA-6000 mass spectrometer, argon/xenon was used as the FAB gas. Results are shown in Fig. 2. Powder X-ray diffraction patterns of polymer samples were taken with a Philips analytical X-ray diffractometer equipped with PC-APD diffraction software and using CuK α radiation in the 2θ range Results are recorded in Table 4. Potential changes during electropolymerisation of aniline were monitored as a function of time. Potential measurements were made with the help of x-t recorder attached with a suppressor (Digital Electronics, India) (Fig. 1). The potential changes were also monitored at different [aniline], [acid] and applied potentials in HCl-aniline system. During electropolymerization potential changes with time were also monitored (i) in presence of light of intensity 40 Lux, in the complete darkness and in presence of light of different wavelengths using different coloured transparent papers. (ii) at different aniline and hydrochloric acid concentrations (iii) at different applied potentials, (iv) in presence of zinc Table 1Growth kinetics data for acid-aniline and ZnSO 4 admixed acid-aniline systems, Field intensity = 0.60 V/cm System Acid (M) Aniline (M) Equation obeyed Slope (m) Intercept ( c ) Corr. coefficient (R) HCl-C 6 H 5 NH w = ce mt HCl-ZnSO 4 -C 6 H 5 NH w 2 1 = m 1 t +c Table 2IR peak positions (cm 1 ) in different polymer systems System >N-H N-quinoid ring N-benzenoid ring Charge delocalization HCl-C 6 H 5 NH (s) 1495 (s) 1111 HCl-ZnSO 4 -C 6 H 5 NH (w) 1477 (br) 1123

3 200 INDIAN J. CHEM. TECHNOL., MARCH 2005 Table 3Thermogravimetric analysis data for acid-aniline and ZnSO 4 admixed acid-aniline systems System % weight loss at different temperatures ( C) HCl-C 6 H 5 NH HCl-ZnSO 4 -C 6 H 5 NH Table 4Powder X-ray diffraction data for polymer aggregates synthesized electrochemically in the absence and presence of ZnSO 4.[HCl]=2.823 M, [Aniline] = M, field intensity = 0.60 V/cm Angle 2θ HCl-C 6 H 5 NH 2 HCl-ZnSO 4 -C 6 H 5 NH 2 Relative intensity Angle Relative intensity [%] (2θ) [%] by the linear plot between log w versus t with the correlation coefficient value of Each aggregate was characterized by IR, FAB mass, TG and XRD studies. IR results show a peak at 1111 cm 1 due to charge delocalization of the polymer. A peak at 3216 cm 1 is observed due to >N-H bond. There is also a peak at 1495 cm 1 due to N-benzenoid ring (Table 2). Mass spectrum of polymer synthesised from HClaniline system was taken at different times of electropolymerization. Results show that the molecular weights of different aggregates are different and correspond to 14 to 18 units of aniline structure, Most of the peaks of the fragments in mass spectra corresponds to m/z value given by m/z = (n 1 a + n 2 b + n 3 c) Fig. 2m/z versus t plot for electrochemically synthesized polyaniline at different time intervals. Conditions: [HCl] = M, [C 6 H 5 NH 2 ] = M, Field intensity = 0.60 V/cm. where n 1, n 2, n 3 are numbers and a,b and c are units of mass (m/z) sulphate (0.094 M) and (v) in presence of acetonitrile in different proportions. Results and Discussion The electrochemically deposited mass, elliptical in shape was collected at different time intervals. The kinetics of the aggregation process was studied by noting the weight (w) of the aggregate at different time intervals at a fixed field intensity 0.60 V/cm. Results indicate that, weights of deposits (w) increase with the increase in deposition time (t) non-linearly obeying the empirical equation w = c.e mt as is evident The m/z values of the molecular cation of the polymer varies periodically with time (Fig. 2). The smallest fragment corresponds to the radical cation

4 DAS et al.: LIGHT INDUCED ELECTROPOLYMERIZATION OF ANILINE 201 The higher species largely correspond to the linear fragments like But cross-linked fragments such as are also formed. Polymerization of aniline in the presence of an acid, as proposed by Jang et al. 6 proceeds as follows The autocatalytic nature of growth has been suggested by Trivedi 7. The variation in the mass of the deposited polymer is due to the degradation of the polymer occurring together with its growth. Influence of light It has been established theoretically and experimentally that light adsorption plays an important role in the macroscopic structure development It was observed that the polymerization of aniline is induced by light of suitable wavelength. Figure 3 shows oscillations in potential when electropolymerization was carried out in presence of light of different wavelengths. Experiment was also performed in the complete darkness. Potential was found to increase with time, after some time, it attains a constant value. It was maximum in the presence of white light (intensity 40 Lux), while minimum in case of blue light or in the Fig. 3Potential changes with time during elrctrochemical synthesis of polyaniline in HCl-C 6 H 5 NH 2 system in presence of (a) sunlight of intensity 40 Lux, (b) green light, (c) red light, (d) blue light and (e) in the complete darkness. Conditions: [HCl] = M, [C 6 H 5 NH 2 ] = M, Field intensity = 0.60 V/cm. complete darkness. The nature of curves (Fig. 3) shows that the process of electropolymerization is autocatalytic and the nature of the apparent slope changes with the wavelength of light used. Influence of acid concentration on oscillatory characteristric was also studied both in light and in dark (Fig. 4). Mass spectral data also show the reduction in molecular weight in the dark as compared to that in the light. In the higher wavelength region (blue), the excitation of aniline molecule may take place as follows In the electrooxidation, the cation radical is formed as follows It is unstable and dimerizes soon to form dimer, tetramer, etc. while photochemically excited molecule

5 202 INDIAN J. CHEM. TECHNOL., MARCH 2005 Fig. 4Potential changes with time during electrochemical synthesis of polyaniline in HCl-C 6 H 5 NH 2 system at different experimental conditions. Conditions: [HCl] = M; (a) in light and (c) in dark; [HCl] = M; (b) in light and (d) in dark, [C 6 H 5 NH 2 ] = M, Field intensity = 0.60 V/cm. does not proceed for polymerization. This explains the results of electrode potential kinetics. This is also in agreement with the discussion on photocatalyzed oxidative polymerization of aromatic diamines by Li et al. 5. In higher polymers, the chances of delocalization of π electrons and hence the chances of n π* transition increase. It also explains the changes in slope of electrode potential kinetics curves. As the polymer is growing, the number of n and π electrons both increases. The degree of conjugation is also increasing from monomer to higher polymers. It is also observed that potential oscillates with time during polymerization. Figure 5 shows the potential changes with time at different aniline concentrations. At higher aniline concentration, polymerization takes place (Fig. 5 curves e,f), potential was also increasing with time. At lower aniline concentrations (Fig. 5, Fig. 5Potential changes with time during electrochemical synthesis of polyaniline in HCl-C 6 H 5 NH 2 system at different [C 6 H 5 NH 2 ]. Conditions: [HCl] = M, [C 6 H 5 NH 2 ] = (a) 0.0 M; (b) M; (c) M; (d) M; (e) M and (f) M, Field intensity = 0.60 V/cm. curves a-d) there was no polymerization and potential was found to decrease with time. Efforts have been made to characterize the nature of oscillation using next amplitude plots. A t+12, the amplitude of oscillation at any time t+12 second were plotted against A t the amplitude of oscillation at any time t second. Results show that the oscillation in potential is periodic when no polymerization takes place while it becomes more like random noise during polymerization. Experiment was also carried out at different field intensities (Fig. 6). Nature of curves depends considerably on the applied field intensity.

6 DAS et al.: LIGHT INDUCED ELECTROPOLYMERIZATION OF ANILINE 203 Polymerization takes place at 0.6 and 0.32 volt/cm (Fig. 6 curves a and b) while there was no polymerization on the electrode at low field intensity (0.20 volt/cm) (Fig. 6 curve c). Influence of ZnSO 4 on electropolymerization Influence of ZnSO 4 on electropolymerization of aniline has been studied under identical conditions and results were compared with that observed in the absence of zinc sulphate. XRD results show that the two polymer aggregates are different as indicated by 2θ value (Table 4). In presence of ZnSO 4 a nonlinear plot between w versus t was observed obeying the empirical equation w 1 2 =m 1 t+c 1, where w 1 is the weight of the deposit at time t, m 1 and c 1 are slope and intercept respectively (Table 1). Results show that a different mechanism is operative in presence of ZnSO 4. Zn ++ inhibits the polymer growth too. In the IR spectrum of Zn ++ admixed polymer (Table 2), >N- H band has been slightly broadened and band due to N-quinoid ring has been weakened and appeared at a lower frequency at 1560 cm 1. The lone pair of electrons on nitrogen present in polymeric chain coordinates with the cation to form >N-Zn bond. Mass spectral data show a peak at m/z = 1042 corresponding to 10 units of aniline and 2 Zn ++ ions incorporated between the linear fragments. Thermogravimetric results of these polymeric samples are recorded in Table 3. Following conclusions may be drawn from TG results. In case of acid -aniline system 90% weight loss was observed at 550 C as compared to 58% weight loss observed at the same temperature when ZnSO 4 was admixed. Polymer obtained from acid-aniline system is less stable than that when Zn ++ was admixed in the solution before applying the electric field. Figure 7 shows the influence of ZnSO 4 on oscillatory characteristics. The slope of the curve and amplitude of oscillations both get reduced in presence of ZnSO 4. Influence of acetonitrile on electropolymerization Another interesting observation was observed when acetonitrile was added to the acid-aniline solution Fig. 6Potential changes with time during electrochemical synthesis of polyaniline in HCl-C 6 H 5 NH 2 system at different field intensities. Conditions: [HCl] = M, [C 6 H 5 NH 2 ] = M, Field intensity = (a) 0.60; (b) 0.32 and 0.20 V/cm. Fig. 7Potential changes with time during electrochemical synthesis of polymers in (a) HCl-C 6 H 5 NH 2 and (b) HCl-ZnSO 4 - C 6 H 5 NH 2 systems. Conditions: [HCl] = M, [C 6 H 5 NH 2 ] = M, [ZnSO 4 ] = M, Field intensity = 0.60 V/cm.

7 204 INDIAN J. CHEM. TECHNOL., MARCH 2005 Acknowledgements Thanks are due to Heads, RSIC CDRI, Lucknow, RSIC, Nagpur and USIC, Delhi University for providing FABMS, TG and XRD facilities. Thanks are also due to Profossor M L Srivastava, Head, Chemistry Department, DDU Gorakhpur University, for providing necessary facilities and Mr. Ashutosh Kumar Singh for helpful discussion. Fig. 8Potential changes with time during electrochemical synthesis of polymers in HCl-C 6 H 5 NH 2 system in presence of acetonitrile. Conditions: [HCl] = M, [C 6 H 5 NH 2 ] = M, [CH 3 CN] = (a) 0.0; (b) 2; (c) 5 and (d) 10%, Field intensity = 0.60 V/cm. before the start of experiment. Oscillatory characteristic was drastically changed when acetonitrile was added to the acid-aniline system in 2, 5 and 10% v/v (Fig. 8, curves b-d). Amplitude and life time both are reduced and ultimately oscillation ceased after some time. It was found that life time depended on the concentration of acetonitrile. Growth was found to be considerably reduced in the presence of acetonitrile. Results clearly indicate that acetonitrile inhibits the polymerization of aniline. It may be due to the formation of H-bond between [acetonitrile], and aniline cation radical as shown in the following scheme References 1 Shoji E & Freund M S, J Am Chem Soc, 123 (2001) Mac Diarmid A G, Chiyang J C, Halpern M, Mu W L, Somasiri N L, Wu W & Yaniger S I, Mol Cryst Liq Cryst, 121 (1985) McManus P M, Cushman R J & Yang S C, J Phys Chem, 91 (1987) Sherman B C, Euler W B, Ren Force R, J Chem Educ, 71(4) (1994) A94. 5 Li X G, Huang M R, Duan W & Yang Y L, Chem Rev, 102 (2002) Jang G W, Chan C C, Hsueh K F, Hariharan R, Patel S A & Whitecar C K, J Phys Chem, 94 (1990) Trivedi D C, Handbook of Conductive Molecules and Polymers: Vol. 2, Conductive Polymers: Synthesis and Electrical Properties, Edited by Nalwa H S (John Wiley & Sons Ltd.), Nitzan A, Ortoleva P & Ross J, J Chem Phys, 60(8) (1974) Das I, Lall R S & Pushkarna A, J Phys Chem, 91(3) (1987) Das I & Pushkarna A, J Non-Equilb Thermodyn, 13 (1988) Das I, Lall R S & Pushkarna A, J Cryst Growth, 82 (1987) Das I, Lall R S & Pushkarna A, J Cryst Growth, 84 (1987) Das I, Pushkarna A & Agrawal N R, J Phys Chem, 93 (1989) Das I, Pushkarna A & Bhattacharjee A, J Phys Chem, 94 (1990) Das I, Pushkarna A & Bhattacharjee A, J Phys Chem, 95(9) (1991) 3866.