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Growth Patterns of Dendrimers and Electric Potential Oscillations during Electropolymerization of Pyrrole using Mono- and Mixed Surfactants Ishwar Das,*,† Neha Goel,† Namita R. Agrawal,‡ and Sanjeev Kumar Gupta† Chemistry Department, DDU Gorakhpur UniVersity, 273009 India, and St. Andrew’s College, Gorakhpur-273012, India ReceiVed: June 6, 2010; ReVised Manuscript ReceiVed: August 16, 2010

Fractal and dendrimer growth patterns of polypyrrole were obtained during electrochemical polymerization of pyrrole in systems (A) pyrrole-sodium dodecyl sulfate (NaDS)-water and (B) pyrrole-NaDS-cetyl trimethyl ammonium bromide (CTAB)-water. Different morphological transitions including compact f flowerlike and fractal f dendrimer f fractal were observed depending on experimental conditions. Growth kinetics during electropolymerization of pyrrole was studied. Growth rate was found to be higher in system A than in B. Effect of [NaDS], [pyrrole], and field intensity on morphology and weight of polymer aggregates was also studied in both the systems. Different empirical equations were obeyed under different conditions. Electropolymerized aggregates were characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), electrical conductivity measurement, and reflectance spectroscopy. TEM studies revealed that the particle size decreased to ∼140-200 nm in the presence of CTAB. The decrease in particle size on addition of CTAB was also observed in XRD studies. Reflectance spectra of the polymer aggregates support the large π-conjugation in the dendrimer. During electropolymerization, oscillations in potential were monitored as a function of time. Results indicated that growth pattern and electric potential oscillations were interrelated. In the case of fractal growth, the amplitude of chaotic oscillation was higher than the amplitude of oscillation during the growth of dendrimer. Growth morphologies and electric potential oscillations have been explained on the basis of modified Diaz’s mechanism. Introduction There is considerable interest in far from equilibrium phenomena such as electric potential oscillations, chaos, and dendritic and fractal growth during recent years.1-6 Such phenomena are also observed during the electrochemical polymerization process, where dendritic hyperbranched polymers are formed.7,8 During the last years, there has been an intensive study on a new type of polymer, called dendrimer.9-11 Dendrimers and hyperbranched polymers occur in many natural phenomena and can be synthesized for several reasons. Because of their shape, these have very unusual physical and chemical properties. The specific properties of dendrimers make them suitable for a variety of technological applications including pharmaceutical and nonpharmaceutical applications12,13 such as in drug delivery, dendrimer nanodrug,12 photodynamic therapy,12,13 and industrial processes.14 Starburst dendrimers are a novel class of macromolecules having a definite molecular composition.15 These macromolecules consist of a central nitrogen initiator core to which radially branched units are grafted in a symmetrical fashion resembling fractals. Conducting polymers have generated a great deal of interest because of their physical and chemical properpties as well as their potential in industrially useful materials.16-23 Among the conducting polymers, polypyrrole, a representative conducting polymer, has been the subject of great interest for the past decade because of its easy polymerization and wide practical applications.24-28 Among the methods of synthesis known to produce polypyrrole, the * To whom correspondence should be addressed. Tel: +91-551-2338731. Fax: +91-551-2342880. E-mail: [email protected]. † DDU Gorakhpur University. ‡ St. Andrew’s College.

electrochemical polymerization method is most useful to obtain polypyrrole having high conductivity.29 Conducting polymer nanomaterials are of great importance for a wide range of applications in chemical and biological sensing.30-36 Surfactants may also play a key role in tailoring the nanostructures of polypyrrole during polymerization.37 At the same time, surfactants are quite cheap as compared with other dopants such as 4-toluene sulfonic acid silver salt. A number of investigations have been devoted to synthesis of nanostructured polypyrrole by solution chemistry methods.33,34,37 However, electrochemical methods involving mono- and mixed surfactants are lacking. In the present investigation, fractal and dendrimer growth of polypyrrole were undertaken by electrochemical method using mono- and mixed surfactants. Growth kinetics under different experimental conditions viz. variation of time, [NaDS], [pyrrole], and field intensity was studied during electropolymerization of pyrrole. Electropolymerized aggregates were characterized by TEM, XRD, electrical conductivity measurements, and reflectance spectroscopy. Electrical potential oscillations were monitored as a function of time during electropolymerization. The effect of [CTAB] on morphology, growth kinetics, and electric potential oscillations was also investigated. Experimental Section Pyrrole (Merck Schuchardt), sodium dodecyl sulfate (s.d.fineChem.), and cetyl trimethyl ammonium bromide (CTAB, s.d.fine-Chem.) were used as such. Electrochemical Synthesis, Morphology and Growth Kinetics. The electrochemical synthesis of polypyrrole was carried out at room temperature using an experimental setup consisting of a Petri dish containing a solution of monomer and NaDS.

10.1021/jp105183q  2010 American Chemical Society Published on Web 09/16/2010

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Figure 1. (a) Microphotographs obtained during electropolymerization of pyrrole at different time at (i-vi) 10, 20, 30, 40, 50 and 60 min. Conditions: [pyrrole] ) 0.1 M, [NaDS] ) 0.05 M, field intensity )3.6 V/cm, separation between electrodes ) 2.5 cm. Duration of electropolymerization was 30 min in each case and (b) corresponding weight of polymer aggregates at different time.

Figure 2. (a) Microphotographs and (b-c) plots of weight of polymer aggregates and electrical conductivity as a function of [NaDS]. Conditions: [NaDS] ) 0.025, 0.05, 0.075, 0.10, 0.125 M (i-v), [pyrrole] ) 0.25 M. Field intensity ) 3.6 V/cm, separation between electrodes ) 2.5 cm. Duration of electropolymerization was 30 min in each case.

Solution mixture (5 mL) was taken in the Petri dish. A cleaned platinum circular cathode was immersed in the solution, whereas the other platinum vertical anode was put at air/liquid interface

at the center of the cathode of radius 2.5 cm. These electrodes were attached to a potentiostat (Scientific, India) to supply constant potential. Polymerization started at the anode as soon

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Figure 3. (a) Microphotographs obtained during electropolymerization of pyrrole at different [pyrrole] (i-v). Conditions: [pyrrole] ) 0.05, 0.10, 0.15, 0.20, 0.25 M, [NaDS] ) 0.05 M, field intensity )3.6 V/cm, separation between electrodes ) 2.5 cm. Duration of electropolymerization was 30 min in each case. (b) Corresponding weights of polymer aggregates at different [pyrrole].

Figure 4. (a) Microphotographs and (b-c) plots of weight of polymer aggregates and electrical conductivity as a function of field intensity. Conditions: [pyrrole] ) 0.25 M, [NaDS] ) 0.05 M. Field intensity was varied in the range 1.2-6.0 V/cm. Duration of electropolymerization was 30 min in each case.

as the potential was applied across the electrodes. Electropolymerization was carried out at moderate potential to prevent the oxidative decomposition of the solvent, electrolyte, and the monomer. Polypyrrole aggregates were collected after different

intervals of time. Field intensity was varied in the range 1.2-6.0 V/cm. The aggregates were washed with water and dried in an incubator at 50 °C. Aggregates were photographed using an “OLYMPUS” microscope fitted with a camera. Experiments

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Figure 5. (a) Microphotographs obtained during electropolymerization of pyrrole in the presence of CTAB. Close views of plates (i) and (viii) are shown as plates I and II, respectively. Conditions: [pyrrole] ) 0.25 M, [NaDS] ) 0.075 M, [CTAB] ) 0.125 × 10-4, 0.25 × 10-4, 0.5 × 10-4, 0.1 × 10-3, 0.15 × 10 -3, 0.2 × 10-3, and 0.3 × 10-3 M (i-viii), field intensity ) 3.6 V/cm. Duration of electropolymerization was 30 min in each case.

were also performed under different experimental conditions viz. [surfactant], [pyrrole], and field intensity. Pyrrole concentration was varied in the range 0.05 to 0.25 M, whereas concentration of NaDS was varied from the range 0.025 to 0.125 M. Microphotographs are shown in Figures 3a and 4a. We studied dynamics of polymerization by recording the weight of the aggregates at different time intervals, field intensity, [NaDS], and [pyrrole]. Results are shown in Figures 1b, 2b, 3b, and 4b. Influence of Cationic Surfactant (CTAB) on Morphology and Growth Kinetics during Electropolymerization. Influence of cationic surfactant CTAB on the morphology and growth kinetics has been studied at different time and [CTAB]. Results are shown in Figure 5.

Electrical Conductivity Measurements. Electrical conductivity of electropolymerized aggregates was measured at room temperature using an experimental setup consisting of a 2 cm long glass capillary (inner diameter ) 1.4 mm) filled with the solid sample, the conductivity of which is to be measured. Two bright platinum electrodes (diameter ) 0.337 mm) were inserted into the capillary such that these touched the sample and connected to a conductivity meter (VSI-India.). Results are shown in Figures 2c, 4c, and 5c. TEM Studies. Transmission electron microscopic images of electropolymerized aggregates obtained from pyrrole-NaDS and pyrrole-NaDS-CTAB systems were taken, and results are shown in Figure 6c,d.

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Figure 6. Influence of CTAB on (a,b) morphology of polypyrrole aggregates and (c,d) TEM images. Conditions: (a,c) [NaDS] ) 0.075 M, [pyrrole] ) 0.25 M and (b,d) [NaDS] ) 0.075 M, [pyrrole] ) 0.25 M, [CTAB] ) 0.125 × 10-4 M. Field intensity )3.6 V/cm. Duration of electropolymerization was 30 min in each case.

Powder X-ray Diffraction Studies. Power XRD patterns of polymer aggregates obtained in the presence and absence of CTAB were taken in the 2θ range 0-100° using Bruker X-ray diffractometer model D-8. Reflectance Spectra. The reflectance spectra of polymer aggregates obtained at different NaDS concentrations were taken in the wavelength range 200-1000 nm. Oscillations in Anode Potential during Electropolymerization. The potential between anode and the reference calomel electrode during polymerization was monitored as a function of time. Potential measurements were made with the help of an x-t recorder attached to a suppressor (Digital Electronics, India). The experiment was conducted in a flat-bottomed Corning Petri dish. Two platinum electrodes at 2.5 cm apart were inserted in the solution. The circular cathode was extended below the surface of solution, whereas the lower end of the anode was put at the surface. Experiments were performed under different concentrations of NaDS and CTAB. Results are recorded in Figure 7. Results and Discussion Irregular polymer aggregates were produced via electropolymerization at different time, [pyrrole], [NaDS], and field intensity in the pyrrole-NaDS-H2O system. Results are shown in Figures 1-4. Results show that growth of the polymer aggregate increases with time linearly, obeying an empirical equation w ) mt + c, as evident by its correlation coefficient value R ) 0.995, where m and c are slope and intercept, respectively (Figure 1). The linear relationship may be due to the continuous electrooxidation of pyrrole monomer present in the medium at a constant rate. It leads to greater molecular mass and larger π-conjugation of the growing polymer chain. Dependence of growth morphology on NaDS concentration is shown in Figure 2a. Growth was very slow either at a very low or at a high [NaDS]. It increased with an increase in [NaDS], attained a maximum value at 0.075 M, and then decreased (Figure 2b). At low [NaDS] (0.025 M), the morphology is

fractal-like with fractal dimension 1.67, a value close to diffusion-limited aggregation. On increasing the [NaDS], a transition from fractal to dendrimer with different characteristic was observed, as shown in Figure 2a (i, ii). Beyond [NaDS] ) 0.1 M, a transition from dendrimer to fractal was again observed. In the dendrimer region (ii, iii), the growth and electrical conductivity were maximum because of large π-conjugation. At higher surfactant concentration, a sheath is formed around every pyrrole monomer, creating hindrance in electrooxidation of pyrrole monomer, which results in a decrease in the growth rate inhibiting the formation of polypyrrole with high molecular weight, large π-conjugation, or both, which in turn is responsible for the decrease in the conductivity. These observations were also supported by reflectance spectroscopy. In the reflectance, spectra peaks at λ ) 885 and 890 nm were observed, corresponding to NaDS concentrations 0.025 and 0.125 M, respectively. A shift in the peak position to λ ) 900 nm was observed in the case of dendrimer at NaDS concentrations 0.075 M owing to large π-conjugation, which in turn is responsible for increased electrical conductivity of the dendrimer. Results are indicated in Figure 2. Dependence of morphology on pyrrole concentration has also been studied, and results are shown in Figure 3. Results indicated a transition in morphology from a compact to flower-like dendritic structure. Growth of the dendrimer is divergent38 in this case. The transitions from compact to fractal and fractal to dendrimer may be due to an increase in the distance from equilibrium.3 Weight of polymer aggregates increases linearly with [pyrrole], obeying an empirical equation w ) m[pyrrole] + c, where m and c are slope and intercept, respectively. Its correlation coefficient value was 0.993. Increment in pyrrole concentration in the medium results in the easy avaibility of pyrrole monomer for continuous electrooxidation and electropolymerization, favorable for the formation of polypyrrole with large π-conjugation, as evident by the increase in the polymer yield with the increase in [pyrrole]. Dependence of growth morphology on field intensity is shown in Figure 4a. In this case, the fractal-like growth was

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Figure 7. Plots showing anode potential changes with time and corresponding growth morphologies on [NaDS] during electropolymerization of pyrrole. [pyrrole] ) 0.25 M, field intensity ) 3.6 V/cm, separation between electrodes ) 2.5 cm.

also observed either at low or high field intensities. On increasing the field intensity, transitions from fractal f dendrimer f fractal occur, as shown in Figure 4a. Under these conditions, weight and electrical conductivity of polymer aggregates were also measured, and results are shown in Figure 4b,c. The applied potential introduces effect on both the structure and properties of electrogenerated polypyrrole and also on the rate of polymer production. It attains a maximum value at critical field intensity 3.6 V/cm. Pyrrole polymerized in the low potential range 1.2-3.6 V/cm, shows an increase in conductivity and may be related to increase in the π-conjugation. The decrease in conductivity at higher potential may be due to the loss of π-conjugation and degradation of polymer. Influence of a cationic surfactant CTAB, which indeed plays a key role in the preparation of polypyrrole nanoparticles on growth morphologies, and growth kinetics has been studied at a different time. Fractal-like growth was observed in each case and the weight of polymer aggregates varies linearly with time, obeying an empirical equation w ) mt + c, as evident by correlation coefficient value 0.996. Values of m and c are 0.3399 mg/min and 0.288 mg, respectively. In the presence of CTAB,

Dependence of growth morphology and growth kinetics on [CTAB] is shown in Figure 5a,b. Morphologies of the polymer aggregates obtained from (A) pyrrole-NaDS-H2O (Figure 5a (i)) and (B) pyrrole-NaDS-CTAB-H2O systems are shown in Figure 5a (ii-vii) along with their microscopic views shown in Figure 5a (I and II). It is observed that both morphologies are different. In the case of I, the growth is broom-like, whereas in the case of II, the growth is dendritic. On increasing [CTAB] in the medium, growth rate was suddenly decreased, and on further addition of CTAB, it attains a constant value (Figure 5b). The electrical conductivity of polymer follows the same trend as that observed in the growth kinetic studies (Figure 5c). It may be due to the decrease in the weight of the polymer and π-conjugation of the growing polymer chain on increasing [CTAB] and then becomes constant because of the nonavaibility of a lone pair of electrons on the nitrogen atom of the polypyrrole. On addition of CTAB in the pyrrole-NaDS-H2O system, the growth and particle size were reduced, as evident by TEM studies shown in Figure 6. The decrease in particle size on addition of CTAB was also supported by an increase in the flex width in the powder XRD pattern. Potential changes with time along with the corresponding morphologies obtained during electropolymerization of the pyrrole-NaDS-H2O system at different [NaDS] were recorded, and results are shown in Figure 7. It is observed that the amplitude of oscillation was high either at low (0.025 M) or high (0.125 M) NaDS concentration (Figure 7a,e), and corresponding morphologies were fractal. The fractal dimension of (a) was found to be 1.67, very close to diffusion limited aggregation. The morphologies were dendrimer (b-d) in the [NaDS] range 0.025 to 0.10 M. In this concentration, the range of the amplitude was lower than those of a and e. It may be concluded that amplitude of oscillation and weight of polymer aggregate are interrelated. It is also clear from Figure 7 that the amplitude of oscillation is related to morphology. The higher the amplitude of chaotic oscillation, the smaller the morphology of the dendrite. Potential changes with time during electropolymerization of pyrrole at different [CTAB] were also studied in the concentration range 1.25 × 10-5 to 3.0 × 10-4. It was observed that the amplitude of oscillation increased with the

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SCHEME 1

increase in [CTAB], in agreement with the above observations. The mechanism of random motion of cation, development of electric potential oscillations, and diffusion-limited growth of dendritic structure of polypyrrole can be understood on the basis of modified Diaz mechanism,39 as follows: Electropolymerization proceeds through successive electrochemical and chemical steps. It includes the oxidation of the monomer to form radical cation. When current is passed through the system, pyrrole free radical is formed from pyrrole with the release of one electron so that free radical is attracted toward anode. This promotes the passage of electrical current through the system. The free radical undergoes chain polymerization, followed by deposit of the polymer on the anode and decrease in potential. When pyrrole cation is in excess, electrical potential would increase. When these are polymerized, the effective concentration of pyrrole ion decreased, leading to a decrease in potential. When enough pyrrole ions get accumulated, the cycle is repeated again and again but in a nonperiodic manner. Therefore, chaotic oscillations are observed. Detailed steps of mechanism can be formulated as follows

CH3(CH2)10CH2OSO3Na + H2O f CH3(CH2)10CH2OSO3- + H+ + NaOH H+ + H2O h H3O+ Oxidation of monomer at the surface of the electrode takes place to form the cation radical. The cation radical thus obtained is further stabilized by resonance.

(ii) Radical cation-monomer coupling:

(iii) Chain propagation via oxidation of the extending polymer: In addition to R-R′ linkage in the polypyrrole, some R-β′ linkage can also take place.40 This can lead to the greater random motion of radical cation, leading to complex fractal growth and dendritic structure. The dimer obtained during electropolymerization was further oxidized to the free radical, as shown in the Scheme 1. Here the electron is localized at the β-position of the pyrrole moieties, so chances of attack of another pyrrole radical cation with greater electron density at the R-position are favored, as shown in the Scheme 2. Now another free radical cation will be generated in the same pyrrole moiety at the R position (structure d), which is attacked by another pyrrole radical cation with release of 4H atoms to give dendrimer (f) of pyrrole, as shown in Scheme 3. This process would be repeated again and again with the attack at R and β positions of the growing polymer chain into a dendritic polypyrrole in its oxidized conducting form.

The electron thus released is associated with the H+ at the cathode

1 H+ + e- f H2 2 The next step involves (i) radical cation-radical cation coupling, (ii) radical cation-monomer coupling, and (iii) chain propagation via oxidation of extending polymer. (i) Radical cation-radical cation coupling: The radical cations having greater unpaired electron density in the R-position react with the radical cation or monomer to form a dimer as shown below.

Conclusions Polypyrrole with fractal and dendritic morphologies were synthesized by electrochemical polymerization method in the

Electropolymerization of Pyrrole SCHEME 2

SCHEME 3

J. Phys. Chem. B, Vol. 114, No. 40, 2010 12895 presence of mono- and mixed surfactants. Morphological transitions from fractal f dendrimer f fractal were observed depending on experimental conditions. Morphologies and growth kinetics both depend on several factors such as [pyrrole], [surfactant], field intensity, and time of polymerization. Electropolymerized aggregates were characterized by TEM, XRD, electrical conductivity measurements, and reflectance spectroscopy. Reflectance spectra, weight of polymer, and electrical conductivity support the large π-conjugation in the dendrimer. Growth and particle size decreased because of incorporation of CTAB molecule in the polymer framework. TEM studies clearly indicated that particle size of the polymer aggregates decreased to ∼140-200 nm in the presence of CTAB. The decrease in the particle size on addition of CTAB was also supported by

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XRD studies. Polymerization involves random movement of radical cations, and aperiodic oscillations were observed during electropolymerization, resulting in the formation of fractals and dendrimers. Anode potential oscillates with time during electropolymerization. Amplitude of oscillation and morphology depends on [NaDS] and [CTAB]. A mechanism for fractal and dendrimer growth was also proposed. Acknowledgment. We thank the learned reviewers for constructive and helpful suggestions, Department of Science & Technology (Govt. of India) New Delhi for supporting the investigation, and Head, Chemistry Department, DDU Gorakhpur University, Gorakhpur, India for providing necessary laboratory facilities. We also thank Prof. S. S. Sarkar, IIT Kanpur for providing reflectance spectra and Dr. R. R. Pandey, NPL New Delhi for providing TEM results. References and Notes (1) Rastogi, R. P. Introduction to Non-Equilibrium Physical Chemistry: Towards Complexity and Non-Linear Science; Elsevier: Amsterdam, 2008. (2) Kaufman, J. H.; Melroy, O. R.; Abraham, F. F.; Nazzal, A. I. Solid State Commun. 1986, 60, 757. (3) Fukami, K.; Nakanishi, S.; Yamasaki, H.; Tada, T.; Sonoda, K.; Kamikawa, N.; Tsuji, N.; Sakaguchi, H.; Nakato, Y. J. Phys. Chem. C 2007, 111, 1150. (4) Nakanishi, S.; Fukami, K.; Tada, T.; Nakato, Y. J. Am. Chem. Soc. 2004, 126, 9556. (5) Rastogi, R. P.; Das, I.; Pushkarna, A.; Chand, S. J. Phys. Chem. B 1993, 97, 4871. (6) Das, I.; Verma, S.; Ansari, S. A.; Lall, R. S.; Agrawal, N. R. Fractals 2010, 18, 215. (7) Das, I.; Agrawal, N. R.; Gupta, S. K.; Gupta, S. K.; Rastogi, R. P. J. Phys. Chem. A 2009, 113, 5296. (8) Umeda, R.; Awaji, H.; Nakahodo, T.; Fujihara, H. J. Am. Chem. Soc. 2008, 130, 3240. (9) Bell, T. W. Science 1996, 271, 1077. (10) Frechet, J. M. J.; Henmi, M.; Gitsov, I.; Aoshima, S.; Leduc, M. R.; Grubbs, R. B. Science 1995, 269, 1080. (11) Dai, T.; Yang, X.; Lu, Y. Nanotechnology 2006, 17, 3028. (12) Sonke, S.; Tomalia, D. A. AdV. Drug DeliVery ReV. 2005, 57, 2106.

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