Chemical Oxidative Polymerization and in situ Spectroelectrochemical

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Ind. Eng. Chem. Res. 2001, 40, 40-51

Chemical Oxidative Polymerization and in situ Spectroelectrochemical Studies of a Sulfonated Aniline Derivative by UV-Visible Spectroscopy C. Sivakumar,† T. Vasudevan, and A. Gopalan† Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, India

Ten-Chin Wen* Chemical Engineering Department, National Cheng Kung University, Tainan 70101, Taiwan

The chemical oxidative polymerization of diphenylamine-4-sulfonate (DPASA) was carried out in aqueous and 0.5 M H2SO4 solutions using potassium peroxodisulfate (PDS) as an oxidant. The course of polymerization was followed by UV-visible spectroscopy. The absorbances corresponding to the intermediate and the polymer were followed for different concentrations of DPASA and PDS and at different times of polymerization. The medium was found to be homogeneous and became green colored. The rate of polymerization (Rp) was determined for various conditions. The observed dependences of DPASA and PDS on Rp were used to deduce a rate equation for the polymerization of DPASA. The rate constant for the formation of poly(diphenylamine-4-sulfonic acid), PDPASA, was estimated. Spectroelectrochemical studies were carried out using indium tin oxide as the working electrode to follow the course of polymerization under electro-oxidative conditions. The polymer, PDPASA, was isolated from the reaction medium and characterized by 1H NMR, FTIR spectroscopy, and thermogravimetric analysis. Introduction Polyaniline and its derivatives have been widely investigated over the past 20 years. Polyaniline is recognized to be an air stable organic conducting polymer with interesting electrochemical properties useful for the development of lightweight batteries, electrochromic display, or micro electronic devices.1,2 Polyaniline exhibits insulator-to-metal transitions and color changes depending upon both its oxidation state and protonation level. The conductivity value of these polymers ranges from 10-11 to about 2-3 S cm-1. It can be synthesized either chemically3 or electrochemically4 as a bulk powder or film. Although it is soluble in various solvents such as NMP, THF, DMF, etc. in the undoped state, the doped form is insoluble in common solvents except in concentrated acids and is therefore considered as an intractable and unprocessable material. To overcome the problem of processability, many researchers have attempted to prepare polyaniline derivatives by modifying the structure of the polymer chain by using post-treatment of the preformed polyaniline base, chemical, or electrochemical homopolymerization of suitable aniline derivatives and copolymerization of aniline with different kinds of ring or N-substituted derivatives. Attempts are being made to solubilize the polyaniline to improve their processability. A majority of these efforts rely on the post-treatment of the polymer, with fuming sulfuric acid to introduce a sulfonic acid functional group in the benzene ring. The first step in this direction was done by sulfonation of * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 886 6 2344496. † Present address: Chemical Engineering Department, National Cheng Kung University, Tainan 70101, Taiwan.

the emeraldine base form of PANI with fuming sulfuric acid.5 The resulting self-doped PANI was reported to be conducting up to pH ) 7 and also 50% of the phenyl rings in the PANI chain was found to have an anionogenic sulfo group.6 Sulfonated polyanilines have attracted recent interest because of its processability and potential industrial applications. Sulfonated polyanilines are self-doped in nature and soluble in water. Rubner and co-workers7 reported the use of partially sulfonated polyanilines in the fabrication of multilayers. These self-doped polyanilines show much higher electrochemical activity and electrical conductivity than the conventional PANI in neutral and basic solutions.8 These self-doped polyanilines can be obtained from electrochemical polymerization of aniline derivatives containing a sulfo group. Kim et al.9 synthesized water-soluble poly(4anilino-1-butane sulfonic acid) by an electrochemical method. A different approach to get self-doped PANI derivatives was developed through electrochemical copolymerization of aniline with polymerizable monomers containing anionogenic groups. This approach seems to be more promising as compared to direct sulfonation of the base form of PANI. Chen and Hwang10 prepared a water-soluble polymer, poly(aniline-co-N-propane sulfonic acid aniline). Kitani et al.11 and Lee and coworkers12,13 investigated electrochemical copolymerization of aniline with metanilic acid. Yang and Wen14 have copolymerized aniline with 2,5-diaminobenzenesulfonic acid. Kilmartin and Wright15 synthesized self-doped copolymers of aniline with orthanilic acid by electropolymerization. The solubility of the sulfonated polyaniline derivatives provides the possibility of monitoring the course of polymerization by suitably employing experimental techniques.

10.1021/ie0005607 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/21/2000

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 41

It is frequently assumed that reactive intermediates are formed in the initial stages of the electro-oxidation of aniline and similar monomers. These highly reactive species subsequently react with solution species, yielding consequently oligomers and polymers. Because the intermediate species are very reactive and leading to fast further reactions, it becomes difficult to detect them with the usual techniques. In situ UV-visible spectroelectrochemistry seems to be a useful tool for studying the early stages of the electropolymerization process for identifiable intermediates. Genies and Lapkowski16 showed the presence of short-lived intermediates during electro-oxidation of aniline by in situ spectroelectrochemistry. In the early stages of electro-oxidation of aniline, a transient absorbance band in the blue region of the visible spectrum far away from the main absorbance band of the oxidized form of PANI was observed by Park and co-workers17 Leger et al.18 used fast scan UV-visible reflectance spectroscopy to investigate the electro-oxidation of o-toluidine. Malinauskas and Holze19,20 have used UV-visible spectroscopy for the investigation of early stages of electro-oxidation of several ring and N-alkyl-substituted anilines. The electrochemical process of conducting polymer formation can be easily monitored by the transient techniques but it usually offers low yields of the polymer. In addition, the chemical process affords higher yields and is suitable for industrial production; however, because of the higher rate of polymerization, there is a difficulty in monitoring the course of polymerization. Therefore, a convenient in situ method is needed for monitoring the chemical polymerization process particularly for following the initial stages of the polymerization to obtain the kinetics of reaction. Various groups have studied the kinetics of chemical polymerization of aniline and pyrrole with a wide range of techniques. The rate of polymerization of pyrrole has been studied by monitoring the rate of monomer depletion using gas chromatography21 and combined in situ Raman spectroscopy and potentiometry.22 Wei et al.23 have studied the chemical polymerization of aniline by open circuit potential measurements. Duran and coworkers24 have explored the kinetics of polymerization of 2-pentadecylaniline at the air/water interface and have proposed a kinetic expression based on the scheme suggested by Tzou and Gregory25 for the conventional polymerization of aniline. Gill et al.26 have studied the kinetics of polymerization of aniline using H1 NMR spectroscopy. Recently, we have studied the course of polymerization of a different type of monomer, 1,6heptadiyne, by UV-visible spectroscopy.27 Kinetics of polymerization of aniline or aniline derivatives have been reported earlier by following the amount of deposited polymer on the electrode surface as a function of time in electrochemical studies.17,21,23 In these works, the peak current of the redox processes of the polymer was correlated with the amount of polymer deposited and used to obtain the rate of polymerization while following the kinetics of polymerization. In the case of chemical oxidative polymerization of aniline or the majority of the aniline derivatives, the polymers were found to be precipitated during the polymerization and the kinetics of polymerization were followed under heterogeneous conditions.22,24-26 The kinetics of polymerization became complicated by the additional influences of autoacceleration caused by the surface of the formed polymer in such conditions.

From the earlier reports,5,6,8,10 it becomes clear that the polymerization of sulfonic acid containing aniline derivatives could produce soluble-conducting polymers. A thorough literature search reveals that a systematic study related to following the kinetics of polymerization under homogeneous conditions has not been made so far. The present investigation is therefore aimed at following the course of polymerization of diphenylamine4-sulfonate (DPASA) under homogeneous polymerization conditions. UV-visible spectroscopy was selected to follow the course of polymerization. Fast scan UV-visible spectroscopy was specifically selected to identify the intermediates formed during polymerization and to follow the kinetics of polymerization by monitoring the amount of polymer formed. Such a comprehensive study has not been made so far. In the present study, we report the course of polymerization of sodium salt of DPASA by chemical oxidative polymerization with potassium peroxodisulfate (PDS) as the oxidant in an aqueous and aqueous sulfuric acid medium as a function of monomer/oxidant concentrations. In situ UV-visible spectroelectrochemistry studies are also made for the polymerization of the same monomer using indium-doped tin oxide (ITO) as the working electrode to support the information obtained in chemical oxidative polymerization. The polymer is isolated and characterized by FTIR, H1 NMR spectroscopy, and thermal studies. Experimental Section Chemicals. Sodium diphenylamine-4-sulfonate (DPASA) (Aldrich), potassium peroxodisulfate (PDS) (Aldrich), and other chemicals were used without further purification. All reagents were prepared with doubly distilled water Chemical Oxidative Polymerization. Chemical oxidative polymerization of DPASA was carried out in aqueous and 0.5 M H2SO4 solutions. The course of polymerization was followed by using Shimadzu UVPC2401 UV-visible spectrophotometer. Before spectroscopic measurements, the baseline correction was made by suitably using the reactants. In a typical polymerization experiment, aqueous or aqueous H2SO4 solution of PDS was added to DPASA solution in such a way to keep a definite concentration of DPASA and PDS in the system. The time of addition of PDS to DPASA solution was noted as the starting time of polymerization. The UV-visible spectrum was recorded at various time intervals. Kinetic studies were hence performed by monitoring the absorption spectra of the reaction medium at definite time intervals. All polymerization reactions were carried out in a temperature-controlled quartz cuvette at 30 °C. In situ Spectroelectrochemistry. In situ spectroelectrochemical studies of the polymerization of DPASA was carried out by using a Shimadzu UVPC-1501 Multispec HyperUV photodiode spectrophotometer. This instrument records UV-visible spectra in a second. Data accumulation and analysis were done through use of the software (Microcal origin). Spectroelectrochemical experiments were done in a quartz cuvette of 1-cm path length by assembling as an electrochemical cell with an optically transparent ITO-coated glass plate as the working electrode (with a specific surface conductivity of 10 Ω/square) installed perpendicular to the light path, a platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. Before spectroelectrochemical

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Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001

Figure 1. Visible absorption spectra for the course of polymerization of dipehenylamine-4-sulfonate (DPASA) in an aqueous medium.

experiments, the ITO electrodes were degreased with acetone and rinsed with doubly distilled water. For each experiment, a new ITO electrode was used. Constant potential (1.0V vs Ag/AgCl) was applied on aqueous/ aqueous H2SO4 solutions of DPASA using Autolab PGSTAT 30 potentiostat/galvanostat equipment (Eco Chemi B.V., The Netherlands). The UV-visible spectra were collected simultaneously and analyzed using the software. Isolation of Polymer. After polymerization, the polymer was separated by the addition of a 3:1 ratio of diethyl ether and methanol. A green-colored precipitate was seen and it was washed with an excess of methanol. The separated polymer (green precipitate), PDPASA, was dried under a vacuum oven for 24 h. Characterization. The infrared spectrum of PDPASA was recorded with a Nicolet FT-IR 560 spectrophotometer using KBr pellets. The H1 NMR spectrum of PDPASA was recorded with an amx-400 NMR spectrophotometer and the chemical shifts were recorded in ppm units with TMS as the internal standard. Thermal properties of the dried polymer were followed using a Perkin-Elmer TGA 7/DX thermal analyzer over a temperature range of 100-700 °C in an inert atmosphere, at a heating rate of 20 °C/min. Conductivity of the separated polymer sample was measured by a four-probe dc method with a Concord (India) instrument. Results and Discussion Chemical oxidative polymerization of DPASA was carried out by using PDS as an oxidant in an aqueous medium. Polymerization was started by the addition of an aqueous PDS solution to the aqueous solution of DPASA. The pH of all the solutions were followed at initial conditions and also during the polymerization. The pH of the solution of DPASA in water was found to

be 6.0. The lowering of the pH from 7.0 is attributed to the disproportion of the sulfonate group to sulfonic acid. When PDS was added for starting the polymerization, the pH of the medium was found to be changed to 3.8. During polymerization, further lowering of the pH from 3.80 to 3.16 was noticed. This informs us that during the course of polymerization there are reactions that involve the liberation of protons. The reason for the changes in pH during polymerization are explained in a later part of this discussion. The course of polymerization was followed by UV-visible spectroscopy. Figure 1 represents the visible absorption spectra recorded at various time intervals for the polymerization of DPASA with [DPASA] ) 2.50 × 10-3 mol L-1 and [PDS] ) 1.67 × 10-2 mol L-1. The spectrum during the course of polymerization showed a broad band at 425 nm without any significant absorption in the other regions up to 5 min. As time progressed, a new band simultaneously started appearing in the red region with an increase in absorption with time. Beyond 25 min this band had emerged as a peak at 553 nm. An interesting observation was noted upon analyzing the behavior of these bands with various time intervals. Figure 2 represents the time dependence of the growth of absorbance at 425 and 553 nm. On one hand, the band at 425 nm gradually increases in absorption with time at initial stages and reaches a saturation or steady level after that. On the other hand, the absorbance at 553 nm gradually and steadily increases. This indicates that the 425-nm peak corresponds to the intermediate formed during chemical oxidation. Hence, it is attributed to the diphenylamine4-sulfonate cation radical formed by the oxidation of DPASA with PDS. The assignment of the peak at 425 nm for the intermediate radical formation is based on the earlier reports available through spectroelectrochemical studies on the polymerization of aniline derivatives. Earlier, Malinauskas and co-workers28 observed a transient absorbance band for the intermediate


Figure 2. Time dependence of the growth of absorbance at 425 and 553 nm.

at 443 nm for N-methylaniline, at 453 nm for N-(3sulfopropylaniline),19 and at 450 nm for N-ethylaniline.20 The appearance of a clear maximum in absorption vs time profile (Figure 2) for the intermediate (λmax ) 425 nm) clearly demonstrates that a consecutive type of kinetics operates for the product formation.28 Malinauskus and co-workers in their spectroelectrochemical studies on N-methylaniline28 and N-sulfopropyl aniline polymerization19 noticed the transformation of the intermediate into a polymer. It is also relevant to note that the UV-visible absorption spectrum of the DPASA monomer has a peak at 307 nm corresponding to the π-π* transition without any absorptions in the visible region. Difficulties have been encountered in following the transient states for PANI polymerization.29 Because the transient characteristics of the intermediates can be seen on a long time scale compared to aniline polymerization, UV-visible spectroscopy becomes a useful tool for following the course of polymerization of DPASA in the present case. In the present study, the intermediate cation radical seems to undergo further transformations into oligomer or polymer formation as the end product, which absorbs at 553 nm. The 553-nm band can be assigned for the exitonic electronic transition of the quinoid rings present in PDPASA.30 A lower value of λmax was noted for PDPASA in comparison with that of PANI. This blue shift may have arisen from the steric effect of the aromatic pendent substituent, which may cause torsional twists in the polymer backbone. This would reduce coplanarity and thus shorten the conjugational length.31 Hence, chemical oxidative polymerization of

DPASA can be explained through Scheme 1. Polymerization of DPASA is proposed to proceed through the formation of a radical cation of DPASA by the oxidation of DPASA with PDS (reactions (1) and (1a)). PDS was reported to initiate the polymerization of aniline/aniline derivatives by these type of reactions.33-37 The DPASACR is further transformed into dimeric species accompanied by deprotonation.21 An electrophilic attack of dimer with DPASACR results in a trimeric cation radical, which upon subsequent deprotonation generates a neutral trimer. A sequence of the above type of reactions finally leads to polymer formation. The deprotonation reaction must result in changes in pH of the medium. This was actually observed in the present study. The pH of the medium was found to decrease when PDS was initially added, from 3.80 to 3.16, as a result of deprotonation reactions. This supposition is authenticated by the following experiment. The polymerization was allowed to proceed for 24 h and the polymer was isolated as described in the Experimental Section. The dark-green-colored product, poly(diphenylamine-4-sulfonic acid) (PDPASA), was again dissolved in an aqueous medium and the UVvisible spectrum was recorded. The band at 553 nm, which appeared during the course of polymerization, was seen in the spectrum of PDPASA and confirms that the absorption band appearing at 553 nm during the course of polymerization is due to the formation of a polymer. For different concentrations of PDPASA in an aqueous medium, UV-visible spectra were recorded. The absorbance at 553 nm was found to increase linearly

44


Scheme 1

Figure 3. Effect of time on Rp for the polymerization of DPASA in an aqueous medium. [PDS] ) 1.67 × 10-2 mol L-1 (A,B,C,D). [DPASA] ) 1.67 × 10-3 (A); 2.50 × 10-3 (B); 3.33 × 10-3 (C); 5.00 × 10-3 (D) mol L-1.

Table 1. Determination of the Molar Extinction Coefficient Visible Absorption Spectra of PDPSA for Various Concentrations concentration, ×101 g L-1

absorption (λmax ) 553 nm)

0.373 0.467 0.583 0.724 0.911 1.139 1.424

0.177 0.221 0.268 0.325 0.396 0.485 0.621

molar extinction coefficient (), M-1 cm-1 1287 1284 1247 1218 1179 1155 1183 E) 1222

with the concentration of PDPASA. The calibration line between the absorbance and concentration of PDPASA (at λmax ) 553 nm) was used further to estimate the amount of polymer formed during the polymerization. The rate of polymerization, Rp, the amount of polymer formed per unit time of polymerization, was determined. Rp values have been determined by earlier workers21,23 by using the peak current values associated with redox processes of the deposited polymerization. In the present study, the changes in absorbance values were used to determine Rp. Hence, UV-visible spectroscopy was found to be useful in following the kinetics of DPASA polymerization. The consistency of the calculated molar extinction coefficient () at 553 nm is evident from Table 1. The average value (1222 M-1 cm-1) of was also used to estimate the amount of polymer formed at any time interval by noting the absorbance at 553 nm. Figure 3 represents the Rp vs time plot for DPASA polymerization. Rp showed an increasing trend with time at the

initial stages and were found to be higher with increasing [DPASA] at any definite time. In the process of obtaining the rate expression for the polymerization of DPASA, a systematic approach was made to establish the dependences of Rp on [PDS] and [DPASA]. The course of DPASA polymerization was followed by changing [DPASA] in the range from 1.67 × 10-3 to 5.00 × 10-3 mol L-1 while keeping [PDS] ) 1.67 × 10-2 mol L-1. The spectra representing the course of DPASA polymerization are identical (not shown) to the one discussed above. However, for a fixed time interval, increasing the [DPASA] increases the optical density at λmax ) 553 nm. The plot of the log Rp vs log [DPASA] (not shown) was found to be linear with a slope value of unity. For verifying the first power dependence of Rp on [DPASA], the plot of Rp vs [DPASA] (Figure 4, plot A) was drawn. This was found to be linear and passing through the origin, confirming the firstpower dependence of Rp on [DPASA]. In the electrochemical polymerization of ANI, a first-power dependence of Rp on [ANI] has been noticed by Tzou and Gregory,25 Wei et al.,21 and Shim and Park.32 In electrochemical polymerization of aniline by potentiostatic or cyclic potential sweep techniques, the initial formation of polyaniline on the surface of the electrode was reported to be slow. The initially formed or deposited polyaniline on the surface of the electrode was reported to accelerate the polymer growth significantly. This effect was noted as self-catalysis or autoacceleration in the electrochemical polymerization of aniline. In electrochemical polymerization both nucleation and autoacceleration steps were reported to occur under heterogeneous conditions. Because of the heterogeneous nature of the autoacceleration step, the rate constant of the second step was directly dependent on the total available surfaces or contact surfaces of the formed polymer rather than the amount of polymer present. Hence, the plot Rp vs [ANI] was not reported to pass through the origin.21,25,32 The presence of an intercept


Figure 4. Effect of [DPASA] on Rp for the polymerization of DPASA in an aqueous medium. Plot A: [PDS] ) 1.67 × 10-2 mol L-1. Plot B: [PDS] ) 2.50 × 10-2 mol L-1.

in the plot has been explained by the autoacceleration effect caused by the heterogeneity of the medium. Gopalan and co-workers33-37 explained the autocatalytic effect under the heterogeneous conditions where the polymer was precipitated. Through several studies33-37 it was concluded that the heterogeneity of the medium resulted in autoacceleration in chemical oxidative polymerization of ANI and ANI derivatives in the presence of added fibers. Hence, the observed linear nature of the

plot of Rp vs [DPASA] (Figure 4, plot A), which was found to pass through the origin, indicates that the autoacceleration effect is not dominant in DPASA polymerization. To get further support for the first-power dependence of DPASA on Rp, another set of experiments were performed with [PDS] ) 2.50 × 10-2 mol L-1 and varying [DPASA] in the range from 1.67 × 10-3 to 5.0 × 10-3 mol L-1. Here again, Rp showed linearity with [DPASA] (Figure 4, plot B). Figure 2, plots A-D, represent the time dependence of growth of absorbance at 425 and 553 nm for different [DPASA]. The plots clearly indicate that a consecutive type of reaction kinetics follows. The intermediate formed in the first step is further consumed in subsequent steps to result in a polymer. Thus, radical cations are increased with increasing [DPASA], resulting in more polymer products. Similarly, when the [PDS] is varied for a fixed value of [DPASA], two different sets of experiments were performed. A typical course of polymerization is given (Figure 5). When the [PDS] mentioned in Figure 5 was varied and also another set of experiments was performed, the first-power dependence of Rp on [PDS] was evident. The plots of Rp vs [PDS] (Figure 6, plots A and B) were stright lines with negligible intercepts. The dependence of PDS and DPASA on Rp were used to deduce a rate expression for the polymerization of DPASA in the present study. In 1992, Gregory and co-workers25 studied the kinetics of chemical polymerization of aniline. In that study, the medium of polymerization became heterogeneous. A kinetic scheme involving the effects of surface and autoacceleration by a polymer was proposed. Wei21 in the electrochemical polymerization of aniline explained the kinetics by indicating the presence of surface effects of the formed polymer. In a simple sense, the following kinetic equation was proposed,

Figure 5. Visible absorption spectra for the polymerization of diphenylamine-4-sulfonate [DPASA] in an aqueous medium (25th minute).

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Figure 6. Effect of [PDS] on Rp for the polymerization of DPASA in an aqueous medium. Plot A: [DPASA] ) 1.67 × 10-3 mol L-1. Plot B: [PDS] ) 3.33 × 10-2 mol L-1.

Rp ) k[M] + k′[M][P]

(1)

where k is the rate constant of the formation of PANI on a bare platinum electrode and k′ is the rate constant on a PANI-coated platinum electrode. Subsequently, Shim et al.32 proposed a kinetic equation for ANI polymerization, indicating the autoacceleration effect as

Rp ) k′[ANI][Oxi] + k′′[ANI][TAS]

(2)

where k′ and k′′ are rate constants for PANI formation on bare platinum and PANI-coated platinum electrodes, respectively, and TAS is the total surface area. The above two equations suit in explaining the kinetics of PANI formation when autoacceleration by the formed polymer and surface effects on polymerization were dominant. From our research group, results have been reported on the establishment of the kinetic equation for the polymerization of ANI and o-toluidine33-37 in the presence of added substrates, while the polymer was precipitating during the course of polymerization. From an analysis of the above reports and an account of the observations made in the present study, a kinetic equation for Rp is deduced. The medium was homogeneous throughout the polymerization in the present study, which confirmed that the formed polymer was soluble in an aqueous medium. The observed green coloration during the course of polymerization and the green color of the isolated polymer justified the solubility of the polymer in the medium of polymerization. Earlier, many of the sulfonated polymerization derivatives were reported to be soluble in an aqueous medium.7,8,10 The presence of autoacceleration in ANI and ANI derivatives resulted in a specific intercept in the plot of Rp vs [ANI] or [ANI derivative]. The intercept was used to find the rate constant of the autoacceleration step. In addition, if a plot of Rp vs [monomer] passes through the origin,

Figure 7. Derivative of absorbance as a function of time for DPASA polymerization in an aqueous medium. [PDS] ) 2.50 × 10-2 mol L-1 (A,B,C,D). [DPASA] ) 1.67 × 10-3 (A); 2.50 × 10-3 (B); 3.33 × 10-3 (C); 5.00 × 10-3 (D) mol L-1.

autoacceleration can be considered as insignificant. Hence, considering the fact that the plots of Rp vs [DPASA] (Figure 4) and Rp vs [PDS] (Figure 6) are linear and pass through the origin and homogeneous conditions exist for polymerization, the following kinetic equation is proposed:

Rp ) k1[DPASA][PDS]

(3)

This obviously means that, for the polymerization of DPASA, a mechanism that is similar to PANI is operative with negligible contribution from autoacceleration by the formed polymer. The reaction rate for describing chemical oxidative polymerization is now explained from the monomer conversion point of view.

-d[DPASA]/dt ) k1[DPASA][PDS]

(4)

where [DPASA] and [PDS] are concentrations of DPASA and PDS at any time intervals. The variation of [DPASA] with time is then defined by

[DPASA] ) [DPASA]0 - n[P]

(5)

where n is the number of monomeric units in the polymer molecule, [DPASA]0 is the initial concentration, and [P] is the concentration of polymer formed at any time of polymerization. Assuming that 2.5 electrons are necessary to oxidize 1 molecule of monomer25 and 1.25 mol of PDS are consumed in the oxidation of 1 mole of DPASA implies that at time “t”,

[PDS] ) [PDS]0 - ([DPASA]0 - [DPASA])/1.25 (6) where [PDS]0 is the initial concentration of PDS. Substituting eq 5 into eq 6 and rearranging,

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 47 Table 2. Evaluation of Rate Constant for the DPASA Polymerization medium/approach aqueous/from Rp

0.5 M H2SO4/from Rp aqueous/(dA/dt)

0.5M H2SO4/dA/dt a

variation

condition

figure

slope

DPASA DPASA PDS PDS DPASA PDS PDS)2.08 × 10-2 mol L-1 PDS)2.50 × 10-2 mol L-1 PDS)2.92 × 10-2 mol L-1 PDS)3.33 × 10-2 mol L-1 DPASA)3.0 × 10-3 mol L-1 DPASA)4.0 × 10-3 mol L-1 DPASA)5.0 × 10-3 mol L-1

PDS ) 1.67 × 10-2 mol L-1 PDS ) 2.50 × 10-2 mol L-1 DPASA ) 1.67 × 10-3 mol L-1 DPASA ) 3.33 × 10-3 mol L-1 PDS ) 1.50 × 10-2 mol L-1 DPASA ) 3.0 × 10-3 mol L-1 DPASA ) 3.33 × 10-3 mol L-1

4 (plot A) 4 (plot B) 6 (plot A) 6 (plot B)

1.50 × 10-2 2.23 × 10-2 1.39 × 10-3 2.74 × 10-3 0.0115 0.002

PDS ) 1.50 × 10-2 mol L-1

Figure 7

k1,a M-1 s-1 3.31 × 10-3 3.29 × 10-3 3.07 × 10-3 3.04 × 10-3 2.83 × 10-3 2.46 × 10-3 3.23 × 10-3 b 3.33 × 10-3 b 3.10 × 10-3 b 3.12 × 10-3 b 2.54 × 10-3 b 2.60 × 10-3 b 2.83 × 10-3 b

Rate constant for the formation of PDPASA. b Statistical average of several values.

Figure 8. Visible absorption spectra for the polymerization of diphenylamine-4-sulfonate [DPASA] in a 0.5 M sulfuric acid medium.

[PDS] ) [PDS]0 - 0.8n[P]

(7)

Substituting eqs 5 and 7 into eq 4, assuming d[DPASA]/ dt ) 0, and rearranging

d[P]/dt ) Rp ) 0.8k1[P]2 - 0.8k1[DPASA]0[P] k1[PDS]0[P] + k1[PDS]0[DPASA]0/n (9) Because the kinetics was studied by following the absorbance of the polymer, eq 7 can be conveniently converted using

A ) 553nmb[P]; b ) 1 cm; [P] ) A/

(9)

Substituting the terms in eq 9 into eq 8 makes

d[A]/dt ) 0.8k1A2/ - 0.8k1[DPASA]0A k1[PDS]0A + k1[PDS]0[DPASA]0/n (10) Figure 7 represents the plots of dA/dt as a function of time using absorbance values at various time inter-

vals. The shape of dA/dt vs time is similar to the plot of Rp vs time (Figure 3) and confirm each other. The rate constant, k1, value appearing in eq 10 was found out by regression analysis for each set of [DPASA] and [PDS]. The consistency of calculated k1 values in Table 2 for a different set of conditions also support the selection of rate expression (eq 4) for Rp. This was also verified by calculating the value from the slopes of the plots Rp vs [DPASA] (Figure 4) and Rp vs [PDS] (Figure 6). The closeness of k1 values among different conditions can also be seen here (Table 2) and confirm that eq 4 is the right choice for representing Rp for DPASA polymerization. The average value for k1 for DPASA polymerization is found to be 3.20 × 10 -3 M-1 s-1 at 30 °C. The pH of the solution of DPASA in 0.5 M H2SO4 was found to be 1.52. Upon addition of PDS, the pH was found to be lowered to 1.50. As polymerization proceeds, the pH was found to change from 1.50 to 1.42. These pH changes are consistent with the proposal of deprotonation reactions as explained earlier in aqueous DPASA polymerization. Polymerization was also carried

48


Figure 9. (A) Cyclic voltammogram of DPASA in an aqueous medium with a scan rate of 20 mV/s vs Ag/AgCl. (B) Absorbance-wavelengthpotential profile for the polymerization of DPASA. The potential was swept between 0 and 1.0 V vs Ag/AgCl; first cycle (scan rate ) 20 mV/s). (C) Absorbance-wavelength-potential profile for the polymerization of DPASA. The potential was swept between 0 and 1.0 V vs Ag/AgCl; second cycle (scan rate ) 20 mV/s).

out in an aqueous 0.5 M H2SO4 medium. Figure 8 represents the spectra for the typical course of polymerization. Here, also, the intermediate formation was seen through the peak at 425 nm, which transformed into product with a similar response to time as noticed in aqueous polymerization. The PDPASA in 0.5 M H2SO4 shows the absorption band corresponding to exiton transition of the quinoid form at 495 nm. The slight blue shift for the peak from aqueous solution for PDPASA suggests that the conjugation length is lower in 0.5 M H2SO4. Such a dependency of this peak on pH was noted earlier.31 Interestingly, a clear band appears at 720760 nm for PDPASA in the acid medium. This indicates that PDPASA gets externally doped or protonated by H2SO4, resulting in an intense bipolaronic band.

Results on the course of DPASA polymerization revealed the first-power dependencies of [DPASA] and [PDS] on Rp (figures not shown). Hence, an expression for Rp similar to eq 4 suits well in explaining the polymerization in aqueous 0.5 M H2SO4. The k1 value was evaluated by adopting the procedure similar to that of the aqueous system. The value of k1 was found to be 2.826 × 10-3 M-1 s-1 at 30 °C. The k1 value was found to be low in the 0.5 M H2SO4 medium in comparison with that of the aqueous medium. In Situ Spectroelectrochemistry of DPASA. Figure 9A represents the cyclic voltammograms, recorded during the electropolymerization of DPASA in an aqueous medium using ITO as the working electrode. An oxidation peak appears at 0.91 V, which may correspond


Figure 10. Visible absorption spectra recorded during the course of electrochemical polymerization of DPASA.

to the generation of a DPASA cation radical. Exhaustive oxidation was performed of a solution of DPASA (1.0 × 10-6 M) at 0.91 V. The charge associated with this processes (2.0 mC) revealed that the oxidation wave corresponded to one electron transfer. On the reverse scan, two redox peaks appeared at 0.525 and 0.329 V. The voltammograms and UV-visible spectrum were recorded simultaneously. Figure 9B presents the threedimensional absorbance-wavelength-potential diagram obtained with a collection of spectra recorded at each 40 mV/s during both positive and negative scans of potentials. As can be seen from Figure 9B, during the positive potential scan, the UV-visible spectra display no specific bands up to 0.9 V, which corresponds to the oxidation peak of DPASA that appeared in the voltammogram (Figure 9A). For potentials higher than 0.9 V, two bands start to appear in the spectra and the intensity of these two bands increases with the potentials. The band at 425 nm corresponds to the formation of a DPASA cation radical as a result of oxidation. The appearance of 553-nm peaks confirms the conversion of the DPASA cation radical to PDPASA. The CV for the second cycle has additional peaks arising from the oxidation of formed PDPASA. The increased absorbance at λmax ) 553 nm in comparison with that of the first cycle indicates that an increased amount of polymer was formed (Figure 9C). Polymer was not found to be deposited on the electrode as evident from the CV of the electrode recorded with background electrolyte. The medium becomes green colored after polymerization. The above information is in-line with our proposal of the soluble nature of PDPASA in an aqueous medium. We applied a constant potential of 1.0 V vs Ag/AgCl for an aqueous solution of DPASA (0.005 M). After the potential was switched, two absorbance bands started appearing at 425 and 553 nm, which are due to the intermediate and PDPASA (Figure 10). Both bands grow in intensity; however, the peak at 425 nm attains

saturation after a certain time while the peak at 553 nm grows continuously. Experiments with different [DPASA] reveal that Rp (determined by adopting a similar procedure to chemical oxidative polymerization) linearly increases with [DPASA]. Characterization of PDASA. The H1 NMR spectrun recorded in DMSO-d6 is shown in Figure 11. The two doublets that appeared between 6.8 and 7.0 ppm correspond to the protons on the pendent phenyl sulfonic acid substituent in the polymer backbone. This assignment was based on the reports of earlier workers.31,38 Armes and co-workers38 and Chen and Hwang10 reported the appearance of a broad signal in the range between 6.5 and 8.5 ppm and assigned it to the aromatic protons in the PANI backbone. The quartets appearing between 7.4 and 7.6 ppm in the present study are assigned to the aromatic protons in the polymer backbone. This assignment was made based on the report by Roy et al.39 for the sodium salt of poly(o-aminobenzene sulfonic acid) in D2O. To confirm the structure, analysis of the FT-IR spectrum for PDPASA (Figure 12) was used. The spectrum shows two strong aromatic C-N stretching bands at 1393 and 1112 cm-1. The band at 1393 cm-1 is assigned to the C-N stretching of the phenyl sulfonic acid substituent.11 Strong asymmetric and symmetric SdO stretching appears at 1178 and 1035 cm-1, respectively. A strong S-O stretching band is seen at 707 cm-1. The CdC stretching of the benzene ring appears at 1460 cm-1. The band at 1593 cm-1 is assigned to the CdC ring stretching of the imino group,40 which is higher than that of polyaniline propane sulfonic acid.10 This can probably be attributed to the lower doping level of the PDPASA. The strong C-H out-of-plane bending vibration of the para-substituted benzene ring appears at 809 cm-1, which indicates that the monomers in the polymer are bonded in a head to tail fashion in-line with previous reports.38

50


Figure 11. H1 NMR spectra of PDPASA.

Figure 12. FTIR spectra of PDPASA.

The TGA thermogram of the PDPASA in a dry nitrogen atmosphere is shown in Figure 13. It shows a weight loss starting at about 190 °C, which is probably due to the elimination of the free sulfonic acid groups. At 280 °C the remaining sulfonic acid groups are lost. At 550 °C the polymer decomposes. The conductivity of the chemically synthesized PDPASA was found to be 0.31 × 10-2 S cm-1. However, the value for PDPASA in the present study was found to be higher than poly(aniline propane sulfonate)41 (1.5 × 10-5 Scm-1) and this may be attributed to the increased extent of self-doping in the present case. The low value of PDPASA in comparison with the PANI (5.0 S cm-1)5 and poly(aniline propane sulfonic acid)10 (1.5 × 10-2 S cm-1) values may be attributed to the reduction in the π-conjugation length for PDPASA as observed through UV-visible spectroscopy.

Figure 13. TGA of PDPASA.

Conclusions UV-visible spectroscopic studies on the chemical oxidative polymerization of diphenylamine-4-sulfonate (sodium salt) (DPASA) by peroxodisulfate (PDS) reveal that polymerization proceeds through a consecutive-type kinetics. This was evident in following the absorbances at 423 and 553 nm corresponding to the intermediate and the formed polymer (PDPASA). The homogeneous conditions for polymerization give a smooth kinetics with a rate of polymerization showing first-power dependencies on both DPASA and PDS. No autoacceleration effect by the formed polymer was noticed. The closeness of the value of the rate constant for the formation of PDPASA determined through different approaches justifies the deduced rate equation. The rate


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Received for review June 5, 2000 Revised manuscript received September 12, 2000 Accepted September 20, 2000 IE0005607

Chemical Oxidative Polymerization and in situ Spectroelectrochemical

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