Materials Chemistry and Physics 85 (2004) 316–328

Electrochemical, spectroelectrochemical and spectroscopic evidences for copolymer formation between diphenylamine and m-toluidine P. Santhosh, M. Sankarasubramanian, M. Thanneermalai, A. Gopalan∗ , T. Vasudevan Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, India Received 22 September 2003; received in revised form 3 December 2003; accepted 19 January 2004

Abstract Electrochemical copolymerization of diphenylamine (DPA) with m-toluidine (MT) was carried out for different molar feed ratios of DPA in 4 M H2 SO4 using cyclic voltammetry. Cyclic voltammogram (CV) of the deposited copolymer film was also recorded in 4 M H2 SO4 . CVs recorded during polymerization of mixture of DPA and MT clearly reveals deposition of copolymer with different proportions of DPA unit in it. The CV characteristics show dependence on the molar feed concentrations of DPA or MT. UV-Vis spectroelectrochemical studies reveal the formation of intermediates/copolymer having units of DPA and MT in the backbone structure with a new characteristic band around 580 nm. Copolymers were synthesized for different molar concentration feed ratios of DPA and the molar composition of DPA and MT units in the copolymers were determined by UV-Vis spectroscopy. Reactivity ratios of DPA and MT were computed by using Fineman–Ross and Kelen–Tudos methods and correlated with spectroelectrochemical results. The copolymers were further characterized by FT-IR spectroscopy and thermogravimetric analysis. © 2004 Published by Elsevier B.V. Keywords: Electropolymerization; UV-Vis spectroscopy; Spectroelectrochemistry; Copolymer composition

1. Introduction Polyaniline (PANI) has been extensively considered as the special member of conducting polymer family as it can be doped by protonic acids and gaseous iodine [1–5] to result in highly conducting material. Besides, PANI has been successfully employed as technologically important material in rechargeable batteries, sensors, electrochromic display devices and corrosion protecting film [6–9], due to its well-behaved electrochemistry [10], electrochromism [11], good environmental stability in air [12] and ease of preparation in both aqueous acidic and organic media. However, the limited solubility of PANI in its highly conducting state, owing to the stiffness of its backbone [13], restricts its usefulness in utilizing for commercial products. This necessitates modification of the PANI structure to achieve better processability. Modifications of the structure of the PANI chain have been achieved by several methods: (i) post-treatment of the parent polyaniline base [14,15]; (ii) homopolymerization of aniline derivatives [16–18]; (iii)

∗ Corresponding author. Tel.: +91-4565-225205; fax: +91-4565-225202. E-mail address: algopal [email protected] (A. Gopalan).

0254-0584/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2004.01.021

copolymerization of aniline/aniline derivatives with different kinds of aniline derivatives [19–22]. Alkyl or aryl substituted PANI show better solubility in common organic solvents [23,24]. Likewise, more soluble PANI type materials have been synthesized by introducing a sulphonic acid group into the backbone structure of PANI [25,26]. Improved processability [16–27], electrochromism and other properties over PANI [28–30] have been noticed for the polymers derived from various benzene ring substituted and N-substituted aniline derivatives. Polymers of N-substituted aniline derivatives, such as N-methyl, N-ethyl, N-propyl, N-naphthyl and N-benzyl were synthesized by electrochemical methods [31–33]. In general, substituted PANIs have been found to show better processability over PANI [34]. Additionally, it has been reported that, the introduction of substituent in PANI backbone alters the torsional angle and the electronic properties. Typically, theoretical studies on PANI type material indicate that ionization potential and band gap of PANI type materials depend on the torsional angle between adjacent rings of the polymer chain and substituent in aryl part or N-atom of PANI [35]. Diphenylamine (DPA), an N-substituted aniline derivative was polymerized by Comisso et al. [36] in a mixture of 4 M H2 SO4 and ethanol. In the electrochemical polymerization,

P. Santhosh et al. / Materials Chemistry and Physics 85 (2004) 316–328

poly(diphenylamine), PDPA could not be deposited continuously as a film during polymerization by cyclic voltammetry on the working electrode, which could be due to dissolution of oligomers in ethanol. Hence, the use of ethanol as co-solvent in electrolyte medium provides an adverse effect on deposition and to be avoided. Also, reports are available on polymerization of N-alkyl diphenylamine, 3-methoxy diphenylamine and 3-chlorodiphenylamine [37,38]. PDPA finds many applications. An amperometric sensor involving PDPA has been reported for simultaneous determination of electro-inactive anions and cations as a detector in ion chromatography [39]. Several common anions and cations such as SO4 2− , Cl− , NO3 − , Na+ , NH4 + , K+ , Mg2+ and Ca2+ were determined using this ion-chromatographic system with satisfactory results. The use of PDPA as sorbent for the solid-phase extraction of some phenolic compounds from water has been reported [40]. The utility of PDPA as sensor for aliphatic alcohols such as methanol, ethanol, propanol, butanol and heptanol has been demonstrated [41]. A negative change in resistance was observed upon exposing the polymers to the vapor of methanol, ethanol or propanol vapors, whereas, a reverse trend has been observed with butanol and heptanol. Schottky as well as p–n junction diodes have been fabricated using spin coated thin film of PDPA [42] and I–V measurements have indicated that the diodes have good stability and high rectification ratio. PDPA exhibits electrochromism as a layer in flexible electrochromic display devices [43]. These devices exhibit great contrast in the visible/near infrared (IR) spectral region with high reflectance in their reflecting state due to morphological changes induced by the movements of ions in the polymer films by using EQCM measurements with fast simultaneous acquisition of frequency and impedance. Recently, Gopalan and coworkers [44] has established the utility of PDPA and its copolymer films for tuning pH sensitivity close to 7 and revealed the possibility of using in biosensor applications. The pH sensitivity, reproducibility and response time were evaluated and compared with that of PANI. Copolymerization provides an alternative and easy approach to modify structure of PANI. Copolymers having proportion of aniline and substituted aniline derivatives in the backbone structure posses different electrochemical characteristics than PANI. In addition, it becomes possible to tune the electrochemical and electronic properties by altering the conditions of copolymerization. Structural modification of PANI with ring or N-substitution, through copolymerization, results in modified electrochemical characteristics [45,46]. Recently, copolymerization of DPA with benzidine (an aniline derivative) [47] and ANI [45] has been performed. Copolymer formation between benzidine (or ANI) and DPA has been reported to occur via formation of –C–(NH)–C– bonds between a nitrogen atom of benzidine (or ANI) and a phenyl carbon atom of DPA. This implies that, phenyl-ended intermediates of DPA can be connected to the nitrogen atom

317

in –NH2 group of ANI or ANI derivative to result the copolymer. Literature reveals that only few reports are available on copolymerization involving DPA as one of the monomers. Recently, Gopalan and coworkers [46,48–51] have reported few spectroelectrochemical studies using cyclic voltammetry and UV-Vis spectroscopy on the copolymerization of DPA with few of the aniline derivatives and explained the copolymer formation through a plausible mechanism. m-Toluidine (MT) is expected to produce a polymer similar to poly(o-toluidine), POT, as C–N coupling of monomeric units could give similar structure for POT and poly(m-toluidine), PMT. However, when MT is polymerized with DPA, mixed proportions of C–C and C–N coupled structures of monomeric units (DPA and MT) would be generated. DPA, a N-phenyl substituted aniline, polymerizes through C–C coupling of monomeric units. Hence, it will be interesting to follow the course of polymerization of mixture of DPA and MT through cyclic voltammetry and UV-Vis spectroelectrochemical studies. The present work reports the polymerization of mixture of DPA and MT using cyclic voltammetry. The course of electropolymerization was also followed by in situ UV-Vis spectroscopy to provide an insight about the mechanism of copolymer formation. Copolymers were synthesized and characterized by FT-IR spectroscopy and thermal measurements to justify copolymer formation.

2. Experimental Diphenylamine, DPA (Merck), m-toluidine, MT (Merck) and sulphuric acid (Ranbaxy) have been used without further purification. 2.1. Synthesis of copolymers Electrochemical copolymerization by cyclic voltammetry was performed as described elsewhere [48] by using EG & G PAR VerSastat II potentiostat/galvanostat. A typical procedure is outlined. A mixture of DPA and MT, having a total concentration of 40 mM was used for the polymerization. The molar concentration feed ratio of DPA was maintained as the ratio of molar concentration of DPA to total concentration of DPA and MT and polymerization was carried out for different molar feed ratios of DPA. Potential was swept in the range 0–1000 mV for 50 cycles with a sweep rate of 100 mV s−1 . Cyclic voltammograms (CVs) were collected continuously while the polymer was deposited on working electrode. Polymerization of DPA and MT were preformed separately in a similar way. The deposited film of polymer (copolymer/homopolymer) was then placed in 4 M H2 SO4 and CVs of the film-coated electrode was recorded after stabilization in the same potential range.

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Fig. 1. (a)–(c) Cyclic voltammograms recorded during the polymerization of DPA with MT in 4 M H2 SO4 . Scan rate = 100 mV s−1 . Molar feed ratio of [DPA] (i) 0.75, (ii) 0.625, (iii) 0.50, (iv) 0.375, (v) 0.25. Total concentration of DPA and MT = 40 mM. (a) First, (b) second, (c) twentieth scan of potential.

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2.2. UV-Vis in situ spectroelectrochemistry In situ spectroelectrochemical studies on copolymerization of DPA with MT were carried out as detailed elsewhere [46]. A Shimadzu UV-2401PC UV-Vis spectrophotometer was used to record the spectra simultaneously during electropolymerization. A BAS-100WB electrochemical analyzer was used to apply potential on the working electrode. 2.3. Thermal analysis Thermograms of homo/copolymer were recorded with a DuPont TA-2050 thermogravimetric analyzer in nitrogen atmosphere, using platinum crucibles with ca. 2 mg of the samples, under N2 atmosphere (30 ml min−1 ) at a heating rate of 10 ◦ C min−1 . 2.4. Infrared spectroscopy Copolymers were pressed as a disk with KBr and FT-IR spectra were recorded using Perkin-Elmer—FT-IR Rx 1 (UK) at a resolution of 2 cm−1 . 3. Results and discussion 3.1. Electrochemical copolymerization and homopolymerization Electrochemical polymerization of diphenylamine (DPA) with m-toluidine (MT) has been performed using cyclic voltammometry (CV) for different molar feed ratios of DPA (0.75, 0.625, 0.5, 0.375, 0.25) on platinum electrode surface in 4 M H2 SO4 by scanning the potentials in the limits 0.0–1.0 V for 50 cycles. Fig. 1(a)–(c) represents the CVs

319

recorded for the first, second and twentieth potential scan while performing polymerization with different feed ratios of DPA with MT. A single anodic peak representing the formation of diphenylamine cation radical (DPACR) and or m-toluidine cation radical (MTCR) was noticed in all the cases. In the reverse scan two distinct peaks were observed at around 0.55 and 0.42 V, respectively. These peaks were assigned to the reduction of the oligomer/polymer products formed by the reaction or cross-reaction between the intermediate species, DPACR and MTCR. The peak current values at any of the redox processes are found to increase steadily with the increase in the molar feed ratio of DPA and with increase in number of cycles (Fig. 1). This indicates the building up of materials on the surface of the electrode. A green deposit was seen on the surface of the electrode. It is also important to note that the oxidation peaks during copolymerization of DPA with MT were found to be different from the peaks corresponding to growth of PDPA [45,49]. This was ascertained by performing homopolymerization of DPA under the conditions similar to the copolymerization. During copolymerization with different feed ratios of DPA, oligomers having different proportions of DPA or MT in the backbone may be formed. The in situ UV-Vis spectroelectrochemical studies (discussed later) clearly support this proposal. The CVs recorded for the second (Fig. 1b) and twentieth (Fig. 1c) scans during the polymerization of mixture of DPA and MT distinctly differ from the CVs recorded during polymerization DPA alone. Also, the twin redox characteristics [45,49,50], that were noticed for the polymerization of DPA (Fig. 2c), were virtually shrinked into a single redox process in the cases of polymerization with the mixture of DPA and MT (Fig. 1c). Evidences for the formation of copolymer with MT units in the structure have also been obtained. CVs of the respec-

Fig. 2. Cyclic voltammograms of polymerization of DPA on platinum electrode in 4 M H2 SO4 . Scan rate of 100 mV s−1 . [DPA] = 40 mM. (a) First, (b) second, (c) twentieth scan of potential.

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Fig. 3. Cyclic voltammograms of PDPA and copolymer films in monomer-free background electrolyte (4 M H2 SO4 ) at a scan rate of 100 mV s−1 . (a) PDPA film; (b)–(f) copolymer film prepared by having molar feed ratio of [DPA] ((b) 0.75, (c) 0.625, (d) 0.5, (e) 0.375, and (f) 0.25). Total concentration of DPA and MT = 40 mM. Inset: effect of scan rate on peak current.

tive polymer films were recorded in the monomer free background electrolyte (Fig. 3). CVs of the films deposited with mixture of monomers are not the simple superimposed CVs of the individual PDPA or PMT films. The CV pattern of the stabilized films are similar to the CV pattern recorded during copolymerization. The stable and the surface bound nature of the deposited films were evident from the linearity obtained between peak current of redox process and scan rate (Fig. 3 insert) [45,51]. Copolymers were prepared from different molar feed ratios of DPA. The molar composition of DPA and MT units in the copolymer prepared with different molar feed ratios of DPA were determined by UV-Vis spectroscopy. (Table 1). On increasing the molar feed ratio of DPA, an increasing

trend in molar composition of DPA in the copolymer was noticed. Clearly, the copolymers prepared from different molar feed compositions of DPA are having different molar compositions of MT or DPA units in the backbone of the copolymer. It is pertinent to note that the redox characteristics of polymer film (Fig. 3a) deposited under different feed composition of DPA are reflective of the differences in the composition of the monomer units in the copolymer. 3.2. In situ spectroelectrochemical studies in the course of copolymerization In situ spectroelectrochemical studies on the copolymerization of mixture of DPA and MT with different molar feed

Table 1 Molar composition of DPA or MT in poly(DPA-co-MT) by UV-Vis spectroscopy εPDPA = 0.316 × 10−2 l g−1 ; εPMT = 1.287 × 10−2 l g−1 ; λmax (PDPA) = 341 nm; λmax (PMT) = 358 nm; λmax poly(DPA-co-MT) = 368 nm Molar concentrations

Concentration of copolymer (10−2 g l−1 )

DPA (mM)

MT (mM)

15

25

2 4 6

20

20

25

35

Average, ε (×10−2 l g−1 )

Molar composition of the copolymer DPA

MT

0.45

0.15

0.85

2 4 6

0.32

0.59

0.41

15

2 4 6

0.22

0.82

0.18

5

2 4 6

0.12

0.93

0.07

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ratios of DPA, inform that the spectral characteristics show variation among them. These variations can be viewed due to the formation of copolymers having different proportions of monomer units (DPA or MT) in it. Fig. 4(a)–(e) shows the UV-Vis spectra recorded during the copolymerization of mixture of DPA and MT with various molar feed ratio of DPA as 0.2, 0.8, 0.4, 0.6 and 0.5. The peak and band position noticed in the UV-Vis spectra (Fig. 4(a)–(e)) are totally different from the band and peak positions noticed for polymerization of DPA alone.

321

Fig. 4a and b display the spectra recorded for the copolymerization with molar concentration of DPA as 0.2 and 0.8, respectively. Two peaks around 422, and 584 nm and a broad band beyond 700 nm could be seen in Fig. 4a. On the other hand, two peaks around 464 and 587 nm and a broad band beyond 700 nm could be seen in Fig. 4b. There are variations in the ratio of absorbance values corresponding to these peaks upon changing the feed ratio of DPA. Hence, the copolymerization with these two molar feed ratios of DPA (0.2 and 0.8) results in different intermediates/polymers. Such

Fig. 4. UV-Vis spectra recorded during constant potential electropolymerization of DPA with MT for different feed ratios; potential = 0.8 V vs. Ag/AgCl; feed ratios of DPA: (a) 0.2; (b) 0.8; (c) 0.4; (d) 0.6; (e) 0.5.

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differences could also be noticed for the electro polymerization with molar feed composition of DPA as 0.4 (Fig. 4c), 0.6 (Fig. 4d) and 0.5 (Fig. 4e). Besides, the positions of the bands and peaks can be compared with the corresponding spectral behavior noticed with the electro polymerization of DPA. Three peaks around 430, 500 and >600 nm have been reported by earlier workers [52]. Based on the literature [51], the band around 430 nm is assigned for aniline type cation radical in the backbone. The other bands around 500 nm (DPACR) and >700 nm are assigned for the generation of diphenyl benzidine type oligomer cation radical (DPB•+ ) (Scheme 1) and N ,N diphenyl benzidine type dication radical (DPB•2+ ) of the oligomer (Scheme 1), respectively. The peak noticed around 580–590 nm in the case of copolymerization was virtually absent in the case of polymerization of DPA. Hence, for the copolymerization, the assignment of the bands are as follows: the peak observed around 430 nm is assigned for the cation radical generated

Fig. 4. (Continued ).

Structures of oligomer of DPA H

.N. Diphenylamine (DPA)

H

H

.N.

.N.

N,N’-diphenylbenzidine (DPB) - dimer H

H

N

N

N

N

H

H

Fully reduced PDPA - oligomer H

H

.

.N+

N +

N

N

H

H

Polaroinc (DPB+.type) structure of PDPA Scheme 1. Structures of oligomer of DPA.

P. Santhosh et al. / Materials Chemistry and Physics 85 (2004) 316–328

323

Mechanism of electropolymerization of DPA with MT H e-

N

..

[

N +.

(A)

H3C

]

N +

H

500 nm

H

H3C

H3C -e-

NH .. 2

H

H

.

NH .+ 2

NH2

+

+

..N

H

(B)

-2H

H

H

H3C

- e-

700 nm

CH3

(OR)

.NH + 2

(C)

580 nm CH3

H

H

-

-e

700 nm

(OR)

H

-2H +

.N+

H2.. N

N +

H2N+

A + D(C)

H3C

..N

NH + 2

N +

+

(D)

580 nm

H3C

H

.+

N

N

..

.N.

H

580 nm

H

H3C

..

N

NH2

..

H

H3C

..

NH

N

.. x Poly(DPA-co-MT) Scheme 2. Mechanism of electropolymerization of DPA with MT.

y

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0.12

775 mV 700 625 550

0.08

Absorbance

0.02

0.01

0.00

0.00 400.00

475

Absorbance

0.01

500.00

600.00 700.00 Wavelength

800.00

400

0.04 325 250 175

0.00 400.00

500.00

(a)

600.00

700.00

800.00

Wavelength (nm)

0.35

0.02

0.30

Absorbance

0.01

775 mV

0.01

0.00

700 625

0.00 400.00

500.00

Absorbance

550

0.25

600.00 700.00 Wavelength

800.00

475 400 325 250 175 mV

0.20

0.15

0.10 400.00 (b)

500.00

600.00

700.00

800.00

Wavelength (nm)

Fig. 5. UV-Vis spectra recorded during the electropolymerization of DPA with MT by sweeping the potential from 0.0 to 0.8 V (vs. Ag/AgCl) at a scan rate of 1 mV s−1 ; molar feed ratios of DPA as (a) 0.2 and (b) 0.8 (↑: anodic sweep direction, ↓: cathodic sweep direction).

P. Santhosh et al. / Materials Chemistry and Physics 85 (2004) 316–328

X1 =

ε12 − ε2 ε1 − ε 2

where ε12 , ε1 , and ε2 are the specific extinction coefficient of the copolymer (1 and 2), homopolymer 1 and 2, respectively. Clearly, the composition of DPA or MT in the copolymer varied with the molar feed composition of DPA or MT used for the polymerization (Fig. 6). The molar compositions of DPA or MT in the copolymer, determined for various feed composition of the monomer, were used to find the reactivity ratios of DPA and MT

0.9

F2

0.6

0.3

0 0

0.2

0.4

0.6

0.8

f2

Fig. 6. Plot of copolymer composition vs. feed composition of MT.

by Kelen–Tudos [54] method as 0.432 and 0.396 and Fineman–Ross [55] method as 0.452, 0.358 for DPA and MT, respectively (Fig. 7). From these results, it is concluded that a change in the feed composition of DPA or MT could alter the compositions of the DPA or MT units in the copolymer.

0.9

0.6 G

from the polaronic transition of aniline type moieties (MT units) in the copolymer. The new peak which was observed around 580–590 nm is assigned for the oligomer/copolymer formed as a result of cross reactions between cation radicals generated from DPACR and MTCR (Scheme 2). A support to the assignment of band around 430 and 580 nm was obtained from dynamic spectrovoltammetric studies. UV-Vis absorption spectra (Fig. 5a and b) were collected while sweeping the potential in the range from 0.0 to 0.8 V at scan rate of 1 mV s−1 on a solution having a mixture of DPA and MT. No spectral changes could be seen in the first anodic scan of potential up to 0.5 V. After that, two absorption bands could be noticed around 420 and 580–590 nm. These bands have also been noticed while performing bulk polymerization with molar feed composition of DPA as 0.2 and 0.8 (Fig. 5a and b) with an applied potential of 0.80 V. The changes in positions of the bands with changes in molar feed composition of DPA can be viewed as due to the variations in the composition of the monomer units (DPA or MT) in the copolymer. The observed variations in the ratio of absorbances at these bands also adds support that changing the composition of DPA gives a copolymer with differences in DPA or MT units in it. Hence, the composition of the two monomer units (DPA or MT) in the copolymer was determined by UV-Vis spectroscopy based on Ramelow and Baysal [53] and extended for conducting polymers in the recent report [46]. UV-Vis spectra were collected for a series of diluted solutions (1 × 10−2 to 6 × 10−2 g l−1 ) of PDPA. The λmax value was found to be 341 nm. Using the values of absorbance, plots were made for absorbance versus concentration of PDPA. These plots were found to be linear with negligible intercepts. Using absorbance, A = εCl, the average value of specific extinction coefficient, ε314 nm was calculated as 0.316 × 10−2 l g−1 . Similarly, for PMT solutions of varying concentrations (1 × 10−2 to 6 × 10−2 g l−1 ), UV spectra were recorded. At λmax (358 nm), the molar specific extinction coefficient, ε358 nm was calculated as 1.287×10−2 l g−1 . Using the spectra recorded for the copolymers synthesized with different molar feed concentrations of DPA or MT, the molar extinction coefficient of the copolymer, ε12 , was calculated (Table 1) and used to determine molar composition of DPA and MT units in the copolymer (Fig. 6). The mole fraction X1 of the monomer 1 in the copolymer has been determined by:

325

0.3

0 0

0.75

1.5

(a)

2.25

F

0.4

0.2 0

ξ

-0.2 -0.4 0

0.2

0.4

(b)

Fig. 7. (a) Fineman–Ross poly(DPA-co-MT).

and

η (b)

0.6

0.8

Kelen–Tudos

1

plots

for

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Copolymers prepared with different molar feed concentrations of DPA were characterized by FT-IR spectroscopy and thermogravimetric analysis and the results are in agreement with the variations in DPA or MT units in the copolymer.

a

b

FT-IR spectra of copolymers are presented in Fig. 8. The absorption band around 3400 and 1325 cm−1 correspond to N–H stretching mode of secondary amine. The absorption band around 1500 cm−1 is characteristic of C–C multiple bond stretching modes of benzene ring. [17]. The strong absorption band at 1600 cm−1 is assigned for the bending mode of aromatic secondary amine [56]. The presence of bands around 1175 and 1152 cm−1 are attributed to the presence of diphenoquinone type units and represents the DPA units in the copolymer. The band around 1236 and 1112 cm−1 indicates the presence of –C–NH–C– link as a consequence of linking of NH2 group of MT with phenyl carbon atom of DPA [57]. The band around 1316 cm−1 is assigned to stretching vibration of C–N groups with partially double bonds characteristics. The absorption peak around 812 cm−1 is assigned to the C–H stretching frequency of 1, 2, 4-trisubstituted aromatic ring. The variations in the ratio of intensity of peaks around 1175 cm−1 (due to the diphenoquinoneimine) and 807 cm−1 can be considered as due to the variations to DPA and MT units in the copolymer. The relative intensity of band

% Tansmittance

3.3. FT-IR spectroscopy c

d

3400.00

2400.00

1400.00

400.00

Wavenumber (cm -1)

Fig. 8. FT-IR spectra of poly(DPA-co-MT) prepared with different feed ratios of DPA: (a) 0.25; (b) 0.38; (c) 0.50; (d) 0.62.

around 1112 cm−1 (C–H in plane deformation vibration) to 1600 cm−1 can be considered as the measure of oxidative level in the copolymer. The ratio of intensities (I1112 /I1600 ) increases with increasing composition DPA units in the copolymer and thereafter decreases. This is consistent with

120

100

Weight loss (%)

80

60

b c de

f a g

40

20

0 100

200

300

400

500

Temperature (0C)

Fig. 9. Thermogravimetric analysis of (a) PDPA, (b) PMT, (c-g) poly (PPA-co-MT) prepared with different feed ratios of DPA: 0.25 (c), 0.38 (d), 0.50 (e), 0.62 (f) and 0.75 (g).

P. Santhosh et al. / Materials Chemistry and Physics 85 (2004) 316–328

the observed changes in copolymer composition (Table 1) and also the changes in electrochemical characteristics as noticed with cyclic voltammetric studies. 3.4. Thermal analysis Fig. 9 represents the thermogram of PDPA, PMT and the copolymer samples with DPA content as 0.25, 0.38, 0.50, 0.62 and 0.75. Homopolymers as well as the copolymers showed major weight changes at the temperature range 200–400 ◦ C. Removal of dopants and degradation of main chain units are mainly occurring at these temperature ranges [58]. The onset temperature of decomposition of main chain and removal of dopants was found to be shifted to higher temperatures for the copolymers with higher proportion of DPA units.

4. Conclusions Cyclic voltammograms representing the polymerization of mixture of DPA and MT, with different molar feed ratios of DPA, are found to show variations in redox peak positions and current values. The electronic transitions as noticed from UV-Vis spectra recorded during electropolymerization with different molar feed concentration ratios of DPA depend on molar compositions of DPA and MT units in the copolymer. The generation of intermediate species having both DPA and MT units, through the cross-reaction between DPACR and MTCR was evident from the appearance of new absorption band around 580–590 nm. The molar compositions of DPA and MT, determined by using UV-Vis spectroscopy were used to find the reactivity ratios of DPA and MT through Fineman–Ross and Kelen–Tudos methods. The dependence of ratio of intensities of bands corresponding C–H out-of-plane bending vibration to C–N stretch of secondary amine and the variations for the onset of decomposition temperature for the copolymers prepared with different molar feed ratios of DPA, confirm that the molar composition of DPA or MT in the copolymer is altered with the changes in the molar feed ratios of DPA.

Acknowledgements The authors gratefully acknowledge the financial assistance from Department of Science and Technology (DST), New Delhi, India (SP/S1-H12/97).

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