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Synthesis, Characterization, andConducting Properties of Poly(thioureaazomethines)L. Ravikumar a , I. Pradeep a , T. Thangaiyan a , R. Mohan a & J.Balachandran aa Department of Chemistry , C.B.M. College , Coimbatore , IndiaPublished online: 14 Feb 2012.

To cite this article: L. Ravikumar , I. Pradeep , T. Thangaiyan , R. Mohan & J. Balachandran(2012) Synthesis, Characterization, and Conducting Properties of Poly(thiourea azomethines),International Journal of Polymeric Materials and Polymeric Biomaterials, 61:4, 288-299, DOI:10.1080/00914037.2011.584224

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Synthesis,Characterization, andConducting Properties ofPoly(thiourea azomethines)

L. Ravikumar, I. Pradeep, T. Thangaiyan, R. Mohan, and

J. Balachandran

Department of Chemistry, C.B.M. College, Coimbatore, India

Three new polyazomethines having phenylthiourea groups were synthesized throughsolution polycondensation of terephthalaldehyde with 4,40-bis(thiourea)biphenyl ether,4,40-bis(thiourea)biphenylmethane, and 4,40-bis(thiourea)biphenyl sulphone. For com-parison purposes, simple polyazomethines were prepared by the polycondensation ofterephthalaldehyde with 4,40-diaminodiphenyl ether, 4,40-diaminodiphenylmethane,and 4,40-diaminodiphenyl sulphone. Poly(imine)s having phenylthiourea groups werecharacterized through IR and 1H-NMR spectroscopic methods and the thermal stabilityof the polymers were evaluated through TGA analysis. Conductivity of polyanilinesynthesized in aqueous p-toluenesulfonic acid was found to be 3.83Scm�1. The conduc-tivity of the polymeric blends with polyaniline dopped with p-toluenesulfonic acid andHCl (20% by weight) were found to be in the range 0.16� 10�3� 5.7� 10�3 Scm�1.

Keywords conductivity, p-toluenesulfonic acid, phenylthiourea, polyaniline, polya-zomethine, solubility, thermal stability

1. INTRODUCTION

Synthesis of plastic conductors having good electrical conductivity has been

the subject of many researchers worldwide. Polyazomethines known as poly

(imine)s or Schiff base polymers have great technological importance due to

Received 16 December 2010; accepted 3 April 2011.The authors wish to thank UGC India for financial support in the form of the PG GrantChemistry.Address correspondence to L. Ravikumar, Department of Chemistry, C.B.M.College (affiliated with Bharathiyar University), Coimbatore 641042, India. E-mail:ravikumarcbm@rediffmail.com

International Journal of Polymeric Materials, 61:288–299, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0091-4037 print=1563-535X online

DOI: 10.1080/00914037.2011.584224

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their practical applications. Interest for this type of materials is due to the fact

that aromatic polyazomethines are isoelectronic with poly(p-phenylene viny-

lene)s, which are the most electroluminescent polymers [1]. The interesting

properties of poly(imine)s are due to the presence of conjugated backbone

and imine sites [2]. Polyazomethines possesses good thermal stability [3],

mechanical strength [4], nonlinear optical properties [5], ability to form metal

chelates [6], and semiconducting properties [7]. Protonation or complexation of

azomethine linkage influences the opto-electronic properties of polyazo-

methines [8]. Azomethine links, when incorporated into phosphonate and

siloxane polymers, showed self-extinguishing and liquid crystalline properties

[9,10]. The presence of nitrogen in the main chain of the poly(imine)s makes it

possible to connect to other hetero atom-containing polymers such as polyani-

line [11]. Synthetic polymer-bearing azomethine groups were reported as

adsorbents for the removal of heavy metal ions from aqueous solutions

[12,13]. A major drawback of linear aromatic polyazomethines is their limited

solubility in organic solvents due to their rigid rod-like chain structure.

Several modifications of the chemical structure have been used in order to

lower the transition temperature and to improve the solubility of aromatic

polyazomethines. To achieve this, it was necessary to synthesize new mono-

mers, either diamines or dialdehydes, or both. Diamine monomers containing

bulky substituents such as 2,5-bis(4-aminophenyl)-3,4-diphenyl thiophene

[14], 3,4-bis(4-aminophenyl)-2,5-diphenyl thiophene or pyrrole [15] have been

used in combination with aromatic dialdehydes to obtain soluble polyazo-

methines. The introduction of dialkoxy substituents in terephthalaldehyde

was another option for improving polymer solubility [16].

Phenylthiourea and its derivatives have attracted interest due to their

excellent combination of properties, such as outstanding thermal behavior,

high corrosion resistance, good chemical resistance, and low inflammability

[17–19]. Polyesters having azomethine and phenylthiourea units were reported

to have good conductivity, corrosion resistance properties and better solubility

[20,21]. The presence of thiocarbonyl groups is known to increase the solubility

of Schiff base polymers [22].

Among the available intrinsically conducting polymers, PANI is found to

be the most promising because of its low cost, ease of synthesis, and tunable

properties. The doping process was found to be the most important factor in

the achievement of a high level of conductivity for PANI. Moreover, PANI

synthesized in different acidic solutions, such as 2-chloroethylphosphoric acid,

5-sulphosalicilic acid, and benzene sulphonic acid, also improves the processa-

bility and conductivity [23]. A better method for successful utilization of PANI

is through blending it with either commercially available polymers [24] or

with polyesters [25]. It can also be used as a conducting filler in suitable

conjugated polymers such as polyazomethines. There is a wider scope for

the practical application of such composites [26,27]. In doing so it is important

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to not only achieve high degree of conductivity but also to make them suitable

for processing.

The objective of the present work was to synthesize new polyazomerthines

having phenylthiourea groups and to study their conductivity and processabil-

ity by blending with minimum percentage of PANI synthesized in aqueous

HCl and aqueous p-toluenesulfonic acid. This study is of particular interest

due to the fact that the blend composition is solution-processable in common

organic solvents.

2. EXPERIMENTAL

2.1. Materials4,40-diaminodiphenyl ether (Himedia), 4,40-diaminodiphenylmethane

(Aldrich), and 4,40-diaminodiphenyl sulphone (Aldrich) were recrystallized

before use. Terephthalaldehyde (Fluka) was used as received. N,N-dimethyl-

formamide (DMF) was dried by the usual methods and distilled under reduced

pressure. Benzene was treated with 20% volume concentrated H2SO4, washed

with NaHCO3, distilled water, dried over anhydrous CaCl2, and filtered. It was

then refluxed with sodium shavings, distilled, and stored over sodium wire. All

other solvents and reagents were purified according to standard procedure.

2.2. Preparation of MonomersPreparation of 4,40-bis(thiourea)biphenylmethane

This monomer was prepared by modifying the procedure reported earlier

[20]. In a 250ml porcelain evaporating dish, 0.01M of 4,40-diaminodiphenyl-

methane, 10.5ml concentrated HCl, 0.04M of ammonium thiocyanate, and

75ml of water were taken and heated in a steam bath for 1 hour and allowed

to cool. The contents were evaporated slowly to dryness over a period of 4–6

hours. The light-yellow-color solid was boiled with ethanol along with activated

charcoal. The product 4,40-bis(thiourea) biphenylmethane was recrystallized

with ethanol and dried. IR (KBr): 3265 cm�1 (-NH2), 3163 cm�1 (-NH),

1065 cm�1 (C¼S). 1H-NMR (dH, d6-DMSO) Fig. 2. d 9.57 (s, 2H, NH), d 7.28 (d,

4H, phenyl), d 7.26 (d, 4H, phenyl), d 3.85 (s, 4H, NH2).

4,40-bis(thiourea)biphenyl ether, and 4,40-bis(thiourea)biphenyl sulphone

were prepared according to the above procedure using 4,40-diaminodipheny-

lether, and 4,40-diaminodiphenyl sulphone, respectively.

2.3. Polymer SynthesisAn equimolar mixture of 0.0126M of phenylthiourea diamine (1d–f) or

simple diamines (1a–c) and 0.0126M terephthalaldehyde dissolved in 20ml

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of DMF were added into a 250ml three-neck flask equipped with condenser, a

collector tube, and a magnetic stir bar. Then, 4ml of dry benzene (in order to

remove water as an azeotrope) and p-toluenesulphonic acid as catalyst were

added. The reaction mixture was stirred for 6 hours at reflux temperature

under nitrogen atmosphere. The product was precipitated in water, filtered

off, washed 3 times with absolute alcohol, and then dried.

Figure 2: 1H-NMR spectrum of monomer 4,40–bis(thiourea) biphenylmethane (1d).

Figure 1: 1H-NMR spectrum of monomer 4,40-diaminodiphenylmethane (1a).

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2.4. Preparation of Polymer/Polyaniline BlendsPolyaniline was synthesized by oxidation polymerization of distilled ani-

line with (NH4)2 S2O8 using aqueous HCl and p-toluenesulphonic acid (TSA).

PANI obtained was washed several times with distilled water and ethanol until

the washings are colorless. PANI thus obtained was filtered and dried in a

vacuum oven and stored in polyethylene containers.

Polymer=PANI-HCl or polymer=PANI-TSA in the required weight percent-

age were weighed and the powder was grounded in crucible for homogeneous

mixture. The powder was put in a hydraulic press for making the pellets. The

thickness of the pellets is in the range (0.124–0.229 cm).

2.5. CharacterizationInherent viscosities of the polymer were determined using an Ubbelohde

viscometer at a concentration of 0.5 gdl�1 in NMP at 30�C. Solubility of the

polymers was tested in different solvents with 25mg of polymer in 5ml of

solvent stirred for 24h. Infrared spectroscopy measurements were performed

on a Perkin-Elmer system 1760 Fourier transform (FT-IR) spectrometer. The1H-NMR spectra were recorded in DMSO-d6 solvent using a Bruker instru-

ment. Thermogravimetric analysis (TGA) was recorded on a Perkin-Elmer

analyzer in nitrogen atmosphere at a heating rate of 10�Cmin�1. Conductivity

measurements were carried out on a four-point probe connected to a Keithley

voltmeter constant current source system. Measurements are made by making

a gentle contact at different positions of the pellet.

Scheme 1: Polymer Synthesis.

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3. RESULTS AND DISCUSSION

3.1. Polymer SynthesisAs outlined in Scheme 1, polyazomerthines 3a–3f were prepared by

solution polycondensation method in DMF with good yield. The inherent

viscosities of the resulting polymers were between 0.32–0.61dL=g. The

solubility of polyazomerthines was studied qualitatively and the results are

presented in Table 1. The solubility of these polyazomethines was related to

the diamine moieties. Polymers derived from diamines containing thiourea

groups 3d–3f showed good solubility in polar aprotic solvents such as NMP,

DMAc, DMF, and DMSO (Table 2). The presence of thiocarbonyl groups in

the polyazomerthines, which decreases the possibility of hydrogen bonding, is

the reason for the better solubility of polymers 3d–3f. Polymers 3d, 3e, and 3f

form brittle films in DMAc.

3.2. Spectral Characterization1H-NMR spectrum of the simple diamine 1a (4,40-diaminodiphenyl-

iaminodiphenylmethane) and that of monomer 1d (4,40-bis(thiourea)biphenyl-

biphenylmethane) are shown in Figures 1 and 2, respectively. The appearance

of NH signal at d 9.57 of 1dwhen compared to the simple diamine 1a proves the

formation of 4,40-bis(thiourea)biphenylmethane. The IR spectra of terephtha-

laldehyde, polymer 3a, and 3e are shown in Figure 3. The disappearance of

C¼O aldehyde stretching frequency at 1682 cm�1 of terephthalaldehyde and

the appearance of characteristic azomethine -CH¼N- stretching frequency in

the polymer at 1619 cm�1 (3a) and 1615 cm�1 (3e) clearly indicates the forma-

tion of a -CH¼N- link during the condensation reaction. The thioamide

-NH-stretching frequency of polymer 3e appears at 1512 cm�1. There are vis-

ible and significant changes in the spectra of polymers when compared with

the spectra of monomers. The -CH¼N- stretching frequency of polymers 3b

and 3c appears at 1617 cm�1 and 1625 cm�1, respectively. Similarly for polymer

3e and 3f, the CH¼N stretching frequency appears at 1618 cm�1 and 1620 cm�1

and that of NH stretching frequency of thioamide appears at 1493 cm�1. Aweak

absorption in region 3350-3361 cm�1 is due to the bending frequency of

Table 1: Inherent viscosity and thermal stability of polymers

Polymer gInh g/dL Yield % T10%�C T20%�C

3a 0.41 88 450 5053b 0.39 92 442 4703c 0.32 90 250 4003d 0.58 92 308 4553e 0.59 88 308 4703f 0.61 87 258 438

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thioamide. The complete disappearance of the absorption bands of the aldehyde

group at 1682 cm�1, NH2 stretching frequency in the region 3268–3175 cm�1

(diamines), and the appearance of -CH¼N- stretching frequency at 1615 cm�1

(in the polymers) clearly indicates the formation of azomethine links in the

polymer.

Poly(imine)s 3a–3c are not soluble in DMSO-d6 and hence their 1H-NMR

spectra could not be recorded. 1H-NMR spectrum of polymer 3e is shown in

Figure 4. The aldehyde proton that usually appears in the region 9.6–10.5

[28,29] is completely absent in the polymer. Moreover, the -NH2 protons of

Figure 3: IR-spectra of 1:terephthalaldehyde; 2:polymer 3a; and 3:polymer 3e.

Table 2: Solubility of polymers

Polymer Pyridine NMP DMAc DMF DMSO Acetone Benzene

3a PS S PS PS IS IS IS3b PS S PS PS IS IS IS3c PS S PS PS PS IS IS3d S S S S S IS IS3e S S S S S PS IS3f S S S S S IS IS

PS: Partially soluble, S: Soluble, IS: Insoluble.

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the thioamide monomer that appeared at d¼ 3.85 (Fig. 2) is completely absent

in the 1H-NMR spectra of the polymer. In the 1H-NMR spectrum of the polymer

3e, d¼ 8.1–8.0 is attributed to the -N¼CH- protons. The aromatic protons

appear at d¼ 7.8–6.14. The broad signals, coupled with multiple splitting

and the integrated values, suggest complete polycondensation between the

-NH2 and -CHO groups of the monomers. The weak signal that appears at

9.9 is due to the -NH- protons.

3.3. Thermal StabilityTG curves of polymer 3a–c and 3d–f are shown in Figures 5 and 6, respect-

ively. The thermal stability data is presented in Table 1. With simple polyazo-

methines 3a and 3b 10% weight loss occurs at 450 and 442�C. In the case of

polyazomethines having thiourea groups 3d and 3e, 10% weight loss occurs at

Figure 5: TG-curves of polymers 3a–3c.

Figure 4: 1H-NMR spectrum of polymer 3e.

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308�C. This observation indicates that introduction of thiourea groups in the

polymer backbone considerably decreases the thermal stability but improves

the polymer solubility. This is anticipated due to the presence of C¼S groups

in the polymer backbone, which is highly prone to oxidation. However, in poly-

mers 3c and 3f, which consist of -SO2- links between phenyl rings, the 10%

weight loss temperature occurs at 250 and 258�C, respectively. This suggests

that among the linking groups between the phenyl rings -O-, -CH2- and -SO2-,

polymers having -SO2- links are least stable.

3.4. Conductivity StudiesFor blending purposes, three types of polyanilines were synthesized. PANI

synthesized in aqueous HCl (PANI=HCl), in aqueous p-toluenesulphonic acid

(PANI=TSA), and PANI treated with 1mol. NaOH solution (PANI=NaOH)

were blended with 10% and 20% weight ratio with polymers 3a–f. The conduc-

tivity measurements are shown in Table 3.

It is well known that PANI synthesized in various acidic media shows

different levels of conductivity. PANI synthesized in styrilphosphinic acid,

2-chloroethylphosphinic acid, and benzene sulphonic acid showed conductivity

in the range 1.6–2.0Scm�1 [19]. In the present case, PANI synthesized in

p-toluene sulphonic acid shows a higher conductivity of 3.8 Scm�1 when com-

pared to PANI synthesized in HCl (present work) and other acidic mediums.

Neither PANI=NaOH nor its blend with polymers showed any conductivity.

Polymers blended with 10% PANI=HCl or 10% PANI=TSA blends also show

no measurable conductance. However, polymers blended with 20% PANI=

HCl or 20% PANI=TSA show good conductance in the range 0.16�5.74�

Figure 6: TG-curves of polymers 3d–3f.

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10�3 Scm�1. It is clear from Table 3 that even though PANI-TSA has higher

conductivity than PANI-HCl, the blends comprising polymer 3a–f =PANI-HCl

show higher conductivity than polymer 3a–f=PANI-TSA. This is probably

due to the better protonating ability of PANI-HCl. Among the PANI-HCl

blends, polymers 3d–f show higher conductivity than 3a–c, indicating that

3d–f is a better host than 3a–c.

The IR spectra of PANI=TSA, the blends 3a=PANI-TSA and 3e=PANI-HCl

are shown in Figure 7. In the IR spectra of PANI=TSA, major stretching fre-

quencies are 1473 cm�1 (N-B-N, stretching), 1290 cm�1 (CN, bond stretching),

Table 3: Conductivity measurements of polymer blends

Polymer blendConductivityScm�1�10�3 Polymer blend

ConductivityScm�1�10�3

3a=20% PANI-HCl 1.1537 3a=20% PANI-TSA 0.17653b=20% PANI-HCl 1.6131 3b=20% PANI- TSA 0.25613c=20% PANI-HCl 1.7124 3c=20% PANI- TSA 0.16543d=20% PANI-HCl 3.8621 3d=20% PANI- TSA 0.59323e=20% PANI-HCl 5.7412 3e=20% PANI- TSA 0.61273f=20% PANI-HCl 5.0682 3f=20% PANI- TSA 0.5128

Figure 7: IR-spectra of 1:PANI=TSA; 2:polymer 3a=PANI=TSA; and 3:polymer 3e=PANI=HCl.

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and 1095 cm�1 (Quinoid ring stretching). When compared to the IR spectra of

PANI base [30], the lower shift of important stretching frequencies observed

with PANI=TSA suggests that it is the half oxidized form that shows higher con-

ductivity. The poly(imine)s having thiourea group (3e), when blended with 20%

by weight of PANI=TSA, show a maximum conductivity of 5.74� 10�3 Scm�1.

The IR spectra of polymer blends 3a=20% PANI=TSA and 3e=20% PANI=HCl

are shown in Figure 7. The weight percentages of PANI samples are only

20% and hence the IR spectra of the blends are compared with those of the poly-

mers 3a and 3e. The -CH¼N-stretching frequency of the polymers observed at

1619 cm�1 shifts to lower values at1590 and 1595 cm�1 in the blends 3a and 3e,

respectively. The thioamide -NH- stretching frequency also shifted to lower fre-

quency. With the inclusion of PANI into amide or ester polymers, the ester or

amide carbonyl band shifts to a lower frequency [31–33]. When this shift

towards lower frequency becomes larger in the blends, conductivity increases.

In the present case, the shift is about 24 cm�1, which explains the higher

conductivity of the blends.

CONCLUSION

New polyazomethines having a phenyl thiourea group were synthesized.

Inclusion of thiourea groups into the polyazomethine backbone decreases

the thermal stability but increases the solubility. PANI synthesized in

p-toluenesulphonic acid shows a conductivity value of 3.83Scm�1. The poly-

mers, when blended with 20% weight of PANI=HCl and PANI=TSA, show

conductivity in the range 0.16� 10�3 – 5.74� 10�3 Scm�1. Polyazomethine-

bearing thiourea groups are better hosts than simple polyazomethines when

blended with PANI-HCl.

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