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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3931–3936

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

rganic conductor: Influence of preparation temperature

im Mei Yeea, H.N.M. Ekramul Mahmudb,∗,nuar Kassima, Wan Mahmood Mat Yunusc

Department of Chemistry, Faculty of Science, University Putra Malaysia, 43400 Serdang, Selangor, MalaysiaFaculty of Chemical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, MalaysiaDepartment of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia

r t i c l e i n f o

rticle history:

eceived 18 December 2007

eceived in revised form 1 July 2008

ccepted 5 September 2008

eywords:

a b s t r a c t

The conducting polypyrrole–polyethylene glycol (PPy–PEG) composite films were produced

at various polymerization temperature ranging from 5 ◦C to 60 ◦C using 1 × 10−3 M PEG,

0.20 M pyrrole and 0.10 M p-toluene sulfonate at 1.20 V (vs. SCE). The polymerization tem-

perature of 5 ◦C appeared as the optimum preparation temperature showing the highest

electrical conductivity of 70 S/cm and the thermal diffusivity of 8.76 × 10−7 m2 s−1. The elec-

trical conductivity and thermal diffusivity exhibited a decreasing trend with the increase

in polymerization temperature in the pyrrole solution used to prepare the composite films.

olypyrrole

olyethylene glycol

hermal diffusivity

lectrical conductivity

The XRD results reveal that low temperature (5 ◦C) typically results in more crystalline films,

which are denser, stronger and have higher conductivity. The optical microscopy of PPy–PEG

shows the globular surface morphology. The surface of the of the solution side of PPy–PEG

film prepared at low temperatures showed a globular morphology.

of the films (Rodriguez et al., 1997). At high temperatures, side-

opant

. Introduction

onducting polymers are organic materials that can exhibitlectronic conductivity. Most of the works with conductingolymers have been focused on three main classes of poly-eric materials. There are polyacetylene and its derivatives,

olyphenylenes and its derivatives and polyheterocyclics suchs polypyrrole and polythiophene. These polymeric materialsan be obtained in various forms like powders and thin filmsBhattacharya and De, 1996). In recent years, conducting poly-

er have been studied extensively because of their potentialpplication in light emitting diodes (Friend et al., 1999), energytorage (Novak et al., 1997; Mermilliod and Tanguy, 1986; Osakat al., 1987), energy conversion (Skotheim et al., 1982), elec-

rochemical capacitors (Belanger et al., 2000), electrochromicevices (Gazotti et al., 1999), gas sensor (Talaie et al., 2000;arris et al., 1997), electromagnetic interference shielding

∗ Corresponding author. Tel.: +60 3 55436343.E-mail address: ekramul@salam.uitm.edu.my (H.N.M.E. Mahmud).

924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2008.09.011

© 2008 Elsevier B.V. All rights reserved.

(Mahmud et al., 2005) membranes (Blega et al., 1999) andcorrosion protection (Iroh and Su, 1999). Among the numer-ous polyheterocycles conducting polymer prepared to date,polypyrrole is by far the most extensively studied. The reasonsfor this intense focus on polypyrrole certainly lie in the factthat the monomer (pyrrole) is easily oxidized, water solubleand commercially available. Hence, polypyrrole presents sev-eral advantages including environment stability, good redoxproperties and the ability to give high electrical conductivities(Rodriguez et al., 1997).

Electropolymerization temperature has a substantial influ-ence on the kinetics of polymerization as well as on theconductivity, redox properties and mechanical characteristics

reactions such as solvent discharge and nucleophilic attackson polymeric radicals cause the formation of more structuraldefects, resulting in lower conducting films. The effects of

g t e

3932 j o u r n a l o f m a t e r i a l s p r o c e s s i n

preparation temperature of PPy–PEG composite polymer filmson the electrical conductivity, thermal diffusivity, molecularorder and morphology have been investigated in this presentcommunication.

2. Experimental

2.1. Sample preparation

The aqueous solution in the one-compartment cell contain-ing pyrrole monomer (supplied by Fluka), p-toluene sulfonateelectrolyte and the insulating polymer PEG, was electrochem-ically polymerized at a constant voltage (vs. SCE) at roomtemperature for 2 h to form PPy–PEG composite films. Thecomposite films thus produced on the ITO glass surface as aninsoluble film was rinsed thoroughly with distilled water andthen peeled off from the electrode. It was then dried in theoven at 60 ◦C for 24 h. The PPy–PEG films were prepared at 5 ◦C,15 ◦C, 25 ◦C, 40 ◦C and 60 ◦C in the pyrrole solution containing1 × 10−3 M PEG, 0.2 M pyrrole and 0.1 M p-toluene sulfonate.The temperature was set and controlled by water bath cir-culator with temperature controller (Protech Yamatake SDC10). The average thickness of the prepared films was about40 �m.

2.2. Characterization of the composite films

The thickness of the film was measured by using a digitalcaliper (Mitutoyo Digimatic Caliper, Japan). The electrical con-ductivity was measured at room temperature using a standard

Four Point Probe technique while the thermal diffusivity wasmeasured by the photoacoustic technique. X-ray diffractionmeasurements were performed using a Philip PW-1390 model.The optical microscopy (Invested Trinocular Microscope VBT-2T) was used to get the micrographs of the samples.

Fig. 1 – Experiment setup for

c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3931–3936

2.3. Measurement of thermal diffusivity

Photoacoustic technique was used to measure thermaldiffusivity of the prepared conducting composite films.Photoacoustic is the production of acoustic waves by theabsorption of light. The open photoacoustic cell techniqueconsists of five main components, namely light source(Helium Neon), detector (Microphone), optical chopper (Stan-ford Research Systems, SR 540) and low-noise preamplifier(Stanford Research Systems, SR 560) and lock-in amplifier(Stanford Research Systems, SR 530). In this experiment, thephotoacoustic signal produced in the sample was detectedby open photoacoustic cell. The detector photoacoustic sig-nal was later amplified by low-noise preamplifier, which willbe processed by the lock-in amplifier. The data acquisition wasrun on a personal computer (PC) by using SR 575 lock-in soft-ware and the communication was carried out using SR 232cable. The output voltage of signal displayed on the front panelof the lock-in also can be captured by personal computer usinga BNC to read and release this voltage upon receiving pulsefrom lock-in amplifier. The experimental setup is shown inFig. 1.

It has been shown that at low modulation frequency thephotoacoustic signal for optically opaque samples is given bythe expression (Naoto et al., 1994).

S = A

fexp(−b

√f ) (1)

where A is a constant and b is related to the thermal diffusivityof sample, and ˛ given as

√

b = ls

�

˛(2)

By fitting the experimental data to the expression (1), thethermal diffusivity of the sample can be easily calculated. The

OPC detection technique.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3931–3936 3933

Table 1 – Physical observation on the variation of preparation temperature of PPy–PEG composite films.

Preparation temperature toproduce the films (◦C)

Physical appearance of the films Average thicknessof the films (�m)

5 Strong, flexible, smooth shiny and deep greenish-black 3715 Strong, flexible, smooth shiny and deep greenish-black 4925 Strong, flexible, smooth shiny and deep greenish-black 3940 Strong, flexible, rough s60 Strong, flexible, rough s

Fa

ai

3

Td6satpti

3e

Tcspeic6ct

fusivity may be due to the fact that at lower temperatures,the polymers have higher doping level and longer conjugationlengths.

ig. 2 – Electrical conductivity of PPy–PEG composite filmsgainst various polymerization temperatures.

verage error in the values of ˛ obtained from these data wasn the order of 1%.

. Results and discussion

he PPy–PEG composite films were electrochemically pro-uced at various temperatures, i.e., 5 ◦C, 15 ◦C, 25 ◦C, 40 ◦C and0 ◦C using 1 × 10−3 M PEG, 0.20 M pyrrole and 0.10 M p-tolueneulfonate at 1.20 V (vs. SCE). A strong, flexible, smooth shinynd deep greenish-black composite film was obtained at theemperature of 5 ◦C, 15 ◦C and 25 ◦C. With the increase in filmreparation temperature (40 ◦C and 60 ◦C), the composite filmsurned rough. The physical properties of the films are shownn Table 1.

.1. Effect of polymer processing temperature on thelectrical conductivity of PPy–PEG composite films

he electrical conductivity of PPy–PEG composite filmshanges with the increase in polymerization temperature ashown in Fig. 2. It shows that the PPy–PEG composite films pre-ared at a low polymerization temperature (5 ◦C) exhibits anlectrical conductivity of 70 S/cm. Later, with further increasen polymerization temperature at 15 ◦C and 25 ◦C, the electri-

al conductivity of the prepared composite films decreased to7 S/cm and 61 S/cm, respectively. This decrease in electricalonductivity may be due to the fact that at lower tempera-ures, the polymer composite films have higher doping level,

urface and deep greenish-black 154urface and deep greenish-black 195

fewer structural defects and longer conjugation lengths. Thissuggests that the materials synthesized at lower temperaturewill be more electrically conductive. This same inverse rela-tionship between synthesis temperature and conductivity isalso observed in template-synthesized polypyrrole fibers (Caiet al., 1991). Later, with further increase in polymer prepara-tion temperature at 40 ◦C and 60 ◦C, the electrical conductivityof the films decreased to 8 S/cm and 1 S/cm, respectively.

3.2. Effect of polymer preparation temperature on thethermal diffusivity of PPy–PEG composite films

The thermal diffusivity of the PPy–PEG composite films showsa decreasing trend with the increase in polymer preparationtemperature as shown in Fig. 3. It shows that the thermal diffu-sivity of the composite films decreases from 8.76 × 10−7 m2 s−1

to 7.88 × 10−7 m2 s−1 with the preparation temperature of 5 ◦Cand 25 ◦C, respectively. Later, with further increase in temper-ature at 40 ◦C and 60 ◦C, the thermal diffusivity decreased to4.68 × 10−7 m2 s−1 and 4.09 × 10−7 m2 s−1, respectively (Fig. 3).Thus, the thermal diffusivity has been found to be inverselyproportional to the preparation temperature of the PPy–PEGcomposite films. The similar effect of increase in prepara-tion temperature resulted in decreased thermal diffusivity ofthe polypyrrole film doped by p-toluene sulfonate has beenreported by Naoto et al. (1994). This decrease in thermal dif-

Fig. 3 – Thermal diffusivity of PPy–PEG composite filmsagainst various polymerization temperatures.

g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3931–3936

3934 j o u r n a l o f m a t e r i a l s p r o c e s s i n

3.3. Effect of polymer preparation temperature on themolecular order of PPy–PEG composite films

Fig. 4 shows the effect of preparation temperature on themolecular order of the PPy–PEG composite films studied byXRD. The PPy–PEG composite films prepared at 15 ◦C, 25 ◦C,40 ◦C and 60 ◦C showed a relatively lower intensity than thoseof the intensities shown by the films prepared at 5 ◦C (Fig. 4).Electrodepositing at low temperatures and low current den-sities typically results in more crystalline films, which aredenser, stronger and have higher conductivity (Warren, 2001).The relatively higher molecular order was found in PPy–PEGcomposite films prepared at 5 ◦C than in the films prepared at15 ◦C, 25 ◦C, 40 ◦C and 60 ◦C. This order can again be linkedwith the conductivity of these films where it is found thatthe films prepared at 5 ◦C showed the highest conductivity(70 S/cm) while the film produced at temperature more than5 ◦C (15 ◦C, 25 ◦C, 40 ◦C and 60 ◦C) showed a lower conductiv-

ity. Structural study of conducting polymers (Pouget et al.,1981) points out that there are crystalline regions within whichthe chains are regularly and densely packed which in turnfavors the electrons to hop from one chain to another. Out-

Fig. 5 – The optical micrographs of the solution side of PPy–PEG40 ◦C and (e) 60 ◦C (magnification: 20×).

Fig. 4 – XRD diffractograms of PPy–PEG composite filmsprepared at 5 ◦C, 15 ◦C, 25 ◦C, 40 ◦C and 60 ◦C.

side of these regions the order in the chain arrangement ispoor resulting an amorphous state. Prigodin and Epstein (2002)stated that in polypyrrole conducting polymer, single chainsare arranged in the space in a very complicated way. Usually,

composite films prepared at (a) 5 ◦C, (b) 15 ◦C, (c) 25 ◦C, (d)

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3931–3936 3935

Fig. 6 – The optical micrographs of the electrode side of PPy–PEG composite films prepared at (a) 5 ◦C, (b) 15 ◦C, (c) 25 ◦C, (d)40 ◦C and (e) 60 ◦C (magnification: 20×).

ti

3m

Thsssps

he higher the degree of order, the higher is the conductiv-ty.

.4. Effect of polymerization temperature on theorphology of PPy–PEG composite films

he surface of the solution side of PPy–PEG composite filmsas been shown in Fig. 5(a)–(e). The surface of the solutionide of PPy–PEG film prepared at 5 ◦C (Fig. 5a), looks partially

imilar to that of the film prepared at 15 ◦C (Fig. 5b). Theurface of the films shows a globular morphology. The filmroduced at 25 ◦C (Fig. 5c) shows globular morphology withome spherical ball. With the increase in temperature from

40 ◦C to 60 ◦C, the morphology appears rough. The morpho-logical study of the electrode side of PPy–PEG composite filmssynthesized at 5 ◦C, 15 ◦C, 25 ◦C, 40 ◦C and 60 ◦C in the presenceof 1 × 10−3 M PEG, 0.2 M pyrrole and 0.1 M p-toluene sulfonateat 1.2 V (vs. SCE) has been visualized in Fig. 6(a)–(e). The sur-face morphology of the electrode side shows that the surfacewas very smooth with a few cracks for the film produced atlower temperatures (5 ◦C and 15 ◦C), while the surface of thefilm produced at higher temperature (25 ◦C and 60 ◦C) has got

cracked surface with less smooth part. Thus, the surface mor-phology of the electrode sides of the films is also influencedby different preparation temperature of PPy–PEG compositefilms.

g t e

r

Talaie, A., Lee, J.Y., Lee, Y.K., Jang, J., Romagnoli, J.A., Taguchi, T.,

3936 j o u r n a l o f m a t e r i a l s p r o c e s s i n

4. Conclusions

The electrochemical preparation of polypyrrole–polyethyleneglycol (PPy–PEG) composite films by potentiostatic methodin aqueous medium at various polymerization temperatureshas been done successfully. The preparation temperature ofPPy–PEG composite films played a vital role on the electricalconductivity, thermal diffusivity, molecular order and mor-phology. The lower preparation temperature was found tooffer higher conductivity and thermal diffusivity. The rela-tively higher molecular order was found in PPy–PEG compositefilm prepared at a low temperature. The globular surface mor-phology of the PPy–PEG composite films was observed whichwas found to be influenced by the preparation temperature.

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