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Synthetic Metals 181 (2013) 27– 36

Contents lists available at ScienceDirect

Synthetic Metals

journa l h om epage: www.elsev ier .com/ locate /synmet

reparation and characterization of chemically synthesizedolyaniline–polystyrene blends as a carbon dioxide gas sensor

. Bhadra, N.J. Al-Thani, N.K. Madi ∗, Mariam A. Al-Maadeedenter of Advanced Materials, Qatar University, Qatar

r t i c l e i n f o

rticle history:eceived 19 April 2013eceived in revised form 15 June 2013ccepted 23 July 2013vailable online 6 September 2013

eywords:onducting polymerhemical blending

n situ polymerizationensoranosphere

a b s t r a c t

This correspondence presents a radically distinctive approach for preparing conducting polymerpolyaniline–polystyrene (PANI–PS) blends doped with HCl by an in situ dispersion polymerizationmethod. Generally, there are two prerequisites for choosing the blended polymers: good compatibilitywith PS molecules and comparatively high electrical conductivity. Therefore, we use different monomerratios of PS keeping the aniline concentration constant while preparing the five blends. The blend films,cast on glass substrates, are characterized by various studies such as SEM (for morphology), XRD (forcrystallinity), FTIR chemical interaction, DSC, and TGA thermal properties. Subsequently, the electricalproperties are studied using I–V characteristics and four probe conductivity measurements. The SEMresults of the above polymerized blends reflect the formation of spherical nanoparticles with diame-ter ranging from 50 nm to 90 nm with a decrease in PS concentration that facilitates the formation ofclusters. Furthermore, the XRD diffractograms show semi crystalline behavior of the blend films wherecrystallinity decreases with decreasing PS concentration. The FTIR spectra establish a new peak in theblend films other than the peaks found in PANI and PS films indicating possible chemical crosslinkingbetween the polymers. It can be observed that the increase in PS concentration increases the intensityof the peak. The TGA of the polymer blends reveals five-step degradation including four due to the purepolymer and one due to the blend formation. The decomposition temperature is found to be more forblend film than that of pure PANI but less than PS decomposition. The DSC results display a peak, each

for water and oxidation of the polymer blend, but do not contain any such for PANI. From DSC data, itis clear that the blend film exhibits the combined physical properties of both the polymer components.The non-ohmic behavior of I–V characteristics by the blend films indicates that these nanocompositefilms may have some utility in electronic device applications; an increase in conductivity is observedwith decreasing PS concentration. Finally, the blend films with high conductivity seem to have goodsensitivity and reversibility when used as CO2 sensors.

. Introduction

Conductive polymers are promising candidates to be used inlectronic devices, viz., vapour sensors, electronic and photonicransistors, solar cells, and nanoscale lasers. They have variousdvantages over their conventional counterparts; some of themre as follows: reasonable price, facile large-scale synthesis, solu-ion processability, and tuneable properties. The development ofanoscale conductive polymers is a step toward device miniatur-

zation and continues to aid in the refinement of charge transportheories of quasi 2 D systems. Conducting polymers contain �-lectron backbones that are responsible for their unusual electronic

∗ Corresponding author. Tel.: +974 4033 5666; fax: +974 4033 3989.E-mail addresses: jollybhadra@qu.edu.qa (J. Bhadra), n.al-thani@qu.edu.qa

N.J. Al-Thani), nmadi@qu.edu.qa (N.K. Madi), m.alali@qu.edu.qa (M.A. Al-Maadeed).

379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.synthmet.2013.07.026

© 2013 Elsevier B.V. All rights reserved.

properties, such as electrical conductivity, low energy optical tran-sitions, low ionization potential, and high electron affinity. Also,they are useful to enhance speed, sensitivity, and versatility ofsensors. Properties of conducting polymers depend strongly onthe doping level, ion size of the dopant, protonation level, andwater content. Conducting polymers find ever-increasing use indiagnostic medical regents [1,2] with distinguishable chemicalmemory, and they are being made into prominent new materi-als for the fabrication of industrial sensors. A number of reviews[3–14] on the use of conducting polymers in fabrication of sen-sors have been reported in literature. Polyaniline (PANI) is foundto be the most widely studied one among the conducting poly-mers; this is preferred in electronic and electrochemical device

applications due to its various excellent features, such as strongstability under ambient conditions, reversible doping/de-doping,change in conductivity and/or optical properties upon poten-tial switching, and simple preparation methods. One of the key

28 J. Bhadra et al. / Synthetic Me

pfi�wAvpwhdaptpmwflsmtbc

2

pps

2

oti

2

p0m0nPtotro

2

s4a

Fig. 1. Formation of conducting polymer nanoparticles.

roblems related to the potential application of PANI is its dif-cult processability either from melt or solution. With a rigid-conjugated backbone molecular structure, PANI decomposesithout melting when heated and is insoluble in common solvents.

number of methods are possible to improve these propertiesiz. doping by functionalized protonic acids [13], blending witholy [epichlorohydrin-co-(ethylene oxide)] [14], high moleculareight polyethylene [15], and poly(methyl methacrylate) [16]. PSas some salient characteristics, such as good thermal stability, aegradation temperature above 400 ◦C, good chemical resistance,nd proficient mechanical toughness. The high processability ofolymers allows simple fabrication for a wide range of geome-ries including smaller sensor designs; this characteristics makesolymer-based thin-film gas sensors more advantageous. Further-ore, the cost becomes lesser due to the ease of its processinghilst the small size and good mechanical properties give a robustexible sensor that could be used in applications where dynamictresses and strains are encountered. Therefore, PANI can be madeore processable by blending it in a high-polymer matrix of PS. In

his article, we present experimental observations of the sensingehavior of films obtained from the dispersion of PANI in differentoncentrations of PS for CO2 gas.

. Experimental

This section describes the materials used, preparation of sam-les, cluster determining techniques to determine the clustersresent in the images that are obtained from SEM, and gas sensingetup in detail.

.1. Materials

All the chemicals and reagents used in this experiment arebtained from Merck (Germany) and Sigma chemical company;he purity level is quite high (99.9%). On the other hand, anilines purified by repeated distillation under vacuum prior to use.

.2. Sample preparation

We use the conventional technique [17] to achieve the dis-ersion polymerization in which the aqueous solution containing.1 M of HCl and 1 M of aniline is added to PS in toluene; differentolarities of PS are taken in this procedure, such as 0.6 M, 0.7 M,

.8 M, 0.9 M and 1 M. Keeping the bath temperature at −5◦C, ammo-ium persulphate solution is added drop wise to the solution ofS and aniline in aqueous HCL. Within a few hours, the mixtureurns into a bluish-green homogeneous solution without any tracef sediment. Then the films are spin-cast on various substrates fromhis solution, dried, washed with methanol, and deionised water toemove the oligomer or unreacted oxidant if present. The formationf PANI–PS nanoparticles is precisely shown in Fig. 1.

.3. Characterization techniques

Scanning electron microscopy (SEM) is employed to study theurface morphology and grain size of PANI using a NanoSEM Nova50. The in-plane I–V characteristics and four probe conductivityre some of the commonly used standard methods to study the

tals 181 (2013) 27– 36

electrical properties [18]. We use Keithley 2400 source meter tomeasure the I–V measurements and conductivity of the blends;whereas an X-ray diffractometer (mode 1, EDXRF 300, Link) isused to perform the study of particle size and crystallinity. Thespectral and crosslinking analysis, being important in polymerblend formation, are studied using a Fourier-transform infrared(FTIR) spectrometer (8101 M, Shimadzu). A Perkin-Elmer Pyris TGAexamines the thermal properties of the PANI by performing athermogravimetric analysis. Also, differential scanning calorime-try (DSC) by a Perkin-Elmer Precisely Jade DSC supplements morefor this cause. Finally, Aeroqual digital multi gas detector, series500 connected with CO2 sensor head (range 0–5000 ppm), NH3sensor head (0–1000 ppm), and ethanol sensor (PASPORT EthanolSensor (PS-2194) (0–3%)), monitors the temperature and humidityconsistency with the change in gas concentration during sensingexperiment.

2.4. Cluster determination techniques

The method used for determining the cluster distributionis called thresholding [19]. Essentially, thresholding is used toextract an object (polymer in this case) from its background (SEMimage) by assigning an intensity value t (threshold) for eachpixel such that each pixel is either classified as an object pointor a background point. In general, threshold is defined as: t =t ((x, y) , p (x, y) , f (x, y)) where (x, y), f(x, y) and p(x, y) indicate pixelcoordinate, local property in a neighborhood centered at a pixellocation and pixel intensity, respectively. If t is a function of pixelintensity (x, y) only, it is called as Global threshold. It is an automatedprocedure for bimodal histograms. Global thresholding using Ostu’soptimal threshold technique is used to calculate the componentconnectivity in this work. The algorithm basically tries to find a cri-terion function that helps determining a threshold to reduce intravariance in foreground and background of the image. The methodworks in the following manner:

(1) Find the class probabilities:q1 (t) = ∑t

i=1p (i) , q2 (t) = ∑Ni=t+1p (i), where p(i) is the

probability distribution, [0, N − 1] is the range of intensity lev-els.

(2) Find the class means:

�1 (t) =t∑

i=1

ip (i)q1 (t)

, �2 (t) =N∑

i=t+1

ip (i)q2 (t)

(3) The individual class variances are calculated:

�12 (t) =

t∑i=1

[i − �1 (t)]2 p (i)q1 (t)

, �22 (t)

=N∑

i=t+1

[i − �2 (t)]2 p (i)q2 (t)

And find the weighted within class variance: �w2 (t) =

q1 (t) �12 (t) + q2 (t) �2

2 (t).

Finally, the threshold t is obtained that minimizes �2w (t).

2.5. Gas-sensing analysis

We canvass the gas sensitivity of PANI–PS nanocomposite sen-sors under the continuum flow of gas at room temperature andrelative humidity of 47%. A schematic diagram for gas detection unit(shown in Fig. 2) consists of two chambers of equal volume where a

J. Bhadra et al. / Synthetic Metals 181 (2013) 27– 36 29

or gas

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3

Xm

3

fitp

baofp

tsottc

phous (pure PANI and PS) to semi-crystalline (PANI–PS blend films

Fig. 2. Apparatus used f

ixing chamber gets connected in series with a working chamberhat contains gas sensor to monitor the change in concentrationuring the process of sensing. Keeping the operating temperaturexed, the temperature and pressure in both chambers are con-rolled and monitored during the process. The following procedures adopted for gas sensing measurement: from the very beginning,oth chambers are evacuated repeatedly until the sample con-uctivity gets reduced to a nearly constant value; presumably theoisture content was reduced to a minimum value. Then we record

he value of resistivity (as a function of time on a Keithley sourceeter model number 2400) for the working chamber. After recor-

ing the steady state resistivity value, the analyt gas is allowed toass from the mixing chamber to the working chamber for 5 min.nce the sample film gets exposed to a predefined concentration ofas for 5 min, it is exposed to air in order to recover initial resistanceor the same duration. This process is repeated using different con-entrations of gas. A four-probe configuration determines the DClectric resistance of the samples exposed to analyt gas. A simplerrangement using copper wires with a silver paste having 2 mmistance between the probes is responsible to provide the electricalontacts. The change in voltage of the samples against constant cur-ent (0.1 �A) is measured by passing gas through the glass chambert a fixed flow-rate.

. Results and discussion

This section accommodates various results on SEM, FTIR spectra,RD, TGA, DSC, electrical properties, and gas sensitivity measure-ent of the PANI–PS blends.

.1. SEM picture study

The results on SEM that describe the morphology of PANI–PSlms are shown in Fig. 3(a)–(e). It is evident from the results thathe grain sizes of PANI particles are nanosized with different mor-hology when a change in the PS concentration occurs.

For 1 M of PS (as shown in Fig. 3(a)), the particles appear toe uniform spherical particles with diameters of 50–80 nm. With

decrease in PS concentration, these particles connect with eachther to form clusters or chains. This chain formation is the lowestor 1 M PS, while for 0.6 M PS (shown in Fig. 3(e)), almost all the PANIarticles combine together to form chains of spherical particles.

Fig. 3(f) shows the MATLAB generated histogram for the clus-er distribution of the particles in the SEM pictures. The histogramhows that with the decrease in PS concentration, the number

f clusters decreases, i.e. more and more particles get connectedogether forming big clusters and reducing the number of clus-ers. The number of clusters seems to be the least for 0.6 M of PSoncentration.

sensing measurement.

3.2. FTIR spectra

Fig. 4 shows the FTIR spectra of PANI–PS films for different con-centrations 0.6 M/M, 0.7 M/M M/M, 0.8 M/M, 0.9 M/M and 1 M/M.In order to check the extent of chemical crosslinking between purePANI and PS, we compare their corresponding FTIR spectra with theblends spectra. Some additional peaks can be observed in the FTIRspectra of blend films with the peaks observed in pure films of PANIand PS. This indicates that there is a chemical interaction betweenPANI and PS molecules forming new bonds which are obvious sincewe have used chemical methods to prepare the polymer blends. Thepeaks observed, only in blend films, are characterized as follows: (a)the irregular groups of lines noted at 3600 cm−1 are due to water,the broad peak at 3420.00 cm−1 corresponds to NH2 stretchingin pure PANI films, on the other hand, the same peak for the blendfilms has been red-shifted to 3120.00 cm−1, (b) the peak detected at2912.00 cm−1 is due to C H stretching in pure PANI films, but at thesame time, there has been blue-shift in blend films to 3010.00 cm−1

[19–22].All the spectra including pure PANI and PS film show the

presence of aromatic skeletal vibration peak which occurs at1580.00 cm−1, however intensity-wise, the level of the peaks inblend films is lower when compared [19]. Because of various fac-tors, such as ring stretching, contraction of all bonds in the ring andinteraction between the stretching modes [23,24], one may observea new peak at 1400.00 cm−1 during blend formation. Furthermore,a small peak due to C N stretching is marked at 1300.00 cm−1

whose intensity decreases with PS concentration. A C Cl stretch-ing peak arises in the region 590.00–700.00 cm−1 because of theuse of Cl as a dopant [22–25]. In addition to these peaks, the peakat 1690 cm−1 assigned to C O stretching may be related to theelectrical conductivity of the polymer sample which depends onthe PANI unfolded structure [22]. This peak is only observed inthe PANI–PS blends with lower concentrations of PS (0.6 M and0.7 M PS) whose conductivity values are the highest. From the FTIRstudy, it can be observed that the increase in PS concentrationdoes not affect the type of bond formation in the blends, but anincrease affects the intensity of the peak; a higher concentrationcorresponds to a higher intensity. The IR spectrum of PS shows char-acteristic peaks at 1559.00 cm−1, 1479.00 cm−1, 1109.00 cm−1, and787.00 cm−1.

3.3. XRD study

XRD patterns provide information on the nature and structuralchanges of the samples. Fig. 5 reflects such changes from amor-

containing 0.6 M/M, 0 7 M/M, 0.8 M/M, 0.9 M/M and 1 M/M) state.The systematic alignment of polymer chain folding or the forma-tion of single/multiple helices (i.e. part of the polymer) could bethe reason behind this semicrystalline nature of PANI–PS blends

30 J. Bhadra et al. / Synthetic Metals 181 (2013) 27– 36

Fig. 3. SEM pictures for PANI-PS blends for different concentrations of PS (a) 1 M, (b) 0.9 M, (c) 0.8 M, (d) 0.7 M, (e) 0.6 M and (f) Histogram for cluster distribution of particles.

[pmsat

26]. Moreover, the conductivity of polymers depends on variousarameters, such as doping level (carrier’s concentration) and for-

ation of polarons and bi-polarons [27] which are obvious in any

emi-crystalline system. The XRD pattern of the PANI sample showsn amorphous hump at ≈20 ◦C, however, the amorphous peak inhe blend film seems to have broken into several small sharp peaks

indicating semi-crystallinity. These peak heights are observed todecrease with decreasing PS concentration, meaning crystallinity

decreases with decreasing PS concentration. On the other hand,PANI reflection peak at 20 ◦C gets overlapped with that of PS at19 ◦C; the widening of this peak indicates that PS and PANI form atwo phase structure in the composites.

J. Bhadra et al. / Synthetic Metals 181 (2013) 27– 36 31

Fig. 4. FTIR spectra for pure PANI, pure PS and their blend films.

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tslblswwattPP4

.4. TGA study

TGA of PANI, PS, and all blends are performed within a temper-ture varying from 30–700 ◦C with a heating rate of 10◦C/min; ahree-step weight loss may be observed for PANI as shown in Fig. 6.he weight loss in the first step starts at approximately 70 ◦C andontinues up to 135 ◦C due to the presence of moisture and toluene,hich are used as solvents in the preparation of PANI, free HCl, andnreacted monomers. The weight loss is approximately 13% of theotal weight of PANI–PS blends. The second step may be attributedo the loss of dopants from deeper sites in the material. The majoreight loss of chemically synthesized PANI–HCl, at approximately

50–220 ◦C in the TGA curves, is imputed to HCl loss. In the thirdtep, moderate weight loss profile, observed for the PANI salt at20–380 ◦C, is due to polymer backbone degradation [28–30]. TheGA studies of PS reveal that PS backbone is stable up to 275 ◦C withhe polymer degradation commencing near 340 ◦C and completingt ≈470 ◦C.

All PANI–PS blends show a five-step weight loss, which includehe ranges similar to those of pure PANI salt and pure PS. The firsttep of weight loss occurs between 35 and 135 ◦C and is due to theoss of moisture, solvent, and free HCl. The second step PANI–PSlends of weight loss occurs at 170–250 ◦C and is attributed to be

oss of dopant used during polymer preparation [29–31]. The thirdtep of weight loss is due to the degradation of the polymer PANI,hich starts at ≈260 ◦C and ends at ≈340 ◦C. The fourth step ofeight loss is observed only in the blend films starting at ≈340 ◦C

nd ending at ≈390 ◦C [31,32]. This weight loss is attributed tohe PANI and PS polymer blend formation with the decomposi-

ion temperature more than that of PANI but lower than that ofS decomposition. The last and final decomposition step is for theS polymer, whose decomposition starts at ≈390 ◦C and ends at70 ◦C.

3.5. DSC spectra

The DSC thermogram of PANI–PS blends is shown in Fig. 7, whichshows an endothermic peak below 100 ◦C due to the elimination ofwater. Another peak above 200 ◦C could be due to the oxidationor crosslinking between the polymers PANI and PS. A continuumtrend from 300 ◦C onwards indicates the possible degradation of apolymer backbone [33,34]. No melting peak is observed for PANIalthough it can exhibit some crystallinity, because, as previouslystated in the literature, PANI’s degradation temperature is lowerthan its melting temperature [35]. From DSC data it is clear that theblend film exhibits the combined physical properties of both thepolymer components.

3.6. Electrical property measurement

Fig. 8 shows the conductivity as a function of the volume frac-tion of PANI and the conducting component available in the blend.The abrupt increase in conductivity at a certain content of PANIis defined as the percolation threshold, which implies that certainconductive routes are formed in the insulating matrix; the percola-tion threshold is 0.7 M PS (37% PANI) in this case. With the electricalconductivity of the composite films ranging from 4.3 × 10−5 S m−1

to 5.2 × 10−2 S m−1, there seems to be an increase in conductivitywith a decrease in PS content.

The significant increase of conducting species, i.e., PANI in thecomposite with a lower PS content, is the main reason behindthis effect. This has been observed in some other PANI [36] com-posites as well. However, our observation on FTIR, SEM and XRD

data depicts the increasing order and increasing proximity to thenanophase with the decreasing PS content. We show the conduc-tivity values for different PANI–PS films in Table 1 along withthe n values. We have also measured the current voltage (I–V)

32 J. Bhadra et al. / Synthetic Metals 181 (2013) 27– 36

Fig. 5. The XRD showing the PANI-PS blends for different concentration of PS.

Table 1Table showing conductivity and n-values of different PANI–PS blend films.

PANI:PS Conductivity (S m) n-value

1:1 4.3 × 10−5 0.861:0.9 6.2 × 10−5 1.041:0.8 2.3 × 10−4 1.071:0.7 1.38 × 10−3 1.091:0.6 5.2 × 10−2 1.10

Fig. 6. TGA thermograme for PANI-PS blends for different concentration of PS.

characteristics of these films on glass substrates. These essentiallyfollow a non-ohmic behavior showing current voltage to havepower law dependence i.e., I˛Vn. The value of n is estimated tobe 0.86–1.10 as PS content is changed from 1 M/M to 0.6 M/M;the n values are also shown in Table 1. The non-ohmic behaviorof I–V characteristics indicates that these nanocomposite films canbe utilized effectively in electronic device applications.

3.7. CO2 sensitivity measurement

In the present study, the sensing capability of PANI–PSnanocomposite blend sensors toward gas has been explored. Thesensitivity (%) of the sensor is defined as

S =[

(Rg − Ro)Ro

]× 100

Whereas, the gas sensor response is defined as

�R/Ro = [(Rg − Ro)/Ro],

Rg and Ro denote the resistance of the sensor measured in the pres-ence of gas and air, respectively. The total response time is definedas the total time required by the sensor to change the resistance

Fig. 7. DSC thermograme for PANI-PS blends for different concentration of PS.

J. Bhadra et al. / Synthetic Metals 181 (2013) 27– 36 33

ft

(Nsc3wcimlt

tdt

Fig. 8. Conductivity vs. PANI concentration.

rom base value to saturation (upon exposure to analyt gas) andhen back to the starting base value (when exposed to air).

An attempt is made to study the selectivity of PANI–PS films0.8 M, 0.7 M and 0.6 M of PS) for a constant concentration of CO2,H3, and Ethanol gas at room temperature. From the bar chart, for

electivity shown in Fig. 9, it is observed that for lower PS con-entrations 0.6 and 0.7 M, the response time lies within the range00–500 S and 500–650 S, respectively in case of all three gases;hereas for the sensor with 0.8 M of PS in particular, it is signifi-

antly high. It may be noted that the room temperature sensitivitys one of the advantages of the conducting polymer sensors over

etal oxide sensors [37]. Furthermore, the response time is theeast for CO2 in each case indicating the applicability of the sensoroward CO2 gas.

The gas sensor responses of all blend films above the percola-ion threshold are measured as shown in Fig. 10. The conductivityecreases upon exposing the sensor to gas, and recovers closelyo the original value on switching back to air for all sensors. The

Fig. 10. Gas sensor responses for 0

Fig. 9. Response time of PANI-PS sensor films to CO2, NH3 and ethanol vapour.

results show that the sensors (made from the blend films) carryinglarger compositions of PANI have better gas sensing ability. Besidesthe sensors with higher PS concentrations (0.8 M PS), it is observedthat �R/Ro value gradually increases upon exposure to analyt gas,but after a long recovery time, it fails to return to the baseline value(initial response value) on being transferred to dry air (as shown inFig. 10) indicating a poor reversibility. Fig. 11 illustrates the repeat-ability characteristics of the sensor (0.7 M and 0.6 M PANI–PS) forthree CO2-air cycle, where the adsorption and desorption time areabout 400–500 s and 500–800 s, respectively. These values togetherare significantly less than a recent reported value, 30 min, in theliterature [38]. This indicates that CO2 molecules are reversiblebound to the polymer and after a period of time, CO2 can be des-orbed when exposed to air. Once the molecules are removed, thepolymer returns to its initial electrical conductivity. Consequently,subsequent analysis on the sensor results using blend films withthe PANI composition of 37 wt. % and above (0.6 M and 0.7 M of PS)

is provided here.

The sensing mechanism shown in Fig. 12 can be explainedby the following compensation effect [20]: in conducting poly-mers, the electrons in �-bonds form the backbone of the chain

.6 M, 0.7 M and 0.8 M of PS.

34 J. Bhadra et al. / Synthetic Metals 181 (2013) 27– 36

F

dtspdaohI�dtstn�ot

obceftop(ofOp

ig. 11. Reproducibility of sensor response to repeated exposures of 1500 ppm CO2.

ominating the mechanical properties of the polymer. Due tohe �-orbital overlap of neighboring molecules of the conjugatedtructure, the �-electrons get delocalised along the entire chainroviding their semiconducting and conducting properties [39]. Inoped PANI, a positive charge and an unpaired electron associ-ted with a quinoid structure are formed as a result of removalf an electron from a polymer chain [40]. This is because CO2as two lone pair of electrons with �-type C O bindings [41].

t may be possible that CO2 molecules form weak bonds with-electrons of the PANI surface, which results a decrease in con-uctivity or an increase in sensor resistance. It may be notedhat the process of adsorption (of CO2) is reversible. When theensor is exposed to air, it requires a longer time for its reset-lement during recovery. This could be due to its chemisorptiveature. During the desorption process, the bond formed with-electrons of the PANI surface breaks and retains the polarityf the PANI structure. This causes a decrease in sensor resis-ance.

In Fig. 13, we show the sensitivity (S%) and total response timen exposure to CO2 gas as a function of PS concentration in thelend. The sensitivity is noticed to increase with the decrease in PSoncentration until 0.7 M of PS where it is found to be the high-st; this could be the presence of more possible adsorption sitesor CO2 gas than sensor with 0.6 M of PS. The sensitivity of a sensorypically depends on two factors: the conductivity and the numberf adsorption sites. The blend containing 0.6 M of PS has com-aratively high conductivity, however, SEM images and histogramFig. 2(a)–(e)) show that the 0.6 M has significantly lower number

f adsorption sites. Consequently, this leads to lower the possibleormation of weak chemical bonds with the composite molecules.n the other hand, for 0.7 M of PS, though conductivity is com-aratively lower but high adsorption sites facilitate more number

Fig. 12. Mechanism underlying the sensor response.

of temporary chemical bonding sites to the blend surface addingan additional advantage for high sensitivity. The observed sensi-tivity percentage values in this experiment are much higher thanthe values reported in the literature [38,42]. However, the trendgets disrupted beyond this concentration and goes on decreasing.On the other hand, the response time is monotonically decreasingwith decrease in PS concentration and is the least for 0.6 M.

The typical sensor response–recovery characteristics of PANI–PSsensor to different concentration of CO2 at room temperature isshown in Fig. 14. Both the sensors containing 0.6 and 0.7 M of

PS show good sensitivity to the concentration change of CO2. Ineach exposure of CO2, the sensor responds promptly and returnsto its baseline value on exposure to air. Furthermore, the increase

J. Bhadra et al. / Synthetic Metals 181 (2013) 27– 36 35

ieossTai

Ft

Fig. 13. Sensitivity and total recovery time vs. PS concentration.

n exposed gas concentration renders an increase in sensitivity forach sensor. This is evident from Fig. 15, where the response valuesf both sensors vary linearly with the CO2 gas concentration. Theaturation effect is apparent at 2000 ppm in case of both the sen-ors which is more than the values reported in the literature [42].he improved response, in presence of PANI nanoparticles, could bettributed to the higher vapour permeability of the nanoparticles,

nduced by an increase in the free volume of the polymer.

ig. 14. Sensors response as a function of CO2 gas concentration at room tempera-ure.

Fig. 15. Response of PANI-PS blend sensors resulting from replicated cyclingbetween CO2 and air at 27 ◦C for 0.7 M and 0.6 of PS.

4. Conclusions

In this paper, conducting polymer PANI–PS blends have beensuccessfully prepared by in situ dispersion polymerization meth-ods. The blends are prepared using five different concentrations ofPS (i.e., 1 M, 0.9 M, 0.8 M, 0.7 M and 0.6 M), and the films obtainedare characterized using SEM, XRD, FTIR, DSC, and TGA. The changein PS concentration significantly affects the morphology and struc-ture of the blended films; this is felt on the SEM pictures whereit renders the formation of nanoparticles. Additionally with thedecrease in PS concentration, the particles combine to form clustersor chains; the XRD diffractograms exhibit semi crystalline behav-ior of the blends. The FTIR spectra depict crosslinking betweenthe polymers indicating the chemical blending formation. The TGAand the DSC graphs reveal different degradation temperatures forblends with an increased degradation temperature for the blendedfilms when compared with pure PANI. The electrical properties arealso observed to increase with a decrease in the PS concentration;the I–V characteristics show some non-ohmic behavior. Conclud-ing the experiment, the CO2-sensing behavior seems to be the mostresponsive characteristics for the blends containing 0.7 M and 0.6 Mof PS. Although the study of gas sensitivity is important, there arestill some other issues of great significance such as long term sta-bility, effect of temperature and humidity, etc. which we plan toexplore in future.

Acknowledgments

The authors express their deep sense of gratitude to Texas Uni-versity for XRD measurements. Authors acknowledge Office of Vice

President for Research for funding the Project No. QUUG-MTU-FALL-11/12-1. The authors are also thankful to Dr. S.P. Dakua forhelping us do the MATLAB programming.

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