Jnrt2009 2 All Papers Tapia

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JOURNAL of NUCLEAR And Related TECHNOLOGIES, Vol. 6, No. 2, December, 2009

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FTIR AND XPS ANALYSES OF THERMALLY AGED POLYANILINE

EMERALDINE FILMS: RELATIONSHIP TO MORPHOLOGICAL

AND ELECTRICAL PROPERTIES AFTER DOPING

A. K. G. Tapia1,4

, E. J. Del Rosario2, B. Basilia

3, and R. V. Sarmago

4

1Physics Division, Institute of Mathematical Sciences and Physics, University of thePhilippines Los Baos, College, Laguna, Philippines

2Institute of Chemistry, University of the Philippines Los Baos, College, Laguna,

Philippines3ITDI, Department of Science and Technology, Bicutan, Taguig City, Philippines

4Material Science and Engineering Program, University of the Philippines Diliman,

Quezon City, Philippines

[email protected]

ABSTRACT

Polyaniline Emeraldine Salt (PAni-ES) was synthesized using standard oxidative

polymerization. The resulting PAni-ES powder was deprotonized using Ammonium Hydroxide

to yield Polyaniline Emeraldine Base (PAni-EB), processible form of PAni. The PAni-EB

powder was dissolved using N-methylpyrrolidone and casted into film on a glass substrate. The

resulting films were thermally aged near the reported glass transition temperature at 65 0C. The

ageing time was 5 to 60 minutes with a five-minute interval. The unaged sample was used as

control. Fourier Transform Infrared Spectroscopy results showed decreasing Quinoid

characteristics upon ageing which is consistent with chemical crosslinking. X-rayPhotoemission Spectroscopy with Synchrotron Radiation as source showed an increase of C-O

attributed to sample oxidation. Sample morphologies were characterized using Atomic Force

Microscope and Scanning Electron Microscope. It was found out that the surface smoothenedand the size of the pinholes decreased with ageing time. These observations are consistent with

crosslinking as well. The doped PAni films from the aged samples showed an increase in

conductivity up to a maximum value of 2.75 S/cm. This was found from the sample aged at 35

minutes. One of the reasons for this is a better surface morphology induced by ageing that

favors electrical conduction. The decrease in conductivity after the optimum value is attributed

to a more dominant decrease in conjugation of the chains. The results suggest that thermaltreatment of PAni-EB films prior to doping yields to optimizing electrical property of the doped

form.

ABSTRAK

Polyaniline Emeraldine Salt (PAni-ES) was synthesized using standard oxidativepolymerization. The resulting PAni-ES powder was deprotonized using Ammonium Hydroxide

to yield Polyaniline Emeraldine Base (PAni-EB), processible form of PAni. The PAni-EB

powder was dissolved using N-methylpyrrolidone and casted into film on a glass substrate. The

resulting films were thermally aged near the reported glass transition temperature at 65 0C. The

ageing time was 5 to 60 minutes with a five-minute interval. The unaged sample was used ascontrol. Fourier Transform Infrared Spectroscopy results showed decreasing Quinoid


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characteristics upon ageing which is consistent with chemical crosslinking. X-ray

Photoemission Spectroscopy with Synchrotron Radiation as source showed an increase of C-Oattributed to sample oxidation. Sample morphologies were characterized using Atomic Force

Microscope and Scanning Electron Microscope. It was found out that the surface smoothened

and the size of the pinholes decreased with ageing time. These observations are consistent with

crosslinking as well. The doped PAni films from the aged samples showed an increase inconductivity up to a maximum value of 2.75 S/cm. This was found from the sample aged at 35

minutes. One of the reasons for this is a better surface morphology induced by ageing that

favors electrical conduction. The decrease in conductivity after the optimum value is attributed

to a more dominant decrease in conjugation of the chains. The results suggest that thermal

treatment of PAni-EB films prior to doping yields to optimizing electrical property of the doped

form.

Keywords: FTIR, XPS, Surface Morphology, Crosslinking, Electrical Conductivity,

Polyaniline

INTRODUCTION

Conducting organic polymers, which are highly conjugated -electron systems, display

electronic properties like that of metals (Kroschwitz, 1988). Polyaniline (PAni) is a conducting

polymer whose electrical properties can easily be tuned by doping. Polyaniline Emeraldine Salt

(PAni-ES) is the doped form of PAni. It has a conductivity of the order of 100 S cm1, which ishigher than that of common polymers (104

S cm1) (Stejskal et. al., 1996, 2004).

PAni films have been grown on flat surfaces such as glass, silicon and noble metals. (Riede et.al., 2002; Ghos et. al., 2001) These films are fabricated by casting (Han et. al., 2001) and

vacuum deposition (Qiu et. al., 2005). Polyaniline-Emeraldine Base (PAni-EB) is the

processible form of PAni. PAni-EB powder can be dissolved by suitable solvents such as meta-

cresol (Lee et. al., 2006) and N-methylpyrrolidone (NMP) (Angelopolous et. al., 1988). The

resulting solution can be used to make films, gels and wires (Gregory, 1988). In order to make

these materials conducting, protonation can be done by dipping the fabricated materials in acid

for 24 hours.

Films are usually post-processed for achieving properties necessary for applications. Thermal

processing can be done on the films to yield certain mechanical, morphological and, especially,

electrical properties. An interesting process that can be induced by thermally treating polymersis crosslinking. This behaviour can lead to rearrangement of polymeric chains that has direct

effects on conduction process of conducting polymers. It was found out in previous studies that

partial crosslinking can also occur at glass transition temperatures lower than 100C. (Ding et.al., 1999; Tsocheva et. al., 2000; Rodrigues et. al., 2002) This means that subjecting PAni films

to thermal treatment at lower temperatures can induce crosslinking.

Crosslinking in PAni can be indicated by the conversion of Quinoid rings to Benzenoid ringsvia a link between Quinoid rings (Ding et. al., 1999; Tsocheva et. al., 2000; Rodrigues et. al.,

2002; Mathew et. al., 2002). Figure 1 shows Scheme 1 for crosslinking in PAni. Also,

crosslinking in PAni films have direct effects on its surface profile and mechanical properties

(Tan et. al., 2001; Liu et. al., 1999).


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Figure 1. Scheme 1 for crosslinking in PAni-EB

Ref.: Ding et. al., 1999; Tsocheva et. al., 2000; Rodrigues et. al., 2002; Mathew et. al., 2002

The crosslinking of PAni can also be described using Scheme 2 as seen in Figure 2, and for

each crosslink formed only two hydrogen atoms are released.

Figure 2. Scheme 2 for crosslinking of PAni

(Ref.: Cronklin et. al., 1995; Scherret. al., 1991)

This paper presents the molecular structure and the elemental analyses of the aged PAni-EB

films as revealed by Fourier Transform Infrared Spectroscopy and X-ray Photoemission

Spectroscopy, respectively. Also, the effects of ageing on the molecular structure are related to

the morphological and electrical properties of the samples. Ageing can be used to optimize the


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electrical properties of the films. This is essential in order to meet characteristics for devices

using conducting polymers such as conductive coating, conducting tapes and other nanodevices(Stenger-Smith, 1998). Lastly, the optimization of the electrical conductivity of the PAni film

via doping of aged PAni-EB films was observed.

METHODOLOGY

Fabrication and Ageing of PAni Films

PAni-ES was synthesized using standard oxidative polymerization (Stejskal and Gilbert, 2002).

A 0.25 M aqueous solution of Ammonium Peroxydisulfate (APS) was mixed with 0.2 M of

aniline in 1 M H2SO4. The mixture was stirred and maintained at 4C in an ice-bath. The green

PAni-ES precipitate was filtered and washed with distilled water and was air-dried. PAni-EB

was made by mixing Ammonium Hydroxide (NH4OH) with the PAni-ES precipitate for 4

hours. The mixture was filtered and the precipitate was air-dried.

The PAni-EB film was prepared by casting method. PAni-EB was diluted in N-methylpyrrolidone (NMP) with the ratio 5% wt/wt. The solution was stirred for 8 hours. This

was poured on glass substrate and was dried in a vacuum oven for 4 hours at 80 C. After

drying, the film with the substrate was dipped into distilled water to allow the water to sip into

the interface. The film lifted off from the substrate after several hours. The PAni-EB film was

then dried and was post-processed by thermal ageing. It was prepared to be aged at 65C which

is around the reported glass transition temperature of PAni-EB (Tsocheva et. al., 2000). Ageing

was done from 5 minutes to 60 minutes with a five-minute interval.

The control (unaged) and aged PAni-EB films were then doped by protonation. This was done

by dipping the samples in 1M aqueous solution of Sulfuric Acid (H2SO4) for 24 hours.

Afterwards, the films were air-dried in a fume ood.

Characterizations of PAni Films

To characterize the morphology of the aged PAni-EB films, Scanning Electron Microscope

(SEM) images were taken using Philips SEM (X130). In order to verify morphological

observations, Atomic Force Microscope (AFM) images were acquired using Atomic Force

Microscope (Digital Instruments Nanoscope) for the surface profile of the aged films. This was

carried out in tapping mode. The AFM scan size is 20 m by 20 m with a scan rate of 0.7004

Hz. The samples observed for surface morphology were the control, samples aged at 20

minutes, 40 minutes and 60 minutes.

The molecular structure of the aged films was investigated using Fourier Transform InfraredSpectroscopy (FTIR) done with the Perkin Elmer Spectrum RX1 FTIR Spectrometer. The

transmittance spectra were from 450 cm-1 to 4000 cm-1. The peaks studied were in the range of450 cm-1 to 1800 cm-1 as reported in this paper. The absorbance measurements were derived

from the transmittance spectra using Beer-Lambert law. For quantitative analysis of the FTIR

spectra, Lorentzian fitting for the peaks was done using Origin 5.0. The area of the curve-fit for

each peak is proportional to the amount of the molecular structure represented by that peak in

the sample. The samples measured were the control, samples aged at 15 minutes, 30 minutes

and 45 minutes.

The X-ray Photoemission Spectroscopy measurements were done using a Synchrotron as

source at the Siam Light Research Institute. The beamline used is for probing electronic

structure of solids and solid surfaces, and phenomena pertaining to processes of materialssyntheses or materials modifications. (Prayoon Songsiriritthigulet. al., 2003) There were three


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samples measured under ultra-high vacuum condition: the control, samples aged at 15 minutes

and 30 minutes. The scans were done for Carbon and Oxygen from 270 eV to 300 eV.

Lastly, electrical measurements were carried out using Keithley 224 as current source and

Keithley 2001 as the digital multimeter. The electrical conductivity of the doped samples

derived from the aged PAni-EB films were measured using in-line Four-point Probe method.The current was scanned and the voltage was read. The magnitude of the current depends on the

resistivity of the sample. If the sample has high resistivity, the range for the current was reduced

to lower values. The typical range used was in the milliampere level.

RESULTS AND DISCUSSIONS

Surface Morphology of the Aged PAni-EB Films

The free-standing film produced was lustrous and mechanically robust. The thickness

measured was 23.06 1.26 m. Figure 3(a)-(d) show the SEM images of the control and aged

samples at 20, 40 and 60 min at a magnification of 10,000. It can be seen by visual inspectionthat as ageing proceeds the surface of the samples become smoother and the presence of the

pinholes diminish.

Figure 3. SEM of PAni-EB films aged at different ageing times (a) control (b) 20 min (c) 40min (d) 60 min


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To verify the observations from the SEM images, AFM images were taken. Figure 4(a)-(d)show the AFM images of the control and aged samples at 20, 40 and 60 mins. It can be

observed that the sample aged longer appears to be smoother. Also, the pore sizes decrease with

ageing time.

Figure 4. AFM Images of PAni-EB films aged at different ageing times (a) control (b) 20

minutes (c) 40 minutes (d) 60 minutes

The decrease in the pore sizes indicates that the free volume of the material also decreases. This

is attributed to increasing interaction between chains that can be caused by crosslinking.

Crosslinking process induces creation of bonds between chains and entanglement of polymeric

chains. Also, since the sample is aged near its glass transition temperature, it is possible that the

chains slip past each other and move to occupy free spaces.


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In order to check the observed smoothening of the films due to ageing process, roughness of

the films were measured using the surface profile of aged PAni-EB films from the AFMimages. Figure 5 shows the mean roughness (Ra) and root-mean-square (rms) roughness of the

film samples based on the AFM images. It can be seen from both parameters that as ageing time

increased roughness decreased. This is again consistent with the SEM images.

Figure 5. Mean Roughness and Root-mean-square Roughness of PAni-EB Film at different

ageing times

In addition, it was earlier postulated that the NMP retained in Emeraldine base films cast from

the NMP solution acts as a plasticizer. The interaction of the NH groups in the Emeraldine films

with the C=O groups in NMP leads to an isotropic film with a smooth surface morphology

(Chen and Lee, 1993).

It was also noted in a paper by Tan et. al. (2001) that the smoothening is due to the effects of

crosslinking which compensates for any increase in roughness that arise from the loss of

hydrogen bonding as the NMP content decreases.

FTIR Analysis of the Aged PAni-EB Films

The changes in the morphology of the PAni-EB films can take place due to the resulting

interaction between chains upon ageing. FTIR spectra were used to verify what happens to the

chains in the film. Figure 6 shows the FTIR spectra of the control and aged PAni films.

The shoulder positioned at 1675 cm-1 is assigned as C=O stretching in NMP. The peaks at 1574

and 1512 cm-1 are ascribed to Quinoid and Benzenoid stretchings, respectively. The shoulder at

1379 cm-1 is assigned to C-N stretching near the Quinoid structure. The nearby peak at 1305

cm-1 is referred as the C-N stretching near the neighboring secondary aromatic amine groups.

The shoulder at 1235 cm-1 is the C-N stretching near the polaronic structure (Milton and

Monkman, 1993). The peak at 1160cm-1 mode is most likely to be an in-plane C-H bending

motion of the aromatic rings. The bands at around 1105 and 645 cm-1

are observed in the filmswhich are reported modes for sulphonate residues in PAni powder (Ohsaka et. al., 1984). The


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tiny waves at 1007 and 955 cm-1 were assigned to the in-plane and out-of-plane vibrations of

multi-substitution on the benzene ring. The peaks at 825 and 515 cm-1 are out-of-plane C-Hbending modes. Specifically, the position of the 825 cm-1 peak is characteristic of para-

disubstituted aromatic rings (Tang et. al., 1988).

Figure 6. FTIR Absorbance spectra of the (a) control and aged PAni-EB films (b)15 min, (c) 30

min and (d) 45 min

It can be noticed that the shoulder for C=O stretching (1675 cm-1) increases with ageing time.

This structure is attributed to sample oxidation as can be verified by the XPS results.

The Quinoid stretch (1574 cm-1) and Benzenoid stretch (1512 cm-1) are shown for all samples.It can be seen that the sharp peaks corresponding to Quinoid and Benzenoid stretches for

freshly-prepared PAni films merge into broad bands upon film ageing. This indicates faster

equilibration between Quinoid and Benzenoid structures as film ageing progressed. This agreeswith the proposed crosslinking scheme in Figure 1.

The peaks in the FTIR spectra were fitted using Lorentzian distribution (Gulmina and

Akcelrud, 2006). Figure 7 shows the Lorentzian Fits of the peaks in the FTIR Spectra of thecontrol PAni-EB sample. The distribution of the fit per peak can be seen (dashed line). It can

also be observed that the sum of the distribution of the peaks (faded line) traces the original

FTIR spectra (dark line). The fits have high fidelities which are around 0.94 (unity means a

perfect fit).

400550700850100011501300145016001750

Wavenumber (1/cm)

Absorbanc

e(a.u.)

1675

1512

1379

1305

1235

1160

1105

1007

955

825

645

515

1574

a

b

c

d


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Figure 7. Lorentzian Fitting of the Peaks in an FTIR Spectra of PAni-EB Film

Areas of the Quinoid and Benzenoid peaks were specified. Figure 8 shows the ratio ofBenzenoid to Quinoid areas (B/Q) with ageing time. It can be seen from the plot that the ratio

increases. This trend indicates that more Quinoid rings are converted to Benzenoid.

Figure 8. Ratio of Benzenoid and Quinoid vs. Ageing Time

0

0.2

0.4

0.6

0.8

1

9001100130015001700

Wavenumber (1/cm)

Absorbance

(a.u.)

Quinoid

Stretch

Area=41.130.46

Benzenoid

Stretch

Area=70.930.42

1.5

2

2.5

3

3.5

4

0 10 20 30 40 50

Ageing Time (min)

B/Q


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The C-N stretching near the Quinoid structure (1379cm-1) fades with ageing time and becomes

dominated by the broadening C-N stretching near the secondary aromatic amine group (1305cm-1). This strengthens the decrease of Quinoid structures observed in Figure 6.

Figure 9 is the magnified portion of the peak between 1400cm -1 to 1253 cm-1. It can be seen

that there is a broadening of the region upon ageing.

Figure 9. Magnified Portion of the Broadening in the 1400 cm-1 to 1253 cm-1 region

This broadening indicates increased interaction of vibrational modes which is previouslyattributed as physical chain crosslinking (Milton and Monkman, 1993).

The peak at 1160cm-1 mode is most likely to be an in-plane C-H bending motion of the

aromatic rings. Scherret. al., (1991) reported that changes in the peak observed at 1160 cm-1 ismost likely due to linkage of imine Nitrogen with neighboring Quinoid ring as seen in Figure 2.

This change is not observed for the aged samples.

Results from FTIR analyses show the possibility of both physical and chemical crosslinking

taking place on the PAni-EB films upon ageing. This verifies the cause of smootheningobserved for the surface morphology of the samples. Lastly, since the ageing temperature is

way below the curing temperature of PAni, it would be safe to say that only partial crosslinking

occurs.

XPS Analyses

Figure 10 shows the XPS spectra of the PAni-EB films. This shows the composition of the

samples surface up to 10 nm level. The deconvoluted peaks at 286 eV and 289 eV are referred

to as C-O and O-C=O groups, respectively (Li et. al., 1997). The results are summarized in

Table 1 for the percent composition of C-O and O-C=O for different ageing times.


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Figure 10. XPS Spectra for PAni-EB Films (a) control, (b) aged at 15 mins and (c) at 30 mins

Table 1. The percent composition of C-O at 286 eV and O-C=O at 289 eV

for different ageing times

Ageing time (min) 286 eV 289 eV

0 (control) 17.36 11.04

15 18.15 10.24

30 27.45 9.02

It can be seen from the values the apparent increase of C-O which supports sample oxidation

(Liu et. al., 1999). This is also consistent with the observed oxidation due to NMP residues seenin the FTIR spectra at 1675 cm-1. This is a stronger quantitative evaluation of sample oxidation

compared with the FTIR measurement. However, the slight decrease of O-C=O still needs to be

verified.

Electrical Conductivities of the PAni-ES Films

Electrical conductivities were then measured for the PAni-ES samples derived from aged films.

Figure 11 shows the conductivity of the samples versus ageing time. It can be seen that the

conductivity increased with ageing time. A maximum electrical conductivity of 2.75 S/cm was

observed for a film aged for 35 min. This is so much larger than the 0.02 S/cm conductivity of

the unaged sample (data not included in the plot). After prolonged ageing, it can be seen that the

conductivity decreases. The process was done for another batch of samples and the behaviour of

the results were the same.


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Figure 11. Conductivity of Doped PAni-EB Film at Different Ageing Time

With increasing ageing time, smoothening of the film takes place as revealed by SEM and

AFM results. This is a possible consequence of physical crosslinking where chains entangle and

rearrange themselves at the glass transition temperature. FTIR elucidates the presence of

chemical crosslinking in the sample due to the decreasing Quinoid characteristics. XPS resultssupported sample oxidation.

These were related to the electrical conductivity measurements. The increase in the

conductivity suggests that a smoother surface with smaller pinholes favor charge conduction by

decreasing the conduction path. It is also likely that when two chains are attached by a crosslink,

the distance between the chains decrease that will aid better inter-chain conduction for charges

to move along the film. On the other hand, prolonged ageing causes the conductivity to

decrease due to the creation of more saturated Nitrogen sites as revealed in Schemes 1 and 2 of

crosslinking. This process leads to decreasing the conjugation of the polymer chain that lowers

the intra-chain conduction. The mechanisms by which crosslinking helps the charges to hop

along the film and, at the same time, decrease the conjugation of the polymer interplay for the

electrical conduction in the sample. Sample oxidation also contributed to the decrease inconductivity.

With these findings, the post-processing done on the PAni-EB films prior to protonation led tothe optimization of the conductivity of the samples. This method can also be done for large-

scale processing of films in which maximum conductivity can be attained.

CONCLUSION

The electrical conductivity of Polyaniline (PAni) film was optimized by doping the aged

Emeraldine Base Film. The PAni-Emeraldine Base (PAni-EB) film was fabricated via casting

method on a glass substrate. The PAni-EB films were subjected to post-processing by ageing atits reported glass transition temperature of 65C. The results from SEM, AFM, and FTIR


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showed indications of partial crosslinking of the sample upon ageing. In addition, XPS results

support sample oxidation. The doped samples of aged PAni-EB films showed an increase ofelectrical conductivity up to an optimum value.

ACKNOLWEDGEMENTThe authors would like to thank the PCASTRD, Department of Science and Technology,

Republic of the Philippines for the funding. We would like to thank the Department of Wood

Science and the Institute of Chemistry, University of the Philippines Los Baos for allowing usto use their vacuum ovens. Lastly, we acknowledge the Siam Light Research Institute for the

XPS measurements.

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Jnrt2009 2 All Papers Tapia