Enhanced luminescence observed in polyaniline–polymethylmethacrylate composites

Enhanced luminescence observed in polyaniline–polymethylmethacrylate composites

Journal of Alloys and Compounds 449 (2008) 176–179 Enhanced luminescence observed in polyaniline–polymethylmethacrylate composites M. Amrithesh a , S...

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Journal of Alloys and Compounds 449 (2008) 176–179

Enhanced luminescence observed in polyaniline–polymethylmethacrylate composites M. Amrithesh a , S. Aravind a , S. Jayalekshmi a,∗ , R.S. Jayasree b a

Division for Research in Advanced Materials, Department of Physics, Cochin University of Science and Technology, Cochin 22, Kerala, India b Department of Radiology, S.C.T.I.M.S.T, Thiruvananthapuram, Kerala, India Received 7 November 2005; received in revised form 9 February 2006; accepted 10 February 2006 Available online 19 January 2007

Abstract Polyaniline (PANI) is one of the highly pursued conducting polymers owing to its high electrical conductivity, interesting optical properties and excellent environmental stability. Recently, it is also being used extensively as hole-injecting electrodes in polymer LEDs. Inspite of its excellent electrical properties, the mechanical strength and processibility are not very attractive. It is reported that making composites of PANI with conventional polymers such as polymethylmethacrylate (PMMA), significantly improves the mechanical strength and processibility. Recently, we have observed that the DC electrical conductivity of HCl doped PANI can be further enhanced by making composites with PMMA. PMMA has been reported to have excellent optical properties such as very good transparency in the visible region and fairly good photoluminescence emission. This prompted us to carry out investigations on the optical properties of this composite, especially the photoluminescence characteristics. In the present work, we have synthesized PANI–PMMA composites using bulk polymerized PMMA. The FTIR spectrum reveals that PANI has been dispersed as an interpenetrating network in the PMMA matrix. The change in the photoluminescence (PL) behaviour of PANI–PMMA composites with different aniline to PMMA feed ratios has been investigated. It is observed that the photoluminescence intensity increases with increase in the PMMA content in the composite, possibly due to greater chances of exciton formation and subsequent radiative decay to the ground state. The PL spectrum of PMMA is also taken for comparison. The enhancement in the PL intensity of the composites with increase in the aniline to PMMA feed ratio is quite comparable with the enhancement in the DC electrical conductivity of these composites. © 2006 Elsevier B.V. All rights reserved. Keywords: Exciton; Interpenetrating network; PANI–PMMA composites

1. Introduction Ever since the discovery by Alan Heeger, conducting polymers have emerged as one of the thrust areas of experimental research. One of the remarkable features of conducting polymers is that it is possible to control the electrical conductivity of these polymers over a wide range from insulating to metallic and even to superconducting by proper doping with suitable dopants. Many of the conducting polymers show good optical behaviour also [1,2]. This finding has resulted in substantial efforts to synthesize conducting polymers for specific technological applications. In addition to good electrical conductivity and optical characteristics, conducting polymers have several ∗

Corresponding author. Tel.: +91 484 2577404. E-mail addresses: jaya [email protected], [email protected] (S. Jayalekshmi). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.02.096

other advantages such as plasticity, low cost, lightweight and ease of fabrication. Inspite of all these attractive properties, many of the conducting polymers have low mechanical strength and are difficult to process [3]. By making composites with suitable conventional polymers, the physical properties such as mechanical strength and processibility can be improved [4,5]. In addition to it, the properties of the guest polymer can be beautifully blended with that of the host polymer. Polymer composites can be prepared either by mechanical blending or by chemical in situ polymerisation. Of the two, chemical polymerisation gives more uniform composition [3]. Polyaniline (PANI) has attracted global interest because of its excellent electrical properties, stability in air and ease of synthesis [6]. In suitably doped form, its applicability as hole-injecting electrode in electroluminescent devices is one of the hot areas of research in the field of polymer optoelectronics

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[7,8]. Its composites with many other polymers are also being pursued for optoelectronic applications [4]. Polyaniline can be prepared in bulk form using chemical oxidative polymerization or in thin film form using plasma polymerization technique, electrochemical deposition, spin coating or solution casting. One of the negative aspects of polyaniline is its comparatively low mechanical strength, which limits its applications to some extent. By making composites with polymethylmethacrylate (PMMA), the mechanical strength of polyaniline can be improved. Also the excellent optical properties of PMMA can be suitably incorporated into the host polymer. Recently, we have observed that the DC electrical conductivity of HCl doped PANI can be enhanced with the addition of PMMA. This observation combined with the excellent optical properties of PMMA is the prime motivation for investigating the photoluminescence characteristics of these polymer composites in the present work. Here, we have adopted a novel synthesis route for the preparation of PANI–PMMA composites. The structural characterization of the composites has been carried out using FTIR technique.

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Fig. 1. FTIR spectrum of PANI–Cl–MMA composite.

3. Results and discussion 2. Experimental

3.1. Spectral analysis 2.1. Synthesis In order to fully exploit the excellent optical properties and the mechanical strength of PMMA, we have worked with bulk polymerized PMMA instead of the widely used emulsion polymerized PMMA. The monomer methylmethacralyte (MMA) was washed first with 20% sodium hydroxide solution to remove the inhibitors and then with distilled water. The initiator benzoyl peroxide was added to MMA and stirred for 2–3 h at around 80 ◦ C till the solution became viscous. The resulting solution was poured into a dish and kept for drying till crystal clear PMMA sheets were obtained. Aniline was mixed with hydrochloric acid. PMMA dissolved in toluene was added to it. Ammonium peroxidisulphate was added drop wise to the resulting mixture with continuous stirring. The temperature was maintained at around −10 ◦ C and stirring was allowed to proceed for 4–5 h. The precipitate was filtered, washed several times and dried. Since the synthesis temperature is low and the molarity of the oxidant high, the amount of unutilized reagents is very small. Hence, the synthesis is very much environment friendly.

2.2. FTIR studies The Fourier Transform Infrared Spectrum of the composite sample was taken using an Avatar 370 spectrometer employing a DTGS KBr detector.

2.3. Photoluminescence (PL) studies Photoluminescence spectra of PMMA, PANI and PANI–PMMA composites with different aniline to PMMA weight ratios were recorded using a Jobin Yvon Fluorolog 3 Spectrofluorometer. (Model FL3-22). The instrument has a 450 W Xenon lamp as source and a PMT as detector (Model R928P). The sample compartment module is T-box type. Double gratings are used for excitation and emission spectrometers. The slit width for excitation is 7 nm and that for emission 9 nm.

2.4. DC conductivity studies DC electrical conductivity studies of PANI and PANI–PMMA composites were carried out using a computer controlled Keithley 236 Source Measure Unit under high vacuum of the order of 10−5 Torr. The samples taken in the form of pressed pellets with silver paste electrode were placed in a custom-made conductivity cell.

The FTIR spectrum of PANI–PMMA composite (Fig. 1) shows all the major peaks of HCl doped PANI as well as PMMA (see Table 1). This confirms the fact that PANI has been dispersed as an interpenetrating network in the PMMA matrix. 3.2. Analysis of photoluminescence spectra The photoluminescence spectra of PANI, PANI–PMMA composites with different feed ratios of aniline to PMMA and PMMA are given in Figs. 2–4. The excitation wavelength used was 300 nm. As seen from Fig. 2, HCl doped polyaniline shows a broad spectrum with very low photoluminescence intensity. PANI–PMMA composites show greater PL intensity as compared to pure PANI. It has been reported that emeraldine base form of PANI (PANI–EB) shows a broad emission spectrum with its maximum located at 400 nm, which is caused, by the reduced benzenoid/amine groups of EB [10]. It is found to get suppressed drastically in intensity when transformed into the highly conducting state by chemical doping. This could be the Table 1 The wave numbers of various vibration peaks observed and the corresponding assignment of vibrations Wave no. (cm−1 )

Assignment of vibrations

3436 1726 1564.54 1467 1298 1241 1107 871 799

N H Stretching Vibration in PANI C O Stretching in PMMA C N Stretching in PANI O CH3 Deformation in PMMA C N Stretching mode of benzenoid unit of PANI C O Stretching in PMMA Quinonoid unit vibration of doped PANI Skeletal vibration of PMMA C C and C H stretch for benzenoid unit of PANI

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Fig. 2. Photoluminescence spectra of PANI and PANI–PMMA composites synthesized with different aniline to PMMA feed ratio.

Fig. 3. Photoluminescence spectrum of PANI–PMMA composite with aniline to PMMA feed ratio 1:1. The emission intensity is enhanced so much that the spectrum is not included along with the other photoluminescence spectra and is given separately.

reason for the broad featureless PL spectrum of HCl doped PANI (Fig. 2) in the present case. For comparison, the PL spectrum of pure PMMA is shown in Fig. 4 and since the intensity of the PL peak of PMMA centered around 450 nm is much smaller compared to that of PANI and PANI–PMMA composites, it is shown separately. Similarly the PL spectrum of PANI–PMMA composite with aniline to PMMA feed ratio 1:1 is also shown separately in Fig. 3 because of its much increased PL intensity compared to PANI and other PANI–PMMA composites. The analysis of the PL spectra shows that though pure PMMA shows a PL emission peak at 450 nm, it is much reduced in intensity compared to that of PANI and PANI–PMMA composites. As mentioned earlier, the PL spectrum of HCl doped PANI is quite broad and featureless. Coming to the spectrum of PANI–PMMA composites, it is seen that the PL emission intensity increases drastically with increase in PMMA content in the composites. The emission peak is centered at around 502 nm in all the composites which could be attributed to a ␲–␲* transition. It is surprising to see that even when both PMMA and HCl doped PANI do not show any intense PL emission spectrum, around the 500 nm region, the composites are showing intense and sharp PL emission peaks around 500 nm.The intensity of the peaks increases with increase in the aniline to PMMA feed ratio which is equivalent to the increase in the PMMA content in the composites. It might be possible that the excitation wavelength of 300 nm chosen in the present PL studies could excite the impurities in PMMA resulting in PL emission and if this is the case the intensity of the emission peak will increase with increase in PMMA content. However, in the present case, definitely, this is not the situation, since the intensity of PL emission in PMMA is much less (Fig. 4) compared to that in the composites. Also the emission peak in PMMA is around 450 nm where as that in the composites is just above 500 nm.The enhancement of the PL emission intensity in the composites with increase in PMMA content can be explained as follows. There are electron donating groups such as NH in PANI and electron withdrawing groups such as C O in PMMA. This combination enhances the ␲ electron mobility in the composites. So as PMMA content increases in the composites the combination probability and hence the ␲ electron mobility increase further. This in turn favours the formation of singlet excitons. The singlet exciton states so formed decay radiatively to the ground state resulting in enhanced photoluminescence [9]. In support of this explanation we are including the results of DC electrical conductivity studies on these samples. 3.3. Conductivity results

Fig. 4. Photoluminescence spectrum of PMMA.

In Fig. 5, the variation of DC electrical conductivity with temperature for HCl doped PANI and PANI–PMMA composites is shown. The room temperature conductivity of HCl doped PANI is 0.044 S/cm while that of PANI–PMMA composite synthesized in the 1:1 ratio is 0.3725 S/cm. From the figure, it is obvious that the DC electrical conductivity of the composites is higher than that of HCl doped PANI and also it increases with increase in PMMA content in the composites. This result is all the more interesting since PMMA is an insulator. The increase in con-

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is the common major factor contributing towards both of these mechanisms. 4. Conclusions

Fig. 5. Variation of DC conductivity with temperature of PANI and PANI–PMMA composites.

PANI–PMMA composites with different aniline to PMMA feed ratios have been prepared and their structural, photoluminescence and conductivity studies carried out. The FTIR spectrum of the composite shows that PANI has been dispersed as an interpenetrating network in the PMMA matrix. It is observed that as the PMMA content in the composite is increased, the photoluminescence intensity also increases, possibly due to greater chances of exciton formation resulting from increased ␲ electron mobility. This is well supported by the DC conductivity results. The DC electrical conductivity of these composites falling in the high semi conducting region (0.044–0.3725 S/cm) coupled with the high PL emission intensity peaked around 500 nm highlights the prospects of application of these composites as emissive layers in polymer light emitting devices. Acknowledgement

ductivity is explained as follows. In PANI–PMMA composites, the conducting PANI regions are interconnected by insulating PMMA regions. By adding PMMA, increase in conductivity occurs due to electronic tunneling through non-conducting PMMA separating mesoscopic conducting PANI islands [5]. For the composite loaded with less amount of PMMA, the conductivity increase is marginal. But as more amount of PMMA is added, the conductivity increases appreciably which can be due to increased enhancement in ␲ electron mobility owing to increased tunneling probability. The composite sample with aniline to PMMA feed ratio 1:1 is showing maximum DC electrical conductivity and PL emission intensity. The enhancement in PL emission intensity with increase in PMMA content in the composites is complimented by the enhancement in DC electrical conductivity with increase in PMMA content. The enhancement in ␲ electron mobility with increase in PMMA content

The J.R.F given by K.S.C.S.T.E to Amrithesh M. is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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