Photosensitive polyaniline colloidal particles prepared by enzymatic polymerization using the azopolymer DMA-co-AZAAm as stabilizer

Materials Chemistry and Physics 124 (2010) 389–394

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Photosensitive polyaniline colloidal particles prepared by enzymatic polymerization using the azopolymer DMA-co-AZAAm as stabilizer M. Güizado-Rodríguez a,∗ , M. López-Tejeda a , J. Escalante b , J.A. Guerrero-Álvarez b , M.E. Nicho a a Centro de Investigación en Ingeniería y Ciencias Aplicadas (CIICAp), Universidad Autónoma del Estado de Morelos (UAEM), Av. Universidad No. 1001, Col. Chamilpa, C.P. 62209, Cuernavaca, Morelos, Mexico b Centro de Investigaciones Químicas (CIQ), Universidad Autónoma del Estado de Morelos (UAEM), Av. Universidad No. 1001, Col. Chamilpa, C.P. 62209, Cuernavaca, Morelos, Mexico

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Article history: Received 13 November 2009 Received in revised form 17 June 2010 Accepted 21 June 2010 Keywords: Polyaniline colloids Enzymatic synthesis DMAAm copolymers Thermo-sensitive azopolymer Photoisomerization

a b s t r a c t Water-dispersible polyaniline colloids with photosensitivity were prepared by enzymatic polymerization using the photo- and temperature-responsive azopolymer: N,N-dimethylacrilamide (DMA)-co-N-4phenylazo-phenylacrylamide (AZAAm) copolymer, poly(DMA-co-AZAAm), as steric stabilizer. The resulting polyaniline nanoparticles showed an interesting photoisomerization behavior in water solution, as indicated by 1 H NMR and UV–vis spectra obtained after irradiation with UV light at 365 nm. Its comparative analysis with poly(DMA-co-AZAAm) was realized. The effect of the steric stabilizer on the morphology of the polyaniline was studied by AFM and SEM analysis of the polyaniline synthesized with and without azopolymer. Additional characterization such as molecular weight and infrared spectra were performed. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Among the conducting polymers, polyaniline (PANI) is probably the most widely studied because it has a broad range of properties derived from its distinct doping mechanism, which results in tunable electrical conductivity [1]. The formation of polyaniline dispersions has been exploited as an effective method of circumventing the limited processability of this material. These dispersions can be used for the preparation of microstructured electrically conducting composite films. Conducting polymers, like polyaniline, would find potential applications in multidisciplinary areas such as electronics, thermoelectronics, chemistry, electrochemistry, electromagnetism, electromechanics, electro-luminescence, electro-rheology [2]. Colloidal suspensions of polyaniline are usually produced by dispersion polymerization [3] using a suitable polymeric stabilizer [4] such as poly(vinyl alcohol), poly(N-vinylpyrrolidine), poly(vinyl methyl ether), cellulose ether, or sophisticated tailor-made copolymers. The resulting colloids range in size from 100 to 300 nm in diameter and this range varies as a function of both the steric stabilizer and reaction conditions [5]. The synthesis of polyaniline colloidal particles is commonly carried out by either chemical [6] or electrochemical oxidation [7] of aniline in the presence of a steric stabilizer. As the ani-

∗ Corresponding author. Tel.: +52 777 3297084x6220; fax: +52 777 3297984. E-mail address: [email protected] (M. Güizado-Rodríguez). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.06.052

line monomer polymerizes, a surface layer of the steric stabilizer attaches to the PANI nanoparticles protecting the particles from aggregation. Hence, a stable dispersion of PANI colloidal particles can be obtained [1]. By this general process various polyaniline derivatives, copolymers, blends or composites have been studied [8] and nanostructures of polyaniline with different shapes, for example, nanowires, nanofibers, nanoshells, and nanotubes have been produced [9]. Recently, Cruz-Silva et al. achieved the enzymatic polymerization of aniline in dispersed media as an environmentally friendly alternative for preparing sterically stabilized PANI colloids [10]. The peroxidase/hydrogen peroxide system used has some advantages over traditional oxidizing agents, such as better control of the oxidation rate and the reduction of oxidation byproducts to water. This polymerization was successfully adapted to prepare pH- and thermosensitive polyaniline colloidal particles. Stimuli-responsive polymers undergo a large change in size and physical properties in response to external stimuli such as temperature, pH or specific wavelengths of light [11]. Polymers that contain azobenzene (azopolymers) are attractive stimuli-responsive materials because of their photoresponsiveness and their potential optoelectronic and photonic applications [12]. Most of their applications depend on the reversible trans–cis photoisomerization of the azobenzene group, allowing their use in optical information storage, light switching devices, surface relief gratings, holograms, and induction of liquid-crystal alignments [13]. Their photoisomerization process has been extensively researched due to both its practical and theoretical interest [14]. In particular, azopolymers

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are very attractive since they are thermally and mechanically more stable than their respective monomers and are the preferred choice for thin film fabrication [15]. The DMAAm copolymers bearing azo groups display dual temperature- and photo-responsive phase behavior [11]. They are water soluble and their composition can be easily adjusted to tailor their physicochemical properties. Herein, we report the synthesis of polyaniline colloidal particles by enzymatic polymerization using a temperature- and photo-responsive azopolymer, poly(DMA-co-AZAAm), as steric stabilizer. The adsorption of poly(DMA-co-AZAAm) confers photosensitivity to the polyaniline particles. The morphology of the colloids was studied by atomic force microscopy and scanning electron microscopy, and their chemical characterization was done by NMR, GPC, FTIR and UV–vis spectroscopic techniques. 2. Experimental Compounds were purchased from Sigma–Aldrich, Biochemika and J.T. Baker and used without further purification. NMR studies were carried out with a Varian Inova 200 instrument. Monomer and polymer were dissolved in CDCl3 (ca. 30–40 mg of monomer and 10 mg of polymer in 0.5 mL) in 5 mm (o.d.) tubes and measured at 25 ◦ C with TMS as internal standard. Chemical shifts are stated in parts per million. IR spectra in KBr disks (4000–500 cm−1 ) were recorded on a Bruker Vector 22 FT spectrophotometer with OPUS 5.5 software. Mass spectra were obtained on a High Resolution MStation JMS 700 JEOL spectrometer. Melting points were measured using a Electrothermal MEL-TEMP of 50/60 Hz and were uncorrected. Elemental analysis was obtained with a Multielemental Analyzer Vario EL. Molecular weights were determined by HPLC using a Waters 600 Controller, 996 Photodiode Array Detector, and a PL Gel column (300 mm × 7.5 mm, pore size of 10,000 Å (fractionation range 4000–40,000 molecular weight) and a GPC 5UM precolumn. The column was eluted with THF at 1.0 mL min−1 at 40 ◦ C. The samples concentration was around 2 g L−1 and the injection volume was 20 ␮L. UV–vis spectra were measured with a Thermo Electron Corporation GENESYS 10 UV Scanning apparatus with Vision Lite software. For irradiations, a Cole Parmer 9815 series lamp (115 V to 60 Hz, 6 W) was used. Polyaniline purification was carried out in a Beckman Coulter AllegraTM X-22 R Ultracentrifuge. Atomic force microscopy (AFM, nanoscope IV multimode scanning probe microscope) and scanning electron microscopy (SEM, LEO Mag = 2 KX, 6 KX; EHT = 6 kV, WD = 10 mm) were used to analyze the surface morphology of the polymer deposited on conductive glass substrates. Conductivity of solutions was measured with a Hanna Instrument HI 991301 Portable pH/EC/TDS/Temperature Meter. AZAAm monomer and poly(DMA-co-AZAAm) synthesis have been reported [11], but we report here some minor modifications, such as monomer/MEO/AIBN molar ratios, and a complete characterization of the products. 2.1. Synthesis of 4-phenylazophenylacrylamide (AZAAm) monomer 4-Phenylazophenylacrylamide was synthesized as follows: 4-aminoazobenzene (1.61 g, 8 mmol) and triethylamine (1.46 mL, 10.4 mmol) were dissolved in diethyl ether (40 mL). Acryloyl chloride (0.8 mL, 9.5 mmol) was dissolved in diethyl ether (15 mL) and added dropwise while stirring at 0 ◦ C under a nitrogen atmosphere. The reaction mixture was allowed to come to room temperature and stirring continued for 4 h. The reaction solution was filtered, then washed 3 times with water to remove triethylammonium chloride. The product was obtained by evaporation of solvent (1.49 g, 5.93 mmol) and dried under vacuum. Yield 76%, m.p. 158–160 ◦ C. Spectroscopic data: Anal. Calc. for C15 H13 N3 O: C 71.70%, N 16.72%, H 5.21%. Found: C 69.33%, N 16.78%, H 4.56%. MS (m/z) = 251 (M+ ), IR (KBr)  (cm−1 ): 3282 (NHst ), 1667 (NHC O, H-associate), 1545 (NHı), 3131, 3066 ( CH2 st), 1603 (C C). MS (EI, 70 eV): m/e = 791 (M+ ). 1 H NMR (200 MHz, CDCl3 , 20 ◦ C, TMS) ı = 7.3–7.9 (m, 9H, aromatic protons), 6.40 and 5.73 (dd, 2H, double bond terminal hydrogens, 2 JH–H(gem) = 1.5 Hz), 6.24 (dd, 1H, double bond hydrogen internal, 2 JH–H(cis) = 9.9 Hz, 2 JH–H(trans) = 16.9 Hz). 13 C NMR (200 MHz, CDCl3 , 20 ◦ C, TMS) ı = 163.8 (C O), 152.7, 149.2, 140.4 (Cipso ); 131.0, 129.2, 128.6, 124.1, 122.9, 122.4 (Caromatic ) 130.9, 120.2 (Cdouble bond ).

three times by precipitation into diethyl ether and dried under vacuum, 1.6 g of orange solid was obtained. M.W. (g mol−1 ), Mn = 1791, Mw = 2588, Mz = 3177, PD = 1.45. Spectroscopic data: IR (KBr)  (cm−1 ): 3465 (NHst and OHst ), 2927, 2862 (C–Hst ), 2372 (C Carmonics ), 1635 (C Ost ), 1499 (C–Cst ), 1255 (C–Nst ), 1145 (NHıip ), 689 (NHıoop ). 1 H NMR (200 MHz, CDCl3 , 20 ◦ C, TMS) ı = 7.4–8.0 (m broad, 9H, aromatic protons), 2.9 (broad, 6H, methyl groups), 3.0, 2.6, 2.4, 1.6 (broad, principal chain protons). 13 C NMR (200 MHz, CDCl3 , 20 ◦ C, TMS) ı = 174.8 (C O), 130.8, 129.2, 124.1, 122.8, 120.2 (aromatic carbons), 37.4, 36.0 (methyl carbons). 2.3. Enzymatic synthesis of poly(DMA-co-AZAAm)-stabilized polyaniline Polyaniline colloids were synthesized in dispersed media using aniline, horseradish peroxidase (HRP) as enzyme and poly(DMA-co-AZAAm) as steric stabilizer. The azopolymer (150 mg) was dissolved in deionized water (5 mL) and cooled to 5 ◦ C. The following reagents were then added to a final concentration 0.1 M: toluenesulfonic acid, ATS (95.1 mg, 0.5 mmol), aniline (46 ␮L, 0.5 mmol) and EDTA (1 mg). The mixture was stirred for a few minutes and the horseradish peroxidase enzyme, HRP (1.25 mg, 0.25 mg mL−1 ) was added. To start the polymerization three additions of 0.1 M H2 O2 (48 ␮L in 5 mL of distilled water) were added under vigorous magnetic stirring over the course of 2 h at 5 ◦ C, after which 1 mL of 1 M HCl was added. Polyaniline particles were purified using several centrifugation–redispersion cycles and repeated water washing. Additionally, 0.1 N NH4 OH solution was used for dedoping (i.e., removal of the toluenesulfonic acid and therefore conversion of the polyaniline to its electrically nonconductive form). M.W. (g mol−1 ), Mn = 1070, Mw = 2044, Mz = 2991, PD = 1.91. Spectroscopic data: IR (KBr)  (cm−1 ): 3446 (NHst and OHst ), 2925 (C–Hst ), 2370 (C Carmonics ), 1632 (C Ost and C–Cst ), 1501 (C–Cst ), 1402 (C Nst ), 1301 and 1252 (C–Nst ), 1143 (N–Hıip or C–Hıip ), 827 (C–Hıoop ), 687 (N–Hıoop ). Polyaniline (reference polymer) was synthesized in a similar way but without DMA-co-AZAAm. IR (KBr)  (cm−1 ): 2923 (C–Hst ), 2373 (C Carmonics ), 1625 and 1500 C–Cst , 1400 (C Nst ), 1119 (N–Hıip or C–Hıip ), 992 (C–Hıoop ), 683 (N–Hıoop ). 2.4. UV–vis spectra The solutions of poly(DMA-co-AZAAm) (60 ␮L at 3 mg mL−1 ) and poly(DMAco-AZAAm)-stabilized polyaniline (0.28 mL at 0.56 mg mL−1 ) in water in a sealed quartz cuvette were exposed to UV light at 365 nm. UV–vis absorption spectra were sequentially recorded after every 12 and 20 s of exposure for both polymer solutions.

3. Results and discussion 3.1. Characterization by UV–vis and FT-IR spectroscopy of poly(DMA-co-AZAAm) and poly(DMA-co-AZAAm)-stabilized polyaniline During the synthesis of polyaniline colloids with poly(DMAco-AZAAm) as steric stabilizer the solution changed in color from orange to brown and finally dark green. After polymerization, the

2.2. Synthesis of temperature-responsive poly(DMA-co-AZAAm) The temperature responsive copolymer poly(DMA-co-AZAAm) was synthesized with a hydroxyl terminus by free-radical polymerization in DMF at 60 ◦ C for 20 h, using 2-mercaptoethanol (MEO) as a chain transfer reagent and 2,2 -azobisbutyronitrile (AIBN) as an initiator (monomer concentration = 2 mol L−1 ). The radio of N,N-dimethyl acrylamide (DMA) (2.4 mL, 22.8 mmol) and 4phenylazophenylacrylamide (AZAAm) (0.56 g, 2.25 mmol) was 91:9. The monomer/MEO/AIBN molar ratio was 100/2/0.4 (MEO (35 ␮L, 0.5 mmol) and AIBN (16.4 mg, 0.1 mmol)). The system was evacuated three times and a nitrogen flow was maintained for 20 min before heating. After 20 h, the product was purified

Fig. 1. UV–vis spectra of (a) polyaniline, (b) poly(DMA-co-AZAAm) and (c) poly(DMA-co-AZAAm)-stabilized polyaniline particles.

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Fig. 2. Infrared spectra of (a) poly(DMA-co-AZAAm) and (b) poly(DMA-co-AZAAm)-stabilized polyaniline particles.

polyaniline were isolated from the steric stabilizer solution by several centrifugation–redispersion cycles. UV–vis spectra of these polymers are presented in Fig. 1. For polyaniline the sample showed two transitions at 381 nm and 684 nm due to the ␲ → ␲* transition and the benzenoid–quinoid transition, respectively [7a,16]. In the case of poly(DMA-coAZAAm)-stabilized polyaniline particles, two bands at 340 and 601 nm were observed. The band at 340 nm corresponds to trans azo isomer of poly(DMA-co-AZAAm). The Fourier transform infrared (FT-IR) spectra of the polyaniline in its emeraldine base form with poly(DMA-co-AZAAm) are shown in Fig. 2. The characteristic peaks from both spectra are:

the peaks at 2927 and 2862 cm−1 corresponding to the C–H stretching mode, the peak at 1632 cm−1 corresponding to C O stretching mode, the peak at 1402 cm−1 corresponding to the C N stretching mode, the peak at 1143 cm−1 due to the N–H or C–H in plane bending mode, and the peak at 827 cm−1 representing the out of plane bending mode. The peaks around 1632 and 1501 cm−1 are assigned to the C–C stretching modes of the benzenoid ring and the quinoid ring. These signals confirm a 1–4 substitution pattern of the aromatic ring and an emeraldine oxidation state of the polymer because no peaks due to ortho coupling or branching were observed, in agreement with previous studies [10].

Fig. 3. UV–vis spectra of (a) poly(DMA-co-AZAAm) and (b) poly(DMA-co-AZAAm)-stabilized polyaniline particles.

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Fig. 4.

1

H NMR of poly(DMA-co-AZAAm) in CDCl3 solution after irradiations.

3.2. Photoinduced properties: UV–vis and 1 H NMR analysis of poly(DMA-co-AZAAm) and poly(DMA-coAZAAm)-stabilized polyaniline Fig. 3 shows the changes in the absorption spectra of poly(DMAco-AZAAm) and poly(DMA-co-AZAAm)-stabilized polyaniline in water during irradiation by UV light at 365 nm. As shown for the poly(DMA-co-AZAAm) spectra, the absorption band at around 346 nm due to the ␲ → ␲* transition of the trans form of the azo bond decreases gradually (max changes from 346 to 335 nm), while the absorption band at 437 nm due to the n → ␲* transition of the cis form of the azo bond increases with light irradiation time. Two apparent isosbestic points are seen at 296 and 414 nm in the copolymer spectra. It takes ∼2 min of total exposure, under these conditions, to complete the photoinduced trans to cis isomerization for the copolymer solution. After irradiation the molecules tend to relax back to the conformations they had before irradiation if the solution is left under atmospheric conditions. However the trans population is not recovered as observed in the UV–vis spectra. Macroscopic properties of this copolymer solutions are not modi-

fied because the change in color is not remarkable due the lower absortivity of the cis form; secondly, because this isomeric form is very unstable and can only be conserved in the darkness during a maximum of two hours before returning to the trans form, which is more stable [17]. This photoinduced trans–cis isomerization was analyzed in the poly(DMA-co-AZAAm)-stabilized polyaniline particles as presented in Fig. 3. It was interesting to verify that these nanoparticles acquired this optical property with the incorporation of poly(DMAco-AZAAm). For this reason the absorption band at 340 nm, due to azopolymer, decreases gradually while the absorption band at 604 nm, due to polyaniline, remains unchanged. Under this polyaniline band is hidden the smaller band at 437 nm due to the cis form of azopolymer. However, poly(DMA-co-AZAAm)-stabilized polyaniline particles did not show the drastic changes with the temperature that we expected. This smart copolymer, poly(DMA-co-AZAAm), has been used to switch reversibly between the expanded and collapsed states in response to temperature changes above and below the lower critical solution temperature in the range of 40–45 ◦ C, pH 5.5

Fig. 5. AFM phase images of (a) poly(DMA-co-AZAAm)-stabilized polyaniline and (b) polyaniline particles.

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Fig. 6. SEM images of poly(DMA-co-AZAAm)-stabilized polyaniline: a (2 KX) and a (10 KX) and polyaniline particles: b (2KX) and b (10 KX), respectively.

[11]. In this work, we observed that poly(DMA-co-AZAAm) begins to precipitate around 60 ◦ C in aqueous solution. 1 H NMR spectra of DMA-co-AZAAm copolymer (aromatic zone) after irradiation at 365 nm are presented in Fig. 4. The C signal increases in proportion to the decrease of the A signal (probably ortho protons are protected because of steric effects in the cis form). The B signal diminished only 1%. These characteristics are due to the presence of a cis–trans azo isomer of poly(DMA-co-AZAAm). 3.3. Conductivity: solution analysis Measurements carried out in solutions with deionized water gave the following values: polyaniline has 0 mS cm−1 (pH 7.2), poly(DMA-co-AZAAm) 0.038 mS cm−1 (pH 6.9), poly(DMA-coAZAAm)-stabilized polyaniline has the highest value, 0.170 mS cm−1 (pH 8.0). These results agree with data reported by Kumar et al. and Sulimenko et al. [18], where a lower pH (4.0–4.5) was required to produce the conducting polyaniline whereas a pH of 6.0 or higher results in a more branched, insulating form of polyaniline. 3.4. Morphology: AFM and SEM analysis of poly(DMA-co-AZAAm) and poly(DMA-co-AZAAm)-stabilized polyaniline The morphology of the polyaniline colloidal particles was characterized by atomic force microscopy (AFM) and is shown in Fig. 4. By using poly(DMA-co-AZAAm) at 30 mg mL−1 spherical particles of approximately 250 nm diameter (Fig. 5) were obtained. These particles sometimes gave conglomerates with a horizontal distance average of 1.3 ␮m (1.1–1.8 ␮m) and the vertical distance average of 190 nm (128–313 nm). However, synthesizing polyaniline without steric stabilizer led to irregular particles with different sizes, from 357 to 735 nm in diameter that also formed conglomerates with a

horizontal distance average of 1.7 ␮m (1.2–2.3 ␮m) and a vertical distance average of 427 nm (330–653 nm). SEM micrographs confirmed the particle sizes as shown in Fig. 6. The deviation from the spherical shape in polyaniline colloids prepared by dispersion polymerization has been attributed either to an excessively high particle growth rate or to a low steric stabilizer absortion rate [6]. In addition, it has been shown previously that the aniline oxidation rate is a key parameter in controlling the shape of polyaniline colloids [19]. In this context, Cruz-Silva et al. showed that the enzymatic polymerization of aniline gives a longer and smoother oxidation stage [10] than chemical oxidation, which makes enzymatic oxidation a very suitable pathway for aniline dispersion polymerization. 4. Conclusions In summary, an enzymatic route for the synthesis of photosensitive polyaniline colloidal particles using the azopolymer DMA-co-AZAAm as stabilizer is presented. This azopolymer protects the particles from additional aggregation and gives optical properties to them. When the azobenzene group is incorporated into polyaniline nanoparticles, a wide range of possible consequences such as photoalignment, birefringence and non-linear optical effects are originated by its photoisomerization. Acknowledgments The authors thank María Gregoria Medina Pintor for FTIR spectra, Daniel Bahena Uribe for his help with AFM images, Rene Guardían Tapia for SEM micrographs and Michael Dunn for reading the manuscript. This project was funded by PROMEP/103.5/07/2674 project and CONACYT CB2007-81383-Q.

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