- Email: [email protected]
Adjusting the inner-structure of polypyrrole nanoparticles through microemulsion polymerization
Materials Chemistry and Physics 98 (2006) 304–308
Adjusting the inner-structure of polypyrrole nanoparticles through microemulsion polymerization Yang Liu, Ying Chu ∗ , Likun Yang Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China Received 14 April 2005; received in revised form 24 August 2005; accepted 7 September 2005
Abstract The inner-structure of polypyrrole (PPy) nanoparticles can be straightly adjusted by a simple alcohol-assisted microemulsion polymerization. The characteristic peaks of PPy in infrared spectroscopy (IR) confirm the formation of PPy. The morphology of PPy nanoparticles was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). UV–vis and Raman spectra exhibit that the conjugation and order of PPy chains were improved directly in the presence of alcohols and adjusted by the alkyl unit of alcohols. These changes in the molecular structure lead to the improvement in the electrical conductivity of as-prepared PPy nanoparticles obviously. © 2005 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Nanoparticles; Microemulsion polymerization; Alcohol; Conjugation
1. Introduction Conducting polymers with conjugated double bonds have been attracted much attention as advanced materials. Polypyrrole (PPy) has received widespread interest due to its good environmental stability, facile synthesis, and higher conductivity than many others [1,2]. It could always be used in drug delivery, rechargeable batteries, supercapacitors, sensors, anhydrous electrorheological fluids, microwave shielding and corrosion protection [3–6]. PPy can be prepared by either oxidatively chemical or electrochemical polymerization of pyrrole. However, many factors influence the properties of PPy such as the nature of solvent, polymerization method and rate, deposition time, pH in solution, monomer and electrolyte concentrations in polymerization [7–9]. Furthermore, as shown in the literature, synthetically conducting PPy is insoluble and infusible, which not only limits its processing and applications in many fields, but also restricts the investigations on the relationship between properties and molecular structure of PPy [10]. Although series of relevant efforts have been paid on the structural changes of PPy [1,11–13], few works
∗
Corresponding author. Tel.: +86 4315269667; fax: +86 4315684009. E-mail address: [email protected] (Y. Chu).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.09.025
focus on straightly controlling the molecular structure. How to adjust the inner-structure of PPy is still in the developing stage. As ideal media for the synthesis of nanoparticles, microemulsion systems have been widely employed to prepare superfine particles [14]. Recently, Wan and co-workers have reported the synthesis of polyanline (PAni) and PPy micro/nanotubules and the self-assembling of sub-micrometer-size tube junction of conducting polymers by using surfactant [15,16]. Jang et al. have fabricated novel crystalline superamolecular assemblies of amorphous PPy nanoparticles and PPy nanotubes through surfactant templating [17,18]. Using adsorbed surfactants as templates, Grady and co-workers have synthesized morphologically controlled PAni and PPy nanostructures on flat surfaces [19]. The preparation of PAni-inorganic nanocomposites has also been accomplished in reverse micelle [20,21]. These achievements indicate that the microemulsion systems are powerful for the fabrication of polymer nanostructures. Furthermore, in aqueous systems it is generally accepted that the presence of alcohols can change the growth and shape of micelles, as well as the dielectric properties of the micellar interface and the molecular order of the interface region [22,23]. In this communication, we developed a simple alcohol-assisted micellar method, by which the molecular structure of the obtained PPy nanoparticles (average diameter about 30 nm) can be adjusted straightly and easily.
Y. Liu et al. / Materials Chemistry and Physics 98 (2006) 304–308
305
2. Experimental 2.1. Materials All of the reactants and solvents are analytical-grade and used without any further purification. Pyrrole was supplied by Shanghai Chemical Reagent Company. Ammonium peroxydisulfate ((NH4 )2 S2 O8 ) was used as oxidant and sodium dodecyl sulfate (SDS) was used as surfactant, which were purchased from Beijing Chemical Reagent Company. Other chemicals were supplied by Changchun Fine Chemical Company.
2.2. Preparation of PPy nanoparticles The typical approach employed is briefly as follows. Three millimoles SDS was dissolved in 30 ml distilled water and stirred vigorously to ensure the complete dissolution of surfactant. Subsequently, to this solution was added a mixture of 0.2 ml pyrrole and 0.4 ml n-amyl alcohol. After a substantial stirring for 30 min, 5.8 ml of 0.5 M aqueous solution of (NH4 )2 S2 O8 was added into above solution. The stirring was carried out for 12 h. At the end of this period the reaction mixture turned black. PPy precipitates were separated from the reaction media by adding excess methanol and filtration. Samples were collected and washed with methanol and distilled water for several times. Finally, the products were dried for 5 h under vacuum at 60–70 ◦ C. In order to study the influence of alcohols on the properties of PPy nanoparticles, different type of alcohols (n-amyl alcohol, iso-amyl alcohol, n-butyl alcohol, iso-butyl alcohol and tert-butyl alcohol) were used in our experiment. PPy nanoparticles were also prepared without using alcohol.
2.3. Characterization FTIR analysis was carried out on a Mafna FT-IR 760 spectrophotometer. Transmission spectra of the samples were obtained by forming a thin transparent KBr pellet containing the materials. Electron microscopy studies were carried out with transmission electron microscope (TEM, JEM-2010, 200 kV) and scanning electron microscope (SEM, XL30 S-FEG, 20 kV). Samples for TEM were deposited onto carboncoated, 3 mm diameter, copper electron microscope grids, and then dried in air. Samples for SEM were mounted onto circular aluminum stubs, air-dried and gold-coated with sputter-coater. UV–vis absorption spectra were measured in transmission by a 765MC UV–vis spectrometer using a 1 cm quartz cuvette. Raman spectra were obtained using a Renishaw 2000 Raman spectrometer employing a He–Ne laser operating at 632.8 nm and 25 mW and a charge coupled device (CCD) detector with 1 cm−1 resolution. Dry PPy powder synthesized under different reaction condition was compressed into disk pellets, 13 mm in diameter and 1 nm thick, and the electrical conductivity of the product was measured on these pellets employing a fourprobe technique with the help of a programmable current/voltage generator (Advantest R6142) and a precision digital voltmeter (Solartron SI 7071) under laboratory condition.
3. Results and discussion A key property of a conducting polymer is the presence of conjugated double bond, which gives rise to the electrical conductivity as it allows the efficient transfer of electrons or positive charges along the polymer backbone. Well conjugation (increasing conjugation length, i.e. number of monomer units and improving ordered degree) throughout polymer chain will allow charge to migrate a long distance. In our experiments, the inner-structure of PPy nanoparticles can be straightly adjusted by a simple alcohol-assisted microemulsion polymerization. The conjugation degree of PPy chains was improved by introducing alcohols into the reaction media.
Fig. 1. IR spectra of PPy nanoparticles obtained in the presence of (a) n-amyl alcohol, (b) iso-amyl alcohol, (c) n-butyl alcohol, (d) tert-butyl alcohol and (e) in the absence of alcohol.
3.1. FTIR analysis The FTIR spectra of the products obtained in the presence of different alcohols as well as the product obtained in the absence of alcohols were shown in Fig. 1. All these spectra show characteristic PPy peaks at 1457, 1303 and 1175 cm−1 , which are due to C C ring stretching, C H in-plane and C N stretching vibrations, respectively [24,25]. The peak around 1549 cm−1 is attributed to C C backbone stretching, which is weak in spectra (e). Peaks ranging 1920–2960 cm−1 are assigned to aliphatic C H stretching mode, depending on long alkyl tail of SDS. The characteristic peaks of alcohols could not be observed in all of the spectra, indicating that alcohols have been completely eliminated from the final PPy nanostructures. 3.2. TEM and SEM The photographs in Fig. 2 represent the general morphological features of the PPy nanoparticles obtained without using alcohol during the experiment process. The formation of spherical particles with average particle diameter ranging from 50 to 70 nm is clearly confirmed by TEM and SEM images. As shown in Fig. 2, these particles conglutinate together. Fig. 3 shows the typical TEM images of PPy nanoparticles obtained when n-amyl alcohol was introduced to the experiment process. As shown in Fig. 3a, the uniform particles are monodisperse and have average diameter of about 30 nm. The observation also reveals the existence of short rods that have almost the same diameter with that of particles (Fig. 3b). The formation of the rods might be driven by the interactions among the polymer chain, including – interactions, hydrogen bonds, and even ionic bonds [16,26,27]. 3.3. UV–vis analysis To our surprised, the inner-structure of the PPy nanoparticles as prepared can be adjusted by the introduction of alcohols. As
306
Y. Liu et al. / Materials Chemistry and Physics 98 (2006) 304–308
Fig. 2. (a) TEM and (b) SEM images of PPy nanoparticles synthesized in the absence of alcohol.
Fig. 3. (a and b) TEM images of PPy nanoparticles in the presence of n-amyl alcohol.
discussed in other works, the typical –* transition peak of PPy presents at 420 nm [28,29]. While in the presence of namyl alcohol, the –* transition peak shifts significantly to 473 nm. The red shift of the –* transition is due to the well conjugation and few contortion of PPy chains. The short-chain n-alcohols (C4 –C8 ) have the similar effects on the molecular structure of PPy nanoparticles. However, the further experiments exhibit that the alkyl unit of alcohols strongly affect the red shift of –* transition. The peak moves to 455 nm when iso-amyl alcohol employed, and using n-butyl alcohol, iso-butyl alcohol and tert-butyl alcohol, the peaks appear at 472, 458 and 437 nm, respectively (Fig. 4). So, it can be seen that alcohols may adjust the conjugation and order of PPy chains directly. This can be proved further by Raman shift. 3.4. Raman shift The scattering peak around 1600 cm−1 in Raman shift is assigned as C C bonds in PPy, which is considered to be an
Fig. 4. UV–vis spectra of PPy nanoparticles synthesized in the presence of different alcohols: (a) n-amyl alcohol, (b) iso-amyl alcohol, (c) n-butyl alcohol, (d) iso-butyl alcohol and (e) tert-butyl alcohol.
Y. Liu et al. / Materials Chemistry and Physics 98 (2006) 304–308
307
Table 1 Conductivity of PPy nanoparticles synthesized in the presence of different alcohols Alcohol
Conductivity
(S cm−1 )
n-Amyl alcohol
iso-Amyl alcohol
n-Butyl alcohol
iso-Butyl alcohol
tert-Butyl alcohol
None
9.60
5.77
8.72
5.83
1.81
0.067
skeletal band (i.e., around 1500 cm−1 ) also gives a qualitative measurement of the conjugation length. A higher ratio means a longer conjugation length [31]. The same conclusion that the conjugation could be adjusted by different alcohols can also be drawn by calculating and comparing these ratios. 3.5. Conductivity
Fig. 5. Raman spectra of PPy nanoparticles synthesized: (a) and (b) in the presence of n- and iso-amyl alcohol, respectively, and (c) in the absence of alcohol.
overlap of the two oxidized structures, and shifts to a higher wave number if the conjugation length is shortened [28,30]. This peak of PPy synthesized without using alcohols presents at 1624 cm−1 and moves to 1594 cm−1 to show lengthening of conjugation after the introduction of n-amyl alcohol during the synthesizing process. When iso-amyl alcohol was selected, the peak moves to 1608 cm−1 . From Figs. 5a, b and 6 it can also be seen that alkyl unit of alcohols strongly affect the conjugation length of PPy chains. Furthermore, calculating the band ratio between the intensity of a band sensitive to the oxidation state of the polymer (i.e., around 1600 cm−1 ) and the intensity of the
The electrical properties of conducting polymers are determined by their structure [10]. Tian and Zerbi [32] proposed the theory of effective conjugation coordinate to explain the correlation between structure and conductivity of polymer. A reduction of the conjugation length will result in the decrease of the conductivity. Although anionic surfactants is not helpful to the increase in the conductivity when only (NH4 )2 S2 O8 is used as the oxidant [10], the conductivity is changed from 0.067 to 9.60 S cm−1 by the introduction of n-amyl alcohol into the reaction system. It is evident from Table 1 that the conductivity of PPy nanoparticles can be improved obviously by adding alcohols into the reaction system and the alkyl units of alcohols also affect the conductivity strongly. This phenomenon agrees with the results of UV–vis and Raman spectra very well. Improving this method to obtain PPy particles with much higher conductivity is very necessary. Introducing other suitable surfactants and lowering reaction temperature may benefit the process. 3.6. Adjusting mechanism Micelles will be formed in aqueous solution when the concentration of SDS comes to 0.1 M. Py monomer has a good solubility in alcohols we used, and it will be dispersed very well in the micellar system in the presence of alcohols. At the same time the alcohols make Py monomer dilute, which may be helpful to the formation of well conjugation. Furthermore, short-chain alcohols can also act as effective cosurfactant to further lower the interfacial energy and optimize the structure of PPy chains during the interfacial polymerization. However, the branched-chain alcohols with bigger volume alkyl units are unfavorable for their entering into the “oil pool” and not effective as cosurfactant either. Although the fundamental basis of structural adjustment for this system has not yet been fully understood, it may be important that the interaction between hydroxyl of alcohols and nitrogen atom of Py also plays the major role in determining the ordered molecular structure in PPy nanoparticles [33]. 4. Conclusion
Fig. 6. Raman spectra of PPy nanoparticles synthesized in the presence of different alcohols: (a) n-butyl alcohol (b) iso-butyl alcohol (c) tert-butyl alcohol.
In summary, inner-structure of PPy nanoparticles can be adjusted directly by a simple microemulsion polymerization at
308
Y. Liu et al. / Materials Chemistry and Physics 98 (2006) 304–308
room temperature. Alcohols used in the experiments have an important influence on the inner-structure and property of PPy nanoparticles. It will give birth to a promising method to the controllable synthesis of other conducting polymers with high conductivity. Acknowledgments This work was supported by the National Nature Science Foundation of China, No. 20173008. We also thank Prof. Yichun Liu for his professional communication. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Y.C. Liu, C.J. Tsai, Chem. Mater. 15 (2003) 320. B.R. Saunders, R.S. Fleming, K.S. Murray, Chem. Mater. 7 (1995) 1082. J.M. Pernaut, J.R. Reynolds, J. Phys. Chem. B 17 (2000) 4080. K. Jurewicz, S. Delpeux, V. Bertagna, F. Beguin, E. Frackowiak, Chem. Phys. Lett. 347 (2001) 36. C.W. Lin, B.J. Hwang, C.R. Lee, Mater. Chem. Phys. 55 (1998) 139. J.W. Goodwin, G.M. Markham, B. Vinent, J. Phys. Chem. B 101 (1997) 1961. K. Dhanalakshmi, R. Saraswathi, C. Srinivasan, Synth. Met. 82 (1996) 237. B.J. Hwang, K.L. Lee, Thin Solid Films 279 (1996) 236. K.G. Neoh, E.T. Kang, K.L. Tan, J. Phys. Chem. B 101 (1997) 726. L.X. Wang, X.G. Li, Y.L. Yang, React. Funct. Polym. 47 (2001) 125. S. Chakrabarti, D. Banerjee, R. Bhattacharyya, J. Phys. Chem. B 106 (2002) 3061.
[12] A.N. Zelikin, D.M. Lynn, J. Farhadi, I. Martin, V. Shastri, R. Langer, Angew. Chem. Int. Ed. 41 (2002) 141. [13] X.J. Zhou, K.T. Leung, Macromolecules 36 (2003) 2882. [14] Y. Liu, Y. Chu, Mater. Chem. Phys. 92 (2005) 59. [15] Z. Wei, Z. Zhang, M. Wan, Langmuir 18 (2002) 917. [16] Z.X. Wei, L.J. Zhang, M. Yu, Y.S. Yang, M.X. Wan, Adv. Mater. 15 (2003) 1382. [17] J. Jang, J.H. Oh, Chem. Commun. (2002) 2200. [18] J. Jang, H. Yoon, Chem. Commun. (2003) 720. [19] A.D.W. Carswell, E.A. O’Rear, B.P. Grady, J. Am. Chem. Soc. 125 (2003) 14793. [20] X.M. Sui, Y. Chu, S.X. Xing, C.Z. Liu, Mater. Lett. 58 (2004) 1255. [21] X.M. Sui, Y. Chu, S.X. Xing, Colloids Surf. A: Physicochem. Eng. Aspects 251 (2004) 103. [22] M.S. Akhter, Colloids Surf. A 157 (1999) 203. [23] Z.H. Kang, E.B. Wang, M. Jiang, S.Y. Lian, Nanotechnology 15 (2004) 55. [24] C. He, C.H. Yang, Y.F. Li, Synth. Met. 139 (2003) 539. [25] G.I. Mathis, V.-T. Troung, Synth. Met. 89 (1997) 103. [26] J. Jin, T. Iyoda, C. Cao, Y. Song, L. Jiang, T. Li, D. Zhu, Angew. Chem. Int. Ed. 40 (2001) 2135. [27] H. Kosonen, J. Ruokolainen, M. Knaapila, M. Torkkeli, K. Jokela, R. Serimaa, G. Brinke, W. Bras, A.P. Monkman, O. Ikkala, Macromolecules 33 (2000) 8671. [28] D.Y. Kim, J.Y. Lee, D.K. Moon, C.Y. Kim, Synth. Met. 69 (1995) 471. [29] W.S. Huang, A.G. MacDiarmid, Polymer 34 (1993) 1833. [30] Y.C. Liu, Electroanalysis 15 (2003) 1134. [31] J. Duchet, R. Legras, S. Demoustier-Champagne, Synth. Met. 98 (1998) 113. [32] B. Tian, G. Zerbi, J. Chem. Phys. 92 (1990) 3892. [33] G. Zotti, S. Zecchin, G. Schiavon, B. Vercelli, A. Berlin, E. Dalcanale, L.B. Groenendaal, Chem. Mater. 15 (2003) 4642.
Adjusting the inner-structure of polypyrrole nanoparticles through microemulsion polymerization Yang Liu, Ying Chu ∗ , Likun Yang Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China Received 14 April 2005; received in revised form 24 August 2005; accepted 7 September 2005
Abstract The inner-structure of polypyrrole (PPy) nanoparticles can be straightly adjusted by a simple alcohol-assisted microemulsion polymerization. The characteristic peaks of PPy in infrared spectroscopy (IR) confirm the formation of PPy. The morphology of PPy nanoparticles was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). UV–vis and Raman spectra exhibit that the conjugation and order of PPy chains were improved directly in the presence of alcohols and adjusted by the alkyl unit of alcohols. These changes in the molecular structure lead to the improvement in the electrical conductivity of as-prepared PPy nanoparticles obviously. © 2005 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Nanoparticles; Microemulsion polymerization; Alcohol; Conjugation
1. Introduction Conducting polymers with conjugated double bonds have been attracted much attention as advanced materials. Polypyrrole (PPy) has received widespread interest due to its good environmental stability, facile synthesis, and higher conductivity than many others [1,2]. It could always be used in drug delivery, rechargeable batteries, supercapacitors, sensors, anhydrous electrorheological fluids, microwave shielding and corrosion protection [3–6]. PPy can be prepared by either oxidatively chemical or electrochemical polymerization of pyrrole. However, many factors influence the properties of PPy such as the nature of solvent, polymerization method and rate, deposition time, pH in solution, monomer and electrolyte concentrations in polymerization [7–9]. Furthermore, as shown in the literature, synthetically conducting PPy is insoluble and infusible, which not only limits its processing and applications in many fields, but also restricts the investigations on the relationship between properties and molecular structure of PPy [10]. Although series of relevant efforts have been paid on the structural changes of PPy [1,11–13], few works
∗
Corresponding author. Tel.: +86 4315269667; fax: +86 4315684009. E-mail address: [email protected] (Y. Chu).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.09.025
focus on straightly controlling the molecular structure. How to adjust the inner-structure of PPy is still in the developing stage. As ideal media for the synthesis of nanoparticles, microemulsion systems have been widely employed to prepare superfine particles [14]. Recently, Wan and co-workers have reported the synthesis of polyanline (PAni) and PPy micro/nanotubules and the self-assembling of sub-micrometer-size tube junction of conducting polymers by using surfactant [15,16]. Jang et al. have fabricated novel crystalline superamolecular assemblies of amorphous PPy nanoparticles and PPy nanotubes through surfactant templating [17,18]. Using adsorbed surfactants as templates, Grady and co-workers have synthesized morphologically controlled PAni and PPy nanostructures on flat surfaces [19]. The preparation of PAni-inorganic nanocomposites has also been accomplished in reverse micelle [20,21]. These achievements indicate that the microemulsion systems are powerful for the fabrication of polymer nanostructures. Furthermore, in aqueous systems it is generally accepted that the presence of alcohols can change the growth and shape of micelles, as well as the dielectric properties of the micellar interface and the molecular order of the interface region [22,23]. In this communication, we developed a simple alcohol-assisted micellar method, by which the molecular structure of the obtained PPy nanoparticles (average diameter about 30 nm) can be adjusted straightly and easily.
Y. Liu et al. / Materials Chemistry and Physics 98 (2006) 304–308
305
2. Experimental 2.1. Materials All of the reactants and solvents are analytical-grade and used without any further purification. Pyrrole was supplied by Shanghai Chemical Reagent Company. Ammonium peroxydisulfate ((NH4 )2 S2 O8 ) was used as oxidant and sodium dodecyl sulfate (SDS) was used as surfactant, which were purchased from Beijing Chemical Reagent Company. Other chemicals were supplied by Changchun Fine Chemical Company.
2.2. Preparation of PPy nanoparticles The typical approach employed is briefly as follows. Three millimoles SDS was dissolved in 30 ml distilled water and stirred vigorously to ensure the complete dissolution of surfactant. Subsequently, to this solution was added a mixture of 0.2 ml pyrrole and 0.4 ml n-amyl alcohol. After a substantial stirring for 30 min, 5.8 ml of 0.5 M aqueous solution of (NH4 )2 S2 O8 was added into above solution. The stirring was carried out for 12 h. At the end of this period the reaction mixture turned black. PPy precipitates were separated from the reaction media by adding excess methanol and filtration. Samples were collected and washed with methanol and distilled water for several times. Finally, the products were dried for 5 h under vacuum at 60–70 ◦ C. In order to study the influence of alcohols on the properties of PPy nanoparticles, different type of alcohols (n-amyl alcohol, iso-amyl alcohol, n-butyl alcohol, iso-butyl alcohol and tert-butyl alcohol) were used in our experiment. PPy nanoparticles were also prepared without using alcohol.
2.3. Characterization FTIR analysis was carried out on a Mafna FT-IR 760 spectrophotometer. Transmission spectra of the samples were obtained by forming a thin transparent KBr pellet containing the materials. Electron microscopy studies were carried out with transmission electron microscope (TEM, JEM-2010, 200 kV) and scanning electron microscope (SEM, XL30 S-FEG, 20 kV). Samples for TEM were deposited onto carboncoated, 3 mm diameter, copper electron microscope grids, and then dried in air. Samples for SEM were mounted onto circular aluminum stubs, air-dried and gold-coated with sputter-coater. UV–vis absorption spectra were measured in transmission by a 765MC UV–vis spectrometer using a 1 cm quartz cuvette. Raman spectra were obtained using a Renishaw 2000 Raman spectrometer employing a He–Ne laser operating at 632.8 nm and 25 mW and a charge coupled device (CCD) detector with 1 cm−1 resolution. Dry PPy powder synthesized under different reaction condition was compressed into disk pellets, 13 mm in diameter and 1 nm thick, and the electrical conductivity of the product was measured on these pellets employing a fourprobe technique with the help of a programmable current/voltage generator (Advantest R6142) and a precision digital voltmeter (Solartron SI 7071) under laboratory condition.
3. Results and discussion A key property of a conducting polymer is the presence of conjugated double bond, which gives rise to the electrical conductivity as it allows the efficient transfer of electrons or positive charges along the polymer backbone. Well conjugation (increasing conjugation length, i.e. number of monomer units and improving ordered degree) throughout polymer chain will allow charge to migrate a long distance. In our experiments, the inner-structure of PPy nanoparticles can be straightly adjusted by a simple alcohol-assisted microemulsion polymerization. The conjugation degree of PPy chains was improved by introducing alcohols into the reaction media.
Fig. 1. IR spectra of PPy nanoparticles obtained in the presence of (a) n-amyl alcohol, (b) iso-amyl alcohol, (c) n-butyl alcohol, (d) tert-butyl alcohol and (e) in the absence of alcohol.
3.1. FTIR analysis The FTIR spectra of the products obtained in the presence of different alcohols as well as the product obtained in the absence of alcohols were shown in Fig. 1. All these spectra show characteristic PPy peaks at 1457, 1303 and 1175 cm−1 , which are due to C C ring stretching, C H in-plane and C N stretching vibrations, respectively [24,25]. The peak around 1549 cm−1 is attributed to C C backbone stretching, which is weak in spectra (e). Peaks ranging 1920–2960 cm−1 are assigned to aliphatic C H stretching mode, depending on long alkyl tail of SDS. The characteristic peaks of alcohols could not be observed in all of the spectra, indicating that alcohols have been completely eliminated from the final PPy nanostructures. 3.2. TEM and SEM The photographs in Fig. 2 represent the general morphological features of the PPy nanoparticles obtained without using alcohol during the experiment process. The formation of spherical particles with average particle diameter ranging from 50 to 70 nm is clearly confirmed by TEM and SEM images. As shown in Fig. 2, these particles conglutinate together. Fig. 3 shows the typical TEM images of PPy nanoparticles obtained when n-amyl alcohol was introduced to the experiment process. As shown in Fig. 3a, the uniform particles are monodisperse and have average diameter of about 30 nm. The observation also reveals the existence of short rods that have almost the same diameter with that of particles (Fig. 3b). The formation of the rods might be driven by the interactions among the polymer chain, including – interactions, hydrogen bonds, and even ionic bonds [16,26,27]. 3.3. UV–vis analysis To our surprised, the inner-structure of the PPy nanoparticles as prepared can be adjusted by the introduction of alcohols. As
306
Y. Liu et al. / Materials Chemistry and Physics 98 (2006) 304–308
Fig. 2. (a) TEM and (b) SEM images of PPy nanoparticles synthesized in the absence of alcohol.
Fig. 3. (a and b) TEM images of PPy nanoparticles in the presence of n-amyl alcohol.
discussed in other works, the typical –* transition peak of PPy presents at 420 nm [28,29]. While in the presence of namyl alcohol, the –* transition peak shifts significantly to 473 nm. The red shift of the –* transition is due to the well conjugation and few contortion of PPy chains. The short-chain n-alcohols (C4 –C8 ) have the similar effects on the molecular structure of PPy nanoparticles. However, the further experiments exhibit that the alkyl unit of alcohols strongly affect the red shift of –* transition. The peak moves to 455 nm when iso-amyl alcohol employed, and using n-butyl alcohol, iso-butyl alcohol and tert-butyl alcohol, the peaks appear at 472, 458 and 437 nm, respectively (Fig. 4). So, it can be seen that alcohols may adjust the conjugation and order of PPy chains directly. This can be proved further by Raman shift. 3.4. Raman shift The scattering peak around 1600 cm−1 in Raman shift is assigned as C C bonds in PPy, which is considered to be an
Fig. 4. UV–vis spectra of PPy nanoparticles synthesized in the presence of different alcohols: (a) n-amyl alcohol, (b) iso-amyl alcohol, (c) n-butyl alcohol, (d) iso-butyl alcohol and (e) tert-butyl alcohol.
Y. Liu et al. / Materials Chemistry and Physics 98 (2006) 304–308
307
Table 1 Conductivity of PPy nanoparticles synthesized in the presence of different alcohols Alcohol
Conductivity
(S cm−1 )
n-Amyl alcohol
iso-Amyl alcohol
n-Butyl alcohol
iso-Butyl alcohol
tert-Butyl alcohol
None
9.60
5.77
8.72
5.83
1.81
0.067
skeletal band (i.e., around 1500 cm−1 ) also gives a qualitative measurement of the conjugation length. A higher ratio means a longer conjugation length [31]. The same conclusion that the conjugation could be adjusted by different alcohols can also be drawn by calculating and comparing these ratios. 3.5. Conductivity
Fig. 5. Raman spectra of PPy nanoparticles synthesized: (a) and (b) in the presence of n- and iso-amyl alcohol, respectively, and (c) in the absence of alcohol.
overlap of the two oxidized structures, and shifts to a higher wave number if the conjugation length is shortened [28,30]. This peak of PPy synthesized without using alcohols presents at 1624 cm−1 and moves to 1594 cm−1 to show lengthening of conjugation after the introduction of n-amyl alcohol during the synthesizing process. When iso-amyl alcohol was selected, the peak moves to 1608 cm−1 . From Figs. 5a, b and 6 it can also be seen that alkyl unit of alcohols strongly affect the conjugation length of PPy chains. Furthermore, calculating the band ratio between the intensity of a band sensitive to the oxidation state of the polymer (i.e., around 1600 cm−1 ) and the intensity of the
The electrical properties of conducting polymers are determined by their structure [10]. Tian and Zerbi [32] proposed the theory of effective conjugation coordinate to explain the correlation between structure and conductivity of polymer. A reduction of the conjugation length will result in the decrease of the conductivity. Although anionic surfactants is not helpful to the increase in the conductivity when only (NH4 )2 S2 O8 is used as the oxidant [10], the conductivity is changed from 0.067 to 9.60 S cm−1 by the introduction of n-amyl alcohol into the reaction system. It is evident from Table 1 that the conductivity of PPy nanoparticles can be improved obviously by adding alcohols into the reaction system and the alkyl units of alcohols also affect the conductivity strongly. This phenomenon agrees with the results of UV–vis and Raman spectra very well. Improving this method to obtain PPy particles with much higher conductivity is very necessary. Introducing other suitable surfactants and lowering reaction temperature may benefit the process. 3.6. Adjusting mechanism Micelles will be formed in aqueous solution when the concentration of SDS comes to 0.1 M. Py monomer has a good solubility in alcohols we used, and it will be dispersed very well in the micellar system in the presence of alcohols. At the same time the alcohols make Py monomer dilute, which may be helpful to the formation of well conjugation. Furthermore, short-chain alcohols can also act as effective cosurfactant to further lower the interfacial energy and optimize the structure of PPy chains during the interfacial polymerization. However, the branched-chain alcohols with bigger volume alkyl units are unfavorable for their entering into the “oil pool” and not effective as cosurfactant either. Although the fundamental basis of structural adjustment for this system has not yet been fully understood, it may be important that the interaction between hydroxyl of alcohols and nitrogen atom of Py also plays the major role in determining the ordered molecular structure in PPy nanoparticles [33]. 4. Conclusion
Fig. 6. Raman spectra of PPy nanoparticles synthesized in the presence of different alcohols: (a) n-butyl alcohol (b) iso-butyl alcohol (c) tert-butyl alcohol.
In summary, inner-structure of PPy nanoparticles can be adjusted directly by a simple microemulsion polymerization at
308
Y. Liu et al. / Materials Chemistry and Physics 98 (2006) 304–308
room temperature. Alcohols used in the experiments have an important influence on the inner-structure and property of PPy nanoparticles. It will give birth to a promising method to the controllable synthesis of other conducting polymers with high conductivity. Acknowledgments This work was supported by the National Nature Science Foundation of China, No. 20173008. We also thank Prof. Yichun Liu for his professional communication. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Y.C. Liu, C.J. Tsai, Chem. Mater. 15 (2003) 320. B.R. Saunders, R.S. Fleming, K.S. Murray, Chem. Mater. 7 (1995) 1082. J.M. Pernaut, J.R. Reynolds, J. Phys. Chem. B 17 (2000) 4080. K. Jurewicz, S. Delpeux, V. Bertagna, F. Beguin, E. Frackowiak, Chem. Phys. Lett. 347 (2001) 36. C.W. Lin, B.J. Hwang, C.R. Lee, Mater. Chem. Phys. 55 (1998) 139. J.W. Goodwin, G.M. Markham, B. Vinent, J. Phys. Chem. B 101 (1997) 1961. K. Dhanalakshmi, R. Saraswathi, C. Srinivasan, Synth. Met. 82 (1996) 237. B.J. Hwang, K.L. Lee, Thin Solid Films 279 (1996) 236. K.G. Neoh, E.T. Kang, K.L. Tan, J. Phys. Chem. B 101 (1997) 726. L.X. Wang, X.G. Li, Y.L. Yang, React. Funct. Polym. 47 (2001) 125. S. Chakrabarti, D. Banerjee, R. Bhattacharyya, J. Phys. Chem. B 106 (2002) 3061.
[12] A.N. Zelikin, D.M. Lynn, J. Farhadi, I. Martin, V. Shastri, R. Langer, Angew. Chem. Int. Ed. 41 (2002) 141. [13] X.J. Zhou, K.T. Leung, Macromolecules 36 (2003) 2882. [14] Y. Liu, Y. Chu, Mater. Chem. Phys. 92 (2005) 59. [15] Z. Wei, Z. Zhang, M. Wan, Langmuir 18 (2002) 917. [16] Z.X. Wei, L.J. Zhang, M. Yu, Y.S. Yang, M.X. Wan, Adv. Mater. 15 (2003) 1382. [17] J. Jang, J.H. Oh, Chem. Commun. (2002) 2200. [18] J. Jang, H. Yoon, Chem. Commun. (2003) 720. [19] A.D.W. Carswell, E.A. O’Rear, B.P. Grady, J. Am. Chem. Soc. 125 (2003) 14793. [20] X.M. Sui, Y. Chu, S.X. Xing, C.Z. Liu, Mater. Lett. 58 (2004) 1255. [21] X.M. Sui, Y. Chu, S.X. Xing, Colloids Surf. A: Physicochem. Eng. Aspects 251 (2004) 103. [22] M.S. Akhter, Colloids Surf. A 157 (1999) 203. [23] Z.H. Kang, E.B. Wang, M. Jiang, S.Y. Lian, Nanotechnology 15 (2004) 55. [24] C. He, C.H. Yang, Y.F. Li, Synth. Met. 139 (2003) 539. [25] G.I. Mathis, V.-T. Troung, Synth. Met. 89 (1997) 103. [26] J. Jin, T. Iyoda, C. Cao, Y. Song, L. Jiang, T. Li, D. Zhu, Angew. Chem. Int. Ed. 40 (2001) 2135. [27] H. Kosonen, J. Ruokolainen, M. Knaapila, M. Torkkeli, K. Jokela, R. Serimaa, G. Brinke, W. Bras, A.P. Monkman, O. Ikkala, Macromolecules 33 (2000) 8671. [28] D.Y. Kim, J.Y. Lee, D.K. Moon, C.Y. Kim, Synth. Met. 69 (1995) 471. [29] W.S. Huang, A.G. MacDiarmid, Polymer 34 (1993) 1833. [30] Y.C. Liu, Electroanalysis 15 (2003) 1134. [31] J. Duchet, R. Legras, S. Demoustier-Champagne, Synth. Met. 98 (1998) 113. [32] B. Tian, G. Zerbi, J. Chem. Phys. 92 (1990) 3892. [33] G. Zotti, S. Zecchin, G. Schiavon, B. Vercelli, A. Berlin, E. Dalcanale, L.B. Groenendaal, Chem. Mater. 15 (2003) 4642.