Preparation and Conductivity of Polyaniline/SiO 2

Transcription

1 Preparation and Conductivity of Polyaniline/SiO 2 Preparation and Conductivity of Polyaniline/SiO 2 Qunwei Tang, Jihuai Wu*, Haiyong Ao, Hui Sun, Lin Lin and Jianming Lin The Key Lab for Functional Materials of Fujian Higher Education, Institute of Material Physical Chemistry, Huaqiao University, Quanzhou, , China Received: 1 June 2007 Accepted: 5 July 2007 SUMMARY A novel polyaniline/ silica composite with a conductivity of 0.26 S/cm was prepared by the polymerisation of aniline monomer and ultrafine silica powder in aqueous solution in the presence of hydrochloric acid as dopant and ammonium persulfate (APS) as initiator and oxidant. The influence of APS, silica and dopant on the volume conductivity of the composite was investigated by X-ray diffraction, infrared spectroscopy and conductivity measurements. The preparation conditions were optimised. The conductivity of the composites remained fairly stable on storage for 50 days. 1. INTRODUCTION Conducting polymers such as polyaniline and polypyrrole have attracted considerable attention in the past two decades for their potential applications in electromagnetic radiation shields, microwave absorbents, batteries, etc Polyaniline (PANI) has been the object of intense investigation because of its good conductivity, stability, electrochemical behaviour and ease of preparation 6. A series of researches into its conducting composites such as PANI/carbon nanotubes 7,8, PANI/ minerals 9,10, PANI/metals 11,12, and PANI/ oxides 13,14 have been reported. Various synthetic methods such as template polymerisation 15, electrospinning 16 and in situ polymerisation 17 have been used to prepare conducting polymers. Since composites often exhibit improved chemical and physical properties over single component counterparts, in previous papers we investigated the synthesis and Smithers Rapra, 2007 conductivity of some novel composite hydrogels based on poly(acrylic acid) and polyacrylamide superabsorbent. In continuation of our studies, nano silica was mixed with polyaniline to form a conducting composite by aqueous solution polymerisation. It was expected that the composite would have a good conductivity and stability. 2. EXPERIMENTAL 2.1 Materials Aniline monomer (ANI) (Shanghai Chemical Reagent Corporation, China, 98.5%) was distilled under reduced pressure prior to use. Ammonium persulfate (APS) was used as a radical initiator for the polymerisation of polyaniline. Ultrafine SiO 2 powder (size: nanometer; specific surface area: m 2 /g) was purchased from the Qingdao Yuminyuan Silica Gel Reagent Factory (China). HCl, H 2 SO 4, H 3 PO 4, and acetic acid solutions were used as received. *Tel: ; Fax: ; jhwu@hqu.edu.cn 2.2 Synthesis of PANI/SiO 2 Composite 2 ml aniline monomer (ANI) was dissolved in 1.5 M HCl solution to form 300 ml solution at ambient temperature. Predetermined amounts of ultrafine SiO 2 powder were added to the ANI solution with a stirring speed of 450 r/ min to disperse the SiO 2 particles evenly. 100 ml ammonium persulfate (APS) solution was added slowly. Then polymerisation was carried out in aqueous solution at 25 C for 8 h. The intte product was vacuum-filtered and washed with deionised water to remove oligomers and any impurities. Finally, the product was vacuum dried at 80 ºC for more than 5 h to obtain a solid PANI/SiO 2 composite. The preparation of doped PANI is shown in Scheme Measurement and Characterisation For the measurement of conductivity, all samples were prepared in pellet form (diameter: 13 mm, thickness: 1 mm) at a pressure of 14 MPa using a Carver model C Press 13. The four probes method was used to measure the conductivity. Polymers & Polymer, Vol. 15, No. 8,

2 Qunwei Tang, Jihuai Wu, Haiyong Ao, Hui Sun, Lin Lin and Jianming Lin Scheme 1. Preparation of doped PANI 21 Powder X-ray diffraction patterns of samples were measured using a D8 Advance X-ray diffractometer (Bruker Co., Germany) with a Cu-Kα source at wavelength nm, running at 40 kv and 40 ma, scanning from 5º to 60º at 6º /min. The samples were identified by IR spectroscopy on a Nicolet Impact 410 FTIR spectrophotometer using KBr pellets. 3. RESULTS AND DISCUSSION 3.1 IR Spectrum and XRD of the PANI/SiO 2 Composite Figure 1 shows the IR spectra of the SiO 2 powder and PANI/SiO 2 composite. The absorption peaks at 3450 cm -1 and 1640 cm -1 belong to OH stretching and bending motions of absorbed H 2 O on SiO 2, respectively. The 1085 cm -1 absorption peak was attributed to Si O Si dissymmetrical stretching, and the 961 cm -1 absorption peak was ascribed to the siliconoxygen bond vibration in Si O H. The absorption peak at 1568 cm -1 was assigned to quinoid ring stretching and 1475 cm -1 to benzene ring stretching. The 1303 cm -1 and 1239 cm -1 peaks were attributed to N H stretching in the connecting bridge between the benzene ring quinoid ring and benzene ring benzene ring of PANI, respectively 22. The disappearance of the 1640 cm -1 and 961 cm -1 peaks from the IR spectrum of the composite indicated the formation of chemical or physical bonds between the OH groups of SiO 2 and NH 2, NH groups on PANI after polymerisation. Powder X-ray diffraction patterns of SiO 2 and PANI/SiO 2 composite are given in Figure 2. No diffraction peaks were found in Figure 2a, confirming that the SiO 2 was amorphous. From Figure 2b, it can be seen that the maximum peak in SiO 2 shifted from 2θ = 21.3 to 2θ = 20.1, and the diffraction intensity of the composite weakened considerably because of the formation of the composite and the further dispersion of the SiO 2 particles. 3.2 Influence of APS on the Conductivity of the Composite It is known that the conductivity of the composite derives from the PANI. The effect of the molar ratio for initiator (APS) to aniline monomer (ANI) on the electrical conductivity of the composite is plotted in Figure 3. When the ratio was lower than 1.0, the reduced quantity of initiator led to a slower polymerisation reaction, the PANI chains could not form effectively, and the conductivity of the composites was low under otherwise identical reaction conditions. On the other hand, APS (ammonium persulfate) is not only an initiator, but also an oxidant. The higher the concentration of oxidant, the faster the reaction and when it exceeded 1, ANI monomer was oxidised too much, which resulted in the breakage of the PANI chains and a decrease in conductivity. According to Figure 3, to obtain a polyaniline/sio 2 composite Figure 1. IR spectra of SiO 2 and PANI/SiO 2 composite (Preparation conditions: molar ratio for APS to ANI = 1, mass ratio for SiO 2 to ANI = 2, HCl concentration 1.5 M, reaction at 25 C for 8 h) 606 Polymers & Polymer, Vol. 15, No. 8, 2007

3 Preparation and Conductivity of Polyaniline/SiO 2 Figure 2. XRD of SiO 2 (a) and PANI/SiO 2 composite (b). (Prepared conditions: molar ratio for APS to ANI = 1, mass ratio for SiO 2 to ANI = 2, HCl concentration 1.5 M, reacted at 25 C for 8 h) Figure 3. Influence of APS on the conductivity of the composites (Preparation conditions: mass ratio for SiO 2 to ANI = 2.0, HCl concentration 1.5 M, reaction at 25 C for 8 h) with high conductivity, a molar ratio for oxidant to aniline of 1.0 is best. 3.3 Influence of Amount of SiO 2 on Conductivity of the Composite The relation between the conductivity and the amount of SiO 2 in the composite is depicted in Figure 4. The conductivity gradually decreased with increasing amounts of SiO 2. The conductivities of the composites fell from 285 to 9.3 ms/cm with the ratio for SiO 2 to ANI varying from 1 to 6. It has been proved that a highly oriented and extended-chain conformation of PANI plays an important role in the conductivity of the composite 23. In this respect, increasing the number of insulating SiO 2 particles means the PANI chains cannot be highly oriented and extended, so the conductivity decreased. According to Figure 4, to obtain a polyaniline/sio 2 composite with higher conductivity, the mass ratio for SiO 2 to aniline monomer should preferably be around 1.0. Figure 4. Influence of SiO 2 on the conductivity of the composites (Preparation conditions: molar ratio for APS to ANI = 1.0, HCl concentration 1.5 M, reaction at 25 C for 8 h) 3.4 Influence of HCl concentration on the conductivity of the composite It is known that the conductivity depends on the structure and on the concentration of dopants. The influence of HCl concentration on conductivity is shown in Figure 5. The conductivity increased with increasing concentration of HCl until the HCl concentration reached 2.5 M. After that, the conductivity decreased because of the reduction in both the concentration and the mobility of polarons with excessive dopant concentration 24. The optimum behaviour can be expected with the formation of bipolarons at high doping levels 25, which has been observed in polypyrrole 26. The polarons and bipolarons were in equilibrium, and the pairing of polarons can occur at high doping levels to form bipolarons 27, as in Scheme 2. According to Figure 5, to obtain a polyaniline/sio 2 composite with higher conductivity, a HCl concentration of 2.5 M is preferable. Polymers & Polymer, Vol. 15, No. 8,

4 Qunwei Tang, Jihuai Wu, Haiyong Ao, Hui Sun, Lin Lin and Jianming Lin Scheme 2. HCl doping mechanism of polyaniline 27 Figure 5. Influence of HCl concentration on conductivity (Preparation conditions: mass ratio for SiO2 to ANI =2.0, molar ratio for APS to ANI =1.0, reaction at 25 C for 8 h) Figure 6. Influence of acids on conductivity of composite (Prepared conditions: mass ratio for SiO 2 to ANI = 2.0, molar ratio for APS to ANI = 1.0, acid concentration 2.5 M, reacted at 25 C for 8 h) 3.5 Influence of Acid on Conductivity The kind of acid used is another important factor when preparing PANI composites with high conductivity. Typically PANI has two stable forms, an emeraldine base (EB) and an emeraldine salt (ES) with different degrees of doping. EB is a nonconducting form of PANI whereas ES is conducting 28. Conductivity can be changed depending on the strength of the acid used for doping and the method of processing 29,30. In this study, the conductivities of polyaniline/sio 2 composites doped with acids were measured and the results are plotted in Figure 6. The order of conductivity in samples at the same acid concentration conditions is: H 2 SO 4 > HCl > H 3 PO 4 > CH 3 COOH. The ES samples doped at a low ph had high degrees of crystallisation and high electrical conductivity, as reported by Pouget 31. The change in conductivity of composites made with different acids is consistent with the acidity, which is in the order H 2 SO 4 > HCl > H 3 PO 4 > CH 3 COOH. Although sulphuric acid is the strongest acid, it is also very corrosive and a very strong oxidising agent, so HCl was chosen. 3.6 Influence of Reaction Time and Temperature on Conductivity The reaction temperature and time also affected the structure and conductivity of the composite. From Figure 7, the conductivity of the composite first increased and then decreased with reaction time, in three temperature curves, and the higher the temperature, the shorter the reaction time for reaching the highest conductivity. Namely, the sample was prepared at 50 C and had its highest conductivity after 6 h, the 25 C sample after 8 h and the 0 C sample after 10 h. Higher reaction temperature normally results in a shorter reaction time, in accordance with the general rule of chemical reactions (Equation (3)). 608 Polymers & Polymer, Vol. 15, No. 8, 2007

5 Preparation and Conductivity of Polyaniline/SiO 2 Figure 7. Influence of reaction temperature and time on conductivity of the composites (Prepared conditions: mass ratio for SiO 2 to ANI = 2.0, molar ratio for APS to ANI = 1.0, HCl 1.5 M) Figure 8. Influence of time on conductivity at 25 C (Preparation conditions: mass ratio for SiO 2 to ANI = 2.0, molar ratio for APS to ANI = 1.0, HCl concentration 1.5 M) temperature on the conductivity, we chose a reaction temperature of 25 C and a reaction time of 8 h. 3.7 Conductivity Stability of the In actual application, it is important that the composite keeps its conductivity without it declining during storage. In order to investigate the conductivity stability of the PANI/SiO 2 composites, samples having different preparation conditions [reacted at 0 C for 6 h (sample a), at 25 C for 8 h (sample b), at 50 C for 10 h (sample c)] respectively were stored in air at room temperature and the conductivities were later measured. From Figure 8, the conductivity of the composites remained fairly stable but the stability was influenced by the polymerisation reaction temperature. For the three samples (a),(b) and (c), the conductivities were maintained at 90%, 80% and 58.0% of their original values respectively after 50 days. The superior stability of sample a may be due to the fact that sample c was prepared at a higher temperature for a longer reaction time, inducing more side reactions and allowing more by-products to occur, causing a decline in stability. (3) But APS is a strong oxidising agent for ANI monomer and PANI. A longer reaction time causes a side reaction involving the oxidation of PANI, which results in the breakage of the PANI conducting chains to some extent, and a decline in the conductivity of the composite. Owing to the exothermic nature of the polymerisation, a higher temperature produces more short chains in the PANI and decreases the conductivity. In order to obtain a polyaniline/sio 2 composite with high conductivity, the reaction temperature has to be kept reasonably low, as reported by other groups 32. Considering the influence of reaction time and 4. CONCLUSIONS A novel polyaniline/sio 2 conducting composite was prepared by polymerising aniline in aqueous solution with ultrafine SiO 2 powder in the presence of hydrochloric acid as dopant and ammonium persulfate as initiator and oxidising agent. When the reaction conditions were optimised at a mass ratio for SiO 2 to ANI 2.0, molar ratio for APS to ANI 1.0, HCl concentration 2.5 M, and with polymerisation at 25 C for 8 h, the polyaniline/sio 2 composites possessed a conductivity 0.26 S/cm. The composites showed good environmental stability. ACKNOWLEDGEMENTS This work was financially supported by the Natural Science Foundation of China Polymers & Polymer, Vol. 15, No. 8,

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