Chemical Oxidative Polymerization of Polyaniline - ACS Publications

Laboratory Experiment pubs.acs.org/jchemeduc

Chemical Oxidative Polymerization of Polyaniline: A Practical Approach for Preparation of Smart Conductive Textiles Nedal Y. Abu-Thabit* Department of Chemical and Process Engineering Technology, Jubail Industrial College, Jubail Industrial City 31961, Saudi Arabia S Supporting Information *

ABSTRACT: Electrically conducting polymers are one of the promising alternative materials for technological applications in many interdisciplinary areas, including chemistry, material sciences, and engineering. This experiment was designed for providing undergraduate students with a quick and practical approach for preparation of a polyaniline-conducting polymer, demonstrating its application as a conductive coating for fabrication of smart, flexible, conductive textiles. Polyaniline was prepared by chemical oxidative polymerization (COP) of aniline monomer using ammonium persulfate as redox initiator. The polymerization was characterized by color change during the slow induction period, followed by sudden heat release during the polymerization stage due to the autoacceleration nature of this exothermic reaction. This polymerization system was used to demonstrate the heterogeneous precipitation polymerization technique. As a demonstration for the practical application of polyaniline in the area of smart conductive textiles, different nonconductive textile samples were coated with polyaniline by in situ COP and used as electronic textiles for operation of capacitive touch-screen displays. KEYWORDS: Polymer Chemistry, Industrial Chemistry, Oxidation/Reduction, Synthesis, Applications of Chemistry, Polymerization, Second-Year Undergraduate, Interdisciplinary/Multidisciplinary, Precipitation/Solubility

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have been awarded the Nobel Prize in Chemistry 2000.4 ECPs are called “synthetic metals” because of their intrinsic electrical conductivity resulting from the full delocalization of π electrons on the long chain aromatic polymer backbone. Owing to their semiconducting nature, ECPs have been used for a wide range of advanced applications such as actuators,5 sensors,6 biosensors,7,8 artificial receptors,9 pH-responsive films,10 electrochromic display devices,11 and energy storage devices.12 Among the ECPs, polyaniline (PAni) has attracted much interest because of its low production cost, mechanical and environmental stability, adjustable conductivity, and direct integration into various devices through different synthetic routes. In the past 2 decades, most of the developed educational experiments related to synthesis and applications of ECPs used electrochemical polymerization procedure.5,7−11,13 Polyaniline can be prepared by COP or electrochemical polymerization methods.14 In the case of electrochemical polymerization, aniline monomer is oxidized by the application of an electrical current and a thin layer of the resulted polymer is deposited on the surface of a conductive electrode.11,13,15 Hence, the application electrochemical polymerization is limited to the conductive substrates such as metals, carbon, and conductive

CHEMICAL OXIDATIVE POLYMERIZATION Chemical oxidative polymerization (COP) is used for synthesis of polymers (oligomers) from aromatic compounds such as aniline, pyrrole, phenols, diphenyl sulfide, thiophenols, and thiophene.1 The monomers used in COP are characterized by pronounced electron donor properties with high oxidation tendency.2 Oxidation of monomer is achieved by using an inorganic (or organic) oxidizing agent or an applied potential (i.e., electrochemical oxidative polymerization).2 During the COP, cation or cation radical sites are generated in the monomer (polymer) molecule, thus initiating polymer growth.2 Polyaniline can be considered as one of the most interesting polymers that can be prepared by COP to illustrate nonclassical or reactivation chain polymerization due to the following facts:3 • The synthesis procedure is simple and quick. • The starting materials are inexpensive. • Chemistry and engineering majors will find a wide range of applications. • Polyaniline is conductive. • Several chemical redox initiators are possible to use in this experiment.

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BACKGROUND FOR ELECTRICALLY CONDUCTIVE POLYMERS Electrically conducting polymers (ECPs) were discovered in 1976 by Heeger, MacDiarmid, and Shirakawa, for which they © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: January 23, 2016 Revised: June 7, 2016

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DOI: 10.1021/acs.jchemed.6b00060 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

oxides. In comparison, in situ COP has been effectively employed to deposit the conductive polyaniline on both conductive and nonconductive substrates.16−19 COP offers a simple, quick, and practical approach for preparation of polyaniline-coated substrates as it avoids the solubility problems of polyaniline that require the use of nasty and toxic solvents in cases such as dip coating or spin coating procedures.

Table 1. Overview of the Main Experimental Tasks Time, min

Task Description Introducing students through lecture to oxidative chemical polymerization, heterogeneous precipitation polymerization, and electronic textiles Describing the experimental procedure, assigning students into three groups, and distributing the tasks among the group members Labeling beakers, preparing monomer and initiator solutions, selecting textile substrates and reaction setup Undertaking the polymerization reactions Filtering and washing substrates Drying the polymer/textile, disposal of organic waste, and glassware cleaning Measuring electrical resistance for textile samples and demonstrating their electrical conductivity Total time for all tasks

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PRECIPITATION POLYMERIZATION OF POLYANILINE The COP of polyaniline can be classified as a heterogeneous precipitation polymerization method.20 In this method, the formed polymer precipitates in the reaction medium and can be collected by a filtration process. Precipitation polymerization enables the deposition of polyaniline coating into various nonconductive hydrophilic and hydrophobic surfaces.21 More information related to precipitation polymerization and its application is provided in the Supporting Information (student handout and lab report).

30 15 15 15 15 30 30 150

detailed experimental procedure is available in the Supporting Information, student handout and lab report.

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HAZARDS The oxidation of aniline is exothermic. Polymerization using aniline concentrations over 1 M, especially when carried out in large volumes (over 0.5 L) can result in the overheating of the system, followed by an explosion.28 Such reaction conditions should be avoided. Aniline is toxic by inhalation, in contact with skin, and if swallowed. Aniline should be handled carefully by students under the supervision of the lab instructor. Aniline shall be used inside the fume hood to avoid vapor inhalation. Proper protective gloves shall be used for handling aniline.29,30 To minimize students’ exposure to aniline during the experiment, the instructor should provide the aniline monomer to students using 1 mL disposal droppers inside the fume hood. An alternative way for safe handling of aniline is to replace it with the solid aniline hydrogen chloride salt.28 Ammonium persulfate is very hazardous in the case of skin and eye contact (irritant). During the experiment, students shall always wear protective eye goggles, proper gloves, and a lab coat to address safety concerns. After the experiment, polymerization solutions, filtrate solutions, and washing solutionswhich contain unreacted monomer and oligomersshall be disposed in the organic waste container.

BACKGROUND FOR SMART CONDUCTIVE TEXTILES Smart textiles are defined as textile products such as fibers and filaments, yarns together with woven, knitted, or nonwoven structures, which can interact with the environment and users.22 Smart conductive textiles represent an important category of smart textiles with a wide range of potential applications.23−27 Smart conductive textiles offer advantages such as low cost, light weight, stretchability, and flexibility; these attributes can be integrated into variously shaped structures in ways that are impossible with traditional electronics technology.22,25 Smart conductive textiles can be prepared by using conductive nanoparticles, conductive carbons, and ECPs.12 Among these methods, ECPs provide a quick, practical, and cost-effective approach for fabrication of smart electronic textiles through simple COP procedures. This laboratory introduces polymer engineering technology students to (i) chemical oxidative polymerization of polyaniline, (ii) heterogeneous precipitation polymerization system, and (iii) fabrication of polyanilinecoated conductive textiles by using in situ COP polymerization approach.

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RESULTS AND DISCUSSION Students were distributed into three groups. The first group carried out the COP of pure aniline monomer without addition of a textile substrate. This allowed for the calculation of the monomer conversion, monitoring the polymerization process, and demonstration of the precipitation polymerization concept in which the polyaniline precipitate was collected by suction filtration. The remaining two groups added a selected textile substrate to the polymerization solution, targeting the preparation of electrically conductive textile samples. The COP was carried out by employing ammonium persulfate as a redox initiator at room temperature. The setup used for the polymerization reactions is shown in Figure 2. The molar ratio between the monomer and initiator was kept to 1:1.25 as per the equation shown in Figure 1. A filter paper was used to check and visualize the presence of polyaniline precipitate during the course of polymerization reaction, Figure 4. The initial temperatures for the monomer solutions were recorded. After that, initiator solutions were added simultaneously by the three groups to achieve comparable results at

EXPERIMENTAL OVERVIEW This experiment was designed and implemented as a part of an industrial polymerization course for polymer engineering technology undergraduates during the third semester and after the completion of a polymer chemistry course as a prerequisite. The experiment was conducted during the last five academic semesters with approximately 25 students per semester. Students were introduced to the theoretical concepts related to polymerization techniques chapter prior to the practical experiment. The experiment was designed to be completed within three laboratory periods (≈2.5 h) as depicted in Table 1. Figure 1 shows a sketch illustration for the COP experimental procedure. Each group starts the experiment by preparing the aniline monomer solution with and without the textile fabric (solution A), and the redox initiator (solution B). Various woven and nonwoven textiles or fabrics can be selected, which provide an opportunity for creativity and student interaction based on their level and background. The B




Figure 1. Sketch illustration of the experimental procedure for preparation of conductive polyaniline and conductive polyaniline-coated textiles through in situ chemical oxidative polymerization approach.

identity, Figure 4a. The amount of the blue precipitate increased gradually, as shown in Figure 4b,c. After a 6 min

Figure 2. Experimental setup for the three groups.

the end of polymerization. During a total period of 15 min, the following parameters were observed and recorded by the students: (i) reaction temperature, (ii) color of the reaction solution, (iii) presence of precipitate, and (iv) color of the precipitate. These parameters were recorded in a separate table provided in the lab report document (see the Supporting Information, student handout and lab report). One attractive and exciting feature during the synthesis of polyaniline is the color change during the initial and slow stage of polymerization (induction period), Figure 3. Upon instantaneous mixing of monomer and initiator solutions, the starting colorless reaction solution turned pink in color. The intensity of the pink color decreased gradually and changed to a light purple color. The light purple color was converted gradually into a blue color and finally developed into a dark blue color after an induction period of 4 min. At this stage, the homogeneous polymerization system turned into heterogeneous and the first precipitate was observed with blue color

Figure 4. Digital images for the filter paper used for checking the formation of precipitates during the course of polymerization: (a) after 4 min, (b) after 5 min, (c) after 6 min, and (d) after 10 min.

induction period, the first rise in temperature was observed, which served as an indication for the starting of the polymerization period. The temperature continued to increase due to the exothermic nature of the redox polymerization. See Figure 5. This sudden and rapid increase in temperature is accompanied by an autoacceleration that is due to the

Figure 3. Change of solution color during the first 4 min of aniline polymerization. C




water-soluble oligomers. In a similar way, polyaniline-coated textiles were removed from the reaction solutions and soaked with 1 M HCl repeatedly until the solution became colorless, Figure 6c,d. After that, polyaniline-coated textiles were washed with acetone and then dried in an oven at 60 °C for 30 min. The green color of the obtained textiles indicated the successful coating process of the conductive polymer during the 15 min precipitation polymerization reaction, Figure 6e. As opposed to most other insulating polymers, the most exciting feature of the ECPs is their intrinsic electrical conductivity; this stimulates students’ interest and inspires them to more effectively participate during the practical session. Incorporating conductive textiles in an educational setting is exciting, as it allows students to build things that they can incorporate into their lives in a very unusual and visible way.32 The final part of the experiment was devoted to the measurement of the electrical resistance of the conductive textiles, demonstration of their electrical conductivity, and their application as conductive fabrics for operation of capacitive touch-screen displays. The sheet resistance of the polyanilinecoated textiles was measured by using a digital multimeter (see the Supporting Information, student handout and lab report). Figure 7a shows the photographs for three different textile

Figure 5. Temperature profile during the polymerization reaction. After the induction period, the polymerization is accompanied by a sudden increase in temperature, “auto-acceleration”, and heat release due to the exothermic nature of the redox polymerization reaction.

mediation of aniline oxidation by the produced polyaniline.31 The autoacceleration provides evidence for the transition of the oxidative polymerization from the slow induction stage to the fast polymerization period, Figure 5. The dark green polyaniline dense precipitate was formed after 10 min reaction, as shown in Figure 4d. Interestingly, the polymerization of aniline continued on the filter paper, as observed in Figure 4d. The polymerization reaction continued for a total time of 15 min, with an average conversion of 65−70%. Higher conversions (≈90%) can be obtained by increasing the reaction time to ≈30 min. As shown in Figure 6a, polyaniline green precipitate was

Figure 7. (a) Digital images for the prepared polyaniline-coated conductive textiles with their sheet resistances; (b) illustration and verification of the electrical conductivity nature for the prepared polyaniline-coated textiles with application of the smart and flexible conductive textiles for operation of capacitive touch-screen displays through (c) touching and (d) writing interactions.

Figure 6. (a) Collection of polyaniline precipitate by suction filtration; (b) collected filtrate solution; (c) removal of the polyaniline-coated textile from the reaction solution to be soaked in 1 M HCl solution; (d) polyaniline-coated textile being repeatedly washed with fresh 1 M HCl until the soaking solution become colorless; (e) dried green polyaniline-coated textile.

samples before and after the in situ COP process, using similar reaction conditions. After the polymerization, the sheet resistances of the polyaniline-coated textile samples were lowered by 4−6 orders of magnitude, which provided strong evidence for the successful polymerization and conversion of the original insulating samples into conductive textiles. Students were requested to verify the electrical conductivity of their coated textile samples by using a battery, LED lamp, and alligator clips, Figure 7b and Supporting Information, video 1.

collected by suction filtration of the dark green polyaniline solution. The presence of water-soluble oligomers was indicated by the observed purple color of the filtrate solution, Figure 6b. The precipitate was washed with 1 M HCl until the filtrate became clear and colorless, followed by a final washing with acetone. The washing step is necessary to remove unreacted initiator, monomer, and low-molecular-weight, D




(3) Wei, Y. Nonclassical or Reactivation Chain Polymerization: A General Scheme of Polymerization. J. Chem. Educ. 2001, 78 (4), 551. (4) Heeger, A. J. Nobel Lecture: Semiconducting and metallic polymers: The fourth generation of polymeric materials. Rev. Mod. Phys. 2001, 73 (3), 681. (5) Cortés, M. T.; Moreno, J. C. Electropolymerized Conducting Polymer as Actuator and Sensor Device: An Undergraduate Electrochemical Laboratory Experiment. J. Chem. Educ. 2005, 82 (9), 1372. (6) Blair, R.; Shepherd, H.; Faltens, T.; Haussmann, P. C.; Kaner, R. B.; Tolbert, S. H.; Huang, J.; Virji, S.; Weiller, B. H. Construction of a polyaniline nanofiber gas sensor. J. Chem. Educ. 2008, 85 (8), 1102. (7) Sadik, O. A.; Brenda, S.; Joasil, P.; Lord, J. Electropolymerized conducting polymers as glucose sensors. J. Chem. Educ. 1999, 76 (7), 967. (8) Lunsford, S. Variable effects during polymerization. J. Chem. Educ. 2005, 82 (12), 1830. (9) Ramanaviciene, A.; Ramanavicius, A.; Finkelsteinas, A. Basic Electrochemistry Meets Nanotechnology: Electrochemical Preparation of Artificial Receptors Based on Nanostructured Conducting Polymer, Polypyrrole. J. Chem. Educ. 2006, 83 (8), 1212. (10) Schmidt, D. J.; Pridgen, E. M.; Hammond, P. T.; Love, J. C. Layer-by-Layer Assembly of a pH-Responsive and Electrochromic Thin Film. J. Chem. Educ. 2010, 87 (2), 208. (11) Sherman, B. C.; Euler, W. B.; Force, R. R. The Modern Student Laboratory: Polyaniline-A Conducting Polymer: Electrochemical Synthesis and Electrochromic Properties. J. Chem. Educ. 1994, 71 (4), A94. (12) Abu-Thabit, N. Y.; Makhlouf, A. S. H. In Industrial Applications for Intelligent Polymers and Coatings; Hosseini, M., Makhlouf, A. S. H., Eds.; Springer: Cham, Switzerland, 2016; DOI: 10.1007/978-3-31926893-4_21. (13) Xie, Q.; Li, Z.; Deng, C.; Liu, M.; Zhang, Y.; Ma, M.; Xia, S.; Xiao, X.; Yin, D.; Yao, S. Electrochemical quartz crystal microbalance monitoring of the cyclic voltammetric deposition of polyaniline. A laboratory experiment for undergraduates. J. Chem. Educ. 2007, 84 (4), 681. (14) Tarver, J.; Loo, Y.-L. In Conjugated Polymers: A Practical Guide to Synthesis; Mullen, K., Reynolds, J. R., Masuda, T., Eds; The Royal Society of Chemistry: Cambridge, U.K., 2014; DOI: 10.1039/ 9781849739771-00248. (15) Goto, H.; Yoneyama, H.; Togashi, F.; Ohta, R.; Tsujimoto, A.; Kita, E.; Ohshima, K.; Rosenberg, D. Preparation of conducting polymers by electrochemical methods and demonstration of a polymer battery. J. Chem. Educ. 2008, 85 (8), 1067. (16) Trey, S.; Jafarzadeh, S.; Johansson, M. In situ polymerization of polyaniline in wood veneers. ACS Appl. Mater. Interfaces 2012, 4 (3), 1760. (17) Abu-Thabit, N.; Umar, Y.; Ratemi, E.; Ahmad, A. PolyanilineCoated Polysulfone Membranes as Flexible Optical pH Sensors. Proceedings of the 2nd International Electronic Conference on Sensors and Applications, Basel, Switzerland, Nov. 15−30, 2015; Sciforum Electronic Conference Series, Vol. 2; 2015; p B001, DOI:10.3390/ ecsa-2-B001. (18) Zhang, K.; Zhang, L. L.; Zhao, X.; Wu, J. Graphene/polyaniline nanofiber composites as supercapacitor electrodes. Chem. Mater. 2010, 22 (4), 1392. (19) Tai, Q.; Chen, B.; Guo, F.; Xu, S.; Hu, H.; Sebo, B.; Zhao, X.-Z. In situ prepared transparent polyaniline electrode and its application in bifacial dye-sensitized solar cells. ACS Nano 2011, 5 (5), 3795. (20) Odian, G. Principles of polymerization; John Wiley & Sons: Hoboken, NJ, USA, 2004; pp 297−298. (21) Fedorova, S.; Stejskal, J. Surface and Precipitation Polymerization of Aniline. Langmuir 2002, 18 (14), 5630. (22) Stoppa, M.; Chiolerio, A. Wearable electronics and smart textiles: a critical review. Sensors 2014, 14 (7), 11957. (23) Atwa, Y.; Maheshwari, N.; Goldthorpe, I. A. Silver nanowire coated threads for electrically conductive textiles. J. Mater. Chem. C 2015, 3 (16), 3908.

At this stage of the experiment, a question can be raised among the students to address the advantages of conductive textiles and their potential applications. As an exciting example, cotton gloves coated with conductive polyaniline can be utilized for operating touch-screen displays in cold climates and winter.33 The application of the polyaniline-coated conductive cotton gloves for operating the mobile capacitive touch screen was illustrated through touch and writing operations, Figure 7c,d, and Supporting Information, video 2.

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CONCLUSIONS This experiment provides a real opportunity for students to strengthen their theoretical knowledge with practical experimentation in the field of polymer engineering technology and polymer chemistry. At the end of the experiment, students realize the possibility of transferring their scientific knowledge into exciting technological applications utilizing inexpensive resources around them. Throughout the experiment, students develop understanding of many concepts related to polymer chemistry, such as redox, precipitation, and heterogeneous polymerization; oligomers; induction period; and autoacceleration and heat of polymerization. Students gain practical skills such as handling chemicals, conducting polymerization reactions, and preparing smart conductive and flexible electronic textiles. The experiment includes a number of interesting features such as synthesis of a conducting (noninsulating) polymer, color change during polymerization, preparation of smart conductive flexible textiles, and their application for operation of capacitive touch-screen displays. These exciting features attract students’ attention and engage them with more effective participation during the practical experimental sessions. This experiment can be adapted and tailored for preparation of conductive textiles and substrates for different applications according to the level of students or the subject of interest.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00060. Student handout and laboratory report (PDF, DOCX) Video to demonstrate the electrical conductivity for the prepared conductive textiles (AVI) Video to demonstrate of the application of conductive glove textile for operation of capacitive touch-screen displays (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest.

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REFERENCES

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(24) Lee, Y.-H.; Kim, J.-S.; Noh, J.; Lee, I.; Kim, H. J.; Choi, S.; Seo, J.; Jeon, S.; Kim, T.-S.; Lee, J.-Y.; Choi, J. W. Wearable textile battery rechargeable by solar energy. Nano Lett. 2013, 13 (11), 5753. (25) Hu, L.; Pasta, M.; Mantia, F. L.; Cui, L.; Jeong, S.; Deshazer, H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Stretchable, porous, and conductive energy textiles. Nano Lett. 2010, 10 (2), 708. (26) Abu-Thabit, N. Y.; Basheer, R. A. Synthesis of highly conductive cotton fiber/nanostructured silver/polyaniline composite membranes for water sterilization application. Mater. Res. Express 2014, 1 (3), 035010. (27) Lee, J. A.; Shin, M. K.; Kim, S. H.; Cho, H. U.; Spinks, G. M.; Wallace, G. G.; Lima, M. D.; Lepró, X.; Kozlov, M. E.; Baughman, R. H.; Kim, S. J. Ultrafast charge and discharge biscrolled yarn supercapacitors for textiles and microdevices. Nat. Commun. 2013, 4, 1970. (28) Stejskal, J.; Gilbert, R. G. Polyaniline. Preparation of a conducting polymer (IUPAC technical report). Pure Appl. Chem. 2002, 74 (5), 857. (29) The National Institute for Occupational Safety and Health (NIOSH). http://www.cdc.gov/niosh/docs/81-123/pdfs/0033.pdf (accessed May 2016). (30) Environmental Health & Safety for the Energy Technologies Area (ETA SAFETY). http://eta-safety.lbl.gov/sites/all/files/ VWR%20Chemical%20Resistance%20Gloves%20Chart.pdf (accessed May 2016). (31) Sapurina, I.; Stejskal, J. The mechanism of the oxidative polymerization of aniline and the formation of supramolecular polyaniline structures. Polym. Int. 2008, 57 (12), 1295. (32) Buechley, L.; Eisenberg, M.; Elumeze, N. Towards a curriculum for electronic textiles in the high school classroom. ACM SIGCSE Bulletin 2007, 39 (3), 28. (33) Youtube. http://www.youtube.com/watch?v=YStXwgSmz_E (accessed May 2016).

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Chemical Oxidative Polymerization of Polyaniline - ACS Publications

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