The Modern Student laboratory Polyaniline-A Conducting Polymer Electrochemical Synthesis and Electrochromic Properties Bradford Charles Sherman, William B. Euler, and R. Ren Force University of Rhode Island, Kingston, RI 02881 Conducting polymers have been under serious study for the last 15 years ( I ) . This class of novel solid-state materials consists of linear oxanic ~olvmerswith extended n conjugatiou. The strikingone-dimensional topology of these svstems allows for an unusual phvsics that begets peculiar ;onlinear charge-carrying particles with the esoteric names soliton, polaron, and bipolaron. After generating charge carriers by oxidative or reductive doping many conducting polymers demonstrate conductivities that begin to rival metallic standards such as copper. These materials have additional value in nonlinear optical applications and as optical memory elements. Conducting polymers are certain to play a role in the evolution of "smart materials" in 21st Centum technolow. -" However. virtuallv none of the marvelous discoveries pertaining to conducting polymers have filtered down to the undergraduate teaching laboratories. Background Polyaniline (PAN)is One of the most studied and interesting conducting pol~mersyet discovered and has 'The lower endofthis vonage range produces better quality films albeit at a slower rate, however, a higher voltage often is used to establish the crucial initial layer d polymer on the substrate. At voltages greater than +0.7V electrolysis of the polymer wmpetes wiih polymerization.
many potential commercial applications (2).APAN battery is already on the market. PAN is also an extremely good candidate for use in an undereraduate laboratow . experi. ment or as a demonstration fo; younger audiences due to the ease of electropolymerizationto prepare polymer films, the dramatic elec&oihromic pmper&es of these films and the relatively low toxicity of aniline. This paper describes the construction, operation, and potential scope of application of a simple electrochemical cell that can be used to, first, synthesize PAN as a thin film and, subsequently, to exhibit the exciting electrochromic properties (color switching) of the polymer. The Four Forms of Polvaniline Current scientific consensus indicates that PAN exists in four different forms (see the table) each ofwhich has a dif. polanerized ferent color (3).Aniline is electrochemically .. . into the green, conducting form of PAN known as emeraldine salt. After the film is synthesized all four forms of PAN can be obtained and reversibly interconverted very quickly electrochemically (i.e., by changing the applied voltage) (4) ~ which makes for a dramatic laboratory experience, I conversion between the green and the emeraldine base blue form can also be accomplished by changing the pH (5). The names, structures, colors, and electrical properties of the four forms of PAN are given in the table. The chem-
The Forms of Polyaniline Name
Structure
ColorC
Comments
3'0
fully reduced insulating
b) Emeraldine Salt
green 320, 420, 800
partially oxidized protonated conducting
cl Emeraldine Base
blue
~artiallvoxidized
d) pernigranilineb
purple 320,530
fully oxidized nsulating
a) Leuwemeraldinea
1
7H
'Four repeat units shown.
bTw repeat unitsshown. 'Numbers given are the absorption maxima in nanometers. A94
Journal of Chemical Education
7
'
?
\
clear
~
~
ical features that distinguishes the various forms are the ratio of benzoid to quinonediimine units, which can be controlled via redox processes, and the degree of pmtonation, which can be regulated by pH. Experimental An important experimental consideration for the use of PAN is the ease of the electrochemical synthesis (6).Aniline is not fussy about reaction conditions. The only chemical requirements are aniline and some acid. The aniline should be moderately pure with a slightly yellow color bug1. A one.battery gesting that some oligomers are ~i~~~~ present). Highly pure, colorless electrochemical cell. Aaniline is harder to initiate to po- conducting glass working lymerize, while dark, degraded electrode. B-copper wireaniline can be quickly cleaned up counter electr0de. by vacuum distillation before use. HC1 is used in this recipe but any comnon mineral or organic acid can be used as catalyst and electrolyte although the electrochemical polymerization rate a i d the subsequent film morpholo& may vary (two areas to robe in more advanced experiments) (7).For a bench top scale experiment add 5 mL bf aniline to 70-80 mL of -3 M HCI (Conc. HCI diluted 4 to 1)in a 150-mL beaker. PAN films are electrochemically synthesized between +0.6 and 4 . 7 V versus standard calomel electrode. (SCEJ.' Typical research investigations use sophisticated three electrode potentiostats to electrochemically synthesize high quality thin films of polyaniline. However, a crude apparatus made from two flashlight batteries, some wire and a variable resistor works surprisingly well. The simplest setup that will work utilizes one flashlight battery as the voltage source (e.g., 1.5-V alkaline D cell) with a lead wire soldered to the terminals a t each end (Fig. 1).Attached alligator clips allow for easy connection of electrodes. Comoletion of the cell is easv. The ~ositiveterminal is connected to the working electmde-a conducting substrate upon which the PAN film grows. Apiece of conductingglass the size of a micmsco~eslide is recommended as the working electrode that will allow full observation of the film color through transmission. We use NESA glass (tin oxide doped with fluoride or antimony).' The negative terminal is connected to a bare copper wire that completes the circuit. The two electrodes are placed in the beaker of aniline and acid, connected to the battery and within 5-10 min a discernible green film will appear on the working electrode. Films of optimum thickness for observing the electmchromic behavior usually are obtained after 20-30 min. If grown too thick, the PAN films will appear excessively dark, and it will be difficult to clearly view the color changes.
'NESAglass is a trademarkof PPG Industries, Pinsburgh. PA, USA. 3 ~ hsystem e can be calibrated by the electrochromicchanges or a wh meter.
Figure 2. A two-battery electrochemical cell. A-conducting glass working electrode. Mopper wire-counter electrode. &lo5 R variable resistor. In order to observe the electrochromic changes, the beaker with aniline and -3 M HC1 must be replaced with an electrolyte solution of 0.5 M KC1 and 0.1 M HC1. The electrochromic properties are obsewed by changing the applied voltage on the PAN film. Using a three-electrode potentiostat fully reduced leucoemeraldine, the clear form, is obtained below -0.2 V, protonated green emeraldine salt occurs between +0.3 to +0.4 V, blue emeraldine base appears a t +0.7 V and full oxidation to purple pernigraniline is obtained a t +0.8 Vrelative to SCE. With the single flashlight battery only a voltage of +1.5 V (with the film connected to the positive terminal) or -1.5 V (with the film attached to the negative terminal) can be applied to the polymer. Therefore, only the fully reduced clear and fully oxidized purple forms can be generated electrochemically with this simple set up. A much more useful apparatus can be constructed by adding a second flashlight battery in series and including a variable resistor in the circuit (Fig. 2). The two batteries allow for both negative as well as positive potentials to be applied without swapping the lead wires and the variable resistor provides control of the voltage by application of Ohm's law. Avariable resistor with a 10' Q capacity is compatible with the +1.5-V batteries. Any appropriate resistor will do, but a 10 turn lOOK pot is recommended and can be purchased for a few dollars at Radio Shack. Note that the working electrode is connected to the wiper on the variable resistor. The two-battery apparatus will not only generate better quality films by allowing the attenuation of the electrochemical polymerization voltage t~ the recommended range3 of +0.6 to +0.7 V versus SCE but also will yield a controllable and fully reversible display of all four PAN film colors. The color chanee is on the order of a few seconds and is reversible through many cycles before film drxrndution occurs. (Elrrtrochromic cycle lifetime is another area for advanced study). Aspectmscopic analysis ofthe electrochromic behavior is perhaps the most powerful aspect of the experiment (8). The relationship between the transmitted color and the electronic absorbance spectrum is easy to establish. In order to obtain the W-vis spectrum of a film, the eonduct(Continued on nezt page)
Volume 71 Number 4 April 1994
A95
The Modern Student lclborcltory ing glass substrates can be cut into strips l-cm wide before coating with PAN. These thinner glass substrates will fit inside an o ~ e n - t o ~ ~ sauare e d cuvet that. in turn. can be inscncd inin a st&dnrd tlr-vis spectrophotometc~samplc holdcr Thc cuvct can bc filled with thc acidic KC1 solution that will keep the film stable while obtaining the spectrum. If a little care is used when connecting the electrodes, the open-topped cuvette can be used as
