In the Laboratory
Easily Built Low-Cost Apparatus To Measure Polymer Conductivity Jonas Gruber* and Henrique de Santana Instituto de Química - Universidade de São Paulo, Cx.P. 26077 - 05599-970 - S. Paulo, SP, Brazil Isaac Gruber Eaton, Cutler Hammer do Brasil Ltda. Divisão Blindex, Diadema, SP, Brazil Wilson Gazotti, Jr. Instituto de Química - Universidade de Campinas, Cx.P. 6154 - 13081-970 - Campinas, SP, Brazil Although conducting polymers have been extensively studied in the last 15 years (1), resulting in countless publications not only in specialized magazines but also in more general ones (2), we notice that undergraduate students still closely relate the electrical conductivity phenomenon primarily to metals, alloys, and graphite. However, the growing importance of conducting polymers and the ease of obtaining some types of such materials as films or in crude form led us to consider including some simple experiments in practical organic chemistry courses, in which the students could prepare, dope, and measure the conductivity of these polymers. For research purposes we have been using a four-point probe apparatus (3) to accurately measure conductivity over a wide range; but for teaching purposes, the less sophisticated model here described offers the advantages of being cheap and readily built, and students can easily understand how it works. Fairly good results have been obtained between 10{1 and 10{5 S/cm. Conductivity Measuring Method (Fig. 1) An electric current is drained through a reference resistor of known value, connected in series to a polymer disk specimen whose conductivity is to be determined (Fig. 1). The voltage drops across the reference resistor and the polymer are indicated by the two voltmeters as shown. Since the drained current is unique through both conductors, the voltage drops are proportional to their respective resistance, obeying Ohm’s law. Namely, Vref = IRref and Vpol = IRpol. Thus
Rpol Vpol = Rref Vref and
Rpol =
Vpol ⋅ Rref Vref
Vpol/Vref is actually the voltage ratio to the above two voltage drops, and this therefore allows this function to be expressed as Rpol = NvRref (Nv being the voltage ratio). As Rref is a given numerical value, say 1000 Ω, this turns our equation a simple linear function: Rpol = 1000 Nv. It is noteworthy that the polymer resistance obtained here is independent of the supplied voltage value as long as this value differs from zero volts. Now, to convert poly*Corresponding author.
418
Figure 1. Simplified conductivity meter.
mer resistance into conductance (G) in mhos, simply obtain the reciprocal value of Rpol: G = 1/Rpol (mhos); and to convert conductance into conductivity (in S/cm), apply the relationship S = G(d/l), where d is the “point” diameter of the probe and l is the thickness of the polymer disk or film, both expressed in the same unit (cm or mm). The power supply for voltage ratio determination (Nv) is constant and regulated (Fig. 2) because in this conductimeter, for simplicity, a single voltmeter is employed that is switched alternately to read the voltage drops across Rref and Rpol. Now if the supply voltage were permitted to fluctuate freely during the switching of the voltmeter to read Vpol and Vref, errors could result in calculating Nv. We have suggested here the use of a 3 1/2-digit LED single chip A/D converter with relevant circuit (4) to serve as a digital voltmeter. This is optional. Any commercial digital voltmeter with a 2-V full-scale range and an input resistance of 1 MΩ or higher could equally serve the purpose. Common analog moving coil multimeters are usually not suitable for this application. These instruments usually come with an input resistance of 20 kΩ/V. On a 2-V scale, for example, the input resistance offered across Rref or Rpol would be 40 kΩ. This would heavily shunt these resistances at their higher values. Since the specimen’s conductivity may vary by 5 orders of magnitude, the reference resistance must change to match accordingly. Otherwise the voltage drop may in some cases be so minute as to fall bellow the minimum voltage the voltmeter can read, and so be obscured by noise. A list of parts indicated in the various figures is given at the end. Two-Point Probe (Fig. 3) We designed a probe for seizing the specimen disks during the taking of measurements. This probe is mounted on a metal base of 7 × 5 cm to which two 5 × 5 × 1-cm parallel ebonite plates are fastened perpendicularly, keeping 4 cm clearance between them. Two copper rods held within the plate’s holes serve as electrodes. The diameters of these
Journal of Chemical Education • Vol. 74 No. 4 April 1997
In the Laboratory
Figure 2. Complete power supply with voltage regulation.
and timing are taken care by this chip. The voltage indication is accomplished through 3 common-anode, seven-segment LED displays and a half-digit LED to display the most significant digit whenever it should show a 1, i.e., when the voltage indicated is equal to or greater than 1 V. The readout configuration chosen for this chip is a 0- to 1.999-V scale. The sole gauging required was to move trimmer R13 so as to equate the displayed value to that of a voltage of 1.900 V applied by us, while monitoring at terminals C and D by a Beckman model 3050 digital mulFigure 3. Two-point probe. timeter. The small number of low-cost components required encouraged us to include this voltmeter circuit in our project, making this conductimeter more self-contained. Reference Resistance Switching Unit (Fig. 4) rods, in the vicinity of touching both sides of the polymer disk or film, are reduced to 1 mm. The left rod, shown in the figure, consists of two coaxial bars with telescoping movement aided by an embedded spring to provide an automatically adjusting gap between the rod points in accordance with the polymer specimen seizure. During measurements, the probe is electrically connected to the electronic part via two alligator clips grasping both outer rod ends. The two copper points can be cathodically coated with platinum from an acidic solution of hexachloroplatinic acid or, alternatively, cleaned regularly with glass paper.
A constant regulated voltage is applied to terminals A and B. A is connected to the wiper of switch CH3, which permits selection of one of the six reference resistors (R4– R9). These resistors are of rated values 1 Ω to 100 kΩ. The common terminal to all resistors and terminal B are connected to the two-point probe. Switch CH4 selects either of the two voltage drops (Vref or Vpol) to be present on terminals C and D, which are connected to the digital voltmeter described below or to any other one (with an input resistance greater than 1 MΩ on a 2-V full-range scale). Digital Voltmeter (Fig. 5) Our suggested voltmeter incorporates an Intersil single-chip 3 1/2 digit LED analog-to-digital converter type ICL 7107. This chip is a sort of all-in-one device. It comprises most circuitry pertaining to analog and digital signal processing, as can be seen in Figure 5. Some external components are still required but none of them are active items. All amplification, comparison, zeroing, LED driving,
Figure 4. Reference resistance switching unit.
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In the Laboratory
Figure 5. Digital voltmeter.
Power Supplies (Fig. 2) Two independent power supplies were used, one for the conductimeter and one (optional) for the digital voltmeter. The first one generates a constant voltage by an LM317 integrated circuit. Trimmer R2 allows calibration of this voltage to values above approximately 1.25 V. We have set it to 1.90 V in order not to exceed the maximum capacity of the voltmeter (1.999 V). The second power supply delivers +5 V and {5 V stabilized by the voltage regulators LM7805 and 7905, respectively. LM317 and LM7805 were mounted on small aluminum heat sinks, each with a surface of approximately 10 cm2 . Although two separate transformers T1 and T2 were used, a single transformer may be employed, if preferred. In that case the transformer will have to contain two separate secondary windings and must sustain the total power consumption relevant to the conductimeter and digital voltmeter. Printed Circuit Layouts Although the whole circuitry could have been mounted on any universal printed board, we preferred to assemble our device on designed prints.1 This yields higher density of components per board area, as well as a neat appearance.
probe set-up and a 4-probe apparatus. The experiments were repeated 3 times for each polymer disk and the average values were used for the final conductivity calculations. As can be seen in the last column, the differences are within approximately ± 20%, which is acceptable for our purposes.
List of Parts
Resistors (all resistors 0.25 W) R1 - 220
R6 - 100
R11 - 22K
R2 - Trimpot 4K7
R7 - 1K
R13 - Trimpot 20K
R3 - 270
R8 - 10K
R14 - 470K
R4 - 1
R9, R12 - 100K
R15 - 1M
R5 - 10
R10 - 150
C1 - 2200µF/16V
C6 - 0.33µF/250V
C11 - 0.047µF/250V
C2, C8, C13 - 0.1µF/250V
C7 - 2.2µF/16V
C12 - 0.22µF/250V
C3, C9 - 1µF/16V
C10 - 100pF
C14 - 0.01µF/250V
Capacitors
C4, C5 - 1000µF/16V
Diodes D1-D3 - 1N4002 PR1, PR2 - rectifier bridge SKB 1.2/08 L1 - 5 mm green LED
Some Measurements and Comparisons with a 4-Probe Apparatus
Q1 - LM317 adjustable voltage regulator
Table 1 presents the conductivities of 4 different doped conducting polymer samples, measured with both this 2-
Q3 - LM7905 negative voltage regulator
Voltage regulators and transistors Q2 - LM7805 positive voltage regulator
Integrated circuits and displays U1 - ICL7107 3 1/2 digit LED A/D converter
Table 1. Conductivities of Some Conducting Polymer Disksa Conductivity (S/cm) Sample
2-point probe
4-point probe
∆ (%)
1
4.9 × 10-3
4.6 × 10{3
+7
2
2.3 ×
1.9 ×
3 4
U5 - PD 387 PA common anode 1/2 digit LED display
Miscellaneous T1 - transformer primary, 110/220 Vac, secondary 6 Vac 1 A T2 - transformer primary, 110/220 Vac, secondary 6+6 Vac 0.5 A
10{4
+21
2.7 × 10-2
3.3 × 10{2
{18
6.2 × 10-5
5.7 × 10{5
CH1 - DPST switch
+9
CH2 - 4 pole double throw slide switch
10-4
a
Samples 1 and 2: poly(diphenylaniline)/NaClO 4 in different oxidation states; sample 3: poly( o-anisidine)/p-toluenesulfonic acid; sample 4: poly( p-phenylenevinylene)/I 2.
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U2-U4 - PD567 common anode 7 segment LED display
Journal of Chemical Education • Vol. 74 No. 4 April 1997
F1 - fuse 0.5 A
CH3 - 6 position selector switch CH4 - SPDT toggle switch
In the Laboratory Acknowledgments
Literature Cited
We thank FAPESP and CNPq for the scholarships and Fábio Harada for the drawings. Note 1. Copies of the printed circuit board layouts can be obtained contacting the authors.
1. Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986. 2. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539–541. 3. Holtzknecht, L. J.; Mark, H. B., Jr.; Ridgway, T. H.; Zimmer, H. Anal. Instrum. 1989, 18, 23–35. 4. Intersil Component Data Catalog; General Electric: Schenectady, NY, 1986; Chapter 6, 37–47.
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