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Doping front migration in intrinsically conductive polymers and its application
Electrochimica Acta 54 (2009) 4258–4261
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Doping front migration in intrinsically conductive polymers and its application Jens Peter Hermes, Meinhard Knoll ∗ Institute of Physical Chemistry, University of Muenster, Corrensstr. 30, D-48149 Muenster, Germany
a r t i c l e
i n f o
Article history: Received 18 December 2008 Received in revised form 13 January 2009 Accepted 26 February 2009 Available online 9 March 2009 Keywords: Conductive polymers Electrochemical doping Chemical doping Migration Smart label
a b s t r a c t Doping front migration is a recently discovered effect occurring in sandwich structures composed of intrinsically conductive polymers. A system based on chemical or electrochemical doping is capable of controlling an integrated display and modifying the electrical resistance of the conductive polymer. The effect does not require a battery and is capable of monitoring time and temperature exposure. Low-cost devices using doping front migration could be the basis of a new class of smart labels for applications such as electronic “best before” labels on food and drink. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Smart labels containing RFID (radio-frequency identification) transponders are commonly applied to products and packages for identification using radio waves [1,2]. A transponder comprising an integrated circuit and antenna is powered from currents induced in the antenna during radio interrogation by the reader system. The device is inactive when not within receiving range of a reader, and is incapable of modifying any visual information printed on the device label. Recently Knoll [3] has described two effects that may enable development of a new class of smart labels: doping front migration and self-writing. RFID transponders based on these effects do not require a battery and are capable of controlling an integrated display or varying the load resistance of the transponder. In this paper we investigate doping front migration based on pH doping and electrochemical doping of intrinsically conductive polymers. We describe the influence of the migration layer composition and the relation between migration length and the uptake of mass in the migration layer. The change in electrical conductance is compared to the migration length. 2. Theory Fig. 1 depicts a sandwich structure comprising an activation layer and a migration layer on a flexible substrate. The activation layer consists of a thin film of conducting polymer (PEDOT:PSS
∗ Corresponding author. Tel.: +49 251 83 63 851; fax: +49 251 83 63 852. E-mail address: [email protected] (M. Knoll). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.086
or PANI:DBSA) deposited on the substrate. This is covered by the migration layer, a polyvinyl alcohol (PVA) matrix. This multilayer is partially covered by an encapsulation layer. When the exposed portion of the migration layer is brought into contact with either water or water vapor in a humid chamber, water is absorbed by the migration layer and advances through it with a sharp front. Beneath the water-exposed region of the migration layer (hatched area) the activation layer can interact with the electrolyte in the migration layer. Chemically or electrochemically induced doping changes at the surface of the activation layer produce a color change in the activation layer. The color change enables clear visualization of the moving doping and electrolyte fronts and determination of the migration length by the naked eye. The utility of this effect depends on a distinct transition at the moving electrolyte front. The migration of the electrolyte front is typically described as a diffusion phenomenon, although diffusion ordinarily does not lead to sharp profiles. However, sharp migration fronts have been observed in solvents and vapors migrating in glassy polymers [4–8]. The sharpness of the front can be explained using a diffusion coefficient that varies with concentration [10–12] in either a stepwise [5,8] or exponential [8,9] manner. The polymer structure is loosened as the diffusing molecules are inserted between polymer segments, increasing the free volume available for diffusion of additional penetrant molecules. This can be interpreted as a chain reaction that leads to a sharp front. Each penetrant molecule moving into the network induces more swelling in the polymer, and the front marks the boundary between the two different states. Long and Thompson [13] found that the mass uptake is pro√ portional to t. This result may be applied to PVA and the effect of doping front migration as demonstrated later. The migration length
J.P. Hermes, M. Knoll / Electrochimica Acta 54 (2009) 4258–4261
4259
Fig. 1. Principle of doping front migration [3].
is described by: √ L=K· t
(1)
in which K is the migration coefficient. K depends on temperature. 3. Experimental Chemical doping is induced by a base or acid in the hydrated portion of the migration layer changing the doping level of the activation layer. In electrochemical doping the depicted cell (Fig. 1) represents an electrochemical half-cell in which the activation layer acts is an electrode and the migration layer acts as an thin electrolyte film. A second electrode is required to enable a redox reaction. In both cases the activation layer is a thin film of a conductive polymer (PANI:DBSA or PEDOT:PSS) and the migration layer is based on polyvinyl alcohol. PANI:DBSA was used for chemical doping devices. Depending on the application, it experiences a color change between the green emeraldine salt and the blue emeraldine base form (Fig. 2a). The emeraldine salt has a higher conductivity. For electrochemical doping devices we employed PEDOT:PSS, which is conductive in its pale blue oxidized form (Fig. 2b) and can be reduced to a deep blue form. In both cases the conductance values increased or decreased as the migration proceeded. A solution of Panipol T (Panipol Oy, Finland) and toluene (ratio 1:1) was used for dip coating PANI:DBSA on a polyvinylchloride substrate. Both the base and salt forms of PANI:DBSA were used. To obtain the emeraldine base, films of the salt form of PANI:DBSA were dipped in a 0.5 M NaOH solution. PEDOT:PSS on polyester foil (Orgacon EL-350, AGFA Gevaert) and PVA films (TF40, Aicello) were used as received. PVA solutions containing CaCl2 were prepared as described by Knoll [3]. The chemical doping cell was prepared by laminating a PVA foil to a PANI:DBSA layer on PVC and covering with a plastic film. Cells containing activation layers made from PANI:DBSA (emeraldine salt) were activated using a solution of 50 mM NaOH
Fig. 3. Migration front and migration length L. (a) PANI:DBSA (emeraldine salt), chemical activation in NaOH solution; (b) PANI:DBSA (emeraldine base), chemical activation in citric acid solution; (c) PEDOT:PSS, electrochemical activation.
and cells containing PANI:DBSA (emeraldine base) were activated using a 50 mM citric acid solution. The electrochemical cell was prepared with a PEDOT:PSS electrode and an aluminum electrode as described by Knoll [3]. Both electrodes were short-circuited and activated using water vapor in a humid chamber. For electrical measurements two electrodes were applied adjacent to the migration area using silver paste. A computer-based measurement system recorded the changes in conductance over time. The migration length could be measured by the naked eye. For the sake of accuracy however we took photographs of the samples at various times and evaluated them using the computer program imageJ. The program calculated the length using a reference scale next to the migration area. Temperatures were recorded using the EL-USB-1 data logger by Lascar Electronics. To determine the mass uptake the weight of the cell was measured on a micro balance after carefully drying the outside of the cell. To compare different values the results were normalized by dividing each measurement by the maximum value. 4. Results and discussion Fig. 3 depicts migration fronts and migration lengths L. Plots of migration length versus time for chemical doping are presented in Fig. 4. The quadratic plot demonstrates the expected t1/2 dependency. The migration coefficient K in Eq. (1) was K = 0.75 mm h−1/2 (a) and K = 1.03 mm h−1/2 (b).
Fig. 2. (a) Different states of PANI [14,15], Emeraldine Salt is deprotonated in alkaline medium to Emeraldine Base or Emeraldine Base reacts with an acid (HA) to yield the Emeraldine Salt. A is an arbitrary anion. (b) Conductive state of PEDOT:PSS [16].
4260
J.P. Hermes, M. Knoll / Electrochimica Acta 54 (2009) 4258–4261
Fig. 4. Migration length versus time, chemical doping. (a) PANI:DBSA (emeraldine salt), activation in NaOH solution, Lmax = 10.0 mm, K = 0.75 mm h−1/2 ; (b) PANI:DBSA (emeraldine base), activation in citric acid solution, Lmax = 31.0 mm, K = 1.03 mm h−1/2 .
In Fig. 5 the relation between the migration length L and mass uptake m is depicted. Both curves can be compared using their normalized values. The time dependency of mass uptake of PVA by water sorption correlates with the migration length. The migration length as a function of time for electrochemical doping was determined from experiments in which PEDOT:PSS was electrochemically reduced in a short-circuited cell (Fig. 6). The calculated migration coefficient was K = 1.45 mm h−1/2 . The results of an electrical method to measure the migration length are displayed in Fig. 7. The electrical conductance was measured between the electrodes a and b. Electrochemical reduction of PEDOT:PSS in the area between x = 0 and x = L resulted in a decreasing conductance. The normalized plots of length (L/Lmax )
Fig. 7. (a) A top view of the reduced PEDOT:PSS area and electrodes a and b; (b) normalized plot of length and conductance value versus time.
and conductance (−G/Gmax ) versus time validate this correlation. This behavior enables the device to act as a variable resistor in an electrical circuit, for instance modifying the load resistance of an RFID tag.
Fig. 5. Normalized plot of mass and length versus time.
Fig. 8. Dependence of the migration coefficient K on the amount of salt in the migration layer.
Fig. 6. Migration length versus time, electrochemical doping, PEDOT:PSS.
Fig. 9. Migration length versus time for changing temperatures.
J.P. Hermes, M. Knoll / Electrochimica Acta 54 (2009) 4258–4261
4261
Fig. 10. “Best before” labels based on doping front migration.
The migration velocity depends on the amount of salt incorporated in the migration layer. This was investigated by comparing the migration coefficients of layers containing various salt concentrations. The plot contained in Fig. 8 demonstrates that the migration velocity increases with higher salt content. Higher concentrations could not be analyzed because CaCl2 is hygroscopic in the dry state and migration began during exposure to moderate humidity in the laboratory rather than starting under controlled conditions in the humid chamber. The migration velocity is also affected by temperature. This is the basic property enabling these devices to be used as time–temperature integrators. This ability was tested by exposing the devices to changing temperatures. Fig. 9 depicts the function L2 = f(t). The slope of the curve is given by K2 which is almost the same for each temperature regardless of the temperature exposure history of the sample. 5. Conclusion and outlook We demonstrated doping front migration based on pH doping and electrochemical doping of the intrinsically conductive polymers PANI:DBSA and PEDOT:PSS. The influence of the composition of the migration layer as well as the relation between migration length and mass uptake in the migration layer were studied. The change of electrical conductance was also compared to the migration length. Doping front migration does not depend on a battery. The device may be read out by eye at any time, and is capable of monitoring time and temperature exposure.
This ultralow-cost system may form the basis of a new class of smart labels containing electrical functions and capable of displaying changing information to the human eye. The device may also be used as a time–temperature integrator that monitors and reports product storage conditions. This may be very useful as a “best before” label on food products. The time–temperature integrator works like a clock which is running faster at higher temperatures. If the cold chain of a food product is broken the time–temperature integral increases and the bar of the device reaches the end of the scale before the date of the shelf life which is printed on the package. Doping front migration could be activated by water vapor diffusing from the product and passing through a gas permeable membrane into the migration layer (Fig. 10). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J.S. Rasul, Microelectron. Reliab. 44 (1) (2004) 135. J. Whitmarsh, Microelectron. Int. 22 (3) (2005) 16. Knoll, Electrochim. Acta 54 (2008) 216. T. Alfrey, et al., J. Polym. Sci. Pol. Sym. 12PC (1966) 249. J. Crank, J. Polym. Sci. 11 (2) (1953) 151. E. Gurnee, J. Polym. Sci. 41 (138) (1959) 119. G.F. Billovits, C.J. Durning, Polymer 29 (8) (1988) 1468. M.A. Samus, G. Rossi, Macromolecules 29 (6) (1996) 2275. L. Perrin, et al., Polym. Int. 39 (3) (1996) 251. G. Astarita, G.C. Sarti, Polym. Eng. Sci. 18 (5) (1978) 388. G.S. Kulagina, et al., Polym. Sci., Ser. A 49 (4) (2007) 425. V.S. Markin, et al., Pharm. Chem. J. 32 (7) (1998) 44. F.A. Long, L.J. Thompson, J. Pol. Sci. 15 (80) (1955) 413. A.G. Green, A.E. Woodhead, J. Chem. Soc. 97 (1910) 2338. R. Cushman, J. Electroanal. Chem. 219 (1987) 335. J. Ouyang, et al., Polymer 45 (25) (2004) 8443.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Doping front migration in intrinsically conductive polymers and its application Jens Peter Hermes, Meinhard Knoll ∗ Institute of Physical Chemistry, University of Muenster, Corrensstr. 30, D-48149 Muenster, Germany
a r t i c l e
i n f o
Article history: Received 18 December 2008 Received in revised form 13 January 2009 Accepted 26 February 2009 Available online 9 March 2009 Keywords: Conductive polymers Electrochemical doping Chemical doping Migration Smart label
a b s t r a c t Doping front migration is a recently discovered effect occurring in sandwich structures composed of intrinsically conductive polymers. A system based on chemical or electrochemical doping is capable of controlling an integrated display and modifying the electrical resistance of the conductive polymer. The effect does not require a battery and is capable of monitoring time and temperature exposure. Low-cost devices using doping front migration could be the basis of a new class of smart labels for applications such as electronic “best before” labels on food and drink. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Smart labels containing RFID (radio-frequency identification) transponders are commonly applied to products and packages for identification using radio waves [1,2]. A transponder comprising an integrated circuit and antenna is powered from currents induced in the antenna during radio interrogation by the reader system. The device is inactive when not within receiving range of a reader, and is incapable of modifying any visual information printed on the device label. Recently Knoll [3] has described two effects that may enable development of a new class of smart labels: doping front migration and self-writing. RFID transponders based on these effects do not require a battery and are capable of controlling an integrated display or varying the load resistance of the transponder. In this paper we investigate doping front migration based on pH doping and electrochemical doping of intrinsically conductive polymers. We describe the influence of the migration layer composition and the relation between migration length and the uptake of mass in the migration layer. The change in electrical conductance is compared to the migration length. 2. Theory Fig. 1 depicts a sandwich structure comprising an activation layer and a migration layer on a flexible substrate. The activation layer consists of a thin film of conducting polymer (PEDOT:PSS
∗ Corresponding author. Tel.: +49 251 83 63 851; fax: +49 251 83 63 852. E-mail address: [email protected] (M. Knoll). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.086
or PANI:DBSA) deposited on the substrate. This is covered by the migration layer, a polyvinyl alcohol (PVA) matrix. This multilayer is partially covered by an encapsulation layer. When the exposed portion of the migration layer is brought into contact with either water or water vapor in a humid chamber, water is absorbed by the migration layer and advances through it with a sharp front. Beneath the water-exposed region of the migration layer (hatched area) the activation layer can interact with the electrolyte in the migration layer. Chemically or electrochemically induced doping changes at the surface of the activation layer produce a color change in the activation layer. The color change enables clear visualization of the moving doping and electrolyte fronts and determination of the migration length by the naked eye. The utility of this effect depends on a distinct transition at the moving electrolyte front. The migration of the electrolyte front is typically described as a diffusion phenomenon, although diffusion ordinarily does not lead to sharp profiles. However, sharp migration fronts have been observed in solvents and vapors migrating in glassy polymers [4–8]. The sharpness of the front can be explained using a diffusion coefficient that varies with concentration [10–12] in either a stepwise [5,8] or exponential [8,9] manner. The polymer structure is loosened as the diffusing molecules are inserted between polymer segments, increasing the free volume available for diffusion of additional penetrant molecules. This can be interpreted as a chain reaction that leads to a sharp front. Each penetrant molecule moving into the network induces more swelling in the polymer, and the front marks the boundary between the two different states. Long and Thompson [13] found that the mass uptake is pro√ portional to t. This result may be applied to PVA and the effect of doping front migration as demonstrated later. The migration length
J.P. Hermes, M. Knoll / Electrochimica Acta 54 (2009) 4258–4261
4259
Fig. 1. Principle of doping front migration [3].
is described by: √ L=K· t
(1)
in which K is the migration coefficient. K depends on temperature. 3. Experimental Chemical doping is induced by a base or acid in the hydrated portion of the migration layer changing the doping level of the activation layer. In electrochemical doping the depicted cell (Fig. 1) represents an electrochemical half-cell in which the activation layer acts is an electrode and the migration layer acts as an thin electrolyte film. A second electrode is required to enable a redox reaction. In both cases the activation layer is a thin film of a conductive polymer (PANI:DBSA or PEDOT:PSS) and the migration layer is based on polyvinyl alcohol. PANI:DBSA was used for chemical doping devices. Depending on the application, it experiences a color change between the green emeraldine salt and the blue emeraldine base form (Fig. 2a). The emeraldine salt has a higher conductivity. For electrochemical doping devices we employed PEDOT:PSS, which is conductive in its pale blue oxidized form (Fig. 2b) and can be reduced to a deep blue form. In both cases the conductance values increased or decreased as the migration proceeded. A solution of Panipol T (Panipol Oy, Finland) and toluene (ratio 1:1) was used for dip coating PANI:DBSA on a polyvinylchloride substrate. Both the base and salt forms of PANI:DBSA were used. To obtain the emeraldine base, films of the salt form of PANI:DBSA were dipped in a 0.5 M NaOH solution. PEDOT:PSS on polyester foil (Orgacon EL-350, AGFA Gevaert) and PVA films (TF40, Aicello) were used as received. PVA solutions containing CaCl2 were prepared as described by Knoll [3]. The chemical doping cell was prepared by laminating a PVA foil to a PANI:DBSA layer on PVC and covering with a plastic film. Cells containing activation layers made from PANI:DBSA (emeraldine salt) were activated using a solution of 50 mM NaOH
Fig. 3. Migration front and migration length L. (a) PANI:DBSA (emeraldine salt), chemical activation in NaOH solution; (b) PANI:DBSA (emeraldine base), chemical activation in citric acid solution; (c) PEDOT:PSS, electrochemical activation.
and cells containing PANI:DBSA (emeraldine base) were activated using a 50 mM citric acid solution. The electrochemical cell was prepared with a PEDOT:PSS electrode and an aluminum electrode as described by Knoll [3]. Both electrodes were short-circuited and activated using water vapor in a humid chamber. For electrical measurements two electrodes were applied adjacent to the migration area using silver paste. A computer-based measurement system recorded the changes in conductance over time. The migration length could be measured by the naked eye. For the sake of accuracy however we took photographs of the samples at various times and evaluated them using the computer program imageJ. The program calculated the length using a reference scale next to the migration area. Temperatures were recorded using the EL-USB-1 data logger by Lascar Electronics. To determine the mass uptake the weight of the cell was measured on a micro balance after carefully drying the outside of the cell. To compare different values the results were normalized by dividing each measurement by the maximum value. 4. Results and discussion Fig. 3 depicts migration fronts and migration lengths L. Plots of migration length versus time for chemical doping are presented in Fig. 4. The quadratic plot demonstrates the expected t1/2 dependency. The migration coefficient K in Eq. (1) was K = 0.75 mm h−1/2 (a) and K = 1.03 mm h−1/2 (b).
Fig. 2. (a) Different states of PANI [14,15], Emeraldine Salt is deprotonated in alkaline medium to Emeraldine Base or Emeraldine Base reacts with an acid (HA) to yield the Emeraldine Salt. A is an arbitrary anion. (b) Conductive state of PEDOT:PSS [16].
4260
J.P. Hermes, M. Knoll / Electrochimica Acta 54 (2009) 4258–4261
Fig. 4. Migration length versus time, chemical doping. (a) PANI:DBSA (emeraldine salt), activation in NaOH solution, Lmax = 10.0 mm, K = 0.75 mm h−1/2 ; (b) PANI:DBSA (emeraldine base), activation in citric acid solution, Lmax = 31.0 mm, K = 1.03 mm h−1/2 .
In Fig. 5 the relation between the migration length L and mass uptake m is depicted. Both curves can be compared using their normalized values. The time dependency of mass uptake of PVA by water sorption correlates with the migration length. The migration length as a function of time for electrochemical doping was determined from experiments in which PEDOT:PSS was electrochemically reduced in a short-circuited cell (Fig. 6). The calculated migration coefficient was K = 1.45 mm h−1/2 . The results of an electrical method to measure the migration length are displayed in Fig. 7. The electrical conductance was measured between the electrodes a and b. Electrochemical reduction of PEDOT:PSS in the area between x = 0 and x = L resulted in a decreasing conductance. The normalized plots of length (L/Lmax )
Fig. 7. (a) A top view of the reduced PEDOT:PSS area and electrodes a and b; (b) normalized plot of length and conductance value versus time.
and conductance (−G/Gmax ) versus time validate this correlation. This behavior enables the device to act as a variable resistor in an electrical circuit, for instance modifying the load resistance of an RFID tag.
Fig. 5. Normalized plot of mass and length versus time.
Fig. 8. Dependence of the migration coefficient K on the amount of salt in the migration layer.
Fig. 6. Migration length versus time, electrochemical doping, PEDOT:PSS.
Fig. 9. Migration length versus time for changing temperatures.
J.P. Hermes, M. Knoll / Electrochimica Acta 54 (2009) 4258–4261
4261
Fig. 10. “Best before” labels based on doping front migration.
The migration velocity depends on the amount of salt incorporated in the migration layer. This was investigated by comparing the migration coefficients of layers containing various salt concentrations. The plot contained in Fig. 8 demonstrates that the migration velocity increases with higher salt content. Higher concentrations could not be analyzed because CaCl2 is hygroscopic in the dry state and migration began during exposure to moderate humidity in the laboratory rather than starting under controlled conditions in the humid chamber. The migration velocity is also affected by temperature. This is the basic property enabling these devices to be used as time–temperature integrators. This ability was tested by exposing the devices to changing temperatures. Fig. 9 depicts the function L2 = f(t). The slope of the curve is given by K2 which is almost the same for each temperature regardless of the temperature exposure history of the sample. 5. Conclusion and outlook We demonstrated doping front migration based on pH doping and electrochemical doping of the intrinsically conductive polymers PANI:DBSA and PEDOT:PSS. The influence of the composition of the migration layer as well as the relation between migration length and mass uptake in the migration layer were studied. The change of electrical conductance was also compared to the migration length. Doping front migration does not depend on a battery. The device may be read out by eye at any time, and is capable of monitoring time and temperature exposure.
This ultralow-cost system may form the basis of a new class of smart labels containing electrical functions and capable of displaying changing information to the human eye. The device may also be used as a time–temperature integrator that monitors and reports product storage conditions. This may be very useful as a “best before” label on food products. The time–temperature integrator works like a clock which is running faster at higher temperatures. If the cold chain of a food product is broken the time–temperature integral increases and the bar of the device reaches the end of the scale before the date of the shelf life which is printed on the package. Doping front migration could be activated by water vapor diffusing from the product and passing through a gas permeable membrane into the migration layer (Fig. 10). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J.S. Rasul, Microelectron. Reliab. 44 (1) (2004) 135. J. Whitmarsh, Microelectron. Int. 22 (3) (2005) 16. Knoll, Electrochim. Acta 54 (2008) 216. T. Alfrey, et al., J. Polym. Sci. Pol. Sym. 12PC (1966) 249. J. Crank, J. Polym. Sci. 11 (2) (1953) 151. E. Gurnee, J. Polym. Sci. 41 (138) (1959) 119. G.F. Billovits, C.J. Durning, Polymer 29 (8) (1988) 1468. M.A. Samus, G. Rossi, Macromolecules 29 (6) (1996) 2275. L. Perrin, et al., Polym. Int. 39 (3) (1996) 251. G. Astarita, G.C. Sarti, Polym. Eng. Sci. 18 (5) (1978) 388. G.S. Kulagina, et al., Polym. Sci., Ser. A 49 (4) (2007) 425. V.S. Markin, et al., Pharm. Chem. J. 32 (7) (1998) 44. F.A. Long, L.J. Thompson, J. Pol. Sci. 15 (80) (1955) 413. A.G. Green, A.E. Woodhead, J. Chem. Soc. 97 (1910) 2338. R. Cushman, J. Electroanal. Chem. 219 (1987) 335. J. Ouyang, et al., Polymer 45 (25) (2004) 8443.