- Email: [email protected]
Synthesis of polyaniline films by plasma polymerization
Synthetic Metals 88 (1997) 213–218
Synthesis of polyaniline films by plasma polymerization G.J. Cruz a, J. Morales b, M.M. Castillo-Ortega c, R. Olayo b,U a
Departamento de Fı´sica, ININ, Apdo. Postal 18-1027, Col. Escando´n, Mexico D.F. 11801, Mexico b Departamento de Fı´sica, UAM-I, Apdo. Postal 55-534, Iztapalapa, Mexico D.F. 09340, Mexico c Departamento de Investigacio´n en Polı´meros y Materiales, UNISON, Rosales y Blvd. Transversal, Hermosillo, Sonora, Mexico Received 20 February 1997; accepted 21 February 1997
Abstract This work presents a study on the formation of polyaniline films by plasma polymerization using RF glow discharges with resistive coupling between stainless-steel electrodes. The polymer was obtained at a frequency of 13.5 MHz and pressure in the range (2–8)=10y2 Torr. Polyaniline and I2-doped polyaniline films were synthesized as thin films that can be adhered to glass and metal surfaces. The IR analysis shows the characteristic polyaniline peaks with evidence that some benzene rings are broken by the discharge energy. The thermogravimetric analysis shows an increase in polymer decomposition as a function of the time of synthesis. This effect, the physical characteristics and solubility of the film suggest crosslinking in the polymers due to the continuous impact of the electronic rain. The way in which the relative humidity affects the electric conductivity of plasma polyaniline films was established. At room temperature and relative humidity of 40% the conductivity was between 10y10 and 10y12 S/cm. When the films were synthesized with I2, the electric conductivity ranged from 10y4 to 10y11 S/cm with relative humidity from 40 to 90%. Keywords: Polyaniline; Films; Plasma; Polymerization; Iodine; Conductivity; Humidity
1. Introduction Aniline is considered as a starting monomer to produce conducting polymers, as homopolymer or by doping with anions. Polyaniline (PAn) is usually obtained in solution by direct oxidation of aniline or by electrochemical oxidation [1]. This process is used to obtain linear PAn and sometimes is combined with simultaneous doping to increase the conductivity. Linear PAn is an electric insulating polymer but can be converted to a semiconductor by doping the compound with halogens and other anions. Crosslinked PAn shows similar behavior with lower values of conductivity. Films of PAn are difficult to synthesize by conventional chemical methods because they form powder when recovered from the starting acid solution. Some PAn films over insulating substrates have been prepared by chemical oxidation using very dilute aqueous solutions of aniline [2]. Thin films of around 1 mm in thickness of chemically synthesized PAn have also been prepared by thermal and/or vacuum evaporation of the polymer powder [3]. Paloheimo et al. [4] recently used the layer-by-layer self-assembly method for fabricating thin films of PAn. U
Corresponding author. Tel.: q52 5 724-4625; fax: q52 5 724-4611; e-mail: [email protected]
Another method of PAn film synthesis less commonly used is by plasma polymerization. In this method, the reaction begins in the gas phase and, as the polymer grows, goes to the nearest wall, where the film is formed. The chemical reactions are promoted by excitation and/or ionization produced by collisions of the monomer molecules with the electrons immersed in a plasma generated by electric discharges. The best suited plasma for polymerization is the glow discharge. In this process the polymerization occurs without the presence of other reagents, such as the oxidant agent and the solvent in the liquid phase, giving a product without major contamination. The reactions can also be combined with other compounds looking for specific characteristics in the final polymer. There is not much information about PAn synthesized with this technique. Bhat and Joshi [5] synthesized PAn by plasma polymerization using the RF inductive coupling mechanism. The plasma polymerization method has also been used to synthesize other conducting polymers [6– 8]. Fig. 1 shows the four idealized oxidation states of PAn: leucoemeraldine, emeraldine, pernigraniline and emeraldine salt. Different structures result in different electrical behaviors of the material. Emeraldine salt is a partially oxidized compound, protonated, with electrical conducting characteristics. Leucoemeraldine is a fully reduced compound with
0379-6779/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved PII S 0 3 7 9 - 6 7 7 9 ( 9 7 ) 0 3 8 5 3 - 8
Journal: SYNMET (Synthetic Metals)
Article: 4987
214
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
Fig. 2. Experimental setup. Fig. 1. Representation of idealized oxidation states of PAn.
electrical insulating characteristics. There are no double bonds between the aromatic rings and the N–H groups. Emeraldine base is an insulating compound, partially oxidized with few N–H groups in the main chain. Emeraldine changes from insulator to conductor when it is protonated with proton donor acids, such as HCl. This change is one of the most interesting properties of PAns. The structure of emeraldine PAn can be changed to emeraldine salt by removing an electron from the N–H group. Pernigraniline is a fully oxidized compound without conducting characteristics. There are no N–H groups in the structure. The level of protonation in the structure causes dramatic changes in the conductivity. Another factor that affects conductivity in doped PAns is moisture. It is known that moisture increases the conductivity in doped PAn by changing the electrostatic interaction between anions and positive charges. The increment in conductivity reported for PAn doped with HCl or camphor sulfonic acid under moisture is around one order of magnitude [9]. In this work we present the process to obtain PAn films and in situ I2-doped PAn films by plasma polymerization using RF glow discharges with the resistive coupling mechanism. We study the influence of the dopant, the synthesis parameters and the relative humidity on the electric PAn properties. Both films, undoped and doped PAns, are studied by thermogravimetry, IR spectroscopy and scanning electronic microscopy to determine the structure and the oxidation states of plasma PAns.
2. Experimental setup The experimental setup is shown in Fig. 2. It consists of a 1500 cm3 cylinder glass reactor, coupled to a system of mechanical and oil diffusion vacuum pumps, a wave generator and an RF amplifier. At the sides of the reactor are located the electrodes, which are made of stainless-steel bars and plates. The glow discharges in the reactor were set by a Wavetek 164 wave generator and an ENI A150 RF amplifier with a resistive coupling mechanism at 13.5 MHz and around 12 W power. The glow discharges were set without carrier gas.
Aniline was Baker reagent grade, previously distilled. Iodine, acetone, acetonitrile, tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO) were also Baker reactive grade. The system pressure was (2–8)=10y2 Torr, measured with an Edwards Pirani gauge. In the reactor the average temperature was around 90 8C between the electrodes. The monomer was supplied as vapor exhausted from the container with liquid aniline. The polymerization reaction times were 60 to 300 min. The IR spectra of the films were taken with a Perkin-Elmer FT 1600 spectrophotometer sampled directly from the films. The thermogravimetric data were taken with a modified Dupont 951 thermogravimetric analyzer. A heating ramp of 15 8C/min, from room temperature to 1000 8C was used. The analyzer was connected to a 2100 console from TA Instruments. The PAn electric resistance was measured in a capacitive arrangement with the sample in the middle, with an OTTO MX620 digital multimeter. A glass bell was used to induce a relative humidity of 35–90% to the films. The film thickness was measured with a Mitutoyo micrometer with resolution of 1 mm. 3. Results and discussion Several runs of plasma PAn syntheses were done. The first part of the results belongs to the synthesis of PAn alone, while the second part corresponds to the synthesis of PAn with I2. With the addition of I2 we looked for an increase of the PAn electric conductivity and its sensitivity to environmental humidity. The result of the polymerization reaction was a thin film formed on the internal surfaces of the reactor. Metallic and glass surfaces were used to observe the adherence of the films. In all cases the film was deposited with approximately the same characteristics. The exception was the surface of the electrodes, where part of the film was burned by the discharge, which is more energetic in this zone. Films for at least six different times were polymerized for each method. 3.1. Solubility PAns are difficult to solubilize. Two of the common solvents of PAn are DMSO and THF. The plasma PAn film is
Journal: SYNMET (Synthetic Metals)
Article: 4987
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
215
almost inert to DMSO and THF. The PAn was washed with acetone, acetonitrile and DMSO. With acetone the film swelled and was separated from the surface. The other solvents were then applied to dissolve the aniline, oligomers and low molecular weight PAn. None of the three solvents dissolved the remaining film, even though the DMSO was heated up to 140 8C with strong stirring. 3.2. Film thickness Fig. 3 shows the evolution of film thickness. The points in the plot represent values obtained in different experiments. It can be noted that the PAn film grew showing a linear thickness increase from 2.1 to 10.63 mm, in a reaction time range of 60 to 300 min. The film of PAn doped with I2, simultaneously at the reaction time, also shows a linear increase from 6.3 to 21.3 mm with a higher slope. The film was homogeneously deposited. It had little variation in thickness in the region between the electrodes. The dopant influence in the film is noted by the greater thickness in doped films compared with the films obtained without dopant at the same reaction time. The points in Fig. 3 represent the average values in the reactor surface. The films were obtained with a pressure of (2–8)=10y2 Torr and 10–12 W power. Although the experiments had some points outside these conditions, most of the time shared this region. Under these conditions the polymerization reaction gave the best results for the film growth in our system. Runs with conditions outside this range resulted in very thin films at the same reaction time. Bhat and Joshi [5] report the plasma PAn film thickness growing in a short time, up to 10 min, using the inductive coupling mechanism at 40 W power. In their experiments the film thickness had a maximum of around 3.3 mm at 6 min of reaction, having a tendency to saturation in that period. In our case, the reaction time was more than 40 times greater using a resistive coupling mechanism. 3.3. Morphology
Fig. 4. Microphotograph of PAn film at 350= (reduced in reproduction by 43%).
over the substrate. The layers have different thickness and are deposited as a group of disordered paper sheets. The sharp edges of the layers resemble fractures in a body subject to tension; this may be generated when the film is removed from the surface. The layer structure may imply a deposit mechanism that fuses the polymer and gives it some cyclic spatial homogeneity. Fig. 3 shows a linear increase of film thickness with time, so the appearance in the layers could not be assigned to a difference in the growing film with time. It should be a periodic change in the structure that can be generated by crosslinking of the polymer. Fig. 5 shows a microphotograph of PAn film at 2500= magnification. At this magnification, the surface is seen as two regions with different appearances: one is the plain film and the other consists of bubbles randomly distributed on the surface. The diameter of the bubbles varies from 1 to 6 mm. The plain surface is the polymer that has grown homogeneously. The bubbles can be attributed to drops of monomer or oligomers trapped between the layers that vaporize, forming small bubbles in the film, and may be the start of the tensions that promote the large fractures. Fig. 6 shows a microphotograph of PAn film at 20 000= magnification. At this magnification the film presents a con-
Fig. 4 shows a photograph of PAn film at 350= magnification. It can be seen that the film is formed by layers stacked
Fig. 3. PAn film thickness as a function of the reaction time.
Fig. 5. Microphotograph of PAn film at 2500= (reduced in reproduction by 43%).
Journal: SYNMET (Synthetic Metals)
Article: 4987
216
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
Fig. 8. IR spectra of PAn films from different regions of the reactor. Fig. 6. Microphotograph of PAn film at 20 000= (reduced in reproduction by 43%).
tinuous surface without discontinuities. This surface is the plain region shown in Fig. 5. The surface shows some microstructure, as a surface made of collapsing particles of different sizes but with a layer structure. 3.4. IR analysis Fig. 7 shows the IR spectra taken from the PAn films synthesized at different reaction times. The spectra show peaks at 3362 and 1633 cmy1 due to N–H vibration and deformation. A peak belonging to the C–H aliphatic vibration is located at 2932 cmy1. The peaks at 1316 and 1173 cmy1 may be due to the C_C and C–C interactions. These medium and small size peaks suggest the presence of aliphatic groups in the main chains, probably due to the breaking of benzene rings. The benzene ring with disubstitution 1,4 is found at 830–840 cmy1. The C–N bond of the diphenyl amine group is observed in the IR spectrum at 755 and 700 cmy1. This bond can be found in leucoemeraldine PAn (see Fig. 1). Fig. 8 shows the IR spectra taken from the I2-doped PAn films at different regions in the reactor. The peaks belonging to the 1,4 disubstitutions in the benzene ring, 832, 752 and 696 cmy1, are more intense in the region labeled as opaque, that is located in the reactor between the ground electrode and the monomer inlet. The films with better performance in electric conductivity are located in this region.
Fig. 7. IR spectra of plasma PAn films at different reaction times.
Fig. 9. IR spectra of PAn films doped with I2.
Fig. 9 shows the IR spectra of PAn films doped with I2. The peaks at 545 and 833 cmy1 belong to the absorption of 1,4 disubstitution in benzene rings. The peaks at 507 and 751 cmy1 can be assigned to 1,2 and 1,3 disubstitution. The absorption at 696 cmy1 corresponds to the 1,3 disubstitution. The intensity of the peaks indicates that there are 1,4 disubstitutions in the benzene rings; moreover, the 1,2 and 1,3 disubstitutions may be present. The intensity of peaks between 840 and 696 cmy1 is bigger in the PAn doped with I2 than in the PAn without dopants. In particular, the peaks of 1,4 disubstitution in the benzene ring are more intense in the doped PAn. This may be the difference in the film conductance ability, which, combined with the presence of I2, makes better semiconductor films. 3.5. Thermal degradation PAn synthesized by a chemical or electrochemical method is a stable powder between 0 and 300 8C. The linear polymer can exhibit thermal decomposition proportional to temperature without large changes in the mass loss tendency. Fig. 10 shows the thermal decomposition of plasma PAn films in nitrogen atmosphere. The curves belong to samples taken after 60–180 min of reaction time. At 100 8C the samples have lost around 3% of the total mass, possibly water and solvents trapped inside. Later, there is a flat zone and, at 150 8C, the major mass loss tendency begins. At approximately 500 8C the tendency changes to a moderate mass loss.
Journal: SYNMET (Synthetic Metals)
Article: 4987
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
Fig. 10. Thermogravimetric decomposition of PAn films.
At the maximum temperature the samples have lost 85–60% of the total mass. The curves show the tendency to lose more mass in the region 200–500 8C as the reaction time is increased. At 1000 8C, the sample for 60 min of reaction time has lost around 60% mass and its behavior presents a slow decomposition. On the other hand, at 180 min of reaction time, the profile of the curve suggests that the polymer decomposes rapidly in many places, between 200 and 500 8C, once the energy to break the first bonds is reached. As the atmosphere is nitrogen, the mechanism of decomposition is assumed to be thermal degradation instead of oxidation. The crosslinking reduces the thermal stability of the samples making the structures more rigid. This phenomenon may originate from the film’s exposure to the electron impact inside the reactor. The electrons, impacting constantly on the film, produce free radicals randomly in the structure that promotes crosslinking. This is a consequence of the exposure to the electronic rain. Thus, the heating produces an increasing structure vibration that breaks the atomic bonds randomly. Smaller fragments volatilize (200–500 8C), but medium size and bigger fragments remain until the bond breaking makes the fragments small enough to volatilize (500–850 8C). Later, the last bigger fragments begin to decompose at 850 8C. On the other hand, linear and small chains, as oligomers, have enough freedom of movement to degrade proportionally to the incoming energy. Other studies of PAn synthesized chemically show linear decomposition beginning from around 300 8C and ending up to 500–600 8C, depending upon the chemical synthesis conditions [10]. 3.6. Electric conductivity Figs. 11 and 12 show the electric resistivity and conductivity of the films measured after separating them from the substrates. The values are between 10y10 and 10y12 S/cm, depending on the reaction time and the relative humidity (RH) to which they were subjected. Fig. 11 shows the variation of the electric resistivity as a function of RH. The values are in the range 109–1012 V cm for reaction times from 60 to 300 min and RH of 40–90%.
217
Fig. 11. Electric resistivity of PAn films as a function of RH.
Fig. 12. Electric conductivity of PAn films as a function of the reaction time.
The plot shows that the resistivity decreases as humidity increases. After 90% RH, the limiting value for three of the curves is close to 9=109 V cm, while the value for the 300 min reaction is almost one order of magnitude less (9=108 V cm). Fig. 12 presents the behavior of the electric conductivity as a function of the reaction time for different RH. The plot shows a little growth in the conductivity as the reaction time increases. The values are in the order of 10y10–10y12 S/cm, typical for PAn synthesized by chemical or electrochemical methods without doping. The curves do not have a clear tendency. As discussed earlier, the exposure to the electronic rain produces crosslinking in the polymer; so when the reaction time increases, the crosslinking is also increased. This effect reduces the conductive properties of the polymer. On the other hand, the same effect increases the polymer’s ability to absorb water, which increases the conductivity of the system. Both effects influence the film’s electric characteristics in opposite directions. Fig. 12 shows the result of the competition at different reaction times and RH conditions. On the films that were doped in situ with I2 simultaneously at the polymerization reaction, the reaction time has the opposite effect on the resistivity compared with the undoped PAn films. The resistivity increases as the reaction time increases. The profile of the curves could be associated with the formation of PAn forms susceptible to being conductive at the
Journal: SYNMET (Synthetic Metals)
Article: 4987
218
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
Fig. 13. Electric resistivity of PAn films doped with I2.
Fig. 14. Electric conductivity of PAn films doped with I2.
beginning of the polymerization. As the time goes on, the continuous electronic rain reduces the conducting capacity because of the increasing crosslinking in the polymer. The electric conductivity after doping with I2 was 10y10– y4 10 S/cm. This dopant has significant influence on the conductivity, as we can see in Figs. 13 and 14, where the electric conductivity increases from 10y10 to 10y4 S/cm for RH from 40 to 90%. The water trapped in the films increases the electric conductivity by six orders of magnitude, while the same conditions for undoped PAn films increase the electric conductivity by only two orders of magnitude.
4. Conclusions The formation of PAn films by plasma polymerization was carried out by obtaining a thin film that can be adhered to glass and metal surfaces. The reaction conditions influenced the growth and structure of the film significantly because a measurable film thickness was only obtained within specific intervals of pressure and discharge power. Outside this interval the discharge produced a small amount of film.
Accordingly with the IR spectra, the plasma PAn is a compound that keeps the benzene rings of aniline with ortho, meta and para substitutions in which the chains grow to form the polymer. Another important point to consider is that benzene rings become broken by the glow discharge energy and that some fragments bond with the main chain of the polymer to form additional groups that make the structure different from the idealized structure of the oxidation states. The thermogravimetric decomposition shows that the polymer apparently grows linearly at the initial stages, but the continuous impact of electrons produced in the glow discharge promotes the formation of free radicals that crosslink the structure with the next monomer groups that are arriving at the surface. Longer reaction time produces more crosslinked structures. The undoped films exhibit electrical conductivity between 10y10 and 10y12 S/cm for low values of RH, but, as the humidity increases from 40 to 90%, the conductivity increases by around two orders of magnitude. There is a difference of one order of magnitude from the data reported by Pinto et al. [9] for PAn doped with camphor sulfonic acid. The conductivity has also the tendency to rise as the reaction time increases, around an order of magnitude in reaction times from 60 to 300 min. When aniline is combined with I2, the behavior of the conductivity changes significantly. The conductivity decreases as the reaction time increases, from 10y10–10y11 S/cm for 40% RH to 10y4–10y9 for 90% RH, conversely to the undoped films. The conductivity increases up to six orders of magnitude with an increment in RH of 50– 90%. This is four orders of magnitude more than undoped films. In this way the PAn films change their position in the conductivity scale, from insulator to semiconductor as a function of the environmental moisture. References [1] E.M. Genie`s, A. Boyle, M. Łapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139–182. [2] K.F. Schoch, W.A. Byers and L.J. Buckley, Synth. Met., 72 (1995) 13–23. [3] T.L. Porter, K. Caple and G. Caple, J. Vac. Sci. Technol. A, 12 (1994) 2441–2445. [4] J. Paloheimo, J. Laasko, H. Isotalo and H. Stub, Synth. Met., 68 (1995) 249–257. [5] N.V. Bhat and N.V. Joshi, Plasma Chem. Plasma Process., 4 (1994) 151–161. [6] X. Xie, J.U. Thiele, R. Steiner and P. Oelhafen, Synth. Met., 63 (1994) 221–224. [7] K. Tanaka, Y. Matsuura, S. Nishio and T. Yamabe, Synth. Met., 65 (1994) 81–84. [8] M. Grunwald, H.S. Munro and T. Wilhelm, Synth. Met., 41–43 (1991) 1465–1469. [9] N.J. Pinto, P.D. Shah, P.K. Kahol and B.J. McCormick, Phys. Rev. B, 53 (1996) 10 690–10 694. [10] K. Amano, H. Ishikawa, A. Kobayashi, M. Satoh and E. Hasegawa, Synth. Met., 62 (1994) 229–232.
Journal: SYNMET (Synthetic Metals)
Article: 4987
Synthesis of polyaniline films by plasma polymerization G.J. Cruz a, J. Morales b, M.M. Castillo-Ortega c, R. Olayo b,U a
Departamento de Fı´sica, ININ, Apdo. Postal 18-1027, Col. Escando´n, Mexico D.F. 11801, Mexico b Departamento de Fı´sica, UAM-I, Apdo. Postal 55-534, Iztapalapa, Mexico D.F. 09340, Mexico c Departamento de Investigacio´n en Polı´meros y Materiales, UNISON, Rosales y Blvd. Transversal, Hermosillo, Sonora, Mexico Received 20 February 1997; accepted 21 February 1997
Abstract This work presents a study on the formation of polyaniline films by plasma polymerization using RF glow discharges with resistive coupling between stainless-steel electrodes. The polymer was obtained at a frequency of 13.5 MHz and pressure in the range (2–8)=10y2 Torr. Polyaniline and I2-doped polyaniline films were synthesized as thin films that can be adhered to glass and metal surfaces. The IR analysis shows the characteristic polyaniline peaks with evidence that some benzene rings are broken by the discharge energy. The thermogravimetric analysis shows an increase in polymer decomposition as a function of the time of synthesis. This effect, the physical characteristics and solubility of the film suggest crosslinking in the polymers due to the continuous impact of the electronic rain. The way in which the relative humidity affects the electric conductivity of plasma polyaniline films was established. At room temperature and relative humidity of 40% the conductivity was between 10y10 and 10y12 S/cm. When the films were synthesized with I2, the electric conductivity ranged from 10y4 to 10y11 S/cm with relative humidity from 40 to 90%. Keywords: Polyaniline; Films; Plasma; Polymerization; Iodine; Conductivity; Humidity
1. Introduction Aniline is considered as a starting monomer to produce conducting polymers, as homopolymer or by doping with anions. Polyaniline (PAn) is usually obtained in solution by direct oxidation of aniline or by electrochemical oxidation [1]. This process is used to obtain linear PAn and sometimes is combined with simultaneous doping to increase the conductivity. Linear PAn is an electric insulating polymer but can be converted to a semiconductor by doping the compound with halogens and other anions. Crosslinked PAn shows similar behavior with lower values of conductivity. Films of PAn are difficult to synthesize by conventional chemical methods because they form powder when recovered from the starting acid solution. Some PAn films over insulating substrates have been prepared by chemical oxidation using very dilute aqueous solutions of aniline [2]. Thin films of around 1 mm in thickness of chemically synthesized PAn have also been prepared by thermal and/or vacuum evaporation of the polymer powder [3]. Paloheimo et al. [4] recently used the layer-by-layer self-assembly method for fabricating thin films of PAn. U
Corresponding author. Tel.: q52 5 724-4625; fax: q52 5 724-4611; e-mail: [email protected]
Another method of PAn film synthesis less commonly used is by plasma polymerization. In this method, the reaction begins in the gas phase and, as the polymer grows, goes to the nearest wall, where the film is formed. The chemical reactions are promoted by excitation and/or ionization produced by collisions of the monomer molecules with the electrons immersed in a plasma generated by electric discharges. The best suited plasma for polymerization is the glow discharge. In this process the polymerization occurs without the presence of other reagents, such as the oxidant agent and the solvent in the liquid phase, giving a product without major contamination. The reactions can also be combined with other compounds looking for specific characteristics in the final polymer. There is not much information about PAn synthesized with this technique. Bhat and Joshi [5] synthesized PAn by plasma polymerization using the RF inductive coupling mechanism. The plasma polymerization method has also been used to synthesize other conducting polymers [6– 8]. Fig. 1 shows the four idealized oxidation states of PAn: leucoemeraldine, emeraldine, pernigraniline and emeraldine salt. Different structures result in different electrical behaviors of the material. Emeraldine salt is a partially oxidized compound, protonated, with electrical conducting characteristics. Leucoemeraldine is a fully reduced compound with
0379-6779/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved PII S 0 3 7 9 - 6 7 7 9 ( 9 7 ) 0 3 8 5 3 - 8
Journal: SYNMET (Synthetic Metals)
Article: 4987
214
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
Fig. 2. Experimental setup. Fig. 1. Representation of idealized oxidation states of PAn.
electrical insulating characteristics. There are no double bonds between the aromatic rings and the N–H groups. Emeraldine base is an insulating compound, partially oxidized with few N–H groups in the main chain. Emeraldine changes from insulator to conductor when it is protonated with proton donor acids, such as HCl. This change is one of the most interesting properties of PAns. The structure of emeraldine PAn can be changed to emeraldine salt by removing an electron from the N–H group. Pernigraniline is a fully oxidized compound without conducting characteristics. There are no N–H groups in the structure. The level of protonation in the structure causes dramatic changes in the conductivity. Another factor that affects conductivity in doped PAns is moisture. It is known that moisture increases the conductivity in doped PAn by changing the electrostatic interaction between anions and positive charges. The increment in conductivity reported for PAn doped with HCl or camphor sulfonic acid under moisture is around one order of magnitude [9]. In this work we present the process to obtain PAn films and in situ I2-doped PAn films by plasma polymerization using RF glow discharges with the resistive coupling mechanism. We study the influence of the dopant, the synthesis parameters and the relative humidity on the electric PAn properties. Both films, undoped and doped PAns, are studied by thermogravimetry, IR spectroscopy and scanning electronic microscopy to determine the structure and the oxidation states of plasma PAns.
2. Experimental setup The experimental setup is shown in Fig. 2. It consists of a 1500 cm3 cylinder glass reactor, coupled to a system of mechanical and oil diffusion vacuum pumps, a wave generator and an RF amplifier. At the sides of the reactor are located the electrodes, which are made of stainless-steel bars and plates. The glow discharges in the reactor were set by a Wavetek 164 wave generator and an ENI A150 RF amplifier with a resistive coupling mechanism at 13.5 MHz and around 12 W power. The glow discharges were set without carrier gas.
Aniline was Baker reagent grade, previously distilled. Iodine, acetone, acetonitrile, tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO) were also Baker reactive grade. The system pressure was (2–8)=10y2 Torr, measured with an Edwards Pirani gauge. In the reactor the average temperature was around 90 8C between the electrodes. The monomer was supplied as vapor exhausted from the container with liquid aniline. The polymerization reaction times were 60 to 300 min. The IR spectra of the films were taken with a Perkin-Elmer FT 1600 spectrophotometer sampled directly from the films. The thermogravimetric data were taken with a modified Dupont 951 thermogravimetric analyzer. A heating ramp of 15 8C/min, from room temperature to 1000 8C was used. The analyzer was connected to a 2100 console from TA Instruments. The PAn electric resistance was measured in a capacitive arrangement with the sample in the middle, with an OTTO MX620 digital multimeter. A glass bell was used to induce a relative humidity of 35–90% to the films. The film thickness was measured with a Mitutoyo micrometer with resolution of 1 mm. 3. Results and discussion Several runs of plasma PAn syntheses were done. The first part of the results belongs to the synthesis of PAn alone, while the second part corresponds to the synthesis of PAn with I2. With the addition of I2 we looked for an increase of the PAn electric conductivity and its sensitivity to environmental humidity. The result of the polymerization reaction was a thin film formed on the internal surfaces of the reactor. Metallic and glass surfaces were used to observe the adherence of the films. In all cases the film was deposited with approximately the same characteristics. The exception was the surface of the electrodes, where part of the film was burned by the discharge, which is more energetic in this zone. Films for at least six different times were polymerized for each method. 3.1. Solubility PAns are difficult to solubilize. Two of the common solvents of PAn are DMSO and THF. The plasma PAn film is
Journal: SYNMET (Synthetic Metals)
Article: 4987
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
215
almost inert to DMSO and THF. The PAn was washed with acetone, acetonitrile and DMSO. With acetone the film swelled and was separated from the surface. The other solvents were then applied to dissolve the aniline, oligomers and low molecular weight PAn. None of the three solvents dissolved the remaining film, even though the DMSO was heated up to 140 8C with strong stirring. 3.2. Film thickness Fig. 3 shows the evolution of film thickness. The points in the plot represent values obtained in different experiments. It can be noted that the PAn film grew showing a linear thickness increase from 2.1 to 10.63 mm, in a reaction time range of 60 to 300 min. The film of PAn doped with I2, simultaneously at the reaction time, also shows a linear increase from 6.3 to 21.3 mm with a higher slope. The film was homogeneously deposited. It had little variation in thickness in the region between the electrodes. The dopant influence in the film is noted by the greater thickness in doped films compared with the films obtained without dopant at the same reaction time. The points in Fig. 3 represent the average values in the reactor surface. The films were obtained with a pressure of (2–8)=10y2 Torr and 10–12 W power. Although the experiments had some points outside these conditions, most of the time shared this region. Under these conditions the polymerization reaction gave the best results for the film growth in our system. Runs with conditions outside this range resulted in very thin films at the same reaction time. Bhat and Joshi [5] report the plasma PAn film thickness growing in a short time, up to 10 min, using the inductive coupling mechanism at 40 W power. In their experiments the film thickness had a maximum of around 3.3 mm at 6 min of reaction, having a tendency to saturation in that period. In our case, the reaction time was more than 40 times greater using a resistive coupling mechanism. 3.3. Morphology
Fig. 4. Microphotograph of PAn film at 350= (reduced in reproduction by 43%).
over the substrate. The layers have different thickness and are deposited as a group of disordered paper sheets. The sharp edges of the layers resemble fractures in a body subject to tension; this may be generated when the film is removed from the surface. The layer structure may imply a deposit mechanism that fuses the polymer and gives it some cyclic spatial homogeneity. Fig. 3 shows a linear increase of film thickness with time, so the appearance in the layers could not be assigned to a difference in the growing film with time. It should be a periodic change in the structure that can be generated by crosslinking of the polymer. Fig. 5 shows a microphotograph of PAn film at 2500= magnification. At this magnification, the surface is seen as two regions with different appearances: one is the plain film and the other consists of bubbles randomly distributed on the surface. The diameter of the bubbles varies from 1 to 6 mm. The plain surface is the polymer that has grown homogeneously. The bubbles can be attributed to drops of monomer or oligomers trapped between the layers that vaporize, forming small bubbles in the film, and may be the start of the tensions that promote the large fractures. Fig. 6 shows a microphotograph of PAn film at 20 000= magnification. At this magnification the film presents a con-
Fig. 4 shows a photograph of PAn film at 350= magnification. It can be seen that the film is formed by layers stacked
Fig. 3. PAn film thickness as a function of the reaction time.
Fig. 5. Microphotograph of PAn film at 2500= (reduced in reproduction by 43%).
Journal: SYNMET (Synthetic Metals)
Article: 4987
216
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
Fig. 8. IR spectra of PAn films from different regions of the reactor. Fig. 6. Microphotograph of PAn film at 20 000= (reduced in reproduction by 43%).
tinuous surface without discontinuities. This surface is the plain region shown in Fig. 5. The surface shows some microstructure, as a surface made of collapsing particles of different sizes but with a layer structure. 3.4. IR analysis Fig. 7 shows the IR spectra taken from the PAn films synthesized at different reaction times. The spectra show peaks at 3362 and 1633 cmy1 due to N–H vibration and deformation. A peak belonging to the C–H aliphatic vibration is located at 2932 cmy1. The peaks at 1316 and 1173 cmy1 may be due to the C_C and C–C interactions. These medium and small size peaks suggest the presence of aliphatic groups in the main chains, probably due to the breaking of benzene rings. The benzene ring with disubstitution 1,4 is found at 830–840 cmy1. The C–N bond of the diphenyl amine group is observed in the IR spectrum at 755 and 700 cmy1. This bond can be found in leucoemeraldine PAn (see Fig. 1). Fig. 8 shows the IR spectra taken from the I2-doped PAn films at different regions in the reactor. The peaks belonging to the 1,4 disubstitutions in the benzene ring, 832, 752 and 696 cmy1, are more intense in the region labeled as opaque, that is located in the reactor between the ground electrode and the monomer inlet. The films with better performance in electric conductivity are located in this region.
Fig. 7. IR spectra of plasma PAn films at different reaction times.
Fig. 9. IR spectra of PAn films doped with I2.
Fig. 9 shows the IR spectra of PAn films doped with I2. The peaks at 545 and 833 cmy1 belong to the absorption of 1,4 disubstitution in benzene rings. The peaks at 507 and 751 cmy1 can be assigned to 1,2 and 1,3 disubstitution. The absorption at 696 cmy1 corresponds to the 1,3 disubstitution. The intensity of the peaks indicates that there are 1,4 disubstitutions in the benzene rings; moreover, the 1,2 and 1,3 disubstitutions may be present. The intensity of peaks between 840 and 696 cmy1 is bigger in the PAn doped with I2 than in the PAn without dopants. In particular, the peaks of 1,4 disubstitution in the benzene ring are more intense in the doped PAn. This may be the difference in the film conductance ability, which, combined with the presence of I2, makes better semiconductor films. 3.5. Thermal degradation PAn synthesized by a chemical or electrochemical method is a stable powder between 0 and 300 8C. The linear polymer can exhibit thermal decomposition proportional to temperature without large changes in the mass loss tendency. Fig. 10 shows the thermal decomposition of plasma PAn films in nitrogen atmosphere. The curves belong to samples taken after 60–180 min of reaction time. At 100 8C the samples have lost around 3% of the total mass, possibly water and solvents trapped inside. Later, there is a flat zone and, at 150 8C, the major mass loss tendency begins. At approximately 500 8C the tendency changes to a moderate mass loss.
Journal: SYNMET (Synthetic Metals)
Article: 4987
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
Fig. 10. Thermogravimetric decomposition of PAn films.
At the maximum temperature the samples have lost 85–60% of the total mass. The curves show the tendency to lose more mass in the region 200–500 8C as the reaction time is increased. At 1000 8C, the sample for 60 min of reaction time has lost around 60% mass and its behavior presents a slow decomposition. On the other hand, at 180 min of reaction time, the profile of the curve suggests that the polymer decomposes rapidly in many places, between 200 and 500 8C, once the energy to break the first bonds is reached. As the atmosphere is nitrogen, the mechanism of decomposition is assumed to be thermal degradation instead of oxidation. The crosslinking reduces the thermal stability of the samples making the structures more rigid. This phenomenon may originate from the film’s exposure to the electron impact inside the reactor. The electrons, impacting constantly on the film, produce free radicals randomly in the structure that promotes crosslinking. This is a consequence of the exposure to the electronic rain. Thus, the heating produces an increasing structure vibration that breaks the atomic bonds randomly. Smaller fragments volatilize (200–500 8C), but medium size and bigger fragments remain until the bond breaking makes the fragments small enough to volatilize (500–850 8C). Later, the last bigger fragments begin to decompose at 850 8C. On the other hand, linear and small chains, as oligomers, have enough freedom of movement to degrade proportionally to the incoming energy. Other studies of PAn synthesized chemically show linear decomposition beginning from around 300 8C and ending up to 500–600 8C, depending upon the chemical synthesis conditions [10]. 3.6. Electric conductivity Figs. 11 and 12 show the electric resistivity and conductivity of the films measured after separating them from the substrates. The values are between 10y10 and 10y12 S/cm, depending on the reaction time and the relative humidity (RH) to which they were subjected. Fig. 11 shows the variation of the electric resistivity as a function of RH. The values are in the range 109–1012 V cm for reaction times from 60 to 300 min and RH of 40–90%.
217
Fig. 11. Electric resistivity of PAn films as a function of RH.
Fig. 12. Electric conductivity of PAn films as a function of the reaction time.
The plot shows that the resistivity decreases as humidity increases. After 90% RH, the limiting value for three of the curves is close to 9=109 V cm, while the value for the 300 min reaction is almost one order of magnitude less (9=108 V cm). Fig. 12 presents the behavior of the electric conductivity as a function of the reaction time for different RH. The plot shows a little growth in the conductivity as the reaction time increases. The values are in the order of 10y10–10y12 S/cm, typical for PAn synthesized by chemical or electrochemical methods without doping. The curves do not have a clear tendency. As discussed earlier, the exposure to the electronic rain produces crosslinking in the polymer; so when the reaction time increases, the crosslinking is also increased. This effect reduces the conductive properties of the polymer. On the other hand, the same effect increases the polymer’s ability to absorb water, which increases the conductivity of the system. Both effects influence the film’s electric characteristics in opposite directions. Fig. 12 shows the result of the competition at different reaction times and RH conditions. On the films that were doped in situ with I2 simultaneously at the polymerization reaction, the reaction time has the opposite effect on the resistivity compared with the undoped PAn films. The resistivity increases as the reaction time increases. The profile of the curves could be associated with the formation of PAn forms susceptible to being conductive at the
Journal: SYNMET (Synthetic Metals)
Article: 4987
218
G.J. Cruz et al. / Synthetic Metals 88 (1997) 213–218
Fig. 13. Electric resistivity of PAn films doped with I2.
Fig. 14. Electric conductivity of PAn films doped with I2.
beginning of the polymerization. As the time goes on, the continuous electronic rain reduces the conducting capacity because of the increasing crosslinking in the polymer. The electric conductivity after doping with I2 was 10y10– y4 10 S/cm. This dopant has significant influence on the conductivity, as we can see in Figs. 13 and 14, where the electric conductivity increases from 10y10 to 10y4 S/cm for RH from 40 to 90%. The water trapped in the films increases the electric conductivity by six orders of magnitude, while the same conditions for undoped PAn films increase the electric conductivity by only two orders of magnitude.
4. Conclusions The formation of PAn films by plasma polymerization was carried out by obtaining a thin film that can be adhered to glass and metal surfaces. The reaction conditions influenced the growth and structure of the film significantly because a measurable film thickness was only obtained within specific intervals of pressure and discharge power. Outside this interval the discharge produced a small amount of film.
Accordingly with the IR spectra, the plasma PAn is a compound that keeps the benzene rings of aniline with ortho, meta and para substitutions in which the chains grow to form the polymer. Another important point to consider is that benzene rings become broken by the glow discharge energy and that some fragments bond with the main chain of the polymer to form additional groups that make the structure different from the idealized structure of the oxidation states. The thermogravimetric decomposition shows that the polymer apparently grows linearly at the initial stages, but the continuous impact of electrons produced in the glow discharge promotes the formation of free radicals that crosslink the structure with the next monomer groups that are arriving at the surface. Longer reaction time produces more crosslinked structures. The undoped films exhibit electrical conductivity between 10y10 and 10y12 S/cm for low values of RH, but, as the humidity increases from 40 to 90%, the conductivity increases by around two orders of magnitude. There is a difference of one order of magnitude from the data reported by Pinto et al. [9] for PAn doped with camphor sulfonic acid. The conductivity has also the tendency to rise as the reaction time increases, around an order of magnitude in reaction times from 60 to 300 min. When aniline is combined with I2, the behavior of the conductivity changes significantly. The conductivity decreases as the reaction time increases, from 10y10–10y11 S/cm for 40% RH to 10y4–10y9 for 90% RH, conversely to the undoped films. The conductivity increases up to six orders of magnitude with an increment in RH of 50– 90%. This is four orders of magnitude more than undoped films. In this way the PAn films change their position in the conductivity scale, from insulator to semiconductor as a function of the environmental moisture. References [1] E.M. Genie`s, A. Boyle, M. Łapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139–182. [2] K.F. Schoch, W.A. Byers and L.J. Buckley, Synth. Met., 72 (1995) 13–23. [3] T.L. Porter, K. Caple and G. Caple, J. Vac. Sci. Technol. A, 12 (1994) 2441–2445. [4] J. Paloheimo, J. Laasko, H. Isotalo and H. Stub, Synth. Met., 68 (1995) 249–257. [5] N.V. Bhat and N.V. Joshi, Plasma Chem. Plasma Process., 4 (1994) 151–161. [6] X. Xie, J.U. Thiele, R. Steiner and P. Oelhafen, Synth. Met., 63 (1994) 221–224. [7] K. Tanaka, Y. Matsuura, S. Nishio and T. Yamabe, Synth. Met., 65 (1994) 81–84. [8] M. Grunwald, H.S. Munro and T. Wilhelm, Synth. Met., 41–43 (1991) 1465–1469. [9] N.J. Pinto, P.D. Shah, P.K. Kahol and B.J. McCormick, Phys. Rev. B, 53 (1996) 10 690–10 694. [10] K. Amano, H. Ishikawa, A. Kobayashi, M. Satoh and E. Hasegawa, Synth. Met., 62 (1994) 229–232.
Journal: SYNMET (Synthetic Metals)
Article: 4987