Influence of Different Organic Solvents and Oxidants on Insulating and Film Forming Properties of PEDOT Polymer Tariq Bashir
a, b*
, Fatimat Bakare c, Behnaz Baghaei c, Adib Kalantar Mehrjerdi c, Mikael Skrifvars
c
Corresponding to: Tariq Bashir a) School of Engineering, University of Borås, SE-50190 Borås Sweden b) Department of Polymer & Process Engineering, University of Engineering and Technology Lahore, Pakistan Email:
[email protected] Phone: +46 33 435 5922 Fax: +46 33 435 4008 Mobile: +46 760 809264
c) School of Engineering, University of Borås, SE-50190, Sweden Email:
[email protected] Phone: +46 33 435 4497 Fax: + 46 33 435 4008 Mobile: +46 708 196146
Abstract Processing of conjugate polymers has always been a challenge because of their poor solubility and infusibility in organic and inorganic solvents. The processibility and applications of intrinsically conductive polymers (ICPs) can be enhanced by producing their solutions or dispersions in different suitable solvents. It can also be achieved by preparing un-doped or electrically neutral polymers, which can further be transformed in semiconductor after oxidation/reduction reaction. The present study focuses on the preparation of active dispersions of poly (3,4-ethylenedioxythiophene) (PEDOT) conductive polymer in various organic solvents. For this purpose, the polymerization of 3,4-ethylenedioxythiophene (EDOT) monomer was carried out in three different organic solvents, ethanol, 1-butanol and acetonitrile with two commonly used oxidants, ferric (III) chloride (FeCl3 ) and ferric (III) ptoluenesulfonate (FepTS). In this regard, the oxidant and monomer solutions with variable molar concentrations (0.25, 0.5, 1.0 M) were prepared in particular solvents and then these solutions
were
mixed
with
different
monomer/oxidant
volume
ratios.
The
obtained
dispersions of PEDOT can readily be polymerized on the surface of different materials after solvent evaporation and a uniform film can be achieved. The effect of molar as well as volume concentrations of EDOT monomer and oxidant on insulating (undoped/neutral) and film forming properties of PEDOT was investigated. These dispersions were applied on a transparent PET film and cellulosic fibers (viscose), dried at room temperature and analyzed using scanning electron microscope (SEM), optical microscope and ATR-FTIR spectroscopic analysis. The electrical characterization of undoped PEDOT-coated fibers was performed on Keithly picoammeter.
This study contributes to
obtain a simpler and instantaneous
polymerization method of PEDOT preparation and to enhance its application area. Keywords: PEDOT dispersions, solution polymerization, organic solvents, PEDOT coating
INTRODUCTION Intrinsically conductive polymers (ICPs) with heterocyclic structures show good electrical conductivity with strong potential for electronic applications, such as in displays, smart windows, capacitors, sensors, and in batteries [1]. Processing of intrinsically conducting polymers has always been a challenge because of their stiff conjugated aromatic backbone structures, which makes them insoluble and infusible in most of the organic as well as
inorganic solvents [2,3]. In general, most of the virgin conjugated polymers cannot be melt processed. The blending of ICPs with other conventional polymers can enhance their processibility, but it considerably reduces their electroactive properties [4-6]. Poly (3,4ethylenedioxythiophene) (PEDOT) is one of the most prominent conjugated polymer because of its environmental stability, good electrical properties, transparency in thin oxidized films, and its use in different sophisticated electronic applications such as, in light emitting diodes (LEDs) [7-9], heat generation [10-12], EMI shielding, and chemical sensors [13-14]. [7-14]. Since PEDOT was first synthesized by Heywnag and Jonas in the early 1990s, many studies have been carried out to enhance its application areas by dissolving it in different organic solvents [15,16]. be
produced
An aqueous suspension of PEDOT, BAYTRON P, is available which can by
the
polymerization
of
EDOT
in
an
aqueous
polyelectrolyte
(polystyrenesulfonic acid) solution [17,18]. It has good film forming properties, but the performance of PEDOT is much reduced. The utilization of undoped or neutral PEDOT can also enhance its processibility. It can be extracted from the reaction mixture during polymerization process under suitable conditions and then after application on different substrates it can be oxidized to get conductive layers. In order to obtain uniform PEDOT layers on different substrates, flexible or rigid ones, it is useful to polymerize the PEDOT directly on the surface of substrates. Three different polymerization routes are available to attain thin uniform PEDOT layers, oxidative chemical vapour deposition (OCVD), electrochemical polymerization, and wet chemical oxidation [19]. The chemical vapour deposition technique has extensively been reported because it can give thin PEDOT polymer films under a controlled environment on a variety of substrates with maximum conductivity i.e. 1000 S/cm [1,19,14,20-24]. The OCVD is advantageous to obtain uniform coating layers on different substrates with smaller surface areas, so the mass production of PEDOT coated substrates is difficult. On the other hand, a very sophisticated technology and instruments are required for controlling the whole process. Electrochemical polymerization and electro-spinning can also be utilized to synthesize PEDOT layers on conducting donor substrate [25-27]. But the requirement of conductive substrates limits the applicability of this process. Laforgue A. et al. [3] utilized both technologies i.e. electrospinning and vapour phase polymerization to produce PEDOT nanofibers. So, the mass production of PEDOT might be possible if it could be available in solution form or at least in the form of stable colloidal. Not only for PEDOT, but also for other conducting polymers, the
addition of polymeric surfactants during the dispersion polymerization gives stable colloidal particles [18], which can be applied on the surface of substrates by simple coating techniques. In this paper, we focused on the preparation of PEDOT dispersions in different organic solvents which can instinctively be polymerized after solvent evaporation. We selected three well known organic solvents, ethanol, 1-butanol, and acetonitrile, whereas two oxidants, ferric (III) chloride (FeCl3 ) and ferric (III) p-toluene sulfonate (FepTS). The solutions of EDOT monomer and oxidants, with variable molar concentrations, were prepared in three above mentioned solvents and then mixed corresponding solutions with different monomer/oxidant volume ratios. The effects of molar as well as volume concentrations of EDOT monomer and oxidant on PEDOT crystallite formation in reaction mixtures were investigated. The prepared PEDOT dispersions were then applied on a transparent PET film and also on cellulose-based viscose fibers and dried at room temperature to determine the film-forming properties of PEDOT. The obtained PEDOT polymer exhibited insulating behaviour in undoped form and hence, shows very low conductivity. The formation of PEDOT polymer chains in reaction mixtures and surface morphology of PEDOT coatings were analyzed with scanning electron microscope, optical microscope and ATR-FTIR analysis. We investigated the electrical properties of produced PEDOT polymer without doping. Conductivity measurement on undoped PEDOT fibers revealed that better conductivity can be achieved with acetonitrile solvent and FeCl3 oxidant.
EXPERIMENTAL Materials In dispersion polymerization, 3,4-ethylenedioxythiophene (EDOT) monomer (CLEVIOUS T M M V2, H.C. Stark) was polymerized with oxidants, ferric (III) chloride (FeCl3 ) (SigmaAldrich, 98%) and ferric (III) p-toluene sulfonate (FepTS) (CLEVIOUS C-B 40 V2, Heraeus) in different solutions. The EDOT monomer and oxidant solutions were prepared in 1-butanol (C4 H9 OH) (Fisher Scientific), ethanol (C 2 H5 OH) (Fisher Scientific), and HPLC grade acetonitrile (CH3 CN) (Sigma-Aldrich) organic solvents.
The obtained PEDOT
dispersion was applied on viscose fibers (1220 dtex, 720 number of filaments, Z100 twist/meter provided by CORDENKA®). All these chemicals were used without of any further modification.
Experimental Method At first, the solutions of EDOT monomer and oxidant (FeCl3 ) were prepared in 1-butanol, ethanol, and acetonitrile with variable molar concentrations i.e. 0.25 M, 0.5 M, and 1.0 M. In order to find out the effect of molar and volume concentrations of EDOT monomer and oxidant on the PEDOT formation, the equimolar solutions of FeCl3 and EDOT monomer in 1butanol solvent with different EDOT/oxidant volume ratios; 1:1, 1:2, and 1:3, were mixed. The different combinations of EDOT monomer and oxidant solutions, we have tried are shown in Table 1. The yellowish colour of oxidant solutions was immediately transformed into the dark blue colour after the addition of EDOT monomer, which indicates the polymerization of EDOT monomer. The reaction mixtures were then heated at 70 C and stirred at 250 RPM for 20 minutes to extend the rate of polymerization. To investigate the effect of different organic solvents on PEDOT formation, the solutions of EDOT monomer (1.0M) and oxidant (FeCl3 ) (1.0M) in particular solvent, were mixed with same EDOT/oxidant volume ratios as described above. Also, other reaction conditions were kept constant. Similarly, the PEDOT dispersions in FepTS oxidant were prepared by mixing 1.0M EDOT solution in 1-butanol. Unstable PEDOT dispersions with variable concentrations of PEDOT particles were then obtained. After that, 5 ml of each type of reaction mixture was applied on the transparent PET films and dried at room temperature (23±3o C) and RH (70±5 %); to determine the film forming properties of PEDOT prepared in different reaction media. For further application, the viscose fibers were dipped into the PEDOT dispersions for few seconds and then dried at room temperature. From these PEDOT coated fibers we can get better idea about electrical properties of PEDOT films. Characterization Attenuated Total Reflectance Transform Infrared Spectroscopy (ATR-FTIR) The production and the properties of produced PEDOT polymer and PEDOT-coated viscose fibers was analyzed with attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR). IR spectra were acquired by using Nicolet 6700 FT-IR spectrometer in the ATR mode in the range from 4000 to 400 cm-1 with 32 scans and 4 cm-1 of band resolution. For better understanding, we observed the IR spectra between 600 and 1800 cm-1 wavenumbers.
Scanning Electron Microscopy (SEM) The surface morphology of PEDOT-coated viscose fibers obtained with different PEDOT dispersions was investigated by scanning electron microscopic (SEM) analysis. SEM images were acquired on gold sputtered PEDOT-coated viscose fiber samples in JEOL JSM-840A scanning electron microscope at 10kV accelerated voltage. Optical Microscopic Analysis Optical microscopic images of produced PEDOT dispersions, PEDOT coated PET films and PEDOT coated viscose fibers were acquired by using Nikon SMZ800 optical microscope with a 10× objective. Electrical Conductivity Measurement In order to find out the electrical conductivity of PEDOT films, the electrical resistance values of 15 cm long PEDOT-coated viscose fibers were acquired by using Keithly 6000 picoammeter high resistance meter with crocodile clips. All readings were taken at ambient conditions (25±5 C and 65±5 % RH).
RESULTS AND DISCUSSION Solution polymerization of PEDOT For solution polymerization, the EDOT monomer and oxidant solutions with variable molar concentrations (0.25 M, 0.5 M, 1.0 M), were prepared in 1-butanol, ethanol, and acetonitrile. We selected the same type of solvents for both of the EDOT monomer and oxidant. To find out the effect of molar concentration and volume ratios of EDOT monomer and oxidant on PEDOT formation, we took FeCl3 and EDOT monomer solutions with 0.25 M, 0.5 M, and 1.0 M concentration in 1-butanol. At first, EDOT monomer and FeCl3 solutions having 0.25M concentration, were mixed with different EDOT/oxidant volume ratios (v/v) .i.e. 1:1, 1:2, and 1:3 (the volume of EDOT monomer was kept constant). Other solutions of EDOT monomer and FeCl3 oxidant with 0.5 and 1.0 molar concentration were also mixed with same fashion. With the addition of EDOT monomer in FeCl3 solution, the colour of the reaction mixture was instantaneously changed from yellow to dark blue, which is the indication of PEDOT polymerization. In order to increase the extent of polymerization, the reaction mixtures were heated at 70 C for 20 minutes with continuous stirring at 250 RPM. After 20 minutes, the
reaction mixture became thick because of the excess of PEDOT polymer particles or longer PEDOT polymer chains. The schematic diagram of PEDOT polymer chains in reaction mixture is shown in Fig. 1. The PEDOT particles (PEDOT dispersions) in all types of reaction mixtures were then analyzed by optical microscope. Figure 1 The optical microscopic images of PEDOT particles in different reaction mixtures were acquired by placing a drop of PEDOT dispersion on a glass strip. As it is illustrated in Fig. 2 (from top to bottom) the polymerization of PEDOT increases with increasing the molar concentration of EDOT monomer and FeCl3 while keeping the monomer to oxidant volume ratios (v/v) constant. On the other hand, with increasing the volume concentration of oxidant (FeCl3 ) , at constant molar concentration of EDOT monomer and FeCl3 , the suppression of PEDOT formation has been noted, as shown in Fig. 2 (from left to right). It is obvious that with increasing the concentration of either EDOT monomer or FeCl3 the rate of PEDOT polymerization will be increased, but it is hard to say about the completion of the reaction, as we took the samples after 20 minutes. It was assumed that the polymerization of PEDOT will be completed after solvent evaporation when it will be applied on the surface of a substrate. If PEDOT dispersions will have higher amount of stable PEDOT particles, then very thick, having some extent of electrical properties, but brittle PEDOT layer will be formed on the surface of the substrate, which will limit its use in different applications. So, we need PEDOT dispersions with optimum concentrations of PEDOT particles, which can give uniform deposition of PEDOT layers on different substrates with reasonable conductivity. Figure 2 Effect of solvents on PEDOT polymerization Type of the solvents has a significant effect on the rate of the reaction and the final properties of the products. The reactivity of solvents depends on two factors: one is the polarity of the solvents and other is the donor number (DN). Solvents with greater polarity and lower DN are expected to be more reactive. We have selected three well known organic solvents, ethanol, 1butanol and acetonitrile having donor number (DN) 32, 29, and 14.1 kcal/mol, respectively [28].
In order to investigate the effect of the solvent type on the PEDOT formation and its properties, we prepared solutions of EDOT monomer (1.0 M) and FeCl3 oxidant (1.0 M) in selected solvents and mixed both of them with different EDOT/oxidant volume ratios i.e. 1:1, 1:2, 1:3. Other reaction conditions were kept constant. In Fig. 3, the optical microscopic images of PEDOT nano-particles in different PEDOT dispersions are shown. The PEDOT crystals in ethanol+FeCl3 , 1-butanol+FeCl3 , and acetonitrile+FeCl3 reaction mixtures are shown in Fig. 3(A, B, C), Fig. 3 (D, E, F), and Fig. 3(G, H, I), respectively. It is clear that the concentration of PEDOT particles in different reaction mixtures increases from ethanol to acetonitrile i.e. ethanol < 1-butanol < acetonitrile at a specific EDOT/oxidant volume ratios. As the donor number of these solvents is in decreasing trend from ethanol to acetonitrile i.e. ethanol > 1-butanol > acetonitrile, so it is expected that acetonitrile is highly reactive as compared to both of the ethanol and 1-butanol. Also, acetonitrile has high dipole moment which makes it an excellent solvent for many organic as well as inorganic solutes. It can also be noted that in ethanol and 1-butanol, the rod like PEDOT particles were formed having enough space between the polymer chains and hence, they are freely moving in PEDOT dispersion. On the other hand, in acetonitrile, most probably very small and ball like PEDOT particles were formed. As the reactivity of the acetonitrile is higher than the ethanol and 1-butanol, so FeCl3 released more Cl-1 anions in acetonitrile which took part in oxidation reaction of monomer to transform EDOT to PEDOT. Since more monomers were oxidized, so a large number of PEDOT chains with lower conjugated chain length, were produced and a very thick paste like liquid was obtained instead of PEDOT dispersion, shown in Fig. 3(G, H, I). Figure 3 ATR-FTIR analysis was performed on the pure solvents as well as on the PEDOT dispersions prepared in these solvents. Figure 4 illustrates the IR spectra of pure EDOT monomer, 1butanol, ethanol, and acetonitrile solvents. These IR spectra are then compared with the IR spectra of PEDOT dispersions in 1-butanol, ethanol and acetonitrile, as shown in Fig. 5 (A), (B), and (C), respectively. We have just notified the new developed absorption peaks on the IR spectra of PEDOT dispersions, which are absent in the IR spectra of pure solvents. The absorbance peaks at 1488 and 1414 cm-1 on the IR spectra of PEDOT dispersions are associated with the (CC) and (CC) bond stretching on the quinoidal form of the PEDOT polymer chains [18,22]. Also the vibrations at 1185, 1187, and 1188 cm-1 are originated
because of the (COC) bond stretching in the ethylene dioxy group. The presence of 985 and 841 cm-1 at acetonitrile and 845 cm-1 at 1-butanol IR spectra represents the vibration modes of the (CS) bond in the thiophene ring [29-31], whereas this peak cannot be seen in the IR spectrum of ethanol, which could be related to the solvents reactivity i.e. acetonitrile > 1-butanol > ethanol. On the other hand, two new absorption peaks at 1653 and 3463 cm-1 can be seen on the IR spectrum of acetonitrile, which might be because of the deformation of dioxane ring and formation of carbonyl (CO) and hydroxyl (OH) groups [32,21,19]. It also indicates the higher reactivity of acetonitrile solvent. Figure 4 and 5 Film forming properties of PEDOT and electrical characterization After getting PEDOT dispersions in different solvents (discussed above), we applied these dispersions on a transparent PET film in order to investigate the film forming properties of PEDOT, which was prepared in different reaction media. We took 5 ml from each type of PEDOT dispersion, applied it on the PET film with a dropper, left it in open environment for 24 hours for drying, and then analyzed dried films under the optical microscope. In Fig. 6, the microscopic images of dried PEDOT films corresponding to the PEDOT dispersions discussed in Fig. 3 are shown. It is clear from Fig. 6 that the better film forming properties can be achieved with PEDOT dispersions in 1-butanol at almost all EDOT/oxidant volume ratios, shown in Fig. 6(D, E, F). If we compare all of these solvents then the most acceptable PEDOT films were formed at 1:2 EDOT/oxidant volume ratios, shown in Fig. 6(B, E, H). With acetonitrile, although very thick PEDOT film was formed but after drying, cracks were produced in the film, refer Fig. 6(G, I), except Fig. 6 (H). It might be because of the shorter PEDOT polymer chains having very high concentration in the reaction mixture. Figure 6 As it has been assumed in previous section that there is enough space between PEDOT polymer chains in ethanol and 1-butanol but almost uniform films were formed with PEDOT dispersions in ethanol and 1-butanol. It was because of the completion of polymerization of PEDOT after the solvent evaporation. It shows that the polymerization of EDOT was not completed during the mixing of EDOT monomer and oxidant at suggested reaction conditions. It might be a desirable property to extent the processibility of PEDOT because if
the polymerization reaction will be completed then the viscosity of the PEDOT dispersion will be increased, so limited will be the processibility. Undoped PEDOT was also deposited on the surface of viscose fibers in order to get conductive threats for different electronic applications. For this purpose, the viscose fibers cut into 15 cm length were dipped in different PEDOT dispersions prepared with variable EDOT/oxidant volume ratio. At first, we took the PEDOT dispersion obtained with EDOT and oxidant solutions prepared in 1-butanol solvent. In Fig. 7, the optical microscopic images of PEDOT coated viscose fibers obtained at different EDOT/oxidant volume ratios are shown. The molar concentration of EDOT monomer and oxidant was kept constant i.e. 1.0 M. It can be seen that with decreasing the amount of oxidant solution the surface roughness increases. It is obviously because of increasing the concentration of EDOT monomer with decreasing the amount of oxidant, and hence, the availability of more reactive species to take part in polymerization reaction. With higher EDOT concentration, the thicker and rough polymer surface was obtained. Figure 7 The influence of volume concentration of oxidant on electrical properties of prepared PEDOT was observed by investigating the electrical properties of PEDOT-coated viscose fibers. In Fig. 8, the electrical resistance values of PEDOT coated viscose fibers prepared at different volume concentrations of oxidant, are shown. It can be seen in figure that minimum electrical resistance value can be obtained with highest volume concentration of oxidant i.e. 1:3. It might be because of the excess of FeCl3 oxidant, which oxidized maximum number of EDOT monomer to form PEDOT polymer chains and then further these PEDOT chains were partially doped with excess FeCl3 . Figure 8 Similarly, the effect of molar concentration of EDOT and oxidant solutions on the surface properties of PEDOT coated viscose fibers was also investigated. In this regard, we took the PEDOT dispersions prepared with EDOT monomer and oxidant solutions having variable molar concentration i.e. 0.25 M, 0.5 M and 1.0 M. The volume ratio of EDOT/oxidant was kept constant at 1:1. In Fig. 9, it can be seen that at very low molar concentration (0.25 M) of EDOT very smooth PEDOT layer was formed on the surface of viscose fibers, whereas,
thickness of PEDOT layer was increased with increasing molar concentration (from 0.5 to 1.0 M). Figure 9 The electrical properties of these fibers are illustrated in Fig. 10. It is revealed that electrical properties of PEDOT coated viscose fibers are increased with increasing molar concentration at specific EDOT/oxidant volume ratio 1:1. From Fig.9 and Fig. 10, we can conclude that at equal volume ratios, the thickness of PEDOT layer and electrical properties of PEDOT were improved with increasing molar concentration of reacting solutions. Figure 10 From our above mentioned results, we concluded that lowest electrical resistance values can be achieved with 1.0 M concentration at specific EDOT/oxidant ratio i.e. 1:3. So, to find out the influence of solvent type on electrical properties, we took the PEDOT dispersions prepared with different organic solvents at constant molar and volume concentrations. In Fig. 11, the optical microscopic images of PEDOT-coated viscose fibers are shown. With 1butanol, Fig. 11(A), very smooth and homogenous PEDOT layer can be formed. On the other hand, PEDOT thickness was increased with acetonitrile and ethanol, ref. Fig. 11(B) and Fig. 11(C) respectively. Figure 11 The comparison between electrical properties of PEDOT coated viscose fibers obtained with different organic solvents is shown in Fig. 12. It can be seen that with acetonitrile, the lowest electrical resistance values can be achieved as compared to ethanol and 1-butanol. It might be because of the highest reactivity of acetonitrile as compared to the rest of the solvents. Since, these PEDOT dispersions were prepared at specific concentration of EDOT monomer and oxidant, so it might be possible that we will get totally different electrical characteristics with other concentrations. Figure 12 Although, we can get some idea about surface morphology of PEDOT-coated viscose fibers by using optical microscopy, but in order to get clear identification about surface roughness and PEDOT agglomerates formed in different solvents, we performed SEM analysis on coated fibers. In Fig. 13, the SEM images of neat viscose (Fig. 13A), PEDOT-coated viscose
fibers obtained with PEDOT dispersion in ethanol (Fig. 13B), in 1-butanol (Fig. 13C) and in acetonitrile (Fig. 13D) are shown. The variation in surface morphologies of PEDOT-coated viscose fibers can clearly be observed. The coating thickness is minimum in case of ethanol solvent as compared to 1-butanol and acetonitrile. For 1-butanol, relatively thick and evenly distributed PEDOT layer can be seen on the surface of coated fibers. On the other hand, in the case of acetonitrile solvent, as it has been explained earlier, the coating of PEDOT polymer is very brittle and maximum thickness can be observed. Also, some giant sized agglomerates of PEDOT polymer on the surface of coated fibers can also be seen for acetonitrile/PEDOT. Figure 13 The influence of different organic solvents on PEDOT formation and its electrical properties was
studied
by ATR-FTIR spectroscopy.
The IR-spectra of pure viscose fibers,
PEDOT/viscose with 1-butanol, PEDOT/viscose with ethanol and PEDOT/viscose with acetonitrile, are illustrated in Fig. 14. The distinctive cellulosic features can be seen in IRspectrum of pure viscose fibers (Fig. 14a). The broad absorption peaks between 1351 cm-1 , 1014 cm-1 and 895 cm-1 are because of the involvement of different functional groups, such as, (CO), (CC) and (COC). On the other hand, the typical absorption peaks related to PEDOT polymer are observed at 1541-1470 cm-1 (CC), 1337 cm-1 (CC), 1228-1223 cm-1 (CO) cm-1 and 840-838 cm-1 (CS) wavenumber in the IR-spectra of PEDOT coated viscose fibers obtained with different organic solvents, (Fig. 14b, 14c, 14d) [29, 30]. The electrical properties of PEDOT polymer are related to the CC conjugation length and stretching at (CC) functional group can be observed at 1608 cm-1 wavenumber on IR-spectra. It can be seen in IR-spectra of PEDOT-coated viscose fibers that absorption peak at 1608 cm-1 is more prominent in Fig. 14d as compared to Fig. 14c and Fig. 14b. We can conclude that un-doped, neutral PEDOT can be obtained with 1-butanol and relatively higher conductivity values can be achieved with acetonitrile, which justifies the electrical characterization of PEDOT-coated viscose fibers shown in Figure. 12. From these analyses, we can conclude that very less conductive (neutral) PEDOT can be obtained with 1-butanol, as explained above. Although the reactivity of the acetonitrile is higher than 1-butanol and ethanol, but it can initiate the other side reactions and PEDOT with lower molecular weight (dimers, trimers, and oligomers) was produced. Because of higher polymerization reaction in acetonitrile, the electrical properties are better than other two organic solvents. In order to get better quality PEDOT, the polymerization reaction can be
optimized by controlling other reaction conditions as well as with the addition of some inhibitors to minimize the acidic effects of oxidant solutions on EDOT monomers. Figure 14 PEDOT dispersion in Fe (III) tosylate solution The PEDOT dispersions in Fe(III)tosylate (FepTS) solutions were obtained by mixing 1.0 M EDOT monomer solution in 1-butanol with 40 wt. % solution of ferric (III) p-toluene sulfonate in 1-butanol with different monomer/oxidant volume ratios i.e. 1:1, 1:2, and 1:3. Other reaction conditions such as, reaction temperature, reaction time and stirring speed, were kept same as mentioned above. The optical microscopic images of PEDOT particles in reaction mixtures are shown in Fig. 15(A, B, C). It is might be possible that high number of PEDOT polymer chains having high molecular weight were formed. It is assumed that polymers with longer chain length can pack very well and show higher PEDOT concentration. When these PEDOT dispersions were applied on the PET films, we got very non-uniform and brittle PEDOT layers, as shown in Fig. 15(D, E, F), and when these films were analyzed under optical microscope, a giant sized PEDOT crystals were investigated. By keeping the amount of monomer constant, the size of the PEDOT particles was reduced with increasing the amount of the oxidant FepTS, as shown in Fig. 15(G, H, I). It was because of the less availability of EDOT monomer to the excess amount of the oxidant. It might also be possible that the excess amount of FepTS was crystallized at higher humidity level in open environment along with PEDOT polymer chains. So, PEDOT dispersions in FepTS oxidant solution cannot be applied directly on the surface of different substrates without of adding some modifiers or surfactants which can suppress the crystallite formation of oxidant such as, amphiphilic copolymers, like poly (ethylene glycol)-ran-poly (propylene glycol) (PEG-ranPPG) [33,34]. Figure 15
CONCLUSIONS The dispersions of PEDOT polymer can be prepared in different organic solvents having different reactivity. These dispersions can directly be applied on a variety of substrates which can readily be polymerized after the solvent evaporation. The obtained PEDOT polymer films show very low electrical properties in un-doped form, which can further be transformed in
semiconductor films. The optical microscopic images of PEDOT crystals in different reaction mixtures give some idea about the PEDOT polymer chains, type of the PEDOT particles and the concentration of the PEDOT polymer. The quality and the film forming properties of PEDOT can be controlled by optimizing the molar as well as the volume concentrations of EDOT monomer and oxidant. The optimum EDOT/oxidant ratio for all molar concentrations at which better film forming properties were achieved, was 1:2. The required electrical properties of PEDOT can be achieved by selecting an appropriate type of the organic solvent. Ferric (III) tosylate is a very reactive oxidant but it gives giant sized particles of PEDOT and hence, very brittle PEDOT layer was formed on the surface of the substrates. The FTIR analysis, optical microscopic and SEM images of PEDOT layers on the PET films and viscose fibers reveals that 1-butanol is most suitable solvent to get neutral PEDOT with better quality PEDOT films, whereas acetonitrile gives relatively better electrical properties. This study can be utilized to simplify the PEDOT polymerization process and to enhance the processibility of PEDOT. These dispersions can be applied on different substrates by using simple coating techniques to get uniform PEDOT layer. ACKNOWLEDGEMENT The authors would like to express their gratitude to DinEl Miljöfond Energi for providing partial funding to this project and to Mr. Hamid Reza Barghi for fruitful discussion.
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Figure Captions
Figure 1. Schematic diagram of PEDOT polymer chains in reactive dispersions Figure 2. Optical microscopic images of PEDOT dispersions in 1-butaonl+FeCl3 solutions having different EDOT/oxidant molar and volume concentrations Figure 3. Optical microscopic images of PEDOT dispersions in FeCl3 solutions prepared in ethanol (AC), 1-butaonl (DF), and acetonitrile (GI); molar concentration of EDOT and oxidant solutions were kept constant i.e. 1.0 M Figure 4. FT-IR spectra of pure EDOT monomer, 1-butanol, ethanol, and acetonitrile Figure 5. FT-IR spectra of PEDOT dispersions in (A) 1-butanol, (B) ethanol, and (C) acetonitrile Figure 6. Microscopic images of PEDOT-coated PET films prepared from the corresponding PEDOT dispersions in different organic solvents, (AC) ethanol, (DF) 1-butanol, and (GI) acetonitrile, explained in Fig. 3 Figure 7. Optical microscopic images of PEDOT-coated viscose fibers obtained at different EDOT/oxidant volume ratio: (A) 1:3, (B) 1:2 and (C) 1:1. The molar concentration of oxidant and EDOT monomer solutions was kept constant i.e. 1.0 M Figure 8. Electrical properties of PEDOT coated viscose fibers obtained at variable EDOT/oxidant volume ratios but at constant molar concentration of reacting solutions i.e. 1.0 M Figure 9. Optical microscopic images of PEDOT-coated viscose fibers obtained with variable molar concentrations of EDOT and oxidant solutions, (A) 0.25 M, (B) 0.5 M, and (C) 1.0 M. The volume concentration of EDOT/oxidant was kept constant at 1:1 Figure 10. Electrical properties of PEDOT-coated viscose fibers obtained at variable molar concentration of reacting solutions at constant EDOT/oxidant volume ratio Figure 11. Optical microscopic images of PEDOT-coated viscose fibers obtained from PEDOT dispersions in different organic solvents, (A) 1-butanol, (B) acetonitrile and (C) ethanol Figure 12. Comparison of electrical properties of PEDOT-coated viscose fibers obtained with different organic solvents Figure 13. Scanning electron microscopic images of: (A) neat viscose fibers, (B) PEDOT/viscose/ethanol, (C) PEDOT/viscose/1-butanol, and (D) PEDOT/viscose/acetonitrile
Figure 14. IR spectra of (a) pure viscose, (b) PEDOT/viscose with 1-butanol, (c) PEDOT/viscose with ethanol and (d) PEDOT/viscose with acetonitrile Figure 15. PEDOT dispersions in FepTS solutions prepared at different EDOT/oxidant volume ratios, (AC) PEDOT crystals in reaction mixtures, (DF) PEDOT-coated PET films, and (GI) surface morphology of PEDOT layers
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