The Effect of Co-Dopants on the Processability of Intrinsically Conducting Polymers.
Using a combination of functionalized dopants was found to be a simple method to improve the processability of intrinsically conducting polymers while retaining relatively high levels of conductivity. Two intrinsically conducting polymers, poly(3-octyithiophene) and polyaniline were co-doped with various combinations of dopants. In both systems, solubility was improved and coherent films were formed directly from common organic solvents without the need for a post-processing doping step. Co-doped intrinsically conducting polymer films exhibited conductivities up to [10.sup.-2] S/cm. Additionally, certain polyaniline complexes were capable of being melt processed without loss of conductivity.
One fundamental characteristic of intrinsically conducting polymers (ICPs) is extensive [pi]-conjugation in the main chain as shown for several families of ICPs in Fig. 1. Unfortunately, [pi]-conjugated structures also give rise to increased chain stiffness and chain interactions. As a result, most ICPs are insoluble and infusible.
ICPs are typically semiconductors in the natural state and exhibit a marked increase in conductivity when complexes are formed between the chains and guest species. This process has been termed doping in analogy to conventional semiconductor doping. Doping creates the charge carriers and mobility necessary for electrical conduction.
Because of the poor processability of ICPs, doping has historically taken place after an item has already been shaped into its final form such as a film, fiber, or molded article. Since an ICP object is commonly doped by exposure to either liquid or vapor phase dopants, the doping step is diffusion limited. Therefore, the time needed for the dopant to saturate the sample and reach the desired conductivity is usually inappropriate for mass production .
An improved procedure is to dope the ICP during the processing step as was demonstrated with a suspension of poly (p-phenylenesulfide) . It is anticipated that doping a solution or melt containing an ICP will permit the dopant to be distributed much more quickly and uniformly than by diffusion into a solid. This would be a major advancement for the mass production of ICPs.
Recently, there have been extensive efforts to improve processability. Of these, the addition of flexible side chains has been an effective approach. Flexible side chains may be attached by the direct chemical substitution of alkyl, alkoxy, or phenylalkyl side groups. The poly(3-alkylthiophenes) is one of the more common families with chemically anchored side chains . Alternatively, flexible side chains have been attached to polyaniline by employing functionalized dopants .
Unfortunately, ICPs with side chains typically exhibit lower conductivities than linear unsubstituted ICPs. Polyaniline doped with d,l-camphor sulfonic acid and processed from m-cresol solution is one notable exception . Further investigation has indicated that the m-cresol is acting as a secondary dopant as well as a solvent . This secondary dopant concept was defined as using a presumably inactive material to enhance desired properties. Unfortunately, the use of m-cresol will be limited industrially because of its toxicity. This secondary dopant concept was extended to the use of a combination of more conventional primary use type dopants. This method is referred to herein as co-doping and is explored as a possible technique to improve the processability of ICPs while still maintaining relatively high levels of conductivity.
The effect of co-doping on the processability of ICPs was determined for two conducting polymer systems: poly(3-octylthiophene). P30T, and polyaniline, PANI. Both ICPs were repurified while all other chemicals were used as received. P3OT (Neste 0y Corp.) was cleaned by washing with 0.3 M EDTA (Aldrich) . Similarly, PANI (UNIAX Corp.) was cleaned by washing with 3% ammonia hydroxide . The P3OT and PANI samples were then completely soluble in chloroform and concentrated sulfuric acid, respectively.
Co-Doped Sample Preparation
Doping was accomplished by exposing the ICPs to various dopant solutions. In the co-dopant technique, one dopant was selected as a processing dopant and the other selected as a conductivity dopant. When co-doped, the ICPs were first exposed to the processing dopant and then to the conductivity dopant.
Dodecylbenzene sulfonic acid, DBSA, was selected as the processing dopant for P3OT and [FeCl.sub.3] was chosen as the conductivity dopant. First, DBSA (TCI) was dissolved in 25 ml of toluene (Fisher). To this solution, 0.210 g of P3OT was dropped in slowly by spatula. After allowing the P3OT to completely dissolve and the DBSA adequate time to dope the P3OT by stirring overnight, a [FeCl.sub.3]*[6H.sub.2]O (Fisher) solution in acetonitrile (Aldrich) was prepared and slowly injected into the P3OT-DBSA solution by syringe. The volume of the [FeCl.sub.3] dopant solution injected was always kept at or below 1 ml. Finally, more toluene was introduced to bring the entire sample solution to a final volume of 30 ml. In order to limit gel formation in this study, the polymer concentrations were maintained at 0.5[c.sup.*] .
In the PANI co-dopant system, bis (2-ethylhexyl) hydrogen phosphate, DiOHP, was selected as the processing dopant and camphor sulfonic acid, CSA, was chosen as the conductivity dopant. First, DiOHP (TCI) was dissolved into 25 ml of chloroform (Fisher). Then, 0.075 g of finely powered PANI was slowly added by spatula. The PANI-DiOHP solution was allowed to stir overnight before injecting GSA (Aldrich) dissolved in chloroform into the mixture. Finally, additional chloroform was added to acquire a final sample volume of 30 ml.
ICP complexes containing various amounts of dopants were prepared by these procedures. The dopant solutions were prepared with the amounts of dopant to provide the desired degree of doping. The degree of doping is referred to in terms of mol% or by a molar ratio of dopant per monomer repeat unit.
The co-doped ICP mixtures were stirred overnight and sonicated (Branson 2200 ultrasonic cleaner) for 4 hours. Following these treatments, each sample solubility was determined. The solubility determination was accomplished by centrifuging until the solution would pass completely through a 1 [micro]m filter (Gilman). The solubility is reported as the percent of the complex remaining in solution. This was estimated by measuring the weight of the insoluble complex recovered from the centrifuge tubes after evaporating all remaining solvent.
Free-standing thin films were formed from each doped conducting polymer solution by pouring each solution into a flat glass dish followed by evaporation of the solvent under a slow nitrogen flow for one hour. The resulting films were then carefully lifted from the dishes and allowed to dry completely under nitrogen overnight.
The ability of each sample to be melt processed was also established. The complexes were evaluated for the capacity to flow and form quality disks. In order to accomplish this, a sample film previously formed from solution was compression molded in a cylindrical mold 25 mm in diameter. P3OT films were molded at 200[degrees]C and PANI films at 150[degrees]C for 5 minutes.
The conductivities of all the samples were determined by use of the collinear four-point probe method . First, a set of strips were cut from a portion of each sample film formed previously. Each strip was approximately 20 by 5 mm. Four copper wires cleaned with steel wool were fastened to each strip using conductive carbon paint (Structure Prove Inc.). Current (Lake Shore Cryotronics 120 Current Source) was applied to each of the strips through the outer wires and the resulting voltage was measured (Keithley 195A Digital Multmeter) across the inner wires of each strip. The average cross-sectional area of each strip was determined by measuring the width and thickness of the strips. The average conductivity of the set of strips for each sample is reported.
The thermal stability of each co-doped sample was checked for conductivity loss and for crosslinking upon melt processing. To determine conductivity loss, a disk from each sample was hot-pressed into a thin sheet and the conductivity was measured as described previously. The amount of crosslinking in a co-doped sample due to melt processing was determined by reimmersing the hot-pressed sample into solvent and then measuring the portion that remained insoluble.
RESULTS AND DISCUSSION
Single vs. Co-Doping
Functionalized sulfonic acids have been demonstrated to render PANI soluble in some common organic solvents . A number of functionalized sulfonic acids were studied to determine if these types of acids would also be applicable as dopants for P3OT. Of those studied, only DBSA showed further feasibility due to the spectral change and conductivity increase observed upon addition to a P3OT solution .
However, the conductivities of P3OT doped with DBSA were found to be low as compared to P3OT doped with [FeCl.sub.3] as seen in Fig. 2. Even for a P3OT-[(DBSA).sub.0.5] complex a value of [10.sup.-3] S/cm is not achieved. Meanwhile, at the same doping level, P3OT doped with [FeCl.sub.3] has a conductivity over 100 S/cm. On the other hand, all P3OT-DBSA complexes were completely soluble in toluene while all P3OT-[FeCl.sub.3] complexes were completely insoluble.
DBSA and [FeCl.sub.3] where then combined as dopants in a P3OT system. It was desired that DBSA would act as a surfactant much like CSA does with PANI for enhanced processability of doped P3OT . Since the conductivities using DBSA alone were quite low. [FeCl.sub.3] would also be utilized to increase the level of conductivity.
When the [FeCl.sub.3] and DBSA dopants were combined, it was found that intermediate levels of conductivity and solubility could be obtained. Table 1 shows that a co-doped P3OT-[([FeCl.sub.3])].sub.0.2]-[(DBSA).sub.0.5] had a solubility of 45 wt%. This is in comparison to a solubility of 0% and 100% for P3OT films doped singly with [FeCl.sub.3] and DBSA, respectively. Additionally, the co-doped P3OT-[([FeCl.sub.3])].sub.0.2]-[(DBSA).sub.0.5] film exhibited a conductivity of 8.3 X [10.sup.-3] S/cm. This again falls between 9.0 X [10.sup.-4] S/cm and 4.5 X [10.sup.-1] S/cm of the singly doped P3OT films.
Under this co-dopant scheme, one dopant may be considered as the processing dopant and the other as the conductivity dopant. The processing dopant functions as an aid to enhance processability while the conductivity dopant is the primary determinate of the electrical properties. For P3OT. DBSA functions as the processing dopant and [FeCl.sub.3] as the conductivity dopant.
Evaluation of Co-Doped P3OT
It has already been shown that the degree of doping can have a large effect on the properties of ICPs. The conductivities and solubilities as a function of the degree of co-doping for the P3OT-[FeCl.sub.3]-DBSA system are shown in Figs. 3 and 4, respectively. As expected from the earlier studies comparing [FeCl.sub.3] versus DBSA, the conductivity improves with increasing levels of [FeCl.sub.3] in the co-doped system. However, it was found that the conductivity diminishes with increasing levels of DBSA. This suggests that the bulky DBSA side groups, while leading to stable doped solutions, also separate the P3OT chains enough to suppress the interchain hopping of charge carriers. This effect is also seen in the poly(3-alkylthiophences) with longer side chains.
Since the co-doping technique improved the solution processing capability of doped P3OT, an additional series of samples were prepared to determine the optimal co-doped P3OT system. The optimal location for this co-doped P3OT system appears to be in the region around P3OT-[([FeCl.sub.3]).sub.0.1]-[(DBSA).sub.0.25]. Conductivities of [10.sup.-2] S/cm and solubilities above 70% were consistently measured for samples at these levels of co-doping. The conductivities at the optimal point are within an order of magnitude as those observed for equivalent levels of [FeCl.sub.3] doping without DBSA present. However, the DBSA allowed good solubilities to be obtained without overly disrupting the conductivity. It is not feasible to achieve further increases in conductivity since the conductivity of [FeCl.sub.3] alone is not much higher. Increasing the [FeCl.sub.3] level while leading to slightly higher conductivities does not improve the response since a higher degree of aggregation would be expected to occur . The i ncreased aggregation force then appears to be too great for DBSA to sufficiently stabilize in order to obtain improved solubility.
Although the co-dopant technique improved the solution processing of P3OT, the method did not significantly enhance melt processing properties. When the co-doped samples were hot pressed, flexible disks were formed similar to undoped P3OT. Additionally, these disks generally had an aesthetic surface finish. However, thermal dedoping and extensive crosslinking is quite apparent. The conductivities and insolubles of some co-doped P3OT samples after melt processing are given in Table 2. The conductivities decreased by about 2 orders of magnitude from the samples formed directly from solution. Moreover, the disks became largely insoluble when reimmersed in toluene. This indicates that the samples have become significantly crosslinked. These observations are typical for P3OT containing [FeCl.sub.3] and then melt processed . In this case, the addition of DBSA in the co-doping scheme did not suppress thermal dedoping nor crosslinking.
Evaluation of Co-Doped PANI
Co-doping was found to be a practical technique in the P3OT-[FeCl.sup.3]-DBSA system for improving solution processing capability while retaining useful levels of conductivity. It would be even more beneficial if the ICP system could be melt processable. One could then take advantage of existing melt processing techniques such as injection molding to produce conductive articles from these materials. However, the P3OT system was not capable of being melt processed without the occurrence of significant crosslinking and a substantial loss of conductivity.
Phosphoric diesters have been found to both protonate and plasticize PANI . For instance, complexes of PANI-DiOHP may be thermally processed and have conductivities above [10.sup.-3] S/cm. Still, these conductivities are several orders of magnitude below those observed for PANI doped with CSA . Therefore, the co-doping technique was evaluated for the PANI-CSA-DiOHP system with DiOHP as the processing dopant and CSA as the conductivity dopant.
The use of two dopants for polyaniline has been reported previously . In this method, one dopant is predominately located at the core of a polyaniline particle while the other is located at the surface. The specific arrangement is chosen primarily to improve thermal stability and/or enhance the compatibility of the conducting polymer complex with a second polymeric matrix.
The effect of co-doping on the properties of PANI processed from chloroform is given in Table 3. It is observed that the processing dopant, DiOHP, does not appear to hinder the conductivity as much as DBSA did in the co-doped P3OT system. However, the conductivities remain lower by about 3 orders than those reported for the PANI-CSA system as processed from m-cresol and by about 1 order for PANI doped in the surface/core arrangement [4, 12, 13]. On the other-hand, the use of co-dopants significantly improves the solubility of doped PANI. It is interesting to note that the co-dopants appear to act synergistically in the sense that at combined doping levels of co-dopants, the solubilities are around 1.5 times greater than that for the dopants individually at equal doping levels. These solubilities are much greater than those observed for for PANI doped in a standard surface/core arrangement [13, 14].
The properties of PANI processed from chloroform as a function of DiOHP doping level are given in Figs. 5 and 6. The use of CSA improves the conductivity by about two orders. Notice that the conductivities remain constant in the co-doped sample until around 30 mo1% DiOHP doping and then begin to increase with increasing DiOHP. The solubilities increase with increasing DiOHP until a DiOHP doping of about 60 mo1%. The observation that co-doped samples exhibit improved solubilities than those with CSA alone is expected since additional non-polar side groups have been attached which contain greater flexibility.
In addition to the improvement in the solution processability of the PANI-CSA-DiOHP system, it was found that samples with DiOHP doping levels of 30 mo1% or greater became plasticized and could be melt pressed into both disks and films. Note that this was also the level of DiOHP doping where the conductivity began to improve in co-doped PANI. When the DiOHP doping level was below 30 mo1% the samples formed disks that crumbled under their own weight. Additionally, coherent films could not be hot pressed from these samples. PANI doped in the standard surface/core arrangement is also not able to be melt processed [13, 14].
Previous findings suggest that different dopants and solvents may result in different conformations of doped PANI chains . The conductivity is expected to depend on the conformation. For example, rods should be more conductive than coils . Different conformations may also lead to different conjugation lengths. For instance, a more coil-like conformation may cause more defects on the chain which shortens the conjugation length and decreases the conductivity . Based on conductivity results for PANI-CSA, doped PANI chains appear to be more coil-like and have a shorter conjugation length in chloroform than in m-cresol. The onset of plasticity in the co-doped samples coincides with the point at which the conductivity begins to increase with DiOHP doping level. Thus, DiOHP appears to not only plasticize the PANI-CSA complex, but also expands the polymer chains resulting in higher conductivities. This effect is similar but not as extensive as observed when PANI-CSA films are exposed to m-cresol vapor . This effect also resembles that observed upon treatment of PANI-dinonylnaphthalene sulfonic acid films with benzyltriethylammonium chloride. The conductivity enhancement in this case was determined to be the self-assembly of conductive PANI particles to form additional interconnected pathways .
Also of importance is the fact that these samples are relatively thermally stable. The conductivities and insolubles of some co-doped P3OT samples after melt processing are given in Table 4. The conductivities of the hot-pressed films from melt formed disks do not differ significantly from films formed directly from solution staying well within an order of magnitude. Also, the amount of insolubles is fairly low. This demonstrates that the PANI-CSA-DiOHP system is able to be melt processed without conductivity loss and may be reprocessed at this temperature.
Using a combination of dopants is a simple method to improve the processability of ICPs while retaining useful levels of conductivity. In the co-dopant scheme, one dopant is judiciously selected to improve the processability while the other is selected to give optimal conductivity. In both P30T and PANI, solubility was improved and coherent films were formed directly from common organic solvents without the need for a post-processing doping step. Co-doped ICP films exhibited conductivities above [10.sup.-2] S/cm.
The addition of chemically anchored side chains allows undoped P3OT to be solution or melt processed. In the past, a post-process doping step was required due to insolubility or thermal dedoping and crosslinking. The concept of co-doping was first applied in P3OT using [FeCl.sub.3] as the conductivity dopant and DBSA as the processing dopant. The addition of further side chains via doping with DBSA resulted in stable doped solutions. Although DBSA is able to screen interactions in solution, it is not able to prevent thermal dedoping nor crosslinking. As a result, melt processing this doped P3OT system is not advised.
The co-doping technique was then applied to a PANI system. Here, DiOHP was chosen as the processing dopant with CSA as the conductivity dopant. DiOHP provides a means of additional solvating strength as well as acting as a plasticizer while CSA gives good electrical properties. These complexes showed improvement in solubility and could easily be melt processed without loss of conductivity and with little crosslinking.
Unfortunately, as observed from handling, the PANI-CSA-DiOHP samples were relatively brittle and would not be able to carry large loads. One way to improve the mechanical properties while still retaining good electrical properties is to blend ICP complexes with other polymers. Further studies will be performed to improve the mechanical properties by melt blending the PANI-CSA-DiOHP complex into insulating polymer matrices. Although the properties of the co-doped samples did not reach the levels that were ultimately desired, the ease of processing under this scheme encourages the further utilization of the co-dopant technique.
The author wishes to thank the Neste Oy Corporation and UNIAX Corporation for their donations of P3OT and PANI. This work was supported by the MRL Program of the National Science Foundation under Award No. DMR-9123048.
(*.) Present address: RTP Company, Winona, MN 55987.
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The Effect of Single vs.Co-Dopants on the Properties of P3OT. Fe[Cl.sub.3] DBSA Solubllity Conductivity Doping Level Doping Level (%) (S/cm) (mol%) (mol%) 0 50 100 9.0 x [10.sup.-4] 20 0 0 4.5 x [10.sup.-1] 20 50 45 8.1 x [10.sup.-3] 10 25 74 1.0 x [10.sup.-2] Thermal Stability of Co-Doped P3OT. Fe[Cl.sub.3] DBSA Conductivity Insolubles Doping Level Doping Level (S/cm) (%) (mol%) (mol%) 20 50 2.2 X [10.sup.-4] 98 10 25 3.4 X [10.sup.-4] 87 Effect of Co-Doping on the Properties of PANI. DiOHP CSA Solubility Conductivity Doping Level Doping Level (%) (S/cm) (mol %) (mol %) 0 50 42 9.1 X [10.sup.-2] 50 0 48 4.1 X [10.sup.-3] 25 25 66 5.2 X [10.sup.-2] 100 0 57 1.1 X [10.sup.-2] 50 50 82 3.8 X [10.sup.-1] Thermal Stability of Co-Doped PANI. DiOHP CSA Conductivity Insolubles Doping Level Doping Level (S/cm) (%) (mol%) (mol%) 25 25 2.3 X [10.sup.-2] 8 50 50 9.7 X [10.sup.-2] 6