In October 2000, the Nobel Prize in Chemistry was awarded to Alan Heeger, Alan MacDiramid and H. Shirakawi for their pioneering work in developing the field of electrically conductive polymers. This laboratory experiment repeats the inital work on the first of these polymers, Polyacetylene.
Directions are given for the synthesis and the study of the properties of Polyacetylene films. The synthesis of cis-polyacetylene (cis PA) requires about 1.5 hours so this material can be prepared and studied a three hour laboratory session. The cis polymer thermally isomerizes to the trans isomer; this can be easily monitored by infrared spectroscopy. The two forms have dramatically different colors and visible spectra: in very thin films cis-PA is red (copper color in thicker films) while trans-PA is blue. PA is a semiconductor; when doped with iodine vapor the conductivity can be observed to increase by 10 orders of magnitude. The fully deuterated polymer is easily synthesized to demonstrate isotope shifts and confirm the IR assignments.
The synthesis is an excellent place to introduce inert atmosphere techniques using either syringe techniques or through the use of a glove box or glove bag We also use the polyacetylene lab as an opportunity to use semiempirical mo programs, to introduce molecular band gap theory (and relate it to visible spectra) and to illustrate the differences between metallic and semiconductor conductivity. It is worth noting the difficulties which have prevented commercial exploitation of PA films. Other conjugated polymers (including polyaniline, polypyrolle and polythiophene) are now appearing as commercial products.
Additional material is to be provided concerning transient photocoductivity and polyacetylene redox electrodes.
This work has been supported in part by a grant to BGSU from the National Science Foundation, DUE-ILI
disclaimer:
The procedures and information were developed for use in a junior year chemistry laboratory. Students can prepare a polymer film and measure its spectra in a single three hour period if the materials are readily available in a suitable form. The preparation of the catalyst involves a relatively hazardous pyrophoric material (triethyl aluminum.) We repackage materials so students handle only small amounts under the direct observation and supervision of an experienced faculty member. Students will also need guidance in inert atmosphere techniques.
The study of properties can take several lab periods; we prefer to have different student groups measure different properties and produce a comprehensive overall report.
The material prepared is not of the high quality needed for many research studies. This is caused by compromises to keep the method simple, the facilities modest and to package the tasks into conventional laboratory periods. The polyacetylene prepared is more than adequate for the measurements and observations we discuss.
When Acetylene gas comes into contact with a suitable catalyst it polymerizes in very good yield to form a thin film of polyacetylene. Polyacetylene is one member of a class of compounds known as conjugated polymers. The conjugated polymers are (or can become) excellent electrical conductors and they exhibit a wide range of electronic and electrochemical properties.
Polyacetylene exists as both the cis and the trans form. If the synthesis is conducted at -78 C, the product is almost exclusively cis-PA. Elevated temperatures will produce polymers with increasing trans-component; room temperature synthesis produces films that are about 75% trans. The cis polymer will slowly isomerize irreversibly to the trans (the half life is several days at 20 C, or about 300 min at 115 C.)
Cis and Trans PA are semiconductors with relatively low conductance. However, the material can be doped to become both p-type and n-type semiconductors. At high doping levels PA is effectively a metallic material. The conductance is comparable to mercury, decreasing with increased temperature.
PA can be reversibly oxidized and reduced in an electrochemical cell. Considerable effort has been devoted to producing a storage battery using PA electrodes. The material can be produced cheaply and it has a low density.
In practice, PA has several significant drawbacks and it has not seen any commercial development.
In 1974 Shirakawa and coworkers reported the synthesis of well formed films of PA. They used a highly concentrated version of a conventional liquid Ziegler-Natta catalyst. Under suitable conditions almost all of the polymer formed as a coherent film, either on the surface of the liquid or on the walls of vessels that had been treated with catalyst. Shirakawa, in Japan, and Heeger and MacDiarmid at the University of Pennsylvania studied these films extensively in the next few years.
In the 1950's Ziegler and Natta ....studied polymers. One outcome was the development of a catalyst which is still widely used. which....
This catalyst containes two important components--an acitive metal (generally triethyl aluminum or trimethyl aluminum). The other component is an transition metal organo-metallic compound (various titaninium and nickel compounds are the most important.) This catalysist is a liquid and the solvent is generally a hydrocarbon.
These two materials clearly undergo reaction as the catalyst forms, although the exact form of the active species is unknown to this date. It is generally believed that...
The Z-N catalyst genrally used has a concentration of a few millimolar in Al and Ni or Ti species. (As noted later, polyacetylene films form only if the concentration is much higher.)
The catalyst is prepared under oxygen free conditions. It is aged, concentrated, and used to coat the walls of a reaction vessel. The vessel is cooled to 195K with dry ice and acetylene gas is added. The polymer forms on the walls in about 15 minutes and at these temperatures it is over 85% cis material. The vessel is then opened, the film is rinsed to remove catalyst, and then the film is lifted from the surface and removed. The film may be stretched at this stage and then it is allowed to dry before recording spectra and electrical characterisitics. (The very thin films required for visible spectroscopy too fragile to be removed from the walls of the vessel.)
The Shirakawa version of the Ziegler-Natta catalyst is made by mixing Triethyl aluminum with Tetrabutoxide titanium in a hydrocarbon solvent such as toluene. The recommended composition is about 50 mmolar in Al and 12.5 mmolar in Ti. This preparation and the handling of the catalyst must be done under inert atmosphere conditions since oxygen and water vapor are very reactive towards these materials.
Chien (1984, p 28-32) has a purification procedure for materials and detailed directions for catlyst preparation. We find these steps to be an excessive burden for a thrre hour experiment and we use much simpler sample handing procedures.
We purchase the triethyl aluminum as a 25% solution in toluene (Aldrich.) It is supplied in a 100 ml septum topped bottle. Prior to student use we transfer (oxygen free) about 20 ml to a small septum topped bottle and we have students handle only the smaller amounts. We use the material without further treatment.
The tetrabutoxide Titanium is a relatively viscous fluid so we prepare a 25-50% solution in toluene. This solution is placed into septum topped bottles and degassed with nitrogen. We use the material as received. After extended storage (several years) we find significant traces of t-butanol, making the material unsuitable for use.
The toluene (or other solvents) are stored over 4A molecular sieve and are degassed with nitrogen prior to use. Samples are generally rinsed in hexane. We use reagent grade materials without further treatment.
The catalyst is prepared using syringe techniques, using glass syringes with Luer lock fittings. We have also done this using a glove box purged with nitrogen. The reaction vessel (figure I) is purged with nitrogen and cooled in a dry ice bath. A measured amount of solvent is transferred from the septum bottle to the vessel via a side arm topped with a small rubber septum. (A typical preparation is shown in the table.) Then tetrabutoxide titanium is added in a similar manner and mixed by simple swirling of the vesell.. The triethyl aluminum is drawn into a syringe and it is transferred to the reaction vessel; the reagent is slowly added over about 1 minute while the vessel is gently rocked to stir the materials. The catalyst mixture changes color as the triethyl aluminum is added. Initially a blue-green color forms but it is quickly replaced with a dark brown material. In the original protocol, the catalyst is formed by chilling the reaction flask in a dry ice bath but an acceptable catalyst can be prepared at room temperature.
The syringe used for the triethyl aluminum should be rinsed immediately with several batches of hexane and the syringe should be disassembled. This is partly to avoid hazardous contact with the reagent and partly to avoid the formation of aluminum oxide deposits which can quickly freeze a glass syringe shut. The rinse solution can be deactivated with a small addition of methanol. We generally rinse the syringes and used glassware with 3M HCl to remove any aluminum oxide film that forms.
The catalyst should be allowed to age before use. We have found acceptable results using the catalyst about 30-45 minutes after it is prepared. This allows time for collecting acetylene and other necessary tasks. If the procedures is to be carried out in a single three hour session, longer aging times become a problem. Current studies suggest aging times of several hours at room temperature for optimal results or aging at elevated temperatures (using a solvent like cumene.)
After aging, the catalyst is concentrated by placing it under vacuum to partially
evaporate and remove some of the solvent. This is not critical, but the thicker catalyst
generally coats the walls better.
We generally prepare the catalyst and the polymer in the same reaction vessel, but it is relatively easy to remove a portion of the catalyst with a syringe and a long needle. The catalyst can be transferred to other reaction vessels. We often use 18 mm heavy wall test tubes capped with rubber septa. In these cases the catalyst can be concentrated by applying a vacuum (via a hyperdermic needle) and it can be coated on the walls by rotating the tube. (We have tried lightly etching the inner walls of the tube to help provide a more uniform coating-- results are uncertain.)
Ideally, the synthesis should be performed with highly purified acetylene gas. In practice we've had acceptable results from a cylinder of welding grade acetylene. The key concern is the presence of acetone vapor. (Acetylene tanks store the gas as a solution in acetone.) We pass the acetylene gas through a dry ice cold trap to remove most of the acetone vapor. We've filled evacuated glass storage bulbs, but a much simpler procedures is to inflate a latex balloon and add a glass or plastic stem which has a small rubber septum. Later we remove and transfer 30 ml gas samples with a plastic syringe.
The vessel containing the catalyst is cooled in a dry ice bath. The vessel can either be partially evacuated or filled with an inert gas at atmospheric pressure. Acetylene gas is added through the septum using a simple syringe. The film can often be observed forming within a minute. About 30-50 cc of acetylene (atmospheric pressure) is required to form 4-5 cm2 of useful film.
Synthesis can occur for 10-30 minutes depending on the thickness desired. At the end of this time we remove the septum and flush the film with small rinses of cold hexane. (If a pool of catalyst is present you need to pierce the edge of the polymer film and drain the catalyst from below . It is generally easier to rinse the film before it is removed from the vessel. Ideally this operation would be conducted in an oxygen free environment, but we simply expose the sample to air at this point.
Thin samples can be peeled from the walls of the reaction vessel. We tend to flush them out with solvent and them collect them on a fine stainless steel screen. Samples are generally air dried between screen to prevent curling.
Because the polymer is a very dark material, samples for visible spectroscopy are treated differently. These are formed on the inside walls of small (13 mm) test tubes, washed in situ, and are allowed to remain on the walls of the tube. The colors and spectra are observed on the films while they remain bound to the walls of the tube. The tubes will fit int a standard spectrophotometer.
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The film will slowly isomerize at room temperature. If one wants to determine the true cis content of fresh film it should be kept cold until the spectrum is measured. In practice, a 30 minute drying time makes little noticeable difference.
The freshly synthesized film is quite flexible and can be carefully stretched to 2-3 times its original length. This stretched film typically exhibits higher electrical contact. We use a simple plastic assembly to clamp the two end of the film ; one of the clamps is on a moving slide. The film is then allowed to dry under tension. (Note that this is done at room temperature with some loss of cis content.)
We use nitrogen gas to provide an inert atmosphere; we do not use further purification. We use latex tubing (PVC like Tygon is somewhat oxygen permeable.)
We cannot describe syringe techniques here and refer you to ----'s classic book on inert atmosphere techniques. It is work noting the double needle method for degassing solution in a septum capped bottle. (Small Parts, Fla is a good source for stainless steel hyperdermic tubing.)
The Infrared Spectrum of PA films can be measured directly; the quality will be dependent on the film thickness. As expected the cis and trans forms show quite different spectra and the %cis can be evaluated from several peaks in the spectrum. The assignments, due to Shirakawa, are shown in the table.
The dominant lines of cis-Polyacetylene are given below
3047 w CH stretch B1u
3044 w CH stretch B3u
1329 s CH in plane deformation B1u
1249 w CH in plane deformation B3u
740 vs C-H out of plane deformation B3h
446 vs C-C-C deformation B1u
1800 m
1015 w CH out of plane deformation B2u
%cis = 100 x1.30 A(740)/[1.30 A(740)+ A(1015]
The dominant impurities in polyacetylene samples are generally due to oxygen. This is especially true for samples synthesized by these directions, where we do not make efforts to rigorously exclude oxygen. Sample which have been exposed to air for extended time will also show oxygen peaks.
The isomerization can be followed by recording spectra over a period of 4-8 days. Because the film will slowly oxidize it may be desirable to store the film in nitrogen between measurements. A much faster change can be followed by exposing the film to a heat lamp (xx watts, xx cm away) and recording spectra every 10-15 minutes.
It is tempting to try to follow the kinetics, but it is not clear meaningful results will be obtained under these conditions.
It is easy to synthesize fully deuterated acetylene from calcium carbide and 1-2 ml of deuterium oxide. A simple side arm test tube with a septum and a small balloon works well. After the reaction the tube is placed in a dry ice bath to remove most of the water (D2O) vapor. The fully deuterated polymer is formed from this gas using the procedure above; gas samples are removed with a 20-40 ml plastic syringe.
The IR spectra of cis and trans forms shows significant differences from the normal polymer. (see table 1.) In practice these films generally show significant oxidation (peaks near xxx) and we are reluctant to use them for other purposes, fearing that impurities would be responsible for many of the observed differences.
We generally form some films on the inside surfaces of 10 mm text tubes as descried earlier. The tubes are evacuated before adding the acetylene to gurantee that the gas fills the tube uniformly in a short time. A film is generally visible after 1-2 minutes, at which point the tube is opened and washed with cold solvent. A cotton swap is used to remove polymer from one side of the tube. The absorption spectrum is then recorded directly by placing the tube into the spectrometer. We generally use a HP8542A Diode Array Spectrometer. If several samples are available, one can be heated in a 100 C oven for 10 minutes to provide a trans-PA sample.
Cis-PA is generally described as a semiconductor with an intrinsic band gap of 1.8 eV; one finds a spectrum with a shoulder at 1.5-1.8eV or xxx nm. Trans-PA has a band gap of --- eV, and one finds the shoulder shifts to approximately 1.5eV or xx- nm. There is additional structure in the spectra, but we choose not to analyze it further.
Polyacetylene can be doped with electron donors and electron acceptors to form both n-type and p-type semiconductors. The simplest procedure is to place the polymer in a bottle which contains a few small crystals of Iodine. The polymer absorbs up to 25% of its own weight (CHI0.3) in iodine vapor; this will require about 4-6 hours. The degree of uptake can be detwermined by weighing the sample. The uptake of iodine is largely reversible (and one should avoid handling the doped film with ones fingers.)
Other dopants are generally favored in the research laboratory, but they are more difficult. AsF5 vapor is often used, but this requires a vacuum line and the material is quite hazardous. Sodium naphthalide is also commonly used.
Note that doping the polymer invariably results in complete isomerization to the trans form. The doped film is generally fairly brittle.
A conventional digital multimeter can be used to show the dramatic change in conductivity of doped PA. A typical sample is about 5 mm wide and about 20 mm long and it has a thickness of about 0.01 mm (???). We generally use a copper printed circuit board to provide two contacts about 15 mm apart and we press the sample against it with a piece of insulating plastic. (The film is too brittle for the usual electrical connectors. Graphite based conductive paints can also be used to make contacts.) Prior to doping the resistance is beyond the range of the meter (100 MOhm.) After doping, a resistance of 100 Ohms is typical.
We generally provide a FET-Op Amp circuit capable of measuring resistances to about 1000 MOhm; the undoped film still reads off scale. The conductance of the material can be calculated:
k = (1/R) (width x thickness /length)
The undoped material has k 10-5 (ohm cm)-1 and the doped material has a value around 10-3 ohm-cm. The literature values for the undoped material are about xxx and dope material has been prpared with values as high as xxx. By comparison, the conductance of mercury and copper are....
It is worth noting that the conductivty of highly doped PA shows a negative temperature dependence, becoming less conductive as the temperature increases. This is characteristic of metallic conductors, while semiconductors exhibit an exponentail growth in conductivity with temperature. Polyacetylene is often referred to in the literature as an organic metal. In fact, Polyacetylene research has spawned a journal called Organic Metals.
As a rule, conductivty measurements are better made using a four point proble, to overcome problems associated with the electrode/film contacts. A potential is applied to the two outer electrodes and the current is measured. The potential drop across two inner electrodes is measured. This is not difficult to implement for conductive, doped material but the current is extremely difficult to measure for the undoped species.
Trans polyacetylene films show a change in conductivity when exposed to light. This behavior shows up at several time scales. If subjected to a brief pulse of light, PA shows a rapid transient ( psec) and a slower response ( microseconds.)
This work is in progress and no results are available. The experimental description
is of the apparatus we are testing This can be demonstrated by placing a piece of PA film (lightly doped with iodine) in a
holder equipped with two electrodes. This is placed in the beam of a pulse nitrogen laser
(PTI 2300, 1.25 mJ/pulse, 800 psec, 337 nm.) Note that we place the film far enough to permit the
laser beam to irradiate the full sample, from electrode to electrode. We use a Tektronix TDS380
digital oscilloscope to record the transient current. A large bandwidth amplifier can provide
better signals. Note that it is important to use fully terminated, doubly shielded cables.
The polyacetylene films generally consist of bundles of very fine fibrils, intertwined.
This can be seen in Scanning Electron Micrographs.
Electrochemical Behavior of PA Films
this section is underdevelopment
Morphology of the Film
last edited March 15, 1997