Preparative Methods and Properties of Well Oriented Electronically Conducting Polymers

Abstract

Orientation effects play an extremely important role in macromolecular science. orientation can determine whether a polymeric material is a useless powder or a high strength fiber, a film easily split or tough, optically polarizing or isotropic. Orientation is also important in determining the properties of electronically conducting polymers. In this conference, considerable attention was given to methods of achieving orientation and resulting properties. After a brief introduction, the discussion of orientation is presented in the following order: (1) methods of achieving orientation, (2) methods for obtaining single crystals.

Chain molecules, including those which contain conjugated unsaturated structures, may be highly oriented due to the manner of synthesis or processing. It is not surprising that electronic motion is found dependent on macromolecular orientation, as high electronic conductivity in organic polymeric solids requires the presence of a conduction band formed by the overlap of pi-orbitais. Orientation in polymers is of great interest, as it may be used to improve polymer properties or to assess mechanisms of electronic transport phenomena.

Methods of Achieving Orientation. The most common ways of achieving orientation in semicrystalline polymers are spinning from a gel, melt or solution, stretching in the solid state, or solid state extrusion. Biaxial orientation is found in blown films or in films prepared by passing a melt over a conical die.

Unfortunately, few conducting polymers are processable by these conventional techniques which generally require the existence of an accessable glass transition or melting temperature. Ionic forces in the solid state tend to make partially oxidized, highly conducting polymers intractable.

One method of achieving orientation in a conducting polymer is the preparation of a prepolymer which is tractable. P.D. Towsand presented results on “Durham” polyacetylene (J. Feast) which is prepared by the thermal decomposition of a percursor polymer. A 20:1 ratio of ℓ to ℓ₀ may be obtained for the polyacetylene prepared in this way.

The resulting (CH)_x exhibits a well defined fiber pattern and provides the opportunity of determining whether intrachain or interchain processes are most efficient for generating photoinduced carriers. Towsand’s work on photoexcitation and charge transport in highly oriented Durham polyacetylene showed that there is a factor of 4 favoring interchain vs. intrachain motion.

H. Shirakawa gave an overview of methods for obtaining highly oriented polyacetylene. He described a new method using a nematic liquid crystalline host as a substrate for film growth. The films were grown in the presence of a magnetic field (H₀ = 10 Kgauss) to give well oriented fibrils. The conductivity parallel to the fiber axis was 1.2 × 10⁴S cm⁻¹, while that perpendicular to this direction was 4.8 × 10³ S cm⁻¹.

H. Naarman reported a new variation on the Zeigler-Natta route to polyacetylene. Naarman utilized a standard Ziegler-Natta catalyst in silicone oil at ambient temperature. This method gives a product which can be highly oriented (to 540%). The conductivity along the stretch direction is the highest reported thus far (1.7 × 10⁴ S cm⁻¹). Furthermore, a study of conductivity vs. time showed that iodine doped, highly oriented polyacetylene retained high conductivity in air for much longer periods of time than either unoriented or “Shirakawa” polyacetylene.

Other conducting polymeric materials exhibit anisotropic behavior. Conducting metal phtalocyanine/Kevlar^R molecular/macromolecular blends (MMB’s) have an anisotropic structure (Inabe, Marks, Wynne), but the conductivity appears isotropic or nearly so (peak conductivity is 5 S cm⁻¹). Fibers of doped MPcI_x/Kevlar^R may be spun out of strong acid solvents reflecting a degree of processability. The solid consist of microcrystallites of MPcI_x oriented along the fiber axis in an oriented crystalline Kevlar^R matrix. At MPcI_x loading levels of 40–50% by weight, where conductivity is nearly Maximum, a high modulus is retained.

Methods for Obtaining Polymer Single Crystals. R. Baughman pointed out that it is not possible to assess orientation definitively on the basis of a measurement such as conductivity, which is sensitive to effective conjugation length. He noted that it is best to combine measurement techniques, e.g., x-ray and spectroscopic methods. For the ultimate in detailed structural (x-ray) and spectroscopic (vibrational, optical) characterization single crystals required. H. Shirakawa noted that (SN)_x and polydiacetylene were the only polymer single crystals with unsaturated electronic structures.

To set the problem of obtaining single crystal conducting polymers in perspective, R. Baughman presented a general outline of methods for obtaining three dimensional order in organic polymeric solids. Three approaches were mentioned in the discussion:

• a) Simultaneous crystallization/synthesis (e.g., Wunderlich,s work on the polyoxymethylene).

• (b) Solid State polymerization-polydiacetylenes (pionneered by G. Wegner).

• (c) Matrix controlled polymerization — cyclohexadiyne in thiourea Baughman commented that even if one had a single crystal starting material, partial oxidation would likely give rise to a less ordered structure. He noted that solid state reactions do not work in general to give an ordered structure. Problems which arise include reaction non-uniqueness and Van der Waals volume changes during reaction (a 25% volume change is typical in going from monomer to polymer and results in loss of template during synthesis).

In the context of this discussion, A. Epstein commented that one must always question the structure of a highly conducting material after doping relative to the structure of the starting polymer. There may be clustering (i.e., structural inhomogeneity) associated with dopant species (cations or anions). Generally, long range order is not observed. With the “new” structure after partial oxidation the question arises as to whether there will be a corresponding “new” magnetic behavior.

Electrochemical growth of charge transfer complexes was noted and A. MacDiarmid speculated that electrochemical growth of a conducting polymer might lead to a single crystal. The consensus was that this would be a most interesting and promising route which seems to have been overlooked.

Single crystals can give definitive physical information and oriented polymers may be useful for composite structures. However, B. Scrosatti pointed out that oriented materials generally have poorer kinetics with regard to electrode processes due to slower ion migration.