Doping of polyacetylene films

Dining doping of polyacetylene films, contact with the liquid tetrafluoride led to ignition of the film. 

N. Koshidaand Y. Wachi, Application of ion implantation for doping of polyacetylene films, Appl. Phys. Lett. 45(4) 436 (1984). 

If the principles, so far outlined, are valid then it is to be expected that n-type doping of polyacetylene would lead to a decrease in stability towards oxidation, and this is indeed so 578). However, the introduction of electrons into the chain can also give a new instability in that the oxidation potential can fall to the point where the polymer is able to reduce water and it becomes hydrolytically unstable. Thus n-type doped polyacetylene reacts rapidly with water and with alcohols, with partial hydrogenation of the chain and a rapid decrease in conductivity 579,580,581). Whitney and Wnek 582) have used the reaction of n-doped polyacetylene with alkyl halides and other reagents to prepare functionalized poly acetylene films. 

The report of the doping of polyacetylene (PAc) films to produce metallic levels of conductivity by Shirakawa et al. (1977) sparked the interest in electrically conductive polymers that has continued until today. While it was not the first example of a conductive polymer, the increase in conductivity, by a factor greater than 107, observed on exposing films of trans-PAc to arsenic pentafluoride and iodine, was dramatic, see Fig. 9.1. The impact of this result was immediate, and created an upsurge of interest in conjugated polymers and the possibility of rendering them conductive. 

Acetylene selectivity polymerizes in the presence of Ziegler catalysts whose components have low Lewis acidity [e.g., Ti(0-n-Bu)4—Et3Al(l 4)J. Cis-polyacetylene forms at low temperature, and trans-polyacetylene at high temperature (Eq. (1)). When doped, a polyacetylene film shows metallic conductivity, and hence the application of polyacetylene to polymer batteries and solar cells is now under intensive study 1-3). 

Figure 1.16. X-ray diffraction pattern of an iodine-doped, drawn polyacetylene film y = 0.09. (Photograph by courtesy of the authors of ref. 26. Copyright (1991) Elsevier Science Ltd, Kidlinglon 0X5 IGB, UK.)...

Infrared spectra of polyacetylene films lightly doped with the alkaline earth countercations Ca Sj.+2 or Ba 2 show two new absorptions at "900cm land at "1400 cm i (33). These absorptions increase with heavier doping. However, increasing the doping... 

To explore more fully the nature of the defects induced by the n>doping of polyacetylene with divalent cations, optical absorption experiments have been carried out (17). Spectra were obtained on thin semitransparent films of polyacetylene grown on fiat glass. The 2000A thick films were synthesized using one-third the normal catalyst concentration and a five second exposure to 60 torr of acetylene gas (10). After washing and drying the thin c/s-rich polyacetylene films, they were thermally isomerized to trans-... 

Polyacetylene, a one-dimensional, conjugated polymer represented as (CH) exhibits electrical conduction upon chemical doping with an electron acceptor or donor [1,2]. The chemical doping transforms the polyacetylene from insulator or semiconductor to conductor. Ordinary polyacetylene film is composed of fibrils that are bundles of polyene chains. Because the fibrils are randomly oriented, the inherent electrical conductivity of the polyacetylene chain is depressed owing to fibril contact resistance. This makes it difficult for polyacetylene to become a complete one-dimensional conductor at the macroscopic level. Today, the primary concern is how to align the fibrils of polyacetylene film in order to achieve higher electrical conductivity. 

The conductivity of iodine-doped polyacetylene first reported by Shirakawa et al. [1] in 1977 was 30 S cm . Since then, the conductivity reported for doped polyacetylene has kept increasing, the highest conductivity obtained so far for an iodine-doped stretched polyacetylene film [17] being > 10 S cm, a value comparable with that of copper (6 x 10 S cm ). 

Oriented polyacetylene films have been synthesized by using a nematic liquid crystal as a polymerization solvent. The maximum value of parallel electrical conductivity of the films doped with AsFs was 11,000 S cm [17]. The use of an aged mixture of Ti(OC4Hc,)4-A1(C2H0. in silicone oil that has been mixed with various quantities of n-butyllithium, as the catalyst for the polymerization of acetylene, yields polyacetylene that is highly regular, compact, and crystalline, in well-defined parallel planes. When it is stretched and doped, the polyacetylene film shows a dramatic increase in conductivity to a value greater than lO S cm [18]. 

It has been reported that AsF,-doped m-polyacetylene films can be prepared by exposure of acetylene gas to a small amount of AsF in a glass reaction vessel at —75° to — 198°C. For example, a polyacetylene film obtained at — 75°C with an initial molar ratio of acetylene to AsFs of 10 had the composition [CH,, (AsFs) oii]x and exhibited a room temperature conductivity of 4.8 X 10 s cm [55]. 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Electrically insulating films of polyacetylene, doped with iodine and sodium, became semiconductive (Shirakawa, MacDiarmid, Heeger, 1976). 

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