Dopants for polyacetylene,

Oxygen is also a dopant for polyacetylene, but on exposure the conductivity rises to a maximum then rapidly declines as oxidation of the polymer backbone occurs, as shown in Fig. 21. We have no data on the diffusion coefficient as the process is rapid and is masked by the reaction of oxygen with the polymer. The kinetics are first-order, implying that the doping reaction is rapid, goes to less than 1 mol%, and is then followed by irreversible oxidation of the polymer. Based on the observa-... 

Iodine has proven to be a very effective dopant for polyacetylene. Conductivities as high as 10 S cm have been achieved [14]. Bromine, IBr and ICl also dope polyacetylene [119], but somewhat less effective... 

Since the lighter alkaline earth metals do not dissolve in liquid ammonia, they are more difficult to use as n-dopants for polyacetylene. However, two methods have been developed which can incorporate Mg 2 jons into polyacetylene. These methods are the addition of magnesium salts to sodium-ammonia solutions and electrolysis. We will treat each in turn. 

The utility of classical antioxidants such as hindered amines, phenols, and nitrones for the stabilization of pristine polyacetylene (29), poly(methyl acetylene) (30), and poly(l,6-heptadiyne) (31) has been examined. Poly(methyl acetylene), although dopable to only low conductivities (10" S/cm), has similar oxidative behavior to polyacetylene and serves as a good model for other polyenes. In general, the improvement in stability of poly(methyl acetylene) was limited, but combinations of hindered phenols and hydroperoxide scavengers resulted in a factor of 5 decrease in the oxidation rate (30) as monitored by the appearance of IR absorption bands attributable to carbonyl groups. These degradation rates are still too high for the use of these polyenes in an unprotected environment. The compatibility of such stabilizers with the dopants commonly used for polyacetylene was not studied. 

Most of the research was done with 0.2 micrometer rated porous polypropylene (Accurel ) membrane, and the concentration of polyacetylene in the composite could be varied from 4 to 43 percent. Larger percentages should be possible. The membranes did not lose their flexibility, and membrane properties such as flux rates and bubble point pressure were not altered (see Experimental Procedure 1). As is the case for polyacetylene alone, the conductivity of these membranes could be varied depending upon the type and amount of dopant. Iodine doped laminates were the most stable of the two doped laminates investigated in this study. 

Doping with these Group VA fluorides can be carried out electrochemically, using the polymer as the anode in a cell containing a solution of the dopant ion, e.g. tetrabutylammonium hexafluoroantimonate in propylene carbonate, or from the vapour phase. In the latter case the oxidative doping of polyacetylene is supposed to involve the following reaction of the dopant (for the case of AsFs) ... 

The dopant molecules may furthermore enter the hose lattice in an organized fashion, giving rise to new, enlarged repeating units and corresponding new reflection peaks in the diffraction patterns. Ordered intercalation is a well known phenomenon for graphite compounds (see for instance [113]) and has also been observed for dopants in polyacetylene [114]. In the present section we shall discuss the actual situation for polythiophenes. 

Since the initial discovery of polyacetylene doping by FeClj [92], ferric chloride still remains one of the most popular dopants for conjugated polymers. In addition, it is a very efficient oxidizing/polymerizing agent for the preparation of polypyrrole, polythiophene and their derivatives [32,84]. The most extensive Mbssbauer effect studies have therefore been carried out for FeCl, doped polymers. 

Figure 16.11. Log a,/a versus time t for polyacetylene heated at 110°C in vacuum for a variety of dopants, a, represents the resistance of the sample at the temperature of the experiment at a given time, (tq represents the resistance of the sample...

The polymer is very stable and can withstand temperatures up to 450°C in air without degrading. It is an insulator in the pure state but can be both n- and p-doped using methods similar to those for polyacetylene. However, as PPP has a higher ionization potential it is more stable to oxidation and requires strong p-dopants. It responds well to AsFs, with which it can achieve conductivity levels of... 

Kaufman reported a lower value of 4 x cm s [21]. Because of the slow diffusion of dopants in polyacetylene, an electrode with a higher surface area is preferable for obtaining a high current density. Padula et al reported that highly porous polyacetylene (foam type) showed a higher current density than an ordinary film in cyclic voltammetry measurements [22]. 

---