Polyacetylene crystalline films

Table 4 lists the MBPT(2) band gaps of polyacetylene calculated with basis set 6-31G and DZP at three different geometries by us [36]. The cutoffs N and K are both 21. The geometries used in the calculations are listed in Table 5. The first two were given by Suhai [53,55] and the last one was an experimentally estimated geometry [97], The band gaps obtained are 4.033, 3.744, and 3.222 eV, respectively. There is no direct measurement of the band gap, defined as a quasi-particle energy difference of the lowest unoccupied and highest occupied orbitals. Instead, the absorption spectrum of polyacetylene crystalline films rises sharply at 1.4 eV and has a peak around 2.0 eV [97]. To explain this measured spectrum, one needs to calculate the density of the system s excited states and the absorption coefficients of the states.

By carrying out experiments on the polymerization of acetylene by using a concentrated Ziegler-Natta catalyst solution, Shirakawa and Ikeda were the first to report on the preparation of partially crystalline polyacetylene (PA) films under well-defined conditions [2]. Some six years later, Shirakawa, McDiarmid, and Heeger and colleagues [3] presented unequivocal evidence that, after doping with iodine vapor, the room-temperature electronic conductivity of PA film was... 

The through-space induction was also demonstrated in the polymerization of acetylene within a chiral nematic liquid crystalline field to yield achiral polyacetylene (PA) films that assume helical conformations with left- and right-handed screw... 

Poly acetylene, the simplest among the linear ir-conjugated polymers, is a semiconductor the conductivity is dependent on the crystallinity [1], the oxygen content [2,3], and the cis-trans content [4]. The electrical conductivity of crystalline films of poly acetylene varies from 10 S cm for the cw-isomer to 10 S cm for the ra/75-isomer [4]. Either cis- or trans-polyacetylene films can be doped with various electron-accepting species, such as halogens (L or Br.), metal halides (AsE, or FeCl,), protonic acids... 

The resulting porous, fibrillar polyacetylene film is highly crystalline, so is therefore insoluble, infusible, and otherwise nonprocessible. It is also unstable in air in both the conducting and insulating form. 

The materials used in most current research are irregular mats of highly crystalline fibrils with diameters of around 10 nm, so that the films are characterised by a very high surface area (around 60 m2 g-1), a problem in some potential applications and an asset in others. The morphology of polyacetylene is sensitive to the conditions of preparation and to ageing and was the subject of much heated discussion in the early development of polyacetylene. 

Copolymers with longer polyene segments were found to be insoluble in the reaction solvent (toluene). In these materials, the polyacetylene fraction was crystalline (97). Copolymers with a low acetylene content were composed of a variety of isolated crystalline structures within an amorphous matrix, whereas those containing 50% or more polyacetylene had a morphology that resembled fibrillar polyacetylene. Dried copolymer solutions and suspensions gave blue films with the mechanical properties characteristic of the carrier polymer. No increase in environmental stability was observed. 

A triumph of the Durham route was the preparation of oriented crystalline polyacetylene (107-109) by stretch orientation of the polymer during the transformation reaction. This material has highly anisotropic optical properties, but the anisotropy of the conductivity of the doped polymer was low. Oriented fibers as well as films were prepared. 

Table 2 contains idealized structures of some CPs with typical dopants and values for the conductivities of thin films. The exact structures of PPy and poly thiophene (PT) are unknown. Polyacetylene is the most crystalline and PANi can exist in several oxidation states with electrical conductivities varying from 10 S/cm to the values reported in Table 2. In its undoped state, PPS is an engineering thermoplastic with a conductivity of less than 10 S/cm. Upon doping with ASF5, conductivities as high as 200S/cm have been obtained after casting a film from a solution of AsFsP ...

Both types of polyacetylene are highly crystalline, intractible solids. The morphology of polyacetylene films consists of fibrils about 200 Angstroms in diameter oriented normal to the surface of the film. 

Also shown in Table lO-l is an (alkylcyclohexylaryloxy)-substituted polyacetylene [77]. Polymers of this general structure have been found to display liquid-crystalline behavior. In contrast to vinyl-based liquid-crystalline polymers, the geometric isomerism of the main-chain double bonds plays a role in determining the type of phase that is found. Advincula et al. have examined Langmuir films of polyacetylenes at the air-water interface [78]. Polyacetylene derivatives are unusual in that the polymer backbone itself acts as a chromophore therefore, in studies such as these, UV-visible spectroscopy can be a sensitive probe of polymer conformation. 

Unfortunately, even for the simplest and most studied case, the polyacetylene film, there is not a homogeneous network [5]. The mixing of the amorphous and the crystalline part makes the average properties observed, much more difficult to interpret. Not only does the very complex structure of the conducting polymer films produce scattered data for the conductivity, but the spectroscopic data are often dependent on the packing and chain conformation. As a consequence, the electronic properties of conducting polymer films may vary from one sample to another. Therefore, a major difficulty arises in deciding whether or not the difference observed was as a result of the chosen chemical structure and polymerisation route or of the way the molecules were packed. 

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