Oriented polyacetylene films

Figure 6.26 shows the chemical structure of PPV. Block letters from A to F show the eight carbon sites in one PPV monomer unit. The principal axes of proton hyperfine coupling of an unpaired tt-electron, defined in Figure 6.18, arc also shown for two proton sites of inequivalent bond orientations. ESR spectra of stretch-oriented undoped PPV films have shown anisotropic lineshapes with an average g value of 2.003 and a spin concentration of about 1 spin/10 PPV monomer units. The observed anisotropic behaviour of the. g value and the linewidth, both larger for the stretch direction, are qualitatively similar to those found in stretch-oriented polyacetylene films mentioned in Section 3.2 and... 

On the other hand, orientation is one of the most important factors to enhance the conductivity of one dimensional pi-conjugated polymers like polyacetylene. Several attempts were made to obtain an oriented polyacetylene [3-5]. Akagi and Shirakawa applied magnetic force into a polymerization system consisting of nematic liquid crystals and obtained a highly oriented polyacetylene. Kahlert and Leising synthesized an oriented polyacetylene film by pyrolyzing prepolymers. Ozaki polymerized acetylene on the surface of benzene crystals and prepared an oriented polyacetylene. 

The author was interested in how much the conductivity of oriented polyacetylene films can be enhanced by stretching compared with that of as-grown films. Polyacetylene films were prepared by using both trialkyl aluminium and tetraalkyl titanate having long alkyl chains. Orientation and conductivity of stretched polyacetylene films have been reported here. 

Stretch-oriented polyacetylene films show one order higher... 

Electrical conductivity of polyacetylene films depends on orientation of polymer chains. Figure 5 shows the relationship between relative electrical conductivity and dichroic ratio of polyacetylene films prepared by a mixture catalyst of Trihexyl aluminium and Tetradecyl titanate. Conductivity of fully stretched or oriented polyacetylene films showed twenty times higher than that of as-grown films. Conductivity of polyacetylene films, prepared by other catalysts systems of trialkyl aluminium and tetraalkyl titanate, showed similar relationship, although increment of the conductivity was lower. 

Arava, K., Synthesis of highly-oriented polyacetylene film in a liquid crystal solvent, Synth. Met, 14, 199-206 (1986). 

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]. 

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. 

Polymerization of extremely thin polyacetylene films (< 1 pm) on crystal surfaces by epitaxial growth (e.g. on frozen benzene) [11] also induces orientation in the deposited polymer layer. Substituted polypyrrole films with a high anisotropy can be produced by the Langmuir-Blodgett technique [12]. 

Relationship between orientation and conductivity of polyacetylene films has been studied. Polyacetylene films were... 

The axis of polyacetylene polymer chains in the fibrils may not be not parallel to the direction of fibrils, even in fully stretched polyacetylene films. If it is so, the peak at 740 cm" should disappear in the parallel polarized FTIR spectrum on fully stretched polyacetylene films (Fig. 3b). But the weak peak at 740 cm " appeared. Akagi estimated the average orientation angle between the axis of polyacetylene polymer chains and the direction of fibrils by the following equation [9],... 

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. 

In crude polyacetylene, the polymer chains are jumbled, pointing in all directions. However, by stretching the film, the chains can be made to line up in a more ordered fashion. The conductivity of doped and oriented polyacetylene chains is greater along the direction of the chain than it is perpendicular to the chain. This result suggests that it is much easier for electrons to travel along a chain than to hop from one chain to the next. 

The Durham route leads to dense continuous films of polyacetylene, and the morphology details of films or fibres produced from the precursor polymer is dependent upon the type of conversion utilized. Consequently, the morphological considerations can vary from amorphous materials to highly oriented stretched films. 

Polyacetylene films produced in this way are well-aligned and show a high de ee of crystalhnity as evidenced by the x-ray photograph shown in figure 2. This diffraction pattern was obtained from a film of stretch ratio 1/1q =11 [17]. Line profile analysis of the various reflections shows that the polyacetylene films are composed of uniaxiaUy oriented paracrystaUites with dimensions of 50A perpendicular to, and 80A parallel to the chain... 

A plot of absorption coefficient, a, versus photon energy for an oriented polyacetylene sample is shown by the solid lines in figure 4. In this case, the sample thickness (d 3 im) prohibited transmission measurements above 1.6 eV for E//, however, the absorption peak has been measured on films of sub-micron thickness, stretched on suitable substrates [13], and its approximate form is shown by the broken line. It is evident from the figure that the transverse absorption is strong, a value of oc i = 1-0 0.2 x lO cm being obtained at 2.2 eV [14]. As shown later, an estimate 6i the absorption anisotropy at... 

VII. Mechanical and Electrical Properties of Polyacetylene Films Oriented by Tensile Drawing [75]... 

The wide-angle X-ray diffraction patterns of drawn polyacetylene films show crystalline order with preferred orientation of the chain axis parallel to the stretching direction (for trans-polyacetylene, see Figure VII.1) the (hkO) reflections are observed only with momentum transfer perpendicular to the stretching direction. The large number of equatorial reflections and off-axis... 

---