Oriented polyacetylene

Shirakawa polyacetylene oriented in liquid-crystal matrix [17b] 1600 1.10 (0.50) 1750... 

Much effort has been expended toward the improvement of the properties of polyacetylenes made by the direct polymerization of acetylene. Variation of the type of initiator systems (17—19), annealing or aging of the catalyst (20,21), and stretch orientation of the films (22,23) has resulted in increases in conductivity and improvement in the oxidative stabiHty of the material. The improvement in properties is likely the result of a polymer with fewer defects. 

A non-electrochemical technique which has been employed to alter the physical characteristics of a number of polymers is that of stress orientation [26, 27], in which the material is stressed whilst being converted to the desired form. This has the effect of aligning the polymer chains and increasing the degree of order in the material, and is obviously most applicable to materials which can be produced via a precursor polymer. With Durham polyacetylene (Section 4.2.1) increases in length in excess of a factor of twenty have been achieved, with concomitant increases in order, as shown by X-ray diffraction and by measurements of the anisotropy of the electrical conductivity perpendicular and parallel to the stretch direction. 

Durham polyacetylene occurs in a highly disordered state on conversion from the precursor polymer [90], but using stretch orientation techniques during the conversion reaction, a high degree of order with long conjugated sequences can be achieved [91-93],... 

Active research of the electronic properties began in the early 1970s, when it was shown that polyacetylene may be synthesized as a flexible film with arbitrary and specially oriented fibrils. 

The same authors 369,3701 also obtained similar results if the liquid crystal solvent was aligned by flow during the polymerization. They showed that the polymerization conditions lead to alignment of the fibrils within the polymer mass and of the chains within the fibrils polymers produced in this way could also be doped to a conductivity of 104 S cm-1 371). The morphology of polyacetylene produced by polymerization in a liquid crystal solvent, aligned both magnetically and by flow, has been studied by Montaner et al. 371). They show that the polymer film is made up of very long fibrils built from microfibrils. In one fibril, the orientation of microcrystalline domains with respect to the fibril axis is very well defined, whilst the orientation of the different fibrils in the sample spreads over 20°. 

Woemer et al. 373) produced polyacetylene with locally oriented regions and an optical anisotropy of 2 x by polymerization on crystals of biphenyl. Yamashita and co-workers 374,375) have recently reported epitaxial polymerization of acetylene on crystals of anthracene, naphthalene and biphenyl where fibrils of cis- or trans-polymer formed, crystallographically aligned with the substrate. Fincher et al. 376) produced a 3 x extension which gave a 4 x optical anisotropy. 

Several attempts to induce orientation by mechanical treatment have been reviewed 6). Trans-polyacetylene is not easily drawn but the m-rich material can be drawn to a draw ratio of above 3, with an increase in density to about 70% of the close-packed value. More recently Lugli et al. 377) reported a version of Shirakawa polyacetylene which can be drawn to a draw ratio of up to 8. The initial polymer is a m-rich material produced on a Ti-based catalyst of undisclosed composition and having an initial density of 0.9 g cm-3. On stretching, the density rises to 1.1 g cm-3 and optical and ir measurements show very high levels of dichroism. The (110) X-ray diffraction peak showed an azimuthal width of 11°. The unoriented material yields at 50 MPa while the oriented film breaks at a stress of 150 MPa. The oriented material, when iodine-doped, was 10 times as conductive (2000 S cm-1) as the unstretched film. By drawing polyacetylene as polymerized from solution in silicone oil, Basescu et al.15,16) were able to induce very high levels of orientation and a room temperature conductivity, after doping with iodine, of up to 1.5 x 10s S cm-1. 

A particularly interesting property of Durham polyacetylene is that it can be stretched to draw ratios of up to 20 during the transformation, to yield a polyacetylene sample with high levels of orientation. This effect was reported by Bott et al. 378) for thin films in the electron microscope and then by Leising et al. 379), who drew single fibres of polyacetylene to a highly oriented /rani-state with a density of 1.06 g cm-3. 

The random orientation of the crystalline order in typical Shirakawa polyacetylene means that diffraction studies are limited to powder methods. For such studies, and many others, it would be very useful to have much more oriented polymers and many attempts have been made to orient polyacetylene, either by mechanical treatment of the polymer or by appropriate modifications to the polymerization reaction. These have been reviewed earlier. 

The effect of this is that the charged defects are independent of one another and can form domain walls that separate two phases of opposite orientation and identical energy. These are called solitons and can sometimes be neutral. Solitons produced in polyacetylene are believed to be delocalised over about 12 CH units with the maximum charge density next to the dopant counterion. The bonds closer to the defect show less amount of bond alternation than the bonds away from the centre. 

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