Polyacetylene Durham

The soluble nonconjugated precursor polymer route to Durham polyacetylene via thermal elimination. 

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],... 

This, the second stage of the Durham polyacetylene synthesis (and presumably identical to the homopolymer [96570-67-1]) is capable of explosion with one sixth the energy of TNT. 

The Durham precursor route to polyacetylene is an excellent example of the application of organic synthesis to produce a precursor polymer whose structure is designed for facile conversion to polyacetylene. Durham polyacetylene was first disclosed by Edwards and Feast, working at the University of Durham, in 1980 227). The polymer (Fig. 6 (I)) is effectively the Diels-Alder adduct of an aromatic residue across alternate double bonds of polyacetylene. The Diels-Alder reaction is not feasible, partly for thermodynamic reasons and partly because it would require the polymer to be in the all m-conformation to give the required geometry for the addition to take placed 228). However, the polymer can be synthesised by metathesis polymerization of the appropriate monomer. 

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 crystallinity of m-polyacetylene has been estimated to be 76-84% by x-ray diffraction while trans-polyacetylene is 71-79 %6). Observations on Durham polyacetylene 445) showed that the diffraction peak narrowed, and the interchain -spacing decreased, during isomerization and annealing. The x-ray coherence length, a measure of the crystallite size perpendicular to the chains, increased from 2.6 nm to 7.1 nm, compared with 30 nm for polyethylene. 

Friend et al. 469,4 70) have reported X-ray diffraction studies of highly drawn films prepared from Durham polyacetylene. They analysed the width of the diffraction peaks to obtain a crystallite size perpendicular to the chains of 5 nm in Durham polyacetylene compared to 10 nm in Shirakawa polyacetylene and distortion parameters... 

Fig. 17. Variation of inter-chain spacing in Durham polyacetylene with annealing time at 90 °C, showing increasing ordering. (Ref. 230))...

Durham polyacetylene has the advantage of being a uniform, dense film and so lends itself much more readiliy to diffusion studies. In addition, the uniform morphology is much better suited to device applications, although the low surface area would limit applications in batteries. We have made extensive measurements on the doping of Durham frans-polyacetylene by gaseous AsF5 514 515), which is believed to dope the polymer to form the hexafluoroarsenate ion and arsenic trifluoride 516 ... 

In our own work on Durham polyacetylene 568) wfe find that the stability of doped polymers depends upon the extent of doping. Thus when AsF6 is the counter-ion, a polymer doped to low levels (< 1 mol %) shows very little change in conductivity over a period of days at room temperature in vacuum or dry air, whereas saturation doping (to about 17 mol%) produces a polymer whose conductivity decays rapidly, with ir evidence for the formation of C—F bonds in the polymer. 

Durham polyacetylene was the Hrst processable polyacetylene, and its use has been taken up by a number of groups [41,115]. One physical property which distinguishes it from... 

The first experimental data show that the tetragonal lattice is not realized for lithium [89]. This can be explained by means of packing calculations. The Li ion is much smaller than the holes in the most closely packed form of the lattice proposed (see Table 1.3 and Figure 1.9). Na is only slightly smaller, K just fits nicely, while Rb and Cs" ions leads to some expansion. Though Li-doped polyacetylene has been called amorphous at first, Leitner et al [95] demonstrate diffraction from Li-doped Durham polyacetylene. A number of possibilities for the location of the lithium ions is considered, based on a unit cell almost undistorted by doping [96]. 

It has been suggested that the three orders of magnitude differences in conductivity between some highly oriented polyacetylene types (e.g. Tsukamoto polyacetylene versus Durham polyacetylene) are related to differences in the tilt angle of the polyiodide ions (Pouget et al. [137]). Dopants may have a crucial bridging function, providing the necessary interchain conduction pathways [138],... 

Figure 16.7. First-order plot of the initial weight uptake of oxygen into a 7 micron thick film of Durham polyacetylene at SO C unstablized film film containing 5% Irganox 1010. Adapted from Polym. Deg. Stab. 19, 323 (1987), permission of Elsevier Science Ltd., Kidlington.

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