Diffraction Structures and Chain Packing in the Crystal

O j-polyacetylene shows 10 reflections in x-ray diffraction 439). The unit cell was identified as orthorhombic, a = 761 pm, b = 447 pm, c = 439 pm with a density of 1.16 g cm-3. In the original x-ray study the b axis was taken to be the chain axis but subsequent electron diffraction studies allowed fibre patterns to be obtained 440). On the principle that, for all polymers, the fibre axis is the chain axis, c was identified as the chain direction although there is some dispute The analysis cannot definitely distinguish between the cis-transoid and trans-cisoid structures. 

For tra/u-polyacetylene, X-ray diffraction yields a monoclinic unit cell with a = 424, b = 732, c(chain axis) = 246 pm and (3 = 91 to 93°, i.e. very close to 

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. 

The isomerization of polyacetylene from the cis- to the trans-form can be readily 

A report on doping of polyacetylene by metal halides 462-463) shows that the interplanar spacing increases with the size of the anion and clustering is inferred to occur at low dopant levels as the dopant reflection appears at about 3 mol% while much of the material is still undoped. It is not totally clear whether similar effects might be the result of a combination of slow dopant diffusion and a diffusion coefficient which is dependent on dopant concentration this is discussed in more detail below. 

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