Rigid-rod polymer hosts (polyacetylene, polythiophene, etc.)

Historically these compounds were among first conducting polymers synthesized and they remain the best understood with respect to their structural ordering[]. Specific examples of these polymers are shown in Fig. 1 and include polyacetylene (PA), poly(p-phenylene vinylene) (PPV), poly(p-phenylene) (PPP), polythiophene (PT) and polypyrrole (PPy). The majority of these compounds exhibit crystalline phases which adopt a herringbone equatorial packing (a packing motif common to many conventional linear polymers such as polyethylene and polypropylene) within an orthorhombic or monoclinic unit cell as shown in Fig. 2. Hence there are two polymer chains per projected two-dimensional (2D) equatorial unit cell with p2gg symmetry. The average angular orientation of the polymer chains' major axis, with respect to an equatorial lattice vector (typically a), is specified by the setting angle $\phi$. Table I summerizes representative reported values for the various unit cell parameters. The ratio, $a / b \sin \alpha$, is particularly noteworthy for it often falls near a value of $\sqrt{3}$, especially for PA and PPV. Hence the lattice formed by the equatorial projection of the polymer chain centers closely approximates that of a triangular grid.

 

Figure 2: Left: Two-dimensional equatorial projection of the most representative chain packing for linear unsubstituted conducting polymers. This structure has p2gg symmetry and two polymer chains per cell with general lattice parameters of a, $b\sin \alpha$ and $\phi$ (the setting angle). Right: Three-dimensional construction of the unit cell.

For some conducting polymers the net crystallinity can be quite large and values exceeding 80% have been reported[13]. Coherence lengths, i.e. the distances over which Bragg periodicity is maintained, are rather modest with typical values ranging from 50Å to 200Å depending on the specifics of the polymer sample[14], the particular synthesis route and any additional processing procedures. In addition to these characteristics there can be pronounced fluctuations in the axial chain to chain registry (parallel to the chain axes) between neighboring chains so that there is appreciable paracrystallinity[15]. This type of disorder is typically inferred from systematic diffuse components along scattering profiles within the various non-equatorial ($\ell \ne 0$) layer lines[16]. These properties, when combined with the powder averaging by polycrystalline mats which typify as-prepared films or powders severely reduce the overall distinguishability of individual diffraction peaks. In many cases, further processing can introduce uniaxial, biaxial, or higher order orientation of the crystallites[17]. Despite these complications standard crystallographic analysis methods are possible[17] and significant amounts of structural detail can be discerned from pristine and doped hosts.