Structure parallel to the polymer chain axis

The ordering parallel to the polymer chain axes has also been closely studied with both the polymer main chain and dopant structures receiving attention. Although the changes in the respective c-axis structure are not as dramatic as those in the equatorial plane, there are some interesting effects.

Doping of conducting polymers always involves significant charge transfer either to or from the $\pi$-conjugated orbitals comprising the skeletal backbone. This process is accompanied by a self-consistent relaxation of the electronic states and the structural ordering. N-type doping (in which charge is donated to the polymer backbone) is associated with an overall expansion in the c-axis repeat while p-type doping (in which charge is withdrawn from the backbone) initially generates a nominal reduction in the polymer lattice repeat. The magnitude of these variations can approach one to two percent of the repeat distance. Analogous behavior is seen during intercalation of graphites[88]. A number of studies[89,90,] have observed these effects in conducting polymers. There are some anomalous features to this lattice relaxation. During n-type alkali-metal doping of PA, it has been observed that the rate of change in the c-axis expansion abruptly increases at about 6% mole weight of the alkali-metal. Detailed total energy calculations[77,] have shown that this effect is essentially coincidental because the changes in the average bond angles and bond lengths arising from the change transfer conspire to nearly cancel out at the lighter doping levels.

The repeat spacings for the dopant ions filling the quasi-one-dimensional channels has also been measured. In samples having channels containing alkali-metal ions a modest distribution of intrachannel ion-ion spacings are obtained. These range from approximately 4Å to 5Å. Most of these values are typically incommensurate with respect to the repeat distance of the surrounding polymer chains (i.e., $4\times$ the nominal 1.24Å projected c-axis CH-CH repeat to yield 4.96Å as shown in Fig. 12(a)). There are, however, a few reports of samples containing commensurate alkali-metal ion repeat distances[65]. In all these cases the scattering spectra yield alkali-metal intrachannel coherence lengths, as determined from the angular peak widths, to be significantly less (typically a factor of two) than those of the surrounding polymer chains[75].

 

Figure 12: (a) Projected structural ordering of alkali-metal ions along the polyacetylene c-axis for commensurate and incommensurate spacings. (b) Projected structural ordering of two different polyiodide ions along the polyacetylene c-axis highlighting the possibility of larger iodine repeat distances.

The analysis of the c-axis structure for molecular dopants is considerably more challenging. Iodine-doped PA is a prime example of a p-type doped conducting polymer system whose c-axis dopant-ion structure has been probed[83,]. In general there is an approximate 3.1Å repeat of the iodine atoms parallel to the PA chains. However, these iodine atoms may belong to various odd order polyiodide ions (see Fig. 12(b)) whose relative proportion is both a function of the overall doping composition and the evolution of time[83]. The presence of these mixed arrays of differing iodine species produces large repeat distances and anomalous broadening effects in the individual peaks widths of scattering spectra taken along the meridional direction (i.e., parallel to the chain axis) in uniaxially oriented PA samples. Although complicated these systemic effects have been analyzed and effectively reproduced in model calculations of representative structures[].