Layered structures

The layered guest-host structures that can also develop undergo a structural evolution more reminiscent of the intercalation process in 2D layered materials (e.g., graphites or silicates). In this case, quasi-2D galleries open up between stacked sheets of the polymers chain as shown in Fig. 8. This type of ordering has been often reported in structural studies[82,83,84,85,86,87] using molecular dopants. Systems showing evidence for layer structures include iodine-doped PA[84] and AsF₅-doped PPV[85]. These layer structures may occur either alone in combination with channel formations[83,87].

The details associated with the presence of layered structures are far more sketchy than for the channel phases. There are a number of compromising factors which complicate a comprehensive analysis of this experimental data. The most problematic characteristic is the significant loss of the sample coherence lengths and the resultant broadening of the scattering peak widths in the doped phase which occurs at the onset of dopant uptake. This pathological behavior is clearly captured in the in situ scattering data of Fig. 11 for iodine intercalation into PA. In general few, if any, individual scattering features can be resolved and subsequently identified as unique lattice reflections. In addition, disproportionation reactions can yield a highly variable mixture of molecular guest ions (e.g., I₃^-, I₅^-, etc.). Detailed and rigorous structure factor refinements that test these various proposed models are rare. In view of these severe limitations much of the structural details for large molecular dopant remains incompletely understood.

 

Figure 11: Experimental (hk0) equatorial scattering data obtained in situ during iodine vapor doping of trans-polyacetylene. The large monotonically decreasing background clearly seen in the (CH)I_0.0 scan is an artifact due to air scatter. ($\lambda=1.542$Å) Adapted from Ref. [87].