Semi-flexible rods (polyaniline)

Polyaniline (PANI), also shown in Fig. 1, is the best known example of this kind of conducting polymer system[2]. Other related polymers include poly(p-phenylene oxide) (PPO) and poly(p-phenylene sulfide) (PPS) although their best reported conductivities are very limited. All three of these polymers have been shown to contain large equilibrium phenylene ring torsional displacements (upwards of $\pm 30 ^\circ$ or more) out of the plane defined by the ring bridging atoms (amine/imine nitrogens in the case of PANI, sulphur for PPS, etc.). The most common crystalline form has an average structure best described within the orthorhombic Pbcn space group[].

The chemical and structural flexibility surrounding the amine/imine nitrogen linkages in PANI creates enormous diversity. Unlike the rigid-rod examples of the last section, PANI has limited solubility in a number of solvents and may be cast to yield free standing films or precipitated into powders[44]. The chemical formula which most broadly describes this polymer is represented by:

([(-C₆H₄-NH-C₆H₄-NH-)_1-x] [(-C₆H₄-N=C₆H₄=N-)_x ])_n

and it may be prepared in a variety of oxidation states ranging from leucoemeraldine to emeraldine to pernigraniline with x fractions of 0.0, 0.5, and 1.0 respectively. In addition to this oxidation chemistry, the imine nitrogen can be reversibly protonated using simple acid-base chemistry.

In terms of structure, the most heavily studied form of PANI is emeraldine base (EB, x=0.5) either as-synthesized or after various acid/base treatments[,,45,,,48,49] PANI-EB samples can range from fully to partially crystalline films or powders. The degree of crystallinity[50] is typically quite low and, even in the best samples, never exceeds 50%. The coherence lengths are also quite small with values of 70Å or less typically reported. In the approximate structure of Jozefowicz et al.[], see Fig. 6, the deviation from planarity by the phenylene rings is sequentially alternated between plus and minus $30^\circ$ as one moves along the backbone. In actuality the chemical repeat is four aniline monomer units long with three benzoidal rings and one quinoidal ring. Single crystal studies of various oligomeric PANI analogs[51] and molecular modelling calculations[52] of the EB intrachain structure suggest a more complex ring torsion structure in which the quinoidal ring is seen to be more planar than the benzoidal rings. Recent data[49] from a direct scattering study of the amorphous polymer are also consistent with this more complex intrachain ring structure. Even within the amorphous state there is evidence of least two locally different interchain structural organizations[] depending on the exact preparation of the PANI sample.

 

Figure 6: Schematic diagram of the four monomer repeat in PANI emeraldine base explicitly showing the deviations from planarity of the four rings, three benzoidal and one quinoidal, according to the approximate structure of Jozefowiciz et al. [43] for crystalline EB and the quoted values from structural analysis of amorphous PANI EB by Maron et al. [49].