Temperature induced variations

There are a limited number of studies which have investigated the structural response of conducting polymers to changes in the ambient temperature and/or pressure[53]. While there is no evidence of a state phase change among the linear unsubstituted hosts, significant microscopic effects have been detected and these observations provide important clues towards understanding fundamentals issues of conducting polymer behavior.

The most obvious response to thermal and pressure variations are the large fractional changes in the crystal lattice repeats in the equatorial direction. Thus the ``free'' volume within which individual chains can traverse is a strongly varying quantity. Any electronic transport properties which are dependent on the interchain and intrachain wavefunction overlap will, in principle, be strongly affected. In addition, rotational displacements of the host main chain, about the chain axes, can also have an important consequence[54]. The presence of strong torsional motions about the chain axis will alter the level of main chain $\pi$-conjugation and thus limiting intrachain transport and altering the band structure. This latter response implies that there is also a dynamical component to this effect. Long wavelength collective torsional oscillations alter the intrachain atom to atom $\pi$ overlap only slightly and would be expected to have a minimal influence. Pronounced local torsions, resulting from phenylene ring librations and ring flips about the axis defined by the two para bonded carbon atoms, would be expected to have a more significant impact.

NMR and inelastic neutron scattering have both been used to study the local dynamical response of the ring motions in PANI[55,56] and in PPV films[57,58]. The NMR experiments are able to detect the two aforementioned types of motion, ring flips and librations. At lower temperatures the ring motion is almost exclusively librational with modest torsional excursions about an equilibrium position. At elevated temperatures there are pronounced flips approaching a full 180$^\circ$of rotation.

Thermal studies of these hosts also impact a secondary side issue. For short chain oligomeric model compounds, there exists a liquid crystalline phase referred to as the rotator phase as seen Fig. 7. In this state the underlying translational symmetry (i.e., hexagonal) of the chain packing is retained but no long-range orientational ordering of the chains is present. In this context conducting polymers may be viewed as an example of an ``infinite'' model host system. Diffraction studies reveal an interesting effect that distinguishes the phenylene ring containing compounds. Systems containing phenylene rings and hence the possibility of local ring motions (i.e., PPV and PANI) show little or no evolution towards the rotator phase while those systems lacking in this motion (PA and the conventional polymer polyethylene) exhibit clear signatures indicative of a transition towards this phase[59].

 

Figure 7: Schematic 2D model depicting the three most common structural responses to conducting polymer intercalation. (a) Transformation to a three fold channel structure; (b) transformation to a four fold channel structure; c) transformation to a layer compound.