Poly(p-phenylene vinylene)

PPV is the only other member within this family of conducting polymers that has been successfully processed in film form to yield oriented samples with extremely high levels of crystallinity. Thus the structure of PPV has also been extensively studied[13,16,37,38,39,40]. Since PPV is typically prepared during thermal conversion of a highly processible precursor polymer, it is possible to control the degree of structural order and film morphology[41]. The unit cell is reported to have P2₁/a symmetry with a large monoclinic angle of $\alpha=123^\circ$. This angle produces a nesting of the PPV chains along the $b\sin \alpha$ direction (in the equatorial plane) so that the phenylene rings along one chain sit centered on the vinyl linkage of the two nearest neighboring chains as shown in Fig. 5. In the perpendicular direction, along the a-axis, there is very poor axial ordering and this leads to substantial smearing of the scattering signal in the non-equatorial data[37].

 

Figure 5: Schematic of the PPV structure within the b-c unit cell plane showing the staggered nesting of the vinylene linkage between phenylene rings of the two neighboring chains which is responsible for the large monoclinic angle, $\alpha$. Right: Edge on view of the PPV chains depicting the mean deviation from planarity as defined by $\theta_T$.

The presence of para bonded phenylene rings in the main chain construction increases the complexity of the intrachain structure. Extended $\pi$-conjugation is best achieved if the polymer chains adopt a fully planar conformation. However the steric repulsion between, in the case of PPV, adjacent vinylene and phenylene ring hydrogen atoms favors a non-planar construction. In PPP these effects become even more pronounced. To further quantify the degree of non-planarity, structure factor refinements of x-ray and neutron scattering data have been used. In PPV[39] and PPP[42] the room temperature mean ring deviation from planarity, defined by $\theta_T$ in Fig. 5, are found to be approximately 5$^\circ$ and 10$^\circ$ respectively.