  Small angle X-ray scattering (SAXS) in
poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) solution
has shown the important role of pi-electron conjugation in controlling the
chain conformation and assembly. By increasing the extent of conjugation from
30 to 100 %, the persistence length (l_p) increases from 20 to 66 Angstrom.
Moreover, a pronounced second peak in the pair distribution function has been
observed in fully conjugated chain, at larger length scales. This feature
indicates that the chain segments tend to self-assemble as the conjugation
along the chain increases. Xylene enhances the rigidity of PPV backbone to
yield extended structures, while tetrahydrofuran solvates the side groups to
form compact coils in which the l_p is much shorter.
