  Small angle X-ray scattering (SAXS) studies in
poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with
varying conjugation, and polyethylene dioxythiophene complexed with polystyrene
sulfonate (PEDOT-PSS) in different solvents have shown the important role of
pi-electron conjugation and solvent-chain interactions in controlling the chain
conformation and assembly. In MEH-PPV, 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. In case of PEDOT-PSS, the chains undergo
solvent induced expansion and enhanced chain organization. The clusters formed
by chains are better correlated in dimethyl sulfoxide (DMSO) solution than
water, as observed in the scattered intensity profiles. The values of radius of
gyration and the exponent (water: 2.6, DMSO: 2.31) of power-law decay, obtained
from the unified scattering function (Beaucage) analysis, give evidence for
chain expansion from compact (in water) to extended coil in DMSO solutions,
which is consistent with the Kratky plot analysis. The mechanism of this
transition and the increase in dc conductivity of PEDOT-PSS in DMSO solution
are discussed. The onset frequency for the increase in ac conduction as well as
its temperature dependence probes the extent of connectivity in PEDOT-PSS
system. The enhanced charge transport in PEDOT-PSS in DMSO is attributed to the
extended chain conformation as observed in SAXS results.
