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 -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 Å. 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 * Electronic address: s_paramita@physics.iisc.ernet.in Fax: +91-80-2360 2602 Tel: +91-80-2293 2859power-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.

The strikingly different charge transport behaviours in nanocomposites of multiwall carbon nanotubes (MWNTs) and conducting polymer polyethylenedioxythiophene-polystyrene-sulfonic-acid (PEDOT-PSS) at low temperatures are explained by probing their conformational properties using small-angle x-ray scattering (SAXS). The SAXS studies indicate the assembly of elongated PEDOT-PSS globules on the walls of nanotubes, coating them partially, thereby limiting the interaction between the nanotubes in the polymer matrix. This results in a charge transport governed mainly by small polarons in the conducting polymer despite the presence of metallic MWNTs. At T > 4 K, hopping of the charge carriers following one-dimensional variable range hopping is evident which also gives rise to a positive magnetoresistance (MR) with an enhanced localization length (∼5 nm) due to the presence of MWNTs. However, at T < 4 K, the observation of an unconventional positive temperature coefficient of resistivity is attributed to small polaron tunnelling. The exceptionally large negative MR observed in this temperature regime is conjectured to be due to the presence of quasi-1D MWNTs that can aid in lowering the tunnelling barrier across the nanotube-polymer boundary resulting in large delocalization.

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.

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The positive and negative magnetoconductance ͑MC͒ data ͓J. Vavro et al., Phys. Rev. B 71, 155410 ͑2005͔͒ in various single-wall carbon nanotube samples are analyzed by taking into account the electron-electron interaction ͑EEI͒ contribution, in addition to the weak localization ͑WL͒ regime. The low field MC data shows an H 2 dependence, in accordance with the EEI and WL models. The contribution from EEI to the total MC is further confirmed from the universal scaling of MC relation ͓͕⌬ / T 1/2 ͖ vs ͑H / T͒ plots͔, showing that EEI plays a significant role at higher fields and lower temperatures. Intrinsic parameters such as inelastic scattering length ͑l in ͒ extracted for barely metallic sample ͑120 S / cm at 300 K͒ follow the T −3/4 dependence due to the inelastic electron-electron scattering in the dirty limit. The l in for highly conducting sample ͑3570 S / cm at 300 K͒ follows a T −0.4 dependence. The various order parameters helps us to characterize the system in a disorder-tuned metal-insulator transition scenario.