The properties of nanocomposites of dodecyl ester functionalized multiwalled carbon nanotube (FMWNT) and four poly(3-dodecyl thiophene) (P3DDT) samples with different molecular weight and chain regioregularity are reported here. Uniform dispersion of FMWNT in the composites is evident from TEM study. pi-pi and CH-pi interactions are evident from FTIR spectra in the low molecular weight (M-w = 8 x 10(4)) and high regiorandom (head-tail = 69 mol %) P3DDT (ILM) composites; however, such interactions are not so prominent for other P3DDTs. Wide-angle X-ray scattering patterns indicate intact interchain lamellar structure of P3DDT in the composites. DSC study shows decrease of main chain melting peak of P3DDT (ILM) with increasing F-MWNT content but the decrease is lower with higher molecular weight and higher regioregular samples. Dynamic mechanical analyzer study indicates that there is no significant change in the T-g, T-beta, and T-gamma transition temperature with FMWNT concentration indicating almost ideal mixing of the components. The percent increase of G' increases with increasing FMWNT loading and also with temperature showing an 84% increase in storage modulus at -20 degrees C for ILM12 samples. The UV-vis spectrum indicates a decrease in band gap with FMWNT concentration for ILM composites, however, it is negligible with other P3DDTs. Photoluminescence quenching and multiple emission peaks occur in the composites. dc-Conductivity of the composites increases by two orders and current-voltage (I-V) curves show memory effect only in ILM-MWNT composites. The I-V curves of other P3DDT nanocomposites exhibit rectification property. Quantitative explanation of electronic properties has been afforded from the band energy and work function values. So, the electronic properties of the nanocomposite can be tuned by changing the molecular weight, regioregularity, and concentration of FMWNT. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1412-1425, 2009