Chain-length dependent nematic ordering of conjugated polymers in a liquid crystal solvent

In this communication we demonstrate the dependence of the solute order parameter on the solute molecular weight for polymer solutes dissolved in liquid crystalline solvents. Using ensemble absorption polarization spectroscopy together with single molecule fluorescence polarization measurements, we have determined the order parameter of the conjugated polymer MEH-PPV (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]) in the liquid crystal 5CB (4-cyano-4-n-pentylbiphenyl) as a function of polymer chain length. Ensemble absorption polarization measurements agree well with results obtained by single molecule fluorescence polarization spectroscopy, indicating a large-scale ordering of the MEH-PPV solute in 5CB. These results demonstrate that the increasing number of defects for larger polymer weights inherently limits the alignment of the polymer solute.