Chain-Length Dependent Nematic Ordering of Conjugated Polymers in a Liquid Crystal Solvent

Abstract

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.

Single Molecule Studies on Dynamics in Liquid Crystals

“…For small contour length S actin approaches the order parameter S fd of the smaller nematic fd viruses. These findings agree with the observation from conjugated polymers in thermotropic 5CB, namely that the orientational order parameter S o depends on the relative number of chain segments [60,64], constituting stiffer or more flexible chains. In the nematic phase of fd , F-actin, wormlike micelles, and neurofilaments, are highly elongated, having a rodlike shape in agreement with predictions from Onsager [65].…”

“…While the solute order parameter S o increased with increasing molecular weight, the conformational order parameter was found to be S c = 0.59 for MEH-PPV, independent of the chain length, see Figure 4. This further validates the previous finding on the dependence of the order parameter on the number of tetrahedral defects (number of chain segments) [60], because the percentage of tetrahedral defects (1–11%) was independent of M as deduced from absorption and NMR spectra [64]. These results demonstrate that the increasing number of defects for larger polymer weights inherently limits the polarization anisotropy of the polymer solute.…”

“…The discrepancy may be a result of several complications with the ensemble experiment, including (a) a greater sensitivity to lower-molecular-weight chains, which may be less well aligned; (b) the scrambling effect of interchain energy transfer, which is more prevalent for the higher-concentration samples used in ensemble measurements; and (c) imperfections in the single-LC structure, which are a more serious complication for the larger sample areas used in the ensemble measurements [20]. The higher sensitivity of the single molecule experiment to longer, straighter, and thus better aligned chains (a) was confirmed by a further study on MEH-PPV conformation in 5CB [64]. A comparison of the conformational order parameters S c obtained from single molecule polarization measurements of MEH-PPV in 5CB and in a PMMA (poly(methyl methacrylate) polymer matrix reveals that the polymer chains are stretched out in an anisotropic solvent (5CB: S c = 0.50 – 0.60, PMMA: S c = 0.40) in agreement with predictions from Onsager [65].…”

“…used the above described single molecule polarization method [59–62] to study the dependence of the solute order parameter S o on the solute molecular weight M ranging from monomeric Rhodamine 6 G to 432 repeat units ( M = 3900) [64]. While the solute order parameter S o increased with increasing molecular weight, the conformational order parameter was found to be S c = 0.59 for MEH-PPV, independent of the chain length, see Figure 4.…”

“…These results demonstrate that the increasing number of defects for larger polymer weights inherently limits the polarization anisotropy of the polymer solute. For applications in colored LC displays, the authors suggest to achieve further increase in the dichroic ratio by use of a stiffer macromolecular solute [64]. …”