Nematic Solvation of Segmented Polymer Chains S. Link, ² D. Hu, ‡ W.-S. Chang, ² G. D. Scholes, § and P. F. Barbara* ,² Center for Nano- and Molecular Science and Technology, UniVersity of Texas at Austin, Austin, Texas 78712, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, and Lash-Miller Chemical Laboratories, 80 St. George Street, UniVersity of Toronto, Toronto, Ontario M5S 3H6, Canada Received June 12, 2005; Revised Manuscript Received July 19, 2005 ABSTRACT We examine the effect of polymer chain segmentation on the recently discovered ability of nematic solvents to elongate and align polymer chain solutes. Coordinated single molecule spectroscopy and beads-on-a-chain simulations are used to study the orientational and conformational order of a series of segmented conjugated polymers, dissolved in the nematic liquid crystal 5CB. The order parameters for alignment and elongation are both observed to decrease with increasing segmentation, reflecting an interplay among conformational entropy, solvation anisotropy, and bending energy of the chain. Onsager 1 first predicted that in a binary “solution” of long and short rods, the long rods experience an enhanced alignment in the nematic environment of the shorter rods. 2 The first compelling experimental evidence for this type of “anisotropic solvation” has only been reported recently for single polymer molecules. 3,4 Large single semiflexible biopoly- mers in the nematic phase of rodlike fd virus were visualized directly by fluorescence imaging 3 and found to undergo a coil-rod transition at the isotropic-nematic phase transition. Independently, polarization sensitive single molecule spec- troscopy (SMS) revealed that conjugated polymer (MEH- PPV) chains dissolved in a single liquid crystal of 5CB are nearly perfectly aligned with the nematic director of the liquid crystal (LC). 4 Here using SMS and beads-on-a-chain simulations, we explore the effect of polymer chain seg- mentation on the alignment and elongation in a nematic environment for a series of conjugated MEH-PPV polymers for which synthetic introduction of single bonds at various double bond locations create a controllable number of rigid polymer segments separated by single bonds. For chains with only a few segments, anisotropic solvation due to the nematic solvent is observed to highly elongate the chains. As the number of polymer segments is increased, the chains become less elongated due to an interplay among conformational entropy, anisotropic solvation, and the bending energy of the polymer. In addition, highly segmented chains in nematic solvents are observed by simulation to possess low-energy “hairpin turn” defects that can dramatically decrease the extension ratio without a significant energy penalty from solvation or bending. Polymer chains of three different MEH-PPV compounds, denoted by MX (i.e., M98, M70, and M45, where X is 100 minus the percentage of tetrahedral defects 5 ) were investi- gated. The chains were isolated at high dilution in a single domain nematic 5CB liquid crystal 4,6 (2.5 cm × 2.5 cm × 50 µm) at 22 °C. Data were acquired in a home-built confocal microscope in either “emission mode” (i.e., two orthogonally polarized detector systems each with an APD detector), or “excitation mode” (i.e., one unpolarized detection channel but two orthogonally polarized excitation beams that were synchronously chopped at 10 kHz). Individual polymer molecules diffusing through the excita- tion volume gave rise to fluorescence bursts. For emission mode the polarization ratio was determined for each burst as P ) (I y,em - I x,em )/(I y,em + I x,em ), where I x,em and I y,em are the x and y polarized florescence intensities. For excitation mode P ) (I y,exc - I x,exc )/(I y,exc + I x,exc ), where I x,exc and I y,exc are the fluorescence intensities of the single APD detector, with x and y polarized excitation. Histograms of P were generated from several thousand bursts and are shown in Figure 1 for M98, M70, and M45. Each panel shows a histogram for three sample orientations where the LC director is either aligned along the x-axis giving rise to negative P values, aligned along the y-axis giving rise to positive P values, or aligned at an angle of 45° to the x-axis leading to P values centered around zero. Single conjugated polymer chains are well described as a multichromophoric system in which a chromophore consists of 10-15 repeat units. 7 The P values are a measure of the ² University of Texas at Austin. ‡ Pacific Northwest National Laboratory. § University of Toronto. NANO LETTERS 2005 Vol. 5, No. 9 1757-1760 10.1021/nl051108l CCC: $30.25 © 2005 American Chemical Society Published on Web 07/27/2005