Simulation of the Conformation of Single Conjugated Polymer Molecules with Varying Conjugation Length in a Nematic Liquid Crystal
EMSL Project ID
9993
Abstract
The optical and electronic properties of conjugated polymers have been studied intensively due to their potential application in molecular optoelectronic devices such as organic light emitting diodes [1]. A prototypical conjugated polymer is MEH-PPV, poly[2-methoxy-5((2-ethylhexyl)oxy)-1,4-phenylenevinylene] The interplay between polymer conformation in different environments and the optical properties of MEH-PPV is of great importance as it often determines the function of device. This intimate structure–function relationship is not only important from a technological point of view, but also raises important fundamental questions in the field of polymer physics where the dominant polymer conformations have long been those of a random coil and a molten globule. Single molecule polarization spectroscopy is ideally suited to explore the effect of polymer conformation and solute alignment with the ability to separate these two factors. Measuring the distributions of excitation and emission anisotropies for single polymers allows one to obtain information on their intrinsic molecular conformation and the external alignment in a host matrix. Single molecule studies together with Monte Carlo simulations have previously shown that for MEH-PPV embedded in an inert polymer matrix a dominant conformation of the polymer is that of a defect-cylinder [2]. In a defect-cylinder conformation the stiff segments of the polymer are folding in such a way as to produce a parallel stacking of the chains with kinks located at tetrahedral defect sites. Synthetic introduction of additional defect sites causes a dramatic change in the conformation of the polymer chain and related optical properties [3]. At the same time the environment is believed to play an important role in the polymer conformation as well because of the energetic interplay between chain-chain and chain-solvent interactions. The alignment of MEH-PPV in the nematic liquid crystal phase of 5CB (4-n-pentyl-4’-cyanobiphenyl) has recently been studied by single molecule spectroscopy [4], and new experiment with polymer samples with varying degree of conjugation reveal that the order parameter for the MEH-PPV decreases from 0.95 to 0.56 when roughly half the C=C double bonds on the polymer backbone are replaced by C-C single bonds. This can be attributed to a decrease of the alignment of the polymer molecules in the liquid crystal host and a decrease of the internal conformational order of the polymer chains. Compared to studies performed on MEH-PPV molecules with a varying extent of conjugation, but embedded in an inert polymer matrix, the measured polarization anisotropy of the excitation and the emission are surprisingly higher than expected and clearly demonstrate the influence of anisotropic salvation on the conformation of the polymers. The scope of the proposal is to explore this effect of anisotropic salvation on the conformation of the MEH-PPV by simulating the polymer conformations which are consistent with the measured polarization anisotropies. The simulation will be carried out by the Monte Carlo method of beads on a chain (bond fluctuation method) to generate an ensemble of conformations for model polymer chains with different number of chain defects, intersegment attraction, chain stiffness, and anisotropic salvation.[1] Burroughes, J.H., et al., Light-emitting Diodes Based On Conjugated Polymers. Nature, 1990. 347: p. 539-541.
[2] Hu, D.H., et al., Collapse of stiff conjugated polymers with chemical defects into ordered, cylindrical conformations. Nature, 2000. 405(6790): p. 1030-1033.
[3] Hu, D.H., et al., Spatial confinement of exciton transfer and the role of conformational order in organic nanoparticles. Nano Lett., 2002. 2(10): p. 1121-1124.
[4] Lammi, R.K., et al., Ordering of Single Conjugated Polymers in a Nematic Liquid Crystal Host. J. Phys. Chem. B, 2004. 108: p. 4593.
Project Details
Project type
Capability Research
Start Date
2004-07-14
End Date
2006-11-08
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
S Link, D Hu, W-S Chang, GD Scholes, PF Barbara: Nematic Solvation of Segmented Polymer Chains. Nano Letters 5 (2005) 1757-60.