Polymer Could Lead to Improved LCDs

David L. Shenkenberg

Researchers have shown recently that an external electric field can change the alignment of the polymer MEH-PPV in liquid crystals. This alignment can polarize the light emitted by the liquid crystals in a display, which increases the brightness and contrast, possibly without the need for a second polarizer. Because MEH-PPV, or poly[2-methoxy-5-(2'-ethyl-hexyloxy)
-1,4-phenylene vinylene], fluoresces in the visible range, it can be used for color displays.

Polarization histograms show that the electric field changed the polarity of the light emitted from the polymer liquid crystals. Reprinted with permission of the Journal of Physical Chemistry B.

The researchers, who were from the University of Texas at Austin and from the University of Wisconsin-Madison, and who included Stephan Link, now a professor at Rice University in Houston, used a model system based on an idealized mixture of short 5CB rods and long MEH-PPV rods.

They placed nematic liquid crystals between two coverslips coated with layers of a material that was rubbed, with a Mylar spacer in between. Physically rubbing the material aligns the liquid crystal molecules in one direction — the planar direction in this case — which allowed the researchers to show that the electric field could change that alignment.

They measured the polarization of the light emitted from the polymer solute in the liquid crystals with respect to the alignment direction with single-molecule precision by using a self-assembled setup consisting of a Carl Zeiss inverted microscope, a Melles Griot continuous-wave argon-ion laser and two PerkinElmer avalanche photodiodes. The researchers interrogated the polymer solute in the presence and absence of an external electric field with circularly polarized 488-nm light from the laser.

Light emitted from the polymer traveled through the 100×, 1.3-NA oil-immersion objective lens of the microscope to a Becker & Hickl photon counter and multiscaler, which measured the fluorescence intensity in terms of photon counts over time in two channels. The emission also passed through a beamsplitter that separated the polarized emission into X- and Y-components, which were detected by the two photodiodes. The entire system operated automatically.

The photon counts and polarization histograms indicated that the electric field caused the polymer liquid crystals to change from a planar to a perpendicular alignment. The researchers said they believe that the length difference between the rods of polymer solute and solvent in the liquid crystal causes the solute and solvent to align cooperatively, increasing the number of molecules aligned in a process that they term “cooperative anisotropic solvation.”

The researchers also demonstrated that the depth profile of the liquid crystal director for the applied electric field can be predicted by employing a conventionally used elastic model called Franck continuum theory.

Journal of Physical Chemistry B, ASAP edition, Nov. 2, 2007, doi: 10.1021/jp076345m.