Aligning liquid crystal molecules

The compact size, ease of use and low power consumption of liquid crystal (LC) devices have made them popular for use in photonics products, such as displays for cellphones, televisions, and tablet computers. A typical LC device comprises a thin LC layer a few micrometers thick sandwiched between a pair of indium tin oxide (ITO) conducting glass substrates (see Figure 1). ‘Alignment layers’ are used orientate the LC molecules with a specific pretilt angle, which is the angle between the director of the LC molecules and the alignment layers. The pretilt angle is very important for LC devices to obtain a defect-free alignment and also to improve their performance, such as response time and viewing angle. In the LC display (LCD) industry, it is common to use polyimide (PI) alignment layers to align LC molecules nearly parallel and perpendicular to the substrates using homogeneous PI and homeotropic PI, respectively. The alignment mechanism depends on the morphology of PI alignment layers and intermolecular interactions between LC molecules and PI molecules.

The required pretilt angle of LCDs depends on the operation mode: near-zero degrees for in-plane switching (iPhone and iPad displays), several degrees for twisted nematic mode (laptop displays), >5° for the supertwisted nematic mode, 45–60° for no-bias optically-compensated bend LCD (fast response applications) and the bi-stable bend-splay LCD (e-books applications), and near 90° for vertical alignment mode (LCD-TV). These operation modes provide different electro-optical properties, such as viewing angle, response time, and memory, for the different product features. Fabricating a variety of devices would be easier if we could control the pretilt angle on demand. There are many existing methods to control the pretilt angle of LCs over a wide range. However, these existing techniques are not easily mass-produced and the materials required are difficult to synthesize.

Recently, we have developed a new approach to align LCs vertically by adding polyhedral oligomeric silsesquioxane (POSS) nanoparticles to LCDs (see Figure 2).^1,2 POSS nanoparticles with nanosized cage structures have been incorporated into polymers for improving their thermal, mechanical, and oxidation resistance. We have demonstrated a novel method for continuously controlling the pretilt angle of LC molecules using conventional homogeneous PI alignment material doped with different concentrations of POSS nanoparticles.³ Adding POSS to the homogeneous PI lowers the surface energy of the alignment layer and generates a variable pretilt angle between 0° and 90°. This method has the advantage of using conventional PI alignment materials, manufacturing processes, and facilities, and so can readily be adopted by industry.

Figure 1. The typical structure of a liquid crystal (LC) device with a polyimide (PI) alignment layer.

Figure 2. The structure of 1,2-propanediol-isobutyl polyhedral oligomeric silsesquioxane (POSS).

To obtain good dispersion of POSS in PI, we used a powerful ultrasonic processor (S4000, Misonix). We filtered the mixture of 0.2% by weight POSS through a 200nm syringe filter. We then diluted the mixture with PI to generate concentrations of POSS in PI varying between 0.01 and 0.07% by weight. We spin-coated the POSS/PI mixture onto the ITO glass substrates to obtain a thin alignment film. The alignment film was first preannealed at 80°C for 10 minutes followed by hard baking at 180°C for 4 hours to allow imidization. We rubbed the surface of the alignment layer once in each direction with a nylon cloth.

We determined the surface energy of the alignment film by measuring the contact angle of distilled water on the film according to the Girifalco-Good-Fowkes-Young model.⁴ To determine the pretilt angle of LC molecules on PI alignment layers, we fabricated antiparallel LC test cells with a cell gap of 6.7μm capillary filled with positive dielectric anisotropic LC molecules (E7). We measured pretilt angles of the LC cells by the modified crystal rotation method.⁵

Surface energy measurements of the PI/POSS alignment layer with different percentages by weight of POSS doped in PI indicate that the addition of the POSS nanoparticles in the homogeneous PI mediates and lowers the surface energy of the alignment layer (see Figure 3). We found the pretilt angle of the rubbed PI film continuously increases with decreasing surface energy for POSS/PI alignment layers (see Figure 3). The decreased attraction between LC molecules and alignment surface molecules results in a higher pretilt angle.

Figure 3. Pretilt angle and surface energy of POSS/PI alignment layers as a function of POSS concentration in PI.

In summary, we have proposed a new method to control the pretilt angle of LCs over a wide range, which has the advantage of fabricating LC devices that require specific pretilt angles. We have shown that our method, of adding POSS nanoparticles to conventional PI alignment material, is effective and also compatible with current LCD industry production methods. We now plan to examine the long-term stability of these modified PI films.

The authors would like to thank the National Science Council of Taiwan for financially supporting this research under contracts: NSC 98-2112-M-009-020-MY2, NSC 98-2221-E-239-003-MY2, and NSC 99RC04. Many thanks to Dr Joanna Carpenter of Form and Content Media for her excellent editorial assistance.