Vertically aligned liquid crystal mode on UV-cured reactive mesogen using imprinting method

1. Introduction

Microcontact printing of polymer-based macromolecules has typically resulted in poorly defined or randomized molecular structures [1]. Here, we demonstrate a novel imprinting process of well-defined, defect-free polymeric layers with an ultra violet (UV) curing technique [2–4] based on polymerizable liquid crystals (LCs) [5–9]. Commercially, the mechanical rubbing method [10,11] on polyimide (PI) is used to provide microgrooves, which result in unidirectional alignment of LCs. However, this rubbing process has some drawbacks like the creation of static electricity due to the friction between the alignment layer and rubbing roller and the introduction of dust onto the layer [12]. To overcome these disadvantages, noncontact methods have been developed, including ion-beam [13–15], photo-alignment [16–18], self-assembled monolayers [19], and SiO oblique evaporation methods [20]. In particular, the imprinting method for the desired alignment states has been widely studied in other research groups [21–24]. These results discussed alignment layers made of polydimethylsiloxane (PDMS) or using a lithography mold. More recently, a variety of processes for making polymeric thin films have been developed for flat panel displays, especially for liquid crystal displays (LCDs) and light emitting diodes (LEDs). However, these processes usually result in polymeric structures with a poor or random orientation of the molecular chains.

H.-G. Park and associates employed nanoimprint lithography to produce a thin alignment layer of LCs [21]. The resulting polymeric structure demonstrated a well-defined nanopatterned surface topography. In addition, the LC molecules were homeotropically well-aligned to the patterned surface. This novel lithographic method was shown to be useful as alignment layers provide hysteresis-free and thermally stable LCDs.

In the present article, we demonstrate a novel soft imprinting technique for the stamping of polymeric layers based on reactive mesogens (RMs) [25–29] that are defect free and show superior electro-optic (EO) characteristics including low threshold voltage (Vth) and fast response times. The method is based on UV curing of RM [30,31] with liquid crystalline monomers, which can create rigid networks. We demonstrate that, by using this UV cured imprinting technique, we can locally apply monolithic structures stamped on the polymer layer with enhanced EO properties, in contrast to nanoimprinting, which requires additional laser controlling equipment and rubbing, creating electrostatic charges. Using our novel imprinting process, a 38.3% decrease in the V_th and a 26.1% reduction in the response time were observed for vertically aligned (VA)-mode LCDs as compared to the rubbing method.

2. Experimental

RM (RMS03-015, Merck) monomers were coated onto a flexible film with a thickness of about 1 µm using a spin coater at 3000 rpm for 30 sec. To induce anisotropy for LC alignment, these films were cured using a linearly polarized ultraviolet (UV) exposure system (Oriel Co.) with a 1-kW mercury lamp source connected to the lamp power supply. In order to obtain uniform UV energy density, the system employed UV exposure control to maintain 80% of the original UV energy. Also, the system employed a convex lens to achieve strong UV energy density. The films were exposed to UV at wavelength of 310-330 nm, and the UV energy density was 7.9 mW/cm². The incident angle of UV exposure was 90°, and UV exposure time was 3 min. For comparison, the rubbing treatment, which is conventional process to induce anisotropy for the LC alignment, was applied to the original UV cured RM films. Using a spin coater, a 50-nm-thick blended polyimides (PIs, JSR Co. Ltd) was coated onto the indium tin oxide (ITO) deposited glass substrates. The blended PIs used for the homogeneous and homeotropic alignment layers were prepared at various concentration. PI films were pre-baked on a hot plate at 80°C for 10 min and then hard-baked in the oven at 230°C for 1 h. After baking, PI films onto the ITO glass were pressed using a UV cured film for 10 min and were then peeled away from the UV cured film. From this imprinting process, an RM monolayer was printed on the PI-coated glass from an RM multilayer on the flexible film. To measure EO properties, we fabricated vertically aligned (VA)-mode LC cells. Atomic force microscopy (AFM) (multimode, VEECO) images were obtained to provide surface morphology on the alignment layer. To confirm of the existence of a printed monolayer of RM and their anisotropic nature, we conducted retardation (REMS-100, Sesim) measurements.

3. Results and discussion

Figure 1 shows the schematic diagram of the fabrication process discussed above. The imprinted substrates were fabricated in VA cells with cell gaps of 5 μm to allow measurement of the EO characteristics. The LC materials were injected into the cell by capillary injection at room temperature.

 

Fig. 1 Schematic representation of fabrication via the imprinting method. An RM monolayer detached and implementation of these imprinted PI layers in the VA mode cell.

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Figure 2 shows the AFM image of homeotropic PI, and imprinted RM films which are polarized UV exposed and rubbed. In the case of pristine homeotropic PI, almost no morphology was observed. However, the imprinted RM surface by polarized UV exposure showed better morphology images on the glass substrate since it came from RM molecules. When the rubbing method was used, groove morphology was observed on the substrate. These morphologies reflect alignment of the LC molecules on the glass substrates.

 

Fig. 2 AFM images of homeotropic PI, the imprinted cell, and the rubbed cell.

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Anisotropy is a key factor of the fabrication of display devices or materials. Measurement of infrared (IR) absorption cannot be used to evaluate the anisotropy of the alignment layer of actual display devices because their substrates are made of glass and are not transparent to IR radiation. However, optical retardation measurements [32] using ellipsometry for visible light can be used to evaluate the anisotropy of a thin membrane, and can be used to evaluate the alignment of the molecules in the alignment material after imprinting. To develop better RM anisotropic characteristics, we used polarized UV light. Figure 3 shows the results of optical retardation. To compare the anisotropic characteristics, we also measured a substrate where rubbing was performed on the blended PI. Non-treated PI measured almost zero retardation, which suggests few anisotropic characteristics. In contrast, retardation increased after the imprinting process. This result indicates that the anisotropic characteristics of the RM stamp were successfully transferred onto the blended PI layers and these were affected by base PI layers. The anisotropic characteristics of the imprinted RM layer by rubbing treatment was greater than those by polarized UV exposure because the groove morphology enhanced the anisotropic characteristics. In addition, the surface of imprinted RM layer by rubbing treatment was uneven due to groove morphology. This result affected the pretilt angle of LC molecules on the imprinted RM layer.

 

Fig. 3 A Rotation angle dependence of the optical retardation of rubbing, imprinting, and homeotropic PI surfaces: a) 60% homeotropic PI, b) 70% homeotropic PI, c) 80% homeotropic PI, d) 90% homeotropic PI.

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It is important to achieve a regular pretilt angle of LC molecules on alignment layers because the pretilt angle affects not only uniform LC alignment but also EO characteristics of LC cells. Thus, the pretilt angles of LC molecules on imprinted RM layers were measured. Figure 4 shows the calculated pretilt angles of LC molecules on imprinted RM layers with polarized UV exposure and rubbing method as a function of the homeotropic PI ratio. As can be seen, high and constant pretilt angles of approximately 90° were obtained in polarized UV exposed sample cells, while gradually increased pretilt angles from 58.65° to 83.84° were obtained in rubbed sample cells as a function of homeotropic PI ratio. The increase of pretilt angles in rubbed sample cells was attributed to the microgroove morphology, which uneven surface made base PIs layer more influenceable to LC alignment.

 

Fig. 4 Pretilt angles of LC molecules on imprinted RM layers with polarized UV exposure and rubbing treatment as a function of hometropic PI ratio.

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We fabricated VA mode cells with imprinted RM films on the blended PIs layers to examine the EO characteristics. The time-transmittance plots for both imprinted and rubbed VA mode cells show the rise and fall response time in Fig. 5. The imprinted VA mode cell had rapid rise and fall times of 8.7 and 11.6 ms at an 80% homeotropic PI ratio, respectively. The rise times and the fall times of VA mode cells were range from 8.7 to 11.4 ms and from 10.5 to 15.7 ms, respectively. In contrast, the rubbed cell exhibited slower response times. Anchoring energies of imprinted and rubbed VA mode cells were almost the same and these values were 1.0 × 10⁻³ J/m². Therefore, the anchoring energy were not affected the change of EO characteristics. In addition, since the rubbed cells had a large pretilt angle, they did not work as well as the VA mode cells. This result indicates that an imprinted RM layer reoriented the LCs in an azimuthal direction when voltage was supplied, and thus molecular collision with trapped defect points was minimized, resulting in improved response time [30].

 

Fig. 5 Response time of imprinted and rubbed VA cells: a) rise time of imprinted VA cell, b) decay time of imprinted VA cell, c) rise time of rubbed VA cell, d) decay time of rubbed VA cell.

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EO characteristics, including a decreased V_th and fast response time, were achieved in VA mode LC cells, as shown in Fig. 6.The voltage-transmittance (V-T) curve confirms that the homeotropically aligned LC could completely switch ON and OFF under supplied voltage above a certain threshold voltage (V>V_th). The lowest V_th of imprinted VA mode cells is 2.306 V at an 60% homeotropic PI ratio. In VA mode, since the percentage of homeotropic PI was increased, the V_th decreased. Since the RM molecules are aligned in the same direction as that of PI, the LC molecular motion improved. In contrast, the rubbed VA mode cells did not work normally because the pretilt angle on imprinted RM surface treated by rubbing was low to operate in the VA mode. Consequently, imprinted VA cells can operate at a low driving voltage at imprinted VA mode cells, which means lower energy consumption.

 

Fig. 6 Voltage-transmittance properties of VA cells: a) imprinted VA cell, b) rubbed VA cell.

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4. Conclusion

In summary, the imprinted RM layer, which is cured by UV light, was successfully aligned to a homeotropic state using VA mode cell fabrication. During the UV light curing, RM medium breaks down to form free radicals, starting a rapid polymerization process resulting in the creation of a cross-linked network of carbon-carbon bonds. Finally, RM monomers are vertically aligned, which provides anisotropic optical properties. The optical anisotropy were demonstrated by retardation measurements. From the imprinting process, the RM monomers were stamped randomly or linearly into the defects on the surface of homeotropic PI. Trapping defects on the alignment layer can result in superior EO characteristics, i.e., reduced V_th and the fast response time of VA mode cells with supplied voltage. This proposed imprinting process provides the advantage of a novel alignment method with controllable switching speed and low energy consumption.