Ultraviolet embossed alignment layers having patterned spacers for flexible liquid crystal display
Introduction
In recent years, liquid crystal (LC)-based flexible displays have attracted great attention since they offer the potential to develop display devices that are thinner, lighter, robust, and conformable compared to the conventional glass-based displays [1], [2], [3], [4], [5]. Flexible displays are basically composed of flexible substrates, barrier layers, conducting layers, and functional thin film materials [6]. Among those components, flexible substrates are the fundamental starting element for the flexible displays. To enable flexible displays, glass substrates should be replaced by plastic or other bendable substrates. For the replacement of glass substrates, plastic substrates are required to provide the comparable properties of dimensional stability, thermal stability, and solvent resistance to glasses [7], [8], [9], [10]. However, the plastic substrates might not be satisfactory for the dimensional and thermal stability to maintain the molecular alignment of liquid crystal. To overcome these problems, we have previously reported the fabrication of ultraviolet (UV) embossed alignment layer without requiring thermal treatment for the flexible liquid crystal display [11]. The UV embossed method with the phase-separated polymer walls can basically provide mechanical stability against bending deformation and also prevent the problems that are encountered with the thermal treatment of the rubbing process on the plastic substrates. However, in the practical use of photopolymerization-assisted manufacturing process to form polymer walls in the flexible LC devices, the chemical stability of the LC phase is as much important as mechanical properties of the alignment layers to keep the quality of devices. In spite of the mechanical stability with the phase-separated polymer walls, there might be some possibilities to have defects due to the remaining residual monomers in the vessel even after UV exposure during the preparation of phase-separated polymer walls, which causes a deteriorative effect on the electro-optic properties of liquid crystal.
In this work, we demonstrated the fabrication of UV embossed alignment layers having patterned spacers which are made by the replication of the structures having both microgrooves and spacers by using durable elastomeric polydimethylsiloxane (PDMS). This fabrication eliminated the problems associated with residual monomers in photopolymerization-assisted manufacturing of LC devices. We investigated alignment properties of LC molecules in UV embossed alignment layers having the polymer walls and the patterned spacers which are column- and line-shaped spacers, and the effect of the anisotropy of patterned spacers on alignment properties of LC in a bent environment.
Section snippets
Experimental
Alignment layers having the patterned spacers are fabricated by UV embossing processes using durable elastomeric PDMS. Fig. 1 shows the schematic illustration of procedures for fabricating UV embossed alignment layers having the patterned spacers. For the fabrication of master molders which have both microgrooves and spacers, 1-μm thick thermal silicon oxide layer is grown on a silicon wafer. The silicon oxide layer is patterned by a reactive ion etching process to make small grooves. Then, the
Results and discussion
The high purity of LC in the cell is important in the viewpoint of chemical stability of the LC phase since LC is easily influenced by the contamination. The formation of the polymer walls was achieved by irradiating selective areas with UV exposure through photomask, and the monomers in the mixture of LC and monomer in the vessel diffuse into the wall by concentration gradient. However, it might be possible that there are residual monomers after completion of photopolymerization in the vessel.
Conclusion
In summary, we demonstrated a method of designing flexible LCDs with UV embossed alignment layers having patterned spacers. The UV embossed alignment layers with patterned spacers could effectively prevent the problems caused by residual monomers in photopolymerization-assisted manufacturing of LC devices and also obtain mechanical stability against external forces. UV embossed LC cells with patterned spacers, especially in the column-shaped spacer which has the isotropic configuration of
Acknowledgements
This work was supported by the 2nd Brain Korea 21 project and LG Chem. Ltd.
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