Polyimide-Free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength in Situ Photoalignment

Abstract

High-quality alignment control of liquid crystals (LCs) for ultrahigh-definition large-sized display is a challenging task. A conventional rubbing method has obvious limitations for fabricating large-sized displays with a small pixel size and an uneven inner surface. To comply with the current trend, we propose a simple and reliable polyimide-less in situ photoalignment. It was achieved using a visible-light-sensitive azo-dye and a mesogenic acrylate, both doped to host LCs. Without using a pretreated alignment layer, mono- and multidomain uniaxial alignments of LC molecules were induced by linearly polarized visible light (LPVL) and subsequently stabilized by unpolarized UV-light irradiation. The stepwise process was monitored by adopting a fluorescent indicator. By loading the mixture into a confined cell, azo-dyes were spontaneously adsorbed at inner surfaces of the cell, whereas reactive mesogens (RMs) were homogeneously dissolved in an LC host. The molecular orientational anisotropy of dyes at the surface, induced by LPVL, aligned the LC director perpendicular to the polarization direction. Upon the second step, UV-irradiation, the RMs in an LC host were photopolymerized into thin interfacial layers, stabilizing the aligned LC director. The overlaid cross-linked RM layers secured a thermal and a radiative stability of LC alignment. The RM layers completely screened the effect of azo-dyes, which can be easily randomized by heat and irradiation. The interfacial RM layer functioned as a permanently stable alignment layer. It provided sufficient azimuthal anchoring strength together with heat and light stabilities, which are essential for practical applications. Such sequential interfacial modifications through dual-wavelength processes can completely avoid interference between forming alignment and stabilization layers, inevitable if the same wavelength light is used. The proposed method provides a simple fabrication process and reliable alignment characteristics by employing effective in situ photoalignment and without using a traditional alignment layer. Therefore, it meets a current trend in the display market toward ultrahigh-resolution and large-area displays.

Highly Stable, Pretilted Homeotropic Alignment of Liquid Crystals Enabled by In Situ Self-Assembled, Dual-Wavelength Photoalignment

“…If the anchoring strength is not strong enough, the relaxation process will become very slow. As reported 47 It is evident that interfacial modification of the ITO surfaces plays a key role for the homeotropic LC alignment. The wetting properties of the RM stabilization layer were therefore investigated using contact angle measurement to confirm the interfacial modification of surface anchoring capability.…”

“…The passive nature of polymer layers makes it resistant to the damaging factors which can perturb the LC alignment conditions. It has been reported that such systems with RM-stabilized LC alignment can maintain the anchoring strength under the applied AC-field stress test for 24 hr, as it is a crucial factor to determine industrial applications of LC devices 47 . If the anchoring strength is not strong enough, the relaxation process will become very slow.…”

“…Stabilization of LC alignment by RM polymerization results in the formation of a thin polymer layer on top of the composite layer of SAA and azo-dye molecules, as shown in Figure 1d. The RM stabilization layer seizes the freedom of azo-dyes reorientation capability influenced by reversible photoisomerization process 46,47 . Here onwards, the new RM layer commands the LC alignment and endures irreversible LC orientation along specific direction.…”

“…The RM polymerization is initiated by a photoinitiator used in trace amounts upon unpolarized UV-light (λ 2 ) irradiation at room temperature. − Stabilization of LC alignment by RM polymerization results in the formation of a thin polymer layer on top of the composite layer of SAA and azo-dye molecules, as shown in Figure d. The RM stabilization layer seizes the freedom of azo dyes’ reorientation capability influenced by the reversible photoisomerization process. , Here onward, the new RM layer commands the LC alignment and endures irreversible LC orientation along a specific direction. The formed RM layer over the composite layer of additives is represented in light brown color, as shown below Figure d.…”

A practical guide to versatile photoalignment of azobenzenes

“…Therefore we have also studied alternative azo dyes for their capabilities as photoalignment layers. Chrysophenine was selected due to the similarity of its chemical structure to that of BY and Congo Red is a suitable candidate [26] due to its absorption maximum close to 500 nm since, for compatibility with other processes, materials with an absorption at longer wavelengths are attractive. The molecular structures as well as the absorption spectra of Brilliant Yellow, Chrysophenine and Congo Red are shown in Figure 8; all three azo dyes, the liquid crystal monomers and the solvents were purchased from commercial suppliers.…”