Construction of Polymer-Stabilized Automatic MultiDomain Vertical Molecular Alignment Layers with Pretilt Angles by Photopolymerizing Dendritic Monomers under Electric Fields

Abstract

The synthesized itaconic acid-based dendritic amphiphile (Ita3C₁₂) monomers and the methacryl polyhedral oligomeric silsesquioxane (MAPOSS) cross-linkers were directly introduced for the construction of automatic vertical alignment (auto-VA) layers in the host nematic liquid crystal (NLC) medium. The auto-VA layer can be stabilized by irradiating UV light. For the automatic fabrication of a polymer-stabilized multidomain VA (PS auto-MDVA) layer with a pretilt angle, Ita3C₁₂ and MAPOSS were photopolymerized under the electric field by irradiating UV light on the multidomain electrode cell. Mainly because of the pretilted NLC at zero voltage, the electro-optic properties of the PS auto-MDVA cell were dramatically improved. From the morphological observations combined with surface chemical analyses, it was found that various sizes of protrusions on the solid substrates were automatically constructed by the two-step mechanisms. We demonstrated the PS auto-MDVA cell with the enhancement of electro-optic properties as a single-step process and investigated how the protrusions were automatically developed during the polymer stabilization.

Figure 1

XPS spectra for the surface of the PS auto-VA cell: (a) carbon (C 1s) and (b) oxide (O 1s) spectra of chemical binding energy at the protrusions on the substrates obtained after washing the unpolymerized host LC medium.

Figure 2

Formation of protrusions and nanosized beads during polymer stabilization.

Figure 3

Schematic illustrations for the fabrication of the PS auto-MDVA cell. (a) Before polymer stabilization without an electric field, Ita3C₁₂ and MAPOSS are migrated on the electrode substrates and induce LC vertical alignment. The self-assembled Ita3C₁₂ dimers are still dissolved in the host LC medium. (b) Before polymer stabilization at the voltage of 5.6 V, directors of Ita3C₁₂ dimers and the host LC medium are reoriented along the applied electric field. (c) Polymer stabilization of Ita3C₁₂ and MAPOSS at the voltage of T₉₀ by irradiating UV light: the self-assembled Ita3C₁₂ dimers and MAPOSS also reacted with each other, and the polymerized Ita3C₁₂/MAPOSS beads in the host LC medium are migrated into the electrode substrates. (d) After polymer stabilization and without electric field: the host LC molecules memorize the pretilt angles (about 2°) from vertical alignment even after removing the electric field.

Figure 4

Topographic SEM images of the protrusions that were polymerized under electric field: on (a) and between (b) the electrodes. (c) Schematic diagram of the protrusions formed on the multidomain electrode substrate and its corresponding SEM image of the protrusions.

Figure 5

Topographic AFM images of the (a) multidomain electrode substrate, (c) surface of the electrode areas, and (e) surface of nonelectrode areas and (b, d, f) their corresponding height profiles, respectively.

Figure 6

(a) Voltage-dependent transmittance curves before and after polymer stabilization at 5.6 V, and (b) rising time vs grayscale for the PS auto-MDVA LC cell before and after polymer stabilization. The time-resolved LC textures at 5.6 V (c) before and (d) after polymer stabilization, where the white scale bars correspond to 100 μm.