Control of Coloration and Colloidal Assembly Based on Molecular Ordering of Reactive Mesogens

Abstract

In recent years, reactive mesogens (RMs) have attracted much attention due to their intrinsic anisotropy as well as a capability of forming highly stable liquid crystalline polymer by a polymerization process. The extended functionality of RM such as photo-sensitivity and thermal-sensitivity can be provided in a simple way of modifying the molecular structure or combining various functional fragments. In addition, the RM films are easily formed onto various kinds of substrate by a solution process. The surface characteristics of the RM structures strongly depend on the molecular alignment of the RMs. Such dependency was hardly observed from the isotropic polymer structures. In this thesis, new types of electro-optical devices and the directional assembly of the colloidal particles using aligning capability of RMs were proposed. At first, the liquid crystal (LC) aligning capability of the RM was experimentally examined in the LC/RM composite system where the patterned RM structures were formed on the bottom substrate of LC cell. In the LC/isotropic polymer composite system, the alignment LC molecules was often distorted due to the monotonic directional preference on the surface of the isotropic polymer structure. On the other hand, the directional preference of LC on the RM structure is identical to the molecular orientation of RM so that the highly uniform alignment of LC molecules across the RM patterns can be achieved by molecular ordering of RM. Next, the surface-initialized polymer stabilization of RM was studied. In usual polymer stabilization process, the photo-initiator was doped directly in the RM/LC mixture. Therefore, the spontaneous polymerization process occurred in the entire region exposed to the external energy. In the approach proposed here, the photo-initiator was doped on the one side of the alignment layer to induce the surface-initialized photo-polymerization. Thus, both the pure LC layer and stabilized RM/LC layer can coexist in a stacking configuration. This concept is directly applicable for electro-optical devices requiring high electrical tunability. Moreover, the self-organized surface topography of the RM film was studied. The depth-wise gradient polymerization of swelled RM film generates the buckling instability to form the surface topography in range from nano to micro scale. It was found that the directional preference of the surface topography appeared when the RM is uniformly aligned before polymerization. Based on the results described above, a new concept of reflective-type liquid crystal display (RLCD) based on the polarization selective reflector (PSR) was proposed. Previously, the color filter was placed in front of the RLCD and the mirror was constructed on the bottom substrate. In such configuration, the ambient light passes the different color filters during incidence and reflection depending on the incident angle. In the proposed approach here, the PSR on the bottom substrate acts as a role of polarization-selective mirror as well as color filter so that the color loss from the filtration can be effectively reduced. The PSR was constructed through the spin-coating of chiral nematic RM mixture and photo-polymerization process with the photomask. The PSR selectively reflects the circular polarization component of incident light in the pre-defined bandwidth of the wavelength and the reflectance can be varied by the applied voltage to change the phase retardation of the upper LC layer. Next, a new way of reducing color distortion in liquid crystal display (LCD) based on anisotropic voltage dividing layer (AVDL) was demonstrated. The voltage dividing layer (VDL)-based method for reducing color distortion involved a planarization processes for the uniform alignment of LC molecules on the VDL. The proposed approach based on AVDL enables to spontaneously align the LC molecules without any additional surface treatment. The color distortion of the LCD in patterned vertical alignment mode was examined by both numerical simulations and the experimental measurements. Finally, the assembly of the colloidal microwires using self-organized surface topography was demonstrated. The surface topography of highly uniform wavy structure was formed along the direction of RM. Using the periodic wavy structures as the templates, the colloidal microwires in single line or multiple lines were assembled. The number of lines was determined depending on the height and the period of the wrinkle in combination with the particle size. In conclusion, the anisotropic aligning capability of RM was explored for the control of the coloration and the directional colloidal assembly. The new concept of device and both theoretical and experimental results provided here will lead to the development of the advanced electro-optical device and the colloid applications.