Polyimide-free vertical alignment in a binary mixture consisting of nematic liquid crystal and reactive mesogen: Pretilt modulation via two-step polymerization

The alignment and order control in an anisotropic system is of interest in broad fields of research and engineering. Although the alignment control technique has been developed for decades in commercialization of liquid crystal (LC) displays, achieving polyimide-free high homogeneity in LC alignment and precise modulation of pretilt remain challenging. Here, we demonstrate a LC-reactive mesogen (RM) binary mixture that can control vertical alignment with neither alignment-inducing additives nor polyimides coated on the surface. A synthesized RM itself plays both roles of a vertical alignment inducer and pretilt fixer. To achieve it, we conduct two-step polymerization. In step 1, UV exposure energy is controlled minimal enough to polymerize RMs partially as LC is well aligned vertically. In step 2, a voltage is applied from patterned electrodes to produce pretilt and then second UV is exposed to fix the pretilt by polymerization of the residual RMs. The result contributes to aiding the efficiency of the material synthesis, fabrication process and enhance electro-optic properties like quicker response time, low-threshold, operating voltage, and on-state transmittance. We believe this approach will open a new way to achieve the pretilt in anisotropic medium from not only engineering but also scientific perspectives.