Anchoring energy enhancement and pretilt angle control of liquid crystal alignment on polymerized surfaces

Abstract

We demonstrate enhanced surface anchoring energy and control of pretilt angle in a nematic liquid crystal cell with vertical alignment and polymerized surfaces (PS-VA). The polymerized surfaces are formed by ultraviolet (UV) irradiation-induced phase separation of a minute amount of a reactive monomer in the vertical-aligned nematic liquid crystal. By introducing a bias voltage during UV curing, surface-localized polymer protrusions with a dimension of 100nm and a field-induced pretilt angle are observed. Experimental evidences and theoretical analyses validate that PS-VA has increased surface anchoring strength by two folds and pretilt angle has been changed from 89° to 86° compared to those of a VA cell. The enabling PS-VA cell technique with excel electro-optical properties such as very good dark state, high optical contrast, and fast rise and decay times may lead to development of a wide range of applications.

Simulation and fabrication of a fast fringe‐field switching liquid crystal with enhanced surface anchoring enabled by controlled polymer topology

“…In this case, the polymer may be formed mainly on the substrate surface and has a relatively small and short polymer size, so the refractive index of polymer and LC molecules are matched, and no scattering interfaces are induced. 12 This proposition will be confirmed by a morphological study.…”

“…In contrast, the dark and bright state images of 1 wt.% polymer cell are very similar as those of 0 wt.% polymer cell; that is, almost no light leakage or scattering is observed. In this case, the polymer may be formed mainly on the substrate surface and has a relatively small and short polymer size, so the refractive index of polymer and LC molecules are matched, and no scattering interfaces are induced . This proposition will be confirmed by a morphological study.…”

“…The type of reactive monomer material and its concentration have a large impact in the electro-optical properties of a polymer-stabilized cell, because it affects the polymer's morphological properties and device performance. 9,12 Thus, selecting an appropriate RM and its concentration can effectively improve the electro-optical performance of a device, such as preventing dark-state light leakage, decreasing the driving voltage and shortening the response time. The RM with a short length of a molecule improves decay time and gives a good dark state for vertical alignment (VA) mode, while RM with a medium length improves both rise and decay time in VA mode.…”

“…It is also beneficial to have a surface‐localized polymer that provides a memory effect to the LC molecules and enables faster LC molecule reorientation in the field‐off state . A non‐uniform distribution of polymer network in the bulk of an LC layer and surface‐localized polymer also has advantages of keeping a good dark‐state quality and very low light scattering by using a small amount of monomer . With this approach, FFS mode LC devices will have improved dynamic response times and sharp moving images with a minimum color shift at gray‐to‐gray switching.…”

“…8,9 A non-uniform distribution of polymer network in the bulk of an LC layer and surface-localized polymer also has advantages of keeping a good dark-state quality and very low light scattering by using a small amount of monomer. [10][11][12] With this approach, FFS mode LC devices will have improved dynamic response times and sharp moving images with a minimum color shift at gray-to-gray switching.…”