A mechanistic picture of the effects of rubbing on polyimide surfaces and liquid crystal pretilt angles

Abstract

A mechanistic picture of the effects of rubbing on polyimide alignment layer (AL) surfaces and nematic liquid crystal (LC) pretilt angles (ΘP), leading to establishing a mechanism of determination of ΘP’s, was proposed on the basis of the study of the change in surface polarity of a polyimide AL with the rubbing force and density, and the subsequent changes in the ΘP and the azimuthal anchoring energy. In the regime of relatively weak rubbing where inhomogeneous, patchy surface modification (the microscopic reorientation of polymer functional groups and the orientation of polymer backbones) occurs, the surface polarity, the anchoring energy, and the ΘP all increase monotonically with rubbing strength. These increases correlate to an increase in the area fraction of the reoriented/oriented AL surface which generates in-plane orientation of the first LC monolayer at the surface and subsequently bulk alignment. Beyond this region in which the AL surface gets modified sufficiently to induce fully developed in-plane orientation of the LC monolayer, the surface chemistry and roughness have dominant effects on the ΘP and the anchoring energy. The increase in surface polarity and the generation of a larger reoriented surface area by high-force rubbing, which enhances the attractive interaction between the LC monolayer and the rubbed surface, contribute to increases in both the anchoring energy and the thermal stability of ΘP and to a decrease of ΘP. This decrease of ΘP with increasing AL surface polarity was verified by studying the relationship between ΘP and surface polarity under constant rubbing conditions by manipulating the surface polarity via the degree of imidization of a poly(amic acid) AL.

Thermal optical non-linearity of nematic mesophase enhanced by gold nanoparticles – an experimental and numerical investigation

“…The errors in determining these variables through fitting are calculated by estimating the deviation when the cell thickness was varied by 10% with respect to the cell gap created by the spacers (50μm). The fitted parameters are in good agreement with literature values for pretilt 28 (typically between 1° and 2.1° for various LC systems) and the value of splay elastic constant for E7 (11.7 pN) as specified in Merck data sheet. The fitted values for the pretilt and the splay elastic constant are used in the simulations to determine the spatial distribution of alignment of the LC in the cells.…”

Small Surface Pretilt Strikingly Affects the Director Profile during Poiseuille Flow of a Nematic Liquid Crystal

“…Here, we present a pressure-driven or Poiseuille flow study, examining the properties of liquid crystal cells filled with 5CB (4-Cyano-4 0 -pentylbiphenyl) having high (almost normal to the flow) initial azimuthal alignment angle ( 0 ) and small surface pretilt ( 0 ) (from buffed polyimide [11]) in both the splayed and parallel aligned states (see Fig. 1).…”

Controlling the Alignment of Polyimide for Liquid Crystal Devices

“…10 for POSS-doped PI and UV-treated PI alignment layers, respectively. The influence of surface energy of PI alignment layers on pretilt angles had been investigated (Paek et al, 1998;Yu et al, 1999;Ban & Kim, 1999). It showed that an alignment layer with higher surface energy gives the lower pretilt angle due to the increased attractive strength between LC molecules and molecules of the alignment layers.…”

“…The pretilt angle is very important and required for LC devices to obtain a defect-free alignment and also to improve their electro-optical performance, such as driving voltage, response time, color performance and viewing angle. However, the applications of conventional PIs in LCDs are limited by a small tuning range of pretilt angle (~ few degrees) either by controlling the rubbing depth or the number of rubbings (Paek et al, 1998). The pretilt angle of an LCD is either near zero degrees or 90 degrees for using the conventional homogeneous and homeotropic PI materials, respectively.…”