Study of the pretilt angle for 5CB on rubbed polyimide films containing trifluoromethyl moiety and analysis of the surface atomic concentration of F/C(%) with an electron spectroscope for chemical analysis

Abstract

We have investigated the generation of the pretilt angles for nematic liquid crystal (5CB) aligned in the cells using rubbed polyimide (PI) films containing trifluoromethyl moieties as a function of rubbing strength (RS). In order to characterize the films, we have conducted an analysis of the surface atomic concentration of fluorine (F) relative to that of carbon (C), F/C(%) with an electron spectroscopy for chemical analysis (ESCA) in the shallow surface region at the depth of 3 and 12 nm. It is shown that the F/C ratio at a depth of 3 nm decreases with RS, and that the observed pretilt angle shows a high value approaching 45° in the weak RS region, and tends to decrease with increasing RS. The existence of the trifluoromethyl moiety in the special PI, and its appearance in the surface region are considered to be responsible for the generation of high pretilt angles at low RS region since a surface covered by fluorine atoms has a low surface energy and the van der Waals dispersion force is thought to be dominant and hence perpendicular LC alignment is expected. With increasing RS the surface concentration of fluorine atoms decreases and repeated triangles will be formed; a combination of the dispersion force and steric interaction between LC molecules and these structures may cause a medium or low pretilted LC conformation.

Electro-optical Properties of Twisted Nematic Liquid Crystal Displays Fabricated with TIPS-pentacene Doping

“…PI layers were rubbed with a cloth to promote homogeneous alignment. The definition of the rubbing strength (RS) has been given in previous papers [26]. An RS of 300 mm was used on the PI surface in this experiment, and represents strong rubbing for LC alignment.…”

Electro-optical Properties of Twisted Nematic Liquid Crystal Cell with Silver Nanowire Network Electrodes

“…The rubbing strength (RS) was conventionally optimized and set at 300 mm. The definition of the RS has been given in previous papers [14]. These substrates were fabricated in an antiparallel configuration and twisted-nematic (TN) cells with cell gaps of of 60 μm and 5 μm, respectively.…”

Liquid Crystal Orientation Mechanism: Competition Between Rubbing and Ion-beam Method

“…An RS of 300 mm was used on the PI surface, which was strongly rubbed for LC alignment [9]. The PI films were irradiated with an Ar IB plasma using a DuoPIGaton-type IB system [17].…”

“…The alignment mechanism on substrate surfaces is a key issue in LCD applications. The rubbing method for nematic LCs (NLCs) on PI surfaces is most widely used to achieve uniform alignment for fabricating LCDs, and the resulting LC alignment properties have been reported by many researchers [1][2][3][4][5][6][7][8][9][10][11]. The rubbing treatment has a few advantages, such as uniform alignment and a stable pretilt angle; however, the treatment introduces debris and electrostatic discharge.…”