Polar Anchoring Strength and the Temperature Dependence of Nematic Liquid Crystal (5CB) Aligned on Rubbed Polystyrene Films

Abstract

A characterization of rubbed polystyrene (PS) film in terms of the optical retardation measurement as a function of the rubbing strength (RS) has been made, and the alignment capability of the rubbed PS films for a nematic liquid crystal (5CB) has been investigated by measuring anchoring strength and its temperature dependence. It is shown that the liquid crystal (LC) molecules orient homogeneously in a direction orthogonal to that of rubbing, the anchoring strength of the rubbed PS films at the strongest rubbing strength for 5CB is about 70 times weaker compared to that for rubbed standard polyimide films, and the anchoring strength decreases with the temperature in a divergent fashion according to the increase of the extrapolation length toward the clearing point (T c=35.3°C).

Monte Carlo simulation of liquid-crystal alignment and chiral symmetry-breaking

“…25 While our simulations did not show that kind of ordering, our results lead us to consider possible explanations. One is that the local smectic order next to the substrate could form with the layers parallel to the ridges, and with the molecules oriented perpendicular to the ridges or at some degree of tilt.…”

Liquid crystal alignment by rubbed polymer surfaces: a microscopic bond orientation model

“…All rubbed polyimides align LCs along the rubbing direction, with the pretilt direction pointing into the rubbing direction. Rubbed polystyrene, by contrast, aligns LCs perpendicular to the rubbing direction with zero pretilt [8,23]. This extreme behavior has to be explained by any reasonable model.…”

“…Conventional linear optics techniques lack surface sensitivity and second harmonic generation cannot be used because the polymer molecules typically have inversion symmetry. Nevertheless, early proposals of the molecular orientation at the polymer surface were based on bulk-sensitive optical and infra-red measurements carried out on thin films [7][8][9][10][11][12].…”

The effect of phenoxymethyl side groups on the liquid crystal alignment behavior of polystyrene derivatives

“…24 The mechanical rubbing of polystyrene (PS) surfaces can also produce homogeneous planar LC alignment layers, where the LC molecules are aligned perpendicular to the rubbing direction. [25][26][27][28][29][30][31][32][33] However, this perpendicular LC alignment produces a very low pretilt angle of about 0 o26,31 as well as a weak azimuthal anchoring energy of less than 10 -7 -10 -8 J/m 2 , 31-33 which is too low to produce a stable LC cell for practical applications in LCD devices. The homogeneity of an LC cell fabricated with the rubbed PS film is lost after 1 day.…”