Chemistry Reference
In-Depth Information
orientation of liquid crystal surface molecules depends on their interac-
tion with the surface they contact. Such interaction, and hence the ori-
entation of the surface molecules, can be easily controlled by coating
applied to the contact surface of a substrate. When amphiphilic mole-
cules or silane couplers are used as coatings, the long axes of the mol-
ecules align perpendicular to the substrate plate. This orientational
state is called “homeotropic molecular alignment” (Figure 6.7 a). When
other coatings, such as polyvinyl alcohol, nylon, polyimide, etc., are
used, the molecular long axes are oriented parallel to the substrate
plates. This state is called “homogeneous molecular alignment”
(Figure 6.7 b).
A “rubbing” procedure is frequently used to orient molecules in one
direction. The “rubbing” makes grooves or scratches on the surface of
the coating material to induce the anisotropy of the orientation of
chains of the coating material molecules.
The most important application of liquid crystal is LCDs. The twisted
nematic (TN)-LCD structure is illustrated in Figure 6.8. The liquid
crystals are sandwiched between two glass plates with transparent elec-
trodes. The alignment agent is coated on the plates, and the molecular
orientation on the surface is fi xed by the rubbing procedure. The rubbing
direction of upper and lower plates is crossed so that the orientations
of the liquid crystal molecules are twisted through its thickness. The
polarizers are placed outside the glass plates so that the polarization
axes are crossed. In the white mode, the initial state shown in Figure
6.8a, the liquid crystal molecules are oriented parallel to the glass plate.
When the incident light is polarized parallel to the molecular orienta-
tion at the lower plate, the polarization of the transmitted light through
the liquid crystal layer is rotated along the twisted orientation of liquid
crystals, and the light can pass through the second polarizer. When
electric fi eld is applied to the cell, molecular reorientation is induced,
(a)
(b)
Figure 6.7
(a) Homeotropic molecular alignment and (b) homogeneous molecular
alignment in nematic phase.
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