
944 Part 5 Special Structures
γ
L
(energy of LC–surface interaction ) >γ
S
(solid sur-
face energy), then a homeotropic alignment is induced;
otherwise, a parallel alignment is induced.
5.1.1.3 Applications of Liquid Crystals
The main requirements on LC materials (LCMs) for
electro-optics are ahigh clearing temperature (T
N-Is
)and
low melting temperature, i. e. a wide temperature range
of definite liquid crystalline phases, and a low viscosity
for reducing the switching time of electro-optical effects.
Wide usage of LCMs in displays became possible after
the discovery of mesogenic cyanobiphenyls. At present
many of homologous LCs are synthesized for display
applications. Phenylcyclohexane- and bicyclohexane-
substituted LCs are used as components in LCMs; they
have low viscosity and, accordingly, fast switching.
Fluorinated three-ring LCs containing two cyclohex-
ane rings and a phenyl ring are used in LCMs with
high dielectric anisotropy, which are used at low volt-
ages. For the application of a LCM as a material in LC
displays, many properties have to correspond to rigid
specifications. For this reason LCMs consist typically
of 7 to 15 components. Sometimes LCMs contain non-
mesogenic additives, e.g. to reduce the viscosity of the
mixture.
In the twist structure, the molecules are parallel to
the cell surfaces and the angle between the boundary
directors is 90
◦
. In the S effect (Frederics effect), a pla-
nar structure is transformed to a homeotropic one, and
in the B effect, a homeotropic structure is transformed
to a planar one. In the twist effect (or twisted nematic,
TN, effect) a twist structure turns into a homeotropic
one. The disadvantage of the TN effect is the necessity
to use polarizers. The “dynamic scattering mode” of use
of LCs depends on the influence of an electric current
on the orientation of the molecules. At a high enough
voltage, transparent nematic cells becomes turbid. A dis-
advantage of this effect is that the lower the voltage, the
longer the time of switching. One of the modern uses
of this effect is use of the dynamic scattering mode for
data storage. After the applied voltage is switched off,
the mode of the planar cell does not return to its initial
state. The cell can be stored in the turbid state for a long
time (from some minutes to some months).
The host–guest effect results from a reorientation of
dye dopants (1–2% in the LC matrix) in an electric field.
In this case the wavelength of maximum absorption of
light is shifted and the color of the LC cell changes.
The sign of the dielectric anisotropy of a LCM de-
termines the type of electro-optical effect. LCMs with
∆ε>0 are used for the TN (twisted nematic), STN
(supertwisted nematic), and TFT (thin-film transistor)
effects. LCMs with low negative values of ∆ε were
formerly used for the dynamic scattering mode. Now
LCMs with negative anisotropy ∆ε are utilized for
MVA-TFT (multidomain vertical-alignment thin-film
transistor) displays. The higher the value of ∆ε>0,
the smaller the working voltage.
Electric current leads to degradation of a LCM
and reduces the lifetime of the display. Impurities in-
fluence the stability of the material and accelerate
electrodegradation. Therefore a multistage purification,
consisting for example of recrystallization, and col-
umn chromatography, to remove conducting impurities
(intermediate products, water, and CO
2
), is necessary.
Usually the specific conductivity of a LCM is lower than
10
−11
–10
−12
Cm/cm and corresponds to the intrinsic
conductivity.
The elastic constants (K
i
) determine the switching
time of the electro-optical effects. The elastic con-
stant K
1
corresponds to the S effect, K
3
to the B effect,
and K
2
to the TN effect; here K
1
corresponds to splaying
K
2
to twisting, and K
3
to bending [1.11,12].
Cholesteryl compounds were the first materials that
found application in thermography.
A huge number of other applications exist, where the
chemical and physical requirements are totally differ-
ent. These applications include reflectors, temperature
measurement with thermochromic materials, nonlinear
optics, polymer materials, SAMs (self-assembled mono-
layers) and LB (Langmuir–Blodgett) films, the use of
liquid crystals in template synthesis of porous materials,
drug delivery, and many more.
5.1.1.4 List of Abbreviations
Cr, Cr
crystalline phases
S smectic
A, B, C, E, F, G, H specific smectic mesophases
C* chiral smectic C (ferroelectric)
D discotic
Part 5 1.1