Liquid Crystals

Chapter 1.5.1 - Bulk Thin Film

1.5.1.   Bulk Thin Film

For nematics, two commonly used alignments are the so-called homogeneous (or
planar) and homeotropic alignments, as shown in Figures 1.18a and 1.18b, respectively.
To create homeotropic alignment, the cell walls are treated with a surfactant
such as hexadecyl-trimethyl~ammoniumbromide (HTAB).20 These surfactants are
basically soaps, whose molecules tend to align themselves perpendicular to the wall
and thus impart the homeotropic alignment to the liquid crystal.

In the laboratory, a quick and effective way to make a homeotropic nematic liquid
crystal sample is as follows: Dissolve 1 part of HTAB in 50 parts of distilled deionized
water by volume. Clean two glass slides (or other optical flats appropriate for
the spectral region of interest). Dip the slides in the HTAB solution and slowly withdraw
them. This effectively introduces a coating of HTAB molecules on the glass
slides. The glass slides should then be dried in an oven or by other means. To prepare
the nematic liquid crystal sample, prepare a spacer (Mylar or some nonreactive plastic)
of desirable dimension and thinness and place the spacer on one of the slides. Fill
the inner spacer with the nematic liquid crystal under study (it helps to first warm it
to the isotropic phase). Place the second slide on top of this and clamp the two slides
together. Once assembled, the sample should be left alone, and it will slowly (in a
few minutes) settle into a clear homeotropically aligned state.

Planar alignment can be achieved in many ways. A commonly employed method is
to first coat the cell wall with some polymer such as polyvinyl alcohol (PVA) and then
rub it unidirectionally with a lens tissue. This process creates elongated stress/strain

Figure 1.18. Nematic liquid crystal cells: (a) homeogeneous (or planar) aligned and (b) homeotropic aligned.


on the polymer and facilitates the alignment of the long axis of the liquid crystal molecules
along the rubbed direction (i.e., on the plane of the cell wall). Another method
is to deposit silicon oxide obliquely onto the cell wall.

In preparing a PVA-coated planar sample in the laboratory, the following technique
has been proven to be quite reliable. Dissolve chemically pure PVA (which is solid at
room temperature) in distilled deionized water at an elevated temperature (near the
boiling point) at a concentration of about 0.2%. Dip the cleaned glass slide into
the PVA solution at room temperature and slowly withdraw it, thus leaving a film of
the solution on the slide. (Alternatively, one could place a small amount of the PVA
solution on the slide and spread it into a thin coating.) The coated slide is then dried in
an oven, followed by unidirectional rubbing of its surfaces with a lens tissue. The rest
of the procedure for cell assembly is the same as that for homeotropic alignment.

Ideally, of course, these cell preparation processes should be performed in a clean
room and preferably in an enclosure free of humidity or other chemicals (e.g., a nitrogen-
filled enclosure) in order to prolong the lifetime of the sample. Nevertheless, the
liquid crystal cells prepared with the techniques outlined previously have been
shown to last several months and can withstand many temperature cyclings through
the nematic–isotropic phase transition point, provided the liquid crystals used are
chemically stable. In general, nematics such as 5CB and E7 are quite stable, whereas
p-methoxybenzylidene-p'-n-butylaniline (MBAA) tends to degrade in a few days.

Besides these two standard cell alignments, there are many other variations such
as hybrid, twisted, supertwisted, fingerprint, multidomain vertically aligned, etc.
Industrial processing of these nematic cells, as well as the transparent conductive
coating of the cell windows for electro-optical device applications, is understandably
more elaborate.

For chiral nematic liquid crystals, the method outlined previously for a planar
nematic cell has been shown to be quite effective. For smectic-A the preparation
method is similar to that for a homeotropic nematic cell. In this case, however, it
helps to have an externally applied field to help maintain the homeotropic alignment
as the sample (slowly) cools down from the nematic to the smectic phase. The cell
preparation methods for a ferroelectric liquid crystal (FLC), smectic-C* for surface
stabilized FLC (SSFLC) operation, is more complicated as it involves surface stabilization.
21,22 On the other hand, smectic-A* (Sm-A*) cells for soft-mode FLC
(SMFLC) operation are easier to prepare using the methods described above.23

 

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