Chemistry Reference
In-Depth Information
Figure 4.15. A nematic texture showing singularities of high strength. The sample is
a nematic polymer with two-dimensional type mesogenic units.
able to observe singularities with
=3
/
2, 2, and 5/2. The unique “two-
dimensional” architecture of the mesogenic units may have brought in a
high energy for splay distortion and “escape” so that the high strength
singularities are significantly stabilized. Figure 4.15 shows such a nematic
texture with high strength disclinations.
Disclination in smectic phases may also show up in the form of schlieren
textures. It was believed that only the whole-numbered singularities are
present in these phases. However more recent work has shown that cer-
tain smectic C
A
phases can also give schlieren textures with half-numbered
singularities (Watanabe
et al
., 1989; 1992; Niori
et al
., 1995). Other
methods such as X-ray scattering may be needed for an unambiguous
characterization of the phase.
Nematic schlieren texture also show up in the form of “inversion walls”
(Nehring and Saupe, 1972). This happens when the preferred orientation is
along the surface and has a change of 180
◦
in crossing the region of a wall. In
the center of the wall a change of 90
◦
in orientation is achieved. Thus, if the
sample is examined under crossed polars, it is observable that a dark line is
sandwiched between two bright lines, or the opposite. With every rotation
by 90
◦
of the polars the dark and the bright lines interchange. Figure 4.16
shows the texture of a domain full of inversion walls (inversion lines) of
a nematic polymer with two-dimensional mesogenic units. For the bottom
picture in the figure the polars have rotated by 90
◦
relative to that for the
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