Chemistry Reference
In-Depth Information
Figure 4.19. Banded texture of a mesogen-jacketed liquid crystalline polymer. The
shearing direction is perpendicular to the direction of the parallel bands.
coworkers (Noel, 1985) found that a more perfect homeotropic alignment
was achieved by the simple treatment of glass slides with boiling chromic
sulfuric acid, acetone and methanol and rinsing with hot distilled water.
Films thus prepared appear completely dark when viewed orthoscopically
but form conoscopic interference patterns as outlined by Figure 4.10. On
the other hand, planar alignment may be obtained by using freshly cleaved
mica surfaces or by directed oblique evaporation of silicon monoxide on
glass surfaces.
A unique optical phenomenon observable for the nematic phase of poly-
mers with significant chain rigidity is the formation of a “banded texture”
(Figure 4.19) composed of parallel bands of alternate brightness (Dobb
et al
., 1977; Chen
et al
., 1987; Xu
et al
., 1993). This texture has not
been observed for low mass liquid crystals but often found for oriented
thermotropic or lyotropic liquid-crystalline main-chain type polymers with
sucient chain rigidity. In the oriented liquid crystal specimens the poly-
mer molecules are packed in a parallel alignment to form fibrils. The
fibrils travel parallelly in a regular zigzag manner, resulting in the unique
alternating light and dark pattern when being observed on a polarizing
microscope. A contraction effect has been proposed to explain the mecha-
nism of band formation. Thus, the rigid or semi-rigid molecules of liquid
crystalline polymers are packed in parallel in the form of fibrils under shear
along the shearing direction. The formation of the banded textures after
