Chemistry Reference
In-Depth Information
shear cessation is not the result of free thermal relaxation of individual
molecules, but the result of contraction and zigzag rearrangement of the
fibrils as a whole along the orientation direction. The elastic energy stored
during shearing may have provided the driving force for contraction, but
the high stability of the parallel alignment of neighboring molecules due
to very limited molecular motion of these polymers is a necessary condi-
tion for formation of the bands. By contrast, in the case of flexible chain
polymers, molecules may also assume a parallel orientation under shear-
ing. However, the orientational relaxation takes place easily as a result of
the fast thermal motions of individual molecules. The external elastic force
may even accelerate this process. As a result, no regular banded textures
should be expected for flexible polymers. Figure 4.19 is a photomicrograph
of the banded texture given by a mesogen-jacketed liquid crystal polymer
(“MJLCP”, Zhou
et al
., 1987; 1996). The polymer is the first side-chain
type polymer that forms a banded texture. This result shows that the poly-
mer has a chain rigidity comparable with main-chain type liquid crystalline
polymers although its molecular structure is more like a side-chain type
polymer.
The texture of polymeric smectic.
As discussed in Chapter 1 there
are many subcategories of smectic phases. The identification of a smec-
tic phase by POM becomes more di
cult with less certainty when the
order of molecular packing in the phase is increased. Other techniques such
as WAXS are often used together with POM for conclusive identification
of smectics (Gray and Goodby, 1984). Because of the complexity in the
molecular structure of liquid crystalline polymers there are fewer types of
polymeric smectics. The polymeric smectics most often observed are the
less ordered types A and C.
The often observed textures for S
A
are the focal-conic fan texture and
the homeotropic texture. When the molecular orientation is homeotropic,
the optic axis is perpendicular to the film surface so that the preparation
appears black (pseudoisotropic) on a crossed polarizing microscope. As with
the homeotropic nematics, the homeotropic S
A
phase can be differentiated
from the true isotropic phase by conoscopic observations. Further more,
if the cover slip of the homeotropic S
A
preparation is slightly moved, the
orientation is disturbed resulting in “oily streaks” in form of bright bands.
The homeotropic S
A
phase may thus be distinguished from the homeotropic
nematic phase.
