Chemistry Reference
In-Depth Information
Fig. 2.14
Illustration of
the arrangement of liquid
crystal into nematic and
smectic orders
nematic
smectic
Anisotropy is not affected by conformational changes. Generally, molecules that are rigid rod like
and elongated or disc like in shape are the type that can form liquid crystal arrangements. Some
biological polymers exhibit liquid crystalline behavior due to their rigid helical conformations.
Among synthetic polymers, on the other hand, rigid rod structures, mentioned above are the ones
that exhibit most of the liquid crystalline behavior. Polymers that form liquid crystals may exhibit
multiple mesophases at different temperatures. Based on the arrangement of the liquid crystals in the
mesophases, they are further classified as
nematic, smectic,
and
cholesteric
[
51
,
52
].
Both, smectic and nematic are parallel arrangements alongmolecular axes. The smectic liquid crystals
are more ordered, however, than the nematic ones. This is a result of differences in the orientations of the
chain ends. In smectic liquid crystals the chain ends are lined up next to each other. In nematic ones,
however, they lack any particular orientation. Also the smectic liquid crystals are layered while the
nematic ones are not. Microscopic observations [
51
] can help distinguish between the two forms.
Smectic elastomers, due to their layered structure, exhibit distinct anisotropicmechanical properties
andmechanical deformation processes that are parallel or perpendicular to the normal orientation of the
smectic layer. Such elastomers are important due to their optical and ferroelectric properties. Networks
with a macroscopic uniformly ordered direction and a conical distribution of the smectic layer normal
with respect to the normal smetic direction are mechanically deformed by uniaxial and shear
deformations. Under uniaxial deformations two processes were observed [
53
]: parallel to the direction
of themechanical field directly couples to the smectic tilt angle and perpendicular to the director while a
reorientation process takes place. This process is reversible for shear deformation perpendicular and
irreversible by applying the shear force parallel to the smetic direction. This is illustrated in Fig.
2.14
.
If the mesogens are chiral, a twisted nematic, suprarmolecular,
cholesteric
(twisted) phase can
form [
51
,
52
]. The achiral nonlinear mesogens can also form chiral supramolecular arrangements in
tilted smectic phases.
Recently, Tokita and coworkers [
54
] reported a direct transition from isotropic to smectic
arrangement in a liquid crystalline polymer and determined experimentally the existence of metasta-
ble nematic orientational ordering that preceded the formation of translational smectic ordering.
A polymeric material was used that exhibits very slow liquid crystalline transition dynamics [
55
].
This enabled use of conventional methods to study the transitions, such as of polarized light scattering
and synchrotron wide-angle X-ray diffraction analyses. It was observed that at high quench rates or
super cooling, metastable nematic (orientational) ordering occurs preceding full smectic (orienta-
tional and translational) order. Also, the occurrence of nematic preordering (high super cooling)
resulted in morphological changes of growing liquid crystalline domains compared to solely smectic
growth. Specifically, samples cooled at rates high enough to exhibit nematic preordering formed well-
oriented or “neat” tactoidal smectic domains. Samples cooled at lower rates, where only smectic
ordering was observed, formed radially oriented or textured spherulitic domains [
55
]. In commenting
on this observation, Abukhdeir and Rey [
56
] point out that through a simulation model the isotropic to
smectic liquid crystalline transition experimental observations of preordering of smectic liquid
crystalline transitions can be studied. Phase transition kinetics results presented by them show that
nematic preordering results from both thermodynamic potential and dynamic differences in phase-
ordering time scales.
