The individual molecules that comprise a liquid crystal are commonly referred to as mesogens. These molecules tend to be 'rod like' or anisotropic structures, with one axis appreciably longer than the other axis. However, in reality, LC phases have been prepared using mesogen of a wide variety of shapes, including ring structures, banana, hockey stick, and T shaped molecules. Even DNA adopts a LC phase. While theoretically, any thermotropic or lyotropic LC can and have been used to prepare an LCE, we discuss some of the characteristics and practical considerations of mesogens used for LCE.
A mesogen used for an LCE contains three distinct components:
Illustrative chemical structures are shown in the image to the right. The different functional groups of the mesogen are colored in the following manner: polymerization group (blue), flexible spacer group (red), LC core (green). Please click on the image to expand.
The structure of the acrylate and vinyl functional groups that participate in polymerization aspect of
LCE are shown above. Polymerization runs through different reaction mechanism for the different functional groups.
Acrylate groups react with neighboring acrylate groups to establish a polymer network as discussed on the previous
page. Mesogens with acrylate groups on both ends function as cross-linkers
Vinyl groups react with the Si-H bond of a methyl siloxane polymer. Very short and long chain methyl-hydrosiloxane polymers
are used to prepare either main chain or side chain LCE respectively.
Some of the advantages of the acrylate system are precisely controllable preparation conditions and relatively fast material
preparation. Using acrylate based mesogen, materials may be prepared in glass cells, drawn as fibers, or printed using an
inkjet printer. However, materials response may be more limited with
acrylate materials. Advantages of siloxane polymer
are large stimuli response and high yield.
The flexible spacer is comprised of the atoms between the polymer backbone and mesogen core.
Typically composed of methylene (CH2) groups and oxygen atoms, the spacer influences both the LC phase transition
temperature and the orientation in the LCE.
With increasing length, the melting temperature (Tm) tends to decrease, while
the clearing temperature (Tc) tends to increase. With an increasing number atoms, LC polymer networks exhibit and
increasing tendency toward crystallization the LC mesogen tends to enter the
crystalline phase. However, the core structure also strongly influences crystallization, so their are no set rules on the
number of atoms before crystallization occurs.
The number of atoms in the spacer also influences the orientation of the mesogen in a phenomenon known as odd-even effect.
Side chain LCE with an even number of atoms in the flexible spacer have the mesogen are directed perpendicular to the backbone
in what is known as the oblate orientation, while an odd number of spacer atoms have the LC molecules directed parallel
to the polymer backbone.
The structure of the mesogen's core differs considerably of
thermotropic and lyotropic LC. Thermotropic
LC usually have an anisotropic shape, and are based upon various ring structures such as cyclohexane, benzene, and biphenol derivatives
thermotropic mesogens. As the number of rings increased, the isotropic phase transition temperature tends to
increases with the caveat
that this depends strongly on the nature of the ring and spacer.
Lyoptropic mesogens are amphiphilic molecules, having both a hydrophilic and hydrophobic component with the hydrophobic
moiety
facing the exterior of the molecule to access the solvent.