Page 175 - Carrahers_Polymer_Chemistry,_Eighth_Edition

138 Carraher’s Polymer Chemistry

The mesogens form the ordered structures necessary to give the overall material anisotropic
behavior. There have been identified a number of different mesogen groupings. Chains arranged

so that the mesogen portions are aligned in one preferred direction with the ordering occurring in
a three-dimensional layered fashion compose one group of arrangements called smectic arrange-
ments. Here, the lateral forces between the mesogen portions are relatively higher than the lateral
forces between the nonmesogen portions allowing a combination of segments that permit “fl owing”
(the passage of nonmesogen portions) and segments that retain contact (mesogen portions) as the

material flows imparting a “memory”-type behavior of the material. A number of different “pack-
ings” of the mesogens have been identified. The most ordered of the mesogenic groupings is called

“smectic B,” which is a hexagonally, close-packed structure present in a three-dimensional arrange-
ment. A much less ordered grouping is called the “Smectic A” phase. Here there is a somewhat
random distribution of the mesogens between the layers.
Nematic LCs offer much less order in comparison to smectic arrangements. Here, the direc-
tional ordering of the mesogen portions along one axis is retained, but the centers of gravity of the
mesogen portions are no longer “coupled.” Thus, the forces between the chains are less resulting in

a generally greater fluidity for nematic LCs in comparison with smectic structures. Nematic LCs
also offers nonlinear behavior.
The chiral nematic assembly is formed by materials that have chiral centers and that form a
nematic phase. Here, a “chiral-imposed twist” is imparted to the linear chains composing each layer
resulting in a three-dimensional helical arrangement.
The molecular asymmetry typically occurs not because of intermolecular interaction, but because
two molecules cannot occupy the same space at the same time. Molecular chains can exist in a ran-
dom arrangement until a given concentration is exceeded causing the molecules to rearrange in a
more ordered fashion to accommodate the larger number of molecules within the same volume.
Often, this occurs such that there is an ordered phase and a more random phase. As the concentration
of polymer increases, the ordered phase becomes larger at the expense of the disordered phase. This
increase in polymer concentration can occur via several routes such as addition of more polymer,
addition of a solution containing a higher concentration of polymer, and evaporation of the solvent.
For crystalline polymer systems, transition from the crystalline structure to a mesosphere struc-
ture occurs, whereas from amorphous polymer systems, the mesophase occurs after the T has
g
occurred. Some polymer LC systems form several mesophases. Mesophases can be detected using
differential scanning calorimetry (DSC), X-ray diffraction, and polarizing microscopy.
Introduction of flexible “spacer” units such as methylene, methylene oxide, and dimethylsiloxane

groups lowers the melting point and increase the temperature range within which the mesophase is
stable. Often these spacer units are introduced by copolymerization. Thus, preformed p-acetoxy-
benzoic acid is reacted with PET introducing a mesogenic unit in a polymer that has fl exible spacer
units (from the ethylene glycol) in it.
Poly(ethylene terephthalate), PET (4.91) Mesogenic unit (4.92)
R O O R O
O
O O O O
R O R

Along with the mesogen units contained within the polymer backbone, the mesogen units can
occur as side chains. These mesogen units can be introduced either through reaction with monomers
that contain the mesogen unit or through introduction with already formed polymers.

Liquid crystal materials have also been employed as films, plastics, and resins. Poly(1,4-benzoate)
has been marketed under the name Ekonol. It decomposes before it melts, hence it does not form

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