Page 174 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polycondensation Polymers 137

Greek word “mesos” meaning intermediate. The initial molecules investigated as LCs were large
monomeric molecules.
Flory, in 1956, predicted that solutions of rod-like polymers could also exhibit liquid crystal
behavior. The initial synthetic polymers found to exhibit liquid crystal behavior were concentrated
solutions of poly(gamma-benzyl glutamate) and poly(gamma-methyl glutamate). These polymers
exist in a helical form that can be oriented in one direction into “ordered groupings” giving materi-
als with anisotropic properties.
Liquid crystals, LCs, are materials that undergo physical reorganization where at least one of
the rearranged structures involve molecular alignment along a preferred direction causing the
material to exhibit nonisotropic behavior and associated birefringent properties; that is, molecular
asymmetry.
Liquid crystalline materials can be divided into two large groupings—thermotropic and
lyotropic. Thermotropic LCs are formed when “pure” molecules such as cholesteryl form
ordered structures upon heating. When LCs occurs through mixing with solvents they are called
lyotropic LCs.
Thermotropic LCs can be further divided into (a) enantiotropic materials where the liquid crys-
tal phases are formed on both heating and cooling cycles and (b) mesotropic materials where the
LCs are stable only on supercooling from the isotropic melt. The mesotropic LCs have been further
divided into three groupings as follows (Figure 4.12).

• Smectic meaning “soap”
• Nematic meaning “thread”
• Cholesteric derived from molecules with a chiral center

Liquid crystal polymers are typically composed of materials that are rigid and rod-like with a
high length to breadth ratio or materials that have a disc shape. The smaller groupings that give the
material LC behavior are called “mesogens.” These mesogens are simply portions of the overall
polymer that are responsible for forming the anisotropic LC and that, in fact, form the LC segments.
Such mesogens can be composed of only segments from the backbone of the polymer, segments
from the side chain or segments from both the backbone and side chain.

Nematic Smectic A

Smectic C

FIGURE 4.12 Different mesophasic structures, where the mesogenic unit is designated by a rounded
square.

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