Civil Engineering Reference
In-Depth Information
of structuring. Consequently, inclusion of mesogene sequences in the polymer matrix,
may lead to materials with anisotropic characteristics of liquid crystals (LC) [29, 30].
h e mesogene groups may be either intra- or extra-chain, as side "grat " of basic
structures. h ere also exist liquid crystal polymer structures with both intra- and extra-
chain mesogene groups (Figure 14.1).
Polymers with mesogene interactions are obtained by condensation polymeriza-
tion processes. h ey can organize themselves in very rigid or semi-l exible structures,
depending on the available space, and on the linking mesogene groups. Synthesis of
polymers with mesogenic groups laterally attached to the polymer backbone can be
achieved by chemical modii cation of a l exible main chain of the polymer, polymeriza-
tion of a vinyl monomer carrier of the pendant mesogenic groups, followed by polym-
erization of the mesomorphic monomers or mesogenic precursors [31].
h e LC polymers with intra-chain mesogenes are generally characterized by excep-
tional mechanical properties, forming the liquid crystal polymer (LCP) class employed
in many applications. For example, if the mesogenes are connected by a short space,
a highly-rigid material is obtained, which is thermally decomposed before melting,
namely prior to the manifestation of the liquid crystaline properties.
Lyotropic polymer solutions evidence a special behavior; at critical concentration,
the solution is separated into two phases, one with a high concentration of liquid crys-
tals, the other with low concentration and isotropic characteristics. At critical concen-
tration, the viscosity of solutions increases sharply.
In addition to aromatic polyamides, the category of lyotropic LCP also includes
cellulose derivatives and polypeptides. Such polymers may be spun into i bers, start-
ing from concentrations below the critical value. When reducing the decomposition
temperature of the rigid structures with intra-chain mesogenes by introducing bulky
side groups or l exible spacer sequences, destruction of the symmetry of the assem-
bled macromolecules occurs. h us, molecular movement may increase, evidencing the
characteristics of the liquid crystal without requiring a solvent. On the other hand, the
behavior of LC is visible at temperatures below thermal degradation.
Up to now, liquid crystalline polymers with side mesogenes have not been employed
for signii cant applications. h e presence of a spacer between the backbone and the
mesogene causes decoupling between the main chain movement and the mesogene.
Intra-chain mesogens Extra-chain mesogens Intra- and extra-chain mesogens
Figure 14.1
.
Schematic representation of liquid crystalline polymers [28]
