Civil Engineering Reference
In-Depth Information
dif erence between them appears in the state of aggregation of each one, namely: solid
for condis crystals and liquid for liquid crystals [12].
h ermotropic liquid crystals were studied for the i rst time in the late 19th century.
Initially, research focused on the structural characterization and classii cation of liquid
crystals [11]. Also, various theories [12] were implemented on viscosity [13], elasticity
constant [14], etc. Major progress has been recorded in the year 1960, driven by practi-
cal applications of liquid crystals with main [15, 16] and side chains [16, 17].
Potential applications of liquid crystals result from the combination of liquid prop-
erties (l ow) with those of solid state (anisotropy of physical properties), but also from
their ability to exist simultaneously in l uid and ordered state. h e liquid crystalline
properties of polymers are improved, compared to those of the corresponding mono-
mers, as macromolecular compounds are also involved.
h ermotropic liquid crystals are important in terms of fundamental research and
technological applications, while lyotropic liquid crystals play an important role in bio-
logical systems and living tissues [18]. h e i rst applications of liquid crystals are found
in materials with high mechanical strength, since, in the processing moment, the prod-
ucts obtained from a liquid crystal state possess a more advanced ordering degree than
that of the obtained materials. For such applications, processing of the liquid crystalline
compound in the nematic mesophase is recommended and not processing in the smec-
tic mesophase, due to its high viscosity, which is sometimes close to that of the solid
material [3]. Another important application of liquid crystals is represented by electro-
optical materials [19], such as display (TVs, computers, laptops, electronic notebooks,
watches, mobile phones, etc.), and optical devices (optical i lters [20], optical i bers
[21], photoelectric switches [22], tension modulators [23], security elements [24]). In
addition, liquid crystals are used in building transistors with i eld ef ect [25, 26] and
light-emitting diodes [27], due to their ability to form highly-ordered i lms with better
electrical conductivity than other materials.
An interesting area, which involves chiral liquid crystal properties, is that of temper-
ature sensors, used, for example, in the diagnosis of skin cancer, as well as in peripheral
blood circulation problems. Another application in the medical i eld is represented by
nematic elastomer i lms or i bers of liquid crystalline polymers with mesogene in the
side chain, that can be used in the manufacture of muscle prosthesis [28]. To optimize
their performance for dif erent applications, the current knowledge on the relationship
between the structure and properties of liquid crystals should be extended. In addition
to further development of liquid crystals and their applications, the liquid crystal theo-
ries represent a sound basis for other areas of interest. For example, liquid crystals can
be used as model compounds for the study of molecular interactions and of their ef ects
on self-organization in supramolecular chemistry.
14.1.1
Liquid Crystalline Polymers
h e i rst known ordered structures of polymers date back to 1956, when Flory demon-
strated that concentrated solutions of rigid polymers, of stick type, can form ordered
structures. Subsequently, studies devoted to the behavior of some macromolecules
in helical conformations (poly(methyl and/or benzyl glutamate type)), associated in
"packets" preferentially aligned in some direction, coni rmed the existence of this type
