Civil Engineering Reference
In-Depth Information
Formation of mesophases is determined by the nature of the selected polymer, as well
as by the backbone chain and length of the spacer. h e stability domain of the meso-
phase is higher as the backbone chain is more l exible. h e size of the spacer inl uences
the nature of the formed mesophase, the ordering degree of the mesogenes incresing as
the spacer becomes longer.
h e structures of extra-chain liquid crystalline polymers present applicative inter-
est today for other reasons than those of intra-chain LCP. Coupling of the mesogene
with basic chain caused a classical liquid crystal behavior. On the other hand, these
structures exhibit characteristics of processability and a mechanical behavior similar
with that of polymers, having the same sensitivity to various external solicitations (elec-
tric and/or magnetic i eld) as simple mesogenes, which recommends the utilization of
liquid crystaline polymers with extra-chain mesophase for electro-optic applications
[29-32] .
14.1.2
Liquid Crystal Dispersed in a Polymer Matrix
Both polymers and liquid crystals belong to a class of matter, sometimes called con-
densed matter, complexing l uids, sot matter [32-34], extensively studied in recent
years in terms of their behavior and physical-chemical properties. Unlike gases and
liquids, sot materials exibit a certain form (polymer) or internal organization (liquid
crystal) but, in contrast to "hard materials," they strongly respond to external mechani-
cal (polymer) or electric (liquid crystal) disturbances.
To obtain materials with applicative interest, some authors have used the liquid crys-
tals as drops of nematic material in isotropic liquid [35] or nematic liquid crystal closed
in microsized cavities inside a solid [36]. h e liquid crystals dispersed in a polymer
matrix (PDLC) are composite materials in which the nematic liquid crystal is dispersed
in a polymeric matrix, generally in the form of droplets of micro-nanosize [37-41]. h e
unique behavior of the material is due to micrometer or nanometer droplet, according
to practical applications, such as optical shutters, [42] displays, [43, 44] smart win-
dows, [45] data storage devices, [46] microlenses [47] and others [38, 40]. Fergason
[48, 49] and Doane [50, 51] introduced a new class of composite materials consisting
of liquid crystal droplets embedded in a polymer i lm, namely PDLC. Sometimes it is
called by the name of nematic curvilinear aligned phase, or encapsulated liquid crystal
(NCAP), and is found in PDLC systems of emulsion type [52]. Ot en, dif erent terms
are used to specify the use as a chiral liquid crystal such as: PDCLC - liquid crystal
dispersed into a polymeric matrix, PDNC - chiral nematic dispersed into a polymer
matrix [53] or ferroelectric liquid crystal or PDFLC - ferroelectric crystal liquid dis-
persed in a polymeric matrix [54]. To specify a dif erent technique of preparation or
operation of PDLC i lms, terms such as homeotropic reverse-mode polymer-liquid
crystals (HRPLC) [55] or holographic polymer-dispersed liquid crystals (HPDLC) are
used [56, 57] .
Combining outstanding mechanical properties of the polymer i lm (high mechani-
cal strength and l exibility) with interesting optical properties of liquid crystal (high
optical anisotropy), the PDLC systems allow their use in various applications as l exible
display systems, privacy or smart glass, projection devices, sensors, etc.).
