260
formation of domains with permanent electric dipole moment in smectic C systems
by doping with chiral mesogens, which exhibited piezoelectric properties. Shear
processing is an effective way to produce electromechanically active LCEs from
uniaxially oriented films.
Another simple synthetic approach to tune the dynamic mechanical response of
polydomain smectic elastomers for possible applications in vibration isolation or
impact absorption is introduction of methyl substituents into the mesogen (Patil
et al. 2007).
11.4.3 Polymer-Dispersed LCs (PDLCs) and Hybrid
LC Materials
PDLCs represent a mixture of LCs and a monomer/polymer in which phase separation is induced by thermal/UV treatment (crosslinking or polymerization of the
matrix) or by solvent evaporation. Another method for the preparation of PDLCs is
the encapsulation of LCs, usually in water soluble polymers. The nematic texture
within the LC domains is randomly oriented, while the application of the external
field (electromagnetic, mechanical, thermal, etc.) causes the orientation of domains
in the preferred direction, ensuring PDLC transparency when the ordinary refractive
index coincides with that of the polymer matrix (Li et al. 2005). PDLCs are widely
used for manufacturing flat-panel displays, holographic films, switchable windows, etc.
PDMS is not commonly used as a matrix in PDLCs, probably due to the chemical incompatibility with the organic LCs, which may lead to uncontrolled morphology. PDLCs composed of PDMS, crosslinked PDMS or polysulfone as matrices,
and an azomethine compound as an embedded mesogen (5–80 wt.%) have been
prepared by Bronnikov et al. (2009) via solvent-induced phase separation as model
systems to study the nematic phase growth kinetics through the isotropic-nematic
phase transition. This transition was described analytically with the universal law
for the cluster growth, which revealed the influence of both flexibility of the polymer matrix and the mesogen content. For the LC/PDMS systems where the components were immiscible through the I-N phase transition, the soft un-crosslinked
polymer matrix did not prevent the nematic phase growth (the mean droplet size
was independent of the LC phase concentration), while in semi-flexible crosslinked
PDMS matrix, the growth of the nematic droplets was restricted (the mean droplet
size decreased significantly with the LC phase concentration).
Racles et al. (2015b) also reported the preparation of PDLC materials by encapsulation in PDMS. These authors first prepared LC siloxane-containing polyazomethine NPs, which were stabilized in toluene using an amphiphilic siloxane oligomer,
then mixed with PDMS, and the matrix was crosslinked. This approach provided
highly flexible films with S max of 600–800%, low Y and increased dielectric permittivity up to 300%, which were primarily intended for electromechanical applications.
C. Racles et al.
formation of domains with permanent electric dipole moment in smectic C systems
by doping with chiral mesogens, which exhibited piezoelectric properties. Shear
processing is an effective way to produce electromechanically active LCEs from
uniaxially oriented films.
Another simple synthetic approach to tune the dynamic mechanical response of
polydomain smectic elastomers for possible applications in vibration isolation or
impact absorption is introduction of methyl substituents into the mesogen (Patil
et al. 2007).
11.4.3 Polymer-Dispersed LCs (PDLCs) and Hybrid
LC Materials
PDLCs represent a mixture of LCs and a monomer/polymer in which phase separation is induced by thermal/UV treatment (crosslinking or polymerization of the
matrix) or by solvent evaporation. Another method for the preparation of PDLCs is
the encapsulation of LCs, usually in water soluble polymers. The nematic texture
within the LC domains is randomly oriented, while the application of the external
field (electromagnetic, mechanical, thermal, etc.) causes the orientation of domains
in the preferred direction, ensuring PDLC transparency when the ordinary refractive
index coincides with that of the polymer matrix (Li et al. 2005). PDLCs are widely
used for manufacturing flat-panel displays, holographic films, switchable windows, etc.
PDMS is not commonly used as a matrix in PDLCs, probably due to the chemical incompatibility with the organic LCs, which may lead to uncontrolled morphology. PDLCs composed of PDMS, crosslinked PDMS or polysulfone as matrices,
and an azomethine compound as an embedded mesogen (5–80 wt.%) have been
prepared by Bronnikov et al. (2009) via solvent-induced phase separation as model
systems to study the nematic phase growth kinetics through the isotropic-nematic
phase transition. This transition was described analytically with the universal law
for the cluster growth, which revealed the influence of both flexibility of the polymer matrix and the mesogen content. For the LC/PDMS systems where the components were immiscible through the I-N phase transition, the soft un-crosslinked
polymer matrix did not prevent the nematic phase growth (the mean droplet size
was independent of the LC phase concentration), while in semi-flexible crosslinked
PDMS matrix, the growth of the nematic droplets was restricted (the mean droplet
size decreased significantly with the LC phase concentration).
Racles et al. (2015b) also reported the preparation of PDLC materials by encapsulation in PDMS. These authors first prepared LC siloxane-containing polyazomethine NPs, which were stabilized in toluene using an amphiphilic siloxane oligomer,
then mixed with PDMS, and the matrix was crosslinked. This approach provided
highly flexible films with S max of 600–800%, low Y and increased dielectric permittivity up to 300%, which were primarily intended for electromechanical applications.
C. Racles et al.