droplet in absence of the external electric field. PDLCs can be formed
from emulsions of liquid crystal and polymer. This combination maintains dual
nature: electro-optical performance of liquid crystals and structural and mechanical advantages associated with polymers. Colored PDLC can be made by
incorporating dyes in the PDLC material. The dye has dual role as it is distributed
between LC droplets and matrix polymer. The latter is first UV cured with
the help of dye. On the other hand, the orientation of the elongated dye molecule
is controlled by the nematic director configuration inside the droplet. The absorbance by the dye is modulated by the nematic director alignment with an
external electric field. The guest-host PDLC (nematic) sample, containing
about 1% dye concentration, shows higher transmission and faster switching
responses.
Holographic polymer-dispersed liquid crystals (H-PDLCs) are usually formed by
the illumination of interference patterns on a mixture of photoactive monomers and
liquid crystals. Local differences in photopolymerization rates and reduced miscibility of LC domains are formed in periodic fashion with the period of the interference pattern, in the area having low-light intensity. An electric field manipulates the
orientation of the LC so that the hologram will appear “hidden,” making the
hologram electrically switchable. The H-PDLC has possible application in a range
of areas including switchable focus lenses, telecommunication, optical filters, flatpanel displays, etc.
Miscellaneous
Photoactive Liquid Crystalline Anisotropic Gel
The oriented liquid crystals (LCs) stabilized by a polymer network is known as
anisotropic gel. Basically the gel comprises oriented LCs whose long-range molecular orientation is permanently held by the network. In anisotropic gels the LC
orientation is lost in the isotropic phase, which is recovered once the sample is
cooled into LC phases. Anisotropic gels is prepared from a homogeneous mixture
comprising a nonreactive nematic LC compound and a reactive azobenzenecontaining diacrylate monomer (5–10 wt %) when exposed to linearly polarized
light and thermal polymerization carried out in LC phase.
In case of cholesteric LCs, the network can stabilize different textures,
while showing bistability required for display applications. The presence of
small amount of network improves shock resistance of the surface-stabilized
ferroelectric LCs.
Anisotropic gels also show bending/unbending behavior. The swelling also
remains anisotropic in good solvent. The enlargement occurs and is higher in
direction parallel to rubbing direction. If UV radiation with appropriate wavelength
(~360 nm) falls on film, there is bending similar to photoactive LCP.
14 Photoactive Liquid Crystalline Polymers
407
from emulsions of liquid crystal and polymer. This combination maintains dual
nature: electro-optical performance of liquid crystals and structural and mechanical advantages associated with polymers. Colored PDLC can be made by
incorporating dyes in the PDLC material. The dye has dual role as it is distributed
between LC droplets and matrix polymer. The latter is first UV cured with
the help of dye. On the other hand, the orientation of the elongated dye molecule
is controlled by the nematic director configuration inside the droplet. The absorbance by the dye is modulated by the nematic director alignment with an
external electric field. The guest-host PDLC (nematic) sample, containing
about 1% dye concentration, shows higher transmission and faster switching
responses.
Holographic polymer-dispersed liquid crystals (H-PDLCs) are usually formed by
the illumination of interference patterns on a mixture of photoactive monomers and
liquid crystals. Local differences in photopolymerization rates and reduced miscibility of LC domains are formed in periodic fashion with the period of the interference pattern, in the area having low-light intensity. An electric field manipulates the
orientation of the LC so that the hologram will appear “hidden,” making the
hologram electrically switchable. The H-PDLC has possible application in a range
of areas including switchable focus lenses, telecommunication, optical filters, flatpanel displays, etc.
Miscellaneous
Photoactive Liquid Crystalline Anisotropic Gel
The oriented liquid crystals (LCs) stabilized by a polymer network is known as
anisotropic gel. Basically the gel comprises oriented LCs whose long-range molecular orientation is permanently held by the network. In anisotropic gels the LC
orientation is lost in the isotropic phase, which is recovered once the sample is
cooled into LC phases. Anisotropic gels is prepared from a homogeneous mixture
comprising a nonreactive nematic LC compound and a reactive azobenzenecontaining diacrylate monomer (5–10 wt %) when exposed to linearly polarized
light and thermal polymerization carried out in LC phase.
In case of cholesteric LCs, the network can stabilize different textures,
while showing bistability required for display applications. The presence of
small amount of network improves shock resistance of the surface-stabilized
ferroelectric LCs.
Anisotropic gels also show bending/unbending behavior. The swelling also
remains anisotropic in good solvent. The enlargement occurs and is higher in
direction parallel to rubbing direction. If UV radiation with appropriate wavelength
(~360 nm) falls on film, there is bending similar to photoactive LCP.
14 Photoactive Liquid Crystalline Polymers
407