first choice as guest as one molecule has the ability to orient large number of LC
moieties in polymers. Liquid crystal display (LCD) is based on the guest-host effect
which utilizes the convenient alignment of LC on application of electric voltage and
the close reorientation of the dye guest. This is responsible for selective absorption
by the dyes. Due to the presence of dye, the LCDs function with only one polarizer
and show a wide viewing angle and fairly good image contrast (Wolarz et al. 1992).
By using fluorescent dyes, the displays can function in both active and passive
modes.
Double guest-host (DGH) displays concept is based on the same operation
principle. However, a second independent switchable layer is employed behind the
first. The cell configuration is such that it acts as efficient polarizer at a selected
wavelength only. The neutral polarizers used in conventional GH displays, having
high absorption problem across the spectrum, can be dispensed with resulting in a
brighter display (Uchida and Wada 1981). The trilayer displays have also been
attempted. It uses different color dye in each layer which can be selectively switched
to yield a range of colors (Castellano 1992).
Photoactive Polymer Dye/LC Blend
The azobenzene polymer network does dual function by stabilizing liquid crystal
and also alters its orientation through irradiation of suitable wavelength. When the
azo monomer is polymerized and simultaneously irradiated, each photoinduced
trans-cis isomerization is followed by thermally activated cis-trans isomerization,
causing a small reorientation of the transition moment. After a large number of transcis-trans cycles, a preferential orientation of azobenzene groups normal to the
polarization plane occurs. The liquid crystals, present in the polymerization medium,
are thus stabilized by a polymer network carrying azobenzene groups. The
azobenzene orientation resulting from irradiation alters the director fields of surrounding liquid crystal molecules, leading to their orientation. This methodology has
become interesting for controlling the average liquid crystal orientation over macroscopic length scales. Unlike surface alignment, the polymer network offers “volume” or “bulk” effects which can align liquid crystals in relatively thick films. As
azobenzene groups are aligned only in areas exposed to irradiation, the orientation of
liquid crystal molecules should occur only in those areas.
Orientation of Photoactive LCP
The photoactive polymers, particularly azo polymers, are productively used for
making alignment layers for liquid crystalline polymers in the display and semiconductor technology, waveguides and waveguide couplers, data storage media, labels
in quality product protection, light-driven artificial muscles, and so on (Ilnytskyi
et al. 2011). All the processes are dependent on orientation of azo group.
The earliest attempt was to rub the polyimide or polyvinyl alcohol coating on
glass surface and cast polymer solution to obtain the polymer film or polymerize LC
monomer inside rubbed cell. Due to dust and other dirt-related problems being
associated with these processes, newer process is explored. Magnetic and electrical
forces can be utilized for orientation. However, these processes are not much
14 Photoactive Liquid Crystalline Polymers
399
moieties in polymers. Liquid crystal display (LCD) is based on the guest-host effect
which utilizes the convenient alignment of LC on application of electric voltage and
the close reorientation of the dye guest. This is responsible for selective absorption
by the dyes. Due to the presence of dye, the LCDs function with only one polarizer
and show a wide viewing angle and fairly good image contrast (Wolarz et al. 1992).
By using fluorescent dyes, the displays can function in both active and passive
modes.
Double guest-host (DGH) displays concept is based on the same operation
principle. However, a second independent switchable layer is employed behind the
first. The cell configuration is such that it acts as efficient polarizer at a selected
wavelength only. The neutral polarizers used in conventional GH displays, having
high absorption problem across the spectrum, can be dispensed with resulting in a
brighter display (Uchida and Wada 1981). The trilayer displays have also been
attempted. It uses different color dye in each layer which can be selectively switched
to yield a range of colors (Castellano 1992).
Photoactive Polymer Dye/LC Blend
The azobenzene polymer network does dual function by stabilizing liquid crystal
and also alters its orientation through irradiation of suitable wavelength. When the
azo monomer is polymerized and simultaneously irradiated, each photoinduced
trans-cis isomerization is followed by thermally activated cis-trans isomerization,
causing a small reorientation of the transition moment. After a large number of transcis-trans cycles, a preferential orientation of azobenzene groups normal to the
polarization plane occurs. The liquid crystals, present in the polymerization medium,
are thus stabilized by a polymer network carrying azobenzene groups. The
azobenzene orientation resulting from irradiation alters the director fields of surrounding liquid crystal molecules, leading to their orientation. This methodology has
become interesting for controlling the average liquid crystal orientation over macroscopic length scales. Unlike surface alignment, the polymer network offers “volume” or “bulk” effects which can align liquid crystals in relatively thick films. As
azobenzene groups are aligned only in areas exposed to irradiation, the orientation of
liquid crystal molecules should occur only in those areas.
Orientation of Photoactive LCP
The photoactive polymers, particularly azo polymers, are productively used for
making alignment layers for liquid crystalline polymers in the display and semiconductor technology, waveguides and waveguide couplers, data storage media, labels
in quality product protection, light-driven artificial muscles, and so on (Ilnytskyi
et al. 2011). All the processes are dependent on orientation of azo group.
The earliest attempt was to rub the polyimide or polyvinyl alcohol coating on
glass surface and cast polymer solution to obtain the polymer film or polymerize LC
monomer inside rubbed cell. Due to dust and other dirt-related problems being
associated with these processes, newer process is explored. Magnetic and electrical
forces can be utilized for orientation. However, these processes are not much
14 Photoactive Liquid Crystalline Polymers
399