reactive discotic or calamitic LCs may improve the angular dependence of contrast
and grayscale inversion. The oriented LC molecules in a monomeric state can be
stabilized by the photopolymerization. Optical properties are finely tuned by applying mechanical force, electric or magnetic fields, and polarized light. Compared to
the conventional mechanical shearing method, a thinner film with diverse optical
properties is obtained.
Blue phase (BP) is promising for LCD because it does not require any alignment
layers and moreover reveals low power consumption with broader viewing angle.
Electro-optical switching time of BP is also fast with the aid of an external electric
field. The optically isotropic blue phase formed between the helical cholesteric phase
and the isotropic phase becomes anisotropic when a strong field is applied. However,
the blue phase typically appears within a very narrow temperature range. The BP can
be stabilized by employing the anisotropic networks, resulting in broadening the
temperature range.
Optical films can be fabricated on the basis of the patternable characteristics of
RM. Locally confined pattern formed through the use of a shadow mask during the
polymerization process can control the director of the LC molecules (Fig. 5). It
implies that the angle between the optical axis of the film and the polarization
direction is locally controllable. The patterned optical retarder is manufactured by
the irradiation of UV light in the RM mixtures exhibiting I and N phases. This optical
film has been used in a transflective LCD to resolve a narrow viewing angle (Liu and
Broer 2014).
Patterns are also inscribed by means of an interference wave pattern. In the LC
and azobenzene chromophore system, the bent-shaped cis-conformer reduces the
order of mesogenic molecules. Therefore, the N-to-I transition temperature (T NI ) of
the mesogens with cis-form is much lower than that with trans-form. Based on the
selective stability, the phase-mode holographic gratings with high resolution are
attained by illuminating a proper wavelength of light. The trans-cis isomerization is
induced in the bright area of the interference patterns. With LMWLC alone, it is
difficult to obtain stable holographic gratings because of high mobility. The composite films consisting of cross-linked anisotropic LC networks by using the RM are
appropriate for the stabilization of gratings.
Conductive Films
The discotic molecules can be self-assembled together by the strong π-π interactions
and subsequently form nano-columns. Such conjugated columnar structures may
facilitate the charge transport at the molecular level. The discotic core is conductive
by π-conjugation, while the outer alkyl groups insulate electrons. Therefore, the
electrical properties of the discotic LC are strongly dependent on the packing state of
the core. The discotic RMs are assembled to an electrically conductive columnar
phase and immobilized in the anisotropic networks. The ease of film formation is
another key feature of RM. Charge carrier mobility of the discotic LC molecules can
be measured using the pulse radiolysis time-resolved microwave conductivity technique, time of flight, and steady-state space-charge-limited current. The fabrication
of defect-free films over a large area is a major challenge to maintain the high carrier
102
D.-Y. Kim et al.
and grayscale inversion. The oriented LC molecules in a monomeric state can be
stabilized by the photopolymerization. Optical properties are finely tuned by applying mechanical force, electric or magnetic fields, and polarized light. Compared to
the conventional mechanical shearing method, a thinner film with diverse optical
properties is obtained.
Blue phase (BP) is promising for LCD because it does not require any alignment
layers and moreover reveals low power consumption with broader viewing angle.
Electro-optical switching time of BP is also fast with the aid of an external electric
field. The optically isotropic blue phase formed between the helical cholesteric phase
and the isotropic phase becomes anisotropic when a strong field is applied. However,
the blue phase typically appears within a very narrow temperature range. The BP can
be stabilized by employing the anisotropic networks, resulting in broadening the
temperature range.
Optical films can be fabricated on the basis of the patternable characteristics of
RM. Locally confined pattern formed through the use of a shadow mask during the
polymerization process can control the director of the LC molecules (Fig. 5). It
implies that the angle between the optical axis of the film and the polarization
direction is locally controllable. The patterned optical retarder is manufactured by
the irradiation of UV light in the RM mixtures exhibiting I and N phases. This optical
film has been used in a transflective LCD to resolve a narrow viewing angle (Liu and
Broer 2014).
Patterns are also inscribed by means of an interference wave pattern. In the LC
and azobenzene chromophore system, the bent-shaped cis-conformer reduces the
order of mesogenic molecules. Therefore, the N-to-I transition temperature (T NI ) of
the mesogens with cis-form is much lower than that with trans-form. Based on the
selective stability, the phase-mode holographic gratings with high resolution are
attained by illuminating a proper wavelength of light. The trans-cis isomerization is
induced in the bright area of the interference patterns. With LMWLC alone, it is
difficult to obtain stable holographic gratings because of high mobility. The composite films consisting of cross-linked anisotropic LC networks by using the RM are
appropriate for the stabilization of gratings.
Conductive Films
The discotic molecules can be self-assembled together by the strong π-π interactions
and subsequently form nano-columns. Such conjugated columnar structures may
facilitate the charge transport at the molecular level. The discotic core is conductive
by π-conjugation, while the outer alkyl groups insulate electrons. Therefore, the
electrical properties of the discotic LC are strongly dependent on the packing state of
the core. The discotic RMs are assembled to an electrically conductive columnar
phase and immobilized in the anisotropic networks. The ease of film formation is
another key feature of RM. Charge carrier mobility of the discotic LC molecules can
be measured using the pulse radiolysis time-resolved microwave conductivity technique, time of flight, and steady-state space-charge-limited current. The fabrication
of defect-free films over a large area is a major challenge to maintain the high carrier
102
D.-Y. Kim et al.