22 Cooperative Molecular Alignment Process Enabled by Scanning …
377
has advantages such as electronics-free, single-step, noncontact, inexpensive, and,
in general, applicable to incorporation directly into existing fabrication production
lines.
22.2 One-Dimensional Molecular Alignment in LC
Polymer Films by Scanning Wave
Photopolymerization
First, we induced one-dimensional molecular alignment by SWaP with unidirectional light scanning with 250-μm-slit ultraviolet (UV) light at the wavelength of
365 nm to photopolymerize the mixture composed by sample 1 including M1, C1,
and P1 with 97:3 molar ratio of M1:C1 in a handmade glass cell (Fig. 22.2a). The
cell thickness was 3 μm, which shows the maximum degree of molecular alignment induced by SWaP. The photopolymerization temperature was 100 °C at which
the resultant polymer exhibits a nematic phase, and then the sample was cooled to
room temperature. The obtained LC film was transparent and had no color. Polarized optical microscope (POM) observation of the film under crossed polarizers
showed brightness at 45° with the light scanning direction. On the other hand, it
became completely dark when the angle between polarizers and the light scanning
direction was 0° or 90° (Fig. 22.2b). Detailed POM observations using a tint plate
with a retardation of 137 nm elucidate that the film is aligned parallel to the light
scanning direction. Furthermore, the resultant film possessed high birefringence of
0.12. This value is comparable with typical birefringence levels required for current
LC devices [40]. This result means that the mesogenic units of the monomer M1
were unidirectionally aligned along the light scanning direction. For further investigation of the molecular alignment behavior, we measured the polarized UV-visible
(UV-vis) absorption spectra of the resultant film (Fig. 22.2c). Polar plots of the
absorption band of cyanobiphenyl moieties exhibited the maximum value along the
scanning direction (Fig. 22.2d). We calculated the planar alignment order parameter
(S) from the equation S = (A || − A ⊥ )/(A || + 2 A ⊥ ), where A || and A ⊥ denote
the parallel or perpendicular absorbance which is the direction of polarized incident light. The order parameter value was found to be 0.52. This value demonstrates
that the degree of molecular alignment is achieved in similar chemical systems with
current photoalignment methods [13, 28–30]. Furthermore, polarized infrared (IR)
absorption spectroscopy allowed one to identify functional groups in the films. As
a result, the IR spectra revealed that carbonyl moieties were aligned anisotropically
besides cyanobiphenyl moieties throughout the film (Fig. 22.2e). On the other hand,
the alignment direction of carbonyl moieties was orthogonally oriented to that of the
cyanobiphenyl moieties. This result is rationalized by the behavior of mesogens in
side-chain LC polymers. The side chains moieties are aligned alongside the polymer
main chain under a shear-flow field [41–43], and thus the carbonyl groups generally
377
has advantages such as electronics-free, single-step, noncontact, inexpensive, and,
in general, applicable to incorporation directly into existing fabrication production
lines.
22.2 One-Dimensional Molecular Alignment in LC
Polymer Films by Scanning Wave
Photopolymerization
First, we induced one-dimensional molecular alignment by SWaP with unidirectional light scanning with 250-μm-slit ultraviolet (UV) light at the wavelength of
365 nm to photopolymerize the mixture composed by sample 1 including M1, C1,
and P1 with 97:3 molar ratio of M1:C1 in a handmade glass cell (Fig. 22.2a). The
cell thickness was 3 μm, which shows the maximum degree of molecular alignment induced by SWaP. The photopolymerization temperature was 100 °C at which
the resultant polymer exhibits a nematic phase, and then the sample was cooled to
room temperature. The obtained LC film was transparent and had no color. Polarized optical microscope (POM) observation of the film under crossed polarizers
showed brightness at 45° with the light scanning direction. On the other hand, it
became completely dark when the angle between polarizers and the light scanning
direction was 0° or 90° (Fig. 22.2b). Detailed POM observations using a tint plate
with a retardation of 137 nm elucidate that the film is aligned parallel to the light
scanning direction. Furthermore, the resultant film possessed high birefringence of
0.12. This value is comparable with typical birefringence levels required for current
LC devices [40]. This result means that the mesogenic units of the monomer M1
were unidirectionally aligned along the light scanning direction. For further investigation of the molecular alignment behavior, we measured the polarized UV-visible
(UV-vis) absorption spectra of the resultant film (Fig. 22.2c). Polar plots of the
absorption band of cyanobiphenyl moieties exhibited the maximum value along the
scanning direction (Fig. 22.2d). We calculated the planar alignment order parameter
(S) from the equation S = (A || − A ⊥ )/(A || + 2 A ⊥ ), where A || and A ⊥ denote
the parallel or perpendicular absorbance which is the direction of polarized incident light. The order parameter value was found to be 0.52. This value demonstrates
that the degree of molecular alignment is achieved in similar chemical systems with
current photoalignment methods [13, 28–30]. Furthermore, polarized infrared (IR)
absorption spectroscopy allowed one to identify functional groups in the films. As
a result, the IR spectra revealed that carbonyl moieties were aligned anisotropically
besides cyanobiphenyl moieties throughout the film (Fig. 22.2e). On the other hand,
the alignment direction of carbonyl moieties was orthogonally oriented to that of the
cyanobiphenyl moieties. This result is rationalized by the behavior of mesogens in
side-chain LC polymers. The side chains moieties are aligned alongside the polymer
main chain under a shear-flow field [41–43], and thus the carbonyl groups generally