22 Cooperative Molecular Alignment Process Enabled by Scanning …
379
lie perpendicular to the polymer main chain [30, 44]. This suggests that SWaP aligns
the polymer main chains along the light scanning direction.
SWaP can also generate unidirectional molecular alignment even above the
isotropic temperature of monomer and resultant polymers, which has not been previously achieved in LCs using other photoalignment systems. Furthermore, the polymer
films recover their alignment after disappearing under heating above the isotropic
temperature of polymer, displaying a thermal memory effect of molecular alignment. The results achieved by SWaP clearly confirm the hypothesis that the polymer
main chain aligns along the light scanning direction and might affect neighboring
mesogens to align in the same direction. On the other hand, a film photopolymerized
by irradiation with uniform light showed a polydomain structure, which is typical
texture of nematic LC polymers. These observations elucidate that the scanning light
is powerful and important method to control the molecular alignment and to emerge
uniform optical anisotropy.
To demonstrate the generality of SWaP, we also tried to induce molecular alignment in various other chemical materials such as monomers, crosslinkers, photoinitiators, and/or nonreactive dye molecules [67, 68]. The photopolymerization conditions
were optimized regarding light intensity, temperature, and scanning rate to obtain
the highest birefringence and order parameter. POM images and polarized UV-vis
absorption spectra of the obtained films revealed that SWaP provided 1D molecularly
aligned polymer films which is composed of abovementioned materials. This results
clearly indicate that SWaP has simple processing and general materials designing
generation capabilities. Although the radical photopolymerization shown above can
be commonly used because of its generality of materials design, oxygen needs to
be removed to prevent photopolymerization from its disturbance. To avoid this critical issue, we explored a sample of a cationic system, and performed two SWaPs
onto a glass substrate and a flexible substrate running under ambient conditions. We
successfully generated 1D molecularly aligned polymer films by SWaP without a
glass cell or a nitrogen atmosphere. In addition to generality, the processing time
for induction of 1D molecular alignment was much decreased by using higher light
intensity and increased scanning rate.
22.3 Arbitrary 2D Molecular Alignment Patterns
by Scanning Wave Photopolymerization
In SWaP, complex and spatiotemporal scanning of incident light offer arbitrary
2D alignment patterns which is directed by light-triggered diffusion. We defined
SWaP as photopolymerization that spatial or temporal scanning light generate the
molecular alignment utilizing mass flow caused by molecular diffusion in the nonequilibrium state. Such characteristics indicates that even patterned photoirradiation
(e.g., a grating, a lattice, a honeycomb) could provide various complex molecular
alignment patterning induced by diffusion at the edge of the pattern.
379
lie perpendicular to the polymer main chain [30, 44]. This suggests that SWaP aligns
the polymer main chains along the light scanning direction.
SWaP can also generate unidirectional molecular alignment even above the
isotropic temperature of monomer and resultant polymers, which has not been previously achieved in LCs using other photoalignment systems. Furthermore, the polymer
films recover their alignment after disappearing under heating above the isotropic
temperature of polymer, displaying a thermal memory effect of molecular alignment. The results achieved by SWaP clearly confirm the hypothesis that the polymer
main chain aligns along the light scanning direction and might affect neighboring
mesogens to align in the same direction. On the other hand, a film photopolymerized
by irradiation with uniform light showed a polydomain structure, which is typical
texture of nematic LC polymers. These observations elucidate that the scanning light
is powerful and important method to control the molecular alignment and to emerge
uniform optical anisotropy.
To demonstrate the generality of SWaP, we also tried to induce molecular alignment in various other chemical materials such as monomers, crosslinkers, photoinitiators, and/or nonreactive dye molecules [67, 68]. The photopolymerization conditions
were optimized regarding light intensity, temperature, and scanning rate to obtain
the highest birefringence and order parameter. POM images and polarized UV-vis
absorption spectra of the obtained films revealed that SWaP provided 1D molecularly
aligned polymer films which is composed of abovementioned materials. This results
clearly indicate that SWaP has simple processing and general materials designing
generation capabilities. Although the radical photopolymerization shown above can
be commonly used because of its generality of materials design, oxygen needs to
be removed to prevent photopolymerization from its disturbance. To avoid this critical issue, we explored a sample of a cationic system, and performed two SWaPs
onto a glass substrate and a flexible substrate running under ambient conditions. We
successfully generated 1D molecularly aligned polymer films by SWaP without a
glass cell or a nitrogen atmosphere. In addition to generality, the processing time
for induction of 1D molecular alignment was much decreased by using higher light
intensity and increased scanning rate.
22.3 Arbitrary 2D Molecular Alignment Patterns
by Scanning Wave Photopolymerization
In SWaP, complex and spatiotemporal scanning of incident light offer arbitrary
2D alignment patterns which is directed by light-triggered diffusion. We defined
SWaP as photopolymerization that spatial or temporal scanning light generate the
molecular alignment utilizing mass flow caused by molecular diffusion in the nonequilibrium state. Such characteristics indicates that even patterned photoirradiation
(e.g., a grating, a lattice, a honeycomb) could provide various complex molecular
alignment patterning induced by diffusion at the edge of the pattern.