This same group of Seki extended the concept to an azobenzene-containing block
copolymer with a non-photoresponsive polystyrene block. Similarly to the case of
PEO, block molecular weights were selected to obtain morphologies of PS cylinders.
Again, thin films of this material were annealed at temperatures within the smectic
phase of the azobenzene liquid crystalline block and above the glass transition
temperatures of the two blocks. This led to cylinders perpendicular to the plane.
Analogously to the PEO case, illumination with linearly polarized light followed by
the subsequent annealing step led to cylinders lying down in the plane and perpendicular to the polarization direction of light. Direction of the cylinders in the plane
can be changed by subsequent treatments with other linearly polarization directions.
An additional degree of morphology control can be added by illuminating the lied
down cylinders with normally incident nonpolarized light. This leads to an orientation of the azo chromophores in the direction of the propagating vector of light since
nonpolarized light normally reaching the film contains all the polarization directions
in the plane. After this irradiation step and the annealing at the mesophase, an
orientation of the cylinders perpendicular to the substrate was obtained back. This
sequence of experiments demonstrates the robustness of the system and processing
methodology being possible to switch by light (and subsequent thermal treatment)
from one morphology to other (Morikawa et al. 2007). Besides PEO and PS, block
copolymer systems with poly(butyl methacrylate) (PBMA) as non-photoresponsive
block were also used to show this photoalignment principle. In addition, photocontrol of the morphologies was demonstrated in systems with a nematic phase,
demonstrating that the smectic phase is not a requisite for the effect.
In all the cases, the rational design of block copolymers leads to a material with
non-photoresponsive nanocylinders embedded in an azobenzene-rich liquid crystalline majority phase with a mesophase at temperatures above the melting or
glass transition temperature of the non-photoresponsive block (Seki 2014). With
these systems in hand, an exquisite control of the nanocylinder orientation can be
exerted by polarized light, opening new opportunities for the preparation of
functional nanopatterned devices.
Light-Induced Mechanical Actuation in Azobenzene Containing
Liquid Crystalline Systems
The large conformational changes of azobenzene chromophores upon photoisomerization make them suitable molecular building units to prepare soft materials
able to transduce light energy into mechanical work. Incorporation of the
azobenzene molecule in polymer network architectures has been extensively
explored to turn the microscopic photoinduced changes of the chromophores into a
macroscopic deformation of the material. Reversible light-induced expansion and
contraction using light of different wavelengths was initially demonstrated in
azobenzene cross-linked amorphous polymers; however, the amount of deformation,
in the order of 0.2%, was far from being interesting for applications (Eisenbach 1980).
Much larger contractions along the director reaching 20% were first induced by light by
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
469
copolymer with a non-photoresponsive polystyrene block. Similarly to the case of
PEO, block molecular weights were selected to obtain morphologies of PS cylinders.
Again, thin films of this material were annealed at temperatures within the smectic
phase of the azobenzene liquid crystalline block and above the glass transition
temperatures of the two blocks. This led to cylinders perpendicular to the plane.
Analogously to the PEO case, illumination with linearly polarized light followed by
the subsequent annealing step led to cylinders lying down in the plane and perpendicular to the polarization direction of light. Direction of the cylinders in the plane
can be changed by subsequent treatments with other linearly polarization directions.
An additional degree of morphology control can be added by illuminating the lied
down cylinders with normally incident nonpolarized light. This leads to an orientation of the azo chromophores in the direction of the propagating vector of light since
nonpolarized light normally reaching the film contains all the polarization directions
in the plane. After this irradiation step and the annealing at the mesophase, an
orientation of the cylinders perpendicular to the substrate was obtained back. This
sequence of experiments demonstrates the robustness of the system and processing
methodology being possible to switch by light (and subsequent thermal treatment)
from one morphology to other (Morikawa et al. 2007). Besides PEO and PS, block
copolymer systems with poly(butyl methacrylate) (PBMA) as non-photoresponsive
block were also used to show this photoalignment principle. In addition, photocontrol of the morphologies was demonstrated in systems with a nematic phase,
demonstrating that the smectic phase is not a requisite for the effect.
In all the cases, the rational design of block copolymers leads to a material with
non-photoresponsive nanocylinders embedded in an azobenzene-rich liquid crystalline majority phase with a mesophase at temperatures above the melting or
glass transition temperature of the non-photoresponsive block (Seki 2014). With
these systems in hand, an exquisite control of the nanocylinder orientation can be
exerted by polarized light, opening new opportunities for the preparation of
functional nanopatterned devices.
Light-Induced Mechanical Actuation in Azobenzene Containing
Liquid Crystalline Systems
The large conformational changes of azobenzene chromophores upon photoisomerization make them suitable molecular building units to prepare soft materials
able to transduce light energy into mechanical work. Incorporation of the
azobenzene molecule in polymer network architectures has been extensively
explored to turn the microscopic photoinduced changes of the chromophores into a
macroscopic deformation of the material. Reversible light-induced expansion and
contraction using light of different wavelengths was initially demonstrated in
azobenzene cross-linked amorphous polymers; however, the amount of deformation,
in the order of 0.2%, was far from being interesting for applications (Eisenbach 1980).
Much larger contractions along the director reaching 20% were first induced by light by
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
469