chain, and it involves massive motion of the whole polymer system. Though the
mechanism of the mass transition is still unclear, we can get ordered patterns through
this method upon holographic recording. Gratings are the products of mass transition, which we would introduce later in this entry (Yu et al. 2005a). Furthermore, the
motion in crosslinked LC elastomer system can be large enough for being visible
with the naked eye (Yu et al. 2011a).
One of the most interesting properties of photoresponsive LCs is photoinduced
phase transition. When azobenzene moieties possess soft alkyl substituents in their
benzene rings, they often show LC phase. Then, azobenzenes may play both roles of
photoresponsive groups and rodlike mesogens, as shown in Fig. 3c. Upon irradiation
of UV light, the well-known trans-to-cis photoisomerization occurs, which is accompanied by the photoinduced LC to isotropic phase transition, since the transazobenzene can be a mesogen and the cis-azobenzene never shows any LC phase
due to its bent shape. As a result, light can be easily applied to control the changes
between the ordered LCs and disordered isotropic states. This is very important for
their photonic applications.
Homopolymers
Homopolymers contain only one kind of photoresponsive mesogens in their macromolecular chains, incorporating LC properties with high-performance polymer
materials possessing a film-forming nature, high processability, easy-fabrication
characteristics, high corrosion resistance, and low manufacturing costs. More interestingly, the photoresponsive mesogens provide the designed LCP homopolymers
with photocontrolled features, as shown in Fig. 4. Among them, photochemical
phase transition, photo-triggered molecular cooperative motion, and photoinduced
alignment are the most important ones (Yu 2014a).
Rodlike molecules usually show axis-selective photochemical reaction, which
leads to the optical and physical anisotropy. Several materials that undergo an axisselective photochemical reaction have been studied, which mainly includes two
types: photoisomerizable azobenzenes and photocrosslinkable cinnamates. Several
factors that influence the photoalignment of LCPs have been reviewed, covering
(1) the photoreaction of the moieties, (2) the interaction preference between photoproducts and mesogens, (3) the azimuthal anchoring energy, and (4) pretilt formation
of the LC orientation (Kawatsuki 2011).
So far, there are two ways to enhance optical anisotropy in LCPs, that is, thermal
enhancement like thermal annealing and photoalignment enhancement such as
exposure to UV irradiation. For photocrosslinkable LCPs, the axis-selective photoirradiation induces small optical anisotropy, while thermal annealing would largely
enhance the parameters, resulting in molecular reorientation. Kawatsuki et al. prepared novel LCP films, in which the alignment could be controlled by irradiation
with UV light and thermal treatment (Kawatsuki et al. 2001). Besides, the direction
of molecular orientation can be adjusted by controlling the incident angle and the
422
X. Li and H. Yu
mechanism of the mass transition is still unclear, we can get ordered patterns through
this method upon holographic recording. Gratings are the products of mass transition, which we would introduce later in this entry (Yu et al. 2005a). Furthermore, the
motion in crosslinked LC elastomer system can be large enough for being visible
with the naked eye (Yu et al. 2011a).
One of the most interesting properties of photoresponsive LCs is photoinduced
phase transition. When azobenzene moieties possess soft alkyl substituents in their
benzene rings, they often show LC phase. Then, azobenzenes may play both roles of
photoresponsive groups and rodlike mesogens, as shown in Fig. 3c. Upon irradiation
of UV light, the well-known trans-to-cis photoisomerization occurs, which is accompanied by the photoinduced LC to isotropic phase transition, since the transazobenzene can be a mesogen and the cis-azobenzene never shows any LC phase
due to its bent shape. As a result, light can be easily applied to control the changes
between the ordered LCs and disordered isotropic states. This is very important for
their photonic applications.
Homopolymers
Homopolymers contain only one kind of photoresponsive mesogens in their macromolecular chains, incorporating LC properties with high-performance polymer
materials possessing a film-forming nature, high processability, easy-fabrication
characteristics, high corrosion resistance, and low manufacturing costs. More interestingly, the photoresponsive mesogens provide the designed LCP homopolymers
with photocontrolled features, as shown in Fig. 4. Among them, photochemical
phase transition, photo-triggered molecular cooperative motion, and photoinduced
alignment are the most important ones (Yu 2014a).
Rodlike molecules usually show axis-selective photochemical reaction, which
leads to the optical and physical anisotropy. Several materials that undergo an axisselective photochemical reaction have been studied, which mainly includes two
types: photoisomerizable azobenzenes and photocrosslinkable cinnamates. Several
factors that influence the photoalignment of LCPs have been reviewed, covering
(1) the photoreaction of the moieties, (2) the interaction preference between photoproducts and mesogens, (3) the azimuthal anchoring energy, and (4) pretilt formation
of the LC orientation (Kawatsuki 2011).
So far, there are two ways to enhance optical anisotropy in LCPs, that is, thermal
enhancement like thermal annealing and photoalignment enhancement such as
exposure to UV irradiation. For photocrosslinkable LCPs, the axis-selective photoirradiation induces small optical anisotropy, while thermal annealing would largely
enhance the parameters, resulting in molecular reorientation. Kawatsuki et al. prepared novel LCP films, in which the alignment could be controlled by irradiation
with UV light and thermal treatment (Kawatsuki et al. 2001). Besides, the direction
of molecular orientation can be adjusted by controlling the incident angle and the
422
X. Li and H. Yu