28 Photomechanical Effects in Crosslinked Liquid-Crystalline …
481
[10]. This reversible photoswitching of molecular order is the basis of the various
photosynergetic effects.
In addition, strong coupling between the alignment of mesogens and conformation of polymer chains plays a significant role in CLCPs [11, 12]. The deformation
of polymer chains is transformed to macroscopic deformation through crosslinks.
Therefore, macroscopic shapes can be altered along with the alignment change of
mesogens. Finkelmann et al. prepared CLCPs with uniaxial alignment of mesogens through two-step crosslinking method [13]. Poly(hydorogenmethyl siloxane) is
functionalized with mesogens and lightly crosslinked through hydrosilylation reaction. Then, the film is stretched to induce uniaxial alignment of mesogens, which
is fixed by further crosslinking. Through this procedure, homogeneous alignment
of mesogens is memorized as initial alignment of the CLCP film. These films show
contraction and expansion upon heating and cooling along with nematic–isotropic
transition. The strain induced in these systems was 20% in the pioneering work,
and has been enhanced to as high as 400% [12, 14]. In situ polymerization method
has also been developed to prepare CLCPs with polyacrylate and polymethcarylate backbones [15–20]. In this method, LC mono- and di-functional monomers are
polymerized in a cell coated with alignment layers (e.g., rubbed polyimide) to form
crosslinked network. As the alignment of mesogens is memorized during the formation of crosslinked network, CLCPs with complicated alignment of mesogens can
also be prepared with the aid of patterning technique [21–23].
By combining the photoinduced order–disorder transition in LC systems
and the strong coupling between nanostructures and macrostructures in CLCPs,
photoisormerization of dyes can be amplified to macroscopic deformation. Finkelmann et al. developed monodomain films of CLCPs with polysiloxane backbone
functionalized with azobenzene moieties [24]. Contraction of a film was successfully induced upon irradiation with UV light, which induces trans–cis isomerization
of azobenzene moieties and disturbs the alignment of mesogens. The initial length
is recovered along with cis–trans isomerization of azobenzene moieties. Furthermore, CLCP films with high concentration of azobenzene moieties, which were
prepared through copolymerization of azobenzene LC monomers and crosslinkers,
were found to show reversible bending–unbending behavior upon irradiation with
UV and visible light (Fig. 28.1b, c) [25, 26]. This bending behavior is attributed
to predominant absorption of UV light by chromophores near the surface of the
film, which leads to trans–cis isomerization, the alignment change of mesogens,
and deformation of polymer chains at the surface region. Furthermore, polydomain
films have been found to show axis-selective bending upon irradiation with linearly
polarized UV light (Fig. 28.2) [25].
As the photosynergetic transformation from nano to macro includes multiple
processes, the output can be affected by various factors. To make the full use of photosynergetic effects to achieve macroscopic response, adequate designs of molecules,
materials, and devices are essential. However, the molecular design of CLCPs has
481
[10]. This reversible photoswitching of molecular order is the basis of the various
photosynergetic effects.
In addition, strong coupling between the alignment of mesogens and conformation of polymer chains plays a significant role in CLCPs [11, 12]. The deformation
of polymer chains is transformed to macroscopic deformation through crosslinks.
Therefore, macroscopic shapes can be altered along with the alignment change of
mesogens. Finkelmann et al. prepared CLCPs with uniaxial alignment of mesogens through two-step crosslinking method [13]. Poly(hydorogenmethyl siloxane) is
functionalized with mesogens and lightly crosslinked through hydrosilylation reaction. Then, the film is stretched to induce uniaxial alignment of mesogens, which
is fixed by further crosslinking. Through this procedure, homogeneous alignment
of mesogens is memorized as initial alignment of the CLCP film. These films show
contraction and expansion upon heating and cooling along with nematic–isotropic
transition. The strain induced in these systems was 20% in the pioneering work,
and has been enhanced to as high as 400% [12, 14]. In situ polymerization method
has also been developed to prepare CLCPs with polyacrylate and polymethcarylate backbones [15–20]. In this method, LC mono- and di-functional monomers are
polymerized in a cell coated with alignment layers (e.g., rubbed polyimide) to form
crosslinked network. As the alignment of mesogens is memorized during the formation of crosslinked network, CLCPs with complicated alignment of mesogens can
also be prepared with the aid of patterning technique [21–23].
By combining the photoinduced order–disorder transition in LC systems
and the strong coupling between nanostructures and macrostructures in CLCPs,
photoisormerization of dyes can be amplified to macroscopic deformation. Finkelmann et al. developed monodomain films of CLCPs with polysiloxane backbone
functionalized with azobenzene moieties [24]. Contraction of a film was successfully induced upon irradiation with UV light, which induces trans–cis isomerization
of azobenzene moieties and disturbs the alignment of mesogens. The initial length
is recovered along with cis–trans isomerization of azobenzene moieties. Furthermore, CLCP films with high concentration of azobenzene moieties, which were
prepared through copolymerization of azobenzene LC monomers and crosslinkers,
were found to show reversible bending–unbending behavior upon irradiation with
UV and visible light (Fig. 28.1b, c) [25, 26]. This bending behavior is attributed
to predominant absorption of UV light by chromophores near the surface of the
film, which leads to trans–cis isomerization, the alignment change of mesogens,
and deformation of polymer chains at the surface region. Furthermore, polydomain
films have been found to show axis-selective bending upon irradiation with linearly
polarized UV light (Fig. 28.2) [25].
As the photosynergetic transformation from nano to macro includes multiple
processes, the output can be affected by various factors. To make the full use of photosynergetic effects to achieve macroscopic response, adequate designs of molecules,
materials, and devices are essential. However, the molecular design of CLCPs has