28 Photomechanical Effects in Crosslinked Liquid-Crystalline …
483
28.2 Incorporation of Amorphous Polymers into CLCPs
Controlling of mechanical properties is essential for application of CLCPs as soft
actuators in various fields. Conventional CLCPs with polyacrylate backbone, which
have been prepared by in situ polymerization method, typically suffer from brittleness due to the high crosslinking density. With the purpose of enhancing mechanical
toughness of photomobile polymer materials, polymer bilayers (e.g., CLCP and
polyethylene [27, 28]) and composites (e.g., CLCP and carbon nanotube [29]) have
been developed. In addition, elastic modulus is a key parameter to be adequately
controlled for a variety of actual systems. Blending of multiple polymer components is a common strategy to improve mechanical properties of polymer materials.
In crosslinked systems, the formation of interpenetrating polymer network (IPN)
leads to blending of multiple components at nanometric scales [30]. Structures and
functions of constituents can be combined by the formation of IPN, as can be seen
in various systems including hydrogels [31, 32] and dielectric elastomers [33]. In
combination of LC polymers with various amorphous and semi-crystalline polymers, self-assembled structure of mesogens should be preserved to exert the function of LC polymers. Formation of IPN can be a powerful approach to construct
multi-component systems preserving stimuli-responsive properties of CLCPs, since
the structure of LC network is memorized by crosslinks. We developed IPN films
composed of crosslinked azobenzene LC polymer (PAzo) and various amorphous
polymers to improve mechanical and photoresponsive properties of photomobile
polymer materials (Fig. 28.3) [34, 35].
We prepared IPN films by sequential formation of CLCP and amorphous networks
[34]. First, a PAzo network was formed by copolymerization of azobenzene LC
monomers and crosslinkers in the presence of inert LCs. By polymerizing at LC
phase in a glass cell coated with rubbed polyimide, we obtained PAzo network with
homogeneous alignment. Then, the inert LCs were removed to yield a PAzo network
with numerous pores (PAzoTP), which could be used as a template for the formation
of IPN structures. After introducing methacrylate monomers—methyl methacrylate (MMA), n-butyl methacrylate (BMA) or n-dodecyl methacrylate (DDMA)—
and crosslinkers into PAzoTP, thermally initiated polymerization was conducted
to form the second network. The resultant films (PAzo/PMMA, PAzo/PBMA, and
PAzo/PDDMA) showed homogeneous alignment of mesogens, which was confirmed
by polarizing optical microscopy and polarized absorption spectroscopy. This result
implies that the uniaxial alignment is memorized by crosslinks in the PAzo network,
which was preserved even after the incorporation of amorphous components.
Photoinduced bending behaviors of IPN films were observed in comparison with
PAzoTP and pristine PAzo films (Fig. 28.4). Deformation of IPN films could be
induced in the same way as conventional PAzo films upon irradiation with UV and
visible light. The rate of bending increased with the length of alkyl chains of amorphous components. Especially, PAzo/PDDMA films showed much faster bending
than the other films.
483
28.2 Incorporation of Amorphous Polymers into CLCPs
Controlling of mechanical properties is essential for application of CLCPs as soft
actuators in various fields. Conventional CLCPs with polyacrylate backbone, which
have been prepared by in situ polymerization method, typically suffer from brittleness due to the high crosslinking density. With the purpose of enhancing mechanical
toughness of photomobile polymer materials, polymer bilayers (e.g., CLCP and
polyethylene [27, 28]) and composites (e.g., CLCP and carbon nanotube [29]) have
been developed. In addition, elastic modulus is a key parameter to be adequately
controlled for a variety of actual systems. Blending of multiple polymer components is a common strategy to improve mechanical properties of polymer materials.
In crosslinked systems, the formation of interpenetrating polymer network (IPN)
leads to blending of multiple components at nanometric scales [30]. Structures and
functions of constituents can be combined by the formation of IPN, as can be seen
in various systems including hydrogels [31, 32] and dielectric elastomers [33]. In
combination of LC polymers with various amorphous and semi-crystalline polymers, self-assembled structure of mesogens should be preserved to exert the function of LC polymers. Formation of IPN can be a powerful approach to construct
multi-component systems preserving stimuli-responsive properties of CLCPs, since
the structure of LC network is memorized by crosslinks. We developed IPN films
composed of crosslinked azobenzene LC polymer (PAzo) and various amorphous
polymers to improve mechanical and photoresponsive properties of photomobile
polymer materials (Fig. 28.3) [34, 35].
We prepared IPN films by sequential formation of CLCP and amorphous networks
[34]. First, a PAzo network was formed by copolymerization of azobenzene LC
monomers and crosslinkers in the presence of inert LCs. By polymerizing at LC
phase in a glass cell coated with rubbed polyimide, we obtained PAzo network with
homogeneous alignment. Then, the inert LCs were removed to yield a PAzo network
with numerous pores (PAzoTP), which could be used as a template for the formation
of IPN structures. After introducing methacrylate monomers—methyl methacrylate (MMA), n-butyl methacrylate (BMA) or n-dodecyl methacrylate (DDMA)—
and crosslinkers into PAzoTP, thermally initiated polymerization was conducted
to form the second network. The resultant films (PAzo/PMMA, PAzo/PBMA, and
PAzo/PDDMA) showed homogeneous alignment of mesogens, which was confirmed
by polarizing optical microscopy and polarized absorption spectroscopy. This result
implies that the uniaxial alignment is memorized by crosslinks in the PAzo network,
which was preserved even after the incorporation of amorphous components.
Photoinduced bending behaviors of IPN films were observed in comparison with
PAzoTP and pristine PAzo films (Fig. 28.4). Deformation of IPN films could be
induced in the same way as conventional PAzo films upon irradiation with UV and
visible light. The rate of bending increased with the length of alkyl chains of amorphous components. Especially, PAzo/PDDMA films showed much faster bending
than the other films.