elastomer can interact with light and can misalign the long chains which make
drastic change of shape up to 60% when irradiated with appropriate light.
Among the LC phases, the nematic one, having approximately parallel orientation of mesogens along their longest axis, is the least ordered and less viscous state.
This enables easy manipulation of mesogen alignment than other LC phases. Hence,
nematic liquid crystalline materials are most commonly used in elastomer structure.
Photoactive liquid crystalline elastomers (LCEs) can be both main-chain and
side-chain type where the mesogenic moieties can be incorporated in the polymer
main chain (main-chain LCEs) or attached as pendant groups to the main polymer
backbone (side-chain LCEs), generally via flexible spacers. By nature elastomer
being highly flexible, additional spacer allows the polymer main chain or side chain
to accommodate the anisotropic orientation of mesogenic side groups.
Main-chain liquid crystal elastomers usually exhibit interesting properties such as
different regimes of elastic response; unconventional stress-strain relationship is one of
them, and the other is shape memory effect. The important parameters are their
macroscopic behavior and the nature of domain structure, microscopic mesogenic
phase structure, relaxation mechanism, and sample history, respectively. Further, they
are also interrelated. Side-chain LCP has the great advantage that the orientation is
decoupled from the main-chain polymer. This allows innumerable maneuverability in
respect to properties and orientation. Further, spacers will also have various options in
determining the properties. As the side on addition is done to the main chain, photochromic groups can as well be connected with ease to make it photoactive. Further, the
spacer and ligand to mesogen can be varied as per demand. Cross-linking can be done
by using either normal cross-linker or an azo-containing cross-linker (Fig. 3c).
Hyperbranched Architectures
Figure 3d represents hyperbranched architecture of azo-based photoactive LCP as
well as bisbenzylidene-based photoactive LCP. Both azo and bisbenzylidene have
the dual nature.
Branched polymers are known by the presence of branch points or the presence of
multiple numbers of end groups and make a special class of polymers lying between
linear polymers and polymer networks. The branched polymers can have hyperbranched architecture which is amorphous, although there are examples of crystalline hyperbranched polymers. These polymers, possessing many unreacted chain
ends, exhibit properties highly dependent on the nature of the end groups. Obviously, the modification of the end groups provides an excellent route for controlling/
modifying the properties of these polymers. Using multifunctional monomers hyperbranched polymers can be designed having selective end groups. These polymers
have additional processing attributes, such as low viscosity and high functionality.
The synthesis methodologies are plenty which include free radical, condensation,
ring opening, etc. Despite the high risk of gelation, the A 2 + B 3 polymerization, the
overwhelming need of industries encouraged researchers to carry out extensive
research to determine reaction parameters required to avoid gelation.
14 Photoactive Liquid Crystalline Polymers
397
drastic change of shape up to 60% when irradiated with appropriate light.
Among the LC phases, the nematic one, having approximately parallel orientation of mesogens along their longest axis, is the least ordered and less viscous state.
This enables easy manipulation of mesogen alignment than other LC phases. Hence,
nematic liquid crystalline materials are most commonly used in elastomer structure.
Photoactive liquid crystalline elastomers (LCEs) can be both main-chain and
side-chain type where the mesogenic moieties can be incorporated in the polymer
main chain (main-chain LCEs) or attached as pendant groups to the main polymer
backbone (side-chain LCEs), generally via flexible spacers. By nature elastomer
being highly flexible, additional spacer allows the polymer main chain or side chain
to accommodate the anisotropic orientation of mesogenic side groups.
Main-chain liquid crystal elastomers usually exhibit interesting properties such as
different regimes of elastic response; unconventional stress-strain relationship is one of
them, and the other is shape memory effect. The important parameters are their
macroscopic behavior and the nature of domain structure, microscopic mesogenic
phase structure, relaxation mechanism, and sample history, respectively. Further, they
are also interrelated. Side-chain LCP has the great advantage that the orientation is
decoupled from the main-chain polymer. This allows innumerable maneuverability in
respect to properties and orientation. Further, spacers will also have various options in
determining the properties. As the side on addition is done to the main chain, photochromic groups can as well be connected with ease to make it photoactive. Further, the
spacer and ligand to mesogen can be varied as per demand. Cross-linking can be done
by using either normal cross-linker or an azo-containing cross-linker (Fig. 3c).
Hyperbranched Architectures
Figure 3d represents hyperbranched architecture of azo-based photoactive LCP as
well as bisbenzylidene-based photoactive LCP. Both azo and bisbenzylidene have
the dual nature.
Branched polymers are known by the presence of branch points or the presence of
multiple numbers of end groups and make a special class of polymers lying between
linear polymers and polymer networks. The branched polymers can have hyperbranched architecture which is amorphous, although there are examples of crystalline hyperbranched polymers. These polymers, possessing many unreacted chain
ends, exhibit properties highly dependent on the nature of the end groups. Obviously, the modification of the end groups provides an excellent route for controlling/
modifying the properties of these polymers. Using multifunctional monomers hyperbranched polymers can be designed having selective end groups. These polymers
have additional processing attributes, such as low viscosity and high functionality.
The synthesis methodologies are plenty which include free radical, condensation,
ring opening, etc. Despite the high risk of gelation, the A 2 + B 3 polymerization, the
overwhelming need of industries encouraged researchers to carry out extensive
research to determine reaction parameters required to avoid gelation.
14 Photoactive Liquid Crystalline Polymers
397