257
siloxane- organic backbones (Fig. 11.6a). The thermotropic behavior had a direct
correlation with the content of mesogenic group, the length of siloxane segment and
the chemical structure of the starting diacids.
Petr and Hammond (2011) described a RT side-chain liquid crystalline polymer
with a rapid photoresponsive behavior, which combined a side-on azobenzene LC
with a low T g polysiloxane backbone (Fig. 11.6b) and exhibited a nematic LC phase
at RT. It was argued that the side-on arrangement (with mesogens parallel with the
polymer’s backbone) is more suitable for highly responsive chain conformations
compared to the end-on side chain LC functionalization. The relatively short
response times are relevant for actuator and responsive elastomer applications.
Siloxane polymers possessing both photoactive and liquid crystalline properties
have been prepared using click chemistry between azo-diazides and a trisiloxanediyne (Pandey et al. 2012) (Fig. 11.6c). Apart from LC behavior, the trans-cis isomerization has led to a photomechanical effect.
Side-chain-end-on and side-on LCPs with polysiloxane backbones have also
been synthesized by grafting mesogenic monomers to poly(3mercaptopropylmethylsiloxane) via thiol-ene click chemistry (Yang et al. 2013).
According to Yang et al. (2013) when mercapto- groups were deliberately left
unmodified, these were additionally used as crosslinking sites to prepare LCE fibers
and films, but the thermal actuation effects of these films were modest.
Another method for the synthesis of side-chain LCPs is the ATRP. Zhang et al.
(2014) synthesized a series of LC block copolymers using a functionalized
PDMS-Br as a macroinitiator (same PDMS block length) and 11-(4-cyano-4′biphenyloxy)undecyl polymethacrylate LC with different block lengths. The transition temperatures increased with increasing the LC block length. These authors,
using transmission electron microscopy (TEM), investigated the morphology of the
diblock copolymers after thermal annealing, thus observing that the microphaseseparated nanostructure exhibits a transition from the lamellar phase to dispersed
cylinder phase with the increase of the LC block content. In addition, higher values
of dielectric permittivity for block-copolymers than for pure LCP were observed,
which was attributed to the increase of orientation order parameter in confined
microdomains. Likewise, the induced orientation of the side-chain mesogenic
groups (perpendicular or parallel to surface) tunes the permittivity. Similarly,
diblock copolymers PDMS-b-poly(6-(4-(40-cyano-phenylazo)phenoxy)-hexyl
acrylate) with well-defined structures and relatively narrow Mw distributions were
prepared by Wei et al. (2014) and exhibited smectic mesophase (Fig. 11.6d). These
authors indicated that the mesomorphic structures of LC subphase of the diblock
copolymers were identical to those of the homopolymer, and the morphology of
these annealed samples also revealed microphase separation, with lamellar or spherical morphology, depending on composition. The photoisomerization behavior of
the annealed block copolymer film depends on the microphase- separated
morphologies.
On the other hand, Racles and Cozan (2014) reported that siloxane copolymers
containing pendant azomethine mesogenic units bound in ~50% of the structural
units (Fig. 11.7) showed single T g , so there was no phase separation. These
11 Reactive and Functional Silicones for Special Applications
siloxane- organic backbones (Fig. 11.6a). The thermotropic behavior had a direct
correlation with the content of mesogenic group, the length of siloxane segment and
the chemical structure of the starting diacids.
Petr and Hammond (2011) described a RT side-chain liquid crystalline polymer
with a rapid photoresponsive behavior, which combined a side-on azobenzene LC
with a low T g polysiloxane backbone (Fig. 11.6b) and exhibited a nematic LC phase
at RT. It was argued that the side-on arrangement (with mesogens parallel with the
polymer’s backbone) is more suitable for highly responsive chain conformations
compared to the end-on side chain LC functionalization. The relatively short
response times are relevant for actuator and responsive elastomer applications.
Siloxane polymers possessing both photoactive and liquid crystalline properties
have been prepared using click chemistry between azo-diazides and a trisiloxanediyne (Pandey et al. 2012) (Fig. 11.6c). Apart from LC behavior, the trans-cis isomerization has led to a photomechanical effect.
Side-chain-end-on and side-on LCPs with polysiloxane backbones have also
been synthesized by grafting mesogenic monomers to poly(3mercaptopropylmethylsiloxane) via thiol-ene click chemistry (Yang et al. 2013).
According to Yang et al. (2013) when mercapto- groups were deliberately left
unmodified, these were additionally used as crosslinking sites to prepare LCE fibers
and films, but the thermal actuation effects of these films were modest.
Another method for the synthesis of side-chain LCPs is the ATRP. Zhang et al.
(2014) synthesized a series of LC block copolymers using a functionalized
PDMS-Br as a macroinitiator (same PDMS block length) and 11-(4-cyano-4′biphenyloxy)undecyl polymethacrylate LC with different block lengths. The transition temperatures increased with increasing the LC block length. These authors,
using transmission electron microscopy (TEM), investigated the morphology of the
diblock copolymers after thermal annealing, thus observing that the microphaseseparated nanostructure exhibits a transition from the lamellar phase to dispersed
cylinder phase with the increase of the LC block content. In addition, higher values
of dielectric permittivity for block-copolymers than for pure LCP were observed,
which was attributed to the increase of orientation order parameter in confined
microdomains. Likewise, the induced orientation of the side-chain mesogenic
groups (perpendicular or parallel to surface) tunes the permittivity. Similarly,
diblock copolymers PDMS-b-poly(6-(4-(40-cyano-phenylazo)phenoxy)-hexyl
acrylate) with well-defined structures and relatively narrow Mw distributions were
prepared by Wei et al. (2014) and exhibited smectic mesophase (Fig. 11.6d). These
authors indicated that the mesomorphic structures of LC subphase of the diblock
copolymers were identical to those of the homopolymer, and the morphology of
these annealed samples also revealed microphase separation, with lamellar or spherical morphology, depending on composition. The photoisomerization behavior of
the annealed block copolymer film depends on the microphase- separated
morphologies.
On the other hand, Racles and Cozan (2014) reported that siloxane copolymers
containing pendant azomethine mesogenic units bound in ~50% of the structural
units (Fig. 11.7) showed single T g , so there was no phase separation. These
11 Reactive and Functional Silicones for Special Applications